Carbon Fiber Reinforced Polymers (CFRP): Running Parameters, Tool Life, & Safety Tips

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Carbon Fiber Reinforced Polymers (CFRP) is a collection of carbon fibers that, when bound together via resin, creates a material with a wide range of application possibilities. It’s strong, durable, and resistant to corrosion, making it an advantageous material for use in several advanced industries, including the aerospace and automotive industries. Despite its unique abilities, however, machining CFRP is not without its set of challenges, all of which machinists must be cognizant of to achieve desired results. Once CFRP is properly understood and the right cutting tool is selected for the job, the next step is to properly set running parameters for your application.

cfrp laminate material

Running Parameters

Comparison of Metal Machining vs Composite Machining

When machining CFRP, the suggested running parameters are to have a high RPM with low feed rates. Feed rates will need to be adjusted to account for heat minimization, while RPMs may need to be dialed back to prevent excessive fraying, tearing, or splitting of fibers when cutting.

In metal machining, the tool cuts away at material, forming chips. This is possible due to the formation of the metal having natural fracture and stress lines that can be wedged by the cutting tool to create a chip. Unlike metals, machining carbon fiber does not peel away material but rather fracture and break the fibers and resin.

Milling vs Drilling Carbon Fiber

Composite holemaking or drilling is found to be more challenging than milling carbon fiber. It generates more dust due to the drilling speed. Using specific tooling for composites will be crucial in effective drilling. When machining holes, the carbon fiber will relax, creating undersized holes which requires extensive adjustments that are best automated for efficiency.

For help mitigating the challenges of composite holemaking, read Overcoming Composite Holemaking Challenges and browse CoreHog’s offering of drills, specially engineered to mitigate all-too-common holemaking headaches. To achieve better finish and avoid delamination, it is recommended to utilize conventional milling over climb milling within composites contrary to what is recommended in metal machining.

corehog cfrp drills

Within the aerospace industry, drilling is the most common application in machining. Like milling, performing operations such as pecking may be preferred even with increased cycle time if it reduces any chances of error that result in scrapping of the part.

Running Parallel to Grain of Fibers

While every part is different, there is a method for reducing fraying, chipping, or delamination by cutting parallel to the fiber direction when possible. This can be like cutting along the grain of wood instead of cutting perpendicular or at an angle to the grain.

Coolant Applications

The use of coolant when machining CFRP can either benefit or negatively affect the part depending on the application. The preferred coolant of choice for machining carbon fiber is typically using water or a water-soluble coolant. This is due to composites having a porous surface that could allow contaminates to enter the part itself. By using water, it prevents any issues after machining where adhesives or paint may need to be applied to the part that otherwise would not have adhered properly with contaminates present.

cnc machine in the cut with coolant

High Scrapping Costs

Many composite parts are unique in shape and size with custom molded designs that create a large initial cost prior to the machining stage. After the part is molded near to its shape, machining is often used to finish the part or drill holes where needed to finalize the part.

Importance of Considering Machining Challenges to Avoid Scrapping

Having a set process that is consistent and reliable is important in helping to prevent scrapping. Eliminating human error with machines that can monitor the entire process while automating tool changes when tools are worn, avoids issues before they can happen. A key factor is ensuring the setup is correct, having the right tooling, tool path, and coolant option to perform the operation effectively and accurately. With some parts serving critical functions and with a high cost, there is no exception for poor finish or incorrect cuts emphasizing the importance of having a procedure that gets the job done the right way.

Composite Cutting Tool Life Management

Wear Rate & its Effects 

Due to carbon fiber’s abrasion on the cutting tools, a rapid decrease in cutting quality will occur as soon as the tool begins to dull. Fibers will be grabbed instead of fractured, causing fraying and damage to the part. Therefore, tool life should be vigilantly monitored to replace the tool before reaching the point of dullness.

Developing a Process for Success

Unlike metal machining where tools may be utilized until they show signs of wear, this method would be unideal for CFRP as the highly expensive part could be ruined or damaged causing scrapping costs and time. It is good practice to take preventative measures by taking note of typical wear of your tools and using that information to set tool changes before it dulls. Noting tool changes and having high interval checks on cutting and dimension quality will aid in avoiding poor finish or scrapping. Some machines are equipped with tool life management systems which will greatly reduce the chances of having to scrap a part because of tool dullness.

Safety Practices When Machining CFRP

Being that chips are not formed when machining CFRP, and instead, the material is fractured, it creates dust that can spread throughout the air and other surfaces. Not only does this cause hazardous conditions for anyone nearby who may inhale the dust, but the dust is also conductive, which can ruin electronics. To avoid these issues, two different extraction methods can be used depending on the needs of the application.

Wet vs Dry Extraction

The two options for dust extraction are using coolant (wet) or vacuuming (dry). Choosing between the two is dependent on the application, but mostly dictated by the size of the application. Smaller scale machining can be contained through vacuuming, but larger applications would require coolant as vacuuming a large area may be challenging. If a lot of heat will be generated, then it is necessary to have a water-soluble coolant. This would also benefit the use of diamond tooling as they will wear faster at lower temperatures in comparison to carbide tooling. Another would be the dust collection would remain contained with the liquid preventing any airborne exposure.

Disposal Considerations

One benefit of vacuuming over coolant is the disposal process. After machining, the coolant/dust mix would require post-treatment to remove excess water before being transferred to a landfill. This would incur additional costs to the process which may cause some to lean towards vacuuming if heat is not an issue.

Conclusion

With CFRP’s wide range of uses and desirable mechanical properties for its applications, comes the effect of its challenges in machining and high cost of scrapping. Refining this process will be essential for the growing demand of carbon fiber machining in the near future. For more information on CFRP, specifically related to material properties and tool selection, read In the Loupe’s complementary post “Carbon Fiber Reinforced Polymers (CFRP): Material Properties & Tool Selection”.

Carbon Fiber Reinforced Polymers (CFRP): Material Properties & Tool Selection

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Carbon Fiber Reinforced Polymers (CFRP) is composed of carbon fibers, bound with resin, to create a groundbreaking material that has proven to have limitless application possibilities in a wide range of composite cutting industries. Due to its attractive properties of strength, durability, corrosion resistance, and its lightweight nature, there has been a rising utilization of carbon fiber in the aerospace and automotive industries. However, it does not come without its own set of challenges in comparison to typical metal machining.

cfrp material

CFRP Properties

CFRP has a high resistance to being deformed without permanent effects (elastic modulus), resistance to tension, low thermal conductivity, and low thermal expansion. While many of these properties are ideal for many applications, there are unique effects that must be considered when machining.

Abrasiveness

CFRP’s high elastic modulus makes it highly abrasive, causing challenges in tool wear and tool life that must be addressed. For example, while milling typical metals, clean chips are formed and ejected. But milling carbon fiber is like sanding, where material is removed in the form of dust particles.

Abrasion is a large issue in carbon fiber machining as it is responsible for poor tool life and dullness of the tool that can cause a part to be scrapped. As soon as a tool begins to dull, it will cause poor part finish and increase the chances of delamination and fraying.

Causes of Heat Generation

Typically, in metal machining, most of the heat is transferred into the chips with a fraction of the heat into the part and workpiece. Due to CFRP’s low thermal conductivity and no chips formed to dissipate the heat, most of the heat is transferred to the tool and part. This heat is unideal, as it will cause more tool wear and potentially cause damage, resulting in delamination.

For more information on composite delamination, read “Overcoming Composite Holemaking Challenges.”

Varying Properties

No composition of CFRP is made the same, meaning that operating parameters can vary. There is no one-solution-fits-all for every application, as it is often found to be less predictable than metals due to its varying properties. These varying components includes the fiber type, fiber density, resin type, layup orientations, thickness, matrix hardness, and heat sensitivity, which all must be taken into consideration.

Methods to Reduce CFRP Issues

Having high interval checks to monitor cutting and dimension quality to catch any errors before they may be irreversible is one way address CFRP machining issues. Another method to tackle CFRP would be through picking the right tool for the application.

Selecting a CFRP Tool

Tooling material type (substrate) and geometry both determine the quality of cutting the carbon fiber and durability of the tool.

Tooling Substrate/Coating

When selecting a tool to machine CFRP materials, it is essential to opt for a tool that is strong, sharp, and resistant to the abrasive properties that CFRP holds. Many machinists opt for solid carbide cutters with diamond coatings such as DLC, CVD, or PCD, as they provide the tooling with increased tool life and improved cutting action. These tools provide added hardness and abrasion resistance to combat the effects of machining CFRP.

Corehog end mill coating chrt
CoreHog’s Coating Chart

While PCD diamond tools are considerably higher in price than other diamond coated tooling options, they provide the longest tool life and performance against abrasion and tool wear. Often, they are more cost effective in the long run due to their longevity, when compared to cheaper options that only save money in the short term.

corehog square pcd end mill
CoreHog PCD Diamond End Mill – Square

 

Corehog ball nose PCD end mill
CoreHog PCD Diamond End Mill – Ball Nose

In comparison to the cost of the part to be machined, the tool may be a fraction of the cost, making it worth spending the extra money to prevent greater costs that come with scrapping the part. Composite cutting tool manufacturer, CoreHog, stocks an array of PCD Diamond End Mills in Square and Ball Nose profiles.

Tooling Geometry

The geometry of a tool also plays a vital role in its machining capabilities of CFRP. There are many different geometry solutions, depending on your application, with different flutes, angle of cut approach, and profile. Corehog’s selection of CFRP Router Bits offer a great solution to your various CFRP needs.

corehog cfrp router bit upcut burr style
CoreHog CFRP Router Bits – Burr Style – Upcut

corehog cfrp router bit end mill upcut
CoreHog CFRP Router Bits – End Mill Style – Upcut

corehog cfrp router bit drill point upcut
CoreHog CFRP Router Bits – Drill Point Style – Upcut

The Benefits of CoreHog’s Assembly Style Tooling in Composites

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Harvey Performance Company brand CoreHog, which focuses on the manufacturing of the world’s most advanced composite and honeycomb core cutting tools, fully stocks an array of “Assembly Style Cutting Tools,” which allow a machinist to build the perfect solution for their specific application’s needs. In doing so, a cutting tool can be optimized for specific materials, densities, and manufacturing styles to increase efficiency, decrease costs, and provide unbelievable machining flexibility.

Corehog tooling for machining composites

How Does Assembly Style Tooling Work?

CoreHog’s Assembly Style Tooling works by taking multiple pieces and tool components, and assembling them together to create one finished cutting tool. The concept of assembling a completed tool allows machinists greater flexibility in choosing cutting edges that are best suited for their application or material type. Further, this type of tooling is often utilized by machinists because it’s often a less expensive alternative to solid round, non-assembled tooling, as a machinist would only need to replace the cutting end components when they begin to dull, and not the arbors or shank pieces.

CoreHog’s offering of Assembly Style Tooling includes Small Size, Medium Size, and Large Size Finishing Tools, as well as Valve Cutters and Modular Rebating Tools. The way in which each system is built varies by tool type.

Finishing Tools for Composites

Small Size Finishing Tools

Optimized to machine small, closed features in composites, such as pockets, joggles, and closed walls, Small Size Finishing Tools are engineered for the superior finishing of honeycomb core materials. This configuration includes a Small Coreslicer with three different edge options: Smooth, Sawtooth, or Staggered Tooth, and an optional Small CoreHogger. The right edge style for the Coreslicer depends largely on the material you’re working in. While a Smooth edge style works well in lighter density honeycomb core materials such as Kevlar®, Nomex®, and Aluminum, Sawtooth and Staggered Tooth options work best for honeycomb core materials with densities of 6 pounds or higher, such as aluminum core, Kevlar®, or Nomex®.

Corehogger Assembly Guide


Key Benefits: Eliminating the risk of material wrapping around the spindle by disintegrating them as they approach the face of the slicer.

Browse Small Size Finishing Tools

Medium Size Finishing Tools

Designed for finishing honeycomb core materials, this assembly style CNC tooling is engineered for shaping smaller complex surfaces, bevels, and external radii. For this configuration, a Medium CoreHogger and a Medium Coreslicer must be utilized and fastened with a screw. Similar to the Small Size Finishing Tool options, this assembly can be used with a Smooth, Sawtooth, or Staggered Tooth Coreslicer edge.

CoreHog Medium Corehogger Assembly Guide

Key Benefits: This Medium Size Finishing Tool offering includes both carbide and high speed steel options. The carbide version is uncoated, whereas the high speed steel version is TiCN coated for extended tool life and improved wear resistance.

Browse Medium Size Finishing Tools

Large Size Finishing Tools

Designed to vastly reduce cycle times while finishing honeycomb core materials, this assembly style tooling removes large volumes of material quickly, while providing excellent surface finish and keeping tool pressure and heat low.


Large Size Finishing Tools require a slightly more complex configuration. This type of modular tool features an Arbor, which includes a washer and screw; Large CoreHogger; and Large Coreslicer. For this assembly, four types of Coreslicer edge options are available: Smooth, Sawtooth, Staggered Tooth, or Wavy. Wavy style options are best utilized in heavier density types of Kevlar®, Nomex®, and Aluminum Core, and are engineered to be useful when machining parts that contain bond lines.

Corehog large corehogger assembly guide

Key Benefits: The Arbors in this configuration are heat treated and finish ground for extremely tight tolerances in runout, concentricity, and perpendicularity. With tighter tolerances, harmonics are minimized while longer tool life and better part finish are observed.

Browse Large Size Finishing Tools

Valve Cutters


Different from CoreHog’s Finishing Tools, Valve Cutters are assembly tooling engineered for machining honeycomb core materials and finishing thin features, such as bevels and knife edge parts. To build a Valve Cutter, utilize an Arbor, a Valve Stem Slicer, and a screw to fasten the two together. Similar to Small and Medium Size Finishing Tools, the Valve Stem Slicer can feature a Smooth, Sawtooth, or Staggered Tooth edge profile.

Corehog valve cutter assembly guide

Key Benefits: The Stem design of CoreHog’s Valve Stem Arbors is optimized for free flowing applications, eliminating grabbing when machining Honeycomb Core Materials.

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Modular Rebating Tools


Machinists may opt to use a Modular Rebating Tool if they are aiming to reduce setup, minimize cost per cutter, and obtain flexibility with varying sandwich panel configurations. For this configuration, an Arbor connects to a Core Insert, Skin Insert, and is fastened with a screw. Here, the Arbor, which features a .500” shank diameter and a 3” overall length, can be paired with multiple sizes of Core Inserts. As of September 2022, CoreHog’s offering of Core Inserts range in diameter from .875” to 1”, with a length of cut spanning from .160” to .312”. All Inserts feature TiAlN coating, which provides high hardness and high temperature resistance. Finally, the Skin Insert features a ½” diameter, and provides a machinist with the option of DLC or CVD Coating. While DLC coating provides optimal performance, true crystalline CVD diamond coating works to significantly extend tool life.

Corehog modular rebating tool guide

Key Benefits: The complex geometry of Sandwich Panel Cutters – Arbors helps to reduce tearing, flagging, and fuzz, while providing a rebated area to allow for edge filling or fasteners, later on.

Browse Modular Rebating Tools

For more information on CoreHog’s Assembly Style Tooling, visit its website at corehog.com.

Machining Nickel Alloys: Avoiding All-Too-Common Mishaps

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Nickel-based alloys are growing in popularity across many industries such as aerospace, automotive, and energy generation due to their unique and valuable mechanical and chemical properties. Nickel alloys exhibit high yield and tensile strengths at low weights and have high corrosion resistance in acidic and high temperature environments. Because of these advantageous properties, nickel alloys have increasingly become popular in machine shops.

Unfortunately, nickel alloys have a reputation for creating issues at the spindle. These metals present themselves to be problematic as they easily work harden. Further, nickel alloys generate high temperatures during machining, and have gummy chips that can weld onto cutting tools, creating built-up edge (BUE). Fortunately, with the correct approach, one can be successful in cutting nickel alloys.

Work Hardening

Across machine shops, nickel alloys are notorious for being difficult to machine. This reputation stems, largely, from work hardening, or a metal’s microhardness increasing due to the addition of heat. According to the Nickel Development Institute, this heat is generated through friction and plastic deformation of the metal. As the metal is cut, the friction between the cutting tool and workpiece generates heat which is concentrated around the cutting area.

Simultaneously, the metal is being physically worked. This means that as it is being machined, it is experiencing plastic deformation, which is a physical property that measures how much a material can be deformed to the point that it cannot return to its original shape.

This physical working of a nickel alloy increases its hardness faster than it does most other metals. The combination of high heat generation and physical work quickly increases the alloy’s hardness, causing tools to dull quickly and fail. This may result in scrapped parts and broken tools.

HVNI End MIll for Machining Nickel Alloys
Shown above is Helical Solutions’ End Mill for Nickel Alloys. This tool, engineered to excel in Inconel 718 and other nickel-based superalloys, is fully stocked in 6 and 8 flute styles.

Tool Adhesion

As nickel alloys are being machined and heat is generated, chips tend to become stringy and weld themselves to a tool’s cutting edge. This phenomenon, built up edge or BUE, rounds the cutting edge of the tool, resulting in poor cuts and increased friction, thus further contributing to work hardening. An example of BUE is seen in the image below.

BUE On End Mill Flutes
On the above tool, chips from the workpiece (Inconel 718) have welded onto the cutting edge, severely decreasing the tool’s effectiveness. Image Source: International Journal of Extreme Manufacturing

Built-up edge also speeds up tool wear, as the rotational forces involved in the cut increase. Now that the cutting edge is rounded from welded chips, a blunt tool is being forced into the workpiece.

With a blunt edge, the cutting motion changes from a shearing action to plowing. In other words, instead of cutting through the metal, the tool is pushing the material, resulting in poor cuts and increased friction.

Excessive Heat Generation

With poor cutting, internal heat of the tool rises, which can cause thermal cracking, defined by cracks that form perpendicular to the cutting edge. The fractures within the tool are created by extreme internal tool temperature fluctuations.

As a cutting tool rapidly overheats while cutting nickel alloys, cracks may form which can lead to catastrophic tool failure. With high temperatures, galling may also occur, which is characterized by pieces of the tool flaking off due to the same adhesion that causes BUE. As the tool is being welded to the workpiece and the machine continues to rotate it, pieces of the tool may start to break off resulting in tool failure.

Overcoming Nickel Alloy Difficulties

Temperature Control and Coolant Usage

The first step to effectively machine nickel alloys is to keep temperatures manageable as the workpiece is cut. Using high pressure coolant is mandatory. Coolant pressure should be 1000 psi or greater. This high pressure concentrating on the cutting zone of the workpiece will dissipate heat within both the cutting tool and workpiece. By doing so, the chances of work hardening lessen.

High pressure coolant will also aid in clearing out chips from the cutting area. Those hot gummy chips are responsible for BUE. Removing them as quickly as possible reduces the risk of BUE forming on the cutting edge. Additionally, chip removal is important to avoid chip recutting.

Chips absorb much of the heat and often work harden themselves. Recutting these hardened chips will dull the cutting edge resulting in poor cuts and decrease tool life. In general, water-based cutting fluids are preferable as they have higher heat removal rates and have a lower viscosity, which is needed for high metal removal operations.

Using the Proper Techniques

To also assist with heat removal, utilize climb milling techniques, where possible. When climb milling, the chip thickness is at its maximum at the beginning of the cut and tapers off until the cut is complete. Due to this, less heat is generated, as the cutting tool does not rub on the workpiece. Most of the heat from the cut is transferred into the chip.

Selecting the Proper Tooling and Coating

The next step is selecting the right end mill. Your end mill of choice should have a proper tool coating, such as Helical’s Tplus coating. Tool coatings are specifically engineered to improve tool performance by reducing friction, increasing tool microhardness, and extending tool life.

Next is selecting flute count. Tools used for nickel alloys need to be rigid to withstand the cutting forces present when machining high hardness alloys. Therefore, higher flute counts are necessary. If using traditional roughing toolpaths, your end mill should have at least 6 flutes. With 6 flutes, there is sufficient flute valley depth to allow for chip evacuation, while having a larger core diameter keeps the tool strong and rigid.

For finishing operations and instances of implementing high efficiency milling, higher flute counts should be considered. A tool used this way should have 8 flutes to provide excellent surface finish.

Helical’s End Mills for Nickel Alloys

CNC tooling manufacturer Helical Solutions’ End Mills for Nickel Alloys product offering, its HVNI tool family, specializes in machining nickel alloys as it exhibits these key tool features.

The tools shown above are Helical Solutions’ End Mills for Nickel Alloys. These tools are coated in Tplus for high hardness, resulting in improved tool life and increased strength,

With its Tplus coating and variable pitch to minimize chatter, these solid carbide end mills are engineered to perform in all grades of nickel alloys. Coupled with their geometry to maximize cutting performance, Helical’s End Mills for Nickel Alloys utilize faster speeds and feeds, which are readily available on the Helical Solutions’ website and Machining Advisor Pro.

For more information about the chemical make-up, uses, and categorization of nickel alloys, read “In the Loupe’s” post “Understanding Nickel Alloys: Popularity, Chemical Composition, & Classification”.

Understanding Nickel Alloys: Popularity, Chemical Composition, & Classification

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Nickel-based alloys have been a cornerstone of manufacturing for decades, desirable for their broad range of varying resistances to heat, oxidation, and corrosion. Nickel alloys also have a high strength-to-weight ratio and superior electrical conductor abilities. Because of these mechanical and chemical properties, they are primarily used in aerospace, oil, electrical, and chemical industries.

Understanding this valuable metal and how to properly machine it is imperative to delivering an optimal final part.

Nickel Alloy Chemical Composition and Classification

Nickel is commonly found in the form of an alloy, as its crystalline structure allows the element to be paired well with other metals. These atoms are arranged in a face centered cubic lattice; this structure is shown in figure 1 below.

Lattice structure of Nickel. Image source: PriyamStudyCentre.com

According to Priyam Study Centre’s Learning Chemistry, an open face lattice has the highest atomic packing number (the number of atoms per unit volume) of any metallic lattice configuration, with an atom present at each of the 6 faces and 8 corners of the cube. This structure is largely responsible for nickel’s strength and ability to create strong metallic bonds to chromium, cobalt, iron, and molybdenum, the most common metals found in these alloys.

According to City Special Metals’ article on Machining Nickel and Nickel Alloys, nickel alloys are organized into five main categories: Groups A through E. These groups are determined through the percentage of nickel present, as well as the most prominent metal that the nickel is chemically bonded with.

Table 1 displays the breakdown of these groups, showing each group’s chemical composition and a few examples of common types of nickel alloys found in that category.

GroupPercentage of NickelPaired MetalsExamples
Group A95% and greaterAlmost pure nickelNickel 200, 201, 205, and 212
Group B29% to 42%CopperMonel 400, Invar 36
Group C70% to 75%Chromium and ironInconel 600, Monel K-500, and Nickel 270
Group D50% to 56%Chromium and ironInconel 718, Inconel 625, and Hastelloy C-22
Group E63%Copper and ironMonel R405 is the only Nickel alloy in this category
Table 1: Categories of nickel alloys and their chemical compositions. Table data source: Machining Nickel and Nickel Alloys: A Guide from CSM; Nickel Based Alloys: Everything You Need to Know.

Understanding your workpiece material is just as important as understanding your machinery and tools. According to Global Market Insights (GMI), the nickel alloy market has been growing over 4% each year since 2017, and this growth is seeing an upward trend. As these alloys increase in popularity and demand, knowing the chemical compositions and classification of your specific workpiece will play a key role in successfully machining it.

Fabricating products made of nickel alloys present common struggles in every machine shop. Learn how to select proper tooling and implement machining techniques to overcome these challenges by reading CNC Machining Nickel Alloys: Avoiding All-Too-Common Mishaps.

We Partnered with Makino & Mastercam to Set MRR Standards with Helical Solutions’ HEV-C-6

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Material removal rate (MRR) is the term for the measurement of material removed from a part during machining, over a set period of time, and is the key identifier in machine, tooling, and software efficiency. Radial depth of cut (RDOC) and axial depth of cut (ADOC) are critical elements to MRR, as they determine how and where your tooling is engaging with the workpiece. Applying High Efficiency Milling (HEM) strategies to your milling practice is the best place to start to increase MRR while also keeping tool wear and overall tool life in check.

Utilizing HEM with Helical Solutions’ HEV-C-6 chipbreaker with Mastercam software, Makino machinists were able to use a Makino PS105 VMC to achieve a new industry standard in MRR for 4140 steel alloy.

Setting a New Material Removal Rate Benchmark with Helical

Engineered for HEM, Helical’s HEV-C-6 was designed for optimal chip evacuation and reduced tool harmonics at high RPMs. Makino machinists utilized these features wit Mastercam’s highly effective tool paths, resulting in a very impressive MRR.

Watch the video below from Makino to see these impressive feats.

Inside the machine, the HEV-C-6 was spun at 10,695 RPM with Mastercam’s impressively efficient toolpaths to achieve an incredible 40in3 MRR. 

Harvey Performance Company National Application Engineer Don Grandt, who worked with Makino and Mastercam on this project, spoke of what went into this effort.

How did this project initially gain traction?

Makino saw a Helical Solutions email highlighting the HEV-C-6 end mill, and wanted to work with us and Mastercam to do a demo to show off the power of their PS105 Vertical Machining Center with a 1/2″ end mill. Using our tool, they were able to max out the horsepower on the machine to get unbelievable MRR.

What considerations were met as you attempted to set this new industry standard?

All tools can only be pushed to the limits of the machine and material. So the goal was to watch the machine’s spindle load to give 100% or greater efficiency of the machine. We also wanted to use a combination of radial step over, axial stepdown, and Feed per Minute to maximize the MRR, as well.

Why was Helical’s HEV-C-6 chosen for this project?

The HEV-C-6 was chosen for a couple of reasons. First, the email that Helical sent was boasting some impressive things about this tool, which intrigued Makino. The other reason was Mastercam was working with us in the past and had an idea that with their tool path (Dynamic), and Makino’s machine power and performance, that this could be an incredible collaboration.

What is next for the performance between these three brands?

We have pushed the limits of the Makino PS105 Machine with a half inch end mill so the sky is the limit with other machines in Makino’s arsenal with greater horsepower and performance. And as far as Mastercam is concerned, they evolve their software yearly with new tool paths and strategies that make it easier and easier on the tool.

For more information on this impressive feat, a webinar hosted by Makino, and featuring Don Grandt, can be found here. Cutting Tool Engineering (CTE) also featured this collaboration in their August 2021 issue as well, as in an accompanying blog post written by Jesse Trinque of Mastercam.

Achieving Success in CNC Woodworking

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Developing a Successful Cutting Direction Strategy

There are a number of factors that can affect the machining practices of wood in woodworking. One that comes up a lot for certain hardwoods is the cutting direction, specifically in relation to the grain pattern of the wood. Wood is an anisotropic material. This means that different material properties are exhibited in different cutting directions. In terms of lumber, there are different structural grades of wood related to grain orientation. If the average direction of the cellulose fibers are parallel to the sides of the piece of lumber, then the grains are said to be straight. Any deviation from this parallel line and the board is considered to be “cross-grain”. Figure 1 below depicts a mostly straight grain board with arrows indicating the different axes. Each of these axes exhibits different sets of mechanical properties. Because of these differences, one must be conscious of the tool path in woodworking and minimize the amount of cutting forces placed on the cutter in order to maximize its tool life.

straight grain wood board with woodworking axes
Figure 1: Mostly straight grain board with arrows indicating different axes

Cutting perpendicular to the grain is known as cutting “across the grain” in woodworking. In Figure 1 above, this would be considered cutting in the radial or tangential direction. Cutting parallel to the grain is known as cutting “along the grain” (longitudinally in terms of Figure 1). The closer you are to cutting at 90° to the grain of the wood in any direction, the larger the cutting force will be. For example, a tool with its center axis parallel to the tangential direction and a tool path along the longitudinal direction would have less wear than a tool with the same center axis but moving in the radial direction. The second type of tool orientation is cutting across more grain boundaries and therefore yields greater cutting forces. However, you must be careful when cutting along the grain as this can cause tear-outs and lead to a poor surface finish.

The Proper Formation of Wood Chips With CNC Woodworking

When cutting wood parallel to the grain, there are three basic types of chips that are formed. When cutting perpendicular to the grain, the chip types generally fall into these same 3 categories, but with much more variability due to the wide range in wood properties with respect to the grain direction.

Type 1 Chips

Type 1 chips are formed when wood splits ahead of the cutting edge through cleavage until failure in bending occurs as a cantilever beam. A large force perpendicular to the shear plane is produced, causing the wood ahead of the cutting edge to split, forming this tiny cantilever beam. When the upward force finally exceeds the strength of this tiny beam, it breaks off.  These types of chips cause comparatively little wear compared to types 2 and 3, as the material is splitting before coming in contact with the pointed edge. End mills with either extremely high rake or very low rake angles often produce type 1 chips. This is especially true when machining against grain slopes that are greater than 25°. Woods with moisture content less than 8%form discontinuous chips and are at a higher risk of tear-out.

Type 2 Chips

Type 2 chips are the most desirable of the three types in terms of surface finish. They are a result of material failure along a diagonal shear plane, extending from the cutting edge to the workpiece surface. Type 2 chips form when there is a proper balance between the properties of the wood, cutting parameters, and cutter geometry. Woods with a moisture content between 8% and 20%have a much higher chance of forming continuous type 2 chips while leaving a good surface finish.

Type 3 Chips

The last type of chip forms when the rake angle of a cutter is much too low. In this scenario, the cutting force is almost parallel to the direction of travel. This causes a soft material, such as wood, to be crushed rather than sheared away, leaving a poor surface finish. Generally, the surface left behind looks like tiny bundles of wood elements, a surface defect commonly known as “fuzzy grain.” This type of chip occurs more frequently in softwoods as the crushing situation is compounded in low-density woods.

types of wood chips in woodworking
Figure 2: Different types of wooden chips

Extending Tool Life When Woodworking

Speeds & Feeds Rules of Thumb

There are several different categories of tool wear that occur when cnc woodworking. General rules of machining still apply as RPM has the greatest influence on wear rate. Over-feeding can increase tool wear exponentially and also cause tool breakage. As with most machining operations, a balance between these two is essential. If you are looking to increase your productivity by increasing your speed, you must increase your feed proportionally in order to maintain a balance that keeps the tool properly engaged in the material.

Proper Management of Heat

When cutting tools are exposed to high heat, they begin to wear even faster, due to corrosion. The cobalt binder within most carbide tools on the market begins to oxidize and break free of the cutting edge. This sets off a chain reaction, as when the binder is removed, the tungsten carbide breaks away, too. Different species of wood and types of engineered wood have different corrosive behaviors at high temperatures. This is the most consistent type of wear that is observed when machining MDF or particleboard. The wear is due to the chlorine and sulfate salts found in adhesives as this accelerates high-temperature corrosion.  As with aluminum, when the silica content of a wood increases, so too does its corrosiveness.

Generally, increased tool wear is observed in wood with high moisture content. This trait is due to the increased electro-chemical wear caused by the extractives in wood., Moisture content in wood includes substances such as resins, sugars, oils, starches, alkaloids, and tannins in the presence of water. These molecules react with the metallic constitutes of the cutting tool and can dull the cutting edge. Carbide is more resistant to this type of wear compared to high-speed steel.

Best Coatings for Extended Tool Life in Wood

If you want a longer-lasting tool that will maintain its sharp cutting edge (and who doesn’t), you may want to consider an Amorphous Diamond coating. This is an extremely abrasive resistant coating meant for non-ferrous operations in which the temperature of the cutting zone does not exceed 750 °F. This coating type is one of Harvey Tool’s thinnest coatings, therefore minimizing the risk of any edge rounding and maximizing this edge’s durability.

Avoiding Common Woodworking Mishaps

Tear Out

Tear out, sometimes called chipped grain or splintering, is when a chunk of the wood material being machined tears away from the main workpiece and leaves an unappealing defect where it used to be. This is one of the most common defects when machining wood products. There are many different reasons that tear out occurs. Material characteristics are something to be considered. Tear out is more likely to occur if the grain orientation is less than 20°relative to the tool path, the moisture content of the wood is too low, or the density of the wood is too low. Figure 4 shows the grain orientation angle relative to the tool path. In terms of machining parameters, it can also occur if either the chip load, depth of cut, or rake angle is too high.

woodworking grain in relation to tool path
Figure 4: Example of grain orientation angle relative to the tool path

Fuzzy Grain Finish

Fuzzy grain looks like small clumps of wood attached to the newly machined face and occurs when the wood fibers are not severed properly. Low rake or dull cutting tools indent fibers until they tear out from their natural pattern inside, causing type 3 chips to form, resulting in a poor finish. This can be exacerbated by a low feed or depth of cut as the tool is not properly engaged and is plowing material rather than shearing it properly. Softer woods with smaller and lesser amounts of grains are more susceptible to this type of defect. Juvenile wood is known to be particularly liable for fuzzy grain because of its high moisture content.

fuzzy grain wood finish
Figure 5: Example of a fuzzy grain finish

Burn Marks

Burn Marks are a defect that is particularly significant in the case of machining wood, as it is not generally a concern when machining other materials. Dwelling in a spot for too long, not engaging enough of the end mill in a cut, or using dull tools creates an excessive amount of heat through friction, which leaves burn marks. Some woods (such as maple or cherry) are more susceptible to burn marks, therefore tool paths for these types should be programmed sensibly. If you are having a lot of trouble with burn marks in a particular operation, you may want to try spraying the end mill with a commercial lubricant or paste wax. Be careful not to use too much as the excess moisture can cause warping. Increasing your tool engagement or decreasing RPM may also combat burn marks.

burn marks from wood cutter
Figure 6: Example of burn marks

Chip Marks

Chip marks are shallow compressions in the surface of the wood that have been sprayed or pressed into the surface. These defects can swell with an increase in moisture content, worsening the finish even more. This type of blemish is generally caused by poor chip evacuation and can usually be fixed by applying air blast coolant to the cutting region during the operation.

Raised Grain

Raised grain, another common defect of woods, is when one or more portions of the workpiece are slightly lower than the rest. This blemish is particularly a problem when machining softer woods with dull tools as the fibers will tear and deform rather than be cleanly sheared away. This effect is intensified when machining with slow feeds and the wood has a high moisture content. Variations in swelling and shrinking between damaged and undamaged sections of wood exacerbate this flaw. It’s for this reason that raised grain is a common sight on weather-beaten woods. Work holding devices that are set too tight also have a chance of causing raised grain.

Differentiating Harvey Tool Wood Cutting & Plastic Cutting End Mills

Woodworking Upcut End Mill
Harvey Tool Upcut End Mill For Wood

https://www.harveytool.com/products/material-specific-end-mills/woodMachinists oftentimes use Plastic Cutting End Mills for woodworking, as this tool has very similar internal geometries to that of an End Mill for Wood. Both tools have large flute valleys and sharp cutting edges, advantageous for the machining of both plastic and wood. The main difference between the Harvey Tool plastic cutters and the woodcutters is the wedge angle (a combination of the primary relief and rake angle). The woodcutter line has a lower rake but still has a high relief angle to maintain the sharpness of the cutting edge. The lower rake is designed to not be as “grabby” as the plastic cutters can be when woodworking. It was meant to shear wood and leave a quality surface finish by not causing tear-out.

Harvey Tool’s offering of End Mills for Wood includes both upcut and downcut options. The upcut option is designed for milling natural and engineered woods, featuring a 2-flute style and a wedge angle engineered for shearing wood fiber materials without causing tear out or leaving a fuzzy grain finish. The downcut offering is optimized for milling natural and engineered woods and helps prevent lifting on vacuum tables.

For more help on achieving a successful machining operation, or more information on Harvey Tool’s offering of End Mills for Wood, please contact Harvey Tool’s team of engineers at 800-645-5609.

Understanding Wood Properties for CNC Woodworking Projects

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Machinists oftentimes confuse wood for being an “easy to machine material” during CNC Woodworking because of how much softer the material is than metal. In some sense this is true, as you can program wood cutting parameters in CNC Woodworking with much higher feed rates compared to that of most metals. On the other hand, however, wood has many unique properties that need to be accounted for in order to optimize the cutting process for maximum efficiency.

Get the Most Out of Your CNC Woodworking Projects With Harvey Tool’s Speeds and Feeds

Types of Wood for CNC Woodworking

There are 3 main categories of wood for woodworking: hardwood, softwood and engineered wood.

Hardwood

The textbook definition of a hardwood tree is an angiosperm, more commonly referred to as a broadleaf tree. A few examples would be oak, birch, and maple trees. These types of trees are often used for making high quality furniture, decks, flooring, and construction components.

Softwood

A softwood is a coniferous tree, sometimes known as a gymnosperm. These are typically less dense than hardwoods and are therefore associated with being easier to machine. Do not let the name fool you: some soft woods are harder than some hardwoods. Harvey Tool’s Speeds and Feeds Charts for its offering of Material Specific End Mills for Wood are categorized by Janka hardness for this exact reason. Janka hardness is a modified hardness scale with a test specifically designed for classifying types of wood.

Softwood is used to make furniture, but can also be used for doors, window panes, and paper products. A couple of examples are pine and cedar trees. Table 1 lists 20 common woods with their Janka hardness.

Common Name:Janka Imperial Hardness:
Balsa90
Buckeye, Yellow350
Willow, Black360
Pine, Sugar380
Cottonwood, Eastern430
Chesnut, American540
Pine, Red560
Douglas-Fir, Interior North600
Birch, Gray760
Ash, Black850
Cedar, Eastern Red900
Cherry, American Black950
Walnut, Black1010
Beech, American1300
Oak, White1360
Maple, Sugar1450
Apple1730
Cherry, Brazilian2350
Olive2700
Rosewood, Indian3170
Table 1: Janka Hardness of Common Woods

Engineered Woods

Engineered wood, or composite wood, is any type of wood fiber, particle, or strand material held together with an adhesive or binding agent. Although some of these materials are easier to machine than solid woods, the adhesive holding the material together can be extremely abrasive. This can cause premature tool wear and create difficulties when cnc woodworking. It’s important to note that some types of engineered woods are more difficult to machine than others, specifically those with a higher amount of binding material. These types should be programmed with less aggressive speeds and feeds. For example, medium density fiberboard (MDF) if more difficult to machine than plywood, but much easier to machine than phenolic.

stack of medium density fiberboard pieces for cnc woodworking
Figure 1: Example of Medium Density Fiberboard

Properties of Wood

Grain Size

Technically speaking, wood can be considered a natural composite material as it consists of strong and flexible cellulose fibers held together by a stiffer glue-like matrix composed of lignin and hemicellulose. If you think in terms of construction, the cellulose fibers would be the steel rebar, and the concrete would be the lignin and hemicellulose. Wood with large cellulose fibers are considered to be coarse-grained (oak and ash). Woods that have smaller and fewer fibers are considered fine-grained (pine and maple). Softwoods tend to be fine-grained and are therefore stereotyped as being easier to machine since they do not have as many strong fibers to shear. It’s important to note that not all hardwood trees are coarse grained and not all softwood trees are fine-grained.

diagram of natural wood fibers for cnc woodworking
Figure 2: Simplified diagram of fibers that constitute natural wood. The cellulose fibers run vertically in this depiction.

Moisture Content (MC)

Moisture content (MC) is one of the most important variables to consider when machining wood. An extremely common problem with building anything with wood is its tendency to warp. Moisture variability in the air inevitably affects the moisture content within the wood. Any change in moisture content (whether an increase or a decrease) will disturb the shape of the workpiece. This is why one must take into account what type of moisture a product will be exposed to in its final resting place.

Equilibrium Moisture Content (EMC)

Equilibrium moisture content (EMC) occurs when wood has reached a balance point in its moisture content. Interior EMC values across the United States average at about 8%, with exterior values averaging around 12%. These values vary around the country due to the differences in temperature and humidity. For example, the southeastern United States have an average interior EMC of 11% while the southwest averages about 6% (excluding the coastal region). It’s important to consider what region and application the final product is going to encounter so that the wood with the correct moisture content can be selected before machining. Most species of flat-grain wood will change size 1% for every 4% change in MC. The direction of warping depends on the grain orientation.

United States map showing average regional indoor EMC
Figure 4: Average regional indoor EMC

Generally, power requirements for an operation rise with increasing moisture content, mainly because of the surge in density. Density of wood increases with rising MC. The additional power may be necessary to push a heavier chip out of the cutting zone during CNC Woodworking. It’s worth noting that, like synthetic polymers, wood is a viscoelastic material that absorbs energy as it becomes wetter. The proportional limit of its mechanical properties intensifies as MC increases.

When machining some types of wood, cutting region temperature will surge with increasing MC, but in other species it will decline. Be safe and avoid rapid tool wear by decreasing SFM when machining a wood with a moisture content above 10%. Harvey Tool Speeds and Feeds Charts suggest a decrease of 30 per MC percentage point. As always, though, it depends on the type of wood being machined and the type of operation being performed.

Temperature change is not the only reason higher moisture content is associated with rapid tool wear. Moisture within wood isn’t just associated with water, but also with resins, sugars, oils, starches, alkaloids, and tannin present within the water. These substances react particularly well with high speed steel, and to a lesser degree with carbide.

Knots and Their Effect on CNC Woodworking

A knot is a portion of a branch or limb that has become incorporated in the trunk of a tree. The influence of knots on the mechanical properties of wood is due to the interruption of continuity and change in direction of wood fibers associated with it. These properties are lower in this portion of the wood because the fibers around the knot are distorted and lead to stress concentrations. “Checking” (cracking due to shrinking) often occurs around knots during drying. Hardness and strength perpendicular to the grain are exceptions to generally lower mechanical properties. Because of these last two exceptions, woodworking machining parameters should be reduced when encountering a knotted portion of the workpiece to avoid shock loading.

typical natural wood knot in hardwood
Figure 5: Photo of a typical knot

Titanium Machining Cost Savings With Helical Solutions

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When the manufacturing team at Geospace Technologies was looking for better tool life and improved performance on a Titanium CNC milling job, they turned to Harvey Performance Company and local Application Engineer Mike Kanigowski to dial in some Helical Solutions End Mills. With Mike’s help, Geospace Technologies, led by Lead Mill Programmer Tranquilino Sosa, achieved massive success and extensive titanium machining cost savings, which led them to completely shift their tooling repertoire to Helical’s high-performance end mills in their shop.

Struggling With Tool Life

Prior to switching to Helical, Geospace Technologies was experiencing trouble with tool life on a job that required both roughing and finishing toolpaths on a Titanium (Ti-6AL-4V) part. For their roughing pass, Geospace was using a competitor’s 4 flute, 3/8” diameter end mill with a 30° helix angle and TiALN coating. In traditional roughing toolpaths, this tool was running at 1,750 RPM with a 10 IPM feed rate. The tool would take four step downs, three with an axial depth of cut of .200”, and a final pass at .100” for a total depth of .700”.

When finishing, the team used a 1/2” version of the same competitor tool, running at 900 RPM with an 8 IPM feed rate. This would take two passes, one at .400” deep and the last down to the bottom of the part at .700”.

geospace technologies fadal VMC 4020

With this strategy and tooling, the team was creating high-quality parts at a cycle time of 15 minutes and 22 seconds per part, but were only seeing the roughing tool last for 60 parts on average, and the finishing tool for around 120 parts. This was causing tool costs to be higher than they would like, and costing the team precious time with frequent tool changes.

Sosa had seen some of the success that other shops were having with Titanium milling using Helical Solutions end mills, and so they reached out to Kanigowski to see how Helical could help them lower their cost per part while achieving an even better finish.

HVTI-6 Ad

Dialing in Tool Selection

When Mike got in touch with the team at Geospace, he knew there were some immediate benefits to changing the toolpaths used in this job. Using their ESPRIT software, the team was able to dial in a new program using high efficiency milling (HEM) toolpaths through ESPRIT’s “Profit Milling” technology.

With HEM toolpaths in place, Geospace was going to need new high performance tools to take full advantage of the programming adjustments. After much testing and evaluating several options from Helical’s extensive line of end mills for Titanium, Geospace settled on two solid tools.

Helical offers many different options for Titanium milling in HEM toolpaths. During testing, the team at Geospace decided on Helical EDP 59424, a 3/8” diameter, 7 flute, corner radius end mill. This tool features variable pitch geometry and offset chipbreakers for optimal chip evacuation, reduced harmonics, and minimized tool pressure, as well as Helical’s Aplus coating for high temperature resistance, decreased wear, and improved tool life.

7 flute chipbreaker
7 Flute Chipbeaker Tools Fresh Off the Grinder

When looking at the finishing toolpath, Geospace decided on Helical EDP 82566, a 3/8”, 6 flute, square end mill from Helical’s well known HEV-6 product line. This tool featured a variable pitch design to help mitigate chatter and leave a superior finish. While Helical also offers several tools for finishing toolpaths in Titanium, during testing this tool provided Geospace with the best finish for their specific part geometry.

Achieve Impressive Efficiency in Titanium Machining Operations With Helical Solutions’ HVTI-6 Cutter

Experiencing the “Helical Difference”

With the new tools in place, Sosa’s team reached out to Helical for help dialing in speeds and feeds. The Helical tech team was able to get them set up on Machining Advisor Pro, an advanced speeds and feeds calculator developed by the experts behind Helical Solutions tooling. With this “miracle worker” application in their arsenal, the team was able to easily dial in their new tools for their specific material grade, depth of cut, and machine setup.  

The team saw immediate positive results and cost-savings on this job. They were able to increase their roughing toolpaths to 4,500 RPM and 157 IPM. The finishing path remained largely the same, but resulted in a much improved final part. In total, cycle time dropped from 15 minutes and 22 seconds per part to 12 minutes and 17 seconds per part, which was great, but the improvement in tool life was where Sosa was most impressed.

Geospace technologies employee inspecting titanium end mills

With the new Helical end mill in the shop, Geospace was able to run both tools for 580 parts with very minimal wear on the tool. This was a nearly 1000% improvement in tool life for their roughing passes and a 483% improvement in tool life for the finishing operation. In total, one roughing tool was able to last more than 42 hours in the cut before needing to be replaced.

Eliminating the need for a tool change every 60 parts was also a significant time-saver. Constant tool changes were causing serious machine downtime, which was eliminated with the longer tool life experienced with the Helical end mills. What seems like a minor inconvenience will truly add up to dozens of hours in saved time over the course of a few months for Sosa’s team.

titanium machining tool wear
A Closeup of the 7 Flute Chipbreaker After 42 Hours In The Cut

Geospace was thrilled with the results they saw on this Titanium job, as they had never experienced long tool life in Titanium with any other competitor brand. Sosa and his team are excited to continue using Helical Solutions product across all of their other jobs going forward and to continue working with Kanigowski and the Helical tech team on dialing in tool selection and speeds and feeds on future projects.

Please see below for a head-to-head breakdown of the Helical end mills’ performance in terms of total costs and productivity gained versus that of the competitor. These numbers are measured per 1,000 parts, taking into account tooling costs, tool change time, labor costs, running parameters, and cycle times.

titanium machining cost savings

Hardenability of Steel

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Many types of steel have a beneficial response to a method of heat treatment known as quenching. One of the most important criteria in the selection process of a workpiece material is hardenability. Hardenability describes how deep a metal can be hardened upon quenching from high temperature, and can also be referred to as the depth of hardening.

Steel At Microscopic Scale:

The first level of classification of steels at a microscopic level is their crystal structure, the way in which atoms are arranged in space. Body-Centered Cubic (BCC) and Face Centered Cubic (FCC) configurations are examples of metallic crystal structures. Examples of BCC and FCC crystal structures can be seen below in Figure 1. Keep in mind that the images in Figure 1 are meant to display atomic position and that the distance between the atoms is exaggerated.

depiction of BBC and FCC crystal structures in steel
Figure 1: Example of a BCC crystal structure (left) and FCC crystal structure (right)

The next level of classification is a phase. A phase is a uniform portion of a material that has the same physical and chemical properties. Steel has 3 different phases:

  1. Austenite: Face-Centered cubic iron; also iron and steel alloys that have the FCC crystal structure.
  2. Ferrite: Body-centered cubic iron and steel alloys that have a BCC crystal structure.
  3. Cementite: Iron carbide (Fe3C)

The final level of classification discussed in this article is the microstructure. The three phases seen above can be combined to form different microstructures of steel. Examples of these microstructures and their general mechanical properties are shown below:

  • Martensite: the hardest and strongest microstructure, yet the most brittle
  • Pearlite: Hard, strong, and ductile but not particularly tough
  • Bainite: has desirable strength-ductility combination, harder than pearlite but not as hard as martensite

Hardening at Microscopic Scale:

The hardenability of steel is a function of the carbon content of the material, other alloying elements, and the grain size of the austenite. Austenite is a gamma phase iron and at high temperatures its atomic structure undergoes a transition from a BCC configuration to an FCC configuration.

High hardenability refers to the ability of the alloy to produce a high martensite percentage throughout the body of the material upon quenching. Hardened steels are created by rapidly quenching the material from a high temperature. This involves a rapid transition from a state of 100% austenite to a high percentage of martensite. If the steel is more than 0.15% carbon, the martensite becomes a highly strained body-centered cubic form and is supersaturated with carbon. The carbon effectively shuts down most slip planes within the microstructure, creating a very hard and brittle material. If the quenching rate is not fast enough, carbon will diffuse out of the austenitic phase. The steel then becomes pearlite, bainite, or if kept hot long enough, ferrite. None of the microstructures just stated have the same strength as martensite after tempering and are generally seen as unfavorable for most applications.

The successful heat treatment of a steel depends on three factors:

  1. The size and shape of the specimen
  2. The composition of the steel
  3. The method of quenching

1. The size and shape of the specimen

During the quenching process, heat must be transferred to the surface of the specimen before it can be dissipated into the quenching medium. Consequently, the rate at which the interior of the specimen cools is dependent on its surface area to volume ratio. The larger the ratio, the more rapid the specimen will cool and therefore the deeper the hardening effect. For example, a 3-inch cylindrical bar with a 1-inch diameter will have a higher hardenability than a 3-inch bar with a 1.5-inch diameter. Because of this effect, parts with more corners and edges are more amendable to hardening by quenching than regular and rounded shapes. Figure 2 is a sample time-temperature transformation (TTT) diagram of the cooling curves of an oil-quenched 95 mm bar. The surface will transform into 100% martensite while the core will contain some bainite and thus have a lower hardness.

graph of sample time temperature transformation
Figure 2: Sample time temperature transformation (TTT) diagram also known as an isothermal transformation diagram

2.  The composition of the steel

It’s important to remember that different alloys of steel contain different elemental compositions. The ratio of these elements relative to the amount of iron within the steel yield a wide variety of mechanical properties. Increasing the carbon content makes steel harder and stronger but less ductile. The predominant alloying element of stainless steels in chromium, which gives the metal its strong resistance to corrosion. Since humans have been tinkering with the composition of steel for over a millennium, the number of combinations is endless.

Because there are so many combinations that yield so many different mechanical properties, standardized tests are used to help categorize different types of steel. A common test for hardenability is the Jominy Test, shown in Figure 3 below. During this test a standard block of material is heated until it is 100% austenite. The block is then quickly moved to an apparatus where it is water quenched. The surface, or the area in contact with the water, is immediately cooled and the rate of cooling drops as a function of distance from the surface. A flat is then ground onto the block along the length of the sample. The hardness at various points is measured along this flat. This data is then plotted in a hardenability chart with hardness as the y-axis and distance as the x-axis.

diagram of Jominy end quench specimen for hardened steel
Figure 3: Diagram of a Jominy end quench specimen mounted during quenching (left) and post hardness testing (right)

Hardenability curves are constructed from the results of Jominy Tests. Examples of a few steel alloy curves are shown in Figure 4. With a diminishing cooling rate (steeper drop in hardness over a short distance), more time is allowed for carbon diffusion and the formation of a greater proportion of softer pearlite. This means less martensite and a lower hardenability. A material that retains higher hardness values over relatively long distances is considered highly hardenable. Also, the greater the difference in hardness between the two ends, the lower the hardenability. It is typical of hardenability curves that as the distance from the quenched end increases, the cooling rate decreases. 1040 steel initially has the same hardness as both 4140 and 4340 but cools extremely quickly over the length of the sample. 4140 and 4340 steel cool at a more gradual rate and therefore have a higher hardenability. 4340 has a less extreme rate of coolness relative to 4140 and thus has the highest hardenability of the trio.

chart of hardenability for 4140, 1040, and 4340 steel
Figure 4: Hardenability charts for 4140, 1040 and 4340 steels

Hardenability curves are dependent on carbon content. A greater percentage of carbon present in steel will increase its hardness. It should be noted that all three alloys in Figure 4 contain the same amount of carbon (0.40% C).  Carbon is not the only alloying element that can have an effect on hardenability. The disparity in hardenability behavior between these three steels can be explained in terms of their alloying elements. Table 1 below shows a comparison of the alloying content in each of the steels. 1040 is a plain carbon steel and therefore has the lowest hardenability as there are no other elements besides iron to block the carbon atoms from escaping the matrix. The nickel added to 4340 allows for a slightly greater amount of martensite to form compared to 4140, giving it the highest hardenability of these three alloys. Most metallic alloying elements slow down the formation of pearlite, ferrite and bainite, therefore they increase a steel’s hardenability.

Table 1: Shows the alloying contents of 4340, 4140, and 1040 steel

Type of Steel: Nickel (wt %): Molybdenum (wt %): Chromium (wt %):
4340 1.85% 0.25% 0.80%
4140 0.00% 0.20% 1.00%
1040 0.00% 0.00% 0.00%

There can be a variation in hardenability within one material group. During the industrial production of steel, there are always slight unavoidable variations in the elemental composition and average grain size from one batch to another. Most of the time a material’s hardenability is represented by maximum and minimum curves set as limits.

Hardenability also increases with increasing austenitic grain size. A grain is an individual crystal in a polycrystalline metal. Think of a stained glass window (like the one seen below), the colored glass would be the grains while the soldering material holding it altogether would be the grain boundaries. Austenite, ferrite, and cementite are all different types of grains that make up the different microstructures of steel. It is at the grain boundaries that the pearlite and bainite will form. This is detrimental to the hardening process as martensite is the desired microstructure, the other types get in the way of its growth. Martensite forms from the rapid cooling of austenite grains and its transformation process is still not well understood. With increasing grain size, there are more austenite grains and fewer grain boundaries. Therefore, there are fewer opportunities for microstructures like pearlite and bainite to form and more opportunities for martensite to form.

colorful glass representing austenite
Figure 5: The colorful glass pieces represent grains of austenite which transforms into the desirable martensite upon quenching. The black portions in between the color portions represent grain boundaries. Sites where pearlite or bainite will form upon quenching.

3. The method of quenching

As previously stated, the type of quench affects the cooling rate. Using oil, water, aqueous polymer quenchants, or air will yield a different hardness through the interior of the workpiece. This also shifts the hardenability curves. Water produces the most severe quench followed by oil and then air. Aqueous polymer quenchants provide quenching rates between those of water and oil and can be tailored to specific applications by changing the polymer concentration and temperature. The degree of agitation also affects the rate of heat removal. The faster the quenching medium moves across the specimen, the greater the quenching effectiveness. Oil quenches are generally used when a water quench may be too severe for a type of steel as it may crack or warp upon treatment.

metalworker quenching casts in an oil bath
Figure 6: Metalworker quenching casts in an oil bath

Machining Hardened Steels

The type of cutter that should be chosen for processing tools chosen for machining a workpiece after hardening depends on a few different variables. Not counting the geometric requirements specific to the application, two of the most important variables are the material hardness and its hardenability. Some relatively high-stress applications require a minimum of 80% martensite to be produced throughout the interior of the workpiece. Usually, moderately stressed parts only require about 50% martensite throughout the workpiece. When machining a quenched metal with very low hardenability a standard coated solid carbide tool may work without a problem. This is because the hardest portion of the workpiece is limited to its surface. When machining a steel with a high hardenability it is recommended that you use a cutter with specialized geometry that is for that specific application. High hardenability will result in a workpiece that is hard throughout its entire volume. Harvey Tool has a number of different cutters for hardened steel throughout the catalog, including drills, end mills, keyseat cutters, and engravers.

Shop Harvey Tool’s Offering of Fully Stocked End Mills for Hardened Steels

Hardened Steel, Summarized

Hardenability is a measure of the depth to which a ferrous alloy may be hardened by the formation of martensite throughout its entire volume, surface to core. It is an important material property you must consider when choosing a steel as well as cutting tools for a particular application. The hardening of any steel depends on the size and shape of the part, the molecular composition of the steel, and the type of quenching method used.