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Causes & Effects of Built-Up Edge (BUE) in Turning Applications

In turning operations, the tool is stationary while the workpiece is rotating in a clamped chuck or a collet holder. Many operations are performed in a lathe, such as facing, drilling, grooving, threading, and cut-off applications. it is imperative to use the proper tool geometry and cutting parameters for the material type that is being machined. If these parameters are not applied correctly in your turning operations, built-up edge (BUE), or many other failure modes, may occur. These failure modes adversely affect the performance of the cutting tool and may lead to an overall scrapped part.

When inspecting a cutting tool under a microscope or eye loupe, there are several different types of turning tool failure modes that can be apparent. Some of the most common modes are:

  • Normal Flank Wear: The only acceptable form of tool wear, caused by the normal aging of a used cutting tool and found on the cutting edges.
    • This abrasive wear, caused by hard constituents in the workpiece material, is the only preferred method of tool wear, as it’s predictable and will continue to provide stable tool life, allowing for further optimization and increased productivity.
  • Cratering: Deformations found on the cutting face of a tool.
    • This tool mode is a chemical and heat failure, localized on the rake face area of the turning tool, or insert. This failure results from the chemical reaction between the workpiece material and the cutting tool and is amplified by cutting speed. Excessive Crater Wear weakens a turning tool’s cutting edge and may lead to cutting edge failure.
  • Chipping: Breaking of the turning tool along its cutting face, resulting in an inaccurate, rough cutting edge.
    • This is a mechanical failure, common in interrupted cutting or non-rigid machining setups. Many culprits can be to blame for chipping, including machine mishaps and tool holder security.
  • Thermal Mechanical Failure (Thermal Cracking): The cracking of a cutting tool due to significant swings in machining temperature.
    • When turning, heat management is key. Too little or too much heat can create issues, as can significant, fast swings in temperature (repeated heating and cooling on the cutting edge). Thermal Mechanical Failure usually shows in the form of evenly spaced cracks, perpendicular to the cutting edge of the turning tool.
  • Built-Up Edge (BUE): When chips adhere to the cutting tool due to high heat, pressure, and friction.

Effects of Built-Up Edge in Turning Application

A built-up edge is perhaps the easiest mode of tool wear to identify, as it may be visible without the need for a microscope or an eye loupe. The term built-up edge means that the material that you’re machining is being pressure welded to the cutting tool. When inspecting your tool, evidence of a BUE problem is material on the rake face or flank face of the cutting tool.

Image Source: Carbide inserts Wear Failure modes. | machining4.eu, 2020

This condition can create a lot of problems with your machining operations, such as poor tool life, subpar surface finish, size variations, and many other quality issues. The reason for these issues is that the centerline distance and the tool geometry of the cutting edge are being altered by the material that’s been welded to the rake or flank face of the tool. As the BUE condition worsens, you may experience other types of failures or even catastrophic failure.                     

Causes of Built-Up Edge in Turning Applications

Improper Tooling Choice

Built-Up Edge is oftentimes caused by using a turning tool that does not have the correct geometry for the material being machined. Most notably, when machining a gummy material such as aluminum or titanium, your best bet is to use tooling with extremely sharp cutting edges, free cutting geometry, and a polished flank and rake face. This will not only help you to cut the material swiftly but also to keep it from sticking to the cutting tool.

Using Aged Tooling

Even when using a turning tool with correct geometry, you may still experience BUE. As the tool starts to wear and its edge starts to degrade, the material will start building up on the surface of the tool. For this reason, it is very important to inspect the cutting edge of a tool after you have machined a few parts, and then randomly throughout the set tool life. This will help you identify the root cause of any of the failure modes by identifying them early on.

Insufficient Heat Generation

Built-up edge can be caused from running a tool at incorrect cutting parameters. Usually, when BUE is an issue, it’s due to the speed or feed rates being too low. Heat generation is key during any machining application – while too much heat can impact a part material, too little can cause the tool to be less effective at efficiently removing chips.

4 Simple Ways to Mitigate BUE in Turning Applications

  1. When selecting a tool, opt for free cutting, up sharp geometries with highly polished surfaces. Selecting a tool with chipbreaker geometry will also help to divide chips, which will help to remove it from the part and the cutting surface.
  2. Be confident in your application approach and your running parameters. It’s always important to double-check that your running parameters are appropriate for your turning application.
  3. Make sure the coolant is focused on the cutting edge and increase the coolant concentration amount.
  4. Opt for a coated Insert, as coatings are specifically engineered for a given set of part materials, and are designed to prevent common machining woes.

Heavy Duty Racing – Featured Customer

Heavy Duty Racing is a manufacturing company based in Stafford, VA, that specializes in motocross, off-road motorcycle suspension, and 2-stroke engine modification. Its owner, Pete Payne, grew up racing motorcycles. Later in life, he even taught classes on how to race. Simply, Motocross and motorcycles became Pete’s passion.

Pete always looked for ways to enhance his motorcycle’s engine, but quickly realized that no shops in his area could design what he was looking for. To get access to the parts he would need, he would have to rely upon companies from far away, and would oftentimes be forced to wait more than three weeks for them to arrive. Because of this, Pete decided he would need to take part manufacturing into his own hands. He purchased a manual lathe, allowing him to make modifications to his two-stroke engines exactly how he wanted them. Quickly thereafter, Heavy Duty Racing was born.

Pete discussed with us his love of racing, how he first got into machining, the parts his shop has designed, and tips and tricks for new machinists.

How did you get started in machining?

Since I was a kid I have been riding motorcycles and racing motocross. I went to a tech school in the ’80s and learned diesel technologies. When I realized nobody in this area could help design the engines I wanted to make, I decided I needed to learn how to do it myself. I have a friend, George, who is a retired mold and die maker that also worked on motorcycle engines, I asked him for some advice on how to get started. George ended up teaching me all about machining and working on engines. I really learned from failures, by trying new things, and doing it every day. I started Heavy Duty Racing in 1997 and we have been modifying and designing the highest performing engines since then.

What machines and softwares are you using in your shop?

We currently have a Thormach PCNC 1100 and a Daluth Puma CNC Lathe (we call it The Beast, it’s angry and grumpy but it gets the job done). We also have a Bridgeport Mill, Manual Lathe, and a Tiggwell. When we were choosing software to use, they had to be easy and quick to learn. We weighed our options and decided to use Autodesk Fusion 360 about 5 years ago. We mostly machine cast iron and steel since most engines are made from those materials.

What sets Heavy Duty Racing apart from the competitors?

We have a small hands-on approach and treat every part with care. We don’t have a cookie-cutter process so we are very flexible when it comes to customer needs. Since each part is different, we don’t have set prices and have custom quoting on each part. We value our customers and tailor every build to the rider, based on the weight, fuel, and skill level of the rider. We make unique components for each rider so they can have the best experience when they hop on their bike. We are just focused on letting people do what they love.

What is the coolest project you have worked on?

In 2016, MX Tech Suspension in Illinois gave us the opportunity to build an engine for them to display at their event. We got to go to California to watch them demo the engine in front of thousands of people. It was very nerve-racking to watch it live but the experience was amazing. The engine was later featured on the cover of Motocross Action magazine. It was very cool to see something we dedicated so much hard time toward get that much recognition.

Why is high quality tooling important to you?

We are making really difficult machine parts so we need tools that can last. Micro 100 tooling lasts and does the job. The thread mills we use are 3-4 mm and 14 mm and they last longer than any competition out there. The thread mills do not chip like the competition and the carbide is super strong. Breaking a tool is not cheap, so to keep one tool in the machine for how long we have has really saved me in the long run. We found Micro 100 one day looking through our distributor’s catalog and decided to try some of their boring bars. After about 5 holes, we realized that these tools are the best we have ever used! Micro has had everything I’ve been looking for in stock and ready to ship, so we have yet to need to try out their custom tools.

Most engine tolerances are no more than .0005” taper. You need the tooling to hold tight tolerances, especially in engines. Just like with tooling, minimizing vibration is key to getting the engine to last longer. We need tight tolerances to maintain high quality and keep engines alive.

If you could give one piece of advice to a new machinist ready to take the #PlungeIntoMachining, what would it be?

The same advice I’ve given to my son: Don’t be ashamed to start from the bottom and learn from the ground, up. Everybody wants to make cool projects, but you need to learn what is going on around you to master the craft. Learn the processes and follow the steps. It’s very easy to break a tool, ruin a part, or even hurt yourself. Don’t be scared of quality tools! Buying the cheap stuff will help you with one job, but the quality tools last and will save you in multiple situations.

Follow Heavy Duty Racing on Instagram, and go check out their website to see more about them!

Understanding Wood Properties for CNC Woodworking Projects

Machinists oftentimes confuse wood for being an “easy to machine material” because of how much softer the material is than metal. In some sense this is true, as you can program wood cutting parameters 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.

Types of Wood

There are 3 main categories of wood: 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 machining. 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.

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.

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.

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. 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

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, machining parameters should be reduced when encountering a knotted portion of the workpiece to avoid shock loading.

Figure 5: Photo of a typical knot

Save Time With Quick Change Tooling

Making a manual tool change on any CNC machine is never a timely or rewarding process. Typically, a tool change in a standard holder can take up to 5 minutes. Add that up a few times, and suddenly you have added significant minutes to your production time.

As CNC machine tool and cutting tool technology has advanced, there are more multi-functional tools available to help you avoid tool changes. However, sometimes it just isn’t feasible, and multiple tool changes are needed. Luckily, Micro 100 has developed a revolutionary new method to speed up tool changes significantly.

What is the Micro-Quik™ Tooling System?

Developed in Micro 100’s world-class grinding facility in Meridian, Idaho, the Micro 100 Micro-Quik™ tooling system is held to the same standards and tight tolerances as all of the Micro 100 carbide tooling.

The quick change tooling system allows for highly repeatable tool changes that save countless hours without sacrificing performance. This system combines a unique tool holder with a unique tool design to deliver highly repeatable and accurate results.

Each quick change tool holder features a locating/locking set screw to secure the tool and a locating pin which helps align the tool for repeatability. Removing a tool is as simple as loosening the set screw and inserting its replacement.

During tool changes, the precision ground bevel on the rear of the tool aligns with a locating pin inside the tool holder. The distance from this locational point to the tip of the tool is highly controlled under tight tolerances, meaning that the Micro-Quik™ tooling system ensures a very high degree of tool length and centerline repeatability. The “L4” dimension on all of our quick change tools, as seen in the image above, remains consistent across the entire product line. Check out the video below for a demonstration of the Micro 100 Micro-Quik™ system in action!

Quick Change Tooling Benefits

The most obvious benefit to using Micro 100’s Micro-Quik™ system is the time savings that come with easier tool changes. By using the quick change holders in combination with quick change tooling, it is easy to reduce tool changes from 5 minutes to under 30 seconds, resulting in a 90% decrease in time spent swapping out tools. This is a significant benefit to the system, but there are benefits once the tool is in the machine as well.

As mentioned above, the distance from the locational point on each tool shank to the tip of the tool is highly controlled, meaning that regardless of which type of tool you insert into the holder, your stick out will remain the same. This allows you to have confidence in the tooling and does not require additional touch offs, which is another major time saver.

By removing additional touch-offs and tool changes from your workflow, you also reduce the chances for human or machine error. Improper touch-offs or tool change errors can cause costly machine crashes and result in serious repairs and downtime. With the Micro 100 Micro-Quik™ system, initial setups become much easier, allowing you to hit the cycle start button with total confidence for each run.

By making a few simple changes to your tool holding configurations and adopting the Micro-Quik™ system, your shop can save thousands in time saved, with less machine downtime and increased part production. To learn more about the Micro 100 Micro-Quik™ cutting tools and tool holders, please visit (URL here to quick change page).