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Tips for Maintaining Tight Tolerances

In manufacturing large production runs, one of the biggest difficulties machinists experience is holding tooling to necessary tolerances in holes, walls, and threads. Typically, this is an iterative process that can be tedious and stressful, especially for inexperienced machinists. While each job presents a unique set of challenges, there are rules of thumb that can be followed to ensure that your part is living up to its accuracy demands.

What is a Tolerance?

A tolerance is an allowable amount of variation in a part or cutting tool that a dimension can fall within. When creating a part print, tolerances of tooling can’t be overlooked, as tooling tolerances can result in part variations. Part tolerances have to be the same, if not larger, than tool tolerances to ensure part accuracy.

Cutting tool tolerances are oftentimes applied to a tool’s most critical dimensions, such as Cutter Diameter, Length of Cut, Shank Diameter, and Overall Length. When selecting a cutting tool for a job, it’s critical to choose a brand that adheres to strict tolerance standards and reliable batch-to-batch consistency. Manufacturers like Harvey Tool and Helical Solutions prominently display tolerances for many critical tool dimensions and thoroughly inspect each tool to ensure that it meets the tolerances specified. Below is the table header for Harvey Tool’s line of Miniature End Mills – Square – Stub & Standard.

tolerances

Tolerances help to create repeatability and specificity, especially in an industry in which even a thousandth of an inch can make or break a final product. This is especially true for miniature tooling, where Harvey Tool is experienced in the designing and manufacturing of tooling as small as .001” in diameter.

How are Tolerances Used?

When viewing a tolerance, there’s an upper and lower dimension, meaning the range in which the dimension of the tool can stray – both above and below what its size is said to be. In the below example, a .030″ cutter diameter tool’s size range would be anywhere between .0295″ and .0305.”

tooling tolerance

Maintaining Tolerances in Holemaking Operations

Holes oftentimes mandate the tightest dimensional tolerances, as they generally are meant to align perfectly with a mating part. To maintain tolerances, start first by testing the runout of both your machine and your tool. This simple, yet often overlooked step can save machinists a great amount of time and frustration.

Spotting Drills

Spotting Drills allow for drills to have a very precise starting point, minimizing walking or straying from a desired path. This can be especially beneficial when machining irregular surfaces, where accessing a hole’s perfect location can be more difficult.

spotting drills

Reamers

Reaming is great for any very tight tolerance mandate, because many Miniature Reamers have much tighter tolerances than a drill. Harvey Tool’s Miniature Reamers, for example, have tolerances of +.0000″/-.0002. for uncoated options and +.0002″/-.0000″ for AlTiN coated tools. Reamers cut on their chamfered edge, removing a minimal amount of material within a hole with the ultimate goal of bringing it to size. Because the cutting edge of a reamer is so small, the tool has a larger core diameter and is thus a more rigid tool.

miniature reamers

Maintaining Tight Tolerances While Machining Walls

Be Wary of Deflection

Maintaining tolerances when machining walls is made difficult by deflection, or the curvature a tool experiences when a force is applied to it. Where an angle is appearing on a wall due to deflection, opt for a reached tool to allow for less deflection along the tool’s neck. Further, take more axial depths of cut and machine in steps with finishing passes to exert less pressure on the tool. For surface finish tolerances, a long fluted tool may be required to minimize evidence of a tool path left on a part. For more information on ways to minimize deflection, read Tool Deflection & Its Remedies.tool deflection

Corner Radius End Mills

Corner radius End Mills, because they do not feature a sharp edge, will wear slower than a square end mill would. By utilizing corner radius tooling, fracturing on the tool edge will be minimized, resulting in an even pressure distribution on each of the cutting edges. Because the sharper edge on a square tool is less durable and more prone to cracking because of the stress concentration on that point, a corner radius tool would be much more rigid and thus less susceptible to causing a tolerance variation. For this reason, it’s recommended to use a roughing tool with a corner radius profile and a finisher with a square profile for an edge tolerance. When designing a part and keeping manufacturing in mind, if there is a potential for a wall with a radius as opposed to a wall with a square edge, a wall with a radius allows for easier machineability and fewer tool changes.

Maintaining Tight Tolerances While Threading

Making threads to tolerance is all about chip evacuation. Evacuating chips is an issue commonly overlooked; If chips within a hole have not been removed before a threading operation, there could be interference in the tool tip that leads to vibration and chatter within a thread. This would decrease the continuity of the thread while also altering the points of contact. Discontinuity of a thread could be the difference between passing and failing a part, and because threading is typically the last application when machining to decrease damaging the threads, it also increases the likelihood of chips remaining within the hole from other applications.

Tolerances Summarized

If you continue to experience troubles maintaining tight tolerances despite this blog post, consult the Harvey Tool or Helical Solutions tech team, as the problem may exist outside of your machine. Temperature and humidity can vary how gummy a material is, and can lead to workpiece expansion and contraction. Additionally, the foundation of buildings can expand and contract due to outside temperature, which can result in upped runout and irregular vibration in a spindle.

Shining a Light on Diamond End Mills

Diamond tooling and diamond-coated end mills are a great option when machining highly abrasive materials, as the coating properties help to significantly increase tool life relative to uncoated carbide tools. Diamond tools and diamond-like coated tools are only recommended for non-ferrous applications, including highly abrasive materials ranging from graphite to green ceramics, as they have a tendency to break down in the presence of extreme heat.

Understanding the Properties of Diamond Coatings

To ensure proper diamond tooling selection, it’s critical to understand the unique properties and makeup of the coatings, as there are often several diamond coating variations to choose from. Harvey Tool, for example, stocks Amorphous Diamond, CVD Diamond, and PCD Diamond End Mills for customers looking to achieve significantly greater tool life when working in non-ferrous applications.

Diamond, the hardest known material on earth, obtains its strength from the structure of carbon molecules. Graphite, a relatively brittle material, can have the same chemical formula as diamond, but is a completely different material; while Graphite has a sp2 bonded hexagonal structure, diamond has a sp3 bonded cubic structure. The cubic structure is harder than the hexagonal structure as more single bonds can be formed to interweave the carbon into a stronger network of molecules.

diamond tool coatings

Amorphous Diamond Coating

Amorphous Diamond is transferred onto carbide tools through a process called physical vapor deposition (PVD). This process spreads a mono-layer of DLC coating about 0.5 – 2.5 microns thick onto any given tool by evaporating a source material and allowing it to condense onto that tool over the course of a few hours.

amorphous diamond coating

Chemical Vapor Deposition (CVD)

Chemical Vapor Deposition (CVD) is a coating process used to grow multiple layers of polycrystalline diamond onto carbide tooling. This procedure takes much longer than the standard PVD coating method. During the coating process, hydrogen molecules are dissociated from the carbon molecules deposited onto the tool, leaving a diamond matrix under the right temperature and pressure conditions. Under the wrong conditions, the tool may be simply coated in graphite. 6% cobalt carbide blanks allow for the best adhesion of diamond and a substrate. CVD diamond coated end mills have a typical thickness of coating that is between 8 and 10 microns thick.

CVD Diamond Coating

Polycrystalline Diamond (PCD)

Polycrystalline Diamond (PCD) is a synthetic diamond, meaning it is grown in a lab and contains mostly cubic structures. Diamond hardness ranges from about 80 GPa up to about 98 GPa. PCD end mills have the same diamond structure as CVD diamond tools but the binding technique is different. The diamond starts in a powdery form that is sintered onto a carbide plate using cobalt as a solvent metal substrate. This is done at an extreme temperature and pressure as the cobalt infiltrates the powder, causing the grains to grow together. This effectively creates a thick diamond wafer, between 010” and .030” in width, with a carbide base. This carbide base is then brazed onto the head an end mill and sharpened.

PCD Diamond CoatingHow Diamond Coatings Differ

Coating Hardness & Thickness

Polycrystalline tools (CVD or sintered) have a much higher hardness, thickness, and max working temperature than Amorphous Diamond oated tools. As mentioned previously, a PCD tool consists of a diamond wafer brazed to a carbide body while a CVD tool is a carbide end mill with a relatively thick layer of polycrystalline diamond grown into it. This grown layer causes the CVD tools to have a rounded cutting edge compared to PCD and Amorphous Diamond coated tools. PCD tools have the thickest diamond layer that is ground to a sharp edge for maximum performance and tool life. The difference between PCD tools and CVD coated tools lies in the thickness of this coat and the sharpness of the cutting edge. Amorphous Diamond tools maintain a sharper edge than CVD coated tools because of their thin coating.

Flute Styles

Harvey Tool’s line of PCD end mills are all straight fluted, CVD coated tools are all helically fluted, and Amorphous Diamond tools are offered in a variety of options. The contrast between straight fluted and helically fluted can be seen in the images below, PCD (top) and CVD (bottom). Electrical discharge machining, grinding or erosion are used cut the PCD wafer to the specifications. The size of this wafer limits the range of diameters that can be achieved during manufacturing. In most situations a helically fluted tool would be preferred over a straight fluted tool but with true diamond tooling that is not the case. The materials that PCD tools and CVD coated tools are typically used to cut produce a powdery chip that does not require the same evacuation that a metallic or plastic chip necessitates.

PCD Diamond end mill

PCD Ball End Mill

CVD Diamond end mill

CVD Ball End Mill

Proper Uses

CVD tools are ideally suited for abrasive material not requiring a sharp cutting edge – typically materials that produce a powdery chip such as composites and graphite. Amorphous Diamond tools have a broad range of non-ferrous applications spanning from carbon fiber to precious metals but ceramics are typically outside their range as they can be too abrasive and wear away the coating. PCD tools overlap their CVD and DLC coated counterparts as they can be used for any non-ferrous abrasive material.

Cut to the Point

Harvey Tool carries physical vapor deposition diamond-like carbon coated tools, chemical vapor deposition diamond tools and polycrystalline diamond tools. PCD tools are composed of the thickest diamond wafer brazed onto a carbide shank and are ground to a sharp edge. CVD coated tools have the diamond grown into a carbide end mill. Amorphous Diamond coated tools have the DLC coated onto them through the PVD process. For more information on the diamond coating best suited for your operation, contact a Harvey Tool Tech Team Member for immediate help.

Why Flute Count Matters

One of the most important considerations when choosing an end mill is determining which flute count is best for the job at hand. Both material and application play an important role in this critical part of the tool selection process. Understanding the effects of flute count on other tool properties, and how a tool will behave in different situations is an essential consideration in the tool selection process.

Tool Geometry Basics

Generally, tools with more flutes have a larger core and smaller flute valleys than tools with fewer flutes.  More flutes with a larger core can provide both benefits and restrictions depending on the application.  Simply put, a larger core is directly proportional to tool strength; the larger the core, the stronger a tool will be.  In turn, a larger core also reduces the flute depth of a tool, restricting the amount of space for chips to exist.  This can cause issues with chip packing in applications requiring heavy material removal.  However, these considerations only lead us part way when making a decision on which tool to use, and when.

flute count core

Material Considerations

Traditionally, end mills came in either a 2 flute or 4 flute option.  The widely accepted rule of thumb was to use 2 flutes for machining aluminum and non-ferrous materials, and 4 flutes for machining steel and harder alloys.  As aluminum and non-ferrous alloys are typically much softer than steels, a tool’s strength is less of a concern, a tool can be fed faster, and larger material removal rates (MRR) is facilitated by the large flute valleys of 2 flute tools.  Ferrous materials are typically much harder, and require the strength of a larger core.  Feed rates are slower, resulting in smaller chips, and allowing for the smaller flute valleys of a larger core tool.  This also allows for more flutes to fit on the tool, which in turn increases productivity.

flute count

Recently, with more advanced machines and toolpaths, higher flute count tools have become the norm in manufacturing.  Non-ferrous tooling has become largely centered on 3 flute tools, allowing greater productivity while still allowing proper chip evacuation.  Ferrous tooling has taken a step further and progressed not only to 5 and 6 flutes, but up to 7 flutes and more in some cases.  With a wider range of hardness, sometimes at the very top of the Rockwell hardness scale, many more flutes have allowed longer tool life, less tool wear, stronger tools, and less deflection.  All of this results in more specialized tools for more specific materials.  The end result is higher MRR and increased productivity.

Running Parameters

Just as material considerations will have an impact on the tool you choose, operation type and depth of cut requirements may also have a big impact on the ideal number of flutes for your application.  In roughing applications, lower flute counts may be desirable to evacuate large amounts of chips faster with larger flute valleys.  That said, there is a balance to find, as modern toolpaths such as High Efficiency Milling (HEM) can achieve extreme MRR with a very small step over, and a higher number of flutes.  In a more traditional sense, higher flute counts are great for finishing operations where very small amounts of material are being removed, and greater finish can be achieved with more flutes, not worrying as much about chip evacuation.

flute count

Flute count plays a big role in speeds and feeds calculation as well.  One common rule of thumb is “more flutes, more feed,” but this can be a very detrimental misconception.  Although true in some cases, this is not an infinitely scalable principle.  As stated previously, increasing the number of flutes on a tool limits the size that the flute valleys can be.  While adding a 5th flute to a 4 flute tool theoretically gives you 25% more material removal per revolution with an appropriately increased feed rate, feeding the tool that much faster may overload the tool.  The 25% increase in material removal is more likely closer to 10-15%, given the tool is exactly the same in all other specifications.  Higher flute count tools may require speeds and feeds to be backed off so much in some cases, that a lower flute count may be even more efficient.  Finding the right balance is key in modern milling practices.