More Than Metal
Modern Machine Shop magazine doesn’t cover only the machining of metals. Here are some interesting tidbits about machining non-metallic materials from a few of my past stories.
Although the title of my column this month reads like the name of a Motorhead song (R.I.P., Lemmy), it actually alludes to the fact that some Modern Machine Shop articles detail the machining of materials other than metals.
In fact, we ask about the types of metallic and non-metallic materials shops machine in our annual Top Shops benchmarking survey. (Consider participating in our next survey that goes live next month, which I’ll detail in my January One-Off column.) It turns out that a high number of shops machine non-metallic materials. Per this year’s survey, 87 percent of all surveyed shops report that they machine general plastics, and 41 percent of companies in the Top Shops benchmarking group machine medical-grade plastics.
There are often unique challenges in machining these types of materials compared to metals. Here are a few examples from stories I’ve written about shops machining non-metallics and some of what I discovered along the way:
PEEK—Polyetheretherketone is fun to say quickly once you get used to it. Its acronym is PEEK, and this plastic material is produced in both industrial and medical grades. The story “Under One Roof” (short.mmsonline.com/dew) describes Dew Technologies’ process for machining medical-grade PEEK. Because it is important to avoid material contamination, this shop dedicates the machine tool, fixturing and tools to machining only PEEK. It also insists that operators wear gloves when handling PEEK to keep oils off the part surface. The material is machined dry, using air blast for chip control, and its machinability is said to be similar to that of Delrin.
Glass-filled plastics—In some cases, materials such as glass in fiber or mat form are added to plastics to improve insulating properties, dimensional stability and mechanical strength. However, this can lead to difficulty machining them. In “The Challenges of Machining Glass-Filled Plastics” (short.mmsonline.com/glass), K&E Plastics explains that the best cutters for glass-filled plastics have positive rake angles and very sharp cutting edges to cleanly cut glass filler while not burning the resin. Plus, tool coatings such as polycrystalline diamond (PCD) extend tool life in machining of abrasive glass filler. Because machining this material creates tiny chips and a good deal of dust, all the shop’s machines are connected to a central cartridge-style dust collection system to minimize dust build up on machine components and maintain a clean facility.
CFRP—Carbon-fiber-reinforced plastics are being used more these days, especially for aerospace applications. “Setting the Stage for Sizable Composites Work” (short.mmsonline.com/royal) describes situations when Royal Engineered Composites will use either waterjet or conventional milling operations for CFRP. One advantage of waterjet is that its abrasive garnet media serve as the high-pressure stream’s “cutting edges.” Because fresh media is continually introduced into the stream, the stream’s cutting edges are always sharp, minimizing the chance for delamination or leaving behind burred edges.
When milling is the only viable process, the shop uses balanced, HSK shrink-fit toolholders to minimize tool runout and prevent cutters (primarily diamond-coated tools) from wearing non-uniformly. Because most of its composite workpieces are relatively thin, it will also routinely alter the depth that a tool reaches through a workpiece to get the most out of a tool’s total available flute length.
Wood—One of my favorite shops I’ve visited was Thorn Custom Guitar (you might not find that surprising given my Motorhead reference earlier). In “CNC Machining Fuels Guitar Innovations” (short.mmsonline.com/thorn), Ron Thorn explains how he prefers a “metalworking VMC” to a conventional router to machine contoured wood guitar bodies and related components, liking that a VMC offers automatic tool-changing capability and an enclosed machining environment to contain sawdust. The tools he uses have a down-cut geometry, in which the flutes spiral down into the workpiece. If an up-cut flute geometry were used, then the action that brings chips up and out of a workpiece would also cause the wood to fray at its top edge.
He also uses an atypical workholding method to secure thin fretboards and inlay material for machining: double-sided industrial tape. The 0.007-inch-thick tape creates a very strong bond between flat workpieces and a tooling plate. In fact, the bond is so strong that acetone must be used to free workpieces from the tooling plate.
These are just a few examples of articles we’ve written detailing the machining of non-metallic materials. You can find others by searching our newly redesigned website at mmsonline.com. If your shop happens to machine some tricky non-metallic materials, let me know. Perhaps we can similarly highlight your successes in Modern Machine Shop.
Matrix Tooling makes injection molds for components that have features you cannot see.
Passivation—the process for making stainless steel parts more corrosion-resistant—is affected by how well the part is cleaned. It’s also affected by the choice of acid. Nitric or citric?
This shop’s experience of successfully machining a micro aneurysm clip in silicon nitride ceramic points out the need to have CAD, CAM and CNC capability at high resolution—plus the commitment to pull it all together.