Machining Data For Thermoset Plastic Extrusions

The availability of Thermoset Composite rods is a great new addition to the Plastics Marketplace. Presented here are suggested cutting speeds and feeds of Thermoset Extruded Composite Rods and tubes. The data is primarily intended to help those who have not previously machined this material. These are the same compounds used in molding parts. The basic compounds involved are Phenolics (Bakelite), Melamines, Alkyds Epoxies and Diallyl Phthalates. They are rigid (by plastic standards) materials that will not melt under any amount of heat. Therefore, they can be cut at the highest possible machine speeds. The cutting angles are very much like those for brass, but the RPM can be a lot higher. Experienced screw machines companies have usually altered their equipment to take advantage of the faster cycle time available as compared to the metals.

Because these plastics do not absorb heat from the cutting tools (the way metals do) an air jet must be used to cool the tool. A tiny jet of high pressure air (#50 drill) is better than a big blast of slower moving air.


Thermosetting plastics can’t be drilled with a “heavy hand” on the feed. Light, uniform pressure is always best, and a mechanical tool feed is always preferable. Where the materials are brittle approach the hole slowly, and then increase the speed gradually after the drill has started cutting. Automatic screw machine cams can be made to accomplish this in production.

Drills especially designed for use on thermosets are available, such as (Cleveland 923-924-925 series and Morse Twist Drill). They are made of high speed steel with highly polished, wide flutes to give a slow spiral and greater chip clearance. They have a 90 degree point, high helix angles and a long taper. There are also carbide drills available.

Long, deep holes require that the drill be removed from the hole several times during the cut to get ride of accumulated chips. This can also be done on automatic screw machines.

Forming and undercutting require fine feeds of approximately .0005 to .001 per revolution.


Thermosetting plastics are being very successfully ground to simple or intricate shapes in high production jobs that are suited to outside diameter work alone. The economies are great using this method as compared to screw machining for forming the outside. Thermosets (because they are very hard and do not melt) are being form ground to .0003 tolerances on a production basis. For more grinding information contact Etcom, Inc.

Surface grinders can be tooled up to produce gear teeth that have accurate profiles for low production or prototype work. In volume these teeth would probably be extruded to size or molded to shape.

Other types of linear grooves can be ground in the rod stock on similarly tooled surface grinders for prototypes and then if approved, be extruded for production.

Ball bearings are being successfully ground to ± .0003 on form grinders that operate on the centerless grinding principal.


Maximum accuracy obtainable in both interior and exterior threading is a 75 percent thread with a class 2 fit. Resilience in plastics calls for particular attention to rakes on taps and dies or single point tools. Positive rake causes the tap, die or chaser to dig in and crowd its lead, resulting in a weak thread. Threading tools should have a negative rake angle up to 10 degrees, depending on size of thread, pitch, and degree of finish required. A ground, high speed steel gun tap having 2 or 3 flutes is recommended; it should be slightly oversize (.0015 to. 002) for general purpose and cotton flock filled Phenolics.

Dies should have a 33 degree chamfer on the lead and a negative rake up to 10 degrees. Speeds up to 200 FPM in G.P. and a cotton flock filled Phenolics will yield satisfactory production runs with high speed steel dies.

Glass and highly mineral filled Phenolics, Melamines, and Alkyds require high speed nitrided and chromium-plated taps and dies. Two or three flute spiral gun taps with a 5 degree negative rake on the front end helps prevent binding and stripping when the tap is removed. Taps .002 to .008 oversize insure full size thread by compensating for the tendency of the resilient plastic to close down after the tap is withdrawn. The entering side of the hole should always be chamfered as much as possible to prevent chipout. Surface speeds of approximately 50 FPM will give good results in these very rigid materials. Some of the mineral and glass filled Phenolics have modulus of elasticities of approximately 2.5 million with very little elongation, and Rockwells of .95 to 100 E scale. In other words they are very, very hard and brittle.


Carbide tipped end mills give the best results due to their ability to hold razor sharp edges at the relatively low RPMs available on standard milling equipment. Speeds vary from 500 to 1800 RPM with feeds to 10 to 20 inches per minute. Use air jets to take the heat away from the cutting tips.


The best results come from carbide cutters with 10 degree negative rake on the blades and lots of side clearance. Speeds up to 1000 surface feet per minute are possible on the G.P. Materials. For glass filled compounds cut this figure in half or one third.


Diamond abrasive saws are the most economical and smoothest cutting saws where a simple, accurate cut off is required. Ordinary abrasive wheels for cutting Phenolic (Bakelite) or the thermosetting plasitcs work very well, but have relatively short saw life. Edges on saw blade chip long before the saw is worn out. Side support discs should always be used to prevent the cutting heat from warping the saw blade. 3M makes an excellent diamond dust saw blade that is intended for cutting glass tubes but works with excellent results on the thermosets (which won’t load up the diamond dust with residue which builds up additional frictional heat).

The above information is intended as a starting reference guide.

There are hundreds of thermosetting compounds available, so the individual machinist will have to experiment to gain the maximum conditions for any one material. Where time does not permit exploration—these conditions should be the starting point—especially the negative cutting angles and high surface speeds. Positive cutting angles, low RPM’s and heavy cuts and pressures will give nothing but disaster. Once you get the hang of it you will find them as easy to machine as brass.


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