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The Fabrication and Cutting of Thermoset Materials
Introduction
As the machining of plastic materials cannot be mastered in one setting, the same can be said, probably more so, of thermosets (FR4/G-10, G-11, phenolics). Glass epoxies and phenolic have traditionally been unfairly characterized as materials that a lot of machine shops or fabricators avoid due to their dust, wear and tear on equipment/tools and overall “toughness” of the material. To the contrary, with a little bit of practice and patience most fabricators can master the art of thermosets. What follows is mostly a guide to get you started in that process. Mastering these suggestions will not immediately make you an expert machinist of these materials but should significantly improve your ways of doing so.
Paper and Fabric Grades
Thermoset materials machine well with proper techniques. As a rule, they machine more readily than metals with standard machine tools such as those used for wood or metal fabrication.
For most machining operations, ordinary high-speed steel tools are satisfactory. However, where production quantity, speed or finish is important, carbide-tipped tools often prove more economical. Cutting tools must be kept extremely sharp to achieve accuracy and fine finish.
Thermosets are machined dry - cutting compounds and lubricants are not necessary. Cooling by air is preferable to the use of liquid coolants which are difficult to remove from finished parts. Machine operators should be cautioned to keep the temperature of the work below 150°C since temperatures above 150°C may distort the material. Cuttings are readily removed by suction.
Epoxy Glass Base Grades
In many cases, the same machining operations employed in the fabrication of metals and wood may also be adapted to glass base grades. However, certain slight changes in tools and the use of proper speeds are necessary.
Diamond or tungsten-carbide tools will give more satisfactory work with much longer, more economical life than high-speed tools.
CIRCULAR SAWING
Paper and Fabric Grades
Circular saws may be used for straight or angular sawing. When smooth edges are required or close tolerances are important, a hollow-ground circular saw without set should be used. For rough cutting, saws with set are satisfactory. Best results are obtained when the saw blade projects a minimum distance above the saw table.
12” saws should be used for material up to 1” thick and 16” saws should be used for thickness up to 2 1/2”. It is important that all teeth be square, of the same height, and free from burrs. The cutting edge should run either directly toward or just back of the center hole. In both circular sawing and band sawing, the work should be fed as rapidly as possible without forcing.
Common industry standards for circular saws are shown on this page.
Epoxy Glass Base Grades
A diamond impregnated wheel with copper body 1/16” thick and 12” dia. run at 3000-3600 rpm will give good results cutting dry with a good exhaust system. Material feed should be as fast possible without forcing the saw. Idling creates friction and heat, which cause excessive dulling and burning. A flood of water, or water-soluble coolants, on the work and wheel can be used when necessary to prevent overheating. Abrasive wheel cutting under water is also recommended.
Circular Saw Data
Application rpm # of teeth diameter N T S
Rough cut* 2150 100 16” 3/8” 1/4” 1/8”
Smooth cut* 2150 100 16” 9/32” 5/32” 1/16”
All purpose* 2860 100 12” 3/8” 3/16” 1/8”
Tubing cuts 2860 100 12” 1/4” .095” 1/16”
* These saws have .019” set. Other saws are hollow ground to prevent binding
TURNING
Paper and Fabric Grades
Ordinary high-speed tool steel can be used in finishing operations for all thermoset grades. However, carbide-tipped tools may prove more economical and will hold sizes more accurately from piece to piece. About .010” stock should be left for finishing. Thermosets can be turned at 400 surface feet per minute with high-speed tools, and about twice that fast with carbides. Tools should be kept sharp, ground, with an included angle of 80° to 100°, and with a 10° to 16° side clearance. Cutting should be done dry.
Epoxy Glass Based Grades
Conventional machining such as turning, boring, and facing can be done on automatic screw machines, standard and production lathes and hand turret lathes.
Carbide-tipped tools and cutters should be used with surface speeds below those used for paper base laminates. Tools should be ground with a zero rake and machining can be done dry with an exhaust system to remove dust. A coolant can be used, but it is not necessary.
MILLING
Paper and Fabric Grades
Standard tools may be used at speeds and feeds similar to those for bronze and soft steel. It may be more economical in spite of higher material cost to use carbide tools. The cutting angle of the mill will give better results if ground with a slight rake.
Epoxy Glass Based Grades
Glass base laminates can be milled very satisfactorily on any conventional metal-working milling machine. Carbide tipped tools should be used. Only climb or down milling should be practiced, as up milling will tend to delaminate the material.
DRILLING AND TAPPING
Paper and Fabric Grades
A standard high-speed drill with lips backed off to provide plenty of clearance is satisfactory for all thermoset grades. However, for long production runs and deep holes, carbide-tipped drills give the best performance.
Drills should be lifted from the work frequently to prevent binding and excessive heating. The feed should be light and uniform and the speed of the drill should be considerably in excess of that used for soft steel. With tungsten-carbide tips, speeds may be as high as 16,000 rpm. Where possible, the material being drilled should be backed up with scrap thermoset or other soft material to prevent chipping out.
In drilling thermosets parallel to laminations, extra care must be taken to prevent splitting. The material should be clamped in a vise or between plates and the drill should be lifted more frequently to remove chips.
Holes 3/4” and over may be drilled in the conventional manner using radial drill presses or the counter bore method in which a pilot hole is drilled first.
Drill size: Because of the nature of plastic material, the diameters of holes drilled in laminates are usually .002” under the drill size. Therefore, the drill selected should be at least .002” larger than the specified diameter of the hole. If the drill is being used dull, the hole
size may be an additional .002” undersize, or a total of .004” less than the diameter of the drill.
The recommendations for drilling also apply to tapping. Taps used for metal are also suitable for thermosets. Tapping heads or tapping machines may be used and for production work, collapsible taps are available in sizes over 1 ¼”.
Tap Size
In tapping thermosets, high-speed taps .002” oversize should be used.
The tap drill size should also be changed to .002” oversize to counteract the tendency of the drill to cut undersize. If the thread is to be used frequently, metal inserts should be used.
For threaded holes over 1/2”, it is often more desirable to chase the thread on a lathe using a motor-driven cutter mounted on the tool post.
Epoxy Glass Base Grades
When drilling glass base grades, a carbide drill should be used. The materials can be drilled dry with a good exhaust system to remove dust. A flood of water, or wash-soluble coolants, on the work and drill can be used when necessary to prevent overheating and dulling of drills. High speed drills, nitrate treated, can be used, but must be sharpened more often. Care should be taken when sharpening that the drill is cut back far enough to the original body diameter of drill. Spindle speed for these grades is 4800 rpm for 1/4” diameter drills.
The methods for tapping these materials are much the same as for tapping paper base laminated plastics. The abrasiveness may cause taps to cut very close to size, resulting in a tendency toward binding when backing out. Standard high speed steel taps can be used on short runs. For any sizeable quantity carbide taps should be used. Taps should be purchased oversize. Coolant can be used, but is not necessary if a good exhaust system is available.
THREADING
Paper and Fabric Grades
For threaded holes over 1/2”, it is often more desirable to chase the thread on a lathe, using a motor-driven cutter mounted on the tool post.
When cutting a 60° thread, it is always advisable to swing the compound reset on the lathe to a 30° angle. The tool is ground to cut on one side only.
For all other threads, standard methods are used with satisfactory results: the speed and feeds are similar to those used in threading soft steel.
Epoxy Glass Base Grades
External threads and internal threads can be cut on a lathe with a carbide-tipped tool, dry. Fine cuts should be taken to give best results. A coolant can be used, but is not necessary.
