Machining Phenolic: Dust, Wear, and Tooling Guide

After centering on the 0.75″ bore, I use a carbide boring tool to make the 3″ feature. I like brazed-carbide boring bars because I can make a very sharp edge and set the tool with a positive rake angle. Due to the low modulus, the material will deflect under the edge if it is not sharp enough. When taking a heavy cut, too much material will be removed. With a light cut, too little material will be removed. If you have fussy features, sharpness and a positive rake matter. I cut this bore at 270 sfm or slower.

The material is abrasive and rapidly wears cutting edges. Since the part is clamped lightly in the chuck, I take light cuts, 0.08″ on diameter per pass, until the bore is 0.02″ or so undersized. Then I check the cutting edge. I dress it with a diamond file if needed and then hit the finished size. Hitting the mark is the art of a machinist.

This material does not make curly chips; it crumbles when cut and makes toxic dust that’s not a good idea to breathe. So if you want to live for a while, take care. I am 75 and don’t plan on checking out soon, so I use a dust mask when doing this lathe work.

The bolt circle is drilled on a CNC milling machine in the usual way. Figure 2 shows the mill setup. Be careful because 6″ milling vises can generate a lot of clamping force. I find the center with an indicator. If I make more than one part at a time, I check the center on each one. The bolt hole size is 0.3125″. I use a high-speed steel, screw-machine-length drill with a peck drilling cycle and Rustlick water-based flood coolant. The coolant takes care of the dust.

Holes are drilled at 700 rpm and a feed rate of 3 ipm. This produces a nice hole with clean edges and no frayed fiber. The combination of abrasive material with a low modulus causes the holes to drill undersized. I use a 0.315″ drill and take care to keep the edges sharp. The finished holes are good at 0.31″ to 0.314″. I check the hole size with gauge pins. A duller drill drills a smaller hole, which clamps on the side of the drill. A little care prevents that. If I made a lot of these parts, I would go to carbide drills, but that would be a whole different story. Like I have said in the past, high production is not my game.