b'\x1b\x1a\x19 \x1f\x18\x1d\x17\x16 Machinists CornerMACHINING PHENOLICBy Brandt TaylorT his month, Ill talk about machin-ing phenolic plastic, also known as Garolite or Bakelite. The mate-rial is hard and strong for a plastic and used for a variety of machinery components. It is reinforced with paper or cotton fiber and is a ther-moset material, which means that it doesnt melt and retains strength at higher temperatures. Being a plas-tic, the modulus of elasticity is much less than for metals. Compared with steel, phenolic is about 50 times less, meaning that an applied force will cause 50 times more deflection in a part made from phenolic. When clamping or cutting a part, this fact is important.From time to time, I get a job toB. Taylormodify a Boston Gear QD60, a spurPieces of aluminum stock are placed between the chuck jaws and the gear teeth.gear made of phenolic. It has a 5" pitch diameter and comes with a 0.75" bore without a hub projec-tion. The gear needs the bore en-larged to 3" (0.001"), as well as a bolt circle added. To resize the bore, I clamp the gear in a four-jaw chuck on a lathe. Figure 1 shows the setup. Pieces of aluminum stock are placed between the chuck jaws and the gear teeth. Clamping calls for a light touch so the teeth arent dam-aged. These gears are pricey, so I dont want to turn one into scrap.After centering on the 0.75" bore, I use a carbide boring tool to makeB. Taylorthe 3" feature. I like brazed-carbideThe mill setup is shown.boring bars because I can make a very sharp edge and set the toolcut, too much material will be re- 270 sfm or slower.with a positive rake angle. Due tomoved. With a light cut, too littleThe material is abrasive and rap-the low modulus, the material willmaterial will be removed. If you haveidly wears cutting edges. Since the deflect under the edge if it is notfussy features, sharpness and a pos- part is clamped lightly in the chuck, sharp enough. When taking a heavyitive rake matter. I cut this bore atI take light cuts, 0.08" on diameter 12SEPTEMBER 2023 MachinistCorner.indd 12 8/15/23 12:38 PM'