Machining Phenolic: Dust, Wear, and Tooling Guide

Quick take: Phenolic behaves differently from metals because abrasive wear, dust generation, and heat all change tool-life priorities. Use this guide alongside cutting-equation and process-planning references when planning phenolic machining.

Related references: Understanding Cutting Equations, Cutting Speed and Feed Rate Guide, and Chamfers and Countersinks That Halt Burr Formation.

This month, I’ll talk about machining phenolic plastic, also known as Garolite or Bakelite. The material 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 thermoset material, which means that it doesn’t melt and retains strength at higher temperatures. Being a plastic, 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 to modify a Boston Gear QD60, a spur gear made of phenolic. It has a 5″ pitch diameter and comes with a 0.75″ bore without a hub projection. The gear needs the bore enlarged 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 aren’t damaged. These gears are pricey, so I don’t want to turn one into scrap.

Pieces of aluminum stock are placed between the chuck jaws and the gear teeth. Image courtesy of B. Taylor

The mill setup is shown. Image courtesy of B. Taylor