Thursday, October 1, 2009

Quickly tapping (threading) holes in plastic

For my day job, I often have to tap (cut threads into) holes in plastic that will receive screws. This can be done by hand with a standard tap wrench, but it's also possible to do with a hand drill. I bought a small 3/8" square drive adapter that has a 1/4" hex shank. This was intended for use in cordless screwdrivers which accept hex shanks, but a drill chuck will grab the hexagon just fine. I also bought a set of adjustable tap chucks that have a 3/8" square drive hole.
The smaller chuck goes from #2 taps up to about #10, and the larger for #10 up to 3/8".

For plastic, I use lots of distilled water as coolant. If I am tapping a blind hole, I usually just fill it up with water. The nice thing about using distilled water is that it leaves no residue after being blown clean with compressed air.

Tapping a hole with the hand drill works best for a certain range of sizes:

In ABS, I would use the drill for any size #2 through 1/4"

In acrylic or Delrin, I would probably only use the drill for #6 through 1/4"

The really small taps sometimes clog up, even with coolant, and the drill doesn't provide enough sensitivity to avoid snapping the tap. ABS is so soft, a clog would just be pushed away, whereas in acrylic or Delrin, it might jam.

For sizes larger than 1/4", I would probably use a tap wrench because they take a lot of torque, and it's difficult to keep the drill straight and have it deliver enough torque to spin the tap.

I like using the chrome-plated taps because the finish will not rust (a concern since I use water as a coolant), and the chrome is supposed to have a lower friction coefficient than bare high-speed steel.

For sizes smaller than 1/4", I would only use a bottoming tap in plastic. There is no reason to use a plug or taper tap because the bottoming tap makes a cleaner cut, and actually produces less friction and heat than plug or taper taps. In harder materials it makes sense to cut gradually, but plastic is so soft, it will just melt if the tap keeps making light passes that rub against the surface.