Hand machined mechanical pencils

Single-tooth 6mm broach for plastic (03/06/14)

I've recently been making lots of delrin (a.k.a. acetal) parts for a new product (stay tuned!) and I needed holes which are a neat sliding fit for 6mm linear rails. I initially did the obvious - drill 5.9mm then put a 6mm reamer through. Unfortunately, because of the "springiness" of the plastic, the hole relaxed slightly, giving a tight push fit. After checking with a micrometer, it turned out that both the reamer and the shafts were 5.995mm (there was a little random variation on the shafts, but that's the average), so there was no way that reamer was going to do.

Solution - I made up a very simple single-tooth broach which is shoved through the reamed hole and enlarges it very slightly. Photos below.


The part on the right (5.980mm dia.) is inserted into the reamed hole. As it's pushed through, the cutting edge, which is 6.020mm dia., enlarges the hole sufficiently to provide a sliding fit for the linear rails. The rest of the tool is slightly smaller, 6.000mm dia., to guide the broach as it passes through.

It's relatively easy to make on the lathe, it just takes a bit of care with a micrometer to sneak up on the final diameters.

Quick video of using it: