Long hexagonal hole in Delrin bushing

[John_Dennis]

I need to make a 2 inch long 11mm hex hole in a Delrin bushing for an 11mm shaft to slide in. I am thinking about purchasing an 11 mm allen wrench and making a crude broach out of it. The problem I foresee is that the hole may be too small for a sliding fit, in plastic, will the hole be larger or smaller than the cutter?. This is for the carriage motor switch on a lathe.

 

[Jake M]

You're talking about a makeshift solution to precise dimensioning. The answer to that is not so much in theory, but in practice. In other words, order enough stock to test it.

7/16 would give you a couple thousandths of clearance over whatever happens with the 11mm attempt. Although in a freehand ground broach, you might well find that the grinding makes a difference too....

 

[Eddyde]

An Allen key might not be long enough to make a decent broach, a piece of 7/16 hex bar-stock might be better.

 

[RJSakowski]

I would expect the hole to be smaller than your makeshift broach.I also would expect that you aren't particularly concerned with a close tolerance fit in this application. I would try the 11mm Allen wrench first and if that is too tight, then follow with a 7/16" Delrin likes very sharp cutting edges and that will help keep the hole on size.

 

[RJSakowski]

An Allen key might not be long enough to make a decent broach, a piece of 7/16 hex bar-stock might be better.

I don't believe that it is necessary to make a conventional style broach for working Delrin. A single cutting surface on the end of the Allen wrench similar to that found on a rotary broach should do the trick. I would drill and broach the hole first and turn or otherwise machine the external features in order to control alignment.
Edit: I have used that approach to broach a 1/4" hex hole in aluminum.

 

[Chipper5783]

It probably doesn’t need to be hex the whole way through - just ~1/2” at each end (then over cut the center region). Also you may not need it to be a perfect fit - pre drill at 7/16” and the little bit of round showing won’t hurt you. Also, start off with a longer blank and make an entry bore at the full size across the points to act as a guide - then cut the guide section off.

 

[Jimsehr]

I need to make a 2 inch long 11mm hex hole in a Delrin bushing for an 11mm shaft to slide in. I am thinking about purchasing an 11 mm allen wrench and making a crude broach out of it. The problem I foresee is that the hole may be too small for a sliding fit, in plastic, will the hole be larger or smaller than the cutter?. This is for the carriage motor switch on a lathe.

What is the od size of the finished bushing?

 

[John_Dennis]

22mm, approximately 7/8 inch OD cylinder about 3 inches long with a outside snap ring on one end