Cutting tenons in plastic

[NGLJ]

As a very recent newcomer to kitless, I am still very much learning. Based upon a number of articles on making kitless pens, the advice that I gleaned was that making tenons 0.1 mm undersize is good practice, even with plastics, ebonite etc.. The argument was that it is less likely to strip the threads. I have to say that my personal experience has been that it can help. What is the general opinion, and why?

 

[duncsuss]

I have two reasons to size the tenon smaller than the nominal major diameter.

First, as you say, it results in cleaner threads. I'm not sure I'd call it "stripping the threads" but there's no question they tend to chip and/or crumble a bit when the blank is full diameter. (A side note: the same is true when making threads the old fashioned way, with a "chasing tool".)

My second reason is that I polish the tenon before cutting threads, and having it undersized leaves behind a polished flat top, as opposed to a sharp point. I like the way it looks much better when there is that shiny bit in between the grooves.

 

[NGLJ]

if you are using the Hinze tenon cutter, how do you best achieve the slightly undersized tenon?

 

[duncsuss]

You'll have to get that from others - I don't use a tenon cutter, just a standard 1/8" parting tool. (Maybe ask Jim Hinze how he'd do it.)

To get the dimensions, I use either calipers or a little template with slots sized for M10x, M12x, M13x and M14x threading - so 9.9mm, 11.9mm, etc. One of the members here cut them on his CNC machine a few years ago (I remember it was Carl ... but not if it was Carl Morrell or Carl Fisher, too long ago!) Should be fairly simple to make one from a piece of stiff material (that won't flex when you test the tenon), a saw and a file.

 

[NGLJ]

For those who happen to use the Hinze tenon cutter, how are the bushings sized, ie. 9 mm bushing is exactly 9 mm, or is it slightly smaller?

 

[hooked]

I will say that I used to use an old version of the tenon cutter. I do not know if the more current one sold through Turner's Warehouse is different.

When I used one, I would set it with the guide and have the smallest space (paper-thin) between the guide and cutter as possible. I would cut my tenon and then take down the last .1mm or so by hand. I personally found that if you put any pressure or do not have the guide perfectly centered on the cutter when trying to set the tool, it was easy to undersize your tenon. I just played it safe and did the final bit by hand.

 

[NGLJ]

I will say that I used to use an old version of the tenon cutter. I do not know if the more current one sold through Turner's Warehouse is different.

When I used one, I would set it with the guide and have the smallest space (paper-thin) between the guide and cutter as possible. I would cut my tenon and then take down the last .1mm or so by hand. I personally found that if you put any pressure or do not have the guide perfectly centered on the cutter when trying to set the tool, it was easy to undersize your tenon. I just played it safe and did the final bit by hand.

Thanks, that's useful information.