Thread chased boxes

[Eric Holmquist]

Playing around with thread chasing has me struggling with a basic internal design issue.

If I allow for 1/8" of wood behind the threads, and 1/16" for the thread interface, whichever half of the box that contains the male threads will have to allow for at least 5/16" of wood at the joint, and you probably end up closer to 3/8".

If the male threads are in the base, this makes for a very thick walled box especially if it is a small box, yet most thread chased boxes I have seen have male threads on the base piece. I can understand why, this way, nothing inside the box can damage the threads.

If the female threads are in the base, the wall thickness ends up being around 3/16", which seems both more asthetically pleasing, and less intrusive, but seems to put the threads in jeopardy of damage from stuff in the box.

Since part of the turning process uses the threads as a drive center, I am hesitant to go much thinnner.

A threaded insert would seem to make matters worse as you need some wall thicknesss outside of the insert.

I am curious how anyone who has played around with thread chased boxes has delt with this issue.

[Peter Fabricius]

You are right about there being issues with wall thickness, especially on the box bottom with the male threads. I have cut a lot of threads on little boxes and when I first started I cut through the wall of the bottom a few times (slow learner) because it was too thin to support the threads. Now, I drill the box with a smaller than needed hole and then cut the threads and hollow the rest of the box out after the threads are cut. You can even hollow out below the threads a little to get more room in the box and make it lighter.
I do the same for the top of the box, leave the OD larger than needed to give more support for the thread cutting and then trim this down when finish turning the whole box. The bottom is in the chuck holding the lid and gets parted off at the very end after sanding and finishing.
Hope this helps. I am using the 16TPI cutting jig from England and it works great.
Good luck.
Peter F.

[Eric Holmquist]

I have not cut through the side yet, but that is only because I have been super cautious and made my wall thickness very heavy until all threading was done, then with the bottom in the chuck, and the lid held by its threads, turned the final outer profile until I was down to as little material on the female side as I dared.

Turning small boxes from 2" square stock, I ended up with a 1 5/8" OD and a 7/8" ID in the base. My wall thickness in the joint area was 3/8"

I am using the 16 tpi thread chasers from the UK, but the same problem would exist regardless of how you cut the threads. It is probably worse with the threading tool that uses a rotary cutter in the headstock because using that, one can more easily cut to the full engagement depth of the threads without the threads starting to get tearout.