Thread cutting in metal work lathe

[CliveK]

I wish to cut a course square thread (approx 5 tpi) in wood - probably pear.

I have a metal work lathe which can be set up to use automatic feed to cut such a thread.

What shape of cutter should I use?

(I have experimented with a few cutters on some scape wood and all tear the wood badly.)

 

[jasonB]

The problem with trying to do a square or even an ACME thread is that you have to use a set tool profile to suit the thread form, your only option will be to grind a tool with a lot of top rake.

If you look at most thread formers for wood they use a very shallow angle, something like 90degree included angle which cuts each face at 45degs to the grain direction, a square thread will be cutting at 90degs to the grain so tearout is almost unavoidable.

Jason

PS.Another option would be to rig up a holder for a small router/laminate trimmer in place of the topslide and turn the lathe over by hand with the lead screw engaged.

 

[dickm]

Two possibilities. The best is probably to use JasonB's suggestion of mounting a small router, or better still a Dremel-type drill, in the toolpost, with a suitably ground cutter in its chuck.
The other would be to mount a conventional woodturning parting tool to cut at a very low angle (near the upper side of the workpiece)and take VERY fine successive cuts with this. If you could grind a recess in one surface of the parting tool so that the eges cut as well as the point (like one of the Sorby parting tools) that would help too.
Remember, too, to incline the tool so that it fits with the slope of the thread, rather than having it vertical as when parting.
HTH

 

[CliveK]

Thanks for the suggestions I'll see what I can do.

 

[Inspector]

http://forums.taunton.com/fw-knots/messages?msg=7987.11

The link tells of a way to do threads on a table saw. If you were to use a saw blade that cuts a flat slot/groove you would get approximately 4 threads per inch (0.125" for blade and 0.125" of remaining wood for each turn) . A "comb" cut at the correct pitch and placed in the out feed side of the fixture to engage the freshly cut thread as it emerges from the blade should work. Might take a few tries to get the fit right but there wont be any tear out. Could also substitute a router for the saw (probably get some tear out though).

The nut remains a problem to be solved though. :?