3/16 BSW is 24tpi.

According to my old Newnes Engineers Manual it gives the smallest standard Acme pitch as 16tpi. But it also gives these calculations.

width of point of tool = (0.3707 / threads per in) – 0.0052

depth of thread = 1/2pitch + 0.010

flat at top of thread = pitch x 0.3707

flat at root of thread = pitch x 0.3707 – 0.0052

So from those calculations you could make a couple of tools to cut your 24tpi.

Errr, both the title and first post state a square thread!

For a full depth square thread the width of the thread, the width of the gap, and the depth of thread are all the same and equal to half the pitch. So if we assume the thead to be cut is 24tpi then the pitch is ~0.04167", and half that is 0.0208".

A problem with square threads, and one reason why Acme threads became popular, is that the can interfere on the root, crest and flank independently.

When I made my square thread brake shafts I hand ground and honed a HSS toolbit to the correct width, using a micrometer. I then cut taps in silver steel. Square, and Acme taps, usually come in a series with increasing OD, with the last tap cutting full depth. I cut the taps with the full OD and the correct depth of thread. My thread OD was ½" so I made a set of three:

When I cut the brake shaft I made the OD a few thou undersize, and cut slightly deeper than theoretically correct. That should sort out the crest and root interference problems. Once I'd cut the square thread in the nut:

I gradually reduced the width of the thread on the brake shaft until the nut fitted smoothly and without shake:

Andrew

Edit: Bother, too darn slow as usual, I'll just crawl back into the cave and get on with some work

Edited By Andrew Johnston on 09/02/2017 14:53:54