Thread: 4mm X 24 tpi?

Something haywire with your mixed specifications.

Standard pitches for 2.8 - 5.6mm are 0.35 (72.6 tpi), 0.5 (50.8 tpi), 0.6 (42.3 tpi), 0.7 (36.3 tpi), 0.75 (33.9 tpi), and 0.8 (31.75 tpi). 5/16" is 7.94mm, not 3.9mm. 24 tpi on a 4mm rod will leave little material for strength.

Cheers,
Joe

The Good Doctor may be referring to 10/24 UNC. The tapping drill for that is No 24 (3.9mm). The major dia is 0.19", or 4.8mm.

Looks like you're trying to make a 10-24 UNC thread, but this thread is 4.9mm dia and anyway generally non-preferred due to it's relatively small internal cross-sectional area. That small a diameter really needs to be around 32 TPI at the least for strength. A 3BA (4.1mm dia) thread has 34.8 TPI and these screws, although now obsolete, were for many years the only real Imperial threads judged strong enough for instruments, small mechanisms and clocks.

Erm, what's wrong with cutting an M4 thread, seeing as you already have a 4mm dia shank to start off with?

Aha! 10-24 is 3/16", not 5/16". Perhaps the Doctor just needs new spectacles.

Cheers,
Joe

Regardless of the thread size, why not just use a cut off tap of the correct pitch as a thread chaser if you want to cut the thread instead of using a die.

Yep, you're all correct, I misread my chart and was looking at the DRILL size! I wanted to get chasers for the coventry diehead which is 5/16. That is it's full capacity, not the SIZE I wanted to cut. I'd like to add a photo of this shaft if I had good camera handy, as it IS 4mm and IS 24tpi! But not cut to the full depth. I've decided now for expediency to just use a 3/16 BSW die in a holder and close it to the smallest setting. The shaft is to be screwed into ebony after tapping that out for about 3/4 of its 18mm length so I won't need to cut a thread to the full depth. I don't necessarily require this to be 24tpi but do prefer a coarse thread over a metric in this case and thanks all for the advice but rather than new glasses, I'll learn to use the chart properly.