measuring thread depth

hello every one,I have just roughed out two lead screws for a jacobs gear hobber 3/8 BSF. I should probably have bought a die when I bought the tap but I forgot, any way I have a set of wire pins for mesuring thread depth, can any one tel me how to use them?

I have measured accross the tap with some pins and got a measurment of .4 inch, I could just cut untill I get the same measurment on the work but how much clearance I will need?

It might be worth mentioning that the tap is oversize by 8 thou accross both sides when measured directley without the pins, I was expecting it to be less then 3/8 inch because of the rounding of the thread crest but it isn't

any thoughts on any of this any one?

David,

Can't help with the pins, but the tap dimension is not unusual. Many thread forms have a clearance in the root of the female thread in order to avoid the risk of binding. To do this requires that the tap is slightly over nominal size. This is particularly pronounced on ISO metric threads; I don't have details of the BSF thread form to hand, but it doesn't surprise me that the tap is over nominal.

David

Edited By David Littlewood on 13/06/2013 00:13:07

.

David,

There is a pretty good description here

MichaelG.

NB The Little Machine Shop calculations cover 60° threads (US and metric) whereas BSF is 55°.

Andy

.

Very true [and clearly stated in the document]

This, from NPL, gives much more information, but the Little Machine Shop notes are a valuable introduction to the method.

MichaelG.

David,

According to the Zeus chart the depth of thread for 3/8" BSF is 0.0320". This is what you should have aimed for when screw cutting the thread. My late Father taught me to always finish of a screw-cut thread with a die to produce the correct thread profile and size.

The simplest way out of your situation is to buy a die as this will produce the result you require.

Eric

Use a thread chaser, or make one from an old 3/8 BSF Tap?

The rounded tops and bottoms of Whitworth thread forms is the main reason I use UNF/UNC for most of my imperial threads, but a flat top on a BSF thread would do no harm. Ian S C

thank you every one for your help i will get on with the job

An old (1943) copy of Machinery's Handbook states that a BSF nut should be 0.002" oversize. The same copy gives the full diameter of a 3/8" BSF nut as 0.3770" to 0.3860", ie, 0.002" to 0.009" oversize. So it would appear that the OP's tap is just in tolerance. Presumably it will get slightly smaller with regrinds. Earlier I measured a 1/4" BSF spiral flute tap and a normal 5/16" bottom tap, both unused, and both are 0.004" oversize.

Regards,

Andrew

I'v got a 1/4" UNC +.012" tap, we used to use them for fitting over size exhaust studs on the Continental aero engines, next stage was to drill out and fit Heli-Coils and standard studs, about 3rd rebuild.

Ian S C