Thread gauge

I have just cleaned up a thread gauge that I unearthed in my shed. It is marked Starrett No 7 there are 26 blades the finest marked 60-021, the coursest marked 4-320. there are 60 teeth on the finest and 4 on the course one.

Question what is it for ?

Roy

First is Whitworth pitch. second is difference between major and minor diameter in thou.

Edited By JasonB on 27/10/2019 20:20:26

Thanks Jason

Roy

Edited By roy entwistle on 27/10/2019 20:45:12

Is that with a sharp pointed tool Jason. eg 2 inch with 4 tpi thread, from touch on the outside diameter, the minor will become 1.680 inches diameter using a 55 deg included angle tool?

Neil

No it isn't, the thread depth for a 4tpi Whitworth thread is 0.16008" from the radius on the crest to the radius on the root. So a sharp pointed tool plunged by the thread depth will not cut the proper thread profile.

Incidentally 2" BSW is 4.5tpi, not 4tpi.

Andrew

Full form from rounded crest to bottom of rounded valley. Depth is 0.6403278 x pitch so double that to get minor diameter.

lets take 2.5" dia x 4tpi as 2" dia is 4.5tpi

Pitch 1" / 4 = 0.250"

0.6403278 x 0.250 = 0.1600818

2 x 0.1600818 = 0.3201636 = the 320 shown on the gauge and not the height of a pointed triangle.

EDIT, Andrew beat me to it for a change

Edited By JasonB on 29/10/2019 18:58:53

I use full profile threading inserts, they are the proper shape for the job.

Sorry about that, I won't do it again, honest.

Andrew

Checking in my Starrett catalogue, I could not see this "gage". The only one listed there with 26 leaves is the no 475 covering 3 1/2 to 32 TPI.

It's in the catalogue shown in this thread from a couple of years ago

Thank you. It seems that they dropped the no 7 between 50th and 100th anniversary editions.

Thank you Roy, for asking!

The answers have explained the same thing that puzzled me, on similar thread-gauges.