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Thread gauge

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roy entwistle27/10/2019 20:04:50
1057 forum posts

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

JasonB27/10/2019 20:20:10
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First is Whitworth pitch. second is difference between major and minor diameter in thou.

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

roy entwistle27/10/2019 20:35:19
1057 forum posts

Thanks Jason

Roy

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

Neil Lickfold29/10/2019 18:37:21
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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

Andrew Johnston29/10/2019 18:56:33
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Posted by Neil Lickfold on 29/10/2019 18:37:21:

Is that with a sharp pointed tool

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. smile

Andrew

JasonB29/10/2019 18:57:58
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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 changesmiley

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

old mart29/10/2019 19:32:55
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I use full profile threading inserts, they are the proper shape for the job.

Andrew Johnston30/10/2019 14:16:34
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Posted by JasonB on 29/10/2019 18:57:58:

EDIT, Andrew beat me to it for a changesmiley

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

Andrew

ega30/10/2019 16:16:18
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Posted by roy entwistle on 27/10/2019 20:04:50:

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

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.

JasonB30/10/2019 16:25:30
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It's in the catalogue shown in this thread from a couple of years ago

ega30/10/2019 16:57:10
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Posted by JasonB on 30/10/2019 16:25:30:

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.

Nigel Graham 201/11/2019 20:30:10
440 forum posts

Thank you Roy, for asking!

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

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