# Starrett screw pitch gauge markings



## jfcayron (May 4, 2014)

I am now the proud owner of a Starrett #6 screw pitch gauge.



On each blade, there are 2 values:
- the TPI
- a decimal number that seems to be 2x the value called "Depth of Sharp V-Thread" in the Machinist handbook.




e.g. 20 TPI - book: 0.04330 x 2 = .08660 - the blade shows .086
24 TPI - 0.03608 x 2 = .07216 - .072
etc.

The Depth (or Height) of Sharp V-Thread is the difference between the root and the crest if the threads were cut perfectly sharp and straight.

What is the practical purpose of that indication on the gauge? Why is it 2x the value above?


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## DMS (May 4, 2014)

I have a fishtail gauge that has "double depth of thread" printed on the back, which, if I am reading your description correctly, is what the thread gauge lists. I use it as ballpark when single point threading. that looks like a nice thread gauge btw.


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## Grumpy Gator (May 4, 2014)

​ Wouldn't that be the difference between the outside diameter and the root of the thread.
******Just Saying*************G****************************************


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## Bill C. (May 4, 2014)

jfcayron said:


> I am now the proud owner of a Starrett #6 screw pitch gauge.
> View attachment 76053
> 
> 
> ...



I have a Starrett with gauges on both ends.  I used mine to double check the number of threads per inch.  And finished depth, mostly with adjustable thread dies.  

The production shop I was in used "GO/NO-GO" thread gauges for both internal and external threads. When a tap has some wear on it the diameter can change. I always kept a bolt handy to double check the depth of a thread as well. Specially when running a tapping head in a drill press.


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## benmychree (May 4, 2014)

The double depth marking can be used to calculate the root diameter of the thread, to determine tap drill sizes and diameter to bore for internal threads cut in the lathe for diameter / pitch combinations that are not listed in handbooks or charts.


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## jfcayron (May 4, 2014)

Thank you all. It helps!


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