Fishtail Guage Markings?

[Fritz1255]

I got a 60 degree fishtail gauge off EBay. It was made by Brown & Sharpe, Providence Rhode Island. Some questions on the markings:

1) There is no marking that denotes the angle, even though it is 60 degrees (I checked it against another gauge). There is a "650" at the point of the gauge, but I can't imagine what this means.
2) There is a set of markings "Double Depth of Amer Nat Thread". The number for 12 TPI is 0.108, whereas double depth of a 60 degree 12 TPI thread should be 0.144. Maybe someone can tell me what the different thread types are?
3) There are four scales on either side of the gauge - 14, 20 24, and 32. Each represents divisions of an inch - 14ths, 20ths, 24ths, and 32nds. What is this useful for?

[PixMan]

The "650" is their catalog/part number.

The scales are for determining how many threads per inch a thread you're checking might be, with the increments being (mostly) ones you don't find on a lot of standard rules.
With the 14 scale you can verify 28, 14, 7, 3-1/2 threads per inch. With the 20 you can verify 5,10, 20, 40 threads per inch, and so on.

[Optics Curmudgeon]

It would be .144 for a sharp V thread, which the American National form is not, Am Nat has a flat bottom.

[3jaw]

The American National thread form has a flat at the root of the thread and a flat at the crest. It does not come to a sharp point therefore the double depth will be less.

[PixMan]

The American National thread form has a flat at the root of the thread and a flat at the crest. It does not come to a sharp point therefore the double depth will be less.

And to further explain, the flat at the bottom of the thread is .25xP, and the height is then .625xP. So for a 12 pitch American National, pitch is 1/12 = .08333, the flat is .08333 x .25 = .0208, and the height of the thread is .625 x .08333 = .05208

Rounded off the .052 x 2 = .104 and that's why that number is on the fishtail gauge.

[rbjscott]

" Fishtail" gauge I had apprentice pipe fitter tell me. " Why machinist don't make as much money as pipe fitters is they don't even know the name of their qwn tools" 1966 J. W. Salter. I know it is picky.

[John Garner]

The "Pitch" of a 12 Thread Per Inch (TPI) single-start screwthread is 1/12 inch, or 0.083 inch to three decimal places.

The THEORETICAL "Single Depth" of a Sharp V threadform would be Cosine (30 degree) x Pitch, or, to three decimal places, 0.072 inch at 12 TPI.

The THEORETICAL "Single Depth" of a US Standard (aka American National, Franklin Institute, and Sellers) threadform, which has 1/8 x Pitch flats at the Major and Minor Diameters is (1 - 1/8 - 1/8) x Cosine (30 degree) x Pitch, or, to three decimal places, 0.054 inch at 12 TPI.

The THEORETICAL "Single Depth" of a Unified (aka ISO Inch) or ISO Metric threadform, which, in their most basic form have a 1/8 x Pitch flat at the Major Diameter and a 1/4 x Pitch flat at the Minor Diameter, is (1 - 1/8 - 1/4) x Cosine (30 degree) x Pitch, 0.045 inch at 12 TPI.

The THEORETICAL "Double Depth" of a screwthread is twice its "Single Depth", and the 0.108 inch value you cite is twice 0.054 inch, the Single Depth of a 12 TPI US Standard / American National / Sellers / Franklin Institute threadform.

At this point, I'll note that the US Standard (etc) threadform was superseded, in 1949, by the Unified threadform as the USA's national standard threadform, and has been obsolescent since then. But, since most of the Major Diameter x TPI combinations were "lifted" from the US Standard, the vast majority of tap-drill charts and fishtail gages were never updated.