Machinist

[Lawrence London <lfljvenaura@gmail.com>]

Using the Starett C391 Center Gauge
From comments at Practical Machinist
====================================


Reading Center "Fish" Gage 101

    OK guys & gals,

    I have a brand new Brown & Sharpe center gage in my hands. It is much the same as the Starrett and others.

    Other than the 60 degree obvious angles, what is the significance and use of the scale on both sides of the "tail" and what is the

significance and how does one use the "chart" of associated numbers (eg 3 1/2 .371, 4 .395, 4 1/2 .289, etc) on the "head" side?

    In other words, this looks like a pretty simple gage/rule, but it doesn't make sense to me how to use it.


rules (scales) on both sides of the fishtail are TPI

You can quickly use it to confirm or check threads. My Starrett has 32,24,20,14 TPI rules on the edges. They are useful for threads that are

double or half that as well, so you can easily read 16,64,40,10,28,7 TPI with the same rules. Edit: also 12 and 48 TPI

The number chart with "double depth of sharp V thread" which is what I think my other Starrett has on it, I never learned of its significance,

hopefully someone else here can expound on that. I've currently have my Whitworth Starrett gage in front of me, it only has the thread rules on

it, no number chart.

The numbers are how deep the cut in diameter will be IF you were using a sharp V thread. If your crossfeed reads in diameter(as mine does) rather

than depth of cut (or radius) then you can use the numbers directly.

The numbers vary a bit amongst gages. As a ferinstance, my Starrett No. C391 fishtail says 16 .081 on it while my American Caliper Co. has 16

.108. Both are for a 16 TPI thread but the Starrett table is for Double Depth of American National Thread while the ACC table is for Double Depth

of Sharp Thread.

Note that it is for the cross slide depth. If you move the compound, say 0.081" (or 0.0405" if yours reads in depth of cut) and it is setup at 30

degrees it the thred will only be 0.0405" X cosine(30) deep or about 0.0351"

I gage (roughly) with the cross slide. I am so used to it that it is almost automatic to check with the cross slide dial once the compound

reaches that depth. I find it a good point to check because I have only a couple more passes to go. Sucks to go undersize.


Not quite right about "Double depth of AMER. NAT. THREAD

    Quote Originally Posted by David Utidjian View Post
    The numbers are how deep the cut in diameter will be IF you were using a sharp V thread. If your crossfeed reads in diameter(as mine does)

rather than depth of cut (or radius) then you can use the numbers directly.

    The numbers vary a bit amongst gages. As a ferinstance, my Starrett No. C391 fishtail says 16 .081 on it while my American Caliper Co. has 16

.108. Both are for a 16 TPI thread but the Starrett table is for Double Depth of American National Thread while the ACC table is for Double Depth

of Sharp Thread.

    Note that it is for the cross slide depth. If you move the compound, say 0.081" (or 0.0405" if yours reads in depth of cut) and it is setup

at 30 degrees it the thred will only be 0.0405" X cosine(30) deep or about 0.0351"

    I gage (roughly) with the cross slide. I am so used to it that it is almost automatic to check with the cross slide dial once the compound

reaches that depth. I find it a good point to check because I have only a couple more passes to go. Sucks to go undersize.

    -DU-
    The angle of the thread in the American National thread is 60 degrees, so a perfectly sharp thread will form an equilateral triangle. The

height of the triangle (or, for threads, the depth) is given by (0.5/TPI)/tan 30, where 1/TPI is the pitch in inches. For the case of 16 TPI, the

depth is (0.5/16)/tan 30 = 0.0541". The DOUBLE depth is twice as much, or 0.108". However, Starrett gives the double depth as 0.081". So who's

right? Starrett, of course.

    The reason for the discrepany is that Amer. Nat. threads are NOT sharp, neither top nor bottom. See http://www.unionmillwright.com/2879.pdf

for a clear explanation. The formula given therein is depth = p X .649519, where p is the pitch. So, for 16 TPI, the depth is (1/16) X 0.649519 =

0.0406 and the DOUBLE depth is 2 X 0.0406 = 0.081", just what's written on the Starrett gauge! So, the fact that the OD of a thread is always

less than its nominal value is not due to poor quality, but, rather, because of the purposely truncated threads.



Let it be noted that the Sellers / Franklin Institute / U S Standard / American National form of screwthread was replaced as the U S national

standard in 1949 by the Unified threadform -- which also became the national standard of Canada and the UK at about the same time. The Unified

threadform is arguably a derivative of the Sellers, having the same 60 degree V angle and flats at both the Major and Minor Diameters, but the

Major and Minor Diameter flats are of different lengths and the whole thread is shallower.

For the Unified threadform, which shared its fundamental geometry with the ISO Metric threadform that became the national standard of most

European nations in 1959, the Major Diameter flat is 1/8 x Pitch, the Minor Diameter flat is 1/4 x Pitch, and the Single Depth is 5/8 x Pitch x

Cosine 30 degree.


Where is my Double Depth engraving. Starrett No. 391 or No. C391

    At what point did Starrett add the Double Depth info to the Gauge? Mine is a plain Starrett No. 391 not the "C" model and I only have the

word TEMPERED stamped on the backside. Are there No. 391's with this data or was it part of the No.C391 entry?







--

Lawrence F. London, Jr.
lfljvenaura@gmail.com

Avant Geared

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