Measuring Solid Thread Ring Gages


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It looks like the CONSTANT in your formula is the same as what I found in the table from Threadcheck, just formatted differently. I assume that you take your constant and subtract it from your "over wires" measurement to get the pitch diameter.

CONSTANT = [1.93335 X (1/PITCH)] - [2.9939 X BALL/WIRE SIZE]

The formula I got from the Threadcheck table, for Acme Screws, is:

E = M -[ 4.9939 G - 1.933357]

Where E is the Pitch Diameter, M is your "over wires" measurement, G is the wire / ball diameter, and n is threads per inch. Multiplying the minus sign through the brackets you get:

(1.933357 / n) - (4.9939 * G)

which matches your "Constant" formula except for the 4.9939 factor instead of your 2.9939.

Calibration Kid

I see.

I have the formula entered into an excell spreadsheet.


E5 being the Pitch(3.5) and E6 being the Ball Size(0.12908")

When I change the 2.9939 to 4.9939 it makes my constant negative(-0.09223) and throws my pitch diameter way too high :(

I will have to ask them where they got this formula. They may not respond to me though, I have already asked them for so much help.


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The formula from P&W for the Labmaster software computes pitch diameter like this:

PD = M - P(cos a) + B

Where PD is the Pitch Diameter
M is the measurement using the ball probe
P is the Pitch (1/TPI)
a is the half angle of the thread [29?/2 = 14.5? for Acne threads]
B is the ball size

Calibration Kid

Manufacturers PD=5.18566"

Using my old formula, PD=5.18046"

Using the formula you found, PD=5.19885"
Does that look like I did it right?^^


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Are you using a Labmaster Universal with a T-Probe? We have one here, but since we do solid thread rings on our OPAL 600 ULM, I haven't bothered to look into the process on our Labmaster yet. I can check it out if you are still having trouble and let you know what I find on mine.

Solid thread rings are checked against a hi-lo setting plug. It's a very simple pass/fail calibration- but they are expensive set plugs.

I don't think you can use the general 3 wire calculation for this process because you are not using wires.

Calibration Kid

Yes, ULM with T probe that has different ball probes that can be interchanged.

And yea still have not resolved the issue.

Exactly, they are expensive. We have a lot of customers with solid thread rings for really random sizes.


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Guessing you must be using a different ball size then what is in the book-

I have a newer Universal- and maybe the ball sizes have changed- I have 6 sets.

A: 0.011811
B: 0.019685
C: 0.031250
D: 0.046880
E: 0.06250
F: 0.093750

None of these are suited to an 3.5 Acme, so Pratt & Whitney must make some larger ones that are not in the book?

Anyhoo- I am not comfortable with the fact that Pratt & Whitney's formula is a straight basic formula. Acme wire constants are not a simple constant calculation like 60? threads- you need to know that size of the thread in order to calculate the constant- so I imagine that it would be the same concept in this case.

Using a wire diameter formula strikes me as incorrect as well because the geometry is different, and then the whole 2 ball, 3 wire thing makes me wonder as well.

You mentioned it before- is it all internal threads you are having a problem with- or just acmes?

Calibration Kid

Yea, it is not a Pratt & Whitney probe. The previous owner had a custom probe made. We have a whole set of different sized ball probes that you can screw into the probe body.

And yes I have had the same thoughts. I just don't know where to find the correct answer. I have searched and searched.

The only internal threads I have had to check so far are ACME unfortunately, so I don't know if all internal threads are giving me problems.
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