How to Calibrate or Verify Screw Pitch Gages

ScottBP

Involved In Discussions
#1
Ok, this is one of the simple things has probably been beat to death before (like verifying tape measures), but a search of the forums didn't turn up anything on how to calibrate or verify screw pitch gages. Any ideas? My first thought is to slap it on an optical comparator and compare with an overlay- but where do I get do thread pitch overlays (or do they even exist? Off topic somewhat- How do I create overlays?) If not with an optical comparator, how else?
 
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Jim Wynne

Staff member
Admin
#3
Re: screw pitch gage

Why are you trying to calibrate them? The pitch won't change......
I think you're mostly right; even if there's wear, the pitch isn't going to move. Also, I've never been sold on the utility of pitch gages, unless you have a bunch of mixed up screws/bolts and need to sort them quickly, in which case calibration isn't an issue. You can verify the correct pitch using a thread ring gage, and it can be calibrated.
 
B

Brad Eickhoff

#4
Re: screw pitch gage

Scott I am not sure what you are asking to calibrate.

Are you asking about calibrating the pitch on a thread micrometer such as a 0 to 1", or are you asking about calibrating the pitch on a plug gage such as a 10-32 2B go and no go gage.

We send our thread micrometer out to be calibrated. Then we use these gages to calibrate our plug gages. To get the specs for the plug gage pitch I use the Machinery's Handbook.

Hope this helps :D .
 

ScottBP

Involved In Discussions
#5
Re: screw pitch gage

It's a folding screw thread pitch gage set made of thin stainless steel leaf stock. I know the pitch ain't gonna change, just like the length of a hardware store grade tape measure isn't going to change, but when one of our customers sends it in to us to be calibrated, we could either reject it and say "you're kidding, right?" or we could put forth a best effort to verify it somehow. If you're a 3rd party cal lab that makes money off of calibrations, what would you do?
 
#6
Re: screw pitch gage

Tell them it doesn't need calibration, Scott. If you do anything else, and one day they find out it doesn't - like reading this post, you're gonna be finished as a 'trusted supplier'. For the actual cost of doing any kind of verification/calibration, I wouldn't risk taking their money. It's borderline unethical, IMHO.

Better to be proactive and help them to manage their calibration. There might be things which need doing more frequently..............;)

Andy
 

Jim Wynne

Staff member
Admin
#7
Re: screw pitch gage

Tell them it doesn't need calibration, Scott. If you do anything else, and one day they find out it doesn't - like reading this post, you're gonna be finished as a 'trusted supplier'. For the actual cost of doing any kind of verification/calibration, I wouldn't risk taking their money. It's borderline unethical, IMHO.

Better to be proactive and help them to manage their calibration. There might be things which need doing more frequently..............;)

Andy
Again, I mostly agree, but perhaps the best strategy would be to give the customer options--tell how much it will cost to do the calibration (which is just verifying that the gage(s) are what they claim to be) and ask if the customer might want to reconsider. It's tough to say that it's not necessary if the customer has a customer who's demanding it. There's nothing that I know of in the Universal Customer Behavior Manual that requires customers to make sense.
 

Govind

Super Moderator
Staff member
Super Moderator
#8
Re: screw pitch gage

Ok, this is one of the simple things has probably been beat to death before (like verifying tape measures), but a search of the forums didn't turn up anything on how to calibrate or verify screw pitch gages. Any ideas? My first thought is to slap it on an optical comparator and compare with an overlay- but where do I get do thread pitch overlays (or do they even exist? Off topic somewhat- How do I create overlays?) If not with an optical comparator, how else?
Scott,
From the technical point of view:
Screw pitch gauge is not suppose to wear like ring gauge. Pitch gauge is for matching and comparing the thread profile and pitch. However, screw pitch gauge can sure wear off over a long period of use. It is difficult to assume that the wear pattern will be on the same direction of the flank. Hence wear can happen on opposite sides of the flank depend on the user mixture (Left hand Vs right users). This can change the pitch.

There are Projector overlays available from companies like Mitutoyo for Metric and non metric threads. You can use them to compare. I don’t think they have limit lines. Another suggestion is that you can create your own overlay using Auto cad/ Solid edge software and use thin pen to plot on non shrinking film using high magnification (greater than or equal to 50 X) so that you can at least see 4 to 5 pitch in one screen. You can create limit lines based on the standard. This way you can make a decision as to whether scrap a leaf or continue with usage.

From the economics point of view:
You may also want to evaluate the cost of purchasing a new pitch gauge set with Manufacturer certificate Vs going through all these calibration (verification) labour.

Regards,
Govind.
 

Jim Wynne

Staff member
Admin
#9
Re: screw pitch gage

From the economics point of view:
You may also want to evaluate the cost of purchasing a new pitch gauge set with Manufacturer certificate Vs going through all these calibration (verification) labour.
While just buying a new set might be a good idea, it doesn't help if a customer is expecting traceable calibration. The idea that a pitch gage could wear to the point of not being able to verify thread pitch is quite a stretch; the thing would likely disintegrate completely before wearing out to that extent. There is very little contact in use of the sort that would cause such changes. Nonetheless, if wear is an issue and traceable calibration isn't, then buying a new set is the way to go.
 

ScottBP

Involved In Discussions
#10
Re: screw pitch gage

The only way I can think of a screw pitch gage changing would be wear from a lot of screw sorting, unless someone decides to use it as a "comb" to brush away grime on the threads... :mg:

Anyway, I came across a procedure from Starrett (for their cat. no. 6, 155, 472, 473, 474 and 484 gage sets). The accuracy is ±.002" between any two teeth, and 60 ± 1 degrees for 20 pitch and coarser, and ± 2 degrees for finer than 20 pitch. Their procedure is as follows:

1. Appearance- visially inspect the gage for finish and proper markings.
2. Inspect for correct number of leaves.
3. Inspect for operation of the locks.
4. Inspect for accuracy.

It's #4 that I was curious how other people did it. I think the quickest and easiest way is to compare it against a known good gage (read: one that hasn't been abused) with the optical comparator, maybe stack them, mesh the teeth together, or make tracing paper overlays with a pencil and straight edge?
 
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