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Verification of positions of threaded holes

Verification of positions of threaded holes

Verification of positions of threaded holes

For any metrologists in this forum, maybe you can teach a design guy a thing or two about controlling the positions of threaded holes. In ASME Y14.5-2009 (as in ASME Y14.5M-1994) if you call out a position of a threaded hole then, unless otherwise specified, the tolerance applies to the thread's pitch diameter. How is this done? Is there some kind of gage that you screw tight into the hole and has a conical recess in the top to probe with your CMM?

I know from a design standpoint the standard is right to apply the tolerance to the pitch diameter since that best represents the threaded hole's function in the part.

Next question: What if you apply the positional tolerance of the threaded hole at, say, MMC thus enabling a bonus tolerance that varies with pitch diameter size.


RE: Verification of positions of threaded holes

First question: I think you basically connect a thread gage to a gage ball. You may be able to buy the combination as a unit.

Second question: The PD tolerance is so tight that there's little to gain by trying to use the bonus tolerance for position.

Mike Halloran
Pembroke Pines, FL, USA

RE: Verification of positions of threaded holes

There's a maker for this: https://www.jo-plug.com/ , though their advertising copywriter needs to be checked. They are supposed to be "in accordance to ANSI/ASME Y14.5 M – 2015"

RE: Verification of positions of threaded holes

I have rare occasions to use the tiny UNM series of threads. These are commonly used in instrument designs. Are gages and matching gage balls available for these threads in your experience?


RE: Verification of positions of threaded holes

If your position tolerance is not totally insane, you might estimate the hole position by inserting a gage pin into the minor diameter, and measuring its location.

Mike Halloran
Pembroke Pines, FL, USA

RE: Verification of positions of threaded holes

On our CMM, we use shoulder bolts with the head cut off, or, a regular bolt with the head cut off.
Tighten bolt until it doesn't move, but not so tight to cause damage.
This is for checking location only.
If it is a large threaded hole, the CMM probe can go into the hole, but, must make sure that points are on minor diameter.
Cylinder command works well, but have to take points by hand drive.
The thread has to be checked with a thread gage.

Harold G. Morgan
CATIA, QA, CNC & CMM Programmer

RE: Verification of positions of threaded holes

But the bolt was not made to gage tolerances. How do you know that isn't skewing your numbers?


RE: Verification of positions of threaded holes


Here is a short technical paper covering the subject. I doubt you will find an off-the-shelf tapped hole position plug gauge for UNM series threads. But there are specialist companies like this one that will make a custom gauge for a reasonable price. They will also supply a calibration certificate documenting the actual tolerance of the gauge.

I can't think of too many situations where a tight positional tolerance of a tapped hole is required. Tapped holes are a poor approach for providing precision alignment/location between mating parts. There are much better methods such as keys, pins, shoulders, etc., if precision alignment is required.

RE: Verification of positions of threaded holes

We use "Tru-Pos" inserts.

In a pinch, you can thread in a shoulder bolt cut down to leave just a bit of the shoulder. This however tends to leave a little bit of error (.001")

We have had the need for carefully located threaded holes when assembling shoulder bolts. It's a last resort - from a design point of view it's always better to have two close-clearance holes on the shoulder bolt's shoulder section and a nut on the threads of the shoulder bolt.


RE: Verification of positions of threaded holes


Actually in the electronics business there are WAY TOO MANY cases where tight feature-relating positional tolerances have to put on threaded holes because the mating components have ultra-conservative, CYA positional tolerances and the mounting holes are not big enough to make up for it.

The result is that the threaded hole positional tolerances as calculated with the fixed fastener formula are very small. MOST mounted components in the electronic business are designed and manufactured with NO MEs on staff to apply reasonable tolerances or anyone to monitor the processes that affect feature sizes and positions.

Many time when I've used the fixed fastener formula with the intended screw size it shows that the mating threaded holes CANNOT BE USED because the required positional tolerance is <= zero! The only safe way to handle it was always use a one-size smaller screw and reduce the threaded hole sizes accordingly.


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