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internal thread positions 1

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Tunalover

Mechanical
Mar 28, 2002
1,179
Folks-
ASME Y14.5M-1994 states that by default a positional tolerance of a threaded hole applies to the pitch diameter. We've been able to find a gage that will center itself in a 6-32 UNC-2B hole by contacting both the minor and major diameter (The Multi-Ball gage by TPC) but nothing smaller.

My company is in the optics business and the extent of optical alignment depends on positional tolerances; the tighter the positional tolerance of an optic mounting hole pattern the smaller the amount of alignment work required.

But how can we accurately inspect pitch diameter positional tolerances of internal threads smaller than 6-32? We use many 4-40 UNC-2B, 2-56 UNC-2B, 1-64 UNC-2B, 0-80 UNF-2B, 1.00 UNM, and even 0.50 UNM threaded holes for optic mounting hole patterns.

Currently our fabricators are using the minor diameter to find the thread position but every dimensional metrologist and D&T practitioner I've talked to advises AGAINST this practice.

Does anyone out there have experience and knowledge to share on this subject?


Tunalover
 
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Threads are composite features with thread form, pitch diameter, helix angle, straightness tolerances which affect the performance of meauring the location. So you have the thread perfectly located yet the PD of the hole is on high limit and the PD of the bolt is on low limit, slop in the joint. If you use a tap does the tap axis stay parallel with the drilled hole? How do you measure the angularity error of the thread? I do not know to what degree of accuracy you are trying to achieve but I think that anything less than .003-.004" dia true position is uninspectable and unnecessary. If you do try to measure anything that close I would like to see the R & R numbers proving the inspection process is in control.
 
In the past we have used a thread with a larger diameter and a shoulder on it, like a shoulder screw but without the head.

By simply finger tightening this will sit square to the surface, unless the tapping is well out of square and give an accurate body to measure to. This seems to give an accurate indication of how a screw would locate, it does not however in any way check the thread.
 
This is a tough one, to say the least. If you are simply verifying a vendors work, can you have some threaded pin gages made up that are a snug fit, and threaded in the hole locations? Are you not actually determining the location where the screw or other hardware will seat itself when assembled, and not necessarily the pitch diameter or other component as a stand-alone dimension?

Now, for my other comment. Have you found it necessary to inspect that precisely because of some incident with their work? If the work is being done on calibrated machinery with tooling that is changed frequently, your accuracy is going to be very good. To actually inspect a thread and break down locational data for the 3 diametral positions (maj,min, and pitch) is a real chore in small threads.

Good luck!
 
Perhaps this won't help you much, but in the automobile industry, threads are gaged according to major diameter on externally threaded parts and with the minor diameter on internally threaded parts. Thread tolerances usually are very small compared to what is needed for the mating components. I can see where optical systems may need better precision, so you may need to spend a lot of time, effort and money to get what you need.

Regards,

Cory

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