internal thread positions
internal thread positions
(OP)
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?
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





RE: internal thread positions
RE: internal thread positions
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.
RE: internal thread positions
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!
RE: internal thread positions
Regards,
Cory
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