huangyhg

postion tolerance on a theaded hole?
has any one ever heard of a postion tolerance on the minor diameter of a tapped hole? if so how do you inspect that?
to me, it just doesnt make sense because there is very little surface contact to inspect. does any one have any ideas why someone would do this? thank you
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yes. add the fcf with position tol and add "minor" to it.
there are gauges to check threads.
chris is right. it is specified in para 2.9 of asme y14.5-1994.
humanbone,
how would you inspect positional tolerances on anything other than the minor diameter of the thread?
jhg
major or pitch diameters (need correct gauges).
humanbone-
if you look on page 1360 of your handy-dandy msc catalog, at the top of the page you will find threaded hole location gages in english and metric threads. they are threaded plugs with two gaging surfaces. the post is a straight plug ground concentric to the p.d. of the thread and the flat is ground perpendicular to the center axis of the p.d.
the only problem with you using these gages is that you are assuming that the minor diameter is concentric to the p.d. if it is tapped, it most likely is. if it is thread milled you can't make that assumption.
-john
minor thread diameters are the easiest of the three to check using a gage pin pushed down the hole. no special gages would then need to be ordered.
all machine shops should carry several sets of pins.
minor diameter is also common when using a cmm to inspect features. thread gauges will give a good estimate of the center axis of the tap, but are subject to the sizing error of the tap, plus a good thread gauge (plug) is very expensive. picture that multiplied by a couple hundred or more of the same sized holes on a plate ... $$$. stating minor dia with the fcf indicates the inspection is to be done using the minor diameter, and this allows either the pin-method for manual inspection as suggested above, or the use of a cylindrical probe on a cmm. the drop-in gage-pin method does not provide any control over the orientation, and most people would only check its location at the surface instead of projecting the tolerance above the surface to see the entire axis. as a result, it could be significantly out of perpendicular to the surface and still be accepted. the cmm method has the advantage of having the cylindrical probe contact the actual inner boundary of the minor diameter, and includes the perpendicularity wrt the cmm bed. while there are errors inherent in this method also, they are comparatively minor and you have significant cost savings over carrying multiples of a large variety of thread gages, installing, removing and maintaining each of the thread gages.
the question of relative accuracy of each tolerancing method (major, pitch, or minor diameter) of a thread, and the different gage and inspection methods was being addressed by sandia national labs a few years ago. if i recall correctly, the preliminary and follow-up results weren't conclusive, and i think they were doing more tests. as i recall, there are some public-domain white papers published on the results.
overall, consider the cost vs risk when deciding which part of the thread to use as the basis of the positional tolerance. if you can live with a little more uncertainty, you can more easily and economically inspect at the thread minor diameter.
jim sykes, p.eng, gdtp-s
profile services
cad-documentation-gd&t-product development
on high volume projects, i would suggest that a pattern of threaded holes should be reflected with "minor dia" below the feature control frame and also at mmc.
one would make a checking fixture with locating pins made at the virtual condition size which would be the tolerance in the feature control frame beyond (smaller) the the theoretical minor diameter. this feature control frame could (should) reference primary, secondary and teriary datums.
this checking fixture would not only check the location of the holes but also orientation to the primary datum which probably would be perpendicularity.
fixtures like this are always used in the automtove industry by shop floor personnel. they are easy and quick!!
this is the only practical way to check or confirm a pattern of threaded holes in high volume work.
a checking fixture should be the most economical verification method in high volume projects, but more & more customers want hard data rather than a go/no-go, largely because they mistakenly think it is always better. the reality is, if they aren't doing statistical processing or tolerancing, then go/no-go is usually adequate. the few automotive facilities i've been in (stamping, assembly, and engine castings/machining) didn't use checking fixtures; they all used cmms. part of the reason is that if a feature is out of spec, it needs to go through a dimensional inspection anyway to determine what/where the defect is, and determine rework potential. also, more suppliers (in all industries) are relying on statistical processing and statistical inspection to qualify workpieces without 100% verification. the reliance upon the process needs continuous numeric inspection data as a monitoring tool.
i've always been involved in small/moderate run production, so i've never justified the cost of a gage fixture. in these situations, with large plates, cmm is the only practical alternative.
jim sykes, p.eng, gdtp-s
profile services
cad-documentation-gd&t-product development
hard data (cmm) is used on sample submission in stamping shops, etc but on an ongoing run, is not practical. i would never see in a control plan that one should take a part to cmm for positional confirmation.
a cmm confirms centres and not the shape of the hole. a checking fixture will confirm the centre and shape. one could have a cmm confirm a marginally acceptable hole but the checking fixture could reject it. the checking fixture (made correctly)supersedes the cmm.
if one was manufcturing an extremely small run, the cmm is best but one must confirm at the top and bottom of the hole and it is difficult if the hole size is relatively small especially contracting minor diameter threads.

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