huangyhg

mmc on datums for runout
i am in need of a clarification to a runout callout.
datum -a- is a flat index surface. (the mating part threads up against this surface.)
datum -b- is a stepped shoulder diameter slightly larger than the external thread major diameter.
i have a runout on a conical surface that is
|ro|.002|a|b|
what i'm wondering is if i can use mmc for datum -b- in the runout feature. by doing so, i can design my gage to only the max diameter of the shoulder (datum -b- ) and not require a collet in order to find the diameter rfs. my theory in doing so is that i end up with more of a functional gage because the mating part indexes against -a- anyway. tolerance stack-up between the shoulder diameter (-b- ) and the mating part may result in a loose fit, so only the pitch diameter of the threads actually contacts. and thus, the runout of my assembly is really going to be based on the flatness of datum -a- and the mating surface so gaging to something similar is a reasonable functional check.
--scott
circular and total runout reference the datum diameter in rfs according to asme y14.5m-94. one cannot use mmc here.
sorry but no attibute gauge here.
if the runout was converted to positional in a mmc mode, then one could reference the datum diameter in mmc and now it would conducive to an attribute gauge.
dave d.
if you are trying to control the conical surface, then circular runout at rfs is in order. as dave said, you can't do what you're trying to do with a runout specification. if you simply must use an mmc modifier on datum b then you should specify profile of a surface on the conical surface and then reference datums a and b.
i don't think position is the answer here but maybe it is. can you shed some light on your thought process dave?
powerhound, gdtp t-0419
production manager
inventor 2009
mastercam x3
smartcam 11.1
ssg, u.s. army
taji, iraq oif ii
we have actually considered chaging it to a positional tolerance, because then it would control circularity as well as straightness. (don't read too much into those in terms of the definition of them via y14.5, i can't think of another word that adequately describes the concept.) but the problem with changing the tolerance at this point is that it is a completely different philosophy of inspection for these parts, and the change to the tolerance stack-up for assembly will also have to be ammended, flowing into all the tooling and gaging for those components as well.
profile on a surface may work very well. i'll have to ponder that a bit and consider how to inspect it. we usually use cmm or optical comparitor for profile inspection. both of those aren't possible. we need hard gaging.
thanks for the clarification on the runout callout. it is the same as i interpreted. just hoping i was wrong.
--scott
powerhound:
i am rethinking my suggestion to go to a positional tolerance.
in a conical situation, we have a height dimension to consider and this is never a factor in a cylindrical pin or hole. if the cone was in relatively true position but the height was too high, possibly the checking fixture would not descend all the way down. it may appear that the cone is out of position and, possibly, it may not be the case.
positional is not a good replacement here. unfortunately, i would stick with circular runout and then measure.
dave d.
use profile of a surface where the limits are the equivalent inner boundary and outer boundary from your original combination of size & runout. inspection is done exactly the same as if it was a runout control, but now you zero your indicator at the basic radius, and watch that your indictor doesn't exceed +/- half of the profile tolerance. one setup and you're done for size, position and runout ... not bad, eh?
jim sykes, p.eng, gdtp-s
that is fantastic. you deserve a start for that. now if only management realized how much money you just saved them!
--scott
tks. do i get 15% commission too?
jim sykes, p.eng, gdtp-s
jim,
i'm confused. the considered feature is conical, not cylindrical - did you not notice that?
evan janeshewski
axymetrix quality engineering inc.
tks evan, i had missed it. it's essentially the same answer though. the only geometric control that locates a surface is profile of a surface. by establishing the datum axis and rotating the workpiece about the datum axis, you still have an absolute location where any point on the surface is supposed to be, and a tolerance zone normal to the conical surface in which the surface must be. vision-type systems are great for verifying cones.
while it is legal to use +/- tols to size a cone, i'd like to see how anyone measures accurately at opposing points. without referencing the datum, there is no way to ensure that the opposed points are directly opposed (i.e. a circular cross section instead of an elliptical one). again, a surface profile provides the size & runout controls in one setup. you zero the indicator (offset from the datum axis at the basic radius at one end) and now instead of moving the indicator longitudinaly along the axis, you move the indicator longitudinaly at half the basic included angle of the cone.
it's been a rainy, chilly day here, so i decided to make some pretty graphics too. note: marvin is from solidworks 3dcontentcentral.
jim,
are these graphics intended to relate to the original post by swertel?
if they are i cannot quite make the association. it would seem that the conical surface should be positioned vertically and rotated about the base rather than the axis.
it is not readilly apparent as to what is the primary and secondary datum feature in the setup. could it be somehow clarified?

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