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MMC position limitations


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We are running into this problem every week and I'm curious if there is a better way and if the software can be updated in the future to handle it.

We see a lot of irregular features that have a linear size tolerance.(ie. 3.5 +- 0.5). Then we will get a positional check of 0.05 M A|B|C.  The problem is with an irregular feature I cannot get a linear size check in GOM in order to develop the MMC condition.


The below sketch is a sample of something we see a lot. We will get a size tolerance from the outside of both circular cutouts of 3.5 +- 0.5. And there will be a position check on that feature of 0.05 M ABC so we should get up to 0.1mm of bonus. It gets even worse if the callout has a zero positional tolerance so all of the tolerance comes from the MMC deviation.


While this is an odd distance callout and positional check, it is legal under ASME. Slots can be dimensioned similar to this in the standard. I don't like the callout, but it is what it is.

There seems to be no way to get a linear size from this distance check so I cannot used MMC. I've tried creating a circle from the mid point and using the radial points to contruct a circle, and that lets me get into the linear size check area, but it won't let me develop any linear sizes.


Using user defined is not an easy solution b/c I need a variable(bonus tolerance) placed into the tolerance field, and you cannot do that either. So here is what I have to do. In user defined the actual is the simple positional measurement. The nominal is the positional tolerance + the bonus tolerance(feature deviation from MMC).  Then I make my tolerance 1mm-0. Therefore anything positive passes and anything negative fails. But the machinists and others can't really understand this b/c it looks different than all the other MMC checks.

Can this be an update in future versions?




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you are correct evaluation of abitrary surface under material requirement is possible under ASME standard. But you will have not such thing as bonus tolerances. Please have a look into the standard. There you can see that is only a boundary interpretation.

Unfortunetely evaluation is today only possible for slotted and rectangular holes (Note: to achieve this you have to create a width and a length check on your hole, afterwards you can create the position tolerance. The width and length check can be found in the check (not in the GD&T section) of the I-Inspect widget) but not for arbitary sections as in your example.

Of course you could split you callout in 3 independent one; due to the fact that you have a full constraint datum system the maximum tolerance value of the single callout would be the same as for the complete one. Note: Of course, this is only true for a datum system with no remaining degrees of freedom where no material requirement is set. Maybe you can use this as workaround. 

Nevertheless your request to support arbitary forms is already on our list, but unfortunetely the list is quite long and this feature is very requested very rare, therefore do not expect a fast solution.


Sorry that I could not give you a better solution.

Christoph Schult

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Thanks for the reply.


The standard doesn't forbid using position with bonus in a method that differs from the boundary concept. That concept is for hard gauging and using the MMC virtual condition for the gauge. For, instance you cannot use the boundary method with LMC b/c you cannot hard gauge an LMC virtual condition. Therefore, if I wanted to control the position of a slotted hole, protecting a thin wall condition using LMC, you are forced to use the typical axis method vs the surface method. I think this is a grey area b/c what are you controlling, a central point/axis/plane?  It isn't very clear.  Profile would be my go to, but I don't design the parts.


A real helpful idea would be to allow for the use of variables in the tolerance fields in the user defined checks?  That would allow me to set up my deviation from MMC/LMC of a slotted hole and use that variable in the tolerance field. Right now the tolerance fields must be hard coded.




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You have nailed the problem, it is uncomplete unclear what you want to control (in sense of point, plane or line). 

Therefore you (or your designer) have to options (at least in my point of view):

  • Split you hole into feature of size, with a clear element to control OR
  • Specify inspection that cannot evaluate without an optical scanner or ct (in your case the LMC with boundary inspection). Having a detailed 3d-data of your mesh you can of course check in any point is violate the LMC of your feature. Of course you cannot construct a real gauge to measure it but it is no problem for a digital measurement.
    I am quite sure that is not describe in the ASME standard due to the fact that it you cannot measured with real gauges, but maybe it would be a solution for your problem.
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There are grey areas in all standards. Another example of something challenging to hard gauge, but is completely acceptable, is measuring a hole pattern at RFS.

A slot position at RFS is real easy regardless if the engineer didn't think it through correctly. And the standard does not address a slot position at RFS, therefore it does not mean it is violating the standard. 

A slot position at RFS for a datum structure only measuring in the length direction could be setup for a single point which would be the division point of the two opposing tangency points that both pass thru a line created from both opposing axis(I'd use the mid point of both axis for a quick setup. Or if you wanted to be more technical you could create more points or perhaps a center axis. Either way, you are satisfying the needs of an engineering team.

In software such as pcdmis, you can manually create a circle for instance that touches both extreme points of the length. This gives you the ability to have a centerpoint which can be positioned based on some datums for a circular tolerance zone.

I love the GOM software and its power, but it would be really nice to be able to meet grey area demands or demands of a completely incorrect callout. Here I just need to be able to create a circle and let that circle dimension flow into the MMR section of the position check. Right now, you must use the linear check area or something similar to get that dimension to flow into the MMR functionality.

If that isn't an option, I need to be able to apply my own generated bonus into the 'tolerance' field of the user defined section. I am currently simply doing this:

(stated geometric tolerance + bonus tolerance) - positional measurement. Then I set the tolerance to 0.5mm - 0.0mm so anything positive the part is good. But not everyone understands this output as it is difference than what we've been doing. Plus, we have statistical software that really doesn't like output different than the prints.

Hope that makes sense. Keep up the good work.

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