MMC on TP

[[Lo...]]

We are evaluating a bracket for a customer. I measured the existing parts they are getting and the parts are out of spec. We don't do a lot of brackets so this is a little different for us. Anyways the customer asked if we change the TP's to what listed on the photo. The problem is Calypso won't give me options to select MMC on certain diameters and datums. It also won't let you select MMC if you are evaluating in just Y or X direction. It wants the TP to be diametrical.

Is this a GD&T thing, or a Calypso thing? The customer is using different software and a arm with a scanner to measure. I cannot get data using TP marked on the photo. Customer is wanting a explanation on why we can evaluate as marked. I know the datums A,B,C are are co planar and the other holes are evaluated in different axis. Anyone have any thoughts?
[img][/img]

TP mmc Holes.jpg

[[Ja...]]

The circle-M modifier on your datum features represents a datum feature referenced at MMB (not quite MMC) and Calypso is infamous for not being able to compute datum feature boundaries at MMB or LMB unless the feature and datum lie in a common workplane (or something along those lines). That is why your option is greyed out. The only workaround would involve advanced 3D geometry, perhaps some linear algebra, and most likely Zeiss' PCM option.

PC-DMIS is the only software that I am aware of that can compute these material boundaries in order to calculate datum shift/displacement.

[[Lo...]]

Thanks Jacob. I'm aware of MMB and LMB, I'm just old school and always resort to mmc / Lmc. Our engineers don't understand mobility vs bonus, but I understand it. Maybe not to the degree of some. I'm not as good as I want to be with GD&T so I thought I would get some advise on here. Another one of my issues is when the CMM wont do or cannot do, then I suddenly become a software engineer and have to try to explain why. Not something I like doing. I'm sure a lot of guys on here are in the same boat.

[[Ha...]]

This is how you create MMC on a slot https://www.ellisontechnologies.com/sit ... th_mmc.pdf

[[Da...]]

Lonnie,

not sure if I could be any of help in this case, but can you (or anyone else here) clear my mind:
Position can control location, orientation and if ISO, form also. Perpendicularity can control orientation and if ISO, form also.
So, what is there to locate? (even if there are all possible basic dimensions)

[[Lo...]]

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The print does have all the basics listed. This is very thin material and is a stamped part. There is not enough real estate to multiple levels of any of the pierced features. Stamping are typically only measured the cut portion.

[[Lo...]]

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Thanks Hai. I have this. The problem is this true position is in one direction for the width, and one direction for the length, its not diametrical. Calypso wont let you choose MMC on the datums is evaluating in one direction.

[[Ma...]]

Non-diametrical tolerance zone, only using the one basic associated with the slot width. Only select the one axis when creating characteristic.

To get MMC, you need to create a theoretical circle lying between the two planes of the slot width. The theoretical circle will be used for your TP characteristic, and you can apply MMC.

It is a complicated process, but once you get it down, it's not hard. I believe someone posted the instructions above.

Good luck.

[[Da...]]

Rather than doing something wrong, you could try to evaluate the positions with the datums at RFS.
The true power of datums at MMB is that it will give you the ability to check a part with a hard gauge. The datum shift mathematics are quite complicated and I could imagine that Zeiss will only release that ability for Calypso when they are convinced that it works. There’s a reason that Wilcox threw the XactMeasure function out of PC-DMIS and replaced it with a completely new function.

[[Ha...]]

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You can use Symmetry point to get the MMC of each length and width

[[Lo...]]

MMC on the feature is not an issue. Its mmb on the datums. Daniel hit the nail on the head. We are proposing using a hard gage to check all of these positions. The engineer wants to verify they are good to the print he wants to propose. We have numerous new employees from a place that had PCDMIS and they are doubting ZEISS and CALYPSO every day. Now this issue with one of their proto parts is not going to be fun. I showed them that Calypso won't let you choose MMB on the datums. Now I guess a have to petition the German Institue of Engineering. I was told that if Calypso doesn't let you do something, normally there is reasoning behind it. I need those reasons.

Our engineer confirmed with one of his guys that PCDMIS can handle these callouts. Anyone hiring LOL?

[[Ri...]]

I'm part of the camp that doesn't believe that MMB should be used when inspecting parts on a CMM (unless under specific conditions).

Depending on your allowable datum shift, you could get a result of 0. That is great for assembly/acceptance, but for manufacturing it doesn't tell you anything. The same argument can be said for MMC on the feature, but at least that doesn't change the result of the evaluation.

There are always work arounds that you could do though if you really need to do it.

[[Me...]]

Another option would be to not worry about what Calypso or PC-DMIS can or cannot do, and use a specialized software for both inspection & manufacturing.

https://kotem.com/evolve-suite/evolve-smartprofile/

https://kotem.com/evolve-suite/evolve-manufacturing/

[[Ri...]]

Yeah. I've used that software before. It's also imbedded into their vision/cmm software now. It "works" in the sense that it says it is doing what it is doing and gives a result. The problem with evaluating MMB/LMB is that your only countercheck is a functional gage, so you can only catch the software wrong if it doesn't assemble and the software shows it in specification, or if it does assemble and the software shows it out of specification.

Like I said, majority of the time MMB/LMB should not be used for measuring softwares.

[[Me...]]

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With all due respect, but you just described all measuring software in that they say their doing something and output a result. The key to remember with any software being used in this manner is that you’re only approximating reality. Even if the math within the software is solid, one could still have issues with component fit-up simply because the features were not “sampled” adequately.

I see no issue in choosing not to apply DRF modifiers, but that decision should not have to be based on a software’s inability to compute it with the supplied data. Options are available, and will likely become more abundant at time goes on given the digital age we’re living in.

[[Ri...]]

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I think you missed the point. If you report a standard true position, you can countercheck the math and the actual results. Good luck doing that with MMB/LMB.

And you last statement hits the nail on the head, if you are trying to evaluate a fit/function characteristic, why wouldn't you use a gage?

[[Me...]]

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It can be done, after all it’s just math right… But why spend all of that time crunching numbers when you can let software do it for you. It’s sort of like why most folks will reach for a calculator instead of pen and paper. Can there be issues with software doing it incorrectly, absolutely! Think about all of the times that there have been issues with Calypso through the years. But that didn’t stop most people from throwing in the towel and replacing all of their CMM’s with hard gages. Why, because hard gages are not always the most practical or cost effective solution.