Zeiss verses Quindos MMC calculation

[[Ja...]]

I'm trying to correlate results with the customer, our results are relatively close but the MMC calculation is not. The part is a sphere in the center of a shaft, we are checking the sphere position back to the journal diameters on both sides of the sphere and the end of the shaft. The sphere is .00083 from high limit of size with a true position of .001 with MMC. I along with the Zeiss software calculate that the positional allowance as .00183 (.001+.00083). The Quindos software is calculating the position allowance as .00111, the customer states that the software is taking form into account (form per there report is .00081)? Has anyone run into this before, if so could you explain the how calculation works?

Thanks!

[[Ke...]]

Well that sounds... weird.
Can the customer explain how their software "takes form into account"
What is the form of the sphere, and what evaluation method are you using (LSQ or Outer Tangential)?

[[Ja...]]

I agree that it's weird, I asked for an explanation and was told it was an internal software calculation and they did not understand what it was doing.
I'm evaluating the sphere LSQ

[[Ro...]]

On an OD sphere, Maximum condition is Nominal Plus high tolerance, as you get smaller you get bonus.
If it's an ID sphere, Maximum condition is Nominal Minus low tolerance, as you get larger you get bonus.

Its really simple once you wrap your head around the term maximum material condition literally means the point at which the part physically has the most mass, which is small holes and large outer diameter. as you move away from the maximum condition you get bonus.

[[Ja...]]

Roberto, I understand and agree with your calculation method. Now the fun of explaining to the customer they are not calculating the MMC correctly which is causing our discrepancies.

[[Ke...]]

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I would be curious to know what the allowable tolerance is, if you evaluate the sphere using an Outer Tangential evaluation.
The difference in MMC bonus could be explained by:how much data is being used to evaluate the sphere (e.g.:6 points vs 5000 points), and what evaluaton method is being used (e.g.: LSQ vs Outer Tangential - which I would think is the most likely suspect of how the software is "taking form into account").
I would try to narrow the gap between these differences, before telling the customer that the software is calculating the bonus tolerance incorrectly.

[[Ja...]]

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I know the customer is taking single points, we are scanning, I'm working on getting more information. Seems like changing the eval method between the size and location wouldn't give them a good understanding of fit & function? MMC on an external feature should be "Tolerance + (high limit - actual)" the customer method does not calculate this way.

[[Ia...]]

If you're working with ASME, show them this. " the customer states that the software is taking form into account" form should only be taken into account for the diameter, nothing to do with positional tolerance.

[[Ro...]]

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Don't get into a technology battle with the people sending you money.
If your customer is taking a 12 or 15 point sphere then you should also be taking a 12 or 15 point sphere.
Don't extend a fight because you believe that scanning is right or better, just take your 15 points and move on with your day.

[[Ke...]]

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Agreed: If the customer is using single points, and they want single points to be used: "This is the way."

I just have a hard time believing that Quindos is using some weird formula that takes form into account, when determining the bonus tolerance (which seems to be the area of concern); it sounds like something a sales rep say, to "dumb it down" for someone who doesn't measure parts every day

[[Ri...]]

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Form is not being calculated to report the position, it is being used to calculate the attribute of additional positional tolerance, the Modifier.
LSQ and Inner\Outer tangential (as an example) pre-assignment for evaluations will have different form values depending on the actual form of the feature and the chosen evaluation.

[[Er...]]

Are the diameter sizes between calculations the same? If they are, it sounds like they could be using LSQ evaluation for reporting the diameter size and possibly using the size from the outer tangential evaluation when calculating the bonus tolerance. I personally haven't heard anyone doing this before now.

As Keith mentioned, I would be curious what the diameter size/bonus would be if you evaluated to an outer tangential evaluation, and if the result from this evaluation is closer to their result.

[[Ia...]]

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The actual diameter and the diameter tolerance are the only elements used to calculate additional tolerance. I understand that form plays a role in the diameter calculation. But if OP and the customer are getting the same diameter reading, there should be no ambiguity on bonus tolerance.

[[Ja...]]

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I just received confirmation that they are using LSQ for size and OT for for position. That makes it tough to tell an operator where there part is with doing it this way!

[[Ke...]]

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Wonder what the justification is for using different evaluation methods... Hopefully one of the GD&T gurus will chime in and let us know if/how this violates the rules.