Position of Bore Pattern with MMB

[[Mi...]]

This is a new situation for me.

Here is a crude drawing, -A- is the face that's perpendicular to the hole pattern. And I know my drawing is missing it but it's a 4x hole pattern.

How do you setup the Characteristic on something with MMB and and tertiary datum? Without -C- I would set it up using View Tolerance, MMB set on -B- and allow both rotation and translation.

Having -C- has me second guessing turning on or off the rotation, does anybody know more? We don't often deal with MMB.

[[Je...]]

I misread the print during the first review and somehow saw the pattern itself as Dat -B-.

So -C- is going to restrict rotation. Use a standard position structuring and select MMB for your secondary datum. If the "MMB" option is greyed out after utilizing -C- then I advise the use of the CY23 GDT BETA. Straight-forward application using CY23

[[Mi...]]

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Unfortunately this is on Version 2018 with no chance of an upgrade in the next week so I need to work with what we have. I very much look forward to the improved position characteristics that can handle uncommon situations that we DO run into often.

Having MMB on the Datums is no problem when BFBP is setup as per the Cookbook using View Tolerance.

My question is: With having -C- datum, do I allow Rotation or not? the math behind View Tolerance isn't clear.

IF -C- also had MMB I would allow both Rotation and Translation I'm almost positive... but without it I don't know enough about what View Tolerance is doing to be sure.

MMB Position 2.PNG

[[He...]]

You need to have rotation turned on. The bestfit need rotation on in order to rotate around C within the limit of B. Otherwise the bestfit will only be allowed to translate in a straight line between B ans C.

[[Mi...]]

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This is what I was leaning towards, without rotation there was almost no improvement in the position result, and -B- on the real part has a huge amount of size tolerance.

With Rotation on everything seemed to move more like I expected... but without knowing what's happening in the software it's hard to know it's correct.

[[Ri...]]

No. If you have Rotation turned on - you are telling the pattern to allow rotation away from the DRF. This is not what you want.

Here is an example showing this for you.




You have a fully constrained DRF. Just turn on the View Tolerance Best Fit Method and allow for MMB on Datum B

[[Mi...]]

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The problem is, when I repeat your test if I have the fit method set to Minimum and Rotation on, it does what you showed. Matching a regular position with no tertiary datum.

The last position result does not change regardless of fit method since there is only 1 hole.

However changing the fit to View Tolerance with Rotation gives a new result - worse than without rotation which I'm not understanding either.

[[Ri...]]

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Michael,

Would you be willing to send this measurement plan to me in a PM. I'd love to show it to you in 23.

[[Mi...]]

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Unfortunately it's for a prototype EV part that I would probably be shot for sharing. And actually, to do it as per the real print it would be even more complicated as the tertiary datum is another pattern of holes, not a single hole, so that would add even another layer to this. That part of the callout is being ignored and we have picked 1 hole to use for simplicity, and well if the engineer and customer understand and sign off on the method, who am I to argue?

Seeing it in '23 won't help me too much as there is 0 chance of getting it, before I have to complete this work. I have to have an acceptable solution in 2018. For us its a 100k upgrade as we have so many CMMs and its more important for them all to be the same version rather than the latest version. However the much improved GD&T Position Characteristic that is really valid for our parts is one of the biggest points I have used in my presentation to corporate begging for the money to upgrade....

[[Ja...]]

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Is it actually rotating, or is it the difference in the best-fit methods? If you look in best fit of bore pattern window is there an amount in the the rotation box at the bottom?

[[Mi...]]

For anybody finding this later, I understand this now.

View Tolerance, Both translations, No Rotation is correct, unless MMB was also applied to the Tertiary Datum. Without that the axis must go through the center of the Tertiary datum as it translates... which means the axes DO rotate as the center point moves.

Turning on Rotation regardless of fit method would unlock the axis from the center of the Tertiary Datum. That's where I was failing.

It also means that in this particular case, MMB on the Secondary Datum is far less helpful than I originally thought as most of the Positional error is due to the pattern being clocked, evenly one angular direction.