True Position on slot with MMC, and datum shift

[[be...]]

All, I have created 2 planes on the 10" length slot, and created a sym plane with the 2, to Datum -A- as a Cylinder. Eng is complaining that I am calculating MMC on the slot manually. What do I need to create to get MMC in Calypso. Also is there a better way to check this TP without using the whole length, I'm thinking this is avg and I won't see if this slot is bowed??? Any thoughts would be helpful. See attached 🙄 thanks

slot.docx

[[Me...]]

Is anyone else confused by this drawing? I'm by no means an expert, but A datum doesn't control positioning of the slot, right? It can be rotated anywhere. I'm assuming I'm wrong, would love clarification from someone who is smarter than me.

[[Ra...]]

Datum -A- controls the orientation of the axis of the feature to the axis of the datum: perpendicularity only - as I see it.

[[Me...]]

The position, I believe is trying to control parallelism, perpendicularity, and centrality to -A-.
The 30 degree is critical also.
Snap lines from 30 left and 30 right, bisect and see how well the slot is central to that.

[[Na...]]

I would think you could do a symmetry point. Then created a feature (Circle) select it to be a theoretical feature then use a formula and use the symmetry point data as X,Y,Z for the circle. and use distance as the Dia. This should allow you to use the circle in your true position call out as MMC and then just use your MMC from you Cylinder as MMC for -A-.

[[Me...]]

I shouldn't have said parallelism. A bit redundant. But the other 2 are critical. You will have a tuff time key it to the shaft if those one of those is off.

[[Ma...]]

In order to get the MMC you will need to add it by using a formula with the tolerance box in the Position characteristic.

Get the actual of the slot width characteristic and subtract it from 0.501 and then get actual of the diameter of -A- and subtract from 2.251 then add you b/p position tolerance the +0.008 and you will now have all the MMC and datum mobility allowed.

Mark

[[Ra...]]

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How does the 30 degree basic control the slot? Am I being dense?

I like Nathan's idea for getting a feature of size to use for MMC. Mark makes a great suggestion, also. However, will these strategies work over 10" of keyway?

[[Me...]]

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I don't believe it does either. I think that basic dimension is only for the hole-to-hole relationship in the bore pattern.

[[Me...]]

Sorry, you guys have way more GD-T than I do, but if I agreed with Kevin, doesn't the 30 degree involve assembly function relative to the keyslot?

[[An...]]

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[[Aa...]]

First, you should not have to use a formula to include "bonus" tolerance. Calypso has a "Symmetry Plane" geometry that is a feature of size. If you use that, and apply a tolerance to the size, MMC should be available.

At the same time, you are not allowed to add bonus tolerance for the difference between the size of a datum feature and the maximum material size of that datum feature. To borrow from the car insurance commercials, "That's not how this works..." Datum shift is not the same as additional tolerance.

In this case, datum A does not inherently control clocking, but all 12 slots are controlled together, so whatever clocking is used, all 12 positions must be satisfied using that same clocking. Moreover, whatever datum shift is used, all 12 positions must be satisfied using that same datum shift. When you use datum shift for bonus tolerance, you fail to ensure this. (The difference between datum shift and bonus is actually broader than that, but that's the more relevant part for the case at hand.)

Unfortunately, Calypso will not provide a means of referencing the datum feature at MMC/MMB for cases like this. You are forced to choose between slopping the datum shift in with the bonus tolerance and forgoing the benefit of datum mobility.

Clocking is problem all is own for situations like this. If you set up your characteristic with a DRF that doesn't include a datum that constrains a Degree of Freedom (the clocking in this case), Calypso will not optimize that DoF for the results. Instead, it will use the parent alignment of the controlled feature to constrain that DoF. You really need that DoF to be optimized. (The parent alignment might already do a pretty good job of this, but it very well might not.) In cases where there is only one controlled feature, I prefer to add the controlled feature as a final datum in the DRF to optimize. If there are more than one (like here), you have to clock them all together, so basically, you want to pick the feature that will best optimize it. How I go about that depends on the situation. In this case, I would try a theoretical feature in a best fit alignment for all the controlled features. At the end of the day, there is no perfect solution.

[[Br...]]

Becky,
To use MMC on a slot using planes you need to create the planes independently as you normally would. Then in your Features_Special Geometry menu select Symmetry Plane, open it up, then recall the feature points of both planes. This will allow you to set the nominal and tolerance of the slot. This will open up the MMC modifier in the true position characteristic. The same technique can be used for symmetry points. I can demonstrate Monday if this is the Becky I know. As far as datum shift in Calypso, that has always been a question of mine. Does Calypso actually calculate Datum shift and if so, does it take into account Simultaneous requirements? If numerous features share the same datum structure, (such as A ,B at Max, C at Max) I believe they would also have to share the same shift magnitude and direction. Can anyone confirm if my question is valid and/or is Calypso capable, if required, of doing this.

[[be...]]

Hi all,
thanks for all the great help. Aaron the dwg has a note 3- Keyway orientation optional. So all that is need is the TP to -A-.
I'm going to try Mark's suggestion to get the MMC and Datum Shift to come out in results. We were just calculating that at the end, but it was to confusing for the QA Eng???? So he wants it added.
My only concern is the 10" inch length of BOTH slot ( sym plane) and -A- (cyl).
We are getting parts that pass, but need to add in the MMC and Datum Shift.
Sym Plane feature does not get bonus.
Hi Brad.....going to White light training this week???

[[be...]]

Brad you are right I tried the Special Geometry for Sym Plane and I got the mmc, Now I need the Datum shift thanks