Position Tolerance Called to Datums with MMC

[jameslaz]

I have a question regarding a position called out to datums that have a MMC modifier. From what I have read is seem like any bonus tolerance shifts the allowable position tolerance location from the stated datum. What I need clarification on is this. Is I have a feature called out with a position to datum A at MMC and say datum A is dimensioned 1.000 +/-.005 from my feature and I get .002 bonus tolerance from datum A. Does that mean I can be 1.007 (1.000 +.005 tolerance + .002 bonus tolerance = 1.007) from datum A and still be good? I hope this is clear enough to understand.

Thank you for any help,
James

 

[Michael_M]

Max material condition adds to the tolerance frame not to the feature size. The tolerance frame dictates 'where' the hole is located. Max material allows for more 'error' in location.

For example, you have a 1.000 +/-.005 hole with a tolerance frame of .005m A Bm C. For our example, the 1.000 hole is right at 1.000

The hole diameter gives an additional .005 so your .005 frame adds another .005 giving a total of .010. So your dimension could read 1.000 +/-.005 TP .010 A Bm C. The tolerance frame changes every part depending on the actual diameter of the 1.000 hole.

If the hole diameter is .997, the feature frame would be TP .007 A Bm C


Bm is a bit more complicated. I will say datum B is another hole. As this diameter gets bigger, you add more tolerance to the feature frame. If datum B is another 1.000 +/-.005 hole, it adds to the feature frame. Depending on they type of feature datum B is, hole, slot, boss, plane, etc, this can really confuse things.

To keep my sanity, I typically only calculate max material from the actual feature itself and not from any bonus datum's (unless I really really need it). However, if I really need the bonuses from the datum's, someone did not make the part well.

 

[Proud Liberal]

The bonus only applies to the positional tolerance. The size tolerance is not increased. The bonus = (actual feature size) - (nominal size @ MMC). Also note the bonus cannot exceed the total size tolerance. The bonus stops accumulating when the size of feature reaches the LMC limit of size.

 

[jameslaz]

Thank you the answers but I still don't understand what is gained by any bonus tolerance from a datum with a MMC modifier.

I have attached a small sketch that I hope will help.

Looking at the sketch. if the datum C dimension is at 2.495 how would that affect the position callout of .005 to A, B and CMMC?

Thank you again,
James

 

[Michael_M]

In essence, the print you have is incorrect. The 2.500 and the 1.500 dimensions should not have tolerance. They would be basic (wrapped in a square).

The true position frame is what gives the 'tolerances'.



Maybe off topic: How to calculate true position.

Take the error of X and square it, add to that, the error of Y squared. Take the square root of the sum and multiply by 2. This number needs to be less than or equal to the feature frame reference.

Example: the 2.500 is actually 2.4983, the 1.500 is 1.5025, and the .500 hole is .502.

The .502 gives us a 'bonus' of .007 as it is .007" over the smallest hole size allowed. This directly adds to the .005 from the frame meaning the frame is .012.

the error of 'X' is .0017 (which we square getting .00000289). The error of 'Y' is .0025 (which we square getting .00000625). We add .00000289 + .00000625 getting .00000914. We take the square root of this number .00302 and double it getting .00604 (we double it because our calculation uses radius and we need to convert to a diameter).

This formula may look familiar as the Pythagorean therm (Asquared + Bsquared = Csquared).

 

[jameslaz]

let me try one more time with a new sketch that shows the actual part (sorry I had to block off a lot of it for proprietary reasons).

As you can see I have a diameter .220 +/-.005 with a position callout of .014 to A, B-MMC and C-MMC.

On the print the A datum is called out as 1.180 +/-.002, the B datum is a diameter 5.1893 +/-.0010 and datum C is a diameter called as .477 reference dimension.

You can see my confusion. We have problems with the parts and I am trying to see if any bonus tolerance can help. I do not know what to do with the MMC for datums B and C.

Thank you,
James