Position Tolerance with MMC and MMB

hirxm96

Registered
Hello everyone. I have made a quick position Excel calculator but I recently came across a position feature control frame that includes a MMC on the position tolerance spec. and a Maximum Material Boundary (MMB) on both secondary and tertiary datums. My question is, when making the calculations, should I add the bonus tolerance of the datums to the final calculated TP tolerance?

I would like to add the file or at least a screenshot but I think it isn't possible, so hopefully my question is clear enough for any of you to help me out.
 

cristiano

Starting to get Involved
Hi,
nice to see you doing it.
With MMC, tolerance would get bigger (if there is any left, depends on the size deviation), with MMB there would be shift. The amount of shift would be subtracted from deviation result.
 

stevegyro

Involved In Discussions
NOT certain it is proper. If the feature in (T.Pos) evaluation is soley dependent on the given Datum, fine. But 9 times out of 10, their are other features dependent upon that datum, so a ‘simultaneous’ requirement becomes important.

Example, you have a ‘pump body’ basically a cylinder, with a flange on one side, and a center bore. The flange is ‘A’ primary datum. In the case with my company, the ‘B’ datum is the center bore cylinder, but since cylinder axis does not restrain part rotation axis ‘z on cmm’, ‘B’ is actually ‘X & Y’ zero (home) for part. Then the (weak) drawing shows one of two pilot holes, as ‘C’ datum.
Soooo, implied here is to construct a line from this ‘bore at 4 oClock’, to center of cylinder/B_datum, then clock the part to that bore, bc no other clocking (about cylinder axis) exists on these drawings. So the two pins, one at 4 oC and the other at 10 oc, locate the subject part (pump) to another component. These pin bores are through the mounting flange, the A_datum. Sorry should have mentioned earlier.

so in addition, are 8 bores, also thru the flange for mounting, where their T.Positions are also called back to the same center bore, and also have MMB to this main bore, the ‘B’ datum center cylinder.

Consider the two pin bores locate this pump with a precision fit, then the (8) larger bores are for secure fastening. This application is high pressure fluids. So you have two pins bores with their TP located to the B_datum, with a MMB, and in addition, you have 8 Bores on a bolt-circle thru the same flange, ALSO TP located to the B_datum using another MMB.

SO with a larger center bore B_datum away from max material boundary, you/we get to take advantage of ‘SHIFT’.
BUT the two ‘pin bores’ may be shifted left (for example), and the eight (8) mounting bores may be shifted to the right.

this is not kosher. Material Boundaries (on datums) do not allow shift in more than one direction, such as the above, unless there is NO RELATION of the two pin bores, to the (8) mounting bolt bores.

It appears that the locator pins NEED to position within the (8) mounting bolt bores, so to take allowance for MMB for both the 2-pin bores, and the 8-mounting bores, does NOT assure that position between these two will work in function.
-stephen g. Metrology-Quality engineering.
 
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