bonus tolerance when using a hole pattern with MMC/MMB modifier as a datum
bonus tolerance when using a hole pattern with MMC/MMB modifier as a datum
(OP)
Hi, I am trying to understand the affect of using a hole pattern with MMC description as a datum feature reference. In the drawing I have a 4 hole pattern of .096"+.005/-.000 holes used as a datum feature, with a MMC tolerance. Then in the lower section of the part I have 2 other thru holes of the same size that are referenced off of datum A and datum B (the hole pattern). My question is when i reference the 2 thru holes to datum B at MMC, do the two geometric tolerances add up? So my 2 thru holes could move within a .020 diameter tolerance zone (.01 at LMC of datums + .01 LMC of 2 thru holes)? If i put regardless of feature size will this eliminate my problem? Where Thanks in advance.





RE: bonus tolerance when using a hole pattern with MMC/MMB modifier as a datum
You would see that as the 4 holes got larger than the MMB the drill plate would be allowed to shift and turn; the combination could result in a large amount of movement for the location of the two holes.
Even if you used magic expanding pins** for the 4 holes as an RMB basis, the four holes are allowed to individually differ in location by the allowed tolerance and when the pins expand, they will force the plate to shift and rotate.
In neither case can the tolerance values be added. If the sets of holes are not related in a chain of assembly, or you don't want them to compound unexpectedly, then make use of simultaneous requirements and give all the holes the same datum references.
**per the standard all the pins are supposed to expand at exactly the same rate until enough of them wedge up to prevent further expansion. Having never seen any such device, I assume these are magic pins. Even a mathematical simulation of the behavior is not trivial beyond very simple applications, but since no hardware can duplicate it there's no value in that simulation beyond giving an inspector some busy work.
RE: bonus tolerance when using a hole pattern with MMC/MMB modifier as a datum
My thinking is that regardless of what the hole diameter is, it is still able to move the center of the datum holes by half of what the positional tolerance / boundary condition is. So at .096 with location tolerance of .005, the center is able to shift by .0025 (and therefore the two holes are able to shift the same amount). Then at .101 with locational tolerance of .010, the center is able to move .005 (.010 / 2, half one way half the other way)(and the two holes are able to shift this same amount).
So it seems like then the two holes (I guess it is more clear to not say the two holes, but the positional tolerance of the two holes), if referenced off of these 4 holes, if you just think about it in an x or y direction, would also be able to move the .0025 or .005 depending on the size of the .096 nominal datum holes. So worst case for both features, you would add the possible movement of the datums at LMC with the possible movement of the two holes correct?
It seems like you are saying that you can add the tolerances but you say you cannot.
RE: bonus tolerance when using a hole pattern with MMC/MMB modifier as a datum
You can mock this up in a few minutes to see the basics for yourself. Scale up the holes and the tolerances to something convenient. If you don't want to make it in hardware, then create a transparent overlay to simulate the drill plate.
RE: bonus tolerance when using a hole pattern with MMC/MMB modifier as a datum
Whether or not MMB is appropriate here is 100% dependent on the design and tolerance of the mating part(s). You want to use MMB if your mating part is going to be functionally similar to the DFS @ MMB (really, the mating part should be able to be completely encased within the geometry of the MMB DFS if guaranteeing fit-up is the goal). If you're not sure if that's the case you probably want to avoid it.
RE: bonus tolerance when using a hole pattern with MMC/MMB modifier as a datum