bonus tolerance when using a hole pattern with MMC/MMB modifier as a datum

[rhmeng]

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.

 

[3DDave]

Think of the motion of the second set of holes as if you made a drill plate to drill thee two holes. The drill plate would have 4 pins to engage the set of four holes and those pins would be the same size as the MMB.

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.

 

[rhmeng]

3DDave, I am trying to visualize what you are saying. I attached a picture of the part I am working in case you wanted to reference it. The pins in the drill plate that you are talking about would be the same size as MMB, so .091 in my case. As the .096 hole grew to say .101, it can now move and rotate about those same .091 MMB pins.

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.

 

[3DDave]

What method are you using to ensure only X and Y translation and no rotation?

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.

 

[patdh1028]

To the OP: You don't get bonus tolerance from MMB in the formal sense. You're right that the DFS for the pattern @ MMB allows the plate to shift and skew so your two-hole pattern's actual tolerance is based on the manufactured condition of the 4-hole pattern. 3DDave is saying you can't just add that potential for shift to the tolerance zone of the 2-hole pattern, like you can add bonus location tolerance to a hole positioned with MMC. The issue with the DFS @ MMB is that you can't plan or predict how the part will actually mate up to the DFS - you can't use MMB to get bonus tolerance, because it's within the condition that there may be zero clearance between the DFS and the part (holes are made at MMC and are at worst-case locations). So as 3DDave said, if you really care about relating those two hole patterns to each other very closely you may want to just make either make the 4-hole pattern RMB which complicates the DFS or rethink your tolerance scheme so that both patterns are toleranced to the same DRF.

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.

 

[rhmeng]

3DDave I think that patdh1028 cleared this up for me, or maybe just taking a break and coming back to this I now see what you are saying. I was thinking about this in the wrong way when I was saying that the tolerances add together. (I was thinking you just add them as if they were dimensioned off of eachother without either being a datum) however the 4 holes themselves are the datum features, so you cannot account for them twice. They do not add together in the sense that I was thinking, but as patdh1028 points out there is a potential shift depending on the mfg'd position of the 4 datum holes. I hope that kinda clears up where my mistake was, and thank you guys for restructuring my view.