ISO GPS Reciprocity 1

[Ryan6338]

I've been reading ISO 2692 and trying to figure out use cases where the reciprocity requirement would be better than just using zero positional tolerance with MMR (or LMR). I haven't really been able to find any examples of RPR usage outside of the examples given in the standard. Is anyone able to provide guidance on the benefits of using RPR and why one would choose it over 0 position with MMR?

Ryan.

 

[3DDave]

Reciprocity seems to say that any time form or orientation variations are not sufficient to exceed the MMVC that remainder can be shifted back to size tolerance.

The reason in ASME Y14.5 use that 0 position isn't used as much as possible is that manufacturing freaks out when told the position tolerance they have to hold is zero. They like not having to calculate how much larger to make a hole to get however much position tolerance will cover the position variation. They dislike having responsibility for balancing those two factors.

With reciprocity the location tolerance and size range is given and if manufacturing screws up and goes over one of the limits this allows inspection to figure out if that is still OK. It appears that the MMVC remains intact either method.

 

[Ryan6338]

Interesting... thanks for the perspective. From a design perspective, it seems difficult to determine how to choose what the positional tolerance should be when using RPR. I would think it's better to use 0 MMR and let the manufacturer work out the nominal position and sizes they want to aim for.

Ryan.

 

[3DDave]

If one is specifying a hole, for example, there are standard drill sizes and a lot of information on the likely range of holes diameters those drills will make. If you use 0 MMR then manufacturing will not have any matching drills for that size.

The work flow is to pick a nominal hole size, look at the smallest hole it will drill, then subtract whatever tolerance of position you expect, give that diameter as the minimum (which doesn't match a drill size) and the maximum hole diameter to manufacturing and they get to reverse the math you did, in order to use 0 MMR.

It's double work which also means double the chance they or you will screw it up. Which is why manufacturing freaks out when it's on a drawing.

 

[Ryan6338]

Good points. That's exactly the kind of example I needed to understand the idea properly.

Ryan.