True Position Gage for feature without MMC

[Ksplice]

Hi,
I've been tasked with designing an inspection gage to check the attached part. The gage needs to check the locations of the 4 tips
within a true position tolerance of 0.35mm and no MMC in the feature control frame is given on our customers drawing. This part fits into 1mm sized PCB holes and then is soldered. The problem I see with this is that if I design the gage holes to MMC of the part and the 0.40x0.24 flat on top is small (but still in tolerance), than the gage will pass bad parts. If I make the gage hole smaller I could be throwing away a lot of parts that meet print, if the pin tips are at max condition. I do not see how this is realistically possible and think other means of inspection would be a better fit for this. I thought that you needed MMC in order to design a gage. Has anyone had to do this before?

 

[djhurayt]

I believe you are correct in that to use a hard gauge the callout MUST be at MMC

 

[greenimi]

Y14.43- the gage standard-- has examples of RFS gages. No doubt if the functional requirements accomodate features to be at MMC, the applicable gage is easier to design and less expensive than RFS one, but neverthless RFS gages can be design and manufactured with no major problems.

 

[mkcski]

Ksplice:

I agree with greenimi, but get you wallet out. The variable size elements(RFS means every size has to be accommodated) in the gauge will cost lots more then fixed one size pins with MMC!

 

[CheckerHater]

Couple more problems:

1. By application of magic, all expanding elements must touch the part exactly at the same time (which is not very realistic).
2. Features in question are not features of size, so neither RFS or MMC applies to begin with.

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[mkcski]

Ksplice:

I missed the "not a FOS" condition, so I agree with CH that material condition modifiers do not apply. I have wrestled with the location of " parallel lines" - two dimensional features like lines on a measuring scale - and found a compromise using profile of a surface on the adjoining surfaces to control the edges (lines) where the surfaces intersect (interface).

 

[3DDave]

The problem with RFS in this situation is that the mating part does not have the ability to move to align itself; in fact the smaller the pins (and the farther from MMC) the more likely they are to enter the holes.

Why will the gage pass bad parts if the parts fit the mating virtual condition gage. If they fit what makes them bad?

CH's concern about FOS is correct. This is one area that is undefined. What would be appropriate is to tolerance the faces of the tapered section with a profile tolerance on those as well as the flat on the ends. This will control the envelope, even if there is no definite sharp corner as required by the drawing; neither is there a need to determine the theoretical sharp intersection of faces that may not be planar.