true geometric counter part and actual mating envelope

[graceljh]

Hello , another question
Is there anyone knows about what is difference between true geometric counter part and actual mating envelope?
'I found a para in the 1994 Standard
in 1994 standard, para 4.5.3, (a) Primary datum Feature - Diameter RFS
the true geometric counter part (or actual mating envelope) '
so, these two concept is exactly same?
I knew what is true geometric counter part (by Fig 4-10,in 1994 standard)
but I don't know what is actual mating envelop exactly.
Is there some knows what is difference between these two concepts ?
I never found any actual mating envelope Figure in the 1994 standard.
Thanks
ps: Happy new year

 

[drawoh]

graceljh,

I would not worry about the terminology here. The actual problem is fairly obvious. You need to fixture to your datums.

If your datum is a feature of size, your fixture must account for variations in the size of the feature. If your datum is called up at MMC, or MMB in ASME Y14.5-2009, your fixture is fabricated to the maximum allowable size. Any clearance in your fixture is your bonus allowance. Your fixture must clear the feature of size.

If your datum is a feature of size called up at RFS, everyone has problems, including the person who writes the GD&T[ ]standard. A very good drill or lathe chuck sort of accounts for variations in diameter, assuming the part was fabricated round. You could drop your part into a fixture, then shove shims in on either side.

If you are designing and preparing drawings, you should not call up inaccurate features of size as datums. I believe the crude rule of thumb is that the fixture, and your FOS must be ten times more accurate than the tolerance you are testing. In other words, if your RFS FOS datum is a reamed dowel pin hole, you don't worry about clearance.

--
JHG

 

[aysuio]

graceljh,

You may get what you are happy with in 2009 Standard about actual mating envelope. They give comprehensive explanation there.

For term true geometric counter part, it was replaced by term, datum feature simulator, in 2009 standard. You'd better go through this standard to understand these terms.

There are a number of links between the two terms you mentioned, but they are of course not "synonyms". Hope these help.