datumshift: MMC size limit or virtualcondition

[RonnyC]

Hello Everyone.
I wonder if someone could shed some light on my problem.

When referencing a MMC datum feature ( datum shift ) where the datum feature has a positional tolerance,
the rule is the datum shift value is the datum feature's MMC virtual condition ( being MMC size limit + positional tolerance )
In case the datum feature is a hole, this implies using a fixed gage pin as true geometric counterpart ( fixed in size and position ).

If the same datum feature would be referenced RFS, it would imply using a variable gage ( variable in size and floating position )

Is there an in between possibility, using a fixed size / floating position gage.
If so how should this be noted on drawing?

See my attached file as example.
( the example is incomplete and oversimplified )
ref D would be simulated using a fixed gage pin of Ø9.4

Using RFS, floating / variable gage would simulated ref D

I would like to use a fixed size / floating position gage Ø9.9

Thanks for any comments

 

[pmarc]

RonnyC,
For your scheme, in order to fix the part for checking position of Ø20.0 hole, you do not have to tie datum feature simulator D in location to A, B, C. You just have to slide Ø9.9 pin to the smaller hole and check position of the bigger one.

In fact a stepped pin with two coaxial fixed in size diameters - Ø9.9 and Ø19.85 (20-0.1-0.05) can quickly verify this positional requirement.

 

[RonnyC]

Hello pamrc,

thanks for your reply.
That was exactly the gaging I would be thinking of in the example drawing, a stepped pin check outside of the main control fixture.
Question would be how to note this requirement on a drawing as the standard notation ( see example ) would, in my opinion require a Ø9.4mm pin

Regards,
Ronny

 

[pmarc]

RonnyC,
I do not know what you mean by saying standard notation. What I told you is the interpretation according to standard.

To have everything clear, if you want to use hard gaging to check both positional callouts, you need 2 separate gages:
1. for |pos|Ø0.5(M)|A|B|C| - Ø9.4mm pin perfectly perpendicular to A and located at true position to B and C;
2. for |pos|Ø0.05(M)|D(M)| - a stepped pin with two coaxial fixed in size diameters - Ø9.9 and Ø19.85.
That is all you need.

Side note, of course the gages have their own tolerances, so the actual values given above will be slightly different for real gages.

 

[RonnyC]

Hi,

by standard notation I mean the notaion on the attached example.

I did some more reading in Y14.5M 1994 and I think I understand now.

4.5.4.1 Size of a primary or single datum feature
Where a primary or single datum feature of size is controlled by a roundness or cylindricity tolerance, the size of the true geometric counterpart used to establish the simulated datum is the MMC limit of size.
...


4.5.4.2 Size of a secondary or tertiary datum feature
Where a secondary or tertiary datum feature of size in the datum reference frame are controlled by a specified tolerance of location or orientation with respect to each other, the size of the true geometric counterpart used to establish the simulated datum is the virtual condition of the datum feature.
...


So in the example ref D is the primary datum in the datum reference frame of the tolerance of hole Ø20, therefore the true geometric counterpart is the limit of size ( Ø9.9 ) , not the virtual condition (Ø9.4 )

This confirm your statement pmarc,

thanks

 

[CheckerHater]

RonnyC,
I have to agree with pmarc – it’s was hard to understand what you are trying to achieve (please don’t take it as an offence)
If by looking for requirement “in between” MMC and RFS you mean tightening your tolerances, you can do just so – tighten them.
If you specify your smaller hole position as [pos|DIA 0 (M)|A|B|C] your size and position requirement will be reciprocal – the smaller the hole the larger positional freedom.
Then you can possibly check your part with fixture shown on the enclosed picture.
Unless your question was already answered

 

[RonnyC]

Hi CheckerHater ( what's in an alias)

thanks for your reply,
My problem is solved.
I was assuming the position tolerance needs to be added to the limit of size to calculate the datum shift.
I now realise this is not the case ( see my reply before )

thanks anyway.