AVG diameter on a non-rigid part 1

[powerhound]

Hi all,

We are currently in the process of specifying some controls for a steel ring that we want to make and I have a question about the direction we are going. The part is basically a thin ring--about 4mm thick or so--that will be machined and heat treated. We know that the part will distort in heat treat but since the assembled part is under a pre-load that will push it back into shape, we don't want to hold the free state part to very strict tolerances. The part is not really a non-rigid part but can we still specify it as if it were? Currently we are doing the same thing as in Fig. 6-54 (1994) to call out the datum. Am I correct in my thinking that a callout like this means that you don't really have an MMB or LMB boundary to work with when the diameter is in the free state? We are working to the 2009 standard but I don't know what the figure number is in that one.

Thanks,

John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[CheckerHater]

If you can force part into different shape, then it is flexible, even if it is steel.

MMB or LMB is usually taken into consideration when part is free to move as it is being assembled. If you force part in place, it will somehow “self-align” itself, effectively making your datum RFS (Although some may argue with this statement).
You may need to add note to clarify it.

If you can find copy of 2009 version, Fig. 5-14 is probably what you are looking for.

 

[powerhound]

Thanks CH. We are adding notes to clarify the intent.

John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[powerhound]

Another related question? Wouldn't the above stated scenario preclude the use of an MMB modifier in the datum portion of any feature related to the datum since there isn't a defined MMB?

John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[CheckerHater]

I could be wrong but I see your datum feature as one REALLY big press fit dowel (or dowel hole, as I didn’t see the part).
In case of press fit it makes no sense to refer to any feature at MMD/MMC, because your shaft is larger than your hole, so your datum shift/bonus is actually negative (whatever that means), and specifying (M), even if it may be legal, has no real advantage (or disadvantage)
But I am also interested to see what others have to say.

 

[pmarc]

I would say it could be called at MMB size. Furthermore, it could be called out at any convenient size, for instance 191.2, if we are talking about fig. 5-14.
Other option - the restraining note could specify a range within which the datum feature could be restrained and that range could be equal to size limits of a mating feature.

This however will not change a fact that since the ring is brought back into its regular shape because of its interaction with mating feature during real assembly, there should be no loose between datum feature simulator and the datum feature during inspection, so it will work similar to RMB concept.

 

[CheckerHater]

I like the idea of seeing Restrained as “fourth condition”, neither MMB, LMB nor RMB, and I am glad I am not alone seeing it behave more in RMB-ish manner

 

[powerhound]

So I think we are all in agreement that the material boundary is not as forthcoming as if it were simply a size dimension with an upper and lower limit. Am I correct in that assumption?

I also agree that "RMB-ish" is appropriate. That's how we're treating it anyway.



John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II