Please help clarify datum shift vs bonus tolerance

[ozzkoz]

Hi,
I read a thread on here regarding datum shift and read through the section in the ASME standard. My understanding is this:

When the MMC modifier is applied to the tolerance in the FCF then bonus tolerance is allowed. If the feature is produced at LMC then the tolerance zone is increased by the difference between LMC and MMC. This is very clear.

When the MMB modifier is applied to a datum (secondary for example) then the MMB datum feature simulator is used to define the datum plane. If the datum was a hole, then you would take a MMB cylinder and place it in the bore and take measurements relative to the cylinders axis. If the hole was made to LMC then the part could shift relative to the datum feature simulator. I still think I am pretty clear to here.

When you check a hole pattern to the MMB datum, I do not think you get bonus tolerance for the difference between MMB and LMB datum feature. I DO think that you are free to adjust the part within the allowance between the datum and MMB datum simulator before inspection, and then once you decide on its position, you make ALL measurements required relative to this datum without re-adjusting the part.

If it is treated as bonus tolerance then I don't see how the fixed faster formula would work given that it doesn't account for bonus tolerance due to datum shift.

Is my understanding correct?

Thanks

 

[fsincox]

The concept hasn't changed since '82 or before, has it?

 

[fsincox]

I mean the concept of MMC, of course.

 

[axym]

Norm,

I can understand specifying a datum feature at MMB if it has a clearance fit with its mating feature. But whenever I see a primary datum feature referenced at MMB, I always take a closer look. If the potential slop is small enough that the primary datum feature will still control the degrees of freedom, then it's okay. But if there could be so much slop that full contact would be made on the secondary or tertiary datum feature, that's where I have a problem. I like the DOF constraint responsibilities of each datum feature to be well defined, maybe that's just me.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[NormCrawford]

I don't disagree. If the design is such that there is so much clearance between the features of size that they "really" never control the "theoretical" degrees of freedom that the feature of size is theoretically capable of constraining, then it does not make sense to use that feature of size as the primary datum feature in the first place.

Norm Crawford
GDTP-S
Applied Geometrics, Inc.

http://www.GDandT.com