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Datum clearance without MMC

Datum clearance without MMC

Datum clearance without MMC

Hi all, first time poster looking for some advice please!

I am designing an inspection fixture for a part. One of the datum features is a clearance hole with a tolerance on its position and diameter, but there is no MMC call-out. The hole has no function other than to locate the part on the fixture.

My understanding is that this therefore means there should be no clearance when the part is loaded onto the fixture. What kind of physical pin would I need for this? Should its overall size be limited to the upper spec limit of this feature?

Thanks in advance

RE: Datum clearance without MMC

Welcome to the forum! The answer to your question lies not so much in how the pin itself is toleranced, but in how it is referenced elsewhere as a datum reference. Look at where that datum is used in a subsequent callout -- does it have the "M" after the datum letter?

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

RE: Datum clearance without MMC

Thank you for the reply and welcome.

There are no material modifiers anywhere on this part - what would be the correct thing to do in this instance please? I am wondering how to correctly design the datum feature simulator.

RE: Datum clearance without MMC

The datum simulating pin on the fixture will need to be of adjustable size, and make maximum possible contact with the actual datum feature hole while constrained relative to the simulators of any higher precedence datums, if there are any.

RE: Datum clearance without MMC

As Burunduk said, since the datums are not modified, the "datum feature simulator" on the fixture would have to be adjustable so that it grabs onto that clearance hole without any wiggle room.
The technical term for this in the ASME Y14.5 standard is Regardless of Material Boundary (RMB). This of course makes the fixture more expensive, but it makes the part being checked more accurate because there is no shifting of the datum.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

RE: Datum clearance without MMC

"What kind of physical pin would I need for this?"

In general, a precision expanding pin.

The closest are such as Carr Lane sells. They can expand to slightly different diameters along the length of the pin.

I also see https://www.miteebite.com/products/modular-xyz-xpa... The problem as an inspection device is the expansion is from one end, and is not guaranteed to mate with the entire hole surface, but they may be sufficient for manufacturing.

Regardless of Material Boundary/Regardless of Feature Size are best evaluated on CMMs, so an expanding pin will add to an error between how the pin expands and what the CMM software would determine.

RE: Datum clearance without MMC

Check "ID Expansion Clamps" by Jergens.

RE: Datum clearance without MMC

Some other thoughts: if the size fits and the tolerances of your datum hole allow it, an expanding bushing. ETP Keyless bushings for example.

If you have air available, you might be able to design a thin-wall tube that expands under pressure. Or a donut-shaped rubber element that expands to fit the hole when pressure is applied. Would be very fast.

Another possibility is an o-ring on a groove on a pin. It won't provide perfect centering but it will avoid off-center locations.

A spring roll pin comes to mind also if the inspection hole is tall enough and toleranced well enough on diameter.

If your part is thin, maybe you could locate it on a tapered post and have the flat surface underneath arranged to adjust elevation to support the part at whatever elevation the part stops on the tapered post. Of course that makes it sensitive to the form and edge break on the bottom extent of the inspection hole, but it could work well for some applications.


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