Tertiary Modifier is B(MMC)- C(MMC)?

[rvenzon]

I've come across a feature control frame for positional tolerance, where the tertiary modififier shows B(MMC)- C(MMC).

What does this imply? If more info is needed, please specify what.

Thanks!

 

[KENAT]

You may need to post it to help us interpret. Sometimes a datum is derived from 2 datum features. However, this is also sometimes mis used so without seeing your application it's difficult to know. Also, what GD&T standard?

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[fcsuper]

To be a brave soul, I'll guess that someone is spelling out MMC instead of using the M-circle symbol.

Matt Lorono
Lorono's SolidWorks Resources & SolidWorks Legion

 

[rvenzon]

Yes fcsuper, you are correct. I will provide a snapshot of the feature control frame when I get the chance.

 

[KENAT]

Sorry, I wasn't confused by that, I meant the actual application on the drawing, what the datum identifiers represent etc.

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[PeterStock]

I would wonder about a compound datum for a tertiary reference. You should only be removing the last degree of freedom, the need to reference two featurs of the part should not be needed for that.

Peter Stockhausen
Senior Design Analyst (Checker)
Infotech Aerospace Services

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[drawoh]

rvenzon,

The datums B and C are (ought to be?) features of size. The tolerance applies when datums B and C are at MMC.

Compound datums allow you to use two or more features to define a face, a centre, or an edge. This makes sense for primary and secondary datums. I do not reject the possibiliy that this could be a valid callup, but I would want to see the drawing.

I am trying to visualize how this might work. The primary datum could be a face. The secondary datum could be a centre. The tertiary datum could be an axis used for clocking?

JHG

 

[KENAT]

My point exactly drawoh, it sounds odd but off the top of my head I'm not sure there isn't some situation it might be valid.

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[rvenzon]

Working on getting a visual for you guys to go by. Thanks for the input so far.

 

[rvenzon]

Here is a snap shot of the feature frame. It occurs several times, all in reference to a hole or bore.

Some additional information:

Datum [A] is 3 surfaces on the same plane.
Datum is a hole with a perpendicularity tolerance to [A]
Datum [C] is a hole with a positional tolerance to [A] &

Sorry I have to be a bit vauge with providing visuals for you guys as I have customer proprietary MBD.

 

[KENAT]

What is the secondary datum feature, it's starting to sound like it may be legit.

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[rvenzon]

Primary Datum [G] is the face that sits perpendicular to the .6247 hole, and Secondary Datum [F] is the face of a surface which sits perpendicular to a separate bore on a different axis than the Datum [G] face.

 

[drawoh]

rvenzon,

Is there any chance you can show us the datum features?

I would say that the FCF is valid, but this is meaningless. Your datums should retain your part in all six degress of freedom, and there should not be redundant constraints.

JHG

 

[rvenzon]

My apologies... I really can't send captures of the actual model showing the datum features. Liability and legality reasons, that I'm sure most of you can understand.

If I still have trouble resolving this, I will mock up a model/drawing to show the locations and concept of whats in question and post to this thread.

Thanks again for the input.

 

[drawoh]

rvenzon,

I think you have provided enough to work with.

Datum A is irrelevant, because you are not calling it up.

By the sound of it, Datums G and F are non-parallel surfaces. These locate your part in two orthogonal displacement axes, and three rotational axes. You need positional reference for your tertiary datum.

The compound datum B and C does not make sense. Your tertiary locator would be a diamond pin picking up either B or C. Picking up both of them controls side to side displacement and rotation. Your datum F already controls rotation.

You could use B-C as a secondary datum and F as your tertiary. That would make sense to me, although it is a horrible fixturing procedure.

GD&T datums are fixturing specifications, not dimensioning references. Most of your FCFs should specify the same datums. Is is the drafter using different datums for each face of the part?

Also, I think the MMC on the Ø.007" positional tolerance is hardly worth the trouble. I do MMC on features with sizes that are sloppy in comparison to the positional tolerance.

JHG

 

[KENAT]

So primary datum is a face, fine.

Secondary datum is a face/surface, fine.

Tertiary datum is some kind of Plane through the axis of 2 holes.

Have I understood correctly?

Starting to sound overconstrained to me.

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[axym]

The FCF doesn't look illegal to me. If B and C are holes, then the tertiary simulator will be a pattern of fixed-size pins. This could constrain the last degree of freedom.

Whether the datum scheme captures the functional requirement is another question entirely.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[KENAT]

Axym, that's where I was losing myself. I could see how you might be able to simulate a plane, even taking acount of the MMC, using pins.

However, I was concerned about the theoretical plane. Usually when you have a plane from a hole it's based on the axis. This is fine for one hole. However, for two holes to form the plane both axis have to be on the plane. I suppose the for the theoretically perfect part this will be true.

So I suppose it is legit, though as you say, whether it reflects function is another matter.

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[axym]

KENAT,

Keep in mind that the theoretical planes are in the simulators, and the simulators are perfect. So the two pins for our tertiary datum feature will have perfect spacing and perfect orientation and location to the higher precedence datums.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[drawoh]

axym,

The only plane that is any use for fixturing is a real one that can be picked up by a pin. A simulated plane may have some meaning on a drawing, but it is useless on an real part. The FCF does not violate ASME Y14.5, but it is not a good specification.

JHG