Bonus Tolerance Question, MMC on a Datum 4

[cfordyce05]

I have looked through ASME Y14.5-2009 and maybe I am blind, but I can't find what I'm looking for. Attached is an example drawing.

How is the true position of the .056 hole affected by the MMC on datum B? The hole has a .001 tolerance zone at MMC, so I can get a little bonus tolerance if I make the hole larger than MMC (to .056), correct? If the hole ends up outside of the bonus tolerance, does the MMC on datum B give me more bonus to keep the hole in tolerance?

 

[Belanger]

On each manufactured part, the datum feature might have an axis which deviates from the true datum. But yes, the datum repeatable because it is the axis of the worst-case cylinder, which is represented by the constant-size gage.

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

http://www.gdtseminars.com

 

[SeasonLee]

From the fit and function will direct us the use of modifier, this is what I know on the feature modifier, but it’s the first time I know this will also apply to the datum modifier selection. Thanks John and Evan.

SeasonLee

 

[axym]

SeasonLee,

No, the constant-size gage element does not result in a repeatable datum axis. It's the opposite. Let me clarify what I mean.

The datum axis is defined in the gage element (datum feature simulator). For MMB (and LMB) datum feature references, the size of the gage element is constant. This means that there will generally be some clearance between the surface of the datum feature and the surface of the gage element. There can be movement between the two(commonly known as datum shift). The datum feature (and therefore the part) can have different relationships to the datum (and therefore the coordinate system). There is not a stable or easily repeatable relationship.

For RMB datum feature references, the size of the gage element is adjustable and shrinks/expands to achieve stable contact with the extremities of the datum feature. There is no datum shift. The relationship between the datum feature and the datum is stable and repeatable.

This is what I mean by the statement that the constant-size gage element does not result in a repeatable datum axis. The relationship between the part and the datum axis is not repeatable.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[Belanger]

I thought of that too, Evan. It all depends on what he means by repeatable. Does it mean an axis we can find each and every time? Surely a fixed gage does that, even if the part itself slops around. On the other hand, an RMB gage (or even a CMM scan) has many more variables that enter the picture. So it's more stable, but it has more chance for something to go wrong.
But I do agree with everything you wrote.

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

http://www.gdtseminars.com

 

[SeasonLee]

A MMC modifier will have a fixed gage, and the part can be measured repeatedly, is that correct?

Also, I noticed that when a hole pattern is used as a datum feature, it is almost always referenced at MMC, why?

SeasonLee

 

[Belanger]

Well, the notion of repeatability is tricky. Yes, a fixed gage can be repeatable in the sense of ease of measurement -- jiggle the part around and you'll be more likely to "repeat" a measurement. But if you wish to quantify repeatability in terms of statistics, then Evan's take is probably more accurate.

As for the pattern datum, I'll again use the illustration of fixed vs. variable gaging. If a group of holes is identified as a datum feature and then referenced at MMC/MMB, then a fixed gage is used. Even if one the holes is not perfectly positioned itself, the notion of datum shift will still allow for the part to fit onto the gage.

But if we omit the M modifier, and the datum is referenced RFS/RMB, then recall the a variable gage is required -- we must have a gage that self-centers on each of the holes making up the datum. Well the only way this is practical is if each hole in the datum pattern is perfectly positioned! If just one of the holes of the pattern is off by a hair, then the gage isn't really finding the true center; it will bind up on one side of the hole first.

So it's mainly for a practical reason. (Sorry I don't have time to create some sketches right now.)

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

http://www.gdtseminars.com