Datum MMC as bonus. Newbie Alert 13

[marvalgames]

Hi all. Brand new and so much to learn. Anyway I have a drawing with a True Position with MMC. It also uses Datum A and C marked as MMC (or is it MMB). From what I understand this is not added as bonus tolerance? However in some cases possible? Not sure because my co-worker and trainer is saying to add the Bonus from Datum B the slot? Pretty confused. Please see attachment.
Thanks for any feedback

 

[mkcski]

its "safer to simply ignore datum shift" while

In my experience this thinking comes from the real world of MFG and QA vs the perfect world in Design Engr (CAD). Most MFG and QA setups (aligning the part to processing equipment (machine tool or CMM) travel/rotation use dial indicators or touch probes. These methods are considered "open-setup" and are size independent. This makes it almost impossible for the MFG and QA to accommodate the (changing) sizes of the datum features when measuring position error. Effectively they "simply ignore" it. In the QA area - sophisticated CMM software can incorporate the actual sizes of the datum features and "overlay" datum shift (if needed to get acceptable poisoner error). Hope this makes sense.

Certified Sr. GD&T Professional

 

[drawoh]

mkcski,

I believe it has been argued in the past by dingy (?) that for convenient fixturing, FOS datums must be specified at MMB, and the fixtures fabricated accordingly. When fixtured, the OPs part has some diametric and rotational wiggle that can be used to (try to) position the hole within its positional tolerance.

I have not thought through CMM inspection. This must be different.

--
JHG

 

[chez311]

Most MFG and QA setups (aligning the part to processing equipment (machine tool or CMM) travel/rotation use dial indicators or touch probes. These methods are considered "open-setup" and are size independent.

mkcski - i probably would have taken this at face value if drawoh had not chimed in. i would not agree that these setups are size independent - even if you are zeroing in a part with an indicator or probe, the way that part is repeatably held in the fixture dictates much of the variation you might see in the features machined/produced in a given operation and if the fixturing is not adjustable (ie: fixed pins or stops) this would be absolutely size dependent. an MMB callout could provide the potential to utilize for example fixed pins vs. expanding pins/mandrels or any other kind of adjustable fixturing. this of course all depends on the tolerances involved, but i would think that it would probably benefit the manufacturing department to at least consider the effects of MMB if it is called out.

drawoh - it might be redundant but i wholly agree with your statements.

When ignoring the datum shift (pretending datum is at RFS) you risk rejecting good parts (in theory) but not accepting bad parts.

CH - okay, this may be true if the scrap rate is acceptable and your QC department is okay with that practice and it doesn't drive too much cost for manufacturing due to fixturing/setup/etc.., but I was really thinking of it from a more general standpoint. if you are in the design department and you have utilized MMB to enable the usage of fixed functional gauges and open up tolerances for manufacturing, you are forced to take it into account when performing stackups to determine your worst case condition. "ignoring" it would not be the conservative option in this case.

 

[3DDave]

I take CH's suggestion as 'ignore the added tolerance' for the feature under consideration, not ignore the additional variability in stack-ups.

MMB should often be used since most mechanical elements are fixed size; bolts, screws, and pins, for example, don't expand to fill holes. Assuming RFS for features that will encounter those or using 'convenient' references that don't contact anything but air in their next assemblies make stack-ups worthless.

 

[axym]

Hi All,

I agree with pmarc 3DDave, powerhound, and any others who said that datum shift should not be added to the position tolerance. MMB modifiers and datum shift never increase the size of the position tolerance zone. Period. Never, not even in special cases. I say this in spite the fact that some GD&T textbooks by reputable authors show examples of adding datum shift to positional tolerance and bonus tolerance.

The example that is generally shown is the special case of the considered feature and datum feature each being a single hole or single pin (the one in CH's first example). The datum shift yields some of the same results as a correspondingly larger position zone, but not all of the same results. I agree with pmarc and the others that the position zone does not actually get larger

The bonus tolerance of 2 mm would allow the distance between the axis of the considered feature and the datum feature to increase or decrease by 1 mm, resulting in a range of 19 to 21. The datum shift of 2 mm would also allow this distance to increase or decrease by 1 mm, resulting in a range of 18 to 22. This is the same range that an overall position zone of 4 mm would allow, so there is the illusion that the position zone has grown to a size of 4. But it hasn't - it's still a 2 mm zone that is movable relative to the part (or the part is movable relative to the zone, whichever you prefer).

If the axis of the actual considered feature is perfectly perpendicular to datum A, as in CH's figure, then the distinction between a larger zone and a movable zone is not obvious. If the axis is tilted relative to A, the difference is apparent. The 2 mm movable zone would allow the axis to be tilted by a maximum of 2 mm. But the 4 mm zone would allow the tilt to be 4 mm, which would not fit over the pin in the gage in CH's figure and would not conform to the tolerance. Another way to think of this is that the distance could be anywhere from 18 to 22, but it can't be 18 at one end of the hole and 22 at the other. This is similar to the effect in CH's second figure with two considered features - the datum shift must apply the same way to both features.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[mkcski]

its "safer to simply ignore datum shift" while

drawoh and chez311: I was not implying that I agree with ignoring shift, just that there are explanations for the thinking. One must ALWWAYS take advantage of all tolerance available to lower costs. You also keep mentioning fixtures. I work in a world of VERY large hydro turbine components - 20 feet plus dia and 100 plus tons. Our drawings have MMB datums. However, these are one of kind pieces, and fixturing - assuming large quantity runs - is cost prohibitive. What I am coming to is: there are too many factors to consider when dealing with datum shift to think there is a right and wrong way. Understanding the concepts and taking advantage of it when you can is what is important.

Certified Sr. GD&T Professional

 

[chez311]

3DDave/mkcski/CH - okay i think i get where you guys are coming from, i won't continue to beat a dead horse. CH thank you for your diagrams, as i said they were very helpful in making things "click" for me with respect to MMB/datum shift.

agree with pmarc 3DDave, powerhound, and any others who said that datum shift should not be added to the position tolerance. MMB modifiers and datum shift never increase the size of the position tolerance zone. Period. Never, not even in special cases. I say this in spite the fact that some GD&T textbooks by reputable authors show examples of adding datum shift to positional tolerance and bonus tolerance.

If the axis of the actual considered feature is perfectly perpendicular to datum A, as in CH's figure, then the distinction between a larger zone and a movable zone is not obvious. If the axis is tilted relative to A, the difference is apparent. The 2 mm movable zone would allow the axis to be tilted by a maximum of 2 mm. But the 4 mm zone would allow the tilt to be 4 mm, which would not fit over the pin in the gage in CH's figure and would not conform to the tolerance.

Thats a very key point I hadn't considered about the tilting axis of the toleranced features, thank you for pointing that out. After having sketched it now I see what you are saying and it further solidifies my visualization of the divide between MMB datum shift and MMC positional tolerance. I think it is probably useful for me to think of it in the terms that pmarc noted - that it essentially loosens the relationship between the datum feature and the toleranced feature. I can think about it like the fixed functional gauge that CH presented in his example ie: the positional tolerance of the 9mm holes (10mm LMC) never changes in relation to a central theoretical datum axis represented by the fixed 12mm central pin however the datum feature on the part is now allowed to float around this 12mm pin independent of the other features.

I don't know if thats confusing or makes sense to other people but it does in my mind - it probably seems like a minor shift in thinking about it but I've found that its been little paradigm shifts like that which have been very important when learning GDnT. I know it was always pounded into my head that datum shift is NOT bonus positional tolerance but thinking about it in the way I described really hammers the point home for me.

 

[semiond]

I'm trying to think how the datum shift "bonus" idea could be relevant to the drawing of the OP. To simplify things a bit, i assume datum A was called out RFS and only datum C was MMB. Now let's say the slot related to datum C was actually produced at it's smallest size (and biggest displacement?) allowed - which is the MMB condition for the slot. In that condition 2 things happen: 1. the part is completely locked in rotation around the axis by a gage key that fits in the slot without clearance, And 2. Beacuse of a small displacement of the drilled hole from it's true position location, a gage pin that checks the hole position doesn't fit in the part. Instead of scraping the part, the manufacturer can enlarge the slot related to datum C a little bit, to make it depart from it's MMB condition, but still remain within it's allowed limits of size, and if after that he can give a small rotation to the part in it's fixture so that the gage pin fits in the hole, then the hole location is OK. Is that a correct understanding of the "bonus" given by MMB?