Simultaneous requirement confusion

[sendithard]

I thought the DRF chapter was hard to get thru, but this position chapter is taxing as hell to read thru.

In the below picture, you see two separate hole patterns. As the diagram shows, it appears this can be one single hard gauge check...But I want to back up for a second...

1st question: Let's assume the bottom bolt hole circle didn't exist, would you then check all the 4 square hole pattern holes individually or would you check them as a pattern? I thought simultaneous meant you check this as a pattern b/c it is not stating 'individually'??? Or is it that the holes are dimensioned from themseleves that it is in essence a simultaneous measurement by default? Just not called a simultaneous b/c it has interrelated dimensions.

2nd question: Since the bolt hole circle has the same Datums we have a simultaneous condition...so could this be created at MMC with a hard gauge I'm assuming?

 

[sendithard]

Seems I'm a bit of an idiot b/c the top square hole pattern is dimensioned by basics from each other so that I guess is a pattern and not technically 'simultaneous' although each hole must be measure simultaneously.

I think I'm confusing a single pattern being measured together which in my book is a simul measurement vs the ASME book definition of simultaneous which is only 'multiple patterns'

Am I right?

 

[Burunduk]

Simultaneous requirement is one type of pattern and is also listed in the pattern definition (section 3). Within each pattern there is also simultaneity in a sense, but the simultaneous requirement as a Y14.5 term is specific to separate position and profile requirements that reference the same datum features in the same order and boundary conditions. In the 7-52 example you brought, it indeed means a single VC gage for the two patterns. If one of the datum features was a feature of size referenced at MMB it would mean that you can't use datum shift separately for each of the two patterns. You could shift the part only once relative to the datum-simulation fixture, and both patterns would have to fit the gage simultaneously.

 

[axym]

sendithard,

Burunduk summed it up pretty well. I'll add a few more thoughts.

The 4 holes are checked as a pattern because of the 4X annotation. Similarly for the 6 holes. It's nothing to do with the basic dimensions or how they're laid out. The nX annotation is one of several methods to create a "pattern" in Y14.5. The effect of the pattern is that all of the applicable tolerance zones must be evaluated in the same candidate DRF.

The 4 holes and the 6 holes are checked as a pattern because of the simultaneous requirement. Simultaneous requirement is another of the methods to create a pattern in Y14.5.

So creating a pattern (using nX, All Around, n SURFACES, and others) applies the same geometric effect as a simultaneous requirement. It requires that all of the applicable tolerance zones must be evaluated in the same candidate DRF. If this is done using a hard gage, it means that the actual part cannot be shifted one way to check some of the features and shifted a different way to check the others.

The geometric effect is relatively simple, but the words get in the way.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca