General Tolerances: Where do you datum from? Please help 1

[kahlju]

Hello,

So my questions is for both ASME and JIS. When you have features that are complex or considered not so functionally important and are not dimensioned (dimension line), we use General Tolerances. What is the rule for General Tolerances? Where is the datum for these dimensions (dimension line)? What standard and section of that standard is General Tolerancing rules specified?

Please include the JIS equivalents as my conversation goes to Japan. I am having trouble explaining that general tolerances have legality but they asked me where the datum was and I responded with, "they are the datums already used on the drawing." I need to back this up with evidence. JIS 0405 says, "General Tolerances only apply to dimensions without tolerance." I don't know if this is poor wording or if they are using the second definition of dimension (space).

Thank you,
~Justin

 

[CheckerHater]

It also may be "all over "as cast" surfaces" excluding machined areas,or something along those lines.

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[kahlju]

CheckerHater,

Thank you for your input. I have one comment though. I'm not talking about "my" drawing, but rather my case deals with how to interpret/enforce a customer drawing submitted to us from a while back. As for the 8015 that's a loosely followed rule because there's always standards such as Toyota, GM, hondas general specifications, checking methods, materials, etc. which are referenced on the drawing but you don't have them listed out on the drawing (impractical, they are 100s of pages of reference documents). Those hold up in court and you could take the same extension to be true of 3D data with a note that says "3D is master." I guess in the future we need to reject the drawings until they come back 100% complete. Everyone I work with was under the impression that 3D is master meant that point by point it is the master datum and general quality/drawing tolerances were based off of that.

Now I'm just going to branch off just a bit and raise a factual thought experiment. So in mass manufacturing you are subject to AIAG standards for PPAP. More GPS means Gauge price goes up. If there is a general tolerance or something we can just measure it once a year/6 months/months/etc. with CMM in a few spots where quality is the hardest to meet and call it good. Correct me if I'm wrong but if we or the customer GPSes the aforementioned areas, it will need to be included in the gauge and every lot. Think if we have a part that is a super complex contour and it would raise the Gauge cost up to 10 grand plus to make a guage this complex. I would prefer we just do the former and not have to check X amount of parts per lot to confirm quality if it's not critical functional dimension (for example your part contours to some exterior part or maybe it's plastic beauty shield that wraps around a part or something). Wouldn't it be better for the business to just put a general tolerance for the huge "fudge factor" areas. I think we're thinking far to simple with the parts everyone is describing. I'll upload a picture of an old part in a minute and get your suggestions for how to GD & T it.

Thank you,
~Justin

 

[kahlju]

This is one of 3.

 

[kahlju]

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[kahlju]

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[kahlju]

This is a foot bracket. I couldn't find an example with more holes (for lightweighting, trim or whatever). Is there a good way to use profiles to do this piece? So you can see the rail connection and the vehicle connection holes are the GPS important features but he rest could be accurate to anything. It doesn't really run any patterned profile in any direction so it is difficult to GPS (especially if you have discontinuities like a hole or a slot in the middle for trim). This is an example of a situation where they might try to just say a general tolerance for the drawing related to the 3D or the theoretical nominal dimensions. How would you do your GD & T for this type of part?

Thank you,
~Justin Kahl

 

[CheckerHater]

This is the illustration from the book by James D. Meadows.

As you can see, overall surface of the part is subject to loose "all-over" profile tolerance, while "critical" features have more strict requirements attached to them.

The problem is that correct interpretation of your drawings is probably hidden somewhere between "100s of pages of reference documents", but the idea must be the same.

Also, ISO and national standard bodies have GT standards for weldments, castings, etc., which may (or may not) be referenced on the prints.

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future