General Tolerances: Where do you datum from? Please help
General Tolerances: Where do you datum from? Please help
(OP)
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
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





RE: General Tolerances: Where do you datum from? Please help
Maybe that's what your very last sentence is asking -- if so, then we should read "dimensions" in that statement from JIS 0405 as real numbers such as length, width, height, diameter.
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: General Tolerances: Where do you datum from? Please help
Lets do an example where something is not functionally important to check with a gauge so you don't need to put GD & T on the drawing to request quality checking. Say it is a body exterior part that takes many turns and twist and has a lot of curvature. You decide to call out the areas around a hole that you want to control with GD & T for contact. The rest you leave to "3D is master" and put General Tolerancing. Where do you measure the tolerancing from?
Maybe I'm misunderstanding. Does General tolerance only apply to dimensions on the drawing that are just void of tolerances or is it a general tolerance to the nominal condition of all points in respect to one another on the part?
RE: General Tolerances: Where do you datum from? Please help
ISO 2768 is dealing with dimensions (on the drawing) without tolerances, not dimensions you query from CAD model.
"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future
RE: General Tolerances: Where do you datum from? Please help
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: General Tolerances: Where do you datum from? Please help
If all you had was a conventional drawing and a part in front of you and you were asked to inspect the part for conformance to the drawing what do you do?
You measure the features that are given dimensions on the drawing. You compare the measurement against the allowed tolerance (either explicit for that dimension, or the general).
You make judgement on the part based on those measurements and those measurements only.
What you do not do is make measurements between randomly selected features and try to assess if they are "in tolerance".
Why would you change just because there is a 3D model?
RE: General Tolerances: Where do you datum from? Please help
That's the only mention of a datum you can find there.
"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future
RE: General Tolerances: Where do you datum from? Please help
The first Question I have from this applies to objects we don't have a good way to dimension or aren't very super critical but still complex such as a thick dip in your part/stamping. It's possible but not necessary to make a check fixture for it. In terms of the amount this part can vary from the 3D, how do we control it? Is there a rule?
Second question, what does "3D is master" actually mean if there is no rule for general toleranceing 3D? It's on almost all OEM's and supplier's drawings, so what does it actually mean? Is there a rule?
Thank you,
~Justin
RE: General Tolerances: Where do you datum from? Please help
Thank you,
~Justin
RE: General Tolerances: Where do you datum from? Please help
In fact (not the case for my example above to be clear), 3D data at some OEMs is being replaced like in the link below by removing 3D so there is no transference issues from 3D to 2D. I've seen this for Honda this way before. Since most design is 3D nowadays to begin, it's very advantageous to make the drawing 3D to avoid confusions and actually have capability to show the envelope. They explain the advantages in the last paragraph.
Thank you,
~Justin
RE: General Tolerances: Where do you datum from? Please help
http://machinedesign.com/archive/dimensions-tolera...
RE: General Tolerances: Where do you datum from? Please help
However I don't fully agree with their statement:
There is already a definition for "Product Definition Data" in Y14.41:
Note that inspection is included.
Note further than this definition is is completely independent of the existence or non-existence of a 3D model. Exactly the same information is needed for a drawing done by pencil as by CATIA. The important difference is that a fully annotated model can contain all of this entirely within the model, whereas the pencil drawing may need to be supplemented by other documents.
The point is, if there is a feature that is important enough to warrant inspection then the product definition data must say so, establish the features to measure and the tolerance.
RE: General Tolerances: Where do you datum from? Please help
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: General Tolerances: Where do you datum from? Please help
This is true initially but after finding out that only dimensions on the drawing on covered by general tolerancing rules, I wanted to know if there is any rule for generally tolerancing dimensions that were left out of the drawing but relating it to the 3D. For example, complex contours that would be financially unnecessary to gauge with a check fixture (GD&T). This also made me wonder what it meant to write "3D is master" on a 2D drawing if general tolerancing is not the purpose. Is this a pointless note?
Thank you,
~Justin
RE: General Tolerances: Where do you datum from? Please help
On the odd ocasion I use partial/hybrid MBD I have a note referencing the model for 'basic geometry' and then for 'general tolerance' put in a surface profile feature control frame that I usually put datum in (and obviously indicated the datum features on the drawing views).
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: General Tolerances: Where do you datum from? Please help
It is your job to specify dimensions and geometric tolerances to your drawing / model. One of the possible ways was described by KENAT - use profile control.
The only "rule" you can find is "Definitive drawing" from ISO 8015:
5.3 Definitive drawing principle
The drawing is definitive. All specifications shall be indicated on the drawing using GPS symbology (with or without specification modifiers), associated default rules or special rules and references to related documentation, e.g. regional, national or company standards. Consequently, requirements not specified on the drawing cannot be enforced.
It's very simple - there are no hidden rules - only the rules you specify on the drawing.
"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future
RE: General Tolerances: Where do you datum from? Please help
But I always have been curious to see how others handle such things.
RE: General Tolerances: Where do you datum from? Please help
It's not the worst thing. As long as you have mathematically defined surface, you can apply profile tolerance to it.
If you look into any GD&T book, profile can be defined with or without datum(s), so not illegal either.
"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future
RE: General Tolerances: Where do you datum from? Please help
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: General Tolerances: Where do you datum from? Please help
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: General Tolerances: Where do you datum from? Please help
"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future
RE: General Tolerances: Where do you datum from? Please help
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
RE: General Tolerances: Where do you datum from? Please help
RE: General Tolerances: Where do you datum from? Please help
RE: General Tolerances: Where do you datum from? Please help
RE: General Tolerances: Where do you datum from? Please help
Thank you,
~Justin Kahl
RE: General Tolerances: Where do you datum from? Please help
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