Tolerances of ISO 2768 cumulative or not?
Tolerances of ISO 2768 cumulative or not?
(OP)
Does anyone here know if ISO 2768 tolerances are to be treated as cumulative or are they non-cumulative (as say, a basic profile without datum references).
Ken had a thread awhile back on a discussion he had with a German engineer who clamed they were non-cumlative. I like Ken had always assumed they would be cumulative between the non-dimensioned features. Does anyone out there know? Can you reference why and where this is stated?
Thanks,
Frank
Ken had a thread awhile back on a discussion he had with a German engineer who clamed they were non-cumlative. I like Ken had always assumed they would be cumulative between the non-dimensioned features. Does anyone out there know? Can you reference why and where this is stated?
Thanks,
Frank





RE: Tolerances of ISO 2768 cumulative or not?
Can you clarify what do you mean by "cummulative" or "non-cummulative" ISO tolerances?
I was not able to find Kenat's thread on this and I have to admit I am lost a little bit on this.
RE: Tolerances of ISO 2768 cumulative or not?
Thanks, You seem to have some experiance working with ISO?
Sorry, it may be my teminology.
Non-cumulative, I believe would be like a part defined with all basic dimensions and an all around profile tolerance. I believe any (2) arbitrary opposed sufaces would be required to be within +/- 1/2 the profile tolerance zone. This inspite of the dimensioning scheme shown on the drawing.
Cumulative, our normal method where the non-specified or implied dimensions will cumulate a tolerance stack-up.
RE: Tolerances of ISO 2768 cumulative or not?
I'd like to help but I can really find a link between your issue and general tolerances (linear or geometrical) according to ISO 2768. I must have a blackout or something :-[
RE: Tolerances of ISO 2768 cumulative or not?
Sorry, I am not explaining it well.
I am asking if someone knows when you reference the general tolerances under ISO 2768 are they only to be applied to the dimensions as shown on the drawing or do they also apply to the nonspecified, implied dimensions?
If (2) holes are dimensioned in the same direction from a common surface with 200mm and 220mm dimensions, does the implied 20mm tolerance between holes themselves apply as if it was directly specified under the 20mm tolerance? I am assuming, for argument sake, the tolerance band for 200mm & 220mm is much greater than that for 20mm, I don't have it in front of me now.
I would not have assumed that the tighter tolerance of the 20mm would apply, but I have heard claims that some believe it would and I would like to know if it is specified, I assume it must be, somewhere.
Any clearer?
Thanks, for trying to help.
Frank
RE: Tolerances of ISO 2768 cumulative or not?
Powerhound, GDTP T-0419
Engineering Technician
Inventor 2010
Mastercam X5
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II
RE: Tolerances of ISO 2768 cumulative or not?
Thanks, I would agree with you and KENAT's original assesment.
pmarc,
Original thread, sorry Ken.
thread1103-196260: Tolerance analysis ISO2768
RE: Tolerances of ISO 2768 cumulative or not?
a) linear dimensions (e.g. external sizes, internal sizes, step sizes, diameters, radii, distances, external radii and chamfer heights for broken edges)"
It seems primarily directed at the dimensions not the features so I would think it is dimensionally dependent.
Frank
RE: Tolerances of ISO 2768 cumulative or not?
Now I got your point
I can only agree with you, powerhound, Kenat and everyone else who was involved in this and the other (Kenat's) thread.
However a short comment:
For location of holes ISO 5458:1998 "Positional Tolerancing" recommends position tolerance together with basic (theoretically exact dimensions, TED) dimensions from datum reference frame instead of coordinate dimensioning. If in your and Kenat's examples dimensioning was done in proper way (according to the standard), there would be no issue at all, because basic dimensions are non-cumulative and there is a clear instruction from which datum point, line or plane they are locating part's features.
RE: Tolerances of ISO 2768 cumulative or not?
Interesting point, I will look for that one.
Frank
RE: Tolerances of ISO 2768 cumulative or not?
Matt Lorono, CSWP
Lorono's SolidWorks Resources & SolidWorks Legion
Follow me on Twitter
RE: Tolerances of ISO 2768 cumulative or not?
On my parts, they didn't reference the second part of iso2768 for geometric controls and definitely didn't have basic dims.
So arguably they weren't using 2768 as intended by the folks that wrote it.
However, it seems a fairly typical way of using it based on other German & ISO drawings I've seen that reference 2768.
(Maybe it's analogous to the over dependence on block tols in the US, but at least with the block tols I know what I means for sure - even if it's wrong wrt function.)
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Tolerances of ISO 2768 cumulative or not?
If 2 holes are dimensioned as 200 and 220 from an edge, The manufacturer can only assume that the 2 holes have a relationship to the edge which is paramount to the correct proformance of the part. ie the 2 holes locate different fixtures correctly relative to the edge. Applying a general tolerance of +/-1mm the 2 holes may be 18mm to 22mm apart.
If those same holes are dimensioned as 200 and 20mm, the manufacturer can assume that the 2 holes hole an individual fixture or 2 fixtures having a functional relationship between them. Applying the same tol. the 2 holes may be 19 to 21mm apart.
However I donot accept tolerance stack. Just because the first hole is 1mm over dim, the second has a tolerance applied to the theorically correct position. therefore the distance between the holes maybe 18mm (first at 121, 2nd at 120+19) This effectivly reduces the 1mm tol to 0.5mm.
Dont we weave a tangled web!
Dazza
RE: Tolerances of ISO 2768 cumulative or not?
I'm going to respectfully disagree with you. If I understood your post correctly, you are saying that if the first hole was called out at 200mm from the edge and the second hole 20mm from the first hole, that the second hole should really be held to 220mm from the edge instead of 20mm from the first hole. Is that what you meant to say?
Powerhound, GDTP T-0419
Engineering Technician
Inventor 2010
Mastercam X5
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II
RE: Tolerances of ISO 2768 cumulative or not?
Thanks,
Frank
RE: Tolerances of ISO 2768 cumulative or not?
"Chaings of single dimensions should be used only where the possible accumulation of tolerance does not impinge on the functional requirements of the part."
Matt Lorono, CSWP
Lorono's SolidWorks Resources & SolidWorks Legion
Follow me on Twitter
RE: Tolerances of ISO 2768 cumulative or not?
Thank you, for your input. It certainly does seem to indicate they are aware of the basic issue itself. Do you work with many of these drawings?
Frank
RE: Tolerances of ISO 2768 cumulative or not?
In general, the recommended way to avoid accumulated tolerances is to use GD&T. I actually wrote an article about that awhile ago (before I took my new job here at SW). The article has ASME underpinnings, but the same rules are fairly universal, unless someone can find a specific statement in a specific standard to the contrary: http://www.fcsuper.com/swblog/?p=2440
Matt Lorono, CSWP
Product Definition Specialist, DS SolidWorks Corp
Personal sites:
Lorono's SolidWorks Resources & SolidWorks Legion
RE: Tolerances of ISO 2768 cumulative or not?
I think Dazza was the closest to explaining the intracacy here but I need to correct him/her. ISO2768 has a sliding tolerance based on the size of the dimension. For a medium tolerance class per the ISO, dimensions over 6mm but up to 30 mm have a tolerance of +/- .2mm. Dimensions above 120 but up 400 have a tolerance of +/- .5mm.
Therefore, the 200 dimension would have a tolerance of +/- .5mm. The 20mm dimension would have a tolerance of +/- .2mm.
Therefore if the drafter wanted the distance of the holes from the edge to be critical, he would have both holes dimensioned from the edge of the part (i.e. 200 +/- .5mm and 220 +/- .5mm).
If he felt the spacing between the holes were critical he would dimension the first hole at 200 +/- .5mm then the second dimension would be 20 +/- .2mm.
Therefore I feel that the drafter decides the tolerance stackup. Nothing else.
RE: Tolerances of ISO 2768 cumulative or not?
I went back and reviewed my copy of ISO 2768-2. It shows an example drawing (B.2) with a "this-means this" implied by the referencing of general tolerances ISO 2768-mH on an example part. The dimensions are shown with the normal implied +/- tolerances on the dimensions so I still believe this must accumulate if you are interested in the distance between (2) features not directly dimensioned. Say the end of the pin and the start of the taper both listed as +/-0.3mm the distance between the (2) should be 22.5mm+/-0.6mm.
Frank
RE: Tolerances of ISO 2768 cumulative or not?