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IT# as a tolerance on feature control frame 1

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SeasonLee

Mechanical
Sep 15, 2008
918
Please look at the IT# on Runout (and parallelism) callout, if I am not wrong, the IT# should be International Tolerance Grades, can we just specify the IT# as a tolerance on a FCF? It’s an ISO print.

Thanks for all comments

SeasonLee
 
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I would think so. SKF & FAG do it that way right now.
 
It is International Tolerance Grade. Tolerancing tables are still considered "legal" in the standard, however I don't support the practice. I favor real numbers on the drawing, without having the user go to a supplementary resource to find values .... too much chance of error.

Jim Sykes, P.Eng, GDTP-S
Profile Services TecEase, Inc.
 

IT grades are used routinely in titleblock tolerances and general notes as in “UNLESS OTHERWISE SPECIFIED HOLES ARE H10, SHAFTS ARE h10, OTHER +/- (IT10)/2”.

But that implies direct tolerancing. To figure actual value of the tolerance you have to have dimension.
Your parallelism applies to surfaces that are 3.1/2.9 mm apart; assuming this is the dimension in question, related IT tolerance is 0.005.

For runout the definitive dimension is probably 16.1/15.9. IT6 tolerance for this size is 0.011, but it may be less obvious – should we use dimension that describes the size of the feature or some dimension that is placed in the direction runout is measured?

All together looks like general case of laziness to me because it doesn’t really save you time in the way general tolerances do and also might possibly create confusion on the shop floor. (And I have seen IT grade being mistaken with pitch of the thread :))
 
CH,
May I know where do the values 0.005 and 0.011 come from? Which standard are you using?
 
Because, as far as I am aware, ISO 286 defines tolerance grades for linear sizes and not for geometrical tolerances.
 

This is why people on this thread, OP included, consider the drawing confusing.
I wonder if MechNorth could elaborate on following statement:
MechNorth said:
Tolerancing tables are still considered "legal" in the standard
 
I was once told German shop people carry a card with the IT tolerances in their pocket and are supposed to be able to calculate them for themselves; of course, those people are considered craftsmen. I would love to know if that is true. Expecting people to be able to achieve a higher level is not a fault in the system, in my book.
 

I have nothing against highly qualified shop people.
I guess we are in slight disagreement if ISO 286 is strictly limited to tolerancing features of size.

To the best of my understanding, ISO 286 is broken into 2 separate books.
ISO 286-2 is providing “system for tolerances on linear size … suitable for features of the following types: 1) cylinders; 2) two parallel opposite surfaces.”
ISO 286-1 provides necessary pre-requisites.

Unfortunately I am not in possession of actual book, so I can only draw my opinion from the following Abstract:

ISO 286-1:2010 Abstract said:
ISO 286-1:2010 establishes the ISO code system for tolerances to be used for linear sizes of features of the following types: a) cylinder; b) two parallel opposite surfaces.
ISO 286-1:2010 defines the basic concepts and the related terminology for this code system. It provides a standardized selection of tolerance classes for general purposes from amongst the numerous possibilities.
Additionally, it defines the basic terminology for fits between two features of size without constraints of orientation and location and explains the principles of “basic hole” and “basic shaft”.

The phrase “standardized selection of tolerance classes for general purposes” looks suspicious to me. I have to agree that IT grade by itself is NOT a tolerance, so it cannot be used with linear sizes anyway (until accompanied by fundamental deviation).

Is anyone out there in the possession of complete book (possibly 2010 version) to clarify how far “general purposes” will extend?
 
I believe the "7" part of say "H7" is an IT7 grade of machining quality.
 

You are right.

If you look at OP’s drawing, it says IT5 without “H” part.

That’s what I said before, IT grade without fundamental deviation (that is H, g, whatever) is not enough to specify TOLERANCE you can apply to feature of size.

As you mentioned, IT grade by itself may be used to specify tolerances associated with manufacturing process ("machining quality"), so here comes “general purpose” use.
 
CH,
You are correct, a stand alone IT grade for linear size is not enough. It only defines the amount of tolerance allowed (depending on the nominal size interval).
Additionally there has to be a fundamental deviation identifier (like h or H) in order to define how the tolerance is disposed "around" the nominal dimension.

However theoretically in case of runout or parallelism geometrical tolerance you would not need to have the fundamanetal deviation identifier because the characteristics do not have their nominal values. Thus looking from this perspective a stand alone IT5 or IT6 could be OK, but...

The problem is that these IT grades shall not be applied to geometrical tolerances at all. Linear size tolerances are something completely different to geometrical tolerances. It is like comparing apples with pears. Notice also that ISO 2768-2 for general goemetrical tolerances mentions nothing about IT grades. Curious why?
 

That's exactly the question: Is using IT grade for calculating geometric tolerances explicitly forbidden, and if it is, then where?

When we say "this process (say, grinding) produces parts to IT4", does it explicitly mean "size" or flatness/parallelism may be implied as well?
 
It is already used in industry as I mentioned, before. Please state where it is forbidden, my current understand of it would say that is exactly what it is intended for.
 
Sorry, if I didn’t make my opinion perfectly clear before: I DON’T LIKE IT.
But I couldn’t shake off the feeling I’ve seen it before somewhere.
The enclosed picture is taken from book printed far away and long ago :). It doesn’t prove anything, just hints that sometime someplace it was OK.
This is why I am so interested in hard evidence, like quote from current standard, to end it once and for all.
 
 http://files.engineering.com/getfile.aspx?folder=d0930ba4-8bf7-4954-86a1-fafedd1396bd&file=Capture.PNG
CH, Frank,
Simply go to clause 6.1 of ISO 1101:2012 or 1101:2004. It states that second compartment of tolerance frame specifies: "the width of the tolerance zone in the unit used for linear dimensions and complementary requirements (see Clauses 7, 8, 10, and 12 to 16)." Even 1983 edition of the standard stated the same. Is it enough?
 
That’s exactly what IT grade does: it specifies “the width of the tolerance zone”! They are made for each other. :)
 
No further comments, CH.
Using this logic I can easily say that "K7" can be placed instead of profile tolerance value in tolerance frame. 7 is for tolerance grade, so for tolerance zone width. K is for how the tolerance zone is distribute around nominal value (theoretically exact profile). Makes sense? Try to prove it doesn't :)
 
CH, Y14.5-2009, "2.2.1 Metric Limits and Fits" allows the use of tolerance tables. Doesn't endorse it though, just allows it. Thus my "legal" annotation. I don't endorse their use on the drawing because unfortunately the bulk of manufacturing facilities don't have the abilities needed to understand them, and the risk of misreading or incorrectly recording the value is high. Also, for Limits/Fits, CAD models are usually created at the nominal (e.g. 12mm for 12H7) size and the limits of size may not even be in the range specified by the tolerance class; as a result, FEA and CNC from model are immediately wrong. It's drafting laziness.

Jim Sykes, P.Eng, GDTP-S
Profile Services TecEase, Inc.
 
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