IT# as a tolerance on feature control frame
IT# as a tolerance on feature control frame
(OP)
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
Thanks for all comments
SeasonLee





RE: IT# as a tolerance on feature control frame
RE: IT# as a tolerance on feature control frame
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: IT# as a tolerance on feature control frame
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
RE: IT# as a tolerance on feature control frame
May I know where do the values 0.005 and 0.011 come from? Which standard are you using?
RE: IT# as a tolerance on feature control frame
ISO 286 and ANSI B4.2 should give you the same numbers.
Why?
RE: IT# as a tolerance on feature control frame
RE: IT# as a tolerance on feature control frame
This is why people on this thread, OP included, consider the drawing confusing.
I wonder if MechNorth could elaborate on following statement:
RE: IT# as a tolerance on feature control frame
RE: IT# as a tolerance on feature control frame
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:
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?
RE: IT# as a tolerance on feature control frame
RE: IT# as a tolerance on feature control frame
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.
RE: IT# as a tolerance on feature control frame
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?
RE: IT# as a tolerance on feature control frame
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?
RE: IT# as a tolerance on feature control frame
RE: IT# as a tolerance on feature control frame
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
This is why I am so interested in hard evidence, like quote from current standard, to end it once and for all.
RE: IT# as a tolerance on feature control frame
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?
RE: IT# as a tolerance on feature control frame
RE: IT# as a tolerance on feature control frame
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
RE: IT# as a tolerance on feature control frame
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: IT# as a tolerance on feature control frame
For the parallelism callout IT5 wrt datum B, I think the basic reference dimension should be the max thickness 3.1mm; but for the runout callout IT6 wrt datum A, is datum A (M8) the basic reference dimension or the max Ø16.1?
SeasonLee
RE: IT# as a tolerance on feature control frame
And some more food to eat: why do you think that 3.1 mm should be used as a reference for parallelism tolerance value? Table 1 in ISO 286-1 defines tolerance grades depending on nominal dimensions, not the max values. Now my question: what is the nominal for 2.9 - 3.1 dimension? Are you clearly able to answer without any assumption? Is it 2.9? 3.0? 3.1? Depending on the nominal value chosen the dimension may fall in different range of dimensions: less than 3 or 3-6, and the allowable tolerance value may change.
RE: IT# as a tolerance on feature control frame
Allow me to introduce use of IT grade in geometrical tolerance done by no one else but ISO itself.
Which basically proves my point that IT grade is better left for general note.
If you have catalog or standard listing 3000 bearings, it is better just mention IT9 once, than take 3000 chances to make a typo.
So, to pmarc: there are no apples and pears, just fruits. To MechNorht: allowed, but really hard to find. To fsincox: thank you for your support. To SeasonLee: I’d rather put actual values into FCF. As you can see using IT next to dimension that already has tolerance may be confusing.
RE: IT# as a tolerance on feature control frame
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: IT# as a tolerance on feature control frame
RE: IT# as a tolerance on feature control frame
At the end of ISO 286-1:2010 and ISO 286-2:2010 there is a GPS matrix model which shows a position of both standards in the matrix, or in other words, which chains of the matrix are influenced by the standards (see link). Only "Size" chain is influenced in chain links 1 and 2. No impact on Orientation, Location, Runout controls, etc. That really does not surprise me...
http://files.engineering.com/getfile.aspx?folder=d...
RE: IT# as a tolerance on feature control frame
The changes ISO went thru during 2010, 2011, 2012 so far look like they just started the snowball rolling. I won’t be surprised if in the future several standards will be edited to harmonize with the whole “matrix” model. So, whoever will see something, please let us know