Positional Tolerance
Positional Tolerance
(OP)
We have a discrepancy between options about the positional tolerance of a drilled hole in a cast part. I would appreciate anothers opinion. The drawing calls out the following tolerance (as closely as I can imitate the actual call outs, I wish I could attach a drawing):
4 HOLES (diameter symbol)9.0 +-0.2
(control frame)|(position symbol)|(diameter symbol)0.3|A|B|E (end of control frame)
CHAMFER (diameter)10.2 +-0.2 * 45
The horizontal and vertical linear dimensions (for one hole for example) are marked as 29 (horizontal from center) and 50.23 (vertical from center). There is no specific tolerance called out on these dimensions. The title block also has a note about unspecified tolerance on linear dimensions equal +-.2.
What linear tolerances apply to this 9mm hole?
Thanks in advance,
Steve
4 HOLES (diameter symbol)9.0 +-0.2
(control frame)|(position symbol)|(diameter symbol)0.3|A|B|E (end of control frame)
CHAMFER (diameter)10.2 +-0.2 * 45
The horizontal and vertical linear dimensions (for one hole for example) are marked as 29 (horizontal from center) and 50.23 (vertical from center). There is no specific tolerance called out on these dimensions. The title block also has a note about unspecified tolerance on linear dimensions equal +-.2.
What linear tolerances apply to this 9mm hole?
Thanks in advance,
Steve





RE: Positional Tolerance
RE: Positional Tolerance
Best Regards,
Heckler
Sr. Mechanical Engineer
SW2005 SP 5.0 & Pro/E 2001
Dell Precision 370
P4 3.6 GHz, 1GB RAM
XP Pro SP2.0
NVIDIA Quadro FX 1400
o
_`\(,_
(_)/ (_)
Never argue with an idiot. They'll bring you down to their level and beat you with experience every time.
RE: Positional Tolerance
RE: Positional Tolerance
RE: Positional Tolerance
Have you had any formal training in the area of GD&T?
RE: Positional Tolerance
1) title block doesn't apply whenever someone goes to the trouble of specifying a tolerance, and
2) no one 'accidentally' adds a positinoal tolerance callout.
I know, never say never, but this would be my starting point.
Things that supoprt it are heckler's points about rfs and the usual notes in a tolerance block that qualify when the block is applied, and the fact that the positional callout is tighter than the tolerance block.
RE: Positional Tolerance
RE: Positional Tolerance
I would make it a separate callout.
Chris
Systems Analyst, I.S.
SolidWorks 06 4.1/PDMWorks 06
AutoCAD 06
ctopher's home (updated 10-27-06)
RE: Positional Tolerance
RE: Positional Tolerance
Best Regards,
Heckler
Sr. Mechanical Engineer
SW2005 SP 5.0 & Pro/E 2001
Dell Precision 370
P4 3.6 GHz, 1GB RAM
XP Pro SP2.0
NVIDIA Quadro FX 1400
o
_`\(,_
(_)/ (_)
Never argue with an idiot. They'll bring you down to their level and beat you with experience every time.
RE: Positional Tolerance
If you are drawing to ASME Y14.5, nothing has changed in regard to simple drawings. It is still standard practice to include the minimum number of dimensions needed to make the part. A good drawing is a concise drawing. Be it a board drawing or a CAD drawing, this still applies.
RE: Positional Tolerance
If the purchasing agent knows something about GD&T, just showing him the situation should be enough without having to provide concrete evidence for your position.
RE: Positional Tolerance
ewh, of course drawings should still be simple and concise. My point was that it is incredibly easy these days to overdo it. It takes 10 seconds to add a dimension. It used to take a few minutes, more if you had removed the drawing from your desk. In the time it now takes to dimension and add callouts, you used to have plenty of time to consider the necessity of a note, callout, or dimension. That time and incentive to think was built in. Now, it isn't. Usd to be learning to draft took a big time investment. Now the time to produce working drawings is much, much shorter. This is one reason why, in my experience at least, we have a lot of drawings being dranked out for production by a bunch of folks with a less than perfect understanding of drawing, never mind GD&T.
RE: Positional Tolerance
RE: Positional Tolerance
are you saying that you still have checkers? Never mind good ones...
I have to badger ppl to check my drawings around here.
RE: Positional Tolerance
RE: Positional Tolerance
4 HOLES (diameter symbol)9.0 +-0.2
(control frame)|(position symbol)|(diameter symbol)0.3|A|B|E (end of control frame)
The center of the hole (9.0) has to be inside a circle of 0.3 where the center of that circle is located at 29 (horizontal from center) and 50.23 (vertical from center) - This assumes the 29 & 50.23 are BASIC(read as "theoretically exact, ideal, nominal"), there should be a note somewhere that states UNTOLERANCED DIMENSIONS LOCATING TRUE POSITION ARE BASIC.
If you want to use plus minus dimensions you would divide the tolerance by 2 and multiply by 0.707: (0.3/2)* 0.707 = 0.11 so the tolerance is now +/-0.11
Doing this would not violate the customer specification of being within 0.15 of the BASIC or nominal hole location.
RE: Positional Tolerance
These are some factors tneed to be taken into account when specifing the tolerances for the holes. with that being said.......
If Y14.5 is referenced.the 0.3 in the callout will be the diametric tolerance.
RE: Positional Tolerance
RE: Positional Tolerance
This is not necessarily an uncommon occurence, sadly.
RE: Positional Tolerance
Heckler had it right; positional tolerance trumps title block tolerance. Positional callouts are basic dimensions, and the title block tolerance only applies if no other tolerance is specified. The positional tolerance called out is tighter than the title block tolerance, not looser. danap's post on the location of the hole is a good explanation. However, you don't need a note saying that the positional dimensions are basic. They are always basic. The tolerancing comes from the positional callout.
RE: Positional Tolerance
In this situation, I agree. However, it is not a good idea to use a blanket statement like "you don't need a note saying that the positional dimensions are basic. They are always basic." Wrong. It may be acceptable to interpret this drawing in this manner, but it is still a drawing mistake. For a correct drawing, you DO need either a note or a block around basic dimensions. Without those, there is ambiguity regarding the dimensions.
RE: Positional Tolerance
RE: Positional Tolerance
A dimension locating the position of a feature is a basic dimension. That is part of the definition of position. If the dimension is not basic, then it does not give you the position of the feature.
RE: Positional Tolerance
RE: Positional Tolerance
1) You need to have a note on the drawing, or in documentation referenced on the drawing, which clearly states what standard the drawing adheres to. Without that, there is no way to interpret a drawing that mixes linear and geometric tolerancing. In this case, if the drawing is from Europe, it's probably ISO, so assuming ASME/ANSI interpretation could be incorrect.
2) A positional tolerance can only be used on a Feature of Size, which this hole is (it has linear size tolerances).
3) A positional tolerance requires basic dimensions to locate the feature wrt the datums. There must be (i)a BASIC box around the dimension, or (ii) a note on the drawing to the effect of "NON-TOLERANCED DIMENSIONS ARE BASIC" (which disallows the use of a linear general tolerance block), or such similar statement must be contained in a document referenced on the drawing. As neither of these are indicated on the drawing, those dimensions cannot be interpreted as BASIC (I know it seems logical to assume they're BASIC because of the positional tolerance, but that's not a path of logic that a court is likely to take).
4) Linear general tolerances in the title block need a statement such as "UNLESS OTHERWISE SPECIFIED", or else they are meaningless, and inapplicable on the drawing.
5) Absent any statement or reference to those locational dimensions being basic, and since they do not have a linear tolerance on them, you would be compelled to use the general tolerances in the title block (providing the "UNLESS OTHERWISE SPECIFIED" note from #4 is invoked)
Intuition is a great thing, but the courts don't tend to buy it as an acceptable reason for doing something. Where the work doesn't follow the rules, courts will tend to follow a line of logic that best follows the rules.
Now the reality; your client wants you to make parts. Period. If they're not willing/able to correct the drawings, your next best solution is to ask which interpretation they want, and then send them an e-mail with detailed minutes of your conversation. If you have the time, YOU should create a drawing with the correct GD&T on it and send it to them.
Jim Sykes, P.Eng, GDTP-S
Profile Services
CAD-Documentation-GD&T-Product Development
www.profileservices.ca
RE: Positional Tolerance