Tolerances on Mech Drawings 9

[Metalguy]

Many years ago when I took a HS course in mech. drawing, the correct way to show dimensional tolerances was to have the same number for the + and - eg., if you want something between 7 and 9 in. long you used 8+/- 1 (I can't figure out how to get the + directly above the - here).

But the foreign co. (Italian) I'm working at has a weird (to me) way of showing them. They'll use anything from 7 +2/-0 to 9 +0/-2. They've even used (I am NOT making this up!) something like 6 +1/+3!!! This is not a small company, but a fairly large but old manufacturer of BIG equipment.

Anyone seen anything like this before?

 

[dparker213]

Your points are all well taken. One point I should have mentioned really has to do with avoiding my own errors. My references on tolerances calls out nominal sizes with tolerances shown as I had posted, i.e. 10 -.0025/-.0037".

If I was asked to dimension ouside of the standard it would require me to do a few calculations to convert the standard to a different system. This in itself could introduce error, especially towards the end of the day when I too am tired and grumpy . I appreciate that there are modelling systems that cannot handle this kind of tolerance, but I am a bit surprised that the newer software cannot.

 

[ewh]

Maybe there are programs to handle it, but I am not familiar with them. I do know that many do clearance analysis, but with a part modeled with a +/+ or -/- tolerance, the nominal size never exists. Thus if modeled at nominal, a clearance analysis would not give you realistic results. I guess you could edit the dimension on the drawing to reflect the nominal size, but this is usually not a good idea. The dimension would lose it's associativity to the model, and would add another detail that anyone editing the model would have to be aware of. I think that that would present more of a chance for error than converting the tolerance.

 

[ajack1]

I see your point ewh, the software we use does allow you to add hole/shaft sizes h7 etc, but only if you model to the nominal size, it would allow it for example on 10 but not 9.996. This obviously has plus and minus, no pun intended, as you can use the system that is standard and most (at least in automotive industries are used to) but does mean that everything is size for size, so as you rightly say does not show a clearance or drive between the two.

None of this really matters if everyone is on the same system, but becomes more of a problem when information goes through a translator, as text and annotation seem to be two of the things they (especially IGES) do not handle that well. As far as I am aware no system allows you to model intent, only text does this.

I wonder if we will have new standards in a couple of years as 3D becomes more and more the norm, probably driven by the largest supplier I guess Catia or Solid works or whether this will still be being debated in a couple of years time?

 

[ctopher]

ewh,
Curious on your opinion. For example, if a part is dim at 9.877 +000/-.002, would you model it at 9.877 or 9.876? I would use 9.876.

Chris
Sr. Mechanical Designer, CAD
SolidWorks 05 SP3.1 / PDMWorks 05
ctopher's home site (updated 06-21-05)
FAQ559-1100
FAQ559-716

 

[ewh]

ajack,
I hope that softwares and translators will support ASME Y14.41-2003 DIGITAL PRODUCT DEFINITION DATA PRACTICES in the near future. As it is, one of our customers (a major aeronautics company) has sent us files that approach ASME Y14.41, and almost all of the part info is in the 3D file. What isn't in the part file is in text files. All of the annotations and dimensions were converted to curves, and there are no drawings. They look quite confusing (and intimidating) at first, until you understand how they are structured.
I don't see this standard getting a huge following for quite awhile, but it can streamline design and manufacture.