Limit style tolerancing vs bilateral 3

[pugap]

We are in the process of writing drafting standards, and one of the debates is in the method of tolerancing. The majority of our old drawings had ± tolerancing, with some limit style tolerances. The group working on the standards is heading towards standardizing on limit style dimensioning only. The reasoning is that when you are inspecting a part, you only care if you are outside the limits, so the nominal does not matter. The debate comes on the question of design intent and the cad geometry. A part with a .4998+.0001-.0005 tolerance has a different design intent than a .4999/.4993 toleranced part.
Looking for feedback.

 

[ctopher]

I agree drawoh

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

Drawoh-

The approach that I like, for the example you show, is do my cad designing at an easy .500 because at that point I am more concerned with arranging the parts than what their tolerances will be. At the detailing stage, with Autocad or Solidworks you can select the tolerance method that the company likes and just plug in the right values to make it come out. The part will still be .500 in the model.

 

[drawoh]

EngJW,

Where were you for thread404-101081?

I prefer to model to nominal dimension too, rather than to MMC. It simplifies top down design in SolidWorks, and it gives me more control over the tolerances on the drawing. If I want to switch from an RC4 fit to an RC5, all I do is change two tolerances on the drawings.

JHG

 

[ajack1]

The last two posts are interesting. In America is it still a recognised standard to tolerance holes and shafts with numbers and letters say H7 for holes and h7 for shafts?

If so do you still model or draw to mean, even although the hole might have limits of +0.0003 to +0.001?

 

[ctopher]

yes
yes

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

 

[ewh]

Yes, it is still a recognized standard, but not used in all industries.
Yes.

 

[ajack1]

I guess I must be doing that wrong then. I figure that the whole concept is you call the hole or shaft up as a nominal size say 25mm even although it may have a plus/plus limit or a minus/minus limit so it could never be 25mm. The modelling system I use also has the option of adding a H7 for instance to “round” figures but not if you have made the shaft say 24.997.

I guess this system pre-dates CAD and I should rethink the way I work.

 

[EngJW]

Ajack1- I don't think you are wrong. I would make it 25mm and let the drawing handle the tolerance, as Drawoh also has said.

If later on you decide to change the tolerance, you just change it on the drawing and would not have to go back and change your layout or model.

 

[drawoh]

ajack1,

ASME Y14.5M-1994 gives you the option of using the ISO tolerance codes to specify metric tolerances. They are very clear about the metric.

eg. 30 f7.

This will not work in any CAD sofware I know of, unless you model to nominal size.

JHG