Tolerancing style
Tolerancing style
(OP)
I thought this might have been discussed before in this forum but I couldn't find the thread.
Under which circumstances are limits vs ± vs +x/-y tolerances used or more desirable?
Under which circumstances are limits vs ± vs +x/-y tolerances used or more desirable?





RE: Tolerancing style
When I put the tolerance that way, I suppose it identifies it to our toolmaker as needing to be that number without being too restrictive, and he usually builds the tool right at z.
If I went somewhere else, or we had a different tool maker, I'm sure our little "gentleman's agreement" might not work, but that seems to work for us right now.
RE: Tolerancing style
Matt Lorono
CAD Engineer/ECN Analyst
Silicon Valley, CA
Lorono's SolidWorks Resources
Co-moderator of Solidworks Yahoo! Group
and Mechnical.Engineering Yahoo! Group
RE: Tolerancing style
RE: Tolerancing style
In the UK where I worked we normally did limits, e.g. .995 to 1.005. Here in the States the place I work mostly uses +- e.g. 1.000+-.005.
I'm not sure there's any widely accepted method of determining which to use. +- Dimensioning is perhaps better for manufacturing, as it gives them a nominal/mean to aim for without calculation. Limits is perhaps better for inspection since it's obvious if a measurement is more or less than the limits without calculation.
However, the calculation is so trivial I'm not sure it's much of an argument, but I suppose for every calc there's a chance of a mistake.
Now as regards non symmetric +- dimensions, the most obvious example I can think of is typically drill tolerances. Depending on size these will be +.005 -.001 or similar, essentially the nominal size of drill tends to produce an oversize hole. This Tolerancing method is to allow easy selection of standard size tools while allowing for the fact that the hole will be oversized. The same idea may be extrapolated to other processes etc, perhaps some types of semi stock dimensions or the like.
I know some people use non symmetric to imply they'd rather be at one end of the range than the other. For instance if they'd like to have a .5 dimension but if push comes to shove they'd accept up to .51 then they'll dimension it .50 +.01 -.00 rather than .505 +-.005 with the hope that most of the parts will end up nearer .5 than .51. However, to the best of my knowledge this idea of preferring one end of the tolerance zone is not supported by the standard, if the entire tolerance range is acceptable then it's acceptable, if not then a tighter tolerance should be specified.
KENAT,
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RE: Tolerancing style
Guess the debate here will go on.
Thanks all for your insights.
RE: Tolerancing style
In general, I like to keep all dimensions nominal with symmetrical +/- tolerances. I only use asymmetric tolerances when it is absolutely necessary such as the ISO system of press fits. I think asymmetric tolerances are way overused. Most manufacturing process these days are going to try to be mean centered to keep their Cpk high.
The places that I have seen that prefer limit dimensions have production operators making manual measurements. With limit dimensions the operator does not have to make any calculations to see if the part is in limits. As a design engineer I hate them as I have to constantly calculate the nominal. But I make fewer calculation mistakes than the operators so I understand the reasoning.
RE: Tolerancing style
Some people argue here it is all the same if you use (1.003/.997) or 1.000+.003/-.003 or .998+.005/-.001. I would like to think if I use +.005/-.001 they are targeting to the .998 dim I have on the print and not the 1.000 average of the tolerance.
RE: Tolerancing style
KENAT,
Have you reminded yourself of FAQ731-376: Eng-Tips.com Forum Policies recently, or taken a look at posting policies: http://eng-tips.com/market.cfm?
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Tolerancing style
For me, it all depends on the end user, the functionalty of the drawing, and the information I want to show off.
In CAD, if your dimension is Z±x or Z+x/-y, presumably, you modeled to dimension Z. Often, someone needs to know this. The Z dimension is your nominal design intent.
I know that machinists on a lathe like to see limit dimensions. This is probably more convenient to inspectors too.
RE: Tolerancing style
This drives some people nuts, particularly when working with standard fits and matching to unilateral dimensions from purchased components. We thus convert limit and unilateral dimensions to "target" dimensions with an equal (or unequal if an odd number) tolerance.
The only advantage I see is that it prompts you think about your tolerance, and not just copy something.
RE: Tolerancing style
Peter Stockhausen
Senior Design Analyst (Checker)
Infotech Aerospace Services
RE: Tolerancing style
RC4 fit on a 2-1/8" diameter...
Shaft (f7): Ø2.125 -.0012/-.0024 = Ø2.1238/2.1226
Hole (H8): Ø2.125 + .0018/0 = Ø2.2168/2.2150
Note how the limit dimension does not show that the shaft diameter is outside the limit dimension values. If you explicitly show the as-modeled diameter, there is an opportunity for someone, including you, to sanitize the model for fabrication.
The jobbing machine shops I deal with read drawings carefully. In my one and only experience with a rapid prototyper, I do not think they even looked at my drawings. The next time I do rapid prototyping, I will change my CAD procedures.
RE: Tolerancing style
RE: Tolerancing style
We do very little MBD, so all tolerances are covered on the drawings. I except to see that changing, however.
Inspectors I've worked with prefer limit dimensions, and I can see machinists particularly lathe operators preferring unilateral dimensions.
RE: Tolerancing style
So what we as designers have to realize is that there are other people that will be making our parts. They may or may not take the time to double check their inputted values into their calculator or just simply make a mistake. Whatever happens, the designer should be responsible for ensuring that the parts are toleranced for manufacturability. So if that means using limits or (a)symmetrical tolerancing then it should be done.
So what I'm saying is that the parts should be toleranced in a way that makes it easiest to manufacture but it should also hold up to the intended design intent.
RE: Tolerancing style
Group - make it nominal in the model and dwg, and use +/- tol, that is easier for everybody except the up front designer. People get hung up on the 2 1/8 dia being nominal, and in this case, it isn't.
Drawoh's example would be:
Shaft 2.1232 +/- .0006
Hole 2.1259 +/- .0009
Easy to read that the shaft is going to go in the hole. Easy to make to model and/or dwg, easy to inspect.
RE: Tolerancing style
Chris
"In this house, we obey the laws of thermodynamics." Homer Simpson
RE: Tolerancing style
Oh well. For once, I did not do the calculations in my head. I used a calculator. Bad typing made up for it. :(
The problem with the Ø2-1/8" is that I normally model to nominal size, then apply tolerances. In SolidWorks, I can set tolerances to the ISO tolerance codes used by the ANSI tolerances. If, half way through the design, I want to change from RC4 to RC5, I go to the fabrication drawings and change the ISO tolerance codes. The model is Ø2.125".
The jobbing shops I go to seem to get this right, so I assume they are reading my drawings. As noted above, if I need rapid prototypes or castings, I intend to wind up with everything modeled at median size, with ± tolerances.
RE: Tolerancing style
RE: Tolerancing style