I see this quite often, and I'm wondering what other people think about it. It's when a feature was modeled at, say, 2.125, then dimensioned on a drawing as 2.13. If you want the dimension at 2.13, shouldn't you model it at 2.13? If you round just due to default tolerances, why not just keep the dimension at 2.125 and add a tolerance to it?
Thanks Jim, but I'm not asking how to fix it. I'm essentially asking for opinions on best practices between models and drawings.
Model what you need; dimension the result.
we set high in the model and using tolerance to adjust the precision on the 2D dwg
Unless there is a must, we tried to round off the number as much as we can in the model and avoid some like (12.32548)
Also, there is no right/wrong here, Jason - just pick a standard practice which fits best for you
If I need a shaft that is 2 1/8 long, I will draw it 2.1250 long. If the shaft length is not critical to function, I may dimension it as 2.13 and let block tolerance (in this example .XX=.01) dictate the precision.
If the shaft length works equally as well if it were 2.1200 or 2.1400 or anything in between, I would not take the extra time to apply an individual tolerance to a three place dimension.
For me it is not Best Practice as much as it is Industry Accepted Practice that has been used since engineers chiseled on stone tablets.
Jason
Our policy here is to model to the exact nominal. If the drawing says 2.13, we expect the model to be 2.13000000. This is handy for making correct tolerance stacks and helps automated machining meet tolerances. We find it also helps prevent modeling errors when parts are edited later or used in an assembly.
Dwight
I tend to think this way myself, and I have seen issues caused from the dimensions not being the same.
Dwight Livingston has it right.
I teach to avoid losing any information to rounding. So, if you want 2 decimals to loosen up the tolerance, instead just add the tolerance you want.
For instance, if I want 2.125 +/- .03 and my sheet tolerances are
x.x +/- .06
x.xx +/- .03
x.xxx +/- .01
I dimension 2.125 +/- .030
Aside from being good practice, I have seen rounding cause manufacturing issues in machine design.
Exactly! There's nothing stopping someone from adding their own tolerance to a dimension.
A few places where I have worked, it really doesn't make a difference how I dimension or model, they round it to whatever they feel like and say, "Well I thought it was close enough" - I am glad for the educators such as Mike, teaching it the right way.
Is offset tolerances not the right way?
A Dayton Vessatile Punch Blank has a shank dimension of .3750 +.0002/+.0004
In response to all of this I give my 2 pennies worth.
Our model template is set to 0.0000 tolerance set for everything.
Our titleblocks have legends with our standard tolerance per operations. Welding, turning, mills, grinding and EDM.
As far as rounding goes if I have 2.1250 as my base dimension depending on the machine operation my tolerance and end use requirement will fluctuate between 1/16" to 0.0002 for my tolerance spectrum.
In the case of weldments it is drawn to needed size and dimensioned and toleranced accordingly so in all reality 2.1250 becomes 2.13 +/- .060 for that print.
I don't disagree with anyone's opinion on this so much as when you get to 2 decimal places that is a pretty loose tolerance as it is. I have never seen a print with 2.13 +.0004/-.0002 for instance. If I did I sure would want to know if that was rounded off or not.
In the end the whole conversation seems to be pointing to a bigger issue: EDUCATION. Or lack thereof.
Reading or drawing a print is becoming a lost art form, it seems more and more tolerances are misread and print dimensions are all over the map. I will admit I am kind of anal about how my prints look. I guess after years of seeing hand prints it makes me nostalgic for the craftsmanship that used to go into a drawing. Another topic for another day.
Another thing to remember is a piece of advice my drafting instructor gave us in school. "Remember, what you design with a micrometer will be laid out with a tape measure, marked with a chalk, and cut off with an axe."
While you are concerned about holding dimensions to .0001 in some cases, it is totally unreasonable and unnecessary in others. For example .0001 might work well when designing an hydraulic pump but 1/8" is much more reasonable for the 60 foot semi trailer it is going on. When designing and detailing it is also important to remember that it costs more money to hold that .0001" tolerance than the 1/8" so the "Best Practice" differs depending on the industry. Also if your shop is using a standard tape measure in fabrication you will not be looked at too favorably if you dimension using decimals and make people convert everything.
On the model I dimension everything to 3 place. When I need it 4 place (It also tells me it's critical), I make it a 4 place. Then the drawing I'll change what I need to 2 plc 3plc or 4plc
Thanks for all of the replies. It's always interesting to see everyone's take on something.
The issue is that your drawings are set at 2 decimals while your model is set to 3 or 4. This would be a discrepancy between the model/ assembly template and the drawing templates. This can be set in the following location.
It can also be edited per dimension here.
Select the dim, select other, and select over ride units.