Tolerance

[User2012]

I'm wondering how I can give the right tolerance (in either Autocad or SW) to a 3D model before its printing! Any ideas .... Just Help!

[ReMark]

Tolerance in AutoCAD is a function of one's Dimension Style. It is merely a notation. Could you perhaps be referring to "precision" (at least as far as AutoCAD goes)?

[User2012]

I'm getting a little bit confused! Before this, I thought that "Precision" in cad disscussions has been just about the number of decimal places!

Let's put it that way: My question is what step I should take before sending my 3D model for the rapid prototype! I assumed that I should set the dimension tolerance (in Autocad: Dimension> Tolerance) (in SW: DimXpert). Should I go with some optional tolerance value! or there is a specific way to do this!

For instance, how should I know the tolerance assigned for some fragile objects (e.g., hinge, latch...) is gonna be appropriate! is this related to the "Material" type? Any experience or suggestion! Thanx!

[Patrick Hughes]

I'm not familiar with SolidWorks or for that matter more recent AutoCAD releases, even if you are able to assign a tolerance to a solid feature (perhaps a constraint?) the tolerance would not have an affect on a rapid prototype 3D print. The best you can do is manually apply maximum and minimum material sizing to the actual model so the parts would fit after printing. Probably not a bad thing to do anyway since the 3D printing is not going to be an exact size.

Example a female part might be 1.02 +/- .01, model the part at 1.03
mating male part .98 +/.01, model the part at .97

BTW that's a lot of slop.

[User2012]

I'm not familiar with SolidWorks or for that matter more recent AutoCAD releases, even if you are able to assign a tolerance to a solid feature (perhaps a constraint?) the tolerance would not have an affect on a rapid prototype 3D print. The best you can do is manually apply maximum and minimum material sizing to the actual model so the parts would fit after printing. Probably not a bad thing to do anyway since the 3D printing is not going to be an exact size.

Example a female part might be 1.02 +/- .01, model the part at 1.03
mating male part .98 +/.01, model the part at .97

BTW that's a lot of slop.

Got it! Thanks a lot for your good info.

[JD Mather]

Let's put it that way: My question is what step I should take before sending my 3D model for the rapid prototype!

That is an entirely different question than your original questions.
But this question begs several other questions.

Will you be making the part yourself on your own RP equipment?
Are you printing 1:1 scale?
Will there be relative motion between parts when assembled?

You have to model the actual tolerance allowance - not just assign a dimensional tolerance (I will explain in a minute).
I always output my models at highest precision (facetres = 10 in AutoCAD). Filesize is not an issue for me.

We have a variety of RP machines where I work.
Each machine is a bit different - requiring different allowances on different machines and different allowances on different features.
Orientation of the part for printing has an effect on final dimensions.

Did you model the parts to exact perfect sizes to begin with (common error), for example, a 1mm pin will not fit in a 1mm hole. In the real world some pins will be a little larger and some a little smaller than 1mm, same for the holes (depending on the dimension tolerancing method. You need to model such that the largest pin will go into the smallest hole plus allowance for clearance.

For your hinge I might model something like .025" between leaf lengths and at least .03 clearance between pin and holes (.015 per side). You can adjust based on experience.

If you are sending this out to a printing house you should contact them about appropriate allowances and processes.

Don't expect machined part tolerances/fits.

If you scale down (to fit RP machines build envelope) that opens up a whole 'nother can of worms. Especially for parts that are already thin. You might have to model much differently than the real parts.

Bottom line - you model these tolerance allowances into the part understanding the RP processes aren't very precise. In modern programs like Autodesk Inventor or SolidWorks you can and model feature tolerances and have the software rebuild the part at max/min/median values. Unless you do this a lot - it takes about as much time to set up the tolerances as it would to simply make the model changes yourself. If you anticipate doing this a lot - set up templates with the tolerances built in and for the specific processes based on actual experience.

[User2012]

That is an entirely different question than your original questions.
But this question begs several other questions.

Will you be making the part yourself on your own RP equipment?
Are you printing 1:1 scale?
Will there be relative motion between parts when assembled?

You have to model the actual tolerance allowance - not just assign a dimensional tolerance (I will explain in a minute).
I always output my models at highest precision (facetres = 10 in AutoCAD). Filesize is not an issue for me.

We have a variety of RP machines where I work.
Each machine is a bit different - requiring different allowances on different machines and different allowances on different features.
Orientation of the part for printing has an effect on final dimensions.

Did you model the parts to exact perfect sizes to begin with (common error), for example, a 1mm pin will not fit in a 1mm hole. In the real world some pins will be a little larger and some a little smaller than 1mm, same for the holes (depending on the dimension tolerancing method. You need to model such that the largest pin will go into the smallest hole plus allowance for clearance.

For your hinge I might model something like .025" between leaf lengths and at least .03 clearance between pin and holes (.015 per side). You can adjust based on experience.

If you are sending this out to a printing house you should contact them about appropriate allowances and processes.

Don't expect machined part tolerances/fits.

If you scale down (to fit RP machines build envelope) that opens up a whole 'nother can of worms. Especially for parts that are already thin. You might have to model much differently than the real parts.

Bottom line - you model these tolerance allowances into the part understanding the RP processes aren't very precise. In modern programs like Autodesk Inventor or SolidWorks you can and model feature tolerances and have the software rebuild the part at max/min/median values. Unless you do this a lot - it takes about as much time to set up the tolerances as it would to simply make the model changes yourself. If you anticipate doing this a lot - set up templates with the tolerances built in and for the specific processes based on actual experience.

I just passed the model to a RP company. I need to investigate more in order to find the right answer for the questions that you brought up and then to follow the steps that you pointed out here! Thanks a lot!

[wieyink]

thank you this makes me a new science

[JD Mather]

Bottom line - you model these tolerance allowances into the part understanding the RP processes aren't very precise...

I should have also noted that all of the considerations discussed also apply to machined parts. We can't manufacture perfect parts - therefore we need to keep in mind the realistic deviations from ideal that will still allow the assemblies to be assembled.

I see way too many parts modeled in CAD software to exact dimensions - ex 10mm hole for 10mm pin, and this is only the size tolerance, the position tolerance must also be considered.
Stack-up of tolerances must be considered.
Beginners who start to pay attention to tolerances tend to over-tighten tolerances worried that parts won't go together, specify the most generous tolerance that will still allow the product to function as intended.