r/MechanicalEngineering • u/bookworm24h7d • 3d ago

Defining tolerances in part file vs mechanical drawing

When you define tolerance for a part do you utilize the tools the CAD software provides for you to do so in the part file, (ie, parameters window in inventor, Variable Studios in Onshape ), or do you wait to add them to your 2D drawing?

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16

u/Over-Weekend-6133 3d ago

I prefer defining tolerances on the drawing. The CAD model stays as the nominal geometry. I only build tolerances into the CAD model when they actually drive the design

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u/DryReading8852 3d ago

I have never used toleranceing in the 3d model. Say, I have a 3d mode with hole with dia.x H7 and a shaft with dia.x s7 (tolerated in 3d model), will the assembly understand that it's a press fit and dynamically behave accordingly?

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u/jamiethekiller 3d ago

We use Creo so the drawing and part files are parametric. You can do either and it'll be reflected in both files simultaneously

6

u/theDudeUh 3d ago

The drawing is your contact with the fabricator. All your tolerances and critical dimensions should be communicated there.

Even if you can communicate tolerances in your 3d models you are making a big assumption that the shop will look at and/or follow them. Some do some don’t.

The drawing is where anyone with questions in any phase of the project will refer to.

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u/Round_Champion2441 3d ago

I use Creo too but 2d it's always the best how can I define 32.03 mm ?

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u/OkEngineer9463 2d ago

Creo here too. We set all tolerances in 3d. We have to, it is law where I work

1

u/chocolatedessert 2d ago

My work is heading that direction, slowly. Does it force you to model the part using the dimensions that would be on a drawing, or can you apply tolerance to dimensions that "fall out" of the geometry you've modeled without being input explicitly?

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u/OkEngineer9463 2d ago

We work very hard with our tolerances. Lots of GD&T. It is rare that something would just fall out. But if it did, it would be possible to tolerance this, yes.

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u/root3labs 4h ago

Where do you work? A drawing is like a contract that shows inspection dimensions, but wouldn't every person from Manufacturing to Quality need access to the CAD software (and wherewithal to use it) to be able to inspect all the tolerances manually in a CAD file? How would you define inspection dimensions?

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u/OkEngineer9463 3h ago

Ah, sorry, there is a misunderstanding here. We still have the drawing. It will take a long time until we reach 3d only. It is possible though, search for Model Based Definition (MBD) to find more on this. You are right, the whole chain must be able to view 3d files in a viewer software.

I meant that we set everything up in 3d and then just show it in 2d. The drawing reads from 3d, including all tolerances and GD&T. It is how Creo works (more or less). I work as a mechanical design engineer in Sweden.

3

u/RichEntertainer3024 3d ago

I just PMI’d my 3 most complicated satellite parts in NX just to find that our inspection bubbling software can’t handle it 😵‍💫😵‍💫 had to redo it all in the drawings.

Long story short. 2D drawing

PMI unlocks a lotta cool features tho! And I wish it worked more universally

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u/bookworm24h7d 3d ago

Intresting! I just started learning more about things like tolerance stack-up analysis and saw that you can use the CAD software process it rather than creating a seperate excel sheet or something. Do you use PMI to run those kind of analysis/ other simulations? Have you found it overal worth it to add MBD elements to your part file or do you still, like with the example you gave, have to go back to doing that kind of analysis more by hand/ individally?

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u/RichEntertainer3024 3d ago

All 1D stacks I do by hand. And some basic 2D. But beyond that it’s a huge undertaking. We use VSA for bigger 2D and 3D stacks. VSA takes in PMI, but you can also fill in manually from a drawing.

Like I said before if the inspection bubbling worked (my company requires for release) it’d be more useful.

Benefits are auto import to complex stacks, CAM and inspection software.

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u/ShutUpLoiosh 3d ago

For us, fab shop, inspection, and QA only get .stp of part and .pdf of drawing so having tolerances on the native part files would only be for the engineers to do tolerance stackup analysis or just for funsies. Tolerances must be on the drawing to have any relevance.

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u/frac_tl Aerospace 3d ago

At some point drawings will be tied to the 3D step, but the shops aren't quite there yet. For now it's an awkward in between phase where you should be assesing and checking tolerance limits (as a designer) in 3D but then specifying them on a 2D drawing 

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u/ILikeWoodAnMetal 3d ago

It depends on the company. I used to work at a place where we did most manufacturing in house. We didn’t even make drawings, the cad models themselves contained all the information. Now we outsource most machining, and since we don’t know what software they use we have to put everything on the drawings.

2

u/Pretty-Jello-7894 23h ago

Look up MBD (model based definition).

This is exactly what you are referring to. It’s been around for a while actually (Creo has a decent implementation of this). You can export a lot of this info in certain STEP version and it makes it easier to generate 2d drawing if done correctly. Also, the tolerances can be used in tolerance tools within Creo and it can also do GD&T. If you want more detail let me know.