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Multiple (very different) parts, but with shared dimensions

marcus_8marcus_8 Member Posts: 3
edited April 2015 in Community Support
I am working on a detailed mechanical model which will have somewhere around 150 unique parts (I believe around 600 total parts). I am working from a set of engineering drawings. The designs were originally made for traditional machining, but I am planning to attempt a 3D printed version. I am doing the initial version for an FDM printer. The tolerances may need general adjustment later, and I also want to keep my options open to adjust all of the tolerances for a different (more accurate) type of 3D printing.

The parts have different fitting requirements. Some of them require a forced fit while others require a sliding clearance fit and various fits in between. An FDM printer can print these mechanical fit types, but they will need different tolerances than the drawings specify. The same goes for other types of printing.

With so many parts and different types of fits, is there any way I can define these various tolerance types (basically dimensions) in one place and alter them in one place, but have all of my parts in this document updated from the one set of values?

Also of note is that I am not a mechanical engineer, but a software engineer by profession so I may be using improper terminology or have some concepts wrong.

Best Answer


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    john_mceleneyjohn_mceleney Onshape Employees Posts: 57
    @mwu when you model in Onshape (and other 3D parametric systems) you typically model to the nominal dimension and place the tolerances on your drawing. If you're going to 3D print the parts, you may need to go through a bit of an iterative process to see what the system will actually produce (comparing the physical model vs the Onshape model). Once you have some of this empirical data you can modify the dimensions of critical dimensions and re-print the model.

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    marcus_8marcus_8 Member Posts: 3
    edited April 2015
    @John_McEleney  yes, I have been modeling to the nominal dimensions so far. I am unfamiliar with a method to place tolerances on the drawing in OnShape, but what Ilya mentioned is exactly what I'm looking for since I prefer to define difference tolerance values once and apply those in multiple places where needed.

    I do plan on there being a little bit of an iterative process getting parts to fit properly, but I hope to limit it after having printed a few simple mechanical fit tests I designed up quickly.

    Thanks for the help!
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    michael3424michael3424 Member Posts: 678 ✭✭✭✭
    I suspect that @John_McEleney was referring to specifying tolerances on a 2D drawing that is normally printed out.  Those aren't available for OnShape yet, but they are working on them and initial release might be as soon as June.

    FWIW, I've been using a $2k FDM printer lately for some mechanical models and have been finding that it is usually best to make holes a little undersize and drill or ream them out.  Also beware your STL export settings - if they are not fine enough you will get facets on curved surfaces rather than continuous curves.


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    kevin_quigleykevin_quigley Member Posts: 306 ✭✭✭
     Now this thread has sparked an idea in my head. If you design anything that has to interface with something else you need to specify tolerances. But 3D modelling systems are useless at this as you model to a nominal medium in the tolerance range. But what you really need to do is to see what effect the extremes of the tolerance has on the whole assembly, and the only way to do this is to model the parts at those extremes then iteratively examine the outcome.

    in SolidWorks we do this with configurations. We pick some key dimensions and configure models at the tolerance extreme. This is a bit of a tedious process but we set up sensors to flag up pass fail scenarios for the overall assembly. But it is still a manual process of turning configs on and off and checking.

    so, moving forward, with Onshape, could we not design to a nominal size, select key dimensions and have Onshape automatically create branches with the models at different tolerance extremes?

    Then we set up our sensor like pass faill scenarios and Onshape runs through the tolerance ranges, populating a table with each result, and flags up the fails. This would be a killer feature for many.

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    andrew_troupandrew_troup Member, Mentor Posts: 1,584 ✭✭✭✭✭
    + 1 for @Kevin Quigley 's suggestion
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    3dcad3dcad Member, OS Professional, Mentor Posts: 2,470 PRO
    edited April 2015
    Good idea @Kevin Quigley 

    How would you set up sensors in Onshape? Would you include collision detection?

    Tolerances and collision detection are difficult area when working with wood since it's difficult to tell the software that it's not a collision when nail or tapped screw contacts wood. Or if connection is based on 'tight fit'.
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