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Top-down workflow, holding frame for Parts

gauthier_östervallgauthier_östervall Member Posts: 24 EDU
This is not a question about a specific feature, it is a question about how I should organize my work in Onshape. I've read a couple of posts from pros here, which advocated the top-down approach. As far as I understand, this implies starting from the whole, then going into details. Some meant that the best was to start in the Assembly, then designing parts as needed. Some others mentioned "Skeletons", I guess the notion comes from other tools, but the principle is the same.

As a beginner, starting on Assembly is very difficult, foremost because the first projects you make tend to be single parts. I'll take this current project of mine as an example, here is a screenshot:

This is going to be an ergonomic keyboard, if everything works out as planned. The main feature is that the keys are placed in a concave well, and that it makes use of thumbs a lot more than a regular keyboard does.

Because these are the main features, and because this was what I thought could make or break the project, I started with these. It's been going well (!).

But it's a bottom-up approach. Now I need to put these two parts into a holding frame, and I'm not sure how I should go about doing that.

My questions are:

1. knowing the frame would be needed anyway, should I have start with that? The general form of the product? Since I was not sure how the shape of the parts would turn out, I felt that "feeling" the general shape would have been difficult. Maybe an early proof of concept on paper, or even in clay...
2. how do you start in Assembly if there are no parts to import?
3. with these parts above in two Part Studios, roughly placed in an Assembly, how would you create the holding frame part? Do I create a new Part Studio, import the existing parts to it and place them there just as in the Assembly above? Or is there a way to import the placement from the Assembly into a new Part Studio?

I'm trying to learn how to think right from the beginning, as well as solving what I have with as little pain as possible.


  • owen_sparksowen_sparks Member, Developers Posts: 2,660 PRO
    edited February 10
    Have a look at the help blurb for "Edit in context".  Essentially you'll be taking a snapshot in time of the geometry contained in your assembly and then showing that in whatever PartStudio you require.  You can then build off the assembly geometry as you see fit.  Frankly its harder to explain than it is to do so I'd recommend that you have a read and jump in and then shout if bits of it don't make sense.

    Owen S.
    Business Systems and Configuration Controller
    HWM-Water Ltd
  • john_mcclaryjohn_mcclary Member, Developers Posts: 2,524 PRO
    Top Down vrs Bottom Up...

    Do what is easier for you to understand.
    Where I work we always do bottom up, only using the top-down tools as time savers here and there. But not fully relying on them.

    We tend to copy parts from job to job and make tweaks to create a unique part. So they need to be independent of any assembly contexts. So top down approach usually won't work in those situations.

    Onshape's part studio environment lets you design somewhere in the middle... You draw a few parts together as a little kit. Then insert those parts into an assembly without having to create contexts or confusing  links between other studios/documents.
  • gauthier_östervallgauthier_östervall Member Posts: 24 EDU
    @owen_sparks Right on! This is what I was looking for. Or rather "Create part studio in context", but you led me right to it. Thanks!

    Now I only need to get a feel of how I might want to build the actual part, but that will be other questions. 

    @john_mcclary Thanks for the insight. The way I did it still makes more sense to me, so I'll try to take info from internet with a larger grain of salt in the future. 
  • edward_petrilloedward_petrillo Member Posts: 38 EDU
    I do a great deal of top-down modeling starting with commercial off-the-shelf (COTS) parts, placing them in an assembly and then creating a parts studio in context to build the "frame".  I've come to realize that the Onshape mate connector is a powerful tool that facilitates this approach.  You can hang them anywhere, in any orientation, using distance and rotational offsets, and use mates to set all of the geometric relationships and motions that your mechanism requires before diving in to create new parts.  Inserting sketches from your parts studio into the top-level assembly provides additional geometry to pin parts to with explicit or implicit mate connectors.  You will eventually develop a feel for when to maintain or break in-context references and add or delete mates and connectors as your model reaches maturity.  
  • gauthier_östervallgauthier_östervall Member Posts: 24 EDU
    @edward_petrillo So that's how you place parts in Assembly with actual measurements?? I've been wondering. 
  • tony_soares459tony_soares459 Member Posts: 86 ✭✭
    It seems both approaches have their place? Can't start from the top if you don't know clearly what the assembly should look like. Can't start from the bottom if you don't know clearly what the parts should each look like.
    You might need a few parts to inform the assembly, and you might need the assembly to inform the parts. You might need to make a few passes, the parts informing the assembly to then inform updates to the parts to then inform updates to the assembly...
    You can only start from what you know. You can think hard about the assembly before you start for a good top-down model, but at some point you've just got to start, and if a bottom-up approach is all you can do when you start, then that seems just fine to me.

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