Top down and the old layout design, whats the best approach?

I am a designer coming from board to Autocad back in '90, now into the SW 3D world. One technique or method I am trying to get used to is
the "top down" (as they like to call it) method of design in SW. When I worked in 2D, I started with a layout of a part of the machine, something like a gearbox or such, and built a mechanism, new parts being designed as required in the layout. Now with SW, the "layout" which I am accustomed to starting a design with, seems to be the assembly file. I am used to starting with centerlines and a general envelope of how big something will be. Many times my "layout" in 2D became my assembly drawing with a border. Now....am I missing something. It looks like my traditional layout, IS the assemlby file that I will be working in. I will be modifying parts in the context of the assembly file.
To those seasoned designers in SW, I am asking if this is the technique or method they use in machine design with SW. I am trying to get myself into the SW world of machine design in the best way. How do the veteran designers in SW do it?
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You and bob z. started at the same time, except bob z. got to go to solidworks in 2000 (luckily!).
The assembly file is your playground. Use it as you wish. Insert a part and draw a sketch in that part. The sketch in that part will be your layout - make it look like what you would draw on a board. Put dimensions all over it. This will be what you want to work to. If you want, you can even mate new parts to this layout sketch. Honestly, mating the parts to this layout sketch rocks. Play with that a little while and you will see the benefits. There are some fancy names for this process but bob z. can't think of any right now. This layout sketch should help you to feel comfortable in this new 3d world.
bob z. tends to do a funny mix of topdown designing and whatever they call the other way of designing. If you get too caught up in the top- down design process, you could end up with an assembly that needs some sub-assemblies and performance will suffer.
You will get some more replies from people way more knowledgable than good ol' bob z. The ways to get this job done are numerous. Read what people suggest and try all of them. You will get comfortable with one or two ways. The key is to relax and not worry so much about how your feature tree looks and let it flow!
Good luck and have fun.
bob z.
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I would tend to agree with what both of you are describing. They like to call it skeletal modeling. The layout sketch is the skeleton that you hang the meat of your design onto. B.Z. took the concept a step further and put the skeleton (layout) sketch into a part. The part is inserted in the assembly. The benefit is that the skeleton can now have planes and bodies and whatever else you may need plus you can put it into other assemblies or subassemblies. The bebefit is that you can have your layout in more places than the main assembly.
Top-down, bottom up, whatever. Middle-out of some kind is usually the reality of the situation. Billyb is on the right track. If you read enough SW documentation and books, you'll see the phrase "design intent". The layout sketch contains the design intent. The rest of the assembly and parts reference the layout sketches and other features in the skeleton part, so that the design intent is captured as much as possible in one place. Try to keep all your references flowing downhill from the skeleton part. When references start going in more than one direction you slow SW down very quickly.
Depending on what you are doing, the skeleton model may not be the best way to go, though some variation is still recommended. The practical reality of large enough assemblies is that top-down design is too cumbersome for SW to handle. After a design grows to a certain size, it will become necessary to break or lock references between parts in order to save time. Unfortunately, experience will be the best teacher on this issue. You will know when it is time.
If things slow down suddenly, then you have probably made a circular reference. SW won't allow this directly, but it can still be done in more subtle and hard to find ways. Usually, an in-context relation from one part to a feature in a part that is determined by an assembly mate is the simplest case. "Excessive" use of configurations will add complexity to these situations.
I don't know of any way to learn to navigate these issues except to suffer through some of the consequences.
Someone who needs love (or has extra to share) should collect a bunch of this sort of information and start a SW FAQ wiki.
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TOP down is a good approach to use in designing any type of system (like a gearbox). However, in SW you really have to understand the implications of the actions that you take in using the top down approach. In ACAD you might make your layout and then copy a bit here and a bit there as a basis for a detail. In SW there is no good way to make your layout (which you later made the assembly) prior to finishing the details. This can be an aggravation and can slow down the design process. We struggle with this every day because we do quote drawings and then have to start from scratch again in SW to actually make the model. Nevertheless, it is possible to approximate your layout methods in SW, though you won't be able to use them as the final assembly drawing by adding a title block.
The implications of doing anything top down in SW is that you create in context relations between the assembly and parts or between parts through the assembly. This means that the components involved in any in context relationship must be solved each and every time SW does a rebuild and that the relationships between the files must be maintained so that there are not circular references, etc. As in life sometimes a strength can become a weakness. The strength is that if the assembly changes so does the part. The weakness is the overhead which requires a lot more thought and resources. The practical outcome is that most people use in context to enable copying features from one part into another and then they remove the in context relations.
There are other methods that don't require in context relations and these involve envelope parts and skeletons. These are very simple parts that carry key features of the layout and can be used both in the assembly and in parts to transfer layout information. These methods are faster and more robust.
Since you are well versed in ACAD let me propose a method that you might find familiar and yet transferable to SW. Suppose that in DWGeditor or in ACAD you created your layout as usual. As you need to create details from the layout you bring those portions of the ACAD drawing into SW via 2D to 3D having previously placed the detail features on a layer to facilitate the importation. You can then use that geometry directly as the basis for a part, or you can create surfaces from it to use as a skeleton in both a part and as an envelope in the assembly. Much more elaboration on just how to do this is needed, but I won't take the time here.
Once you have all your parts, make the assembly, check fit and interference and create your final drawing from the assembly. The most obvious reason to do this is that SW will allow you to create section views and auxilliary views as well as your axonometric projections much more quickly.
One thing you will probably find in using SW over ACAD is that in SW you will start finding things that you did in ACAD that just don't make sense in 3D. That is one reason it is worth the effort. You will also find that it takes you a bit more time on the design end to do it 3D, but that if you factor in the time saving from having parts that actually fit the first time you will be ahead.
It is a shame that SW never considered making drawings able to parametrically drive components like some other CAD packages but it is more an aggravation than a roadblock.
TOP
PS before doing top down, read all the help on incontext, envelopes, importing parts into parts and 2D to 3D conversion.
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TOP,
Good points, as usual. It looks like we were composing at the same time. But...

I'm not sure what you mean by this. If you use "insert model items" to put the model dimensions on the drawing, you can drive the model from the drawing. Is this not what you are referring to? Myself, this has always seemed to be a marketing novelty. I've never used it except as a curiosity.
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Dale,
As I was typing the thought had occured to somehow get the layout parts onto a drawing in which case this might be done. I haven't worked it out so I didn't want to suggest something that I couldn't answer the details on.
TOP
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Oh, ok. I read you now. Importing dimensions of a sketch that is absorbed into a feature. Right. That can't be done without a workaround.
Gene DiMonte is an advocate of reusing model dims (even did a SWW presentation on it, IIRC), and he told me about a workaround for layout sketches in parts. Extrude the layout sketch to make a feature, then delete the feature. Once the sketch has been absorbed by a (solid?) feature, it is somehow marked so that "insert model items" can retrieve it. I don't know if a similar workaround exists for assembly layout sketches, or layouts in skeleton parts in assemblies, or layouts in envelope parts.
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Maybe I've misunderstood you, but I can insert sketch dimensions into a drawing without the sketch having been used in a feature.
John H
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Perhaps the situation has changed. The last time I tried it I was probably using 2003. I distinctly remember not being able to import models dims for a layout sketch into any view.
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I just re-reread your post, again. Other CAD systems allow you to create layouts in a drawing, and use THAT as the basis of a model?
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I'd agree with the most of the previous posts, because there are so many different ways of doing it that all of them will be right under some circumstances!
If the project is for designing bespoke machinery, or maybe even some consumer device where pretty much every part is unique to that design, then using in-context relations or a skeleton part should cause few problems and might save a stack of time.
However, if the project uses many existing components plus some new ones, then in-context relations can be a minefield to pick your way through - especially if the model was created by someone else! I often use "empty parts" (i.e. just the standard planes + any useful reference geometry) to fill in the gaps in the structure (and show up in a provisional BOM) and to constrain other parts to. I'll then add the geometry to them at the appropriate time. One golden rule is that the "quickest" method to get something that looks reasonable in the short term, is almost always the slowest method in the longer term.
From past experience, the area where it is hardest to decide the approach is when making "sales drawings" where you might need to quickly produce a shaded image/eDrawing/drawing that looks vaguely believable to the customer. How I tackle it depends on :- a) whether I think the enquiry is highly speculative from a potential customer or alternatively a "racing certainty" from an existing customer b) whether it is basically a re-working of a previous design or alternatively involves a lot of conceptual work.
At one end of the scale is a speculative enquiry involving conceptual design work. Here I would tend to model parts that combine several/many real-world parts - this saves on the overhead of naming/saving multiple files, adding many components to an assembly, creating loads of mates, and obviates the need for in-context relations. Obviously if parts have to move relative to one another then they have to be separate. The downside is that if the enquiry becomes an order, you'll have to remodel it all as separate components. However, if you use multibody parts and are careful to use reference geometry where possible, then you might be able to salvage much of the geometry.
At the other end of the scale, are enquiries from existing customers that are re-works of existing designs. Here I would make a copy of the top-level assy and drawing and then selectively "save as" on any components/drawings that needed to be new items.
If there is no well-constructed assembly as a starting point, then I'll use a "bottom up" approach to add existing components to sub-assys which I'll then add to a top level assy. Getting the structure right from the start can save hours of heartache later. I'll also use a top-down method to create the new parts.
John H
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