Approaching a Complex Model: Part2 – Multibodies

multibody where multiple parts shape a common overall shape

Multibodies is an area of the software that has had a lot of consistent development over the last decade. It was added in 2001. I have to say that I wasn’t one of the people who really wanted this to happen. I felt that everything that you can do in multibody modeling, you could already do with in-context modeling. And that’s true, or was true. The part I maybe didn’t understand was that in-context modeling was more risky and more costly than I thought. I was afraid that multibodies would be misused a lot. And they are. Noobs get their hands on multibody and they just want to make lazy assemblies. I guess you’d only do that once. After you try to get a BOM or a drawing out of a multibody, you wouldn’t try that again. If you really want to work that way, just use virtual parts in the assembly. It will pay off in the end.

I think some people have some misplaced idealism, and try to say multibodies “should” work some way, like do away with assemblies, or model everything in a single part file. But the fact is that the software “does” work a certain way, which is probably not the way it “should” work. I don’t know about “should”. There are too many different ways of using this stuff for anyone to say how it “should” work.

First you have to decide if you have a valid application for multibodies. Are you making a clock with a lot of moving parts? Then no, you should probably not use multibodies. Do the parts have a lot of relationships that can’t be captured with a couple master sketches? Are there complex shapes that include multiple parts? Multibodies are great for that. Do you really need to use model dimensions on drawings? Multibodies and drawings don’t play well together. Someone may have told you that you can now make drawings of bodies, and that’s true, but you’ll only do that once. Or you’ll only do it with something really simple.

assembly issues aren't always easy to solve when your assembly was modeled as a single part

Next you have to understand if you have the patience required to work with multiple bodies. Do you get tired of bodies getting hidden for no reason? Are you afraid of learning new interface options? Do you like really complicated feature troubleshooting exercises? Do you like really mind bending file management questions? Multibodies may not be the picnic you’re envisioning. If you throw configurations on top of multibodies, you get a lot of power, but you also get a lot of headaches.

Multibody as tools

Here’s my take on multibodies in SolidWorks. Multibodies are almost always a tool – something I use to copy geometry, create geometry relative to another part, or as a “tool body” say to indent a part. Surfacing is inherently a multibody sort of workflow, while solids by default try to merge with other solids. At the end of a feature tree, I usually end up with only a single solid body, although there are exceptions. Often my last feature is Delete Bodies. Just in case you’re wondering, multibodies and translations are not always a great combination. A translated part can easily turn into an assembly on the other end, even if the other bodies are hidden.

Master face model with configurations to create a family of assemblies

Multibody as master model

When multibodies are not a tool, they are a means to an end. For example, when I use a master model approach. I have two different ways that I use master model: first, I might just model the main faces of parts in an assembly using surfaces, add sketches, and then distribute that “master model” to each individual part file to start building parts. Second, I might model everything as a single part, then break up the part into bodies and save the bodies to individual files.

Inseparable subassemblies

There is an isolated circumstance where you might want the end of the file to have multiple solid bodies. That for me would be an inseparable subassembly. Like a screw with a captive washer. You could model it as a single body, but it would be a cheat. For me these are rare. First of all, when you give a multibody part to the machinist, what does he say to you?

Summary

I look at multibody methods as a means to an end, not as an end in itself. It often helps avoid the need for in-context modeling. So this is kind of a 180 from my original idea where I believed that you could use in-context to avoid multibody. I still believe that they are easy to abuse, though.

File management with multibodies can be a little mind bending, too, but they have improved this quite a bit in the last several years. The thing is that it can be kind of confusing. Did you know there are 4 different commands that you can use to insert one part into another part? Name the 4 commands, and describe the differences between them. Now name the weaknesses of each. This gets to be a difficult game, and this is where just running the CAD software is what adds the complexity. You’ve completely forgotten about your design at this point, you’re just trying to run the CAD software. Wasted brain power, if you ask me.

 

20 Replies to “Approaching a Complex Model: Part2 – Multibodies”

  1. Insert Part and then show ‘Unabsorbed Sketches’.

    For me this is probably the most important tool in SolidWorks.

    Yes it is directly lifted from what I’ve hear Pro-E people talking about, but it allows ONE single secure and safe source for all the common geometry shared between Assembly parts without the infernally awful ‘assembly in context’ relations.

  2. How would you distribute sketches from your “master model” to each individual part file? Insert part?

    I’m and new to SW and I’m coming from the ProE world where I would use “publish geometry” and “external copy geometry”

    Thanks,
    Matt

  3. @Dwight Livingston
    Dwight, I’ve had that same frustration, too—even with custom-named bodies in the feature tree (such as “top housing” or “bottom housing”)! Maybe “how” SolidWorks keeps track of bodies is less relevant than “whether” SolidWorks keeps track.

    Unfortunately, I find SolidWorks sometimes confuses surface bodies, too, which causes trauma with downstream trims, knits, split lines, etc. In one case I found that every other CTRL-Q rebuild would simply alternate which surface ID was given dominance (and which downstream features would fail as a result), since apparently, two surfaces had exactly the same face ID! How does that happen? In attempt to be a more tricksy hobbit I rolled back before the failed features and re-split one of the confused bodies with a split line and that seemed to “reset” the face ID confusion by assigning new face IDs to each “new” surface.

    This sort of thing certainly distracts from the ability to focus on the design—ostensibly what my clients pay me to do.

  4. Matt

    I use multibodies often, though not often for assemblies. One downside I find is that SW tends to loose track which body is which, particularly if you go back in the tree and edit. That can cause a lot of problems. I’d be interested in more insight as to how SW does keep track and what to do to avoid such problems. There’s also the hide/show toggle for bodies, which keeps suprising me whenever I move around in the features tree. I’d also like to know how that is supposed to work.

  5. I have to agree with a couple of the people above. I use multi-body parts for weldments on a regular basis and I see no reason for an I-beam welded to a plate to be two parts; and drawings are not a problem, especially with the Select Bodies button that was introduced in SW2010.

  6. @Knut F

    @Cvan

    Most of the stuff I do is consumer or medical type stuff—commonly requiring molded plastic (or metal) parts. Coming from that background, I discovered my methods for multi-body modeling (using a master model, and then saving bodies out as their own parts) worked great for weldments. The only thing is that you’ve got a sort of “lost” master part that doesn’t fit in anywhere in the BOM, but you simply exclude it from any BOMs, hide it in any assembly, and you’re good to go.

    Here’s a project where I made great use of this practice and it really saved me huge time in making quick, concise edits to the sub-assemblies:
    http://www.industrialdesignhaus.com/PDF/AggreScreed-Grader.pdf

    Generally I find the parts of this type to be geometrically simple, prismatic parts. But even for more complicated stuff, each part can be detailed further after being saved into its own file. General changes to geometry are made at the multi-body, master-part level and trickle into all the saved parts (wherever they go).

  7. If you take Weldments out of the conversation, then I can see where MB can get confused for lazy assemblies as Matt mentioned (although I rarely see it in the field). I’ve also seen many uses of master models – some good and some not so good. I like your analogy of means to an end. Makes good send with product design usage.

    If you bring up Weldments (which came out tin SolidWorks 2004 – the release after MB was introduced) then you open up a huge resource to SolidWorks users. This is where I see most of MB usage and to great effect. As Rick explained, it makes creating frames very quick and efficient, especially since the Weldment usually has a single part number – no part number for each structural member. I wish using the Relative View was a more prevalent since that’s the first question asked when bringing up detailing.

    Also, the many cabinet and furniture designers have a great tool. One of my better cabinet shops have been able to automate a lot of their process via Weldments. A few custom property hiccups are keeping from them going full out the way they want, but they are extremely happy with what they can do now.

  8. @Nathan
    Hi Nathan,

    there are some basic Multi-body tutorials built into your version of SolidWorks. If you go to Help>SolidWorks Tutorial look in the “consumer Product design” or “advanced part design” or something like that. Sorry you’ll have to search around a bit but there is one in particular that uses a simple computer mouse to explain the three different methods to master model and multi-body design. (sorry I can be more specific but I install all my version of SW w/o the help files and samples etc.)

  9. Multibody parts provide a compact way to design an assembly with complex geometry. I like to express the design intent in one place.

    I designed an aircraft nose gear trunion with oleo-pneumatic damping, steering, and retraction mechanism mostly as a multibody part. The model was compact. The complicated geometric interactions were quick to sort out. When it came time to build the part it is a complete disaster. Every part needs a unique number, a drawing, and a 3D model for the cam system. Every assembly needs a number, a drawing, and a bom. The parts must be frozen, safe from accidental changes.

    I have now re-modeled every part into separate parts. Each part has at least 6 different files: sldprt, igs, slddrw, edwg, dwg, epart, cosmos epart, spreadsheet, and notes.

  10. @Knut F
    I recently had a good sized weldment assembly, where three subassemblies were stacked on each other welded together. Then after everything was welded up (as an official assembly), a tsunami of valves, hoses, npt fittings, motors, etc. had to be added as well (in yet a higher level assembly). This had to be done relatively quickly and not take up too much time. Each “subassembly” of about 8 parts were in fact multibody parts, with configurations showing each part by itself. The drawing had multiple sheets with one configuration per sheet including all their part dimensions, as well as a sheet showing dimensions for the welder to position all the parts together prior to weld.

    This saved a lot of modeling time and if the welder turned out to also be the machinist and stock sourcer/sawer, would be quicker for him to print out one drawing and go, not worrying about missing a part.

  11. Mark should be looking forward not back. SWv6 will be the greatest thing since sliced bread. 😀
    BTW Mark I’ve been making a macro to get .obj out of SW for other renderers and whoever wrote the UV unwrap part of the api code should be shot for the results it produces. Too late in the big scheme of things to do anything about it though. 😉

  12. @Knut F
    The tools exist to make drawings of individual bodies, although they are relatively new. Pardon me for my cynicism, but I don’t trust features that haven’t been hammered on for years. SW uses paying customers with released software to find their bugs.

    @mark biasotti
    Mark, I don’t think people use SE to do the kinds of things that push the boundaries in product design in SW. They do have some multibody capability, but it works from a different set of assumptions. It would be safe to say that multibodies are less developed in SE ST4, although they have been in there for longer.

  13. @mark biasotti

    Mark, you can do exactly the same products in SE, although with a different workflow. For small assemblies, I agree that MB may be useful and can speed up the modeling process. But for large assemblies, MBs may become real nightmares! And you know that handling of large assemblies is a strong point for Solid Edge. But I agree that MB can be real useful in some cases. So, how about we just a wait a bit to see what’s coming?

    CM

  14. Very interesting reading. If you also add weldment to multibodies, I guess it can only make it worse? If you model a welded part as a multibody, so you can make a drawing of each plate detail as well as the welded part, what do you think of this approach? We tought it would make life easier with fewer files…….

  15. Hi Matt,

    Thanks for taking the time to write about multi-body – a subject near and dear to my heart. Rob Segal was the original SW develop and is still with us. The way SW implemented it is dare I say, better than any other system I know of. BTW, for the life of me – how can you do similar products in SE because to my knowledge they don’t support MB’s?

    BTW MB was first introduced with SW2003.

    Mark

  16. You should write a tutorial on multibodies. I came here to learn how to use them and left a little disappointed. Great site though. Nice list of links.

  17. Multibody modeling is very useful for top down modeling where you have complex surfacing defining the OML of the part. Though in truth, this is just a work around because the the issues that we have encountered in large assemblies that use in context modeling (>500 individual parts, >1gig). The problem with large in context assemblies is that they become memory hogs, and in several cases parts become corrupted because references to other parts become lost. The advantage of the multibody part is that the memory and stability issues seem to be better, but the disadvantage is a very complex feature tree.

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