Multi-body or Assembly?

Discussion in 'SolidWorks' started by JKimmel, Nov 18, 2006.

  1. JKimmel

    JKimmel Guest

    Is there anyone else out there making extensive use of multi-body
    modeling, not counting weldments?

    For example: I will typically make a sheet metal stamped part plus the
    die over which the part is formed as a single multibody part. (These
    are one-off parts, not production tooling). I also frequently model
    simple assemblies as multibody parts which I then split and either
    reassemble into an assembly, or just add features to individual parts,
    which often means converting them to sheet metal parts.

    I used to do these tasks in the traditional way, but after watching a
    multi-body demonstration at a user group meeting, I tried it out myself
    and it seemed to solve a lot of problems that I've had (and continue to
    have) with assemblies: disappearing external references, moving or
    renaming parts, changing computer or server configurations. My
    experience is that it's a much faster and more robust method of modeling
    small assemblies than the traditional way. I don't use Solidworks BOM
    or a PDM (yet), and I still do a lot of traditional assembly modeling.

    The Solidworks Establishment seems to be very much opposed to this
    method and I'm coming under a lot of pressure to stop, mainly from
    people with little or no Solidworks experience. Any arguments for me
    either way?

    John Kimmel

    "Cuius testiculos habes, habeas cardia et cerebellum." - When you have
    their full attention in your grip, their hearts and minds will follow.
    JKimmel, Nov 18, 2006
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  2. JKimmel

    TOP Guest

    It all depends on how you do this. There can be glitches with the split
    part feature but there are also ways around that are robust. Who is the
    SW "Establishment"?
    TOP, Nov 18, 2006
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  3. TOP wrote:
    Who is the
    A VAR representative and a Community College Solidworks instructor.
    John P Kimmel, Nov 18, 2006
  4. JKimmel

    matt Guest

    There are a few things that you cannot do when you model all of the
    parts in an assembly as bodies in a single part:

    - interference detection
    - dynamic motion
    - auto BOM population
    - exploded views
    - display states
    - you cannot mix sheet metal with other types of parts

    There are also some things that become seriously hampered by modeling
    this way:

    - troubleshooting
    - isolating the features and geometry of a single part
    - for more than a few parts your rebuild speed is going to take a
    serious hit
    - moving parts becomes a major pain
    - getting data to manufacturing can be a problem depending on how you do
    - making drawings can be a bit of a pain
    - using custom properties for parts becomes a problem
    - managing visibility of bodies or keeping bodies that are separate from
    merging are both going to cause you some headaches

    I'm sure there is a long list of other things I am missing here.

    In contrast, the advantages are that it simplifies file management, and
    that external references are simplified (essentially the file management
    issue again).

    Multibody modeling is like in-context in that you should avoid doing a
    lot of it.

    Purists may call this a technique for excitable beginners which you will
    out-grow after a while. If you do anything with more than a few parts in
    it, the multibody approach becomes unmanageable quickly.

    I don't believe anyone will tell you that it technically can't be done,
    just that it's a bad idea even though aspects of it look attractive. UG,
    Mechanical Desktop and AutoCAD R10 AME use that approach.
    matt, Nov 18, 2006
  5. JKimmel

    TOP Guest

    VARs don't necessarily speak ex-cathedra on these matters nor do
    Community College instructors. What they should be doing is showing
    you when and where and what limitations there are on any given method
    and not dismissing it outright. They may be raising a flag which you
    would do well to investigate, but you still have to look at the quality
    of the source and the use for the technique. There is a lot of
    variation between VARS and even at a particular VAR there may be
    several opinions prevalent. "Established" opinions can be wrong. I've
    had my share, many times due to the fact that SW changes with time.
    TOP, Nov 18, 2006
  6. Matt,

    You forgot mass caculations, FEA, Mold flow, CG, the list goes on and on

    Mark Mossberg, Nov 19, 2006
  7. JKimmel

    TOP Guest


    I think he knows what the limitations are. He has stated that he uses
    this in a limited way for quick and dirty. BTW you can do a limited
    form of interference detection in Multibody in that if you try to
    combine two bodies using intersection and there is no interference the
    combine won't result in anything. Since he is exporting to an assembly
    he can still do a lot of the stuff on your list once he puts them in a
    real assembly.

    Also in a multibody you can use the delete body feature to get rid of
    the extraneous bodies for mass props.

    It sounds like he is into stamped parts and of course the other good
    use of multibody there is to get the waste material.

    As John described it he is primarily using a master model approach and
    he is using split body to get the bodies out and into a real assembly.
    As far as his use with the die and part design that makes a lot more
    sense than using incontext or cavity through an assembly IMHO.
    TOP, Nov 19, 2006
  8. JKimmel

    matt Guest

    I agree that the tooling makes more sense than just modeling general
    assemblies as a single multibody part, although I'm still not a big fan
    of it.

    SW does the Mold Tools as multibody, which has pluses and minuses. Split
    Works does the same thing as an assembly, which I think works better. I
    really don't like jumbling the features of multiple parts in a single
    feature tree. That makes SW far more difficult to use, and the
    performance overhead of putting everything into one pile, as well as
    reuse issues... If you like to use Insert Model Items in drawings, all
    of that data is lost if you do multibody and break into an assembly.
    Plus, I don't think the arguments against assemblies are very
    compelling. Also, any of the best practice part modeling stuff regarding
    planes, the origin, symmetry is thrown out the window when modeling an
    assembly as a part.

    I do plenty of multibody work, and am more than a little disenchanted
    with it. There are certainly times when it is appropriate, but I find
    myself doing it grudgingly now. I've had it happen that if a feature
    accidentally doesn't get the Merge switch flipped off, unexpected
    parent/child relations can be created due to SW automatically merging
    planar/cylindrical faces. This can make editing very difficult.

    A project I did last year was a very complex shopping cart basket. It
    was started as a single part master model with all of the major faces
    built as surfaces, and this original part was then inserted into 3 other
    parts (representing areas formed by separate sections of tooling) which
    were built individually from the master model as separate parts having
    approx 300, 400 and 600 features respectively, and then brought back
    together as a single part using Insert Part. This was done so that I
    didn't have a single part with 1300 features in it. The performance was
    still slow, but not nearly as slow as it would have been.

    This is really the reverse technique of modeling an assembly as a part.

    Then of course are all of the problems discussed at length here in the
    ng regarding the Split, Save Bodies, Insert Part, Insert Into New Part
    functions which are nowhere near being reliable enough to base a whole
    method for modeling on. I recommend against going too far down the
    multibody route and finding some bug or limitation that puts the brakes
    on your whole operation.

    Obviously, John can do what he likes, but to me the multibody method is
    something that looks good on the surface, but under that it falls apart
    rather quickly for real work.
    matt, Nov 19, 2006
  9. JKimmel

    ed1701 Guest

    I do use multibodies in a single part to model separate components ...

    For instance, I am working on a make-or-break project for a new company
    right now, and because of the schedule and budget I concluded that I
    could not ethically or responsibly consider working this project any
    other way.

    Here are the rules that I use to decide whether to do
    master-model/multi-body based on its impact to the delivery time and
    overall editability/flexibility/usefulness of the dataset.

    1. I only model multiple components in a single part when those
    components are so intimately intertwined that in-context is a terribly
    complicated or unworkable option.

    Two metal plates that relate to one another? (or other simple shapes,
    like the elements of the wood furniture I've been working on for
    myself) - go in-context.

    Components of a more elaborate product that, on analysis, still have
    only a few relations to each other? - go in-context.

    Multiple components of a molded or cast product that require dozens
    (and dozens and dozens...) of relations to each other - multi-body.
    On my current project, I am using a mix. Some in-context, some
    master-model. It depends on the component and where I feel its best to
    have the features that make it.

    2. Use multi-bodies if the relations are handled best with a sense of
    'history'. In-context refs are always to the final state of the
    referenced model after it has been rebuilt.

    Most folks with experience know to make in-context refs to things that
    can't be changed by subsequent features - sketches and planes (we
    learn that edges and faces can be fouled by fillets, split lines,
    drafts, etc) But sometimes its such a spiders web of stuff that its
    best to deal with refs via a linear feature tree that you can
    absolutely and intuitively control, which is what multi-body master
    models affords you.

    3. Use 'Master model' only until the refs between components are
    established, then finish the parts in the offspring.

    I want to keep the rebuild time of the master model as short as
    possible - its too much stupid overhead to have all the drafts,
    fillets, etc. required to finish components in the master model because
    then all the other components also have to rebuild it (then discard
    it!) to find out what they look like.

    I also have engineers who want dims in models so they can work with
    them and ultimately insert them into the drawing. Dims from the master
    model don't transfer to the child parts - dims in the child parts
    are there for everyone to use. I put as many CTF (critical to
    function) dims into the child parts as possible so they are available
    for the engineers/drawing.

    4. I may be tempted to do a fast, down-and-dirty assembly as a master
    model instead just to get a concept out. Heck, if we are presenting a
    concept, there is a really good chance it will change as we develop it,
    and an excellent chance that it will be discarded altogether (if we
    present five concepts and they pick on, that's an 80% rejection rate)
    But even so, if I suspect it might go further, I will be careful and
    start to apply the above rules. That's a decision made from
    job-to-job based on economics, experience, hunches, a little voodoo,
    etc. And, of course, I have learned from experience that if it goes to
    the next level, it saves time and money to rebuild stuff in separate
    part files * that really needs to be in separate part files * instead
    of trying to keep the master model when it is not consistent with the
    above rules (pennywise/pound-foolish).

    and... oh, I bet I missed a few other good rules.


    Post script:
    It was odd to hear you say that the SWx establishment is opposed to
    multi-body modeling. I thought that the establishment was moving
    towards multi-body modeling - there are multiple, distinct ways of
    doing it (if it wasn't supported, they wouldn't' keep coming up
    with ways to pass bodies onto new parts, like split, RMB save body to
    new part, insert part, etc) there are features/modeling techniques like
    'weldments' that rely on it, and at SWx word I've seen sessions
    by SWx employees that unapologetically demonstrate it. Thanks for
    clarifying that it was your VAR and community college instructor. In
    Concord, I know that it is considered to be a very important part of

    It would be good if those folks gave you the context for their opinions
    or let you know what informed their opinions. There are a lot of
    really good reasons not to use it - its 'seeming' convenience is
    not justification.

    1. There are important stability/recoverability issues.
    Hint-use insert part to make your child parts < used to be called
    insert base part> because it is always recoverable (though you can use
    anything if you are really clever and dedicated, like matt, who can
    make anything work.) Split part where you save bodies out has a lot of
    KNOWN PROBLEMS (hint), same with RMB save body to new part. Example -
    I had a client in last week whose file had gone FUBAR because of that
    last one, and we had to use 'insert part' in the child part to save
    2. There is a big file-management negative (how does a file
    management system deal with a part model that doesn't actually get a
    part number, because it is the parent of a number of other parts? Boy,
    I have run into issues about that with customers)
    3. There is an education/responsibility issue. If you work with
    someone who isn't familiar with it, they can easily (and innocently)
    screw everything up.
    4. There is an overhead issue - if the assembly is initially
    modeled in a single part, each component essentially carries the
    rebuild time of all of the other parts of the assembly (again,
    depending on how you do it)
    5. Then there is lots of stuff that matt mentioned - drawings
    being a big one, sheetmetal being one I don't recall being mentioned
    (no multi-bodies in sheetmetal). I could go on at length. (and did, to
    some extent, at last SWx world - files on our site)

    If someone tells you not to do something, I would hope they would also
    take the time to tell you why.
    ed1701, Nov 20, 2006
  10. JKimmel

    ed1701 Guest

    Useful tip on VERY involved parts.

    Thought experiment:
    We use subassemblies to make our assemblies manageable and don't bat an
    eyelash about it.
    There are times when judicious use of 'sub-parts' to make a complete
    part makes the final part creation manageable.

    Back in SWx 99, I had a part with three distinct zones. I made a
    master model, used insert-base-part to create three parts representing
    each zone, added detail in those sub-parts, then used 'join' to bring
    them all together. There was just no other way to do it efficiently
    (and economically - its all about money) at that time with the
    available processing power.

    I haven't needed to do that in a while, but in cases like matt's, it
    makes sense and its good to have in your bag of tricks.

    The important thing is that though the performance might still be slow
    when bringing them all together, the performance when working/designing
    any individual chunk of the product might be a ton faster.
    Hopefully, most of your time is spent designing and trying out design
    iterations - this is a way to maximize your productivity during what
    you focus your day on.

    BTW - only experience/hunches/a little voodoo will inform what is the
    right approach. But if you are sitting waiting five-ten minutes for a
    part to rebuild, it becomes a little more obvious.
    ed1701, Nov 20, 2006
  11. JKimmel

    JKimmel Guest

    Thanks for all the responses. My local user group is going to get Jack
    Sanford's SWW presentation on "master model techniques" on Dec 2nd.
    J Kimmel

    "Cuius testiculos habes, habeas cardia et cerebellum." - When you have
    their full attention in your grip, their hearts and minds will follow.
    JKimmel, Nov 21, 2006
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