【转帖】question for surfacing guys

yang686526

question for surfacing guys
as an assembly equipment machine design guy, i deal mostly with prismatic parts.  anything you can't easily model with extrude cuts and revolves usually makes for difficult to fabricate parts, so we try to keep it simple.  however, i'm now in a situation where i need to model something to closely (3mm or so) fit to the profile of a product.  we'll make our part by rp, so no machining cost worries.  i have a dumb solid of the product (actually an assembly of a few parts), so my first plan of attack was to do offset surfaces and then trim them up into a model.  however, due to a lot of inside radii, i end up with self-intersecting surfaces when i try to do the offset.  any tips for this type of operation? surfacing pitfalls to avoid?  will i just have to carefully pick my surfaces and then trim up from there?  of course, due to ip i can't post any files on this one.   

-handleman, cswp (the new, easy test)
handleman,
i would stick with the offsetting/trimming tactic.  even if you've got to be selective when picking surfaces to offset in order to avoid self-intersecting surfaces it will be easier (and undoubtedly faster) going this route than trying to surface up what you need.  especially so if you're not all that versed in surfacing.
you may have to generate some surface patches in areas where you just can't get an offset to work, but you'll still be better off, imo.
oooh--that's going to be tedious.  i'd recommend deleting that small fillet stuff and doing a simple shell, then re-adding those after the shell, but it sounds like this might be more complicated than that.
if you can offset major surfaces, pick clusters that make the most sense, that will offset properly.  this will generate lots of individual surface bodies.  then extend edges of surfaces to past where the small intersecting problems happened previously (i'd recommend ignoring the offsetting of the small fillets for now).  so the small fillets aren't included in your offsets and you can use either the extend or trim features to get the desired inner surface you need, then use the cut with surface feature to core things out.
you might even find rebuilding (instead of offsetting) some of the fill-in-the-gap stuff to be quicker.  after getting your main surfaces offset, hide your solid body so you can see what you're doing.  i tend to prefer the mutual option when trimming surfaces.  you may need to use the knit feature to make sure your surface is singular before attempting the cut with surface.  also, you'll probably need your surfaces to extend past the solid body in the shell hole area--so you may need to extend a flange or some such thing through the base of the part (or whichever surface will be "open" in coring out the solid) before making the cut, again, making sure it's all knit together as a singular surface.  if the surface body doesn't go past your solid stuff, solidworks will get confused as to the boundaries of your cut with surface.
  
jeff mowry
handleman,
to follow up on jeff's post, you can also try to make use of the untrim command to extend surface boundaries once you've got offsets created.  it will depend on the geometry you need to create whether you untrim the interior/exterior boundaries (or both).
i used to have to do this all the time in a previous life (albeit in a different cad package).  i don't envy you.  best just to resign yourself to the fact that this will take a while to do and, depending on the quality of the geometry you're starting with, has the potential for an extremely high pita factor.
however you look at it, reverse engineering the profile into an interfacing solid model is going to take a lot of time.  however, i suspect you will be a pro by the end of the project.   here's some quick points in addition to what's above:
surface features till patch holes (one will often work where the other does not):
boundary (use existing geometry)
surface fill (can also use 3d sketches to create surface)
knit to join up surfaces and hopefully produce a solid model.
use sw 2009 if you can.
matt lorono
cad engineer/ecn analyst
silicon valley, ca
i can't help but wonder if you are making this too difficult, but without the file...
i'm thinking insert>part, maybe with scale, maybe an intermediate part with thickened faces... combine with subtract...
thanks, guys!  especially the part about picking (smaller) clusters that make sense.  i had started out by picking as many surfaces as i could and offsetting them all in one feature.  i guess it was fear of a long feature tree.  anyway, i was getting totally lost trying to patch them together.  it seems to be going better now that i've started picking a few surfaces, offsetting them, then patching/trimming/knitting them together into a single surface body before moving on and offsetting more.  it's a bit humbling how you can be so comfortable with a large portion of sw, but then feel so lost when you go outside that area.  i know i'd be in the same boat if i had to do sheet metal, simulation, photoworks, etc.
-handleman, cswp (the new, easy test)
handleman,
instead of inserting surfaces, i normally try to extrude (in context) "up to surface" or set the end condition to "offset from surface."  this may or may not work depending on the geometry you are working with.
joe
sw office 2006 sp5.1
p4 3.0ghz 1gb
ati firegl x1
extruding to an offset from surface, thickening, shelling, and the like will all fail if the surfaces you're referencing won't offset.  handleman is on the right track of offsetting groups of surfaces and knitting.  
-b
haven't posted in so long, just lurking in the shadows. less and less time in solidworks, and more and more in circuit board layout. figured i'd post and maybe spend some more time with my eng-tips friends seeing as people around here are driving me nuts lately. not that anyone missed me.
i'm with rollupswx on the scale approach. when things wont offset or thicken because it creates intersecting surfaces, surfaces that collapse on themselves, eliminates surface elements, or creates 0 radius, there is always the option of scaling the surface geometry. its worked for me on many occasions, where for instance, i'll create a carrying case for a handheld device, where the outer surfaces of the device won't offset for the forementioned reasons.
rfus