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Why won’t this surface offset?

S

speckofdustin

New member
Long-time lurker, first time poster, but hopefully my question will create some beneficial discussion. I'm researching a problem I had on a recent project. I figured out a workaround on that project, but now I'm hoping to understand why the issue happened in the first place.

I was trying to get what I thought was a simple well-built group of surfaces to offset or shell predictably. But whenever I tried, the surface boundaries on the offsets went crazy (see image) and the feature failed or gave an ugly result. I'll try to attach a re-creation of the problem to this post. If anyone has encountered (and conquered) anything like this before, I'd love to hear how you did it!


View attachment 2263<a href="uploads/speckofdustin/2006-05-10_230255_part.prt.zip" target="_blank">
</a>2006-05-10_230401_part.prt.zip
 
hi,

ok.

i selected all your outer surfaces. Control c, control v.
edit- offset.
in the feature --- options.. "automatic fit"

presto.


can i ask what the part is :) looks like the top of a stapler
 
Wow, thanks for your help, guys!



puppet, I hadn't used automatic fit before. Thanks for pointing it out!



hlf, I haven't seen your method before either, but it works
nicely. Thanks for raising the issue of uniform thickness, as
well. If you have a moment, do you mind explaining why the free
form features make the shell possible? I'm thrilled that it
works, but I don't understand why.



Thanks again!



Edited by: speckofdustin
 
Oh, and regarding what the part is, my employer probably wouldn't
appreciate me divulging everything, but you were close in terms of size
when you said stapler. It is a hand-held product.
 
I'll jump on that bandwagon, HLF, can you explain what the freeform surfaces do?


I've taken a look at the model and don't knowwhy it won't offset.. have you tried building it just using Pro/Surface and throwing in a few more control curves? I'd do it but I'm a little short on time..


Cheers,


James
 
hi


in proe


1. topology of surface sometimes is not so good.


2. a gap which issmaller than absolute accuracy can be producted between two shuface which are merged.


meanwhile there are other problems. some of them can be soveld by use "free from" command


my english no good, so I can't express exactly, hope you can understand
 
I can help I think. Accuracy in ProE has quite a bit to do with what will and won't offset properly for you. Sometimes when you quilt together a nice set of surfaces you won't notice a problem but when you go to offset them (especially with only relative accuracy set) the problems that proE has decided to ignore become to big to ignore. It all boils down to math. I suggest any time you plan on modeling in surfacing using proE you set your accuracy to an absolute accuracey at about ten times your manufacturing tolerance. This way your part will be created to the right dimensions from your model.


You will notice this more when you are trying to make a complex curvature surface offset to a smaller total area. sometimes the surfaces want to dissappear completely and ProE can have issue with that!


As a note, automatic fit is nice, but you will have not idea what surface you are actually getting. If you loosen or tighten your tolerances on your part file you will notice that eventually you will be able to offset to the correct dimension!
 
I had a quick look at it during lunchtime, but could not immediatly find what caused the problem. Why do you use a ribbon, and dont create the front surface in style along with the sidewall and put it normal to a plane? It is symmetric anyway no?


All our startmodels and customer start models are changed to 0.01 absolute accuracy. We work alot with skeletons to create the enveloppe of a cab interior. If for example its a complex one and you have a small storage lid 'far' away from the default coord system, and when the part where you copy it to is in relative acc. it happens alot that in that part you cant create the thickness, but you can in the skeleton ...


When your creating compact injection parts, never use auto fit for thickness. In rare cases and its to much of a hassle to investigate why you cant create thickness AND if you are creating a thermoformed or roto moulded part AND if the quilt you modelled is tool side AND if there are no otherparts connecting to it i would consider autofit if the deadline is getting nearer ;) The surface created by the tickness is not important when non tool side





greets


nick
 
Thanks for your replies hlf, ctolman, and dojo. Your information and tips have been really informative.



So from what I gather by everyones comments and my own research, the
freeform command rebuilds the surface in some way that allows it to
offset. I exported a couple of IGES files of the model (with and
without freeform used) and opened them in Alias Studio so I could
examine the surface data a little more closely. The
parameterization of the back surfaces is definitely different in
the two versions. I still don't know what was wrong with the
original surfaces, but freeform certainly made them behave better.



Thanks also for the comments on accuracy. Whether or not that was
the issue with these surfaces, your explanations reminded me of some
other problems in the past that probably could have been solved by
better control of the accuracy setting.



Dustin
 
The geometry is imploding on itself. If you have used conics or parabela's, the offset can only be as large as the minimal radii of any point along the conic. In other words you can only offset a surface of a min radii of 1 = to R1. Any less would be =<0. Use Model Analysys to determine the minimum radii of the conic.







Edited by: donha
 
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