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ragde

3d Modeling Challenge - March 2007

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Guest Marc UK

Eric.

 

I read what you're saying, but in actual fact it usually isn't so straightforward as you make it sound. With many models, when you convert them from poly (sub-d for example) to NURBS, there's so much work to do on them to get them right for manufacturing, that often it's just as easy to start from scratch in NURBS. If only it were so easy as to just click on a command....(I'm dreaming!).

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Guest qbipq

OT : maybe some1 could explain to me why nurbs models ARE production ready and polys NOT!?...

 

anyhow, i really wanted to enter this time but i need to travel a little these days... maybe when i'll be back, hopefully there will be enough time left to join in.

 

cheers everyone!

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Guest Eric Lagman
Eric.

 

I read what you're saying, but in actual fact it usually isn't so straightforward as you make it sound. With many models, when you convert them from poly (sub-d for example) to NURBS, there's so much work to do on them to get them right for manufacturing, that often it's just as easy to start from scratch in NURBS. If only it were so easy as to just click on a command....(I'm dreaming!).

 

Yes for some models I am sure it might be like you said. Only time I would consider doing it is for something like this chair. I would never try to model say a housing for a lawn mower, computer mouse or something with multiple components in sub d then convert. Mine as well start with nurbs or parametric data like you said. 95% of the time you have to anyway. For something simple like this one part chair though it is a perfect candidate for doing this and it was in fact a one click operation in rhino using the t-splines plugin. Sorry if there was confusion earlier suggesting it would be wise to use sub-d and convert to nurbs on any project. Most projects it would be very unwise to do this when you need it to be made. I meant for this specific one.

 

gbipq to answer your question as best I can. Polygons are just triangles. The only place they are smooth are on your computer screen becuase the software intorpolates between the triangles to give it a smooth appearance or something along those lines. The data in the file will give you small facets all over your surface like a sterolithography model. Im no expert on this though but I do know you just wouldn't want to do it.

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Guest qbipq

hmm... but i thought that nurbs shells STILL get converted to polygon meshes, when used in milling or whatever....

am i wrong ???

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Guest robbie

Great work Ragde! Nice renders too! Look forward to the walk through! Can you give a few tip son th erender set up as well? Simple yet effective! :specool:

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Guest robbie

Axel, thanks for trying to upload the files, Ragde's quite good with that sort of thing if you e-mail the files to him. :)

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Guest Marc UK

Eric. We agree. The chair is a perfect candidate for executing as sub-d > NURBS, given the right software. In actual fact, yours is the only model I've seen so far that faithfully reproduces the the surface transistions at the shoulder of the forelegs. As I'm sure you appreciate, I was just making a general point to answer Daan's question. (However, I'd be really interested to see your T-Spline based Rhino model to check the quality of the surfaces...).

 

Someone else asked 'don't milling machines convert NURBS geometry to triangles, anyway'. The answer is no. They are translated into vector curves / paths. By contrast, PRT solutions such as stereolithography , sintering, and anything based on .stl files are triangulated simplifications of the NURBS data.

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Guest Daan
Eric. We agree. The chair is a perfect candidate for executing as sub-d > NURBS, given the right software. In actual fact, yours is the only model I've seen so far that faithfully reproduces the the surface transistions at the shoulder of the forelegs. As I'm sure you appreciate, I was just making a general point to answer Daan's question. (However, I'd be really interested to see your T-Spline based Rhino model to check the quality of the surfaces...).

 

Someone else asked 'don't milling machines convert NURBS geometry to triangles, anyway'. The answer is no. They are translated into vector curves / paths. By contrast, PRT solutions such as stereolithography , sintering, and anything based on .stl files are triangulated simplifications of the NURBS data.

 

Thanks for all this useful info. I'm glad I'm using SW for a reason after all! :D

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Guest qbipq

Mark UK > so then :D, is it safe to say that anything destined to end in .stl format is indeed a very good candidate for subdvs models ?

 

well, in any case, could somebody (an insider maybe :) ) point out the cases in which .stl files are called to duty, or, respectively when not? (regarding materials, fabrication process etc)

 

and one more thing please, what values of tolerance (positional, tangential, curvature) you recommend for this model?

 

thanks, and a very good day to everyone!

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Guest Marc UK

Dear gbipg..

 

Eric's last post pretty much answer's your question accurately...ie: most of the time it wouldn't be either appropriate nor efficient to model your product or component in sub-d's. This is the state of play at present. However, I think we will see significant developments in the linking between sub-d's and NURBS modeling processes over the next couple of years that will make modeling in sub-d's prior to translating to NURBS an increasingly attractive strategy for IDer's. (watch this space....)

 

Usually, the generation of .stl files is an intermediate stage in the product development process, simply produced by exporting your NURBS model in this format, for RPT and subsequent evaluation. For manufacturing the product / component, or whatever, it will need to be represented as pure geometry (NURBS) as I said earlier, so really it makes sense to do all your detailed design work in NURBS rather than mess about first producing a poly model intended for RP and then rebuilding from scratch as a NURBS model for manufacturing. Actually, this whole area of discussion is much more subtle and complex than would allow for easy generalisations, but is certainly a very worthwhile topic to discuss and explore somewhere else in this forum (this thread...I seem to remember....was actually to do with this months challenge set by Ragde....so perhaps we ought not to get too far off track here! :D )

 

Anyway, to summarise, you can't really control your design 'intent' with sub-d's the way you can with NURBS. This will specifically have major ramifications when designing multiple components that need to fit together (as Eric alluded to), or when certain geometric conditions neeed to be met.

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Guest John Layne

My attempt, modelled in SolidWorks and rendered in Maxwell. The render could use a more interesting environment, this one took less than 5mins to set up and was left to render for 5 hours overnight.

post-5758-1175975725.jpg

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Hi friends:

This is the BD of my final model...

It is almost the same than the first one, only some modifications to some profiles and other things.

 

Normally I try to model as simple as I can, just to get the feeling of the job, and then refine the design or change the approach drastically if needed. In this case I am not looking for a lot of accuracy, just to get to the shape as simple and fast as I can.

 

If the model were going to be made for a real job, then accuracy and fidelity to the sample is a must.

I know that the transition from the seat to the legs is the most demanding, and there are many ways to get that shape very accurate to the sample. (I will show you something later). But now...

post-4562-1176342874.jpg

post-4562-1176342897.jpg

Some settings for rendering to come...

 

Ciao

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