3D Model Creation For CNC - How? Do I Use NURBS? If not, what method?

[tommypants]

Hello:

As a former video game artist, I am very familiar with working in 3D and using 3D Studio Max but: I made objects using polygon meshes which I know won't work well (or at all) when making objects to be manufactured with a CNC machine.

My question is what method of 3D model creation is used when making a CNC-ready object?

Do I use NURBS?

Some other method?

I am a complete noob when it comes to CNC so any help from you vets would be greatly appreciated.



Thanks in advance!

Tom

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[txcncman]

A lot of solid models are composed of closed contour geometric shaped that are "extruded" or "revolved" into solids. NURBS (splines) only come into play when you have curved shapes that can't be defined with normal arcs. Once you have a base solid, you can make other solids to boolean add to or boolean subtract from the base solid.

Hope that helps.

[tommypants]

A lot of solid models are composed of closed contour geometric shaped that are "extruded" or "revolved" into solids. NURBS (splines) only come into play when you have curved shapes that can't be defined with normal arcs. Once you have a base solid, you can make other solids to boolean add to or boolean subtract from the base solid.

Hope that helps.

Hmmm... I apprecite your reply, txcncman. But I didn't quite follow you (I should have mentioned that while I have a lot of familarity with 3DS MAx, I am really an art guy and technical terms confuse me quite easily).


Say if I was making a cube and I wanted to round the edges and corners a little.

Would making it out of a poly mesh be readable to a CNC machine?

Again, I am totally new to CNC and simple things like this are stumping a newbie like myself.

[mcphill]

Depends. Many CAM programs can work with STL files just fine, but others can't use it at all. BobCAD "standalone" works fine with STLs, but the BobCAD plugin to SolidWorks can not use STL - this is due to SolidWorks' inability to handle pointclouds well.

I don't know what packages can handle STL and which can't, so it depends on what you get or have access to.

[tommypants]

What I am trying to avaiod is the "faceting" that one gets with making objects out of polygon meshes.

When I have seen objects made in other CNC programs (like Autocad), they seem to be made with splines or NURBS and not the meshes that I am familiar with.

So I was wondering how I should build things in 3DS Max using something other than polygons.

Any help would be appreciated.

And again, technical things like this easily confuse me (maybe my mom and dad were siblings, I don't know) so simple explanations using as little technical terms as possible would be very much appreciated.

Thanks

[txcncman]

I replied to your private message, but wanted to include that answer here in case someone else needs the information. Please know that this is based off my experience and understanding of CAD, CAM, and graphic images.

Raster graphic images and vector graphic images are not readable by the CNC machine control. I hope you have a basic understanding of the differences between the 2 graphic image formats and understand that even raster and vector graphics are not interchangeable. For the CNC machine control, you have to generate cutting tool paths. These tool paths are generated in a similar way to vector graphic images, based on the Cartesian coordinate system (X, Y, Z). To generate these tool paths, most use CAM software to select lines, curves, or surfaces to "drive" the tools around on (think road map). Computer-wise, your poly-meshes do not contain lines, they just cover areas. I know, these areas are represented by lines, but the CAM software usually cannot recognize them as such. If you have made your model with poly-meshes, some way you will need to convert it into vector graphics made of lines, curves, and surfaces. In your cube example, the cube itself would be defined by 12 lines and 6 surfaces for vector graphics. For raster graphics, your cube, depending on resolution, might be made of 100's or 1000's of triangles of the poly-mesh. I am going to suggest you draw some simple 2D geometry with a CAD program and start generating your G-code from that. Then later you can try some 3D stuff.

Adding, the faceting your are witnessing is the fact that these poly-meshes are made of small, flat triangles. So, when going around a curved surface, you literally have a lot of flat spots and not a continuing curve. Graphic artists love poly-meshes because they are in fact inexact and easy to work with within the resolution of the computer monitor/human eye interface. Vector graphics on the other hand, are usually much more precise, well beyond this resolution mentioned. That is about the best I can explain faceting when working with poly meshes.

[awerby]

TX's advice above is good as far as 2d work is concerned. If you're basically concerned with cutting out simple shapes, then polygons are useless and vectors are what you need.

But as I understood it, you were interested in artistic 3d work, which isn't the focus of many of the machinists here. In that case, polygon models work just fine. Your faceting problems are because you're coming from the animation world, which is all about low poly-count models. In animation, you don't see the facets because they're obscured by texture maps. But in CNC machining, the facets are all you get, so you need to use a lot more of them, into the millions. At that point, the individual polygons are so small that they don't show up on the finished carving.

There are plenty of CAM programs that can handle polygon mesh formats like STL. 3DS Max should work fine for generating your models. NURBS surfaces are more economical of computer memory, but few CAM programs can deal with them directly - many of them will mesh them in order to be able to generate toolpaths from a NURBS model. So just go ahead and make your models the same way you did, just make sure the polygon count is high enough - if you look at your shaded model close up and it looks smooth, then it will probably machine that way.

Andrew Werby
www.computersculpture.com

[ger21]

if you look at your shaded model close up and it looks smooth, then it will probably machine that way.

Just make sure the surface isn't using "smoothing".

[tommypants]

TX's advice above is good as far as 2d work is concerned. If you're basically concerned with cutting out simple shapes, then polygons are useless and vectors are what you need.

But as I understood it, you were interested in artistic 3d work, which isn't the focus of many of the machinists here. In that case, polygon models work just fine. Your faceting problems are because you're coming from the animation world, which is all about low poly-count models. In animation, you don't see the facets because they're obscured by texture maps. But in CNC machining, the facets are all you get, so you need to use a lot more of them, into the millions. At that point, the individual polygons are so small that they don't show up on the finished carving.

There are plenty of CAM programs that can handle polygon mesh formats like STL. 3DS Max should work fine for generating your models. NURBS surfaces are more economical of computer memory, but few CAM programs can deal with them directly - many of them will mesh them in order to be able to generate toolpaths from a NURBS model. So just go ahead and make your models the same way you did, just make sure the polygon count is high enough - if you look at your shaded model close up and it looks smooth, then it will probably machine that way.

Andrew Werby
ComputerSculpture.com — Home Page for Discount Hardware & Software

Thank you for your reply, awerby.

Actually, I mentioned that I knew polygon meshes weren't the normal (or best) way of making objects for CNC machines to read.

I was asking about what the standard/acceptable way of making objects in 3D was other than poly meshes.

But your second half of your reply helped out greatly and I thank you.

[ger21]

Actually, I mentioned that I knew polygon meshes weren't the normal (or best) way of making objects for CNC machines to read.

For mechanical type parts, no, but for most decorative carving and artistic type work, meshes are the normal (or most common) method used.

For mechanical type parts, parametric solid modelers like Solidworks and Inventor are the norm.

[tommypants]

For mechanical type parts, no, but for most decorative carving and artistic type work, meshes are the normal (or most common) method used.

For mechanical type parts, parametric solid modelers like Solidworks and Inventor are the norm.

Thank you very much, Gerry. This is exactly what I was looking for.

I have gotten myself Autocad with Inventor and will begin learning it this weekend.

Cheers!
Tom

[ger21]

In the PM you sent me, you mentioned cabinetmaking. I work in a cabinet shop programming CNC's. In the cabinet industry, most people use 2D .dxf files for their CNC parts. These are typically generated by high end (expensive) cabinet software packages.

[BurrMan]

Most toolpath algorithms "do" work from a meshing (triangulation) scheme, but letting them do their thing on a Mathematically perfect NURBS surface, is prefered. Using a poly mesh is forcing it to work on your specifically defined topology of triangles...

For most CAM packages, NURBS will be prefered. Some CAm packages are Mesh only.

[tommypants]

Most toolpath algorithms "do" work from a meshing (triangulation) scheme, but letting them do their thing on a Mathematically perfect NURBS surface, is prefered. Using a poly mesh is forcing it to work on your specifically defined topology of triangles...

For most CAM packages, NURBS will be prefered. Some CAm packages are Mesh only.

Thank you for replying, BurrMan!

I am in the midst of learning AutoCAD.

It is SUCH a non-intuitive program compared to what I am used to (3DS Max) and it is slow going even learning such basic drawing techniques but I am making progress.

[txcncman]

I started with AutoCad at version 2. I abandoned it at 2000. From 14 to 2000 it got worse, not better. I have seen 2002 in action. It was worse still. I can't imagine what people are going through now to try to use it.

[ger21]

From 14 to 2000 it got worse, not better

How does it get worse when all they do is add features??

I've been using it since R14, and imo it gets better with each version. 2013 is outstanding.

[txcncman]

Different opinions? More complex interface, not simpler? Comment based on previous post?

[tommypants]

I started with AutoCad at version 2. I abandoned it at 2000. From 14 to 2000 it got worse, not better. I have seen 2002 in action. It was worse still. I can't imagine what people are going through now to try to use it.

That wouldn't surprise me in the least, txcncman. Many of the 2D programs that have been the industry standard since their inception, I avoid whenever I can because they are so clunky and non-intuitive (i.e.: Adobe Illustrator) and have found much better ones to use.

I tried using Inventor Fusion but for me, the terminology and processes in that program were just so freaking bizarre that I abandoned it before I chucked my computer out the window. It may be better than AutoCad or it may not. I am not experienced enough to know.

I just know that so far AutoCAD is sliiiiiightly more understandable to me but is still frustratingly arcane but I am going to stick with it for now until I find a program that is easier to learn.

[ger21]

I hate the ribbons. In 2010, AutoCAD started to look like the newer Microsoft Office.
Fortunately, the interface is fully customizable. All I have on my screen is a single toolbar, and it hasn't changed in the last 15 years, regardless of what version I'm using. Other than some changes to the icons, AutoCAD 2013 on my machine looks very similar to the way R14 did.

[cadcam]

You not find allot of manufacturing shops that use Autocad these days with software's like inventor, Solid works or Space claim. I would put my attention to solid base software's like these skip the Autocad if you can.