2d, 2-1/2d, 3d

[Chunky]

Just read a post about 2d versus 2-1/2d versus 3d. I read another explanation online that explained it very differently. One of 'ems wrong. I really don't need to know every little thing, but I'd hate to lay down good bucks for a CAD program and then learn that it won't work with my CNC router (doesn't exist yet) because it's the wrong "d". I want to machine things that are more than just profiles. To me- a profile is something you'd cut out of plywood with a jig saw. It has a width (X) and a height (Y). The thickness (the Z axis) is the thickness of the plywood and I had nothing to do with it. I want my CAD to allow me to draw things, twirl them around so I can see what they look like from different views, and ultimately give all that information to my router so it will know how wide, how long, how tall to cut the various aspects. It would seem to me that I would need no less than a 3d program yet I see many recommendations for CADs that are not. My goal for today is to clear this up.

[ger21]

The other post you read here is correct. Unless you have a machine that doesn't have a Z-axis, for all practical purposes 2D and 2 1/2D are the same thing, or can be done in the same CAD/CAM program.

I think the thing your missing, is that, for example, in the other post here, 2-1/2D was compared to stairs. But, you Don't have to draw it in 3D to cut out the parts. How about this. If during your X or Y movements, if the Z-axis is always at a fixed height, the CAD drawing can just be a sreies of 2D lines. Every line can have a different Z height, which may be assigned different ways in different programs, but it can still be done in 2D.

If you want to cut something with curved surfaces with constantly variable Z heights, then you need 3D. Think a ball, or a bowl.

Most inexpensive (under a few thousand $$) 3D CAM programs generate toolpaths this way. You pick an axis, say X for example, and the tool will go to a certain Y location, and travel across the surface of the part, moving up and/or down to follow the contour. When it gets to the other side of the part, the y axis will move over a fixed distance (known as the stepover), and the tool will return back to the other end. Keep going back and forth until the part is finished. This is usually done with a ballnose tool. The closer the stepover, the longer it takes. Also, as the tool moves along the X axis, it doesn't follow curves exactly. They are broken down into very short, straight segments. You shouldn't be able to notice this, though. Once the part is complete, it will appear that the machine moved in arcs, as the part should be smooth.

This process can take several hours on even small parts, if a very smooth surface finish is required. Because of this, whenever possible, 2-1/2D machining is usually utilized. If you look carefully at an object, you can usually break it down into a series of 2D operations, with possibly different Z elevations. With careful though, you can actually make quite complex "3D" parts using a 2D CAD program. Many times at work I've made curved crown mouldings on our router, just using a 2D drawing.

Most CAD programs out there will let you draw your objects in 2D or 3D. Enen inexpensive ones like DesignCAD and TurboCAD. But, for milling purposes, you won't want to use the 3D drawing (model) for your CAM program. You draw a 2D drawing and send it to the CAM program. A simple part that might take 1 hour using 3D CAM, might take 5 minutes using 2-1/2D CAM.

Hope this makes sense.

[Chunky]

Hope this makes sense.

Hmm. I'll have to read that a few dozen times. As usual-- I need a picture. I'll come up with something.

[Chunky]

pic
Obviously either of these can be machined with repeated changes in the Z. Does this picture make it any easier for you to tell me how 3d CAD would make one easier than the other?

[ger21]

It's really a CAM issue more than it is CAD. If you use a 3D CAM program, like MeshCAM, http://www.grzsoftware.com , or any inexpensive 3D CAM, typically you can't specify what to mill and what not to mill. So the entire part would get machined, even the square block at the end.

The example you show on th left with the steps is actually pretty close to what 3D cam would give you. substitute a round nose tool for the straight tool you show, and make each pass so that the tip of the tool is always tangent, and you'll get a finished part like you show on the right with the same technique you show on the left.

Basically, if you have horizontal surfaces that are curved like you show, it is a candidate for 3D CAM. Be aware, however, that it may not be as simple as you'd like. At least without spending a LOT of money.

This probably didn't help much, but I'm getting ready to go on vacation and don't have a lot of time right now.