Re: Re: How to go from 2.5 D to real 3 D ??

>Message: 20
> Date: Thu, 22 Nov 2001 18:14:50 -0000
> From: dougrasmussen@...
>Subject: Re: How to go from 2.5 D to real 3 D ??


>A machine RATED as 3D has to do circular interpolation with
>simultaneous moves in all 3 axii. Quite a difference from just DOING
>3D machining. (Helical interpolation, by itself, doesn't qualify a
>machine as 3D since that's a special case).

Doug, I suspect that a salesman was trying to sell you a bill of goods to get you to buy his machine. 3 axis is 3 axis. Linear movement qualifies. Few controls actually support 3D arcs in any but the normal planes, as defined with G17,18,19. Certainly none of the hobby class machines. The output from most high end Cad-Cam systems is actually short linear moves. That is one reason that the folks at Northwood designs have been successful. They produced a program that could fit arcs to a CNC program that was created by a CadCam system with short linear moves. This makes a greatly reduced program in that a thousand lines can be replaced by a single G18, arc command. In the days of hardwired controllers with out drip feeding hard drives this kind of programming could slow down the best machines on the market. The optimization with arcs made many older machines capable of 3D surfacing in a much more effective way. It takes the stutter out of the processor limited controllers.

Here is a reference to their programs, Metacut: http://www.nwdesigns.com/oldsite.htm


Now, here's the reason that I opine that 3D is 3D. The cutting of a 3D arc at the lowest level is simply the issuance of a series of X-Y-Z movement commands. That is the basis of stepping or servo control. An arc is always cut as some minimal resolution, but still a digital representation. If you take any 3D arc and replace it with a series of short linear moves, at some point the actual cutting that takes place is identical between the two methods. That is why I contend that 3D is 3D. If a machine can cut simultaneous 3 axis linear moves, the addition of 3D arcs does not any further enhance the controller's capabilities.

Best Regards, Fred Smith- IMService