Re: Contouring and CVV

> Date: Thu, 27 Dec 2001 08:41:11 -0000
> From: "ballendo" <ballendo@...>
>Subject: Contouring and CVV

>And the point about centerline vs. "tool" velocity is well taken. I
>think most experienced programmers are aware of this and DO
>make/insert F/R changes when programming c/l, but it sure would be
>easier if it was a CAM "menu" item/option :-) (Or in the control, but
>That's not your "bread"<g>)
>

Only works if the control is aware of the tool size. Some stepper machines presently do something similar with the "ramping" settings. But....It's buried DEEP internally to the controller as they just barely work and if they let the customer play with the kinematics, somebody has to answer the phone to help the customer reset his system to a usable configuration. Using external feed rate control utilizes the controller setup, which should be optimized, and also allows the user to "play" and then to optimize for the particular application, rather than the generic or average ones that the OEM has used to determine a usable setup.

>Thank you for this post! I'll be watching for a Vector update...
>

It's available for OEM use now.

>Ballendo
>
>P.S. What form of contouring do you consider the "best"? I hope my
>post was clear that when I use the term "CVV", I mean the control
>will attempt to maintain as "constant" a velocity as possible,
>toolpath/machine physics considered. So the actual f/r may be slower
>than is programmed at certain "tight curve" areas in the toolpath,
>but it will ONLY be slower for as long as physics dictates.

The ideal contouring is probably quite different from constant anything, except

1) for roughing, constant HP, including the effects of "crack the whip", and depth of cut.
2) for finishing, constant feed per cross sectional cut area(similar to constant HP, but finer control), with some adjustments for long cutters making varying depth of cut and then fluid(smooth, gradual, non-jerky, etc) velocity changes for cutting around corners that impact geometric accuracy due to the inertia and "strength" of the system, motors, frame, etc. Some servo systems are quite good at making the "fluid" movement, but then at the same time leave these big arcs, even turning a circle into an oval shape at high speeds(because of inertial differences between the X and Y axes). Maybe call it Constant Cutting Velocity or optimal. OCV

Constant HP assumes full knowledge of cutting conditions, including tool size and spindle speed. I view constant velocity contouring as a compromise over what people will spend for an option that potentially only 5-10% of a controller manufacturer's customer base will use.


Best Regards, Fred Smith- IMService
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