Re: 3D Contouring - Loss of Accuracy

>    From: Peter <prenolds@...>
> Subject: 3D Contouring - Loss of Accuracy
>
> Hi all,

<s>

> Is it possible that even a servo system can occasionally lose a step or
> two per million?  If so, could that be why commercial CNC machines use
> glass scales mounted on each axis to provide an absolute position
> feedback loop in addition to a (motor) shaft mounted encoder?  I'm not
> quite sure how to incorporate such a thing, but I think it may be
> possible with G-Code Controller and Indexer.  
>
> Anyone with experience in CNC contouring of large 3D g-code files or
> how commercial CNC machines are made - can you help with any
> suggestions? Thanks.

Hi Peter

I don't qualify with experience in contouring of large 3D files but I can
say that most commercial CNC's do not use glass scales for feedback nor
do servo systems with full feedback loose steps occasionally. A loss of
position on this type of servo system indicates a serious problem.

I'd be thinking that maybe the timing of the step and direction pulses
from your PC don't overlap quite enough for the Gecko and occasionally
leave out a step during a rapid change of direction.

Ray