Re: [CAD_CAM_EDM_DRO] Expected precision from acme thread????

Hi Bill;

Why not consider putting rotary encoders on your leadscrews or motors?
The advice you where given is pretty much accurate. Even with ball
screws you have to take care.

I once worked on diamond turning machines where linear encoders where
used for bulk high speed movements and very high resolution rotary
encoders for cutting the optical surfaces. Fun machines to debug.
This does not mean that high resolution linear encoders do not work,
but backlash and machine slop has to be non-existant.

Of course the answer is always somewhat dependant on what you are trying
to do. I could see where for some machining operations this would be
fine, but I think you will want to use rotary encoders for now. The
thing to understand is that backlash and a host of other mechanical and
thermal issues will always be with you. These problems are best
minimized mechanically as much as possible and then dealt with by the
CNC code after that. You can have the best ball screw in the world
and still end up with measurable errors in positioning, it is a sum of
the parts issue. Acme screws though will most likely have to much
backlash to run as you describe with the type of gains most people want
in their cnc systems.



Dave


Bill Kichman wrote:

>I just spoke with (the owner I believe at) Gecko Drives, and discussed linear encoders and such, in connection with closed loop servo control. I was very disappointed to hear that using a linear encoder to alleviate backlash problems will not work well. He indicated that the inertia and hunting that will go on with using servos in closed loop with linear encoders, jumping between the ends of the backlash, make this an undesirable approach. Has anybody got some real life experience as to what effective precision one can attain when keeping the old acme threads, IF THEY ARE NEARLY BACKLASH FREE? My full size milling machine is in great shape for a WWII vintage Van Norman, and I was able to get the backlash adjusted out to nearly zero. Of course the effort required to turn the screw nearer the ends of the table travel are greater naturally due to lesser wear. Am I taking a poor approach in leaving these in? What expectation of precision can I expect?
>Thanks for the input.
>
>Bill Kichman
>103 Old Furnace Road
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