Homebrew PCBs

While I don't fully know the exact details, I can tell you right 
away, you cannot rely on software to eliminate the backlash within a 
CNC system.  For basic positioning, perhaps a little, but in 
practice, no.

While software can compensate for the static deviation between where 
it told the motor to position and where it really is based on 
encoder feedback, the "Slop" is still there.  During cutting, the 
dynamic cutting forces will cause the axes to move around within the 
mechanical slop.  When drilling, the position will be off and the 
holes will not be on size.  When milling, all sorts of dimensional 
problems pop up and in the end, you will have wished you took the 
time the eliminate the mechancial backlash.

While there is backlash compensation within all CNC systems I have 
used in the industrial world, it is primarily only used to 
compensate for a very samll amount of backlash .001" or less 
usually. Beyond that and it is time to correct the mechanical 
problems.

So, don't waste your time or effort in thinking of a software 
solution to backlash - backlash is a mechanical problem and must be 
designed out of the system for even the most basic of machine 
performance.








--- In Homebrew_PCBs@yahoogroups.com, "Stefan Trethan" 
<stefan_trethan@g...> wrote:
> On Tue, 28 Jun 2005 08:36:23 +0200, mikegw20 <mikegw20@h...> wrote:
> 
> > Now I have never built a CNC anything and I am not overly 
mechanical.
> > Right that is my qualifications out of the way...
> > I was thinking about high torque motors to drive a linear thread 
and I
> > thought about those really cheap cordless drills.  That is idea 
pt1.
> > Now of course you need some sort of feedback for the position, 
the
> > normal way would be to put a shaft encoder on the drive.  My 
thinking
> > is that if you used a linear encoder then you would get absoulte
> > positioning thereby negating the need for backlash 
compensation.  Now a
> > quick look around ebay reveals that linear encoders are not 
cheap.  So
> > has anyone used optical mice for a linear encoder? (idea pt b)
> > Here endith todays musings.
> > Mike
> 
> 
> cordless drills usually use a motor what the model-vehicle 
builders would  
> call similar to a "speed 600".
> Not a bad motor, and you can get replacements really cheap as well 
as  
> better motors the same size.
> 
> I would try to find drills with metal gears for this purpose. I 
recently  
> bought a very cheap one with plastic gears, it is still working, 
but i'm  
> always expecting it to fail any time. It was cheaper than a 
replacement  
> battery for a better drill.
> There are often ebay lots of many drills.
> 
> 
> As for linear encoder, there are those striped plastic strips in 
printers,  
> but a much easier solution would be to use digital calipers and 
use the  
> data-out, 'cause you would get absolute position not only 
relative.  
> Probably more expensive than shaft encoders and certainly more 
expensive  
> than steppers.
> 
> 
> ST