>When drilling, the position will be off and the
>holes will not be on size.
I don't agree with this. Perhaps with a light table and a high runout
drill such as the ubiquitous Dremel one may have an issue. With a
heavy table (the X axis and saddle in my setup weighs over 40 lbs)
and a low runout drill this is not an issue. Backlash compensation in
software is very effective when it comes to precision drilling IMO
provided you know what you're doing. Milling and routing is another
story.
--- In
Homebrew_PCBs@yahoogroups.com, "lcdpublishing"
<lcdpublishing@y...> wrote:
> 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