Re: Climb Milling (again)

Smoke,

A value for the backlash or "reversal" compensation for each axis is
entered manually into a table in the controller. The value is pre-
determined by the operator or machine installation tech. A value of
acceptable accuracy can be determined with nothing more than an
accurate dial indicator, sometimes a method of putting a repeatable
load or drag can be helpful, too. On systems with a great deal of
wear the actual value of the compensation factor would be different
at different positions along the lead screws. In that case you would
have to choose an acceptable average value and live with it.

Also, it seems some here are thinking in terms of large amounts of
backlash. When I got my biggest mill it had pre-entered values of
less than .0005" for X,Y,Z from the factory. Since the X table on
that machine must weigh well over 500 pounds it's hard to imagine any
cutter could make it "jump" under normal conditions.

thanks,

Doug

--- In CAD_CAM_EDM_DRO@y..., "Smoke" <gordonr@r...> wrote:
> I've been looking at some of these responses for "software
compensation for
> backlash".
> Real HOGWASH IMO. If the machine "jumps" due to excessive load
under
> backlash, ho the H is the software going to know how much "jump" to
> compensate for? Furthermore, how's the software going to know how
much
> backlash you have in your machine...or how much extra backlash to
compenste
> for due to wear or poor machine adjustment?
>
> Smoke
>
>
> >Ron,List,
> >
> >Your snipped quote below brings up a good point. Most software
> >backlash comp is done at the "mathematically correct" point.
> >
> >I have turned off b/l comp in most pkg's I have used, but I got to
> >looking HARD at the problem when writing my own code.
> >
> >I came to the conclusion that one of the problems is that the b/l
> >comp was being applied at this "mathematical" point, when in fact,
> >the axis would not ACTUALLY change direction for at least a few
more
> >steps. This often happens in arcs, since the finite set of points
> >"available" to the system do not often coincide with
> >the "mathematically correct" set of points for the curve.
> >This results in the backlash comp INTRODUCING a "bump" since it
> >applies the correction before the axis changes direction.
> >
> >NOTE:If the cutter forces are strong enough, when compared to the
> >forces required to move the table; then the cutting forces
WILL "take
> >advantage" of any backlash available in the machine. However, it is
> >good to remember that cutting is a dynamic action with many forces
> >and resistances at work, including mass, stiction, inertia and
> >momentum. In more than a few cases, these factors will(or can be
made
> >to) work in FAVOR of the desired toolpath, even where backlash is
> >present on the mill.
> >
> >Back to my point: One easy way to see this effect is to draw
circles
> >arbitrarily on graph paper, then use the vertices of the graph
paper
> >as "achievable" points in the system, and draw the "best"
> >approximation of the circle (the one first drawn) that you can.
> >
> >Not only will this be an eye-opener for many OUTSIDE of the b/l
comp
> >implications, it will show that the mathematically correct point
> >(where the s/w typically inserts b/l comp) is NOT the same place as
> >the change of axis direction!
> >
> >One way to "fix" this is to set a flag (in the s/w code) when a
> >quadrant boundary is crossed, but NOT to APPLY the b/l comp UNTIL
the
> >dir bit changes... I can state that this DOES do a better job on
> >arcs; compared to simply adding steps in at the quadrant boundary.
> >
> >Admittedly, there are still going to be problems with any "loose"
> >machine. But we need to look at how we can BEST compensate for such
> >limitations (including using the s/w), rather than just calling the
> >whole thing "bogus", IMO. And as I mentioned previously, not all
> >machines controlled by these s/w pkgs. are "milling", and/or
working
> >against(relatively) high cutter loads...
> >
> >Hope this helps.
> >
> >Ballendo
> >
> >>--- In CAD_CAM_EDM_DRO@y..., ron ginger <ronginger@r...> wrote:
> >>After all the good explanations and horror stories of the results
of
> >>climb miling and backlash let's hear from the guys that think
> >>software compensation for backlash is a good, or even useable
thing!
> >
> >>Sure, the software can cause the motor to backup to the right
> >>mathematical spot<snip>
> >
> >>A great bit of proof why Ive always maintained software
compensaton
> >>for backlash is bogus.
> >
> >
> >
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