Re: CNC setup - setting accurate number of counts per unit..

Thanks for the advice.

I'll ponder that and give it a whirl...

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Jon Elson <elson@p...> wrote:
>
>
> dspinnett wrote:
>
> >This may be a dumb question, but I have converted my mill to CNC,
> >and ball screws, and it all works fine.... I'm not sure however
if
> >it is accurate or not.
> >
> >I tried to set up the CNC my moving 1.0" per a dial indicator and
> >then adjusting the software counts to match. When I put a tape
> >measure on the table and move the full travel, its off a bit...
How
> >do I get it set accurately? apparently a 1" travel dial indicator
> >isn't enough, and I'm not comfortable 'eyeballing' an tape
measure
> >over the 12 and 24" of table travel..
> >
> >
> Is it repeatably off the same amount each time, or does it vary?
> Did you make your 1" measurement with both moves in the same
> direction? If not, then you are measuring the move amount MINUS
> the backlash. Don't assume that just because you put a ballscrew
in
> the machine that there will be no backlash. Even if the ballscrew
> does have zero backlash, there is leadscrew bowing, yoke and
> screw end brackets flexing, screw end play, belt flex, and one
> more, rocking of the table. Unless the leadscrew applies force
> exactly in-plane with the slideways, a torque that will attempt to
> rock the table is developed.
>
> It is a bit tricky to make the travel measurement all from one
> direction, but
> by putting something in the quill that can be raised to clear the
> indicator, you
> can do it. You move so that the dial plunger approaches the
object in the
> quill, and bring it in until you have a movement of the dial.
Zero the dial
> indicator at this spot, then zero the axis, too. Now, command
your machine
> to make the 1" move, and see what reading you get.
>
> (You can do this in reverse if you are afraid of smashing the
indicator.
> Push the plunger almost all the way in, then bring the table and
> indicator up to the
> quill. zero the indicator and CNC axis. Jog away from the
indicator and
> back, and you can read backlash. Jog indicator a little closer,
then go
> back to original CNC position, and rezero the indicator. now, do
CNC
> move 1" farther away, and you read the 1" move without backlash.)
>
> Jon