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

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