Re: [CAD_CAM_EDM_DRO] Just How Accurate Is CNC?

At 12:06 PM -0600 11/9/04, Jon Elson wrote:

>Chuck Rice wrote:
>The sole reason is COST! Price a 1000 cycle/rev shaft encoder.
>Surplus, you
>can get them under $100, new, maybe almost $300. With a 5 TPI screw,
>1000 CPR gives 4000 counts/rev, * 5 TPI = 20,000 counts/inch or
>.00005"/count.
>
>Try pricing a linear encoder with the same resolution, and you are
>talking about
>$3000, minimum. Now, a standard DRO scale is .01 mm, or roughly .0005"
>per count. This is really too low for proper CNC control even to .001"
>positioning.

I can well understand the cost issue, but motion measurement has
changed in the last few years. Much has been done with inexpensive
lasers. I do not know a whole lot about it, but it seems that if I
can buy a optical mouse for $5, there might be a more precise version
of a laser measuring device that could be mounted to the bottom of a
table to precisely read table movement. The optical mouse is not that
precise, but it also runs in an unknown environment. The table read
surface could be much more consistent. Or you might use a laser in
the saddle reflecting off the end of the table and measure the
trip-time per pulse.

>A second reason is that just because you can READ the machine position
>with a linear scale on the table, doesn't mean you can CONTROL the position
>to that accuracy. If you put the position feedback sensor on the table,
>there
>better be VERY little backlash in the screw and nut, or the servo will
>rattle
>back and forth across the backlash as fast as it can, possibly even burning
>out the motor or servo amp.

Yes, I can see how a damping algorithm would be important.

I have a lot to learn! Thanks. -Chuck-