Re: [CAD_CAM_EDM_DRO] Which to trust, home-built DROs or hash marks?

John A. McFadden wrote:

>I recently completed fabricating and installing the Y axis of my home-built
>DRO (using a 25 cycle per mm linear strip from Encodertech.com, which gives
>a DRO resolution of 0.00039xxxxxx inches per jump of the encoder after
>quadrature) using Art Eckstein's DRO program and interface plans (I used
>linear strips instead of rotary disks because I dont have a lathe to turn
>the bushings).
>Now that I have the Y axis complete, there comes the question of
>calibration. My mill is an Enco 16 3/8" Mill/Drill, and I found that when I
>milled two flats onto a billet of aluminum such that the flats would be
>1.500" apart (using the hash marks on the mill's Y handle to determine size)
>using a 1/2" endmill that the following measurements resulted:
>The Mill/Drill's hash marks read out 2.000" between centers (so the flats
>should have been 1.500" apart)
>My digital calipers (cheap chinese) said its 1.514" between the flats
>My standard calipers (not dial) said its 1.515" between the flats
>and my digital readouts (I calculated the incremental value using the 25
>cycles per millimeter and calculated it out to be the 0.00039xxxxxx number
>which I then plugged directly into the program, rather than using a scale
>and needle for the calibration because I just ordered the scale on Thursday
>and it hasnt shown up yet) said that the endmill moved 2.0164" (saying that
>flats were 1.5164" apart).
>
>

OK, your calipers are probably the most accurate, although a 1-2" micrometer
would be better. Why does the part measure larger? Because of tool
deflection!
The reaction to the cutting forces deflected the tool probably 1 - 2
thousandths
of an inch. Did you compare making the same cuts with both conventional
and climb milling? How about with removing .010" stock on the finish pass,
and .001" for the finish? Have you miked the cutter itself? They are often
slightly off size, especially if it is a lo-cost import.

(now the embarrasing part) Did you make sure that both positions were made
approaching from the same direction, to eliminate backlash? And, of course,
you have to lock the axis after making the setting, as the cutting
forces will
push the table back. Since you have a DRO, now, you can measure the screw
backlash directly. I'm guessing from the above measurements that you did
NOT compensate for backlash. You have to make a move of several inches
and compare the dials to the DRO. If the error appears relatively constant,
that most likely is your backlash. if it is cumulative with distance, that
is probably a thread accuracy issue.

>Now the question comes up, what to trust? Should I just accept the 1% error
>in the hashmarks on the mill and only use the DROs (the X and Z axis parts
>should be arriving on Thursday)? Or should I investigate further to find out
>if there's any other error in the system?
>
>

You should always investigate fully until you have a good understanding of
the errors, and where they come from. A set of gauge blocks is
invaluable in
making these measurements. A straight edge clamped to the table, and a
dial test indicator in the spindle makes a very good accuracy testing rig.
You put in a gauge block against a stop, and move the table until the dial
indicator touches the gauge block, and you get some particular reading on
the indicator. then, zero your screw dial or DRO, and move away. Drop in
a gauge block, and bring back into contact and the same mark on the
indicator. If you put in a 1" gauge block, you should see a 1" change in
the handle dial or the DRO. If not, there is an error to be found. If you
use gauge blocks, they are more accurate than anything else you can get
your hands on, so you know all other errors are in the machine.

Jon