Repeatability

[richards]

I had some extra time on my hands, so I decided to run a 'repeatability' test on my PRT-Alpha 120X60. The test was very simple. It consisted of using two magnetic based dial indicators at opposite ends of a 12" X-axis travel. It was amazing to see that the X-axis was repeatable to much less than 0.001" at all speeds ranging from 2-ips to 10-ips. It was also a little disheartning to see that very minor finger pressure against the gantry could easily change the end-points by more than 0.075".

[pfulghum]

Mike,


I bet you get repeatable readings when you always approach the dial indicator from the same direction. Do this experiment....

The dial indicator has about 1" of travel.

repeat:
mx 1
#setup dial indicator to be barely compressed.
mx 0.9
pause
# take reading going decreasing x
mx 0.8
pause
# take reading going decreasing x
mx 0.5
pause
# take reading going decreasing x
mx 0.4
mx 0.5
pause
# take reading going increasing x
mx 0.8
pause
# take reading going increasing x
mx 0.9
pause
# take reading going increasing x
goto repeat


When I was running the 1:1 Alphas I would get repeatable results down to .001 at each measurement point, but depending on the direction of approach (increasing or decreasing X) the points were NOT identical.

I will rerun this experiment later tonight with my 7.2:1 alphas.
-- pat

[richards]

Pat,
I followed your suggestion and ran the following file:

SR
MS,10,05
LOOP:
MX, 12
PAUSE
MX, -0.1
PAUSE
MX,-0.1
PAUSE
MX,0.2
PAUSE
MX,-0.3
PAUSE
MX, 0.3
PAUSE
MX, -12
PAUSE
GOTO LOOP

The maximum error was 0.0015 inch, non-cumulative. After 6 repititions, I ended up exactly at the 0" mark on both ends of the test. The error showed up in the small 0.1" moves. I picked 0.1 inch moves because my dial indicator moves one full revolution per 0.1 inch.

My machine is not quite stock. I have 3:1 belt drive gear boxes on both X-axis motors and on the Y-axis motor. Also, I proved again that the machine is robust. I'd forgotten that I used the SR command to set relative mode and gave the command MX, 12 after the MX,-0.3 command. That caused the gantry to overrun one of the dial indicators and jump the rails. No harm done - except to my ego.

[pfulghum]

Mike,

I too have great repeatability, but I still have a question..

Approach the SAME point from two different direction, and never loose pressure on the dial indicator.

For example.

' setup dial indicator to show some pressure.
pause
zx
loop:
mx 0
mx -0.1
' take Reading at Point #1
pause 4
mx -0.201
pause 1
mx -0.198
pause 1
mx -0.1
' take reading at point #2
pause 4
mx 0
goto loop

Both measurement points are at the same "X Value" and I bet your get repeatable values for each measurement point #1 and for measurement point #2....BUT....

What is the difference in the values between point #1 and point #2? This difference is a measure of your "backlash".


With my Alpha 1:1 motors I got about .003 with my 7.2:1 I see zero/nada/zilch difference between point #1 and Point #2.

I'm wondering what you get with your belt drive.

-- Pat

[richards]

Pat,
I've run your test ten times. The greatest variation that I get is when I take the reading after doing the MX, -0.201 command. At that point I'm less that 0.0005 inches short (less than 1/2 a tick mark on my 0.001 reading dial indicator). The only changes that I made to your program was to take out the timed pauses, leaving the command, PAUSE. That gave me time to waddle over to the machine and closely examine the dial indicator after each move.

All in all, I'm pleased with the results. On my machine, there are two places that can cause backlash. 1). The spur gear to rack tension, and 2). The timing belt tension. It appears that the factory suggested 3-1/2 turns on the turn-buckle are about right to keep the spur gear and rack from being sloppy. It also looks like I've adjusted the tension about right on the timing belt.