Re: backlash testing

>Message: 25
> Date: Sun, 04 Feb 2001 02:00:42 -0000
> From: "tauseef" <alieron@...>
>Subject: backlash testing
>
>hey guys
>I have some backlash compensation through CNCPro but have figured out
>that I really don't know what is the best way to figure it out. The
>way I did before was to out a dial indicator in the vise and that on
>the mill. I then put the point of the indicator against the spindle
>and moved my axis back and forth a few mm and then checked to see if
>it returned back to "0."

I will throw out a suggestion & maybe others can also comment if they have found good methods.
A dial indicator has a plunger style actuation and a test indicator has a lever type actuator.

1) Backlash can only be checked on one axis at a time, and it must be done AFTER
gib adjustment, ball nut tightening/adjustment, etc. In other words, you set up your
machine so that it doesn't wobble or have much side play, if you push on the end of
either X or Y axis.

2) To check backlash the key is to do it under programmed feed conditions, NOT by jogging.
And you might also experience different amount of back lash under freewheeling
vs cutting conditions.

Lets assume that your finish cuts are light & that is what you want to be really
accurate, and that roughing will just be called "deflection". ;-)

Your dial indicator must be perfectly parallel to the axis that you are checking.
Use a test indicator in the spindle to make sure that the axis of the dial indicator
is perfectly parallel to the axis to be checked.

Set the indicator against a pin in a chuck or against the chuck itself, and preload
it so that it has at least as much pressure as you expect the backlash to be. If you
think it is .010, set to .015 travel. Set the axis to zero at this position, and zero out
your dial indicator.

Rapid at least 1 inch away from the contact point, then use a small feed rate of say
1 inch per minute feed back to the start point.

Record the reading as a delta from the original zero point.

Next move 1 inch away along the other axis, then rapid 1 inch past the indicator tip,
and use the same 1 ipm feedrate to move back to the starting point.

Now program the spindle to return along the other axis to the point at which it started,
opposite the indicator. You may want to use your hand or a tool to push the indicator out
of the way to avoid any side pressure on the indicator stem. Record this value as a delta
from the original zero.

The total of both readings is your backlash.

Adjust your software parameter by the amount of remaining backlash. Test again to make
sure that the adjustment was correct.

Do the other axis.

Recheck both to be sure that there is no interaction between the backlash adjustments.

An alternate might be to use a test indicator, mounted in the spindle. Tram in a dowel pin,
mounted vertically in a vise.

Perform the same program moves, but lift the spindle so that the indicator can clear the vertical pin.
This may allow you to just program the moves within a single axis and not have to move off
to the side to get the direction from "behind" the dial indicator.



Best Regards, Fred Smith- IMService
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