Re: Measuring backlash more accurately?

For positoning purposes Backlash is always a bad thing,
there is nothing good about it, the more you have the worse
it is.
Software ability to compensate helps a bit dealing with it but
that dose not in any way eliminate the problems backlash causes.
A new Acme screw and nut will have some clearly defined
backlash that is taken up when the direction of motion changes.
The weight of the Machine table is enough resistence to make
it do that reliably and precisely messurably.
As wear and tear take it's toll things get more sinister so.
Increased wear will cause ridges groves and burr's to build up
on the mating components that cause them to resist the proper
takeup of backlash durring reversals.
The weight of the table alone may not be enough to cause backlash
to be taken up reliably.
The screw will sometimes just hang on to the ridge it has dug itself
and will pop only after sufficient force is exerted upon it.
Needless to say that at this stage operation will become highly
erratic and seamingly random.
You will have to make the jugement call when it is time to replace
components for a more reliable operation here.
Good Luck


--- In CAD_CAM_EDM_DRO@yahoogroups.com, Randy Gordon-Gilmore
<zephyrus@r...> wrote:

> I have been fighting a problem on the gantry mill. The Y axis

seems to

> "drift" so that over the course of macining a contoured part (say

10 depth

> passes of outline and internal pockets) it has moved 20 thou or

so. For a

> part that has 30-thou walls, this has created great frustration.
>
> After completely rewiring the controller box, fully separating

signal from

> power grounds, minimizing wire runs, twisting pairs, etc. I made

another

> trial run this morning, with no help.
>
> I realized that the backlash settings might not be right. (That's

about

> the only thing left...) I had measured the backlash on each axis

by the

> conventional method (move + increment, zero indicator, move +

increment

> again and then - increment and write down the reading) but after a

program

> that has maybe 100 Y moves, it seems to me that I need a more

accurate way

> of measuring the backlash.
>
> Is there a way to cycle the axis so the backlash accumulates? I

could then

> run the cycle 10 or 50 or 100 times and divide the resulting

accumulated

> backlash by the number of cycles. I can't think of a cycle though,

that

> just doesn't run back and forth in the backlash window.
>
> The only other thing I could think of is to input a backlash value

that I

> know is wrong, and run the machine back and forth and see how

it "drifts",

> tweak the backlash value, run it some more and repeat until it

doesn't

> drift.
>
> I have a Starett "Last Word" indicator with half-thou graduations.

I can

> estimate to maybe one-tenth but maybe more reliably to 2-tenths.
>
> Any advice?
>
> Thanks!
>
> Randy
>
> Randy Gordon-Gilmore ,----.___________ ______________

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