Re: [CAD_CAM_EDM_DRO] Re: Measuring backlash with a ohm meter

Pete,

I guess I should do a little more reading and a little less thinking..lol. I re-read his post and now I see what he meant. Thats another version of an electronic edge-finder of sorts. I agree, that wouldnt work for most mills. I'm surprised that it does read different resistance.

Ron

vavaroutsos <pete@...> wrote:


Ron, I may have mis-understood what Chuck was saying, but I think he
is measuring the electrical resistance from the work to the spindle,
not the resistance to movement on the axis. This would require that
the work be electrically isolated from the mill or the probe be
electrically isolated. Things may have worked ok in Chucks case due
to the construcion of the mill not providing much electrical
conductivity from the spindle to the table, but I don't think this is
a valid assumption for all mills.

~petev
--- In CAD_CAM_EDM_DRO@yahoogroups.com, R Rogers
wrote:

> Interesting concept. However what may seem absolute is often not.

Measuring resistance upon contact is measuring the effects of a slow
minute crash. Contacting a surface to the point the drive begins to
struggle would not be touching off on an edge. Due to deflection. I'm
not ruling out that this would be accurate. It would require testing
involving the method you perscribed versus a conventional edge
finder. If the results parallel that would be a great edge-finding
technique. Design into a machine control software to enter a setup
screen that when resistance is sensed upon a drive it would
automatically zero that axis, spindle rotating as this is more
accurate. Would need to be able to distinguish between touching and
fast jog resistance measured. Could even have a compensation preset
for the radius of the stylus. None for Z obviuosly. Possibly an
extremely stiff die spring in the shank of the edge finder to prevent
damage on over-travel. This would be alot faster than present

> method. Sometimes when I'm in a hurry I just use the end-mill for

the cut as an edge finder, step-jog to touch and then add the radius.

>
> For measuring backlash, a good dial indicator is pretty tough to

beat. It's not something that is measured all the time.

>
> Ron
>
>
> Chuck Rice wrote:
>
>
>
> I just tried something and it seemed to work well, but since I am a
> newbie, I wonted to pass it by the list to see if it might be

useful

> to others.
>
> I wanted to measure the backlash on my Sherline, so I attached an

Ohm

> Meter to my mill. One terminal to the bed, and the other to the
> spindle. It read a resistance of about 2k ohm. Then I lowered Z

till

> a bit touched the surface of the milling plate and the resistance
> went to zero. Then zeroed Z and micro-stepped back up till the
> resistance went back to 2k ohm. Z showed .0012. To test, I lowered

to

> zero again and as Z hit zero, the resistance went back down.
>
> Using this information, it seems that you could devise an
> edge-finding routine for any metallic object that would position

the

> bit exactly in the X, Y, and Z planes with respect to the object to
> be milled.
>
> Has this been tried before? -Chuck-
>
>
>
>
>
>
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