Re: DRO

Joe,

Lets start from the botto, I also felt the same way when I started this
project. However I am using a "cable" that appears to be plastic coated
multi-strand wire. It is the stuff that is used on a parrallel bar for a
drafting table. Its tough and so far, does not appear to be slipping.

Check the diameter of the shafts on you encoders as mine were advertised at
3/16", but are actually 5mm. Makes a big difference when you order the
drill rod for the shaft and try to stuff it in the hole???

So far, on the two axis that I set up using this with the 5mm shafts (no
diameter reduction), I am getting a consistant 0.000177" resolution for my
units to count by. On the z axis, I am attempting to grouve the shaft to
reduce the diameter so I can get that magical 0.0001 resolution; but will
settle for 0.000125. I am afraid of the slippage problem at such small
diameters along with the possibility of bending the shaft due to tension of
the cable. It appears we are talking physical diameter of the shaft in the
bottom of the grouve on the order of 0.0988" ! But I had to buy a 3'
length of drill rod so I guess I can afford to try as I don't see any
additional projects in the near future that will use 5mm rod for anything
and I use only about 2 1/4" of rod per set of axels.If I start having
problems with this type of wire, may try some of the heavier guitar strings
(don't ask what note, I ain't no musician;_) )as it appears these have a
circular wrap around a central core thus giving the effect of a "tire
track" which would be parallel to the axis of the shaft rather than the
strands that are perpendicular to the the shaft.

Hey, I'm just a country redneck that is doing this for fun and the fact
that the numbers on the computer screen are eaiser to read than the ones on
the dial and I have trouble trying to read a scale even with a loupe! So
even if I am off a thou or so, that is still at least 20-30 times more
accurate than I can do with a scale and a scribe!!!!!

All fun aside, I want it to be as accurate a I can make it. Considering
what I am starting with:

1 An oriental mill/drill which we all know has its problems in the accuracy
department.
2 Acme thread screws that have a ton of backlash in them.

Yes, I desire positional absolute accuracy, but more often than not a
higher desire is to have relative accuracy. For instance, if I want to
mount a motor with a bolt circle fastening pattern, I want the central
mounting hole and the bolt holes to be really accurate. If it is off-center
of the plate I am machining by .050, big deal. In the case of the blocks
being machined for the DRO project, it is important to have consistant
accuracy between the bearing hole and the mounting holes etc. Exactly how
far from the edge of the block is of minor importance.


Another point on the encoders; when I ordered the encoders, I also got some
bearings they had advertised. Well, I ordered 10 and destroyed one of the
motors in trying to see how everything was put together. Got down to the Z
axis, and needed another bearing...Had run out of the loose bearings so I
went to dig the one out of the encoder end of the motor I destroyed. It has
a different OD than the one in the nose of the motor which was the same
size as the loose ones that I ordered. So If you are planning on useing
those bearings, please be aware of this small problem.

Art

At 09:08 PM 02/23/2000 -0800, you wrote:

>From: Joe Landau <jrlandau@...>
>
>Art,
>
>I am starting to build a DRO for my mill--have bought motors
>and encoders from Goldmine, and am planning the setup. I'm
>curious how your wire loop worked out, --what path did you
>use, and what kind of wire. I remember reading that Steve
>was worried about slippage. Have you experienced any, and
>how have you prevented it?
>
>Intuitively, I'd think a steel wire on a very small steel
>shaft could slip very easily.
>
>--Joe Landau

OLDER THAN DIRT

Country Bubba

(Actually the inventor of Country and Bubba)