Dro Updated

The original DRO has worked very well, but as always, you
want more resolution. Doug Fortune on the DRO e-mail list
suggested
using mechanical advantage to increase the resolution. After
studying his message for a while and taking measurements, I
decided to give it a try. This was done by using the original
blocks and adding some pulleys in the middle of the mill. In
my case, I used 5/16" bolts turned down to hold the 5mm ID
bearings and the bolts screw into the same holes as the
original stop plate. Upon observation, it was noted it would be
necessary to put large "tires" on the bearings to be able to
make everything line up and to miss the lockdown bolts for
the X axis slide.

To make the shaft to hold the bearing, I made up a holder to
go in the lathe which will center the bolt. Then carefully
taking very light cuts, the head of the bolt and the shank was
turned down to the 5 mm required for the bearing shaft. After
this was done, the end the shaft was threaded to accept a
10-24 nut to hold the assembly together. By using a nut to
lock the shaft to the mill table, I was able to adjust the pulley
to the required distance from the face of the table so the
wire going to the encoder shaft would be parallel to the table
all along its length.

This is a picture of the finished shaft ready to receive the
bearing. There is a slight shoulder which can be seen just to
the right of the nut that is use to lock the shaft in place on the
mill and the small nut is to hold the bearing onto the shaft.

To make the shaft to hold the bearing, I made up a holder to go
in the lathe which will center the bolt. Then carefully taking
very light cuts, the head of the bolt and the shank was turned
down to the 5 mm required for the bearing shaft. After this was
done, the end the the shaft was threaded to accept a 10-24 nut
to hold the assembly together. By using a nut to lock the shaft
to the mill table, I was able to adjust the pulley to the required
distance from the face of the table so the wire going to the
encoder shaft would be parallel to the table all along its length.

Here are pictures of the x axis assembly on the Mill.

Make sure that all cables are parallel or you will introduce
inaccuracies in your readings. By using this arrangement,
we have now achieved resolutions of less than 0.0001"
(.000089" to be exact) without the reduction in diameter of
the encoder shaft as was done on the z axis. I believe this
results in more reliability as the larger the shaft is, the less
tendency there is for the cable to slip on the shaft.

Here is a line are drawing of the layout of
the various components of this update. By
placing the encoder on the remote line,
you obtain the maximum cable movement
around its axle. Hope this helps you in
deciding how you will do it. There are
several ways it can be done, but; this
setup gives a 2:1 mechanical advantage
on the encoder.