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Adding Low Cost DROs to my Lathe

December 2013

There is a newer entry into the DRO mix.  These are capacitive pickups similar to the technology used on digital calipers.  They are very inexpensive and have a reasonable accuracy of about .001 to .002 per foot of scale length.  Due to their very low cost and acceptable accuracy, these are finding a wide acceptance in the hobby and home machine shop markets.  I decided to use this series of scales on my lathe, as I blew my budget on the mill system.  I bought several of these DRO scales at a cost ranging from about $21 for the 6 inch ones to about $35 for the 36 inch unit.  These units have a small digital readout at the end of a several foot wire.  I plan to mount the displays on a post behind the headstock of the lathe.

This is a small DRO from iGaging Co. which has a printed circuit pattern under the black
stripe.  The sensor is able to capacitively read the locations within this pattern and convert
them to a length measurement.  As you can see it has a relatively small readout panel and the
readout resolution is .0005 inches with an absolute accuracy  of about .002 per foot.  It is
about 1/10 the cost of a standard DRO.

I plan to mount a scale with 36 inch travel on the rear of my lathe to measure the carriage movement, and a second short one on the cross feed to measure my depth of cut.  I doubt I will be able to fit a third unit on my compound rest, as it is just too small.


It took very little time to mount my 36 inch scale on the rear of my lathe.  There is plenty of room and to test it out, I used two very powerful Neodymium magnets, removed from obsolete computer hard drives.  Mounting consisted of placing a magnet on each mounting bracket and placing it on the lathe.  It holds very well.  I then made a formed sheet aluminum bracket which attaches to an existing bolt on the carriage and I had a working DRO for the Z axis.  I know!  Z axis sounds wrong, but technically the Z axis is defined as the axis parallel to the spindle on a machine tool, therefore the carriage moves in the Z axis.

In operation it works great.  It displays the position to the nearest 1/2 thousandth.  You can use a relative mode and zero the scale anywhere you like without disturbing the primary readings.  It displays inches, millimeters, or inches and fractions.  The latter seems totally useless to me, as it reads in 64's of an inch which is about 30 times its minimum resolution (and who can look at fractions in 64's and instantly comprehend the magnitude?).  The biggest problem I see with these scales is that they use small enclosed batteries which risk early discharge if you forget to turn them off when you are finished.  I plan to look into an AC powered supply for them after getting further into the project.

The first scale I mounted was the lathe Z axis.  This is a 36 inch scale.  I have
initially mounted it to the lathe with 2 strong magnets.  The read head is attached to the carriage
with a small aluminum bracket.  Now that I have proven it out, I will replace the magnets with
tapped holes and screws.  In this picture the readout is still plastic wrapped and is laying in
 the foreground.

I also temporarily mounted a much shorter scale on the cross slide, but there are problems there.  As presently configured there is really only one place to mount the scale.  That is on the spindle (left) side of the cross feed, with the slider between the ways.  In this position, it is quite possible to run the read head into the chuck if you are working close to that end (fortunately, I never did!).  There is not adequate material at the rear of the cross slide to mount the scale, and in this position, it accumulates a major portion of the machining swarf.  This is particularly obvious when I machine steel, as each end, where the magnets hold the scale, builds up a large ball of chips.

The temporary cross slide scale is also mounted using magnets.  This mounting is not
satisfactory as the read head will hit the chuck if the carriage is moved too far.  Also it is
mounted  in the area where the most chips fall, many of them hot enough to melt plastic.

This mounting method was just not satisfactory, so I made some changes to the cross slide and mounted the DRO on the right side of the carriage.

Final DRO Mounting March 26, 2014

To mount the scale on the right side of the cross slide, I had to machine a true mounting surface.  I then mounted a strip of aluminum as a backing plate for the DRO.

I used a length of drill rod clamped in the dovetail to align the cross slide.  I then
took a surface cut of about .050 where the aluminum is to mount, to make a
uniform mounting surface.

I have mounted the backing plate.  As the plate covers one of the gib adjusting
screws, I provided a clearance hole.  Also after taking this picture, I beveled
the end of the plate to allow allen wrench access to the compound clamping screw.

The left picture shows the aluminum backing plate mounted to the cross slide.

The right picture shows the scale mounted to the plate.  I verified that it is parallel to the travel within .002.  The read head is attached to the carriage
using a thin aluminum bracket, machined to allow flexibility in the two minor axes, and good stiffness along the axis.

I then went back and removed the magnets from the Z axis scale, drilled and tapped the mounting holes and mounted and aligned the scale.

The longitudinal axis DRO scale is now mounted with screws instead of the magnets.
I never had a bit of trouble with the magnetic mounting, but this is secure.  You can
also see the narrow flexible bracket driving the X axis DRO in the upper left of
the picture.  The finger will readily flex sideways and vertically, but is very stiff
in and out, driving the read head.

I mounted the DRO heads and my tachometer on a temporary mounting bar.

As part of the "Replacing my lathe motor" section, I mounted the DRO heads in a multifunction control box instead of the bar of metal I was temporarily using.  This is much more satisfactory!

The DRO heads are mounted in a control box along with the MachTach and the
VFD and lathe motor controls.

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Richard S. Mason  12/2013