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Adding a Digital Readout to my Milling Machine



I have always envied machinists who have a digital readout (DRO) on their machine shop equipment.  On my milling machine, I can make very complicated shapes and hole patterns, but it is a tedious and error prone process.  For instance if I want to move to a location several inches away, I have start at my known location and very carefully count how many turns I make of the crank handle (0.200 inches per turn) and stop at the correct division of the dial.  It works very well, unless I forget how many turns I have made or miss the stopping point or misinterpret the starting position.  I also have to make sure I always approach from the same direction due to the backlash in the lead screws.  With a digital readout, your exact position of each axis is constantly displayed regardless of the direction of approach.  In addition, you can do many things, guided by the DRO that would be very difficult manually.  Examples of these would include drilling a series of holes in a circular or arc pattern, a series of uniformly spaced holes along a line, a grid of holes (and these last two are at any angle), machining an arc, including rounding corners, by moving the machine to a series of XY (or XZ, or YZ) coordinates, and a number of other, less used functions.  If you are performing operations normally, all based on a common 0 point, and you want to move a certain distance from where you are, you can change to the relative mode, move until the readout shows the distance you want, then you can switch back to the main coordinate system. It includes a calculator and allows the results to be transferred to any axis you want.  In short, it is a very useful, labor saving, error minimizing device.

A Digital Readout system or DRO, consists of a display unit and 2 or 3 scales depending on the size of the system.  The display head constantly displays the actual locations of all 3 axes (or 2 in smaller systems) and has a keypad or buttons to command many different operations.  There is a scale mounted on each axis.  The scale is a device which contains a measurement standard and a movable read head.  The read head must have at least as much travel as the movement of the machine in that axis.  The scale electronics read the position of the read head, interpret it and send the data to the display so it can display the position within .0002 inches.

This discussion is based on a vertical mill application, but DROs are available for lathes, grinders, EDM machines and other similar equipment.

The price of quality DROs has dropped considerably over the last few years and I decided to see about getting one to install on my mill.


This Rockwell vertical mill is the object of this current project.  The DRO will measure
the longitudinal table position (X axis), the in/out position (Y axis), and the height of
the table assembly on the rear column (Z axis).

I buy my DRO

After doing quite a bit of research looking at various options from a number of vendors including Asian imports on Ebay, whose prices start at under $400, I selected a system from a company called DRO PROS.  They have a wide line of quality units priced not that much higher than the Ebay cheapies for comparable systems, but what appear to be a very high quality systems with excellent English language support!  They are located in Vacaville California and do nothing but sell and service DROs.  (I fondly remember Vacaville as the former location of the Nut Tree airport and restaurant.)  The unit I selected is a glass scale, 3 axis unit with a 3 year guarantee.  They have an awesome website (www.dropros.com) with vast quantities of helpful information on all aspects of selecting and installing DROs.  Their site also has many very well done "how to" videos.  In the process of looking I did find another American company whose prices were quite competitive with DRO PROS.  I emailed them a fairly simple question about their units and have still not received an answer.  DRO PROS answered a similar email within 1 hour.

Scales

They sell systems with two types of scales: glass and magnetic.  The optical glass scales are the most common, but can suffer from contamination if not properly protected.  They consist of a glass strip with very accurately etched graduations and are enclosed in an aluminum extrusion.  There is a read head inside which runs on 5 ball bearings to keep it accurately aligned, which optically monitors the graduations, and there is a "trolley" outside the extrusion that moves the read head.  They are built with a double seal where the trolley runs.  By mounting them with the opening down and placing a protective cover over them there is not a problem.  The vast majority of DROs in the field use glass scales and they just keep working year after year.



A glass scale has a rectangular cross section and has a movable trolley
which slides along the bottom to sense the distance to be measured.

Thanks to dropros.com for permission to use these 5 pictures.



    
This shows the cross section of a glass scale.  You have to look carefully, but you can see the end of the glass glued into the slot at the top.  The read head and trolley are also shown.  Note the double seals.
Here is the read head and the trolley.  They are connected together with a wire which attaches to the read head in a ball joint.  This allows a certain amount of latitude aligning the trolley without affecting the read head.



Magnetic scales have a magnetic tape mounted to a shallow extrusion.  The tape is covered by a thin stainless steel cover.  The read head runs along the stainless steel cover and picks up very accurate magnetic pulses from the tape similar to the way the optical pulses are picked up on the glass scales.  I would have liked magnetic scales, but they cost almost 50% more, and for my installation on the X axis, the dimensions did not work as well. Even though they are the smallest scales made, the read head sticks out from the face of scale instead of being below it, which would make my installation thicker.  They do have the advantages of being almost immune to contamination and of being able to be cut to the exact length you need.  Glass scales cannot be cut.


This is what the magnetic scales look like.  They are shorter, thinner, and narrower
than the glass scales and are resistant to liquids and contaminants.




The kit I received looks pretty much like this.  However mine is spread all over my living room floor, so this is a picture
from the dropros.com website.  They spent much more time making it all look pretty than I was willing to do.

In the foreground are 3 scales of different lengths for the 3 axes.  Each one has a graduated glass scale within and a read head
that moves along the length.  The position of this read head is transmitted back to the display panel (back row) and reads out
the position to 0.0002 inches. 

They supply a good assortment of brackets and mounting hardware to cover a wide range of installation configurations, but often
the end user also has to make specialized brackets and mounting hardware depending on the way his machines are built.

The kit came with an educational DVD which appears to be a compilation of all the online video tutorials they have made.

The manuals are written locally by "English as a primary language" authors and are much, much more readable than examples I
have seen from the Ebay type units, and they contain clear color pictures to illustrate their message.  So far I am very pleased
with what I have received.


Getting Started

The first thing I needed to do was to figure out which scales I needed and how to mount them on the 3 axes of the mill.  I started this well before ordering my kit.  I ran into several challenges!

Selecting my Scales

 To select the scale length, I went to the table on the dropros.com website and selected the shortest scale that had a recommended travel greater than my axis travel.  For the X axis with a travel of 16.5 inches, the 450 mm scale was the correct choice with a recommended travel of up to 17.7 in.  The scales have a maximum travel greater than the recommended, but it is unwise to encroach very far into the "safety margin", as a crash into the end will destroy the scale.  My Y axis has a travel just a tad over 6.5 inches, so the 200 mm scale was appropriate with a maximum recommended. travel of 7.8 inches.  The Z axis has the same travel as the X, so the same length scale is proper at 450 mm.  The DRO Pro kits allow you to select any scale up to 1000 mm (39.3 inches travel) for each axis at no extra cost.  They have longer ones available for a small surcharge.  The kit I bought also allows me the choice of standard or slim line scales.  Their less expensive similar kit only allows standard scales.  After selecting the scales, I also needed to verify that they will fit in the appropriate mounting locations.

For the most critical applications, they also have 1 micron scales available (the standard ones are 5 micron) in both the standard and the slim line models.  Instead of the conventional readings down to .0002 inches, these scales measure down to .00005 inches.  There is a surcharge for these scales.

My final selections were:

    X axis - 450 mm slim line, 5 micron
    Y axis - 200 mm conventional, 5 micron
    Z axis - 450 mm conventional, 5 micron

Planning the Mounting

First, on the X axis, I found I could not mount the scale on the rear of the table like I wanted, as with the table all the way back there is only about 1/2 inch clearance.  If I were to mount the scale there, it would reduce my already short 6 1/2 inch travel of the Y axis.

My solution is to mount the scale on the front of the table.  As it turns out, this only involves a slight sacrifice.  The front of the table has a T slot holding a pair of movable stops and a small casting on the saddle where these stops hit when you reach your desired limit of travel.  I will have to eliminate these stops, but I have very seldom used them, and with a DRO they will not be nearly as important.  I carefully laid out the front area of my table on my CAD software and decided it would work quite well.  The scale will mount on the front of the table, and the trolley for the read head will mount on a machined block bolting to the same holes that the stop casting used.

In order to mount the X axis scale on the front of my table, not interfere with the saddle, and stay below the table top with the cover and its screws, I needed to select the "slim line" scale.  The only disadvantage of this series of scales, which have a somewhat smaller cross section in each direction, is that they only have a single seal around the trolley opening.  The conventional scales have two.  Since I am operating in a fairly clean atmosphere, the opening is mounted pointing down, and I will have a cover over it, there should be no problem.  I selected conventional scales for the Y and Z axes.


I had hoped to mount the X scale on the rear of the table where it would be out
of the way.  As you can see, there just isn't room without having to reduce the
Y travel.  I am not willing to do that.


X axis

   
Here I have test mounted the scale on the table.  The trolley is over the aluminum block I made that is fastened to the saddle.  A small plate will connect the trolley to this block.  The scale will be covered with an aluminum cover that was supplied with the kit.  I had to drill and tap holes in the front of the table to mount the scale.  That is not a fun job!  Of course the cable from the read head will be routed around the rear of the machine and connect to the display head.

No matter how neatly I coil the metal sheathed cable from the read head, a couple of coils invariably spring off and do their own thing.
I have now final mounted the scale, aligned it with a dial indicator and connected the trolley to the saddle.  The angled block bolts to the saddle using the original bolt holes from the stop casting.  The front plate bolts to it and to the trolley.  I also reversed the scale from my trial mounting so the cable exits to the right.  I feel the cable run will be simpler this way.  When all is complete, it is easy to tell the display unit which direction is positive and  which is negative.








The magnetic scales have a tapped hole which can be used to mount a cover, but
the glass ones only have a through hole which is blocked by the mounting bolt.  To
solve this problem, I machined a couple of L blocks which shim the scale out slightly
and provide a tapped hole to mount the cover.  Moving the scale out slightly assures
that the trolley will not rub the table surface.



Here is the X axis scale completed and with the cover mounted.  The cover as
supplied interfered with the trolley bolt heads, so I cut about 3/16 off one edge
using a slitting saw.

Mounting the Display

Now that the X axis is complete, I decided to mount the display next.  This will give me an operational 1 axis readout.  As I complete the mounting of the other 2 axes, I will activate them also.  The biggest problem mounting the display is that where I decided to mount the bracket, the column slopes at a 13 degree angle.  I had to make a wedge to make the mounting arm level.


   
The display mounting bracket needs to mount on a vertical surface, but where I will be mounting it the casting slopes at a 13 degree angle off vertical.  This wedge block takes up the difference.  I included a pair of grub screws.  These are used when mounting on an uneven surface such as a bare casting.  They give a stable mounting.  I aligned these with the original grub screw holes on the bracket so I can reach through those holes to tighten these screws.  I have a cheap metric tap and die set.  It was about all I could do to tap the two grub screw holes.  Good tools really would have been worth it!
Drilling using a hand drill and tapping holes in cast iron are not among my favorite things to do.  Here are the two holes for the display bracket.  The job was not made easier as I drilled and tapped the holes horizontally, which made them at an angle to the surface and even with good center punch dimples, it was hard to keep the hole from wandering.








And here is the finished assembly.  I mounted the display and connected it to the
completed X axis.  As I get the other axes mounted I will  just plug them in and they
will be usable too.  This view also shows the wedge plate behind the display bracket.


Y Axis

The Y axis scale needs to be mounted on an as-cast surface, so I need to mount a backer bar first, adjust it to be true using grub screws, then mount the scale to it.  I was not expecting it, but the kit included backing plates for each scale.  That saves a lot of work!  Thank you DRO Pros!

    
The first thing I did on the Y axis was to drill and tap 2 more holes and mount the Y axis backing plate.  This bar is made of cast aluminum.  I am making all my cast iron tapped holes Unified National Coarse threads (American, not metric).  I am using temporary bolts until I get a supply of socket head cap screws.
I have now test fitted the scale to the backing bar.  It all looks good.




Next I drilled yet 2 more holes and tapped them.  This will mount the brackets
which drive the trolley of this axis.



This is the bracket to drive the Y trolley.  The 1/2 inch plate bolts to the saddle
and the 1/8 inch plate holds the trolley.


   

Now it is all installed and aligned.  At the left of the scale you can see an extra block bolted to the base plate.  In a similar manner to the X axis, I created an extra block to mount the cover.
This view from quite low shows the total Y bracket bolted to the saddle, holding the bottom plate which is attached to the trolley.




I am not going to mount the cover until I verify that the Z axis does not interfere with
 it.  Here I have just laid the cover in place to get a general idea how it will fit.  The
final cover will not stick out the front, but be about flush.



My mill now has a fully functioning 2 axis DRO.  Machining the brackets for the
Y axis gave me my first experience using the X axis DRO.  It was wonderful!  The
only problem was when I had to move the Y axis I still had to look at dial graduations
and count turns of the handle.

I have also mounted the backing plate for the Z axis scale.   Since the backing plate
is several inches longer than the flat area of the column, I am bolting it at the very
top and bottom of that flat area.   I located the backing plate as low as I could to the
bottom slope.  I will have about 3 inches of the backing plate cantilevered at the top.
Of course the new top mounting bolt and its associated grub screws must not protrude
above the surface of the backing plate, as the scale mounts flush to it.  After taking
this picture, I counter bored the mounting screw hole, drilled and tapped the grub
screw holes, and shortened the grub screws.

After mounting it, I indicated it along both the face and the  near edge (X and Y axes).
It is close enough that I should be able to align the scale using the grub screws on the
backing  plate and the slotted mounting of the scale.  No shimming needed.  I will  also
have to use the grub screws  to rotate it a bit.  The cast  surface it mounts on is slightly
convex and  looking down, the scale is rotated clockwise from where it needs to be.

Z Axis

The Z axis consists of a scale mounted vertically on the column of the mill on a pre-aligned backing plate, described above, a large block which mounts to the knee casting, and a bracket and a plate to connect the block to the trolley.  The block bolts to the casting and has 4 grub screws to properly align it.  Bolted to the block is a 1/2 inch thick bracket which offsets the trolley by about 4.5 inches to properly position it to stay within its allowable range.


This is the main mounting block for the Z axis brackets.  It bolts to the side of the
knee on as-cast material.  The 4 grub screws are for aligning it there.  This is the
first part I have made using a 2 axis DRO and it is really nice to work with.  All the
measurements are so much easier and faster to set up.

When I test installed it, the clearances were less than I liked and I would have had to
trim the cover to fit, so I fabricated a 3/8 plate the same shape as the top of the block,
and added it as a "shim".

Where the block mounts, the cables from the X and the Y axes slide over one of the
corners as I move the Y axis, so I rounded the corner.  I used the "Simple Arc"
function of the DRO to guide me.  It took 37 separate steps, each positioning it for a
plunge cut.  For each step the DRO gives a pair of coordinates.  I then move the
machine to zero out the numbers then make my cut.  I press a button and it gives me
the next set of locations.  It really goes fairly quickly.  The photo shows the curve
as it came from the mill - no sanding or polishing!



The Z axis scale is installed and aligned.  The addition of the bracket will make
it operational.  I had a little trouble mounting this scale, as the mounting centers
didn't quite match between the scale and the backing plate.  I thought about opening
the slots of the scale slightly, but didn't want to kill the warranty.  I ended up turning
down the diameter of the mounting screws a few thousandths from the head down the
thickness of the scale, not touching the active threads.  This was enough to allow
a slight clearance.


   
The Z axis bracket is similar to the Y one, except there is a several inch vertical offset to properly locate the trolley on the scale.  This keeps the trolley at least 1/2 inch from the bottom end and almost 1 1/2 in. from the top during full travel of the knee.
The Z bracket is fully installed and aligned. 




All three axes are now installed and aligned. 



The job is now functionally complete.  It is all connected and checked out and the
covers are installed on the Y and Z axes.  I had a slight problem on the Y cover,
The inside flange of the plastic end cap was hitting the mounting block and caused
the cover to stick out beyond flush.  I solved it by attaching 1/8 in spacers inside the
cover, lifting the flange above the block.  I also had to shorten the cover about an inch.



I quickly found that the amount the display extended from my mill limited access
to my band saw and made it hard to reach the wrenches on my wall.  I shortened
the mounting bar by 4 inches to help alleviate the problem.  I think for a small mill
this is a more appropriate length anyway.

The display came with a custom fitted clear vinyl cover to help keep it clean.  The
cover does not interfere with reading the displays or affect the touch of the buttons.



Having an additional 4 inches really made it easier to work in that area.  In addition
I made it so the entire monitor will fold in against the side of the machine.


   
I have adjusted the pivot bolt so the arm swings fairly easily.  This stop bracket limits the back travel when the display is in the operating position and keeps the display stable while pressing the buttons.  I initially had a problem tightening the bolt enough that the display would not move when I pressed the buttons.
When not in use, I can now easily swing the display completely out of the aisle.



All three cables gather together into a single service loop which provides sufficient
slack for all table positions.


   
I had to put the excess cable somewhere, so I gathered it in a loop and hung it from the rear of the column.
I noticed that there was a possibility of the cables snagging on the bottom of the Z axis cover, so I made an anti-snag endcap.



Well, I think this job is done!

I am very well pleased with how the installation turned out and am anxious to do
more projects using my new DRO.


Addendum of 12/29/2015

Quill DRO

See how I later added a low cost DRO to the quill of my milling machine.  Please click here.




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R. S. Mason 12/1/2013