Over the past few weekends I have been making (slow) progress preparing to install my new DRO on
my mini milling machine. A DRO (digital readout) displays the exact location of the milling machine work table so that cuts can be made without having to make tedious measurements again and again. The DRO also has many built-in programs that make machining easier. With the DRO on this inexpensive mill, I will be able to do accurate work.
 |
| Optical DRO kit. |
High end DROs can cost $thousands$, but cheap Chinese systems are readily available. I purchased
my DRO from
Vevor.com. I paid $176.99 for the kit and it was delivered a few days later in two boxes. One box contained the display unit, and the other box held the scales.
This kit is a good value!
Because the scales were much longer than the movements of my little milling machine, I would have to shorten the scales. Cutting the optical scales to length is tricky because the aluminum extrusion contains a strip of glass 1/2-inches tall, 1/16-inches thick, and it runs the length of the scale. I viewed several videos on youtube.com to see how other people attempted this. Individual tactics varied, but I finally decided how I was going to do it.
I began by unscrewing the tiny screws that held the end cap on the extrusion. I pulled the read head out and set it aside. Next I pulled the four silicone wipers out of their channels and laid them aside. Now I had an aluminum extruded channel with a strip of glass in the bottom of it. The glass is secured in a slot with 1/2-inch-long strips of silicone pressed in periodically.
I resolved to cut my scales on my compound miter saw. But I took a few precautions before I went to the saw. First, I measured the scale and marked a line on the outside in pencil where I believed the cut should be made. Next, I packed the area inside the channel around the cut line with putty. I used Loctite brand
Fun Tack, and I pressed it in with a popsicle stick. The putty would stabilize the glass during the cutting, and since the channel was packed to the top, it would keep aluminum swarf and bit of glass out of the rest of the channel.
Painters tape was used to seal the rest of the open side of the aluminum channel. This was done to keep dirt and saw swarf out.
The cut on the saw went quickly and without a hitch. I pushed the putty out and inspected my work. The glass and the aluminum were cut in a clean straight line.
Now, I needed to trim the glass back another 3/8-inch so that the end cap could be reinstalled. I packed the putty into the channel again, but farther back this time. I took the scale to my work bench and using my Dremel tool with a diamond cutter installed, I scored the glass scale multiple times to make a groove. Then I spat on the glass (cutting glass has to be done under water!), gripped the end of the glass with my needle nose pliers, and snapped it off!
I removed all putty and tape, blew the rest of the channel out with compressed air, and wiped the channel out with a cotton-tipped swab.
Now I had to put the scale back together again. First I slid the silicone wiper strips back into their grooves. I had to trim these to the new, shorter length. Next I carefully re-inserted the reader head into the channel and connect its cable to the display unit to verify that it was still working. It was.
Finally, the end cap had to be installed. Originally the end cap had four tiny screws holding it on. I think the factory just drilled the holes and then forced the screws in to cut their own threads. There was no way I was going to try to drill and tap those tiny holes. So, I opted to close the scale back up with JB Weld on the end cap that was removed. If I ever have to gain access to the inside again, I can simply unscrew the opposite side end cap.
Now this had to be repeated for two more scales. There is a scale for the X-axis (side-to-side), the Y-axis (front-to-back), and the Z-axis (up-and-down).
With the scales cut to appropriate length, I spent some time thinking about how the scales and readers would be installed on my small milling machine.
I decided that the read heads would be stationary on my milling machine and the scales would move when the table or cutting head moves. Doing this would shield the open end of the scales from any chips or coolant being flung off of the table. The side of the scales with the wipers would either be facing away from the table, or down.
The kit comes with aluminum angles to serve as shields, but they are much too big and they would further reduce the amount of machine travel available, so I am going to go without them. This is a very small milling machine, in a hobby workshop, in a garage, used on weekends. The scales will be fine.
Because the Y and Z axes scales are longer than any possible support structure on the mill, I elected to cut 1-inch-wide by 1/4-inch-thick aluminum bar to support those scales. The X-axis scale is fastened to the back of the table by an 8-32 NC screw through each end cap, and the read head is fastened to the saddle with two 8-32 NC screws. I had to tap drill and tap the threaded holes. I disassembled the milling machine so that the holes could be drilled nice and square on the drill press, and then I moved the parts to the vise to hold them while I worked the tap.
The aluminum bars that support the Y-axis and Z-axis scales are fastened with 1/4-20 NC countersunk socket head screws. Both of these scales are cantilevered out past any machine part that could support them. There are 8-32 NC tapped holes in the ends of these bars that correspond to mounting holes in the end caps of the scales.
The toughest scale and reader pair to install was the Y-axis. This scale is cantilevered toward the rear of the machine from its mount on the right side of the saddle. The trickiest part was the mount for the reader. This was to be mounted to the base casting, but castings usually do not have straight sides. Oh the sides are straight enough, but they do not go straight up and down. The casting has about 3° of draft, and I would have to find a way to support the read head as it is mounted to this sloping side.
 |
| Looking from the rear right corner. |
My first thoughts were to shim it or grind a 3° tapered wedge to butt up against the side. I am not confident that I could have ground that shallow of a wedge accurately. During this past week I discovered spherical self-aligning washers. These washers can take misalignment of 3-4°. These washers are commonly used on grinding wheel arbors. I ordered four pairs (one male and one female) for a 5/16-inch bolt from
McMaster-Carr and they were delivered to me the next day.
Initially, I was going to try to tap drill into the base casting and thread the holes, but after discovering the spherical washers, I elected to drill a clearance hole and use nyloc nuts on the inside. I got four sets because it is recommended that the washers be used with the nuts on the inside surface of the casting as well as the outside surface.
The spherical self-aligning washers worked very well. I used an engineers square to ensure that the mounting bracket was perpendicular to the mounting surface and tightened the screws down.
With the X and Y axes installed, I adjusted the scales so that they were parallel to the reader heads and tightened everything up. I checked to see that the X-axis scale was 1/4-inch below the top of my work table so that it would not interfere with my vise.
I hooked up the cables for X and Y axes to the display unit and tested the X-Y table. Good results so far! I worked on this project all day Saturday and this morning. I still have to install the Z-axis, but I do not anticipate any difficulty. It cannot give me more difficulty than the Y-axis gave me.
When it has been completed, I will post another picture (or a video) showing the completed system installation.
