Going Digital, Part 2

In Part 1 of this saga, I installed iGaging digital readouts (DROs) on the Y (in-out) and Z (up-down) axes on my mini-mill. I saved the X axis (left-right) for last, because it's the most complicated. Phil Oles and I had decided that the best place to install the track was on the front of the mill, to avoid compromising the already-limited travel in the Y direction. But that adds a lot of complications, because there is already a lot in the way there. Here's a photo of the mill table - the DRO track needed to be mounted along the length of the table.

You can see, in the center of the table, that there are four adjustment screws that must be avoided. Therefore, I had to add brackets to space the track out from the table. I started on the right side, because it was easier. I found a likely bit of aluminum scrap, and made this:

It looks simple, but it is carefully designed and made. The two outer holes are for screws to hold the bracket to the mill table. They are countersunk so that the screw heads won't interfere. The center hole is threaded 10-24, and is to mount the bracket at the end of the DRO track.

Any time I need to make something, even a simple thing like this, I make a basic drawing. It shows the exact measurements and relationships among all the holes and other features:

The "NTS" indication means "Not To Scale." It's just a rough sketch, but the numbers are real! Once the part was made, I used it to "spot" the outer holes on the right end of the mill table, and drilled and tapped them 10-24. I later mounted the bracket, and that end was done.

By the way, I actually had to make that bracket twice, because the tap broke off in the hole just as I was finishing the part. That's when the value of that drawing really paid off - I just made it again to the same specs. Here's the sad broken tap - the first one I've broken since I started this hobby a few years ago. It's a rite of passage, I suppose.

On the left end, there is a removable end piece that is just too thin to successfully tap. I decided to make a quite thick replacement, with dimensions suited to my new goals, plus some planned future enhancements. I started with a rough piece of scrap that I bought at Cabin Fever this year for about a buck (left photo). I was rough on every edge, and the top had a serious angle. After crosscutting on one saw, trimming approximately square on another, using a end mill to true the edges and a fly cutter to mill the faces, I had a very nice, very square aluminum block (right photo):

I then carefully measured the existing mounting holes, marked the block and drilled. It's so gratifying when it fits! The measuring technique is worth mentioning. I threaded screws into the holes, with enough clearance that I could measure from the outside of one screw to the outside of the other. Then, I carefully measured the diameter of one screw. The distance center-to-center from one hole to the other is the outside measurement minus the screw diameter of one screw. I used the technique multiple times in mounting the three DROs, and it worked perfectly every time.

I mounted the block on the mill table (left photo) after adding some blue machinist's dye to the inside face, and then I could scribe the thickness of the right bracket onto the overhang (right photo). The scribe mark is hard to see - click the photo to see the enlargement.

I milled the end of the bracket until I was close to the line, and then used the "cut and try" method to get it exactly right. That's when patience is key. I would remove a tiny amount, remount it and measure again. I was able to get it identical within 0.001". Then I could remove it one more time, and with more careful marking and measuring, drill and tap a matching hole for the left DRO track bracket:

Once it was done, I could finally mount the track - here are the left and right mounting points:

Before mounting, though, I also needed to craft a bracket to mount on the back of the sliding head, in the same manner that I used for the Y and Z axes. It looks like this:

In that photo, do you see the tapped hole just to the right of the bracket? There's another one at the far right end, and that's where I wanted to attach the bracket. I pondered how I could possibly get that hole aligned, and I had what I thought was a very good idea. I made this on the lathe:

That is a bolt of the correct M6 (metric, 6mm) thread for that hole. I cut off the head, and then turned a point on the end. I put it in the mill with the point facing out, added more blue dye on the back of the bracket, and then scribed a line on the bracket using the point. It showed the exact centerline of the bolt hole. I chose a spot along that line, drilled, made a precisely dimensioned spacer to go behind it, and installed everything like this:

Before scribing that line, I needed to ensure that the track was perfectly level. I used a dial test indicator to do so, just like I did for the Y axis in Part 1. Phil stopped by just as I was starting that process, and his help was invaluable in getting things all lined up.

Finally it was time for a test, and it worked great. The published tolerance for that DRO is 0.002" over 3 inches of travel, and Phil and I found 0.0005" - half a thousandth! That is very cool. 

Just as we were celebrating that, Allen Forbes dropped by for his first visit to the Grant St. Garage. Phil had to go on to Make717 to teach some other beginning machinists, but Allen stuck around as I tried to find a way to mount the DRO displays.

The three displays are designed to attach to each other, and they have magnets on the back to mount them. It's important that it remains easy to gain access to the back of each readout, because they are battery powered and the battery compartment is on the back. To complicate matters, the thumbscrews that hold the readouts together stick out, so you have to have clearance slots for them.

I cast around and found an iron bracket that had been part of a garage door. It had four slots in it, and one of them was appropriate for one of the thumbscrews. I cut it to length, milled another slot for the other thumbscrew, and it fit fine. I continued by making a wooden angled stand with the metal part mounted on it. Since I haven't decided how I want to permanently mount the displays, I nailed a base piece to it and used it to clamp the stand to my mill table:

And here it is with the displays mounted - looks good, and very stable when pressing the buttons:

There's a bit more work to do, deciding on secure cable routing and completing the stand for the displays when I decide how I want to make it. But it works now! I hope this will help me increase the accuracy of my work, and cut down on the dumb mistakes. We'll see.