Due to a couple of issues I have with the Z-axis of my Smithy Granite 1324 Mill-Lathe I have always wanted a Digital Read Out (DRO). First of all there is a lot of backlash which is kind of unavoidable due to it's drill press like design. The real problem is that it doesn't use a centigrade scale (WTF). The course feed is 1.00"/rev x .05" but the fine feed is .120"/rev x .001" That combined with gravity pulling the quill down causes no end of problems during down feeds.

So, I came up with this solution after looking through a giant industrial catalog, as I am want to do, and noticed that the sensors on some of the DRO scales look just like those handy Digital Calipers, just without the jaws...

Smithy does sell a separate Z-axis DRO for $100.00 but though that was a bit expensive and less interesting.

Step 1: Materials / Tools

Raw Materials Needed:
~ 6" 3/8" steel rod
~ 6" 1/4" steel rod
~ 6" 1/4 threaded rod
1 3/8" nut
7 1/4" nut
1 Cheap 6" digital caliper

Tools used:
Dremel (Cut off wheels, > 1/4" ball grinding stone, > 1/4" cylinder grinding stone)
Hack saw
1/4" tap
1/4" die
3/8" tap
3/8" die
3/16" drill bit
1/4" drill bit
Cordless drill
Small screw driver (for the digital caliper)
File
Drill Press
Lathe

(The thread pitch used for the taps/dies doesn't really matter. They just need to match each other.)

Step 2: Preparing the Caliper

Begin by removing the thumbwheel and back of your digital caliper. Remember that evil designers often hide screws under labels.

Use a cut off wheel to remove the jaws.

Next, make a threaded hole in the back of the movable carriage. Be careful as it is quite thin and probably cast iron, thus brittle.

Now, make a hole in the (jaw) end of the beam. This could be a problem because it's likely hardened. You can use a specialty bit or with a little extra time you can use a ball end grinding stone. It will take a while but it will eventually get through. You will notice that I cut away the scale insert piece. I was worried about damaging it when I was making the hole. It's probably better to leave it because the 1/4" nut that will go in there will get hung up on the sides and can't turn freely. When I took it all apart to document this project I epoxied a washer in the gap.

Note: some of the pictures are of a different caliper I have, they differ slightly. I made this modification before I found Instructables so I had to recreate some of the shots.

Step 3: Prepare the Millhead

I have made a simple model in SketchUp to show the locations of the hole. The top hole is offset 3/4" from the center of the face. The lower hole is right on the center.

You can see I have spot faced the holes, but it's not really necessary.

Step 4: Making the Mounting Brackets

Top section:
1) Thread about 1" of the 1 1/2" x 3/8" rod.
2) Drill a 1/4" cross hole in the non threaded end.
3) Locktight a nut on a 1 1/4" section of the threaded rod. Ensure that when you screw this part into the back of the caliper body it doesn't interfere with the beam.

Bottom section:
1) Cut 1/4" threads on a 1/2" section the 1" x 3/8" rod.
2) Drill a 1/4" cross hole in the non threaded end.
3) Locktight a nut on a 2" section of the threaded rod.

Note: I came up with a nice way to lock the nuts on the threaded rod. I drilled a hole through both parts, then slightly enlarged the hole on each side of the nut. I inserted a finishing nail in the hole and sniped it off leaving a bit extending out. Applying a hammer turned the nail into a rough rivet. It works and is very strong, but in a case like this, a thread locking glue would be fine.

Step 5: Assemble

I suggest making the threaded sections a bit on the long side so there is plenty of room for adjustment.

Step 6: Adjust and Enjoy

You will need to adjust the beam of the caliper so it is exactly parallel with the quill in order to prevent binding and cosine error.

Enjoy you new DRO!