I bought a refurbished caliper on Ebay for a great price, but it was completely dead on arrival. The seller refunded my money, and didn't want the caliper back. I took it apart, re-flowed a couple solder joints, cleaned everything, and.. still doa.. So now I have a perfectly good caliper, it just doesn't measure. But what if I turn it into an old-school vernier caliper? Will that even work? Let's find out.
Step 1: General Approach
I did some reading for how a vernier scale works, and it's pretty nifty: A big shout out to user Margrove, who does a great job explaining it here. Briefly, a one-inch scale with 10 divisions is contrasted against a .9" scale, also with 10 divisions. The closest point that the two scales align will always indicate the nearest 1/10th of an inch.
I started by drawing up a mockup in CAD, then moving the scales around to confirm that what I was reading matched what the program measured. Then I printed out a 1.9" scale with 10 divisions, and confirmed that my printer scale was accurate. I then printed out a 1.9" scale with 100 divisions. The lines are pretty tiny, but I think I can read them well enough.
Then it's time to remove the digital portion off of the caliper. Remove the sticker from the back, then the 5 screws holding the digital part on.
I had some leftover pieces of 3/32 brass plate from a previous project, and I'll be using it to make a sandwich. One piece will lay flat on the face of the caliper's sliding jaw. The other piece will "hang" underneath to act as a holder for the fine adjustment thumbwheel.
Step 2: Make a Brass Sandwich
Starting with the underneath piece, I laid out and cut the brass piece. I'm using a carbide scriber to make clean, precise lines. The brass can be cut with a hacksaw, a bandsaw with a fine tooth blade, or even a scroll saw. Everything is then filed to final shape, and finished off with sanding.
Next the upper half of the sandwich. Lay out and mark the shape you want, cut it to rough shape, then file it to final shape. I also filed a chamfer on the upper piece right where the scale lines will be glued.
Next I soldered the two halves of the sandwich together. Simply align the two pieces where they need to be and strike some scribe marks to act as a guide when soldering. I put one piece in a "helping hands" fixture, spread a thin layer of flux, heated it up with a butane torch and applied a little bit of solder. Note that the solder picture was taken later, after the holes had been drilled. After the solder cools, file the rough edges of the sandwich to a smooth joint. Now is the time to make any final adjustments or modifications, followed by final sanding and edge filing.
Step 3: Attach the Brass Sandwich
Holding the piece in its final position, I used a small drill bit to accurately mark where the holes would need to be drilled. The smallest thread tap that I have is an M3.5-5. I dug through my screw container and found some screws that match that, probably left over from some kind of hard drive. The existing holes in the stainless steel caliper had to be drilled out just a little bit for the tap. The stainless steel is very hard and tough, and broke my drill bit. I resharpened it and tried again using less pressure. That worked better. After drilling I tapped the holes very slowly and carefully, backing up and trying again whenever the tap got too tight. You do not want to break a tap off in these tiny hardened holes.
Next I drilled the holes in the upper brass piece and countersunk them to receive the screws. I had to play with the countersinking a little bit, because it has to be just deep enough for the screw heads to be completely flush with the brass surface. Start the countersink a little shallow and work your way deeper a little bit at a time.
After screwing in the three screws, I filed the screw heads to leave a smooth chamfered surface where the paper scale will be glued on.
Step 4: Finish and Test
Trim the paper scale down to fit the shape of the brass piece. Then with the caliper locked closed, use rubber cement to glue the paper scale in place. The "0" line of the paper scale should align with the 0 line on the caliper, while the "10" line of the paper scale should align with the 1.9 inch line on the caliper.
Now for a test: I grabbed a precision shaft that I had handy and measured it. According to the vernier caliper, the shaft is .393 inches. I checked it with the digital caliper and: .3935 inches. It works!
Of course I'm showing you my best result. I tried 3 or 4 other objects, and on various items the vernier can be a real challenge to choose which thousandths line is most closely aligned. A couple times I read "3 thousandths" when it was actually supposed to be "5 thousandths". Still, accurate to within 2 or 3 thousandths is not too bad! I'm calling it a win.