Digital Caliper for Drill Press

This project describes adding a digital readout to a drill press.

There are lots of such projects on the web so why, you might ask, write another one? AnotherBrian has published two excellent projects.

The reason is to warn you about the digital calipers I used. They don't have serial output. That's why I'm publishing it the the "Circuits" channel rather than the "Workshop" channel.

I bought two of the calipers for another project in which I wanted to connect the serial output to an Arduino. Once again, there are lots of projects using the serial output including a digital readout for a drill press.

But it turns out that not all calipers produce the output. The ones I bought have a "backwards italic" font and semi-circular push-buttons.

What should I do with two "useless" calipers? I know, I'll add one of them to my drill press.

The only unusual thing about this project is that I added an external battery, a switch and an LED to illuminate the scale.

Open up the caliper: peel off the label on the back of the slider "box" and you'll find four screws underneath.

Remove the 1.5V cell and solder wires onto its holder. The outside of the cell is positive. Make a hole so the wires can exit the "box".

It's a little dark on the front of my drill press so I added a white LED. 

It's best to add an external battery to replace the LR44 (AG13) cell. The On/Off button does not turn off the circuit - just the display. So the cell only lasts a year. An AA cell will last very much longer and be easier to replace.

A battery box with a switch with two AA cells can provide 1.5V for the caliper and 3V for the LED. Solder a wire to the plate that joins the two AA cells together.

The LED has a forward voltage of 2.7V and takes 30mA. So a series resistor of (3-2.7)/0.030 = 10ohms is required.

How you attach the caliper to the drill will depend on the details of your press drill. But here's what I did:

I removed the piece of springy metal that gives the slider a little friction. I sawed off the jaws and drilled a hole in the end of the "main scale" slider.

I cut a piece of sheet steel (from the box of some scrap electronics) and attached it to the belt box of the drill with M3 screws. 

I screwed-and-glued the battery box and the slider box to the sheet steel.

I drilled and tapped a long M4 screw into the red plastic ring that used to hold the eye guard and the original, useless scale-pointer thingy.