Lathe Tachometer

I purchased a very old Myford ML7 metal lathe last year, and it's a great little machine. I do miss the speed display from my previous mini-lathe. I'm also planning to swap out the motor with a DC variable speed motor in the future, so I'll need to know what the actual speed is then.

This project will take you through the process of installing a basic Tachometer onto my lathe.

1. Tachometer Kit
2. Acrylic Sheet
3. Aluminium Angle Bar
4. DC Power Supply
5. Epoxy Glue
6. Small Bolt

It's worth spending a bit of time thinking through where you should install the sensor. It needs to be somewhere out of the way and away from any of the metal cuttings, as they will stick to the magnet. I opted for inside the gearbox on the side of the aluminium pulley.

I started by measuring the diameter of the magnet, so I knew what size to make the hole. I used a centre punch to mark the location of the hole and drilled a small pilot hole in the pulley. I gradually used larger drill bits to take the hole up to 10mm in diameter. Be sure to only drill the hole a couple of millimetres deep.

Now, using a file, remove any burrs from drilling the hole. After removing most of the burrs, I let the lathe run a little bit while holding the file in place to smooth everything out.

Next, you need to thoroughly clean the pulley and magnet to remove any oils. I used some brake cleaner and then acetone. Once this had dried, I mixed up some 5-minute epoxy and used it to hold the magnet in place and carefully wiped the excess glue off.

I cut off a small piece of the angled aluminium and drilled a hole for the Hall effect sensor. I had to bend the aluminium slightly by squeezing it in the vice. I could now bolt the hall effect sensor in place with a nut on each side.

I was trying to avoid drilling any holes into the lathe, but I just couldn't think of a way to hold it in place reliably. I drilled a 6mm hole through the mount at the back, in the aluminium sensor mount. I drilled another 6mm hole and used a small file to make the hole into a square for the head of the bolt.

I could now secure the Hall Effect sensor in place with the M6 bolt.

I used a piece of acrylic sheet for the mount. I started by cutting a piece out with a saw, and then used a Dremel to cut out the hole for the display. Once I had the basic shape I wanted, I used a hot air gun to heat up the acrylic and bend it into the shape I wanted. I didn't heat it up quite enough in one spot, and it developed a small crack. I just roughed up the acrylic and chucked some hot glue on there. I went back and trimmed the acrylic a bit more so it would fit onto the post a bit better.

Lastly, I drilled the mounting holes to align with the switch and bolted it into place between the switch and the post.

I snapped the display into place and quickly wired it up to test that everything worked as expected.

I tried a few different power supplies until I found one that fit into place nicely. Unfortunately, I did mix the terminals up once and blew up the display. It turned out I just cooked the voltage regulator and could solder a replacement one in place. My electronic skills are pretty rusty, so I was very happy I managed to fix this.

I made a couple of mounts for the power supply out of the angled aluminium and used a couple of cable ties to stop it from moving.

The mains side of the power supply is wired after the switch for the lathe, so it automatically powers on the tachometer when you start the lathe. I did put a connector between the switch and the power supply so I can remove it if needed.

The DC side of the power supply was trimmed to length and connected to the tachometer display.

The data wire was too short and was likely to get snagged at some point, so I decided to extend it. I cut the cable, soldered in an extension, and sealed it with heatshrink. I then realised I had transposed a couple of the wires, so stripped off the heatshrink, connected it correctly and used more heatshrink. Oh well, that's another one for the blooper reel!

The last step was to use some cable ties to secure everything in place and make sure it was as tidy as I could make it.

The tachometer runs correctly, and it's a nice little addition to my lathe.