Making Lathe Tachometer - Part 1

Friday, February 27, 2015

A few posts ago I offered some general ideas on adding a tachometer to TouchDRO setup that covered some theory and a basic example circuit. Since then I've received a ton of emails asking for more details, so in the next few posts I will provide a detailed start-to-finish example of building a tachometer for my Jet 1024 Lathe. In the post "DIY Tachometer for Your Mill or Lathe" I covered the basic of tachometer operation and provided a basic tachometer circuit using a IR emitter/receiver couple. A number of people wanted to use a Hall effect sensor with a magnetic encoder disk, so I will go that route for this design. Along the way I will use this project as mini-tutorial for some of the basic TouchDRO functionality.

You might recall that a tachometer works by counting the number of revolutions per minute. This is done via a circuit that counts the number of pulses provided by an encoder. In this case the encoder uses a disk with one or more small magnets and a Hall effect sensor. Generally speaking the resolution is proportional to the number of magnets on the disk. In order to work the DRO firmware needs a few pulses per second. With a single magnet the slowest RPM the DRO will read is about 100 RPM or so. Since I run my lathe at very low speeds quite often, I've decided to use 8 magnets.

Short of building a circuit from scratch, the least expensive approach I've found is to use a Sainsmart Hall Effect Sensor module (sold by Amazon and a number of other retailers) and size small neodymium magnets. The parts, including shipping, set me back a bit under $20 (USD); the whole project took a solid evening to complete.

Design Overview

The encoder disk should be made out of nonferrous material. It can be plastic, really, but I like to over engineer things so I decided to turn mine out of aluminum. The size of the disk isn't critical as long as it's large enough to fit the manets and fits into the required space. In my case the only real constraint was the thickness of the disk: it had to be under 0.7" or so for the gear cover to close. The only critical dimension was the bore that had to provide a snug fit over the 1.400" spindle outer diameter.

I found a short piece of 2.5" aluminum stock at the local scrap yard, so it became my starting point. The magnets are a bit under 1.2" thick and 3/16 in diameter. To give myself enough "meat" to mount the magnets and have enough thread for the set screws (if needed) I ended up with the design shown below.

Machining the Encoder Disk

I started by setting the datums for the X and Z axes. To set the Z axis I took a facing cut and without moving the carriage long-pressed the "Zero Set" button.

Long-pressing the "set zero" button sets the absolute origin

Long-pressing the "Zero Set" button set the absolute origin for the current workspace, whereas short-pressing it sets the relative origin. The former is used as a datum for the point (if any added to the workspace, etc.). Generally it's useful to set the absolute origin to the point that is the datum for the current workpiece. This way if the workpiece needs to be moved it will be easy to re-locate. You will know that you long-pressed the button when a confirmation dialog appears warning you that you are about to set the workspace's origin, as shown in the screenshot.

After a light cut the part was measured

This set the absolute origin for the current workpiece, which is correct for the Z axis but not for the X axis [yet]. To address the X axis I took a light turning cut and without moving the cross slide measured the outside diameter, which happened to be 2.468". Using the "Set Dimension" function, accessible by long-pressing the axis readout, I entered 2.468 as the dimension, made sure to check the "Set absolute dimension" checkbox, and applied the change. I didn't have to divide the dimension because the DRO was in "Diameter" mode; if that were not the case I would've entered 1.234 instead. Either way the end result would be the same: the datum would be set to the center of the parts face.

Axis Dimension dialog can be used to set absolute preset

Please note, by default touch DRO start in "Radius" mode. In other words the X axis of the DRO will display the travel of the cross slide. I prefer to work in "Diameter" mode for general turning work. Once TouchDRO's machine type is set to "Lathe", the "Rad/Dia" button switches between the two modes.

First step was to machine the step (no pun intended)

Next I cut the step as shown in the picture. Since I like to work towards zero (it's a good practice in general), I set the final depth of the step as the incremental 0 first. On the X axis I did this by moving the carriage until the DRO read the desired diameter of 1.750"; once there I long-pressed "Abs/Incr" button to set X to 0. For the Z axis I set the point by long-pressing the Z readout and entering -0.35 as the dimension. Once the point was set, it was a matter of taking a series of turning cuts until the DRO read 0 on both axes.

Boring the Hole

With the OD finished it was time to bore out the hole for the spindle. As usual, I started with drilling the hole with progressively larger drill bits, all the way to 1". At that point it was time for the boring bar. To start with, I took the first light cut and without moving the X axis retracted the boring bar. Using a telescoping gage I measured the diameter of the resulting bore and set it as the X axis dimension preset. As I already mentioned, I like working towards zero, so I simply moved the X axis until the DRO read 1.399" and set that point to X's zero by long-pressing the "abs/incr" button.

To set the boring bar position a light cut was taken

From that point on it was a matter of boring out the hole until the X axis read 0.

Parting Off

A piece of cigarette paper was used to touch off the parting tool

The last step was to cut off the part. To set the parting tool to the right position I first needed to touch it off the face of the part. At that point I set the Z dimension to -0.06-0.6 (the tool width is 0.060 and the desired part with was 0.600"). TouchDRO can handle arithmetic expressions in the dimension entry fields, so the resulting dimension will be -0.660".


As you can see, the lathe work for the tachometer disk was pretty simple and should not pose any challenges even without a DRO, especially since there was only one critical dimension. Frankly I "inflicted" the rest of the dimensions on myself to demonstrate how I use TouchDRO. Doing this on a non-critical part I didn't have to concentrate much on the machining operations.


  1. are the wring and setup instructions for this sensor posted here? I cannot seem to find them. I wired the sensor reading a 3 inch plastic disk with 4 magnets, I get a tach reading of 4-17 on a lathe running from slowest to fastest with the direct output and no reading with the other output. any help would be appreciated.

  2. Hello? Anyone there? Looking to complete this project. Is there someone, or somewhere for me to go to? I need to know how to wire this sensor.