RPM Spindle Speed Readout for Mini Mill

I broke a lot of bits when I first started machining because I was impatient and I went too fast.

Over the years, I learned to slow down. But, cutting too slowly has disadvantages besides wasting time. It can unnecessarily wear down the cutting tool or leave a rough surface on the workpiece.

Nowadays, I mix it up -- slow feeds and moderate RPMs to remove material, followed by fast RPMs to leave a smooth surface. But, “slow”, “moderate”, and “fast” are subjective terms and I’d rather be scientific about it.

Expert machinists (and computer-controlled machines) optimize operations by following well-researched feed and speed charts. The data tells them the rate at which a cutting tool should spin and be fed into the workpiece.

The speed and feed variables include:

The hobbyist milling machines don’t come with an RPM (revolutions per minute) display which would tell the operator how fast the cutting tool is spinning. On the mill’s variable-speed dial, there is a sticker that specifies the maximum speed. However, without knowing the approximate speed for any other dial position, it is difficult to target or recreate a particular spindle rate to match a speed and feed chart.

Given my prior background with making optical non-contact tachometers, I decided to make an attractive, dedicated electronic tachometer for the MicroLux milling machine from MicroMark.

Digital tachometer with blue display on a MicroLux milling machine.

Digital tachometer with blue display on a MicroLux milling machine.

The outside of the tachometer is black. No modifications are required to attach the tach to the mini-mill. The tachometer simply replaces the black spindle cap.

My wife thinks it looks a little nefarious. (Due to the paranoid and sensitive age we live in, I'll avoid the specific descriptive keyword she used.) I think it looks cool.

Lite-On LTS-2301AB blue four-digit numeric display.

Lite-On LTS-2301AB blue four-digit numeric display.

The tachometer features a beautiful blue, 10mm tall, 4-digit, 7-segment numeric LED display. It uses Lite-On LTS-2301AB parts that I purchased from DigiKey. These are similar to the one’s I used in the temperature display for the dual fan controller, but these have individual digits instead of pairs.

There isn’t an on or off switch or any buttons. The non-contact tachometer detects when the spindle stop rotating and shuts off the display. When the spindle starts again, the display turns back on. It couldn’t be simpler for the user.

Let’s see how the tachometer optically determines the speed of the milling machine...