A cheap tachometer for the lathe

Work in progress

This is a cheap project for a tachometer for the mini-lathe. Parts are a 4-digit / 9999Hz digital frequency meter bought on ebay for about 13 EUR with shipping from HK, a sensor from a old HP deskjet printer, a interrupter wheel made on the lathe and a 5V power supply.

Making the interrupter wheel

The digital frequency meter measures pulses per second. For it to measure RPM all that's needed is to multiply pulses by 60 (60 seconds = 1 minute). So the needed interrupter wheel must have 60 teeth. Without a mill or rotary table, decided to improvise a simple indexer to cut the teeth on the lathe using a slitting saw on the lathe spindle. For making a interrupter wheel there is no need for accuracy, so this will work ok.

The wheel for the tachometer is a 60mm aluminium disk, 2mm thick and bored to fit the spindle. A 60mm wheel has a 188.5mm circunference. Dividing the circunference by 120 (60 teeth + 60 intervals) is equal to 1.57mm. I will use a 1/16" slitting saw that is close to that.

A basic indexer to cut the tachometer wheel on the lathe

Made a template with 60 divisions on computer and printed it on self adhesipe paper. The very basic indexer is shown above. A pivot with a M10 threaded end is firmly bolted on compound by using a lathe chuck. The wheel is secured centered between the two large steel parts, tightened by a M8 bolt. To index wheel positions, the self adhesive template is placed centered on the wheel and the setup is tight in each position by a M6 SHCS that pushes a 5mm ball onto the pivot.

Setup for making a tachometer wheel on the lathe

To center the pivot on compound with the slitting saw blade I used a pair of small steel rulers. By the time I was preparing this setup, decided to drill and tap another hole for the M6 SHCS, opposite to this one. Otherwise it will be very dificult to adjust positions for all the 60 teeth.

Cutting the tachometer wheel teeth on the lathe

Here is the teeth cutting process, relying only on my visual acuraccy. It was very easy to make the wheel for the tachometer this way and I'm happy with results.

Will continue soon.

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