Comments on: Inexpensive DIY tachometer for a mill or lathe

By: Ted

Ted — Wed, 31 Aug 2011 23:02:23 +0000

try the sunding SD-548C wireless

HERE YOU CAN SEE RPM ON DISPLAY.
included on cyclocomputer

http://www.ebay.com/itm/Wireless-waterproof-Bicycle-Odometer-Speedometer-SD-548-/290601362843?pt=AU_Sport_Cycling_Accessories&hash=item43a930e59b

By: Scot

Scot — Sat, 09 Jul 2011 02:17:22 +0000

I just bought a Bell F20 bike computer on eBay for $14.50. It allowed me to set the wheel size to 0268mm and units to MPH with no problems.

By: Rob

Rob — Tue, 14 Jun 2011 21:21:08 +0000

Sorry Andy, but I agree with Adam and JPR’s calculations because I’ve worked out the math myself. I also bought a $12 cycling computer at Walmart that had no problem accepting a circumference of 268 mm and it works very well as a tachometer.

I haven’t finished it yet (the story of my life sometimes) but I’m working on an aluminum mount for it that will attach under my mini-mill’s head with magnets. The speedo display will be mounted on the outside and its wire and sensor will be neatly stored inside. The magnet I’m using is attached to the spindle with a couple of layers of electrical tape.

Adam’s idea is really useful if you have a CNC mill, which can be much less tolerant of incorrect spindle speeds than a manually operated mill.

By: Andy

Andy — Mon, 30 May 2011 23:57:20 +0000

Congrats on a very interesting website.

A minor point: not many cycle speedos will allow you to set the wheel circumference to 166.7mm, because cycles with wheels 53mm (about 2″) in diameter are a little impractical. Better to use 1667mm, when a reading of (for example) 45.6 kph represents 456 rpm. However, if your speedo doesn’t expect you to pedal at over 99.9kph (60mph!), it may only read up to 999 rpm. My speedo thinks I’m trying for a 120mph cycling world record, because it goes up to 200kph (=2000 rpm).

Andy

By: jpr

jpr — Sat, 13 Mar 2010 22:27:13 +0000

The original cyclometer is a tachometer, its just doing some math to its RPM measurement before printing the number to the LCD. If we mount the magnet on the hub of the wheel, or the rim of the wheel, its still passing the sensor the same number of times per minute. And giving us the same speed reading.

The bike display is showing us:
KPH readout = rpm * 60min/hr * circumference(m) * 0.001m/km

ie if we had a 1m circumference wheel turning 1rpm, it would turn 60 rotations per hour. 60 rotations is a distance of 60 * circumference or 60m. 60 meters is 0.06km, which means the readout would show 0.06KPH.

To get a KPH readout of 1 for 100 rpm:
1 = 100 * 60min/hr * circumference * 0.001m/km
circumference = 1 / (100 * 60 * 0.001)
circumference = 0.1667m
or
circumference = 166.7mm