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ammcoman2
07-07-2008, 06:23 PM
A few years ago I found this article and figured it may come in useful one day.
http://www.homemetalshopclub.org/news/jun02/jun02.html#tachometer
Well I set one up on my lathe last year using a 1/4" diam magnet fixed to the spindle by an aluminum ring.
http://i56.photobucket.com/albums/g189/ammcoman2/DSCN5411.jpg It is hooked up to a "Filzer dB21" bike computer but has a limitation on max rpm. It kinda loses track above about 800rpm. I put it down to the fact that it was cheap.

Now that my Mill has also been upgraded with a 3 phase motor and VFD, I figured I would try another make to see if I could get a hgher threshold before petering out. This time I bought an equally cheap "SIGMA BC506" and set it up using the same size magnet on the spindle pulley. These shots show it in action

http://i56.photobucket.com/albums/g189/ammcoman2/DSCN5409.jpg http://i56.photobucket.com/albums/g189/ammcoman2/DSCN5407.jpg

I am very pleased with this one as it is able to track up to 1500rpm (haven't yet tried any higher) but appears to be slightly optimistic. I guess cyclists don't mind one bit!

On the particular pulley setting I am using, at 60Hz the readout is 99.5 (I used a wheel setting of 1670 in the Km mode) which equates to 995rpm. Using a Starrett mechanical Tach I get 988 (timed for 30 seconds). The calculated value is 980rpm. So pretty close.

One other benefit is to put the setting in Elapsed Time mode so one can use this as a run time meter for servicing (what's that!). It even doubles as a shop clock but I have to peer at it carefully.

The upside of the one on the lathe is that it is just about dead nuts on. I suppose I could play with the wheel diam setting to get the Sigma more accurate but that can wait.

Geoff

retusaf99
07-07-2008, 09:38 PM
The model airplane guys have tachs that go up to 30,000 rpm. Cost is $40 or 50. Check your local hobby shop.


ETA Oopps...less than $30

http://www.centralhobbies.com/tools/tachs.html

Doug:)

lakeside53
07-08-2008, 01:38 AM
Your VFD may have a feature where it will output 0-10V depending on frequency. My Hitachi units do. A simple 4 position switch (assuming you have 4 pulley sheaves) can be used to normalize each range (preset pots or 2 resistors). Feed the output to your voltmeter, or, buy a cheap bare panel meter (less then $20). Flip the switch to your corresponding pulley selection.


I bought a Shimpo panel tach, and a proximity probe for my BP... then I found out that my $20 hand held laser tach pointed at the hex draw bar end works fine - just divide the output by 6!

ammcoman2
07-08-2008, 09:26 AM
Both of the bike computers I purchased were under $20cdn. I thought about the output 0 - 10v option but it would end up being more expensive once switches etc were added into the equation.

Those prop tachs sure are good value, though but do they measure in actual rpm's or in 10's?

Geoff

RobbieKnobbie
07-08-2008, 10:49 PM
The bike computer looks like a great idea, but how do you get it to read RPM?

I looked at Sigma's website and at the instruction booklet and it only lists functions for speed, distance, time, etc. Did you calibrate 1 revolution to equal 1mile or something?

ammcoman2
07-09-2008, 08:37 AM
Hi

To make the speed read the equivalent of rpm you input the wheel diameter to 1670 in the set up mode. This value applies if you have set Km's as the speed. The home metal shop article is a good reference but I elected to use 1670 rather than 167 since my first bike computer wouldn't read over 100km/hr. I know that the Sigma does. Now I have got used to seeing 600rpm as 60.0 - similar to a tach in a car.

Geoff

retusaf99
07-10-2008, 08:47 PM
Both of the bike computers I purchased were under $20cdn. I thought about the output 0 - 10v option but it would end up being more expensive once switches etc were added into the equation.

Those prop tachs sure are good value, though but do they measure in actual rpm's or in 10's?

Geoff
Geoff,
The prop tachs use an optical pickup and measure actual rpm. Point it at a flourescent light and it will measure 3600 rpm in the US (60hz x 60 sec). That's how to check calibration. They just need some contrasting paint or tape to pick up on.

Haven't tried one on tools yet, but I'd think a couple dots of white or black paint, not sure which would work better, and you now have a 2 bladed prop on your mill or whatever.

The bike computers I've seen need a magnetic pickup for cadence rpm. Maybe they have improved, but that seems pretty inconvenient, adding the magnet to something spininng way faster than leg power. (At least my leg power). Maybe Lance Armstrong spins away at 2,000 rpm...but I don't think so.

Doug