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View Full Version : About to give up on my tachometer.



MotorradMike
12-17-2010, 09:44 AM
Hi guys:

I'm going to need a tach when I get the new motor going and I thought I might make it permanent.

I already have a digital counter with a good display, it has update and inhibit inputs so with a bit of logic I can get it to update about once a second.

The problem is I need to count '60' each revolution to get the fast updates.

I printed out a 24" zebra pattern I can wrap around the big 8" pulley but it's going to be difficult and unreliable.

Any ideas?
Or should I throw in the towel and figger out how to get HF to ship me one of these (http://www.harborfreight.com/digital-photo-sensor-tachometer-66632.html).

RobbieKnobbie
12-17-2010, 10:15 AM
I'm not sure what you're looking for with your integrated tach setup, but I can tell you from experience that photo tachs are the absolute bee's knees. Once you get one in your hands you'll be checking the speed on anything that moves.

I can't speak for the HF version though, $40 is insultingly cheap for something like that.

camdigger
12-17-2010, 10:48 AM
I got one of the cheap tachs from the local discounter for a project. The only issue I had was adequate flagging on the turning shaft. sticking a piece of aluminum tape on a freshly machined surface didn't work well. I switched to black electrical tape, which gave good results.

Willy
12-17-2010, 11:00 AM
I got one of these (http://cgi.ebay.ca/Digital-Laser-Photo-Tachometer-Non-Contact-RPM-Tach-205-/370431851998?pt=LH_DefaultDomain_0&hash=item563f757dde), although not from this particular store.
For the price of a six-pack I have a tach that I can use all over the shop, these things literally have a thousand uses, handier than a shirt pocket.

As far as picking up a signal for optical tachs I've always just used a white spot of paint from a metal paint marking pen, works great.

Waterlogged
12-17-2010, 11:26 AM
Cheap chinese tachs work fine for home use or look up tachulator.

Seastar
12-17-2010, 11:45 AM
I bought one of the HF tachs and don't know how I ever got along without it.
I have two drill presses, three lathes and a mill.
All of them require belt/pully changes to change speed.
I found that the posted speeds were way off on some of the machines.
The tachs got me to the proper surface speeds and my finishes have improved and things just cut better.
Buy one - it's a bargin.
Bill

Evan
12-17-2010, 12:02 PM
Wrap magnetic tape around it and use a hall sensor. Magnetic tape has field reversals in a very regular pattern. You can also use an optosensor and room light. If you drill 60 shallow holes in the rim of the pulley it can be read with a hall sensor.


Easiest if you insist on doing it yourself is to just count 1 pulse per rev and use that to gate a 555 timer that is producing a high frequency pulse stream. Set the counter to count down and then you can make it produce a direct rpm readout by adjusting the 555 pulse rate.

MotorradMike
12-17-2010, 02:59 PM
Easiest if you insist on doing it yourself is to just count 1 pulse per rev and use that to gate a 555 timer that is producing a high frequency pulse stream. Set the counter to count down and then you can make it produce a direct rpm readout by adjusting the 555 pulse rate.

Not keen on drilling 60 holes in the pulley.

Your 555 idea seems clever but I don't get it.
The counter will only count up so maybe it won't work.

Are you suggesting the single bright spot on the pulley toggles the 555 output gate so pulses get through between consecutive spots?
Then I'd get more counts with lower RPM.
Hmmmm, maybe that's why it has to count down.
What happens at zero RPM?

camdigger
12-17-2010, 04:01 PM
I bought one of the HF tachs and don't know how I ever got along without it.
I have two drill presses, three lathes and a mill.
All of them require belt/pully changes to change speed.
I found that the posted speeds were way off on some of the machines.
The tachs got me to the proper surface speeds and my finishes have improved and things just cut better.
Buy one - it's a bargin.
Bill

By coincidence, are some of your measured speeds off by +/- 20%?

I've faced that issue before. AC synchronous motor speed is tied to input frequency. Parts of the world use 50 Hz as the standard, others 60 Hz. With all the international commerce, it is within reason that a machine was shipped to you with the wrong size pulleys...

Evan
12-17-2010, 04:15 PM
At zero rpm it counts down to zero. For best results you need to double buffer the count like this circuit. I built it, it works.

http://ixian.ca/pics8/tach.png

MotorradMike
12-17-2010, 04:35 PM
Evan:

That's more work than I'm willing to do for this.
Thanks though, it's always nice to learn a new trick.

Camdigger:
I tried calculating the existing speeds that are claimed, I'm thinking of removing the middle pulley and using a long belt with only 2 speeds, high, and low for the VFD setup.

I found the speeds are way off on both ends. Highs are way too high and lows are way too low. I'm not sure where they got them unless they didn't include the center cone pulley.
All that prompted the want of a tachometer.

Bill736
12-17-2010, 09:46 PM
I bought an inexpensive photo tach with a digital readout. It's supplied with reflective tape , which you stick to a rotating part. It worked fine until one day I tried it on my riding mower engine shaft, and the tach didn't work. I went around to the other side of the engine, and the tach worked ok. The difference was that on one side of the engine, I was holding the tach close to the enclosure which houses the ignition points and condenser. There was apparently an electrical interference which disabled the photo tach, and moving the tach a few inches away allowed it to work properly.
( This was a Kohler K321 engine with the points mounted down low near the crankcase). So, if your photo tach stops working, there may be a local electrical interference. Try moving it a few inches . For the same riding mower, I also bought a digital mini-tach and engine hour meter, which I mounted to the dashboard. They are widely sold for small engines. For the rpm pickup, you simply wind the input wire around the spark plug wire, and set the tach for the appropriate ignition pulses per revolution . It works well enough for this application, which is a governor controlled engine running at relatively steady speed. I have only two criticisms ; the battery is not replacable ( potted in epoxy) and will last about 5 years. (Granted, at my age, I won't have to buy many new tachs.) Also, the tach does not respond quickly to changes in rpm, and actually can take three or four seconds to stabilize after a sudden large rpm change, during which time the reading becomes wildly inaccurate. So, it's not at all suitable for shift points or the like on an automobile. Considering the slow response, the "one half second" sampling rate advertised is meaningless. Even so, for this application it's ok.

darryl
12-18-2010, 12:37 AM
I've played with a couple of photo tachs, one of which I bought. I find that in some cases you need to know roughly what rpm you're running at, in case the photo tach is giving a false reading. If the result it shows is way out of line, you might have to play with the way the reflective spot is arranged. I was getting fairly stable readings that were double what the actual rpms were. I don't know if ambient light was reflecting off the 'sensor' and confusing the pulse counting circuitry, but that's possible.

The worst problem I have with my tach is that it takes aa cells- since I use them a fair amount, I'm always robbing the tach. Then when I go to use it, I have to scrounge up the cells first.

gr8life
12-18-2010, 01:15 AM
Cheap from LMS-accurate checked it w/ a really good strobe tach-easy to install

http://i461.photobucket.com/albums/qq336/gr8lifeLV/Grizzly%20G4003G/Lathewithvarspdtach004Small.jpg
http://i461.photobucket.com/albums/qq336/gr8lifeLV/Grizzly%20G4003G/Lathewithvarspdtach001Small.jpg
thanks
ed

Seastar
12-18-2010, 12:37 PM
By coincidence, are some of your measured speeds off by +/- 20%?

I've faced that issue before. AC synchronous motor speed is tied to input frequency. Parts of the world use 50 Hz as the standard, others 60 Hz. With all the international commerce, it is within reason that a machine was shipped to you with the wrong size pulleys...
My small mill (6x26) is the only import machine and it was off all over the place depending on the pulley choice.
Two of my lathes are logans and were pretty close. The drill presses are old Craftsman and were off 30% in one case and about 15% in the other - one down and the other up.
My small Atlas lathe was on the money.
Go figure

danlb
12-18-2010, 12:48 PM
A trick to using the cheap photo-tach from harbor freight.

I was getting odd readings while looking at the chuck while it spun. I put a piece a black tape all the way around the chuck with just a 1/2 inch gap. I got very good readings from that point. Once I marked the dials I put the tach away and have not used it since.

Dan

Gently
12-18-2010, 01:47 PM
[QUOTE=gr8life]Cheap from LMS-accurate checked it w/ a really good strobe tach-easy to install

Ummm, if something were to happen to you :eek: ......Can I have your stuff? please? :o

just kidding, I seen those pictures and well, physicologial changes came about and my wife thought I was looking at porn!!!!! :D

mickeyf
12-18-2010, 10:09 PM
Easiest if you insist on doing it yourself is to just count 1 pulse per rev and use that to gate a 555 timer that is producing a high frequency pulse stream. Set the counter to count down and then you can make it produce a direct rpm readout by adjusting the 555 pulse rate

I have a Franken-bandsaw project in the works which involves the frame from a 14 in. woodworking bandsaw and a variable speed drive system. In this case I don't so much care about R.P.M. as F.P.M., and I figured I'd do something along these lines but adapt it to read that.

Evan
12-18-2010, 11:06 PM
The circuit above is the complete unit and by adjusting the pot on the first 555 it will read in any units you wish from an incoming pulse stream. It's good into the megahertz.

Add a zero to the number shown.

http://ixian.ca/pics8/rpm2.jpg

gary350
12-18-2010, 11:21 PM
I know the peope at the motor repair shop I been doing business with them for 32 years. I have a small 6 cylinder radial engine that I built I tried my tackometer but could not get a good reading. I took the engine to the motor shop they put a black spot on the flywheel with a black marker then the electronic type tachometer looks at that black spot as it goes past the tach. When the tach read 30,000. RPMs. I freaked out.........that can not be right. They said, yes it is. They double checked by testing a 1725 RPM motor and 3400 RPM motor to prove it was reading right. So we tested my motor again this time 30,110. RPMs.

Buy one of those new electronic tack gizmos they work great.

Here is the video.

http://www.youtube.com/watch?v=1d33uTjTXW8

gr8life
12-19-2010, 12:29 AM
The nice thing about the LMS unit is it's ability to read out in sfm, so when reducing large dia. pieces you can keep a constant sfm as the dia. reduces.
thanks
ed
do what works 4u