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About to give up on my tachometer.

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  • About to give up on my tachometer.

    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.
    Mike

    My Dad always said, "If you want people to do things for you on the farm, you have to buy a machine they can sit on that does most of the work."

  • #2
    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.

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    • #3
      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.
      Design to 0.0001", measure to 1/32", cut with an axe, grind to fit

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      • #4
        I got one of these, 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.
        Home, down in the valley behind the Red Angus
        Bad Decisions Make Good Stories​

        Location: British Columbia

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        • #5
          Cheap chinese tachs work fine for home use or look up tachulator.

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          • #6
            HF tach

            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
            I cut it off twice and it's still too short!

            Comment


            • #7
              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.
              Free software for calculating bolt circles and similar: Click Here

              Comment


              • #8
                Originally posted by Evan
                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?
                Mike

                My Dad always said, "If you want people to do things for you on the farm, you have to buy a machine they can sit on that does most of the work."

                Comment


                • #9
                  Originally posted by Seastar
                  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...
                  Design to 0.0001", measure to 1/32", cut with an axe, grind to fit

                  Comment


                  • #10
                    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.

                    Free software for calculating bolt circles and similar: Click Here

                    Comment


                    • #11
                      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.
                      Mike

                      My Dad always said, "If you want people to do things for you on the farm, you have to buy a machine they can sit on that does most of the work."

                      Comment


                      • #12
                        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.
                        Last edited by Bill736; 12-17-2010, 10:12 PM.

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                        • #13
                          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.
                          I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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                          • #14
                            Cheap from LMS-accurate checked it w/ a really good strobe tach-easy to install



                            thanks
                            ed

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                            • #15
                              Originally posted by camdigger
                              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
                              I cut it off twice and it's still too short!

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