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  1. #1
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    Default Interesting phone app measures rpm

    Was googling for one of those 60hz tacho charts to measure rpm on my bandsaw and stumbled across this

    https://play.google.com/store/apps/d...agneticcounter

    Might be of some use to others - basically stick a magnet on your rotating wheel and the the phone detects it every time it swings past. Clever. Will give it a spin tomorrow.

    Found this as well but not nearly so interesting and costs $20

    https://www.amazon.com/dp/B000I5LDVC...koverflow17-20

  2. #2
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    I recently purchased one of those laser tachos $13.50 free delivery (ebay), works a treat. Stuff on Amazon is really cheap, not.
    With the phone app how do you get on if what you want to measure is not magnetic?
    Regards
    Bradford

  3. #3
    BobL is offline Member: Blue and white apron brigade
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    Quote Originally Posted by BRADFORD View Post
    I recently purchased one of those laser tachos $13.50 free delivery (ebay), works a treat. Stuff on Amazon is really cheap, not. With the phone app how do you get on if what you want to measure is not magnetic?
    Use double sided tape.
    Mind you I do have some concerns about attaching a small magnet to a small diameter curved surface like a spindle and then spinning it at 3000 rpm. The laser tachos are much safer in that regard.

    BTW I see the latest lowest price for those laser tachos is $11.27 in delivery. I bought mine nearly 15 years ago and it's still got the same battery.

  4. #4
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    Default

    Quote Originally Posted by BRADFORD View Post
    how do you get on if what you want to measure is not magnetic?
    Stick the magnet on with double sided tape?

    Phil

  5. #5
    BobL is offline Member: Blue and white apron brigade
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    The other way to do it would be to use Velcro dots. That way the magnet could be easily removed if it got in the way.

  6. #6
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    The bandsaw has a 14" aluminium wheel, the magnet was held on by placing 1 magnet either side of the spoke where the Al web was thinnest. The app has absolutely no documentation but it was fun to watch the sine waves and interesting to figure out how best to make it work. App measured 146rpm which was only slightly different from rpm estimated from motor speed and pulley diameters.

    The phone must have a magnetometer installed in order for the app to work.

    For a thin shaft doing 3000rpm you could perhaps use a lot of duct tape rather than double sided tape or make up a collar with a pocket to hold the magnet.

    How do the laser ones work ? Do they need a target on the spinning object or do you simply hit the spinning object with a red dot. They sound like a lot less hassle than the magnetic one. Some people use the magnet method to check the speed of record player turntables.

    Bill

  7. #7
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    Hi Guys,

    I bought one a while back that uses a laser. The tacho comes with several self adhesive reflective strips that you cut to size and stick on the item you want to measure the speed of.

    Doing a comparison between the tacho fitted to the mill and a known speed source shows that the mill display is about 10 - 12% slow. Not really an issue I suppose.
    Best Regards:
    Baron J.

  8. #8
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    About the target for a laser tacho: in another recent thread/forum about tacho's the following methods were mentioned (apart from the included reflective tape).
    - aluminium foil
    - aluminium tape used for ducts
    - white electrical tape
    - chalk mark

    Haven't used these myself, just thought I would mention them. If these work, it should be possible to come up with similar solutions....
    Peter

  9. #9
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    Hi Peter,

    Yes they all work, as does black tape on a shiny surface, black permanent marker on same. Basically anything that creates a contrast between the two surfaces.
    Best Regards:
    Baron J.

  10. #10
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    Default

    I use one of those laser pointer RPM things too. Yhey work pretty well for a quick reference. However, if I want a fixed digital tacho so I don't have to bother with a hand held thing then I use these two items and make a permanently mounted rpm display;

    Display: https://www.ebay.com.au/itm/DC8-24V-...4AAOSwuwlbTt67

    Sensor: https://www.ebay.com.au/itm/NPN-Indu...0AAOSwO~VaUCdz

    Once set up, they work beautifully, provide ongoing stable rpm measurement. Maybe a little more work initially but for about the same price they work well.

    I have one on my bandsaw and one on my gear head mill. The one on my bandsaw has been set up and calibrated to read metres/min.

    Simon

    Simon
    Girl, I don't wanna know about your mild-mannered alter ego or anything like that." I mean, you tell me you're, uh, super-mega-ultra-lightning babe? That's all right with me. I'm good. I'm good.

  11. #11
    BobL is offline Member: Blue and white apron brigade
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    I've mentioned this before but will do so here in case others haven't seen it.

    One problem with the laser tacho is that the laser may reflect off a surface or detect a patch of dark that you did not notice and that effects the rpm reading. The way to check this is to add on reflector and take an RPM reading and then add a second reflector and the RPM reading should double, if not you have and unwanted contrast indicator.

  12. #12
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    Quote Originally Posted by simonl View Post
    The one on my bandsaw has been set up and calibrated to read metres/min.
    That is a good idea. How did you go about doing that?

    Sent from my SM-G935F using Tapatalk

  13. #13
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    The led display that i linked has several modes of operation and it also includes input for the number of pulses per revolution. So if you were using a 30 tooth gear and the induction sensor for example then you would set it to 30 pulses per revolution.

    Wrt my bandsaw, i worked out the circumference of the wheel. It also has 3 spokes generating 3 pulses per rev. With a bit of trial and error i was able to play with the pulses per rev setting and achieve a figure that was within about 5% of the actual blade speed in M per minute.

    I hope i explained that ok?

    Simon

    Sent from my SM-G900I using Tapatalk
    Girl, I don't wanna know about your mild-mannered alter ego or anything like that." I mean, you tell me you're, uh, super-mega-ultra-lightning babe? That's all right with me. I'm good. I'm good.

  14. #14
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    Yeah that makes sense, I just didn't know they were configurable like that. I've bought a few of these in the past.

  15. #15
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    Default Any manuals with those gadgets ?

    Hey Simonl,

    Do those gadgets come with some instructions or a web link to a manual or did you have previous knowledge to call upon and then simply mucked around with them until they did what you wanted.

    Would you expect an electronics newbie to be able to do what you did ?

    Bill

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