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Thread: Tachometer

  1. #1
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    Default Tachometer

    Saw this on Ebay and thought it was worth a separate thread for anyone wanting a tacho on their lathe/mill.

    This one's different as the sensor is included in the kit.

    http://www.ebay.com.au/itm/Hall-Prox...item3a88a3221a

    Many of use have already added these displays to equipment and they work well and are very handy, using a variety of sensors, and have created plenty of discussion on which of those is best

    Cheers

    Rob
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    But at least you tried.



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  3. #2
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    Hi Rob,

    That's pretty cheap considering the sensor comes with it. Bear in mind that even though it says it's good to 9999RPM, the sensors maximum response is 100Hz. With a single magnet and producing 1 pulse/rev that's a maximum of 6000RPM (assuming the frequency response is as good as they say). Now if you have 2 magnets then it's 3000RPM and 3 magnets it's 2000RPM.

    While even with 3 magnets fitted producing 3 pulses/rev, one would be inclined to think that 2000RPM is a good range but my experience with these cheaper sensors is that their frequency response is much less. I have had some that would not perform after about 60Hz.

    I came unstuck when converting my mill to VFD. I thought I would be clever and make a disc with 3 evenly spaced notches and use it with an induction sensor. The specs stated a frequency response of 150Hz. I figured that at 3000RPM (more than I will ever need) it will produce a signal at 150Hz which is within spec. Turns out the tacho peaked at about 900RPM and then went haywire since the sensor couldn't handle the pulses. I since changed the sensor for a flat plastic package and these seem to be very good.

    Anyway, something to bear in mind when making the sensor disc with the magnet(s).

    Cheers.

    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.

  4. #3
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    Default

    Any suggestions on attaching the magnet(s) to the shaft of a lathe?
    My mill has a tacho built in to the DRO but something on the lathe might be handy.

  5. #4
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    Quote Originally Posted by snowyskiesau View Post
    Any suggestions on attaching the magnet(s) to the shaft of a lathe?
    My mill has a tacho built in to the DRO but something on the lathe might be handy.
    I stuck a permanent magnet onto the spindle between the two bearings with a bit of clear silicon adhesive - this is for a Chinese lathe where the spindle is fully hidden in an enclosed headstock.

    I also put a very small cable tie around it to be sure it didn't fly off at high speed, as I mill with my lathe at max speed (1900 rpm) with carbide.

    By using silicon I can easily scrape it off if I ever have to pull the spindle out.

    The sensor is mounted on a bracket to bring it in close to the magnet - again all hidden in the headstock.

    Works well.

    Rob
    The worst that can happen is you will fail.
    But at least you tried.



  6. #5
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    Quote Originally Posted by snowyskiesau View Post
    Any suggestions on attaching the magnet(s) to the shaft of a lathe?
    My mill has a tacho built in to the DRO but something on the lathe might be handy.
    Magnets on machine tools attract chips and iron dust.

    I think you could easily keep the display unit, but replace the Hall sensor with an inductive proximity sensor. They are a bit more expensive than Hall sensors, but still very affordable (Here a cheap example, there are several dozen if not hundreds always on eBay: http://www.ebay.com.au/itm/DC-6-36V-...item54039afeb2).
    No magnet needed. An inductive sensor can pick up a change in metal. For example on my mill, I just inserted 3 small M3 screws at the bottom of the belt pulley. The pulley is aluminium, the screws are steel, the inductive sensor sees the difference. Keep in mind that to pick up a change in metal, the distance between sensor and the items must be very small, in my example about 0.5mm. On the Hercus lathe, I set the inductive proximity sensor to detect a grub screw protruding from the spindle gear, works perfectly and the distance can be 2mm or more.

  7. #6
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    Quote Originally Posted by cba_melbourne View Post
    . . . . .Keep in mind that to pick up a change in metal, the distance between sensor and the items must be very small, in my example about 0.5mm. On the Hercus lathe, I set the inductive proximity sensor to detect a grub screw protruding from the spindle gear, works perfectly and the distance can be 2mm or more.
    The sensor distance claim on that ebay item says 8m detection distance. Are you suggesting that this is a bit exaggerated?

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    Quote Originally Posted by BobL View Post
    The sensor distance claim on that ebay item says 8m detection distance. Are you suggesting that this is a bit exaggerated?
    I think you might have misread that, the data says 8mm...

    In any event, I'm not sure that particular inductive prox would be fast enough. The data sheet for it says 0.5 KHz but then says 25Hz for AC? 500Hz is ok, but 25 Hz not so much..
    http://www.chinahwe.net/handler/down...Series%201.pdf

    By comparison, a similar IFM prox switches at 2000 Hz. http://www.ifm.com/products/aus/ds/IE5282.htm not as cheap however..

    If you buy a cheap proximity switch, check the data sheets for switching speed. Let's say you have 4 pulses per rev, and a maximum speed of 3000 rpm, 3000/60 = 50 revs/second x 4 = 200 Hz.

    Ray

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    I see there are several all inclusive kits available now at a similar price.

    Here's another one, with a proximity switch.

    http://www.ebay.com.au/itm/Blue-4-Di...item20dc16f327

    Rob
    The worst that can happen is you will fail.
    But at least you tried.



  10. #9
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    Hi Rob,

    That last kit you showed has an induction sensor similar to what I have used. I have found these sensors true to their word WRT frequency response or response time. They seem to be a very good sensor.
    I agree with Chris, Induction sensors are the way to go.

    Edit: I can't seem to find any way of selecting the number of pulses per revolution. It looks like it may assume 1 pulse/rev and not have any flexibility. This is OK if it's mounted on the spindle but it makes it difficult if the sensor is placed on one of the gears and you need to calculate the correct ratio setting.

    Cheers,

    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. #10
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    For a machine with a geared or even a fixed pulley size and belt drive train I am not sure of the need for a tachometer. The only machines I have found it useful on are things with variable speed drives of any description, like a Schaublin lathe and a turret mill I once used. I can see the need with VFD's. For fixed drive machines, observing the cutting tool, if it is chattering or overheating etc dictates the speed, not the book value of a RPM. If you need to confirm the speed chart of a machine with a fixed drive train a hand held tachometer may be better for a once off. Once the chart is confirmed or a new one drawn up a tacho isn't really required after that, is it?

    For those with tachometers on machine tools what are the benefits?


    I would like a tachometer on a fertiliser spreader spinners that have a hydraulic drive. I used a hand held tacho to get the speed set but as we change operators, settings, tractors and oil temps change the RPM does vary. We have a 12v supply and I would like to have a display in the tractor cab so that we can monitor the speed. It needs to be close to 800 RPM. We could get away without having a connector in the wiring between sensor and display as it could stay attached to the machine. All of our tractors have places to run cords through into the cabs and removable mountings for controllers. What tacho system would you suggest?

  12. #11
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    In my opinion, this combination is very hard to beat for price and performance. The display allows for a multiplier so that any number of pulses can equate to 1 revolution. This allows for multiple pulses per rev on the spindle (if you wanted very stable low speed results) or placed inside on one of the gears and allows for the ratio.

    All for under $20 delivered. I use this setup on both my mill and my bandsaw. Eventhough the on the bandsaw it picks up the 6 spokes from the wheel, with the multiplier I was able to do the calc and convert it so that it reads in Metres/min to within about 5%.

    http://www.ebay.com.au/itm/0-56-DIGITAL-Red-LED-Frequency-and-Tachometer-Rotate-Speed-Meter-Two-Function-/261336338796?pt=LH_DefaultDomain_0&hash=item3cd8dc116c#ht_3902wt_1124
    http://www.ebay.com.au/itm/DC-10-30V...ht_2053wt_1362

    Cheers,

    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.

  13. #12
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    Quote Originally Posted by BobL View Post
    The sensor distance claim on that ebay item says 8m detection distance. Are you suggesting that this is a bit exaggerated?
    I use an Omron E2E inductive proximity switch with only 4mm diameter. These are pricy if bought new, but with some patience can be found as "new old stock" on ebay. For these one can download exhaustive data sheets, which diagrams to determine detection distance based on type of metals to be detected. Download this data sheet and have a browse through, it will give you a general understanding of what to look for in an inductive proximity switch, shaow the differences in detection distance and sensor size etc etc:
    http://www.ia.omron.com/data_pdf/dat..._ds_csm446.pdf
    Generally, I have found a short detection distance (1mm and less) requires a well designed installation, as a collision with the sensor may mean its destruction. But you can use very small and lightweight objects for detection, and the sensor itself is very small. Sensors with larger detection distance are usually larger, but less demanding to install, and requirer larger/heavier objects for detection. Larger and/or cheaper detectors may have a lower detection frequency. Then there is the choice between the detectors that are watertight embedded in a stainless enclosure, and those much cheaper ones in a plastic enclosure - depends if you can keep the sensor free of coolant and oil etc. Also the output configuration matters, weather the output is a positive or negative pulse, supply voltage etc. Also watch out, not all detectors have a built-in LED light to show visually when an object has been detected - but I found this a useful feature when setting the sensor up.

    The detection distance given in eBay ads for generic Chinese sensors is simply to detect a chunk of Iron in an otherwise empty space, such as an approaching garage door or the like. Not the distance to detect a chunk of iron embedded in Aluminium.

  14. #13
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    Quote Originally Posted by Burner View Post
    I would like a tachometer on a fertiliser spreader spinners that have a hydraulic drive. I used a hand held tacho to get the speed set but as we change operators, settings, tractors and oil temps change the RPM does vary. We have a 12v supply and I would like to have a display in the tractor cab so that we can monitor the speed. It needs to be close to 800 RPM. We could get away without having a connector in the wiring between sensor and display as it could stay attached to the machine. All of our tractors have places to run cords through into the cabs and removable mountings for controllers. What tacho system would you suggest?
    Slightly off topic, but still a tacho.. I used to manufacture a super spreader controller ( another lifetime ) inductive proximity switch pickup works fine in this environment,

    We used to feed ground speed, belt speed as well as spread width into an embedded computer system to control the belt speed feeding the spinner. Never got into the GPS based systems, that came along much later but allows you to program the exact distribution over a whole farm.. If you just want spinner speed, then one of the above systems with an inductive proximity switch will do nicely.

    Ray

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    Quote Originally Posted by cba_melbourne View Post

    The detection distance given in eBay ads for generic Chinese sensors is simply to detect a chunk of Iron in an otherwise empty space, such as an approaching garage door or the like. Not the distance to detect a chunk of iron embedded in Aluminium.
    I don't see the issue ?

    Are you saying they won't work/do the job?

    Whether it's 1 mm or 5 mm is pretty academic IMHO.

    Rob
    The worst that can happen is you will fail.
    But at least you tried.



  16. #15
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    Quote Originally Posted by cba_melbourne View Post
    . . . . Generally, I have found a short detection distance (1mm and less) requires a well designed installation, as a collision with the sensor may mean its destruction.
    They were also my thoughts

    The detection distance given in eBay ads for generic Chinese sensors is simply to detect a chunk of Iron in an otherwise empty space, such as an approaching garage door or the like. Not the distance to detect a chunk of iron embedded in Aluminium.
    Thanks for that info.

    Do these proximity sensors generally work with the same displays as the magnetic ones use or do they require a special display?
    I see they usually have 4 choices (Dry contact、PNP、NPN、Pulse) presumably the display needs to be wired to suit?

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