Rev. Counter

Hi All, Sometime ago now, I fitted a 4 digit rev.counter, so I could have some idea of the spindle speed when using the mill. It worked fine untill an unfortunate incident resulted in the Hall sensor bit being de-capitated. I have no wiring dia. of the kit I bought, but looking at the ones currently on ebay,it seemed I would need a NPN hall sensor,voltage range 6-40. I found one UK sourced. It's fitted and working, but I can't convince myself that the visual speed of the spindle is reflected in the rpm shown on the display. Was there some other factor that I should have been aware of? I've googled the couple of numbers that appear on the damaged sensor, but nothing comes up. So, electronics experts help me please. Regards Lynne

For the price of a new kit is it worth the hassle?

On my lathe the Hall Sensor is mounted perilously close to the magnets, of which there are four. They sit in shallow milled cavities about 3mm diameter.

Suggestions:

• The new sensor isn't mounted close enough to the spindle, or the magnet(s) don't pass directly underneath and it isn't being triggered reliably.
• One or more magnets has been lost (assuming more than one), causing a miscount.
• If a magnet was dislodged in the accident, maybe it's been replaced upside down (wrong pole facing the sensor)

Dave

Don't you know the full speed of the lathe? That should be easy enough to check.

Possible alternatives:

Borrow a rev counter from a friend? See if it the chuck strobes at expected speeds under fluorescent lighting?

Slow the lead screw to its minimum and measure the rpm from timings and the reduction ratio?

why has your rev counter got 4 magnets?

4 Digit not Magnet so it can display upto 9999

.

What am I missing, Jason ?

MichaelG.

It's badly worded but I think the comment refers to the four magnets referenced by SoD. The answer is it gives four pulses per rev.That can mean a number of things:

* Faster response to a change of speed

* Same update rate but better averaging

^ Better S/N ratio equals more robust in the event of noise

Andrew

Postscript: Beaten to it

Edited By Andrew Johnston on 13/07/2022 21:08:05

Bit fuzzy with Covid at the moment but thought Bernard was questioning the OP

Push the hall sensor closer to the centre. If the magnets are not quite on a perfect pitch circle it may be seeing 3 but

 missing the fourth.

Edited By Peter Cook 6 on 13/07/2022 21:31:12

... or even the mill

You can set the RPM at the lowest and count the revolutions manual.

If you have an Arduino UNO, Mega, etc, use a proxy and a magnet to measure the RPM. There are a lot of examples on how to do this. Beware, the magnet polarity (N-S or S-N) must be OK and depends on the proxy used. If it doesn't work, flip the magnet. The led in the proxy signals if the magnet is detected.

I have bought a cheap IR rpm meter from ebay. It doesn't work reliable on small diameters. I have made a DIY version using a TCRT5000, and an Arduino that works a lot better.

My mill and lathe both have a RPM meter that uses 4 magnets

Oops!

I actually like my rpm indicator/counter on my lathe - one single magnet used. Not important but I fitted a dummy on the opposite side of the shaft to keep it balanced.

My portable rpm counter is an optical machine - I can easily calibrate it from any mains socket with a neon indicator. I reckon the mains is close enough to 50Hz for anything I might need…

My mill will run slow enough for me to count its revolutions on its slowest speed setting, when running at 50Hz.🙂

Edited By not done it yet on 14/07/2022 08:32:55

RC flyers have some neat optical RPM sensors, handheld, that work quite well. Aimed at the plane's prop and it senses the break in rear lighting and seems to work well. They are also available in reflective type used often to find RPM of brushless outrunners by placing a 5mm or so section of black or silver tape on the motor housing and aiming at it. They are iffi at low RPM I find - below 20 rpm or so - I suspect mainly due to the jitter in RPM in sensorless brushless motors at such low RPM's - Ans IC RC engines may not work that well at 20rpm...

Check the local hobby RC shops..

just replacing the Hall sensor would not alter the rev display unless you disturbed the magnets(if you have more than one).

if you have more than one magnet (4), then rotate the spindle by hand and you should get 4 flashes of the led situated in the Hall sensor mounting (per revolution.)

there is no need to go to all the elaborate procedures posted as a new rev counter (with Hall sensor) can be had for next to nothing from China.

Edited By Howi on 14/07/2022 09:52:47

Agree with Howi - the unit widely advertised on line works great, Easy to set up and seems reliable (well mine is). Quite a range of prices for exactly the same unit so just hunt around a bit!

Really not worth the bother fiddling with your existing setup in MHO

regards, Mike

Hall sensors do not have an indicating LED on their own, if it has an LED it is part of a unit. There was an article and update in MEW about 2 years ago on this matter that may help you. A complete unit from the east is about £12. IF all you have done is change the hall effect device and the display reads, what makes you think it is reading incorrectly ? Noel.

S.O.D. N.D.I.Y.and others. Read my post. I refer to 'spindle speeds when using my mill' so why introduce things relating to lathes? My only question was, is there factor I should have considered when ordering the sensor, eg, frequency for example?. It was l ess hassle to fit just the sensor, than to have to get into the box I had made to house the controls for the VFD. I will look into investing in an optical RPM counter so as I can cross check the revs. When the mill was single phase, yes I knew the spindle speeds from the chart giving the pulley settings reqd. NOT now though.

Noel, what makes me think it is not reading correctly, purely subjective really, 'That spindle does'nt look as though its doing 350revs. the reality is that it is doing just that. Anyway thanks to all Regards,Lynne

I did read the post! Perhaps unfortunately I described the magnetic tachometer on my lathe rather than the optical tachometer unhelpfully fitted to my mill. But the machine is irrelevant: magnetic tachometers work the same way whatever they're fitted to: lathe or mill, similar answer.

I hoped I'd answered the question by describing the three things likely to cause a magnetic tachometer to misread RPM. Frequency or other Hall Effect weirdness is unlikely.

My co-defendant is innocent too! NDIY's neon light is the classic way of calibrating the RPM of any shaft; works OK on both mills and lathes. Other methods available, but probably easiest to buy a cheap RPM meter and compare results.

Dave

And I tried to just give you a method so that you could convince yourself.

Sorry about the lathe - but that goes round , just like my mill, so the principles are exactly the same. My second post might have provided some more thought provoking info, but nevermind I failed.🙂