Jet 3520B Speed Display Faulty - NOW FIXED

[Duncan A]

The digital display on my 3520B lathe has suddenly started displaying "0" regardless of actual speed or direction of rotation.
The speed is sensed by a proximity sensor (proxy) picking up the four lobes of a rotor as it passes the proxy.

I have checked the air gap between the proxy and lobes; about 1mm, unchanged from previous.
There is no dirt on the lobes or proxy.
There is no visible damage to the proxy or associated electronics.
A red LED on the proxy remains lit whenever the lathe is powered up whether the lathe is rotating or not.

Speed control is unaffected, and speed display is not really needed anyway, but it's annoying me so I want to fix it. The proxy is probably an inductive device, which are known to fail occasionally, but I'd be grateful for any suggestions as to the likely cause or fixes.

Thanks in advance
Duncan

[Wood spinner]

http://www.brimarc.com/contact

 

[Duncan A]

Thanks Woodspinner, that is the plan if initial efforts fail. The lathe is 4 or 5 years old - well out of warranty unfortunately.
Duncan

[Wood spinner]

Did you get this sorted out ?

[Duncan A]

Hi Woodspinner, not sorted out yet.

I checked the voltage to the speed sensor board - 220Vac as it should be.
Next I checked the voltage to the speed pickup itself - about 16Vdc which is within the working range of the device, but I don't know whether that is the correct voltage for this setup.
With the device powered up, the on-board LED should be off when no metal is within the sensing range, on when it detects the presence of metal. It actually stays on regardless of the presence of metal and, since this is the normal failure mode of these devices, I have taken the view that it is the pickup that is faulty, not the associated PCB.
Perhaps foolishly, I didn't ask Axi for advice, just price and availability of a new pickup.

For the record, the original is a BAKS DA1805NO which I have identified as being NPN type, NO contacts, 10 - 30V, 600Hz switching frequency, 150mA max, 0 - 4mm operating range, size 18 x 18 x 36mm.
These all seem to be fairly standard specs so I fitted a £5 cheapo speed sensor from eBay - but that didn't work although the specs seemed OK.
This morning I ordered the genuine item from Axminster, £35.

I'll let you know in a couple of days!
Duncan

[Paul Hannaby]

That seems to be greatly underspecified for its intended use - 600hz switching rate for a spindle with a speed range up to around 3000 rpm??? No wonder they fail!Or is the speed sensor vane geared down from the spindle?

[Duncan A]

Paul, 600Hz is 36000 rpm since Hz = passes per second (or cycles/s, revs/s etc depending upon the application).
Since the rotor passing the proxy has 4 lobes, at 3000rpm lathe speed, the passing frequency is 12000 rpm, well within spec.

Heard good things about your recent demo at Mid Wales Woodturners, although not a member myself.

Duncan

[Paul Hannaby]

Yes, so it is I will try to engage brain before the keyboard next time! I overlooked the fact that one was minutes and the other was seconds 

[Duncan A]

Paul, we've all done it at one time or another. Minutes/seconds; inches/cm; pounds/kg, you name it, I've done it!

However, the good news is that the proxy arrived this morning, just 24 hours after ordering, so a pat on the back for both Axminster and Royal Mail for excellent service.

The even better news is that it works perfectly. I tested it before fitting and the LED remained off when the lathe was powered up and the proxy held in air, turned on when a screwdriver was placed close to the face - as I had hoped for. Once fitted, a run to full speed gave the correct readings of about 3200 rpm.
My cheapo proxy is obviously duff, not sure it's worth claiming £4 back though.

Duncan

[burywoodturners]

If everyone claimed their four quid back the firm would either go bust and not inflict their faulty goods on us, or they would sort it out so it does work. Either way if we all complain at poor design hen we will be the winners.
Ron

[Duncan A]

Ron, you're undoubtedly right but if the price is too low,as it was in this case, there is clearly an element of risk. Having said that, everything that is sold should be fit for purpose.
On the subject of too cheap online purchases, we are all losers because many of these outfits import direct from China without paying taxes, but most of us - me included - prefer not to ask too many questions.
Not meant as a contentious comment, just an observation relating to the hazards of online purchases and what you should expect for your money.
Duncan

[Wood spinner]

Well done Duncan glad to hear its now sorted