RPM sensor question

[woodiedonald]

Hi,

I'm thinking of retro-fitting an rpm sensor to my lathe, had a look on ebay and came across this one

http://www.ebay.co.uk/itm/4-Digital...299?pt=LH_DefaultDomain_0&hash=item2a3644455b

My question is how do I calibrate it to the diameter I have the magnet set at, surely it will read faster if I was to put the magnet further out from the centre so how does the thing know what diameter the magnet is at?

Cheers
Donald

 

[bugbear]

Hi,

I'm thinking of retro-fitting an rpm sensor to my lathe, had a look on ebay and came across this one

http://www.ebay.co.uk/itm/4-Digital...299?pt=LH_DefaultDomain_0&hash=item2a3644455b

My question is how do I calibrate it to the diameter I have the magnet set at, surely it will read faster if I was to put the magnet further out from the centre so how does the thing know what diameter the magnet is at?

Cheers
Donald

It's counting RPM, not peripheral miles per hour (or feet per second)

BugBear

 

[Spindle]

Hi

It registers a pulse from the magnet once per revolution - it doesn't matter how far out you mount the magnet but for security as far inboard as practicable is best due to reduced centrifugal forces tending to detach the magnet from it's fixing.

Regards Mick

 

[John Brown]

Although the linear velocity of the magnet would be greater when mounted further from the centre, Hall effect sensors don't measure that. You can think of a Hall effect sensor as a device in which the magnetic field distorts the path of a current flowing through the device, but the amount of distortion is proportional to the strength of the (usually static) magnettic field. So the RPM sensor is simply counting pulses over a fixed period of time(or measuring the elapsed time between consecutive pulses).

If I measure the time elapsed between reading the OP and typing my reply, I find that it's equal to two other posts. That's because I type slowly.

 

[Naxie]

Hi Donald, I made a post about something similar a while back involving a bike speedo,

https://www.ukworkshop.co.uk/forums/cheap-lathe-tachometer-t70937.html

The maths is correct......lI think, works for me anyhoo.

Hth,

Kind regards,

Ed

 

[Spindle]

Hi Naxie

Good post but the OP references an electronic RPM tacho as opposed to a speedo which you have used - I don't think your calculations, (correct as they are), will apply.

Regards Mick

 

[sparkymarky]

why not glue one of these http://www.ebay.co.uk/itm/Digital-Laser ... 1e88a089c8 to a block of wood pointed at your chuck, spindle, bowl ect.

hth, mark