RPM reading

I designed & built one a while back based on a PIC microcontroller and an LCD display. Lester Caine

now sells it in kit form. If you are not skilled in the art of soldering, he has been known to offer his repair service before delivering the kit ;-)

Regards, Tony

Here's a slightly more useful link:

And the text of the MEW articles can be found here, if you fancy doing a home-brewed version:

Regards, Tony

I know what DOA means, then this should be DBA (dead before arrival). :-)))

Nick

I bought a frequency counter (0 - 9999) module off eBay from some guy in HK for around £15 and I've put a disc with 60 holes on the spindle. Still got to wire it up....

It's so important that I still haven't got round to it!

You can often find cheap digital multimeters that have a frequency counter function & these can work very well as RPM meters.

Regards, Tony

I use a bicycle computer - attach the small magnet to the rotating shaft with a bit of super glue or epoxy and set the "wheel diameter" in the computer part so that the readout is in RPM. Works well.

Peter

Peter, The bike computers I have seen (kids bikes) have used a reed switch and magnet to sense wheel rotation. I would have thought that the mechical response time of the switch would mean that in your novel application, the maximum mandrel speed over which it would work would be quite low. Top speed on a bike is going to be say 200 rev per minute. Maybe the bike computer you are using is a more sophisictaed device maybe using a Hall effect (electronic/semiconductor)?

Do you know if this is the case? What sort of top speeds are you working with?

TIA

Bob

The more modern (even cheap modern) cycle computers use a hall effect sensor which has no moving parts and thus a much faster response time. Probably the hall effect sensor is now cheaper than the reed switch!

Alan

The bike computer works well for me. It is a hall effect so no moving parts - I would assume that all bike computers as the same these days.

I have set the "wheel diameter" to 1666mm so that on the readout an indicated speed of 20kph equals 200RPM, 40kph equals 400 RPM, etc. - just add a zero to the speed reading in kph and you have RPM (near as damm it). (we are metric here in Australia - you guys can work whatever system suits you!!)

It is not instantaneous however as the computer needs to sample a few revs to work out the speed but this is totally irrelevant for my applications.

Bike computers are readily available that measure speed up to 200kph (as a keen bike rider I shudder to think what sort of ride THAT would be - about 60kph is about my thrill limit !!) which means that we machinists can read up to 2000 RPM - more than adequate for my purposes which is mainly on the milling machine and I get nowhere near that speed.

After using the computer for a while I find that I can now fairly accurately judge the speed of the mill machine by sight and sound and usually only look at the computer as a check or if I am setting up some tooling I have not used for a long time. I still tend to set the speed by the way the cut is going rather than the computer or any tables of recommended speed - a sort of practical method which gives good results - there are too many variables in the home workshop to get too fussy about absolute speeds.

If you are interested visit a bicycle shop and check out what they have - we can get them here for about $35 (about 15 quid??) - check the upper speed limit as that may be important for your application.

regards

Peter

Ooopps... I said "wheel diameter" - I meant to say "wheel circumference"....sorry if that confused those of you with a mathematical bent.

Peter

Ooopps.. that should be "wheel circumference" not "wheel diameter"

(I sent correction a while ago but it seems to have disappeared into a virtual hole - apologies if it turns up twice)

Peter

Assuming you are adjusting the speed with a potentiometer, a cheap way would be to fix a scale on the pot knob and calibrate it with reference to the frequency reading from the VFD (ie. VFD frequency of 50 (in UK) means motor is running at normal speed, 25 is half speed, and 75 is 1.5x speed. Its then a simply matter to translate that into spindle speed depending on the gearbox settings, but you would need a scale for each possible gearbox setting. Like I say, cheap and cheerful, but probably accurate enough.

there you go

all the best.mark

I read about this device on ebay and decided to buy one to see how good it was. It arrived today in Australia from Hong Kong one week after I did the purchase - cost about $AUD35 - no problems with the transaction at all. It seems to work very well - all it requires is to attach a small piece of reflective tape (supplied) to the shaft in question and point the laser beam at this point as it rotates. Dead easy.

I guess I will continue to use my bike computer method described above as it gives a hands free continuous reading and it is permanently mounted on the milling machine which seems to be the one most sensitive to speed.

Nice to have both methods.

regards

Peter