Low cost rev counter and cutting speed display

[SkarvenParticipant @skarven]

I really liked the article “Inexpensive rev counter” by Michael Cox in MEW No.178, so I bought a Bicycle computer for about £9 and mounted it under the DRO of my lathe with a simple aluminum bracket.

I then destroyed a “fridge magnet” to get the “magnet”. A small, very powerful one. I payed about £1.50 for 10 of them.

I then put the magnet on the back part of the spindle of the lathe. Experiments showed it would stay there with it’s own force, whatever revs I tried. The sensor was fastened with double-sided tape after some cleaning.

This worked great! And for a short while I was very satisfied. Looking at the display though, I felt that most of it was giving no information.

After all, the RPM was some small digits in the lower part of the display, while the speed in km/h was displayed wit large digits in the upper part.

After some thought, I realized that setting the wheel diameter to 7 inches, the speed display in km/h would be almost exactly (a few % error) the cutting speed for a diameter of 10mm or 1cm. This would be a very quick Cutting speed calculation method. I find it really useful. 2cm -> twice the display….

 

For imperial guys, a setting of 27 inches for the wheel diameter will give app. the speed in feet / minute for HALF an inch turning diameter. (You cant set the wheel diameter to 54 on my bike computer. Maybe there are just a few velocipedes around these days)

 

Now the whole display gives meaningful information!

 

 

[SkarvenParticipant @skarven]

Additional functionality to Michael Cox’s rev counter

[adyParticipant @ady]

So does an indication of the raw speed give you any advantage?

I always listen to the cut and took it from there, faster or slower.

 

I can get a massive advantage from stiffness.

 

I’m at the low end of the scale, a quarter to half a horsie

Edited By ady on 24/11/2011 00:19:16

[SkarvenParticipant @skarven]

Hi ady

In a few years, I will probably listen to the cut like you. As for now, I’m a beginner, so my ears don’t tell me much, except when things go really wrong

I also think this is a help when I try to remove a lot of metal without destroying the tool.

Actually, I do not really need the rev counter, as I have a geared head lathe.

The size of the motor is either 1.1kw, 1.5kw or 2.2kw. It is listed as follows.

 

Motor power output S1 100%	1,1 / 1,5 kW (400 V)
Motor power input S6 40%	1,5 / 2,2 kW (400 V)

Does this mean different ratings for different intermittence, as in a welding machine?

I’m planning to install a VFD, and then the rev counter will come in handy.

[swarfmakerParticipant @swarfmaker]

If you enter 268mm wheel diameter for a mph readout or 167mm for kph readout, the large display will read rpm on the large display without the trailing zero, works fine.

[SkarvenParticipant @skarven]

I still think that the cutting speed display is rather useful. It saves a multiplication with PI

 

I found out what the S1, S6 is.

 

S1 is a continuous rating.

S3 is an intermittent where the motor is stopped when not running at load.

S6 is an intermittent where the motor is running idle when not loaded.

Period is 10 minutes if nothing is given.

I imagine the S6 would be higher than the S3 when the motor is not cooled by the fan (if it has one)

[Old TimerParticipant @oldtimer90253]

I understand that there are speed counters available to hold in your hand, with a special tap put on the mandrill of the lathe, you can read off the speed of rotation, would any one know where I may purchase one. I am a wood turner, but I have an electronic variable speed motor with a inverter, ,also the facility to change the drive belt to the mandrill, this is why I would like to know the speed it is going at any time

[V8EngParticipant @v8eng]

Hi Old Timer.

I remember that CPC list hand held reflective digital tachometers, which might be worth a look.

**LINK**

Cannot find my catalogue at the moment, so try searching for Digital Tachometer (or similar words).

[chris jParticipant @chrisj]

How about one of these ? LINK

[Keith LongParticipant @keithlong89920]

Hi Old Timer

I think you might find item 290742053674 on E-Bay a more economical proposition. I've got one for checking motor and machine speeds and it works just fine. Only drawback I've found is that the battery still drains in storage so if you're not going to be using it for some time, it's best removed – 10 second job. The above item number is just one of several different but similar items avaiable.

Keith

[Gone AwayParticipant @goneaway]

This might be of interest too:

Laser Tach

Ooops, sorry – crossed with Keith's post there.

I have one of these in transit.

Edited By Sid Herbage on 13/07/2012 01:19:40

[BazyleParticipant @bazyle]

All this electronics is just more stuff to go wrong in a few years. How about a couple of wieghts on arms, a spring, and a pointer.

If you go the VFD route just link the speed pot to the cross slide and it will change speed automatically as you change the diameter.

[The Merry MillerParticipant @themerrymiller]

Couldn't agree more Bazyle, you can't beat the good old ones' (well you can sometimes)

Just won this Starrett tacho on E-bay, god was the bidding fierce.

I eventually swept away the rival bidders when I went for the kill and bid 99p (yes, 99 pence) they had no answer to that bless'm.

I'll tidy it up a bit and try it out later before I buy a Laser one!

Len. P.

[Ian S CParticipant @iansc]

You could make one, as I did for my hot air engines, it's just a 125 (I think) tooth gear out of something I pulled to bits one day, and a single start worm, its all mounted in a little wooden box with the shaft of the worm protruding with a rubber button on its end, hold that to the center of the shaft, and time the rotation of the worm wheel, I'v got a chart stuck on the box, a stop watch is handy. Price, an hour or twos' work. Ian S C

  No., of teethX no of turns  div by  time in sec X 60 = rpm

Edited By Ian S C on 13/07/2012 15:19:52

[Gone AwayParticipant @goneaway]

I'm surprised, Ian, I would have thought that with a limited power/torque device like a hot-air engine, any kind of contact device would affect the rpm. Even one with a relatively low loading torque as you describe.

A non-contact (the dreaded unreliable electronics – where did that rumour start?) method would seem to be ideal in this situation.

Edited By Sid Herbage on 13/07/2012 15:39:04

[KWILParticipant @kwil]

When you move up to a VFD, use a 10 turn pot and a geared knob with indicator (RS can supply) you can divide you speed range into 100 parts. You will know the maximum speed from whatever pulley or gear you are using, say 2000rpm, 1 division = 20 rpm which must be good enough for most people. The variation is reasonably linear.

Edited By KWIL on 13/07/2012 19:13:30

[Ian S CParticipant @iansc]

The tacho is OK on the 4 bigger motors, but even a motor of 5W or so has a slight speed reduction when the tacho is used. I would like to get a non contact / laser rev counter. Ian S C

[Old TimerParticipant @oldtimer90253]

Hi, To you all, in My quest for a speed counter ,/ rev counter, I found a smiths one on Ebay, it has been around for some years, but still works well, thank you all for your help, I also obtained the digital version, that you just stick a reflective strip on the item that is revolving, and the handheld device tells you the revs , good" A" I feel that more and more Lathes are now coming with a speed readout, that in the future it may be offered as an accessory to add to any lathe. yours Old timer

[Lawrie Alush-JaggsParticipant @lawriealush-jaggs50843]

I can get a massive advantage from stiffness.

I thought we were not having Viagra adds here……..

[blowlampParticipant @blowlamp]

Posted by Lawrie Alush-Jaggs on 07/08/2012 10:44:06:

I can get a massive advantage from stiffness.

I thought we were not having Viagra adds here……..

Yeah. That one got by the moderators alright

Martin.

[FMESParticipant @fmes]

I bought a hand held unit from e-bay a while ago, laser unit non contact. Just checked and they are still available for less than a tenner with free postage.

Lofty

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