Working out lathe speeds?

[IanB]

Hi all, I wonder if anyone can advise on calculating my lathe speed?

I have a lathe with a 3 phase motor running off a domestic power supply. It has a control knob to regulate the speed, but only with numbers 1 to 10. I would like to be able to work out what speed the lathe is running at on any particular setting of the control knob.

Below is a photo of the motor plate, most of which I don't understand but it does include "1400/min", which I assume is the rpm of the motor at full power. From this I assume I can calculate the maximum rpm of the lathe spindle by measuring the circumference of the two wheels on the belt drive and doing the right sum.

Does that sound right? And if so, my main question is, can I assume that the control knob increases the speed on a linear scale of 0 to 10? Eg at setting 4, will the speed be 4/10 of the maximum spindle speed?

Any advice appreciated!
Thanks,
Ian

 

[Phil Pascoe]

Why does it really matter? What do you hope to gain by knowing the RPM? As you get used to adjusting the speed to suit the job in hand the actual RPM becomes rather meaningless.

 

[marcros]

I agree with Phil, but if you really do want to know, you can buy a cheap rpm gauge from ebay and the like.

https://www.ebay.co.uk/sch/i.html?_from=R40&_trksid=p2380057.m570.l1313&_nkw=rpm+meter&_sacat=0

 

[IanB]

Well that might be fine with more experience, but when some instructions or a piece of kit says do not exceed 600 rpm for example, it would be good to know what that is in practice

 

[marcros]

I think that you will know very quickly when you start using it.

if it is too fast, it will wobble and vibrate. If it is way too fast, the lathe will attempt to fall over! if it isnt vibrating it could go faster, so you use instinct to balance it between almost wobbling too much but not quite!

 

[Richard_C]

Unlikely it will be 1400 at the chuck unless it's direct drive. Assuming some kind of belt drive you could measure diameter of drive and driven pulleys and do a few sums. But as others have said its not that important especially with variable speed, start slow and get faster.

Remember the speed of the work where the tool is cutting, is a factor of rotational speed and workpiece circumference. As a relative beginner and by nature cautious (belt change lathe not variable speed) I tended to use too slow a speed on small diameter work and life got easier when I used a higher speed.

 

[CHJ]

Hi all, I wonder if anyone can advise on calculating my lathe speed?

Use these discs mounted on discs of card or MDF in your chuck. (simple bolt through the middle) and strobe with a standard mains powered filament bulb.
And this chart to give you a rough guide to safe turning speeds.

 

[LaymarCrafts]

I assume you are driving the motor via an inverter? Most inverters have a readout that gives the Hertz at full speed this can be 50Hz or can be 60Hz or even higher depending how it was programmed.

It may also be programmed with a minimum Hertz setting typically 10Hz to ensure you have adequate motor cooling.

The Motor plate indicates that it runs at 50Hz and 1,400rpm.

If you can read the Hz then you can calculate the speed, i.e if 50Hz = 1,400rpm then 25Hz will = 700rpm.

You of course need to calculate the actual lathe spindle speed first and if you have a step pulley this needs to be calculated for each step.

The potentiometer [speed control knob] can be either linear or logarithmic, although linear is the mote common option.

 

[mcgranag]

Go for the cheapo rpm meters as mentioned above. The strobe and patterned discs would be a good idea too, but you'll need a strobe light, the incandescent filament wont work as its pretty much a constant non flickering source.

 

[Bob Chapman]

When I was demonstrating I always used to say that there are only three lathe speeds. One is 'too fast' by which I meant that the work was vibrating, the lathe was wobbling or simply that the whirring sound was making me feel scared. The second speed was 'too slow', the wood wasn't cutting cleanly and the surface was lumpy and bumpy despite sharp tools. The third speed was 'about right' - the lathe feels and sounds secure, the wood cuts easily and cleanly and leaves a good smooth finish. 'Just right' is somewhere between 'too fast' and 'too slow' and it's the speed I always tried to use (still do, in fact). Take it steady at first and experience will soon teach you the correct speed. You don't need numbers.

Bob

PS Since retiring I've given up my website. The address below still works but I can no longer update or change the site.

 

[CHJ]

...........The strobe and patterned discs would be a good idea too, but you'll need a strobe light, the incandescent filament wont work as its pretty much a constant non flickering source.

Sorry to disagree,
A standard old fashioned filament bulb is fine either mains or low voltage as long as it's run from 50HZ mains.

In fact if using standard filament bulbs as machine lights powered direct from 50HZ, strobing is a risk that needs consideration.

 

[Duncan A]

600 rpm is the max speed stated on many sets of Cole jaws. Perhaps that is why the OP is asking - as a beginner he, quite rightly wants to play safe.
Duncan

 

[u38cg]

It depends on the type of filament; some decayed very quickly meaning that it was possible for them to strobe. Most did not in the last years of production. The strobe risk is mainly from fluorescent tubes.

 

[Phil Pascoe]

and cheap LEDs.

 

[Richard_C]

I thought leds had to be rectified and right plus minus-y-ness so didn't strobe, but that might be faulty memory.

 

[Phil Pascoe]

Nope. I've had LEDs that strobe - cheaper ones, and possibly quite old as I bought some six or seven years ago. Better now, probably.

 

[IanB]

Hi all, thanks to everyone for their input, much appreciated!

Use these discs mounted on discs of card or MDF in your chuck. (simple bolt through the middle) and strobe with a standard mains powered filament bulb.
And this chart to give you a rough guide to safe turning speeds.

Thank you @CHJ, that is interesting and sounds a good and cost effective way to get a good idea of the speeds I am getting, I'll check that out

I assume you are driving the motor via an inverter? Most inverters have a readout that gives the Hertz at full speed this can be 50Hz or can be 60Hz or even higher depending how it was programmed.

It may also be programmed with a minimum Hertz setting typically 10Hz to ensure you have adequate motor cooling.

The Motor plate indicates that it runs at 50Hz and 1,400rpm.

If you can read the Hz then you can calculate the speed, i.e if 50Hz = 1,400rpm then 25Hz will = 700rpm.

You of course need to calculate the actual lathe spindle speed first and if you have a step pulley this needs to be calculated for each step.

The potentiometer [speed control knob] can be either linear or logarithmic, although linear is the mote common option.

Thanks @LaymarCrafts, that's helpful, and good to know that potentiometers are usually linear

600 rpm is the max speed stated on many sets of Cole jaws. Perhaps that is why the OP is asking - as a beginner he, quite rightly wants to play safe.
Duncan

Spot on @Duncan A!

 

[Ttrees]

Are you turning just one size object?
I can't see what use it is, knowing what the speed is, if the work is going to change this, or when turning a table leg that might be square at the top and thinning out down the leg.
Maybe finding the optimum speed for drilling for a particular janka.
Doesn't seem that helpful to me as I presume a bit of trial and error may be needed for some sort of production batch.

Tom

 

[flh801978]

Sorry to disagree,
A standard old fashioned filament bulb is fine either mains or low voltage as long as it's run from 50HZ mains.

In fact if using standard filament bulbs as machine lights powered direct from 50HZ, strobing is a risk that needs consideration.

Have you tried this Chas?
a filament lamp is by its nature a continuous light source...it cant possibly warm up and cool down 50 times a second
flourescent tubes are used for strobe discs or neon tubes in turntables that had a stobe pattern on the circumference of the platter
You shouldnt have flourescent lights illuminating rotating machinery.

 

[CHJ]

Have you tried this Chas?
a filament lamp is by its nature a continuous light source...it cant possibly warm up and cool down 50 times a second

I can assure you a standard filament bulb is capable of cooling down sufficiently at 50HZ to strobe. Your eyes may not be capable of reacting to the flicker but some folks can detect it in their peripheral vision where the retina has less persistence.

I have used these discs to calibrate the speed indicator dial on my current machine and have use this principal for the last 40+ years to spot check machines with variable speed drives.

In fact thinking about it, my Goldring Lenco GL75 transcription Record Deck which I have had for some 55+ yrs uses this very method, built in, to calibrate its speed gates with normal room lighting. ( I now use a desk lamp with a filament bulb if necessary because of modern low energy main lighting)

In machine shops using filament lighting we had to ensure the lighting banks and/or machines were shared across all three phases to reduce the risk.