Reducing lathe speed

[Peter907]

I have a 550w variable speed lathe with a minimum speed of 500 rpm. Can I reduce this speed further by plugging it into a 4000w 250 v AC mains PWM voltage regulator variable speed motor controller? Could this cause any damage?

 

[Paul Hannaby]

It's doubtful it would work in the way you anticipate because the way I understand it, the output of the PWM regulator would be (chopped) DC rather than AC.
Some of the smaller lathes have DC motors and if so, you may be able to replace the existing controller entirely with the one you have. If it's an AC motor, that isn't going to work either!

 

[Peter907]

Thanks @Paul Hannaby
You are correct.
I've now looked a bit more closely into this controller and see that it is only suitable for fans, water heaters, or bulbs, and states that it is not suitable for electric machinery.
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[alex robinson]

I have a 550w variable speed lathe with a minimum speed of 500 rpm. Can I reduce this speed further by plugging it into a 4000w 250 v AC mains PWM voltage regulator variable speed motor controller? Could this cause any damage?

What sort of variable speed is it? Is electronic, or a come drive?

I used to have a come drive Axminster that could be turned significantly slower than the minimum 500 rpm by just moving the handle past the minimum speed and wedging it in place with some mole grips.

 

[Sachakins]

Don't mean for this to sound daft, but have you checked the pulley set up?

I have a herald with variable speed control, but if want really slower speeds I still need change the belt to the lower of 3 positions and controller to matching ratio number, 1,2 or 3.
My older lathe, a Jet, i thing or one the badge clones of it, also had a 2 position pulley for hi and low ranges, as its Controller too cold not go below about 600 on the higher pulley ratio but would go down to about 90 on lower pulley setting.

 

[Peter907]

I have the Charnwood W824. It has two pulley settings and an electronic variable speed control giving 500 to 2000rpm and 1000 to 4000rpm. According to the digital spindle speed readout, the minimum is about 530rpm.
It would be useful to be able to have it running slower than this for sanding and applying coatings.

 

[Sachakins]

I have the Charnwood W824. It has two pulley settings and an electronic variable speed control giving 500 to 2000rpm and 1000 to 4000rpm. According to the digital spindle speed readout, the minimum is about 530rpm.
It would be useful to be able to have it running slower than this for sanding and

Bit Heath Robinson, but works

 

[MikeJhn]

Not sure if is applicable, but to slow my swimming pool pump down I used a Variable Frequency drive unit, can't think of the name at the moment, but when I left France I took the unit off and brought it back with me, now where did I put it!

 

[Phil Pascoe]

I have the Charnwood W824. It has two pulley settings and an electronic variable speed control giving 500 to 2000rpm and 1000 to 4000rpm. According to the digital spindle speed readout, the minimum is about 530rpm.
It would be useful to be able to have it running slower than this for sanding and applying coatings.

It does seem a rather odd choice of speed range - I'd think maybe 250rpm to 500rpm would be of far more use than 3000rpm - 4000rpm to the the average turner.

edit - of course it would probably need a torquier motor to run at a low speed.

 

[Peter907]

@Sachakins
That's a brilliant idea thanks . I reckon even I could turn a set of wooden pulleys to fit on the Charnwood to get me some more useable speed ranges.

When I first scrolled down and read "Bit Heath Robinson, but works", I thought you were being a bit cheeky about my Charnwood. But having examined other people's quality lathes that I aspire to, then perhaps you have actually made a reasonable assessment of my lathe .

 

[Peter907]

It does seem a rather odd choice of speed range - I'd think maybe 250rpm to 500rpm would be of far more use than 3000rpm - 4000rpm to the the average turner.

I agree.
When I first got the lathe, it was set on the 1000 to 4000rpm pulleys, but I soon found that 500 to 2000rpm covered nearly everything I want to do. So it has stayed on that setting.

 

[Peter907]

@Phil Pascoe
I don't think the torque would be a problem since I only want to go below 500rpm for sanding and wiping on oils.
Don't think I'll be turning 20" bowls on the little Charnwood

 

[ChaiLatte]

I reckon even I could turn a set of wooden pulleys to fit on the Charnwood to get me some more useable speed ranges.

Be aware that you will need a new, longer belt if you do this. The only way to retain the same belt is if you reduce the small pulley at the same time you increase the big one. There is a practical minimum on the small one.

It would be wise to do some calculations before making too much: set the motor in the middle of the belt adjustment range and measure the spindle centre distance. Then work out the pulley ratio to give your desired speed. Calculate a new spindle pulley size that produces that ratio. An online belt calculator will then tell you the length of belt you need. You can then refine your initial pulley size calculation to suit belt availability and price.

You do not want to end up in a situation where the only belt that will fit within the mechanical adjustment range is a rare size and hence expensive.

 

[Peter907]

@ChaiLatte
Very good points thank you.
I will do the calculations and check belt sizes first to make sure it is a feasible option.

 

[Phil Pascoe]

@Phil Pascoe
I don't think the torque would be a problem since I only want to go below 500rpm for sanding and wiping on oils.
Don't think I'll be turning 20" bowls on the little Charnwood

Certainly, but the lower speed being available would tempt many people to put larger out of balance stuff on it that it wasn't designed for. Not a sound move from a manufacturer's point of view.

 

[Peter907]

Certainly, but the lower speed being available would tempt many people to put larger out of balance stuff on it that it wasn't designed for. Not a sound move from a manufacturer's point of view.

Yes, I agree. Some people would push the limits and it could easily get dangerous.

 

[Paul Hannaby]

There are a number of reasons why some variable speed lathes don't run very slow. The motor is generally cooled by a fan on the end. If the motor runs slow, so does the fan which can cause the motor to overheat. The controller/motor combination is tuned to give reasonable power across the speed range. Outside that range, the output power might be significantly lower making the lathe less effective.
Keeping the motor running at a reasonable speed and slowing down the spindle by changing the pulley rations is probably the best way forward but obviously means some modification.

 

[MikeJhn]

Not sure if is applicable, but to slow my swimming pool pump down I used a Variable Frequency drive unit, can't think of the name at the moment, but when I left France I took the unit off and brought it back with me, now where did I put it!

For those who may be interested I bought an Invertec drive " Invertek Optidrive E3 IP66 0.75kW 230V 1ph to 1ph AC Inverter, SW, C1 EMC" : Documentation For Optidrive E3 Variable Frequency Drive | Variable Speed Drive | AC Drive | Invertek Drives from "The Inverterdrive Supermarket Ltd" mailto:[email protected] if anyone is interested I may let it go at a reasonable cost.