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How to control the speed of a lathe


clockboy

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I have recently (if you follow my posts) been re-pivoting and I have been using my lathe (Star) to drill very small holes into hardened pivots.. One issue that I have learnt is the speed of the lathe is critical. With very small drills slow is the way because of heat and regular swarf clearance a must for a successful outcome. The issue I had was my lathe is controlled via a foot pedal and I really struggled to keep a slow consistent slow speed. My question is can I purchase a speed controller for the motor. I have looked online and I am a bit confused as to what is possible.

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Hi Clockboy    You can purchase 240v/50Hz speed controllers , How you fit them is dependant on your present fittings, is the foot pedal intergrated with the power cord or is it detachable via an in line plug. If thats the case just fit the controller in line instead of the foot pedal and adjust the controller for the required speed.   I build a fly line loader to fit lines and backing to fly reels tedious by hand, on the same principal altering the resistance of the coil using the potentiometer it went fron crawl speed to max no problem, Its probably more consistant than the foot pedal.  have a google on speed controlers and se what fits the bill there are probably hundreds for different applications. I take it it is powered via a 240v/50Hz motor if so no problems.

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Thanks watchweasol. It has one power plug which feeds directly to the foot pedal and a wire then runs from the pedal to the motor that drives the lathe. Can I therefore just fit a different controller directly to the motor. If so where do I purchase.,

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Hi Clockboy     you might have to cut the wire from pedal to the lathe and insert an in line connection. Male/female so as to be able to insert controller and the foot pedal if so required.   A cheap way of doing it is using a domestic dimmer and a socket. a length of cord with plug atttached into a box wired to the dimmer then wire the dimmer to the socket also on the box then a cord from the box with in line connector connected to the lathe. The dimmer amps  needs to equate to the motor or the fuse will go.

Altenativly have a look on the bay  i noticed there were some arround the £11 mark. The thing to watch is the amperage they can handle. Got to sort my own out I have  Twitchey right foot (years of driving).

 

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The type of speed controller you need depends on the power rating (wattage) and type of motor you are using ("universal" brushed, vs brushless etc).  Mostly the motors used are similar to sewing machine motors, so you might get away with using the sort of foot pedal speed controller that is used for sewing machines.

Edited by AndyHull
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They mainly use a "universal" motor, and some variation of this "classic" design.

 

Sewing-Machine-and-Mixer-Motor-Speed-Control-circuit.png.jpeg.028563d82f3c9ebeda90a863502712f3.jpeg

 

Older sewing machine or food mixer speed controller designs actually used a slightly dubious arrangement of compressible carbon disks as a form of variable resistor.  You don't need to know this of course, unless you are planning on pressing in to use, an old Singer speed controller from the 1970s or earlier.  

If you pick up something on ebay, make sure it is suitable for UK line voltage, some of the cheap Chinese ones are only suitable for 110V. They may work on 240V, or they may release the magic smoke, and sulk.

Whatever arrangement you come up with, I would suggest you ensure that any existing earth pin protection is preserved, and that you use it with an RCD protected outlet or adapter. I wouldn't want you to have any hair raising experiences.

Edited by AndyHull
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If you want to risk literally blowing a fiver, you could try one of these.

https://www.ebay.co.uk/itm/2000W-SCR-Dimmer-240V-AC-Voltage-Regulator-Heater-1000W-25A-Fan-Speed-Control/292181746872?hash=item440763a4b8:g:z38AAOSwd3dZZ0Py:rk:2:pf:0

It claims to be good for 2kW, resistive load, or 1kW inductive (fan motor) but I would say 500W or less is more likely.

SpeedController.thumb.jpg.1e4116af576fdd9d03e49dfb53070ded.jpg

It look pretty similar to the schematic I showed above. No instructions with it, you appear to wire it as follows.

Screw terminals, left to right...1. Neutral(in) 2. Live(in) 3.  Live(out) 4. Neutral(out)

 

Stick that in a suitable plastic project box. Pass the earth through separately and you are good to go (or perhaps good to go bang, but that's half the fun of these things).

 

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On my lathe motor I use a circuit (DIY) very similar to the schematic above. Works like a charm from zero to nearly full speed, and, the best, full torque compensation esp. in low speeds.
The foot pedal I use for on/off switching only.
Frank

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I've got a multifix motor where you set the set speed on the motor and away it goes.  I have been looking at ways to get foot control as its such a pita doing general lathe work without a foot control - the convenience of the on/off function without reaching for it.

I suppose the takeaway is it would be ideal to have a set up where you could swap back forth.

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When I started out I had a foot pedal sort of rheostat control. I changed within a year to a DC motor with a knob to control the speed and reverse switch, which allowed crawling the speed down to something ridiculous like 10 rpm if I wanted. I hate the "variableness" of the foot pedals. Now for the last 15 years I have a neat little 3 phase motor with countershaft that's smooth as silk and with a couple belt shifts a bunch of different speeds. I use my lathe several times a day, every day, sometimes for the whole day. A foot pedal would have sent me to the looney bin a long time ago.

If I was going to kit out another machine and didn't have easy access to 3 phase power I'd go DC again. You can get a Sherline motor with controler for pretty cheap.

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2 hours ago, nickelsilver said:

If I was going to kit out another machine and didn't have easy access to 3 phase power I'd go DC again. You can get a Sherline motor with controler for pretty cheap.

for this I highly recommend the consew motors.  they are DC servo, so when you set a speed, it stays that speed - more load, it sends it more juice to keep rpms consistent.  they're cheap, like $120 US.  Its a very simple mod to convert them to speed control via a rheostat if that's your preference (done it, literally takes on rheostat wired in place of the hall sensor).  Also, they are powerful. 3/4hp.  That doesn't matter much at speed but when you electronically reduce speed (DC or VFD) you won't have much low rpm power unless you start with a lot. (with electronic control power goes down as speed is reduced)

Thinking this through, maybe what is best is a rheostat control up top and a foot operated on/off switch?  I've got up top speed control and its a pain constantly reaching to turn the motor on/off to check things

 

Edited by measuretwice
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That certainly looks like it would do the trick. The only assumption is that you are using a universal motor. Unless you stripped down a modern washing machine, or got your motor from some other weird source, then my assumption is almost bound to be correct.

I see you managed to find one even less expensive than the example I found. Your ebay foo must be stronger than mine :D

Basically you do exactly what you described. You feed power in as described above, and power comes out all nicely chopped up to reduce the available power to the motor. 

There is a far better explanation of how this works here.
 

I'm sure he also has a video of the exact board we have been talking about, but I found this one while looking for that.

The larger unit he shows here would be capable of running a much larger motor, and might also produce finer control at very low speed.

I also came across these -> https://www.ebay.co.uk/sch/i.html?_odkw="4kW"++power+controller&_sop=15&_osacat=0&_from=R40&_trksid=m570.l1313&_nkw="4kW"++speed+controller&_sacat=0
 

Which appear to be a development of the unit that Bigclive talks about. They have a couple of push buttons to set the output power, which I'm in two minds about. Rotating a control gives more immediate feedback than a digital readout in my opinion, so I would be inclined to go for the simpler version, unless you have a *huge* motor to control.

Speaking of which, do you happen to know what the wattage of your motor is? This is probably stamped or printed somewhere on the motor. It may instead give a current rating. Either would be useful.

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I used the force... I see your £3.74  and I give you an even cheaper version at  £1.41

 

https://www.ebay.co.uk/itm/AC-50-220V-2000W-AC-Motor-Dimmers-SCR-Controller-Knob-Switch-Speed-Control-Tool/264039601746?hash=item3d79fc9652:g:5nIAAOSw1T1b69sH:rk:1:pf:0

.. or if you want the variable resistor on some flying leads, so you can mount it off the PCB you can splash for this one at £2.06

https://www.ebay.co.uk/itm/AC-220V-2000W-SCR-Voltage-Regulator-Speed-Light-Dimmer-Temperature-Controller/332671812528?hash=item4d74c903b0:g:sxcAAOSwUl5bEf84:rk:1:pf:0

 

Here's a little tip for anybody trying to find Chinese "things" cheap on ebay. Find the cheapest version on ebay.co.uk, then try ebay.ca (Canada) you may well find it that little bit cheaper still. This used to work with ebay.com (USA) but the "Trump" effect has kicked in, and things on ebay.com are often much more expensive than they are on .co.uk or .ca 

You will need to check that the item works on the UK mains voltage, and that the supplier actually will ship to the UK.

If so, change the ebay.ca url to ebay.co.uk and see if you still see the item.

So for the first example above, I picked..

https://www.ebay.ca/itm/AC-50-220V-2000W-AC-Motor-Dimmers-SCR-Controller-Knob-Switch-Speed-Control-Tool/264039601746?hash=item3d79fc9652:g:5nIAAOSw1T1b69sH:rk:1:pf:0

Which comes in at C $2.39 (Canadian dollars) or  approximately £1.41

I then changed the url to ...

https://www.ebay.co.uk/itm/AC-50-220V-2000W-AC-Motor-Dimmers-SCR-Controller-Knob-Switch-Speed-Control-Tool/264039601746?hash=item3d79fc9652:g:5nIAAOSw1T1b69sH:rk:1:pf:0
 

.. which didn't throw any errors. I then ordered one, 'cos you can never have too many really cheap but useful electronic bits and bobs in the workshop.  I think I'm going to fit it to my drill press. :biggrin:

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55 minutes ago, AndyHull said:

That certainly looks like it would do the trick. The only assumption is that you are using a universal motor. Unless you stripped down a modern washing machine, or got your motor from some other weird source, then my assumption is almost bound to be correct.

I see you managed to find one even less expensive than the example I found. Your ebay foo must be stronger than mine :D

Basically you do exactly what you described. You feed power in as described above, and power comes out all nicely chopped up to reduce the available power to the motor. 

There is a far better explanation of how this works here.
 

I'm sure he also has a video of the exact board we have been talking about, but I found this one while looking for that.

The larger unit he shows here would be capable of running a much larger motor, and might also produce finer control at very low speed.

I also came across these -> https://www.ebay.co.uk/sch/i.html?_odkw="4kW"++power+controller&_sop=15&_osacat=0&_from=R40&_trksid=m570.l1313&_nkw="4kW"++speed+controller&_sacat=0
 

Which appear to be a development of the unit that Bigclive talks about. They have a couple of push buttons to set the output power, which I'm in two minds about. Rotating a control gives more immediate feedback than a digital readout in my opinion, so I would be inclined to go for the simpler version, unless you have a *huge* motor to control.

Speaking of which, do you happen to know what the wattage of your motor is? This is probably stamped or printed somewhere on the motor. It may instead give a current rating. Either would be useful.

The motor has brushes which I replaced last year. Is this a universal motor, sorry me & electrics do not get on !!!!!.

On the motor is has H.P 1/12  also V.T. 230

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The motor has brushes which I replaced last year. Is this a universal motor, sorry me & electrics do not get on !!!!!

Almost certainly, yes.


For what it is worth, this is what a universal motor means. -> https://en.wikipedia.org/wiki/Universal_motor

The other most common type of motor used for relatively low powered single phase operation is the shaded pole motor, but they don't have brushes. https://en.wikipedia.org/wiki/Shaded-pole_motor


You often find shade pole motors in really cheap applications, small single speed unidirectional fans for example or occasionally in older stuff like some electric typewriters, where the motor runs at a constant speed, and load is applied mechanically. Shaded pole motors have the advantage of no brushes, but the simplest ones don't generally have the ability to reverse direction. 

There are a few other motor types, but they either require multi-phase AC, or fancy electronics to control them. You are very unlikely to find one of those running a small lathe.

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On the motor is has H.P 1/12  also V.T. 230 

The V.T. 230 means that it is designed for 230V nominal mains voltage. i.e. you can plug it in to the wall. 

The H.P most probably relates to electrical horse power (slightly different, but not by much from mechanical HP).

If we assume 1/12 HP is in electrical horse power, then we can convert that to Watts.

1/12 HP = 0.083333333 HP


That equates to around 65W so that little triac board should easily be able to cope.

If on the other hand, it is 1/2 HP, then we get 373 W, which the board should still be able to handle, but might make it run slightly warmer.

You can protect the outlet with a 5A fuse in your mains plug, and even in the unlikely event that the board throws a hissy fit, it won't do much more than let out a slightly pathetic puff of smoke, and pop the plug fuse. 

Actually it will probably trip the breaker in a modern household, since if you so much as sneeze next to a modern household breaker it will trip :devil:. Either way, fit a 5A fuse to be on the safe side.

One other thing the fuse will do, I suspect is pop if you stall out the motor. Better that than burning out the field coils I would suggest.

If you saw in the bigclive video, you can (very carefully with a suitably insulated screwdriver) adjust the minimum speed to avoid the motor "chattering" and overheating if you try to run it too slowly.

We can re-visit this when you come to test the thing.

Incidentally the fuse wont stop you from getting a shock if you do something stupid (but the breaker might), its purpose is to stop you burning the house down, or blowing up the breaker in the local substation.

 

Electrical horsepower to watts

One electrical horsepower is equal to 746 watts:

1 hp(E) = 746 W

So the power conversion of horsepower to watts is given by:

P(W) = 746 ⋅ P(hp)

 

Edited by AndyHull
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BTW Are you sure it says 1/12 HP and not 1 1/2HP i.e. 1.5 (e)HP

1.5 HP is around 1119 W which would draw just over the 5A than I suggested, so a 10A fuse would be more appropriate. 

The triac should be able to handle that current, assuming it is the same as the one in bigclives video.

You would need to load up the motor pretty hard to be pushing 1kW of energy into your work piece though, so I think we would be pretty safe to assume this will work without any issues. 

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5 minutes ago, praezis said:

The device in the video will not compensate torque when load is applied, I am afraid. Speed will slow down - different from the circuit in the above schematic.

Frank

I suspect you are correct, you would have less than perfect speed control, however there are a couple of points to consider.

First, the load on the motor on the lathe is relatively low, since the parts you are cutting are relatively small. If you were driving a large motor very hard, for example in an industrial sized lathe, or you need extremely precise rpm control, for example if you need to co-ordinate two motors to perform some precise interaction like in a CNC machine, then lack of torque compensation or precise control of RPM under load would be an issue.

Second, this is a low cost solution to the problem. If you throw more money at the problem, you will get more precise speed control.

In this case however I suspect the solution is perfectly adequate for this problem. You can probably stall the motor (assuming the belts don't slip before that), if you jam a very robust cutter in to a very robust work piece, hence my suggestion to add a fuse to limit the stall current, and my suggestion to adjust the trimmer potentiometer to set the minimum run speed, so we don't have the motor fluttering and over heating at the very low power setting.  

You will probably see the motor slow down, when running at low RPM under heavy load, but this is probably not going to cause any major issues. It is certainly a lot more control than you will get with just the foot control. For a couple of quid I think it is certainly worth trying it, and I suspect it will work fine in reality.

 

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Hello Andy,

this kind of circuit was factory mounted in the German made Praezima watchmakers lathe. And users kept complaining about collapsing speed under load. As I use this lathe, too,  I changed the electronics per your first schematic. Now I am fully satisfied as mentioned above. 

You yourself posted a suitable circuit, why not stick to it? :)

Frank

 

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