Mini Lathe brushless motor query

[John HinkleyParticipant @johnhinkley26699]

I’ve recently acquired an Amadeal AM714B mini lathe and am not liking the way it starts.  Instructions are to turn the speed control to minimum before switching on the spindle.  Fair enough.  If I then select a direction (forward or reverse), the spindle immediately starts to turn in the required direction at about 50-90rpm.  It will not start at zero rpm, as I would expect and ideally want.  It’s not what I’m used to and, particularly for thread and auto-fed turning, not exactly ideal.  What I would like to know is – if anyone else has the same lathe, is this how yours behaves?  I have contacted Amadeal, and they assure me that is the standard mode of operation.

I’m in the middle of converting the lathe to an electronic lead screw and would like to get this sorted before I start cutting metal in earnest.

An alternative solution is to fit a 3-phase motor with VFD control as I had on my previous lathe, something I’m reluctant but prepared to do.

John

 

[JasonBModerator @jasonb]

The Brushless SX2.7 starts at about 40rpm and my brushed warco at about 60rpm.

Both makers state a speed range and neither start at zero, I assume Amadeal would be similar.

[John HinkleyParticipant @johnhinkley26699]

Thanks, Jason.  That’s what I suspected.  I’ll adopt the procedure of: switch on then select direction, then speed when ready to turn/thread and then reduce speed pot to minimum before using the forward/stop/reverse switch to stop the chuck rotation.  If something goes bang, that’s when I’ll dust off the credit card and buy a vfd and motor combo.

John

 

[Andy_GParticipant @andy_g]

I’ve had an Amadeal AM714 lathe with the brushless motor for about 4 years. With the speed control fully anticlockwise the spindle doesn’t move, but as soon as I turn the control up it starts running at ~80 RPM.

I did look at increasing the reduction ratio of the drive belt, as I seldom run it over 1000 RPM and it will run to 2000+ but gave up trying to fit something in without significant modifications. In practice, it hasn’t really been a problem (within the constraints of the lathe).

[JasonBModerator @jasonb]

The bench top machines with their variable electronic speed sneed a bit of a different approach to what people may have been used to with industrial machines or been taught as an apprentice.

I’m not sure what you were used to that ran slower maybe a Myford in backgear and a VFD with the wick turned down but even if the brushed and brushless machines could run down to say 20rpm they would be quite gutless at that as the motor is barely running.

So where you may have selected slow backgear to turn that cast iron flywheel at a snail’s pace you need to wind up the speed pot, go at it with **GT insert and take a lighter cut which won’t take any longer to do the job due to the higher speeds. For example I did the two flywheels for the RLE earlier this week and cut at 400rpm on the 165mm castings.

Threading again need not require slow speeds. As you now have a chuck that won’t unscrew itself you can happily run in reverse with the tool on the opposite side of the work. This means 200-300rpm is quite comfortable as you have the rest of the length of the lathe bed to disengage the lead screw as you will be cutting AWAY from shoulders or the bottom of blind holes

I’ve also never turned the pots on the WM280, X3 or SX2.7 down before stopping or starting and the first two have 15yrs or so old.

[John HinkleyParticipant @johnhinkley26699]

Andy_G,

That’s interesting. I wonder if the “problem” – as I perceive it – lies with the potentiometer?  I have a few spares lying around in a drawer somewhere.  I may put a meter on the one in the lathe and see how it compares with the ones in stock.

 

Jason,

Again, thanks for the input.  In trying to make my query succinct, I probably didn’t give enough background.  My previous lathe was a generic Asian BL200 9×20 lathe, the same as the Axminster BL20, which I bought when I retired to France.  I modified it somewhat and eventually fitted a Clough42 ELS and Huang Yang VFD coupled to an uprated 3-phase motor.  All that was controlled from a remote pendant.  With that set up, I would switch on the spindle, (with the speed pot fully anti-clockwise), select the required chuck rotation direction and then turn up the speed to whatever was needed for the process being done.  That is, starting at zero rpm and increasing as necessary.  When turning a thread, I would always leave the half-nut engaged and either slow the chuck with the pot to a stop, or just switch from forward rotation to off.

As it stands with the “new” lathe, I can’t do this any more.  When the ELS is fitted, however, there is provision for semi-automatic threading to be done, simply by pressing a couple of buttons on the control panel and applying infeed and tool retraction manually.*  I’ll stick to this method to start with and progress to the 3-Phase/VFD route at a later stage.

Thanks, all. I think I’ve got things sorted in my mind, now

John

*  Have a look at this explanation of the ELS that I’ve purchased:

ELS demo

The threading demo starts at around 37 mins in.  It’s in French, but there are subtitles and its pretty much self-explanatory anyway.

[SillyOldDufferModerator @sillyoldduffer]

I made a hand-crank for threading on my mini-lathe; it provides much better control and torque than the motor at slow speed.

My guess is controllers deliberately start hobby lathe motors at moderate speed to avoid stall damage.   If a user turns up the speed control starting from zero, and the motor doesn’t get enough oomph to overcome friction and turn, the windings will cook and maybe the electronics too.

Dave

 

 

 

 

 

[John HinkleyParticipant @johnhinkley26699]

Dave,

I would tend to agree with you, but the manual specifically says to turn the speed pot down to minimum before starting the lathe, suggesting that dire consequences will result if these instructions are not followed.

John

[Andy_GParticipant @andy_g]

That’s interesting. I wonder if the “problem” – as I perceive it – lies with the potentiometer? I have a few spares lying around in a drawer somewhere. I may put a meter on the one in the lathe and see how it compares with the ones in stock.

 

On mine there is a switch on the pot, and the lathe won’t start unless the pot is on zero with the switch ‘clicked’ off, so I have to set it back to zero every time. (The pot on mine is also a bit of an odd-ball as the switch opens when the pot is turned clockwise, rather than closes.) It sounds like yours is a newer design.

[John HinkleyParticipant @johnhinkley26699]

Andy_G,

I must admit, my first thought when I started the lathe for  the first time, was  “what if I replace the pot with one with a switch and wired that in series?”  I dismissed that simply because I don’t know enough about electronics to determine what effect it might have on the circuit board.  I may try that later.  Thanks for replanting the idea, though.

John

 

[John Hinkley]

On John Hinkley Said:

When turning a thread, I would always leave the half-nut engaged and either slow the chuck with the pot to a stop, or just switch from forward rotation to off.

As it stands with the “new” lathe, I can’t do this any more.  When the ELS is fitted, however, there is provision for semi-automatic threading to be done, simply by pressing a couple of buttons on the control panel and applying infeed and tool retraction manually.*  I’ll stick to this method to start with and progress to the 3-Phase/VFD route at a later stage.

Thanks, all. I think I’ve got things sorted in my mind, now

John

*  Have a look at this explanation of the ELS that I’ve purchased:

ELS demo

The threading demo starts at around 37 mins in.  It’s in French, but there are subtitles and its pretty much self-explanatory anyway.

This is bit I really don’t understand, Amadeal has a threading indicator (£16) for the mini lathe which means you can keep the motor running and disengage the half nuts, wind back with the apron wheel and re-engage the half nuts when the indicator gets to the appropriate number. Mind you they do make you twitch a bit when the dial stops turning with the nuts engaged the first time you see it.

[Dave Halford]

On Dave Halford Said

This is bit I really don’t understand, Amadeal has a threading indicator (£16) for the mini lathe which means you can keep the motor running and disengage the half nuts, wind back with the apron wheel and re-engage the half nuts when the indicator gets to the appropriate number. Mind you they do make you twitch a bit when the dial stops turning with the nuts engaged the first time you see it.

Dave,

The AMA714B is supplied as standard with a TDI, so no further purchase would be required.  The main point of fitting the ELS is to do away with the necessity for both the TDI and all the change gears.  This removes the source of a huge amount of clatter from the gears and provides threading capability by selecting from TPI, Imperial, metric and BA threads, all at the press of a button (or two).  The only rotating parts are the motor and spindle and the lead screw and stepper motor, both systems driven through near-silent toothed belts.

The spindle will run in either direction by the use of the standard switch, and the direction of rotation of the lead screw is selectable on the ELS screen.  No need for gear calculations and swapping and all the rigmarole that that entails.  Additionally, feed rate is available “on the fly”, again via the ELS screen.

Does that clear your confusion?

John

 

[Dave HalfordParticipant @davehalford22513]

I get the ELS and the change gear issue, though I would still keep the TDI and avoid all the rigmarole the video takes setting up.

That said I have a 48 speed Norton box so it’s relatively easy for me.

[John Hinkley]

On John Hinkley Said:

Dave,

I would tend to agree with you, but the manual specifically says to turn the speed pot down to minimum before starting the lathe, suggesting that dire consequences will result if these instructions are not followed.

John

We may have got ourselves cross-threaded!

Setting the speed pot to minimum is recommended because. on starting, motors briefly draw a heavy inrush current which might damage the electronics.   Turning the pot down reduces the risk of making magic smoke.   How high the risk of producing magic smoke is depends on the design of the controller: an expensive industrial unit is likely to be well-protected, whilst a cost cutting hobby lathe isn’t.

But that wasn’t my point!  I’m saying it’s not a good idea to start a lathe motor from zero RPM because doing so risks the controller not starting the motor, and the absence of a back-EMF means a high current could flow in the motor long enough to cook the windings.  A stalled motor takes much higher current than a rotating one.

The KBLC-240D motor controller is a popular replacement for a burnt out Chinese control board.  The circuit diagram is arranged to stop the operator from setting the speed either too high or too low.

The red square shows the 3 terminals  P1, P2, and P3 to which the exterior speed control pot is connected.   However, the motor’s speed range is controlled by this and two other pre-set pots in series located ‘nichtgerfingerpoken’ on the board inside the box.

R3 (lower Green ellipse) is labelled ‘Min’ and it sets the motor’s minimum speed which generally isn’t zero.   The upper green pot, labelled ‘Max’ sets the maximum speed.     The Min pot could be adjusted so the motor does start from zero RPM, but I wouldn’t cos’ I’m a cowardly cat!

If you want to try, see if a Min pot can be identified on your board.  With luck the pots are obviously labelled but not always so, making it necessary to reverse engineer the board to make sure.

Don’t guess! The KBLC-240D circuit shows 5 pre-set pots, and I’m not sure what the two marked in Blue are for.  I think that the one on the far right sets over-current protection, and is needed to save the electronics if the owner overloads the lathe.   The other probably works with R4 (next to P2 red box on left) to manage Acceleration and deceleration, protecting the machine from a user who turns the speed up and down over enthusiastically.

The KBLC-240D board is just an example – other motor controllers may be significantly different.

Dave

 

[SillyOldDuffer]

On SillyOldDuffer Said:

We may have got ourselves cross-threaded!

Rather more than that! The original thread title refers to brushless DC motors whereas the schematic shown by SoD is a basic PWM controller for a brushed DC motor.

A DC motor controller contains a 3-phase bridge identical to the core of a VFD, but driven in a different way. In general running a motor at zero rpm but full torque is a rare requirement. An exception can be electric vehicles, where it is the equivalent of holding on a hill by slipping the clutch.

In theory a brushless DC motor needs a position sensor so that the timing of the three phases can be output correctly. More recent controllers are sensorless. simply starting at a random point and looking at the back emf on the unpowered phases to determine rotor position and then adjusting the drive as needed.

At zero rpm, with a DC signal applied to one phase clearly there will be no back emf to measure.

Andrew

[John HinkleyParticipant @johnhinkley26699]

Sorry, chaps.  You might as well be speaking in ancient Greek! I can’t pretend to understand what you are trying to explain, at least not above the most basic level.  As that is the case, I think I’m safest putting up with situation as is, don’t you?

I’ll carry on with the ELS installation for the moment and see how I get on with it when that’s finished.  It may well be that I will have to modify my working practices rather than attempting to bend the lathe to my will. Life’s too short!

Thanks for the input and patience!

John

 

[Anonymous]

JH: A fundamental question is do you have a brushed, or brushless, DC motor?

Andrew

[JasonBModerator @jasonb]

I believe the “B” in AM714B signifies it is Brushless. Most likely to have about 8 wires, 3+earth and the 4 control wires for sensor etc.

[John HinkleyParticipant @johnhinkley26699]

Andrew,

Jason is correct.  It is a brushless motor.

John

 

[Anonymous]

In which case it is probably less important to turn the speed control down before starting. A brushless motor controller will almost certainly involve a processor so it is fairly simple, and cheap(!), to include a ramp up feature on start up. It may be that the manual has not been properly updated between versions of the lathe.

As an aside it is good practice to turn the speed down when changing jobs. You don’t want an out of balance large job on the faceplate starting at maximum speed.

Andrew

[John HinkleyParticipant @johnhinkley26699]

That’s reassuring, Andrew.  I will have to change my working practice to suit the lathe and not the other way around.  I feel more reassured by your reply to use the forward/stop/reverse switch to control the spindle stop and start.

John

 

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