brushless DC motors for mini lathes

[small.planes]

I’m not convinced a foot control is essential or even desirable. Can someone explain why it might be needed?

I have a normal VFD controlled motor on my 8mm lathe.
It’s comically large, but not in the way and has enough power.

Dave

[Mcgyver]

I’m not convinced a foot control is essential or even desirable. Can someone explain why it might be needed?

have you used yours much for watchmaking, i.e. making a balance staff? That's where you want it as this is hand graver work and is a lot of start, cut, stop, inspect while your head is buried in the set up, an inch from the work. it would be a real pita to reach for a pot/switch every time. in my opinion, that is why to have one of these machines, not that super useful a lathe otherwise, so I think of it as a given to have it set up with a foot switch, almost every watchmakers lathe and set set up I've seen does so.

[Brian H.]

What about something like this?



The spec sheet claims ~330W output, so about 1/2 HP. Enclosed, so no worries about airflow or chip infiltration. I bought this one to use with a Universal Pillar Tool build, for which purpose it's undoubtedly overkill on power but otherwise seems like it will fit the bill nicely. It's 2-1/4" diameter by 3" long, with a 1/4" x 1" cylindrical shaft.

$17 at all-electronics. You would still need your own driver and power supply. Note also that this motor has built-in hall effect sensors; I believe that using a driver with hall effect inputs will smooth out the lower speed ranges a bit.

https://www.allelectronics.com/item/dcm-459/3-phase-brushless-dc-motor/1.html

[MaxHeadRoom]

What about something like this?
. Note also that this motor has built-in hall effect sensors; I believe that using a driver with hall effect inputs will smooth out the lower speed ranges a bit.

Unfortunately, hall commutation does not improve the 'cogging' on low RPM on a BLDC motor, it also requires a PID feedback loop (e.g. CNC etc) to remove this completely.

[The Metal Butcher]

I'm rather thinking that if a foot pedal was used for the speed control that controlling the speed would become pretty automatic.

Henry, what have you used up to now? I'm seeing a lot of 100 to 150 watt rated regular sewing machine motors that come as a kit with a matched speed control. Granted they are not brushless but any time I'm using my old black Singer one stitch fits all for something it seems pretty easy to modulate. And when the belt is off the motor it runs very smoothly. Any shake, rattle and roll is either in the machine or the operator.

A foot pedal would actually be kind of dope for watchmaker operations. Want to file just a tad bit? Drop your speed all they way. Need a quick polish? Put your foot down. Chatter started, back up a bit. I could see that actually being excellent for operating. Might get kind of tiring standing on a foot and heel all day, but if you only did it on occasion it would be great IMO.

[BCRider]

I may be wrong on this but if the setup uses the hall sensors then it will push whatever current is needed to try to keep the set RPM. So no way to "feel" or hear the change due to the load on the motor.

Running it "open circuit" without the sensor being hooked up, provided the motor driver is a sensorless operation type, would make it more like the old AC motor but with more speed options.

Henry, do you have another lathe which could be used to make a new smaller motor pulley? That would be one possible fix.

As for cost don't forget that you'll also need a suitable high current DC supply. That Allelectronics motor does not list the watts or max current. And those are pretty small wires. But assuming it's good for a something like 200 watts at 12v that means you'll also need a power supply that can provide at least 200/12= 17 amps. So call it a 20 amp supply. You won't load it down to the full current all the time but it has to be there for when it's needed or the supply will be overloaded and fail on you. Or it needs to be a supply that can self limit itself such as a voltage and current limited test bench supply.

[BCRider]

Unfortunately, hall commutation does not improve the 'cogging' on low RPM on a BLDC motor, it also requires a PID feedback loop (e.g. CNC etc) to remove this completely.

That is also the reason I see some "chatter" on some of the model motors when slowly advancing the throttle from cutoff. But we're talking about the lower 10% of the motor's RPM range where there is VERY little torque anyway.

TMB, for work with a foot pedal I would assume that the operator would be seated on a stool of some form. My Dremel tool uses a foot pedal and for anything needing some care I sit on a stool to use it so I don't dance around on only one planted leg.

[MaxHeadRoom]

I may be wrong on this but if the setup uses the hall sensors then it will push whatever current is needed to try to keep the set RPM. So no way to "feel" or hear the change due to the load on the motor.

Running it "open circuit" without the sensor being hooked up, provided the motor driver is a sensorless operation type, would make it more like the old AC motor but with more speed options.
.

You either require some kind of feedback, halls.or equivalent in order to successfully commutate the motor, other methods are obtaining the rotor position at power up, by slightly exercising the motor wherein the controller then knows the armature position.
You cannot run it succesfully open loop.

[MaxHeadRoom]

But we're talking about the lower 10% of the motor's RPM range where there is VERY little torque anyway.

.

The continuous torque range of a DC brushed and BLDC motor is maximum at Zero rpm, and stays fairly level across the continous rated range.

[J Tiers]

That is also the reason I see some "chatter" on some of the model motors when slowly advancing the throttle from cutoff. But we're talking about the lower 10% of the motor's RPM range where there is VERY little torque anyway.

...........

The continuous torque range of a DC brushed and BLDC motor is maximum at Zero rpm, and stays fairly level across the continous rated range.

Talking apples and oranges. And are both correct.

The issue is when the optimistic designers, typically of mini-lathes, attempt to cover the entire speed range in one turn of the dial, no belt change/gear change.

They end up using the lowest power area of the motor, without any torque enhancing means, for the highest TORQUE requirement job. Ther IS a lack of torque relative to the need, even though torque is constant.

To maintain POWER torque would have to increase, but it does not.

[MaxHeadRoom]

A servo typical torque curve

[BCRider]

You either require some kind of feedback, halls.or equivalent in order to successfully commutate the motor, other methods are obtaining the rotor position at power up, by slightly exercising the motor wherein the controller then knows the armature position.
You cannot run it succesfully open loop.

Technically you're 100% right. But there's more than one way to sense the phasing. The controllers for brushless motors in the boat, car and airplane hobbies only rarely use motor mounted sensors any more. Most of the time it uses measurements made within the ESC from the windings not being powered at the moment to sense the phasing and vary the frequency to run the motor. So yes, it's feedback which you correctly point out is needed.

But it's so common now to find these internally sensed speed controllers that I slipped up and called it "open loop" in my post.

As for the torque that might be the case if we hit them with full power. In that case it's full max torque right from 0 RPM. But the speed controllers chop the power to the motor to control the speed and in doing so they are actually controlling how much power the motor sees. And the reduced power means reduced torque. Who here hasn't felt this when tightening up a hand drill's chuck by just holding the outer barrel and pulling part way on the trigger?

By rights I suppose the speed controls would be more accurately called torque controllers. And as the torque rises so does the speed which the motor and device can run.

[J Tiers]

A servo typical torque curve

Yep, shows the problem clearly.

The more expensive lathes have always used back gears for a reason. Their motors are pretty much constant torque also, but since they are also constant speed, the use of speed reduction by pulleys or gears (or both) was never questioned.

People always seem to complain about "tiny chinese horses", and never stop to think that they may be comparing a motor designed to produce its power at a high speed, but which is being run slowly, with a motor geared down to properly increase torque and maintain constant power. That particularly happens when folks put a DC motor on their machine, and, like the Minilathe designers, decide "hey, I can get that slow rpm without needing those old-fashioned pulleys and gears." Also with some folks putting a VFD on a machine, and assuming they no longer need the pulleys etc.

Problem is, they want to run the DC motor at several hundred to 1000 rpm, and it was designed to run around 4000 (or an AC motor at 400RPM when it is a 1750 rpm motor). Naturally a "1 HP" motor will then put out perhaps as much as 1/4 HP.

They generally see pretty soon that the "old fashioned" stuff has a definite purpose and how it fills the requirement. That's if they don't give up and sell the machine in disgust.

There is a great example of this. One of the "This old Tony" videos about the minilathe shows the machine unable to take more than a tiny thread of material off a 2 or 3" diameter piece of steel at 50 rpm or so. Stalls if any sort of cut is taken. The actual power applied to that cut may be 20 or more times lower than the motor would produce at its design speed.

That happensto be a brushless DC motor (BLDC), but it is not the motor's fault. It is just a drastic mis-application of the motor. Probably done because people who buy a minilathe are assumed not to know the difference.

A BLDC motor can provide perfectly acceptable power for a lathe, or mill, so long as it is applied correctly. Do not ask for something that the motor is not designed to do, and all will be fine.

[MaxHeadRoom]

By rights I suppose the speed controls would be more accurately called torque controllers. And as the torque rises so does the speed which the motor and device can run.

This is where CNC or closed loop can use the full potential of the motor, and take advantage of that flat torque curve, for the self-built systems I used, I used the PC based Galil cards, they allowed the use of simple torque mode drives (transconductance) amplifiers, These could be programmed to take advantage of that continuous torque down to zero RPM.

[small.planes]

have you used yours much for watchmaking, i.e. making a balance staff? That's where you want it as this is hand graver work and is a lot of start, cut, stop, inspect while your head is buried in the set up, an inch from the work. it would be a real pita to reach for a pot/switch every time. in my opinion, that is why to have one of these machines, not that super useful a lathe otherwise, so I think of it as a given to have it set up with a foot switch, almost every watchmakers lathe and set set up I've seen does so.

Yes. Thats why I have a watchmakers lathe. (I also have 3 other sized lathes and a second watchmakers one... Latheaholic )

Balance staff (grain of rice for scale) (I know I made it the 'wrong' way round) :



Winding Stem for an movement transplant:



Pipe for a secondhand:



Ive also used it to make seconds hands with integral pipe, dials and case backs - although I prefer the CVA (or Unimat) for case work as they have to have a saddle/crossslide.

I have no problem with the VFD pendent as a means of control. Not sure if my foot would work as steadily for speed as a pot. Still different people do things differently.
My motor is 3 phase, 2730 rpm nominal and 120 watts (0.16 hp) continuous rating. Plenty of power for the size of things a watchmakers lathe should be used for.

I suspect that RC plane motors are not really rated for running longer than a battery pack lasts - certainly when I flew helicopters the motors got very warm.

Dave