variable speed lathe?

[Bill Wyko]

What if someone were to set up the motor of a lathe like a sewing machine. With a pedal for speed control. Industrial sewing machines have very big motors and are very easy to control speed. Would this be of any use? I know there have been times when I'd like to run my mini slower than 500 rpm.

[Dario Octaviano]

Problem is you might move in a jerky motion (get startled or anything like that) and that can cause accidents.

I won't do it personally.

Something like this may be good for a drill press though but turning...where you stand and shift positions regularly for hours...

[TYLER WOOD]

If your motor has brushes it is not a problem. You can get a rheostat or some other variety of power adjusting system and put it on there. You will loose torque the more you reduce the speed though. If the motor does not have brushes, do nothing!!!! You will burn the motor up. If the lathe has pulleys on it, you could rig up a jack shaft.

[Bernie Weishapl]

It will not work on your lathe and if you switch to a motor with brushes you will not have any power at all when turning slow.

[Bill Wyko]

I have a consew double needle sewing machine but I never use it so I was looking at ways to use the motor w/o canabalizing it.

[Paul Engle]

Bill you could set it up for buffing, or friction finishing, I am in the process of doing that with some retro fit old DC motors from work with speed controls, one for friction finish and one for buffing and a third for what ever happens to 1) or 2). made adapters for the shafts and waaaa laaaa, 2) is threaded for my G3 chuck ( 1-8)so i can go right to it, now for spare chuck I will buy a larger nova of some sort.

[George Tokarev]

It will not work on your lathe and if you switch to a motor with brushes you will not have any power at all when turning slow.

Hate to disagree, but one of the reasons DC motors are used instead of their AC brothers is that they produce nearly full torque at slow speeds. Steppers and the VR technology, not to mention the motor/generators in the electric cars are there for their low-end torque. DC motors with permanent magnets are some real torque makers. You'll find them on jigsaws and feeding the stock on my thickness sander. Different kinds, of course. http://www.answers.com/topic/direct-current-motor

I can't imagine the circumstance where I would want to change speeds while turning like a sewing machine does for turning corners and such. I know that surface speed diminishes going in toward center, but if the tool is sharp, it doesn't seem to create that much of a problem.

[Charles McKinley]

I think the best way to go for variable speed on the lathe is with a 3 phase motor and a VFD or a DC motor and a speed control that you set by hand and then it stays there.

[Rex Guinn]

Bill;
I have a video of Del Stubbs using a foot control for speed, but he uses it via the motor lifting up and down on the pully with a dowl pin to keep the belt tight against the pully. For him it works great. But that is not quite what you are thinking of doing.

[Bill Boehme]

Hate to disagree, but one of the reasons DC motors are used instead of their AC brothers is that they produce nearly full torque at slow speeds. Steppers and the VR technology, not to mention the motor/generators in the electric cars are there for their low-end torque. DC motors with permanent magnets are some real torque makers. You'll find them on jigsaws and feeding the stock on my thickness sander. Different kinds, of course. http://www.answers.com/topic/direct-current-motor

I can't imagine the circumstance where I would want to change speeds while turning like a sewing machine does for turning corners and such. I know that surface speed diminishes going in toward center, but if the tool is sharp, it doesn't seem to create that much of a problem.

First of all, shame on McGraw-Hill -- technical literature is alleged to be their long suit -- must have been some tech writer [the bane of engineers and why I am bald] who came up with that nonsense comment about DC motors being able to produce lots of torque while drawing little current. Torque is in direct proportion to current, that is:

T = k * A

where T = torque, A = current, and k is a gain constant that is a function of conversion factors and motor characteristics.

Secondly, Bernie and you are both correct, but he is talking about power and you are talking about torque. I will post more information tomorrow about this since it is very late, but suffice it to say that it takes power to do work and a motor that is producing torque, but not rotating is not producing any power. Torque is important, but neither torque nor power alone tell a complete story.

Bill

[John Hart]

Bill;
I have a video of Del Stubbs using a foot control for speed, but he uses it via the motor lifting up and down on the pully with a dowl pin to keep the belt tight against the pully. For him it works great. But that is not quite what you are thinking of doing.

Rex...I sure would like to see a picture of that configuration! I don't have an interest in foot controlled devices but I sure would like to incorporate something like you're describing into the lathe I'm building. Are there any pics? or is it only available in video?

[Chris Barton]

I currently have my copy of Del's "Bowl tunring" loaned out but John, if you can look at a copy of that video it shows how he uses a rig to create a foot control of sorts on the lathe he uses in this very old video. In the video he appears to be using a very old General lathe and the motor is slung under the ways and connected to the headstock by a belt. The motor is on a hinge and will only provide sufficient friction to run the bowl when allowed down all the way. If I remember correctly he would step on the lever to lift the motor and thus remove the friction so he could look at the piece and stop it briefly if needed.

[John Hart]

Ah...well that doesn't sound like what I'm after Chris. Although it sounds pretty good for emergency stop. Thanks

[Bill Boehme]

It will not work on your lathe and if you switch to a motor with brushes you will not have any power at all when turning slow.

This basically correct, but it does depend on what the pulley ratio is along with the power output of the motor. For a variable speed mini lathe (which uses a DC motor), this is quite correct.

Hate to disagree, but one of the reasons DC motors are used instead of their AC brothers is that they produce nearly full torque at slow speeds. Steppers and the VR technology, not to mention the motor/generators in the electric cars are there for their low-end torque. DC motors with permanent magnets are some real torque makers. You'll find them on jigsaws and feeding the stock on my thickness sander. Different kinds, of course. http://www.answers.com/topic/direct-current-motor

I can't imagine the circumstance where I would want to change speeds while turning like a sewing machine does for turning corners and such. I know that surface speed diminishes going in toward center, but if the tool is sharp, it doesn't seem to create that much of a problem.

George, you are also correct, but you are talking about torque being at its peak at stall which is not a normal operating condition and could lead to overheating for a typical DC motor. Since you mentioned torque and Bernie mentioned power, I feel that for the benefit of some turners who may be interested to clarify what these mean as far as lathe performance is concerned.

The following figure that I created in Excel shows representative normalized DC motor performance characteristics.



There is a lot of data in the figure, but I will try to sort it out with creating too much confusion, hopefully.

The horizontal axis is motor output torque -- the "twisting force" that the shaft creates. Typically, this value is stated in inch-pounds or inch-ounces since it is very small in comparison to an automobile engine where the torque is stated in foot-pounds. The axis is "normalized" which means that the full scale value is shown as 1.0 rather than an actual value.

On the vertical axis there are values for four different graphs overlaying each other and they are all normalized also to make it easier to compare their relationship under various operating conditions.

The relationship between output torque and motor speed is given by the straight green line. This means that torque and speed are inversely related to each other. As George mentioned, the motors peak torque is reached at zero speed -- this is referred to as the stall torque. At the other end of the green line, you see that the speed is maximum when the load torque is zero -- this is referred to as the no-load speed. On the chart you also see that the green line is marked "Constant Voltage Line". What this means is that the green line gives the relationship between speed and torque for a particular constant voltage. The constant voltage line shown in this figure represents the maximum allowed operating voltage. There is actually a whole set of parallel green lines to the lower left of the one shown where each line would represent the speed-torque relationship for lower voltages. A generalization that can be made about the speed torque relationship is that for any fixed value of load torque, the motor speed is directly related to the applied voltage.

The red line represents the relationship between motor current and load torque. These two terms are directly related and the slope of the straight line is referred to as the motor torque constant. Its units would be expressed as ounce-inches per Ampere or sometimes pound-inches per Ampere. I ought to mention that if you increased motor current, the output torque would not automatically increase while still maintaining the same speed. This is the case because the motor output torque is always exactly equal to the load torque as long as the motor is running at a constant speed. Whenever the two get out of balance, it will cause the motor to either accelerate or decelerate depending, of course, on which value is larger until equalization in motor output torque and load torque are once again in balance at a higher or lower speed. Therefore, for any given constant speed, torque is directly related to the motor current.

The horsepower relationship to speed and torque is not a nice straight line like the previous two parameters because it is a function of output torque multiplied by the square of the motors speed and the result is a curve called a parabola. Three important things about the horsepower curve are that:

1. When the speed is zero and the output torque is zero, the horsepower is also zero -- this is another way of saying that the motor is OFF.
2. As speed and output torque increase, the horsepower also increases until it reaches a peak at one-half of the stall torque. Beyond that point, increasing the load torque results in decreased motor speed and less horsepower output. At this point, we are said to be operating "on the back side of the power curve. In this region, increasing the input voltage to gain back speed will only serve to drive the motor closer to stall. The only way to get to the front side of the power curve is to decrease the load torque.
3. Finally, the speed reaches zero and output torque is at its maximum value. The motor has stalled and the output power is zero.

While we are talking about output power, notice the purple curve labeled "Efficiency" which is the ratio of mechanical power output to electrical power input. Since we do not want the motor to overheat and burn up, it is a very good idea to choose operating conditions that are close to the peak efficiency. It is also desirable to operate the motor in a relatively linear portion of the power curve so for continuous operation, the operating envelope of the motor is kept in the gray-shaded area where the maximum output power point is approximately 30 percent stall torque and approximately 75 percent of no-load speed. Short term operation (meaning a few seconds to one minute normally) might typically be allowed for load torque up to about 45 percent of stall.

So much for the theory -- what does it tell us that is useful. Here are a few useful points:

1. Power is important as Bernie has observed on his mini lathe. Cutting wood requires work and work is the product of power and time. It is possible for a machine to produce torque and yet not be rotating. A machine that does not move gets no work done.
2. Torque is also important in being able to move heavy loads as George observed. There are motors with a respectable output power, but produce very little torque because they are running at a screaming high RPM. Changing the drive ratio by using pulleys would not work in such a situation because belts have a maximum top speed of about 6000 feet per minute. Beyond that they have difficulty maintaining sufficient contact with the pulleys.
3. The actual operation of DC motors are in the shaded gray area of the performance curves. George mentions some motors that are able to pack a lot of torque in a small package. However, even though the motor may be running slow and producing abundant torque, that should not be mistaken for the area near stall at the lower right corner of the figure. Remember that the constant voltage line running from nop-load speed to stall torque is for the peak operating voltage. There is another constant voltage line that is much closer to the origin (labeled low voltage) and that is the operating area under those conditions. What this says is that if you operated that same motor at full voltage and loaded it to stall, you really would see some tremendous torque -- briefly -- followed shortly by smoke.

Also, I would like to state that three-phase AC induction motors driven by a variable frequency drive can easily match or exceed DC motor performance. Using sensorless vector control they can produce full torque from around 150 RPM up to base speed, ~1750 RPM. Above base speed, they operate at full horsepower capability up to the physical maximum design speed of the motor. If using true vector control (i.e., a position feedback device), they are able to extend their performance even further. For instance, I have a couple Baldor three phase AC induction vector motors and vector controllers that allow full torque all the way down to zero speed (not a stall condition, but a commanded torque) and above 1750 RPM they can provide full horsepower up to 6000 RPM. The speed of AC motors is controlled by the driver frequency, but in DC motors, the speed is entirely load dependent. However, the better DC controllers do employ some current sensing to help stabilize speed and if tachometer feedback is employed, then performance will be greatly enhanced.

The one area where DC motors shine is in feedback control applications. Because of their low rotor inertia, theya re able to accelerate must faster than most AC motors and they also have a smaller time constant, which means that they react quicker to stimuli.

A gray area that is somewhat of a hybrid between AC and DC motors is called a BLDC (Brushless DC) motor. They employ rare-earth samarium cobalt magnets in the rotating armature and use controllers somewhat like the AC motors except that they must have commutation for the field windings which they do by using magnetic field detecting Hall-effect sensors. In this sense, they are like an inside-out PM DC motor.The BLDC motors have very low rotor ineria and are able to be used in high bandwidth control systems.

Bill

[John Hart]

Nice dissertation Bill. You talk pretty. In your experience, do you see any performance improvement using a PWM drive over a straight Potentiometer controlled DC configuration?
Just for an example... using a scenario of a standard Permanent Magnet 50VDC Motor....would the performance be different when using 50% PWM signal versus a 25 volt straight input? (let's assume in this hypothetical question that there is no speed or current sensor feedback on the PWM circuit)