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I am working on a design for a small lathe that could double as a horizontal drilling machine like a Westhoff.

My plan is to have specific geared speeds, rather than continuously variable speed.

I am wondering what the best type of motor would be. Cost is no object, but it needs to be compact, no more than 6" in diameter and hopefully less.

  • Startup torque is not required, because in this type of application you can always start the motor unloaded; in fact, even if the motor had zero startup torque I could potentially add a small starter motor to start the main motor, if that would be better.

  • It would run on normal AC household current

  • My thought is to find a motor with a high peak torque rating and then just run the motor at that ideal speed, using gears to adjust speed as necessary

  • Obviously, the more horsepower the better

Some additional points to consider:

  • For the class of lathe/mill intended, the target HP would be between 1 HP and 2 HP

  • A lighter or more compact motor is desirable, so that the overall footprint of the lathe is minimized. Therefore, a motor that has relatively high HP for its weight/size is desirable.

  • High-end lathes use a slipping clutch to protect the motor and gearing from excessive loads. The clutch is set to a torque such that the maximum possible load would not break any of the teeth in any of the gears.

  • A single speed motor is fine because a high-end lathe/mill will have a gearing system that determines the RPM of the machine and allows it to be varied.

  • It is important that the motor have strong inertial force because when metal is being cut sometimes a hard inclusion will be present so that the load on the motor can unexpectedly spike up for a fraction of a second when the cutter hits the inclusion.

  • It is important for the lathe's speed to be as consistent as possible because if the lathe has variability in speed as the load varies, then what will happen is the quality of the finish of the cut will be degraded. What is wanted is for that that motor to keep turning at exactly the same speed regardless of the variability of the load.

  • Cost is not a factor, because other parts of the lathe will be much more expensive to construct than the motor, so the motor is a relatively minor part of the cost to construct the lathe.

What type of motor best fits these characteristics?

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If money is no object then there are a lot of advantages to using a DC motor for a lathe. The ability to have continuously variable speed control is a huge advantage for a lathe. Compared to other machine tools a lathe has the specific problem that the circumference (and thus proper cutting speed inevitably changes as you work. Consider turning a 30mm bar down to 10mm, the circumference changes by a factor of 3 so any fixed cutting speed is going to be a compromise, the alternative being changing gears mid job.

It is also very useful to be able to fine tune cutting speeds on the fly when cutting threads or turning difficult materials. Being able to precisely adjust cutting speeds also lets you get more effective capacity out of a given motor power.

The alternative is an AC induction motor, these are cheaper and don't require electronic control but by the same token don't allow for speed control in single phase.

There isn't much to choose between the two in terms of startup torque. Larger AC motors are generally split phase which gives high starting torque.

You limitation on power will be down to what your supply can deliver. Single phase motors generally go up to about 4hp but this requires a 240V 20A supply.

To give you some sense of power requirements a 500W lathe with a DC brushless motor will cope fairly well with drilling holes up to about 16mm diameter and just about manage 25mm holes with care in metals with good machinability (ie leaded or sulphurated mild steels or brass) and cuts up to about 1mm at a single pass.

For a small lathe (say 120mm chuck) a 1200W motor would probably be ideal and that's about the limit for a domestic power supply.

In general DC motors tend to be a bit more compact for this sort of power requirement than AC induction motors also the ability to add speed control greatly simplifies the transmission design, although having a few gears is still an advantage.

My experience is that for a small lathe running off a single phase supply a DC brushless motor is a lot more usable and efficient in terms of overall production rates than an equivalent AC one. A 3 phase machine gives the best of both worlds but that is not an option here.

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A commonly used motor for lathes is the DC shunt motor. The big advantage is that the speed remains constant irrespective of the load.

The working principle of a DC Shunt Motor is that whenever it is turned ON, then DC current flows throughout stator as well as the rotor. This current flow will generate two fields (namely pole as well as the armature).

The torque speed characteristics of a DC shunt motor and a DC motor in series is presented below:

DC shunt motor DC Series
Speed Torque Characteristics enter image description here enter image description here
source Rumrum Banerjee iceet
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