Installing a VFD on a geared headstock lathe

I've seen claims that you get better threads on a lathe with a VFD drive. I'm trying to visualize how the VFD is connected to the gear box. On my 12 X 36 the motor interfaces with the gear box via pulleys and a belt. Does the VFD drive the input gear directly and how is it mounted. Just asking the question makes me think I am totally ignorant of what is going on here and am so far off base that I can't even see the catcher. Sorry for the baseball analogy.

well on a lathe that doesn't have a low enough gear you can make it turn slower
so it's easier to run up to a shoulder.

Well, my lathe will go down to 70 rpm. My question is how does the installation of a VFD improve the finish of the cut. I've gather that it is supposed to reduce vibration significantly. But I have no understanding of how this is accomplished. Hence, my question. Is it merely the reduction in rotation speed? Does it somehow replace the pulley and belt linkage?

adamsgt said:

I've seen claims that you get better threads on a lathe with a VFD drive. I'm trying to visualize how the VFD is connected to the gear box. On my 12 X 36 the motor interfaces with the gear box via pulleys and a belt. Does the VFD drive the input gear directly and how is it mounted. Just asking the question makes me think I am totally ignorant of what is going on here and am so far off base that I can't even see the catcher. Sorry for the baseball analogy.

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A VFD is not a motor. It only controls electrical power TO the motor, nearly always a three-phase motor.

A VFD can be used to supply three-phase from single-phase, change a single-speed motor into a motor with variable speed, set limits on its power, and stop and start it gradually.

Nothing else changes. There is no mechanical presence - it could be many feet away from the machine-tool at the end of a power cord - even mounted in another room. It may be mounted somewhere on or in the machine if there is suitable space, but need not be, as it will have the ability to support remoted controls for at least selecting FWD and REV, starting, stopping, and motor speed.

No intrusion into belts or gears at all.

Bill

The supposed improved finish has something to do with using a 3ph motor vs a 1ph motor. Somehow, a 1ph motor can produce a pattern in the finish that is related to the 1ph power Hz. The way the VFD uses the 1ph power to control a 3ph motor is said to either reduce or eliminate the appearance of such a pattern. I could not say how big a deal this is and how effective a VFD will be in controlling the appearance of such a pattern.

Mechanically, the 3ph motor takes the place of the 1ph motor without any changes to the physical drivetrain. Electrically, 1ph power comes to the VFD, the VFD controls the motor directly (no switches, contactors, disconnects, etc. between the VFD and the motor), and the lathe controls are wired to inputs on the VFD to control things like RUN/FWD, RUN/RVS, and STOP. The VFD may have a variable-speed control built in, or one may be added remotely.

And Bill said a lot of that already, only differently. What I wrote in no way is intended to contradict what he wrote.

adamsgt said:

Well, my lathe will go down to 70 rpm. My question is how does the installation of a VFD improve the finish of the cut. I've gather that it is supposed to reduce vibration significantly. But I have no understanding of how this is accomplished. Hence, my question. Is it merely the reduction in rotation speed? Does it somehow replace the pulley and belt linkage?

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Some of the Asian machines equipped with a single phase motor show a faint vibration pattern on fine cuts including threads. This can be mitigated by improving the mounting of the motor to isolate vibration from the machine base and / or replacing the motor with a higher quality (Baldor, etc.) motor. If your machine is already equipped with a 3 phase motor you likely do not have this problem.

I think the reason for the claim is due to the ability to run at relatively slow speeds without using back gear. Some older machines are pretty noisy in back gear and this may affect the finish when threading. Contemporary Asian lathes popular with gunsmiths often do not have a "back gear" per se, but still tend to be noisier in the lowest gear setting. I think the real advantage of using a VFD drive is in being able to use a 3 phase motor from a single phase power source plus having almost infinite speed control so that you can "tune out" harmonic vibration by adjusting the speed in small increments.

Scott Roeder said:

you can "tune out" harmonic vibration by adjusting the speed in small increments.

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Unwanted "resonance" is an issue for everything from tall buildings to aircraft props and control surfaces. Single-phase motors may get the blame for it on a machine-tool, not always fairly. They are not really built as 'steppers' and are smooth enough.

The more valuable gain from 3-P, even without VFD, is longer motor life (no start or run caps, nor switching of them), plus essentially 'instant' reversing without need of complex external add-ons.

VFD ability to alter RPM or add 'soft start' ramp-up is a bonus, not a necessity, on a(ny) machine that left the factory working well enough without either.

Use of a VFD, per-each machine instead of an RPC shared across several so as to provide 3-phase to a(ny) machine(s) already 3-P motored can be a good choice. "Can be" is not guaranteed always.

Re-motoring a single-phase machine and having to add a VFD? More likely to be time and money wasted for rather small gain in performance.

Wiser to trade-up to a better machine that was 3-P equipped to begin with.

Bill

There are two possible causes for the "pattern". One is low quality gears. Lathes need precision ground helical gears for maximum smoothness. I doubt they are present on any of the consumer grade Chinese lathes like Grizzly, etc.

The second cause is single-phase induction motor torsional vibration. It is inherent in the physics of the motor and cannot be designed out. It is the reason you often see appliance and air handler motors mounted in rubber collars on each end of the motor. Then, a belt is used to transfer the motion to the machine. This reduces the noise to an acceptable level for most indoor uses. If you use a hard mounted single-phase motor to drive a spur gear headstock, there will be certain gear combinations that will amplify the torsion vibration and produce problems at the spindle.

All the above goes away if a 3-phase motor is used. There is no motor vibration except that caused by an out of balance rotor. This is very unusual in a good quality motor. Even spur gears behave because there is no torsional vibration. The icing on the cake for machine tool applications is to add a VFD to control the motor. A good quality 4-pole motor ( 1750 RPM) can be run from less than 30 RPM to 3200 RPM without problems. This means that the mechanical speed selection on the lathe in question can be set at some convenient point in the middle of the speed range and left there. All speed variation from that point forward is done by the VFD's operator station speed pot. While it is true a standard motor's torque is limited at about 30Hz and below. The operator always has the option of using the lathe's headstock speed selection to allow the motor to run at 60 Hz while getting that high torque at low speed for some large boring, turning job or some such.

RWO