Factor to Consider When Buying a Lathe

One potential feature of a lathe that tends to be ignored when making buy decisions is the availability of multiple speed ranges, usually obtained by shifting a drive belt between pairs of pulleys in the spindle drive. This is a very valuable feature, particularly when selecting a lathe to run on 120 V AC, where one is limited to about 1.5 HP or less. A close parallel is an auto that has only direct drive (no gear selection). It is fine at highway speeds, but is severely challenged when climbing steep hills.

A VFD motor will deliver its maximum torque when operating at an RPS (revolutions per second) that is a little less than the frequency of the AC power generated by the VFD. This maximum torque will be nearly the same for all settings of the VFD. Motor horsepower is proportional to the product of torque and rotating speed. Therefore, the maximum horsepower will occur at the highest speed setting and will fall off proportionally as the speed is reduced. In other words, a 1 HP motor operating in a VFD system with a maximum speed of say 3000 RPM will deliver only 0.1 HP at a speed of 300 RPM. Compare this to a system that with a belt change can gear down the maximum spindle speed by say 4:1. In this case, the HP available at 300 RPM would be 0.4 HP and the maximum torque available would be four times larger than in the first case. Ideally there would be several speed ranges so that one can maximize the available torque at the desired speed for a particular project.

 

Do you know how a switched reluctance motor, Nova DVR, compares?

 

Doug, I have not delved into switched reluctance motor much. I know that it is a cousin of a stepper motor, but I have not run across any performance information. In any case, unless its torque somehow increases as the speed is reduced, its performance will benefit from using a multi-speed transmission in the lathe.

 

I guess my old General 260 with Reeves drive and retrofit vfd has some advantage then although I would expect the Reeves is inherently less efficient than a stepped pulley drive.

 

No, pretty much equal. Both are bending the belt around the pulley. A flat belt vs the v belt used for a reeves drive might have a little advantage, but very small.

 

I had a lathe with a Reeves drive many years ago. Yes, if it could be made more reliable, durable and with a wider speed range it actually would be better than a VFD that has only a few gear options. It effectively would provide an infinite number of speed ranges without the necessity to move a belt to different pulleys

 

I also had a lathe with a reeves drive...it ate belts...

 

One of my lathes is a DVR3000 and it has good torque throughout its range. It senses the load and adjusts accordingly. With my last upgrade it now spins at 5000 rpm top speed It is 20 years old and still one of my favorites.

 

Wouldn't the fact that there are two belts be a factor?

This one doesn't seem to. I replaced the belt and headstock bearings on general principles when I got the lathe and it is working fine with I don't know how many hundreds of hours since.

The Reeves drive is a bit noisy and has to be adjusted while running but otherwise not problematic. It runs smooth at 50 rpm and I have plenty of power for roughing with a 2 hp motor at about 200 rpm.

To the OP, if I were buying a new lathe parts availability would be important- no joy for my old horse. Proprietary controls or motors would be a deal breaker. Solidity and reliability would be key.

 

I have a Nova 1624 with the DVR upgrade. The base 1624 has "choose a pulley" speed control with six pulley pairs. I have the DVR motor turning one of the middle pairs and the available speed range is 50 to 3000 RPM. If a person wanted to do so one could use different pulleys and get a different speed ranges. I don't understand Nova's Digital Variable Reluctance motor to know if that would affect the torque, but I assume so. So maybe a useful feature to someone else.

Thing is whatever the torque is, it's enough for me. I'm not stalling my lathe (except for when I left the spindle lock in).

 

Bill, I guess I did not make myself clear. Except for friction losses, the pulley system does not, in itself, affect the delivered horsepower. However, it allows the motor to run at a higher speed, where it can deliver higher horse power. In a VFD system, the induction motor produces about the same maximum torque for all settings of the speed dial. The maximum available horsepower (hopefully the rated horsepower) occurs at the maximum RPM. As the speed is reduced, the motor torque remains the same so their product, which determines the delivered horsepower, goes down.

 

A common practice with the implementation of VFD's used on lathes is to drive the motors above the normal 60Hz rating. Powermatic and probably jet overdrive the 4 pole motor to about 135Hz. Motor torque falls off with speed above the 60Hz due to limits of the VFD. Sometimes the phrase used is, constant torque below 60Hz and constant horsepower above. A reputable company would not stay in business selling a lathe that the VFD power output was not a reasonable match the motor capabilities.

 

Joe, I had not seen any reference to “overdriving” a motor in a VFD system. Even if it is done in a particular system, the advantage of multiple transmission ratios remains. Assuming the 60 Hz motor is overdriven to 135 Hz, the portion of the operating range below 60 Hz will benefit. It is usually in the low-speed range that high torque is most needed.

 

I believe I have seen some reference to overdriving motors. I do wish mfrs made it easier to determine gear ratios and some of the inverter settings. Overdriving the freq is an advantage - more range of speed and allowing a lower gear range for good low speed performance. The dropoff of torque is inconsequential - the higher speed brings HP to get through a cut.

 

Here is data from my PM3520 Lathe using the high speed range. The Delta S1 VFD is capable of displaying frequency output. Minimum spindle rpm 122, frequency - 5.0Hz. Maximum spindle rpm 3120 frequency is 134 Hz. The 4 pole motor is rated 2HP at 1720 rpm. At 1712 rpm spindle speed (can't get 1720 rpm) the frequency is 72.5Hz. The radius of the motor pulley is 3/8" less than the spindle pulley therefore under driven which correlates to the previous sentence.

A few years ago I had my name on a Robust Sweet 16 that was going to be available after one of the woodworking shows. In talking with Brent regarding my concern as to what I considered a single pulley speed limitation, he assured me that he and customers have used the full capacity (32" diameter) with little problem. Just take a lighter cut. Sure you can stall the motor if you wish to do it. I asked what was the maximum Hz output of the drive and IIRC he said 90Hz. IF the pulley ratio was 1:1 it would take closer to 100 Hz. i don't remember if I asked the ratio. At that time I believe the motor and vector style variable frequency drive were both from Leeson. The point of this is that better performance can be obtained with added $'s and coordinated VFD and motor drive packages. The search for ratios and settings for evaluations and comparisons is not a simple task, especially considering offshore manufacturers.