At a recent club meeting the following statements were made, and I'm interested in comments/corrections.
It is torque, not horsepower that cuts the wood. I can accept that as torque is a measure of rotational force.
With a Reeves drive at low rpm you gain torque over high rpm. This is what I learned riding 10 speed bikes and driving a standard transmission.
With electronic variable speed you lose torque at low speeds, this is why the bigger lathes have 3 hp motors, to make up for the lost torque. This one I don't buy, but I don't know if it is right, wrong or partially correct. From posts in rcw, I though with DC motors you lost torque at low speeds, but AC (3 phase) motors maintained torque at low speeds.
Any corrections or clarifications to these three statements would be greatly appreciated.
True. Horsepower lets you cut the wood *faster* though. Imagine a lathe with a worm drive instead of gearing. The wood would only turn at a few RPM but it would be pretty much unstoppable.
Torque is foot-pounds. For a given torque, the closer to the axis you are, the more linear force is available for cutting.
True. Torque is foot pounds; you can change torque with gearing.
A given motor can generate UP TO a given horsepower. Speed controls can only REDUCE the amount of horsepower. An ideal controller/motor combo would maintain peak horsepower, but vary only the frequency. Unfortunately, because the coils in such motors are inductors, lower frequency means less resistance, so you have to reduce power at low speeds to prevent burnouts. This means less horsepower. I suspect that at lower frequencies you have to run the motor at a lower magnetic offset angle, which reduces the torque the magnetic fields apply to the armature, but I'm just guessing on that one. A really smart motor/controller combo would detect the amount of torque needed to maintain a given RPM, and adjust the angle and power accordingly.
The gearing determines the ratio of horsepower to torque, which is why big lathes still have step pulleys.
Big lathes have bigger motors because you want the motor to overpower the wood, and still provide usable torque at the largest supported diameters. Big lathes allow heavier and larger diameter wood, hence bigger motors.
Part true/Part False. Both torque and horsepower cut wood. For a given horsepower there is a certain speed where you can take a certain depth of cut with a certain tool in a certain type of wood. A 3 horsepower can take basically 3 times greater cut than a 1 horsepower if both are running the same speed. Another factor is that having excess horsepower helps keep the speed of cut consistent, which improves the quality of the cut.
Horsepower = Torque x RPM / 5252
False. When you turn down the speed, you are turning down the horsepower. So a high horsepower motor allows you to have good horsepower through a wide range of speeds. If the rated speed of a motor is 1750 rpm, then you will get the full horsepower rating there (for example a 3 horsepower motor will deliver 3 hp). At 875 rpm motor speed, the motor gives 1 1/2 hp. At 437 rpm, you only get 3/4 hp. If you had a 1 hp motor, also rated at 1750 rpm, you would only be getting 1/4 hp at 437 rpm which doesn't allow you much of a cut. Having a Reeves drive or step pulleys allows you to adjust the speed of the spindle while keeping the motor at full speed or near full speed, thereby maintaining motor horsepower output.
An interesting feature of variable speed AC electronic drives is that they can run the motor at over the typical rated speed. So a 1750 rpm rated motor can be run at 3500 rpm in some cases. Generally the motor is fully capable of being run at this amount of overspeed. In this case, the motor runs at the same horsepower from 1750 rpm to 3500 rpm. So this means that the motor torque drops off to half the torque at 3500 rpm that is has at
1750 rpm. The useful feature of this capability is that you can get full horsepower over a wide range of speeds without changing pulleys. For example if the motor has a 3" pulley and the lathe spindle has a 6" pulley and the controller runs the motor down to 10% of rated speed and the motor is 3 hp with a rated speed of 1750 rpm, you could have spindle speeds of
1750x3/6x10%=88 rpm to 1750x2x3/6=1750 rpm. Between 875 rpm and 1750 rpm spindle speed you have 3 hp. At 88 rpm you have 10%x3hp = 0.3 hp. If you had used a 3 hp motor with a rated speed of 3500 rpm, you would have to use a 12" spindle pulley and a 3" motor pulley to get the same range of speeds. Then you will have 3500x3/12x10%=88 rpm as a low speed. However, you probably don't want to run this motor at 7000 rpm, so if you leave it at max of 3500 rpm then your high speed is 875 rpm. So a motor with a high rated speed gives you less flexibility.
Thank you Derek, As you turn down the speed, the horsepower decreases linearly from your example, and thus the torque is constant from the above equation. If I understand all this right, I can take the same heavy cut (amount of material removed per revolution) at full speed or 1/4 speed, I just have to adjust how fast I move the tool along the tool rest. Just to be clear, I'm still talking about electronic speed control here.
That's right. You can take the same amount of material per revolution either way. But at 4x the speed (given the same pulley setup) you can remove 4x the wood per time period. Of course, there are certain non-linearities in the way controllers work, but that is the basic proportion. For an understanding of the practicality of how horsepower works, watch a Mike Mahoney video and see how fast he roughs a bowl with his
3 hp Vicmarc lathes. Yes, horsepower really does cut the wood, as does torque!
example, and thus the torque is constant from the above equation. If I understand all this right, I can take the same heavy cut (amount of material removed per revolution) at full speed or 1/4 speed, I just have to adjust how fast I move the tool along the tool rest. Just to be clear, I'm still talking about electronic speed control here.
We've seen some engineering stuff as it pertains to a 3-phase motor and VFD combination. But, what it feels like is really the test. A properly set-up system senses the motor WANTING to slow down as the load increases and will make up for it seamlessly. It will also drive the motor at above its rated torque for a time if it feels it is safe to do that.
That said, you still need the mechanical advantage given by properly designed pulley ratios and that is the very thing which is missing when one goes to any direct drive system.
Once you have turned on a well designed VFD-based system, there is no going back -- I don't think that anyone can argue with that.
SNIP .......... ========================== Martin, Torque and horsepower are intertwined functions. Horse power is defined as to performing a given amount of work in a set amount time. The original rating was set by having draft horses lift a weight with a pulley. So the pulling (turning) force is adjusted for the amount of time applied. If you look at automotive engines, they have both torque and horse power ratings. The two ratings occur at different locations in the RPM range. The torque, or kick in the seat, usually rises quickly and peaks in the midrange (3000-3500 RPM typical in US cars). Then due to air intake restrictions, cam specs, exhaust, etc., the torque starts to drop. However, since the HP is a function of Torque X RPM, the horsepower continues to rise on into the higher rev range (4500-6000 RPM depending on tuning). Transmissions and differential gearing increase torque applied to the ground. A single gear ratio would severely limit the versatility of the auto, either limiting the top speed of the car, or causing a very long acceleration time.
All this can be applied to the lathe. A low HP rating means you have to have a wide range of gears or pulleys to prevent bogging it down. A 3HP machine will allow you to cut at a much faster rate (depending on your skill level) than a 1/2 HP unit. A three phase unit has an inherent advantage over a single phase drive due to the power applied over the AC line frequency curve. If you overlay the single phase sine wave with the 3 phase wave forms, you'll see that the 3 phase applies power at a higher average level and has a much lower fall-off in between pulses. This gives you a much smoother power application, as well as better torque. Variable Frequency Drives on 3 phase systems can pay games with these factors by utilizing various sensing circuits to monitor speed, current draw, etc., and adjusting the power application accordingly. Hope this helps some.
Not sure how they work (I'm not a motor engineer), but in the last few years routers have come to market that ramp up speed on startup, and the manufacturers claim they compensate for larger loads by "sensing" a slowdown and automatically increasing power, to maintain nearly constant speed. Do any of the lathes out there do this?
Yes, they do. We are using the Cutler Hammer sensorless vector drives. They detect slippage and will do what they can to make up for the problem. The method is different from that used with the universal motors on routers but the effect is very much the same.
Torque determines if you have enough rotational FORCE to sever the wood against the gouge. Horsepower determines how much WORK you do, or in our case how many cubic inches of shavings you remove, per unit time.
"Martin Rost" wrote: (clip) It is torque, not horsepower that cuts the wood.(clip) ^^^^^^^^^^^^^^ Let's play with this a little. Suppose you were using a lathe that developed enough *torque* to cut the wood, but did so at 1 RPM. It's true, the torque would be cutting the wood, but you would be wishing for some
*horsepower" so you could finish and go in to dinner.
Thanks to all that replied with explanations, examples and an equation. You have all made this issue crystal clear to me. As has been said before, you're a great bunch of people here at rcw. Thanks Martin PS Could a few more of you look at my other post "Survey - Club Lathes"
InspirePoint website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.