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The trouble with "muscle lathes"..........

Bill Boehme

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An ac induction motor windings have very little resistance. When the line voltage is applied, without the armature spinning, the current through the windings is very high. But, when the armature is spinning at a speed that is synchrounous (or nearly so) the armature spinning in the magnetic field of the windings, generates a voltage in the windings in opposition to the line voltage. This generated voltage is called Counter Electro Motive Force (CEMF). The CEMF is usually within a few volts of the line voltage, and the current through the windings is reduced to what would occur without the rotor spinning if only a volt or two was applied. An example, a 10 HP 3 phase 220 Volt motor draws about 30 amps at full load (1780 rpm), about 120 amps with a locked rotor, and about 2 amps when running unloaded (1795 rpm). The cooling fan on the motor contributes to the two amps.

When a load is applied to the motor, and the armature slows, the rotor cuts fewer lines of magnetic force, the CEMF is reduced, and the current through the windings increases as a result. The drop in speed to cause the increase in current needs to be very slight, typically only one or two percent drop in speed will cause the current to go from an unloaded value to a full load value. This happens automatically without any sensors. The motor does not (can not)produce more power than needed at any time. If the motor was producing more power than needed, then the speed would run away until the motor self destructed (running away can happen to series wound DC motors, but they are a different animal).

I've no knowledge of how the DVR motor works, but from reading the info on the links, it sounds similar to a stepper motor with computer control and monitoring. Quite a different system than an induciton motor.

Thanks for your post. I thought about writing something to explain that the motor's output power can't be greater than what is needed and then noticed that you answered the question perfectly. BTW, the largest component of the NL current is the magnetizing current. There are also friction losses, windage losses, iron losses (flux leakage and hysteresis), resistance losses, and eddy current losses.
 
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I always set the lathe speed with no cutting.
The lathe tools should never vibrate. I cannot control a vibrating tool.
Any vibration from the lathe tools needs to be corrected. Tool vibration is most often too much bevel drag, dull tool, tool rest too low, tool rest too far from the wood, or too heavy a cut, In general corrections for tool vibration don't involve lathe speed.

Have to span the parenthetical to realize that the statement is about what makes the wood vibrate, not the tool. The tool doesn't flex, the wood does. Movement is what allows vibration. Not sure what kind of differential thickness you'd need to flap the tool rather than the wood. Maybe a cylindrical quarter inch tool hanging three inches out driven by over an inch wood? If you feel the tool bouncing it's you, not the metal. Considering Newton's third, reducing the rpm will better enable you to counter irregular inputs.

Also, if your lathe is "vibrating" before you touch the tool to the work it's in that category of weight and balance - centrifugal. You lack rigidity somewhere or the thing's walking. Time to fix it. One of the best ways to do that is to reduce rpm.
 
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On vibrating wood, I saw some thing interesting at the Symposium this year. Mike Mahoney and Stuart Batty were doing their '2 ways to turn a bowl' demo. They turned a bowl about 14 inch wide by 8 or so deep. They were taking finish cuts on the inside, with wall thickness about 1/8 inch, and they were not using their hands or a bowl steady. Point is most of us over ride/too much pressure on the bevel of our tool. I have tried to put that more into practice. The bevel is a guide, and you don't push on it, you feel what it is doing.

robo hippy
 

john lucas

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Reed I have discovered that as well but learned it on spindles. When I get chatter, usually caused by either the summer winter wood density differences or flex in thin spindles, I can get rid of it by being extremely light on the bevel. I find it works well on thing bowls as well. Stuart and Mike do a fantastic demo and well worth sitting in on if anyone ever gets a chance to see it.
 

Bill Boehme

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... Are you implying that other variable speed systems also monitor the spindle speed and are trying to do the same thing? I thought this was unique to the DVR....

Well, it wasn't really implied ... I thought that it was plainly stated that not only variable speed systems, but any AC induction motor running straight off the power line as Dale Miner went into some detail explaining.

Depending on the quality of the motor, the slip frequency may be more or less (slip frequency is the difference between line frequency and rotational frequency). The slip frequency in a standard AC induction motor determines the motor current. When unloaded, motor current is at a minimum while at full rated load, the current will be the nameplate full-load current. Most motors can handle overload currents for brief periods of time.
 
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Bill,

If you can, describe Variable Reluctance as it applies to a motor. The description in the Nova website makes me think that the motor has a wound armature and wound poles similar to a syncronous motor with some type of shading to start the rotation. To me, reluctance is the tendency of a magnetic material to resist being magnetized, and that is a function of the material. Where does the Variable part come in?
 

hockenbery

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MichaelMouse said:
Also, if your lathe is "vibrating" before you touch the tool to the work it's in that category of weight and balance - centrifugal. You lack rigidity somewhere or the thing's walking. Time to fix it. One of the best ways to do that is to reduce rpm.

all lathes I've used vibrated. I have probably only used a couple hundred so it is possible there is one out there that does not vibrate.

Next time you rough a bowl Put a glass of water on the ways and watch the surface.
The ripples you will see are made by the vibration of your lathe.

If that water sloshes out you have excess vibration.

All hollow turners know turning tools vibrate.
Some folks even use this fact to do chatter work with spindle gouges and skews.
Some folks do the chatter work unintentionally.

al
 
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Sounds like it may be time to take your car in for some maintenance. :D

It depends on what you mean by "simple". Here is my version of "simple" -- any given mass attached to the end of a spring of a given stiffness has a natural frequency of vibration. If you change either the mass or the spring then that frequency also changes. If you want more, then ....

Suppose that we "excite" this spring-mass system by poking on it with something (a bowl gouge, for instance), the spring-mass system will respond by vibrating back and forth (or perhaps, to and fro) at its natural frequency until the vibrations eventually die down. (The vibrations die down because there is a third component to this system that we initially overlooked called a damper that is absorbing the energy of the vibration).

Now, suppose that we decide to poke this spring-mass-damper with our blunt instrument at exactly the same rate as the system's natural frequency of vibration. We will see that the amplitude of the vibrations grows ever larger until something breaks (mass comes loose from spring, smacks our investigator holding the blunt instrument and then rolls around on the floor.

Seeing that our previous action was a bad idea, we decide to find out what happens if we poke the spring-mass-damper system with the same blunt instrument at a rate that is slightly slower or faster than the system's natural frequency of vibration (a.k.a., "resonant" frequency). We discover after much wear and tear on our investigator that as our chosen exciting frequency approaches the resonant frequency of the system, the amplitude of the oscillations (i.e., the to and fro vibrations of the mass on the end of the spring) grow in magnitude. We also discover that when we excite the system at a frequency sufficiently removed from the system's natural frequency that the exciting frequency no longer has much of an effect on the amplitude of vibration of the system.

In the real world we find that masses, springs, and dampers rarely look like the classic examples seen in freshman physics textbooks. We can often identify a mass, but the spring and damper may not be as obvious. To make the picture even muddier, these systems rarely live in isolation. Most everything around us (a woodturning lathe, for example) consists of a large collection of masses, springs, and dampers that all interact with each other to some degree. An out-of-balance hunk of wood spinning on a lathe is both a source of excitation and a mass coupled to another mass which consists of the lathe body through a rather stiff spring consisting of the drive train. At the same time, the drive train itself is a spring mass-damper-system consisting of the spindle, bearings, belt, and motor. And the motor itself when being powered under load is working to maintain a certain slip frequency by changing the current through the field windings to provide the needed torque. When the load (i.e., the out of balance spinning hunk of wood being intermittently cut with with a blunt bowl gouge) is not reasonably constant then the drive train reaction also becomes part of the overall system stability. The most obvious dampers in this overall system might be your bags of sand, cement, or deer corn on the bottom shelf of the lathe, but the complex geometry of the lathe body itself usually provides most of the damping.

-e-, where have you been? Good to see that you are still alive.

Thanks, Bill. I like the first paragraph, the rest is why I hated physics!!!!:D

PS I sold the car eons ago.
 

Bill Boehme

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Bill,

If you can, describe Variable Reluctance as it applies to a motor. The description in the Nova website makes me think that the motor has a wound armature and wound poles similar to a syncronous motor with some type of shading to start the rotation. To me, reluctance is the tendency of a magnetic material to resist being magnetized, and that is a function of the material. Where does the Variable part come in?

Dale, as you probably know, reluctance is the magnetic version of electrical resistance. The variable part of he name doesn't seem quite as logical, but it sounds impressive.

In a previous post you guessed that a VR (variable reluctance) motor sounded somewhat like a stepper and you are exactly right. In a nutshell, VR motors (also known as, switched reluctance motors) are discrete position devices. This discrete position jumping has a disadvantageous characteristics of rough running and being noisy. Also, steppers can't run without a controller. There were many complaints about those two issues on the early DVR lathes. More sophisticated design improvements in motor design along with better controllers using more powerful microprocessors to process more sophisticated control algorithms has done much to improve the performance of these VR stepper motors. Steppers were originally used as the poor man's servo motor. Steppers are very simple motors and don't have the precision of a servo, but for the money they are often good enough. One characteristic that they share with servo motors and DC motors in general is that their peak torque is produced at zero speed while the torque is near zero at maximum speed. This means that peak horsepower occurs at approximately 50% of maximum RPM where torque is also about 50% of the zero speed torque.

For anyone interested in further information about VR steppers, here are some links that do a good job of providing non-technical descriptions:

http://zone.ni.com/devzone/cda/ph/p/id/287

http://www.wisc-online.com/objects/ViewObject.aspx?ID=IAU14208

http://www.solarbotics.net/library/pdflib/pdf/motorbas.pdf
 
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Bill,

Thanks. The variable part still seems somewhat of a misnomer, but I now have an understanding of the difference from the stepper motors I've had experience with.
 

Bill Boehme

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Bill,

Thanks. The variable part still seems somewhat of a misnomer, but I now have an understanding of the difference from the stepper motors I've had experience with.

The term "switched" also sounds like a misnomer. I think that originally the term "minimal reluctance" was used, but using diminutive terms in advertizing doesn't do much for boosting sales. The wordsmiths got busy and transformed minimal into variable.
 

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I believe I'm going to revise my thinking on this........:D

Over the past few days, I roughed out aproximately 35 bowls of various species, sizes and moisture content. I paid particular attention to monitoring the surface heat of the wood in relation to the heat felt in the shavings. I've concluded that another person who posted in this thread got it right when he said the majority of the heat is carried away with the shavings......this I now believe to be true, and have confirmed with personal experiment.

Also, the heat generated can easily be increased/decreased with the amount of aggressiveness of the cut, and the force applied. As was noted earlier, sharp tools and the pressure applied between bevel and wood is also a factor.

As a result of this thread, and follow-up on the lathe, I'm going to feel more comfortable to take strong heavy cuts for wood removal, as I once did.......without much concern over the heat produced being absorbed by the wood.

I'll note that precision cutting is still done best with light, more carefully executed tool work. Heavy cuts only benefit for wood removal, when finish cuts are not the primary objective.

This is just one more example of how I have benefited from interaction with other turners on this forum.......thanks to all!

ooc
 
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I have to agree with Dale on this one Odie. I use a custom lathe with a capacity of 32 in the 12 inch long gap and 26 x 78 over the bed, and typically turn logs with an endgrain orientation. They are green, heavy and probably larger diameter than the average turner uses. Thus, I turn at slower rpms and by vitue of using bigger pieces of wood, I uses bigger tools to rough them out. Since the wood I use is soft and moist I do tend to take aggressive cuts and very frequently slow or stop the 2 hp motor because of the reduced power at reduced rpm's.

During the roughing stage heat is not an issue. When not roughing out, the spinning of the wood causes a far greater drying effect than anything else (with green wood). Also, sanding pieces while they turn probably creates more heat than cutting (although perhaps not as concentrated in one area.
 

odie

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I have to agree with Dale on this one Odie. I use a custom lathe with a capacity of 32 in the 12 inch long gap and 26 x 78 over the bed, and typically turn logs with an endgrain orientation. They are green, heavy and probably larger diameter than the average turner uses. Thus, I turn at slower rpms and by vitue of using bigger pieces of wood, I uses bigger tools to rough them out. Since the wood I use is soft and moist I do tend to take aggressive cuts and very frequently slow or stop the 2 hp motor because of the reduced power at reduced rpm's.

During the roughing stage heat is not an issue. When not roughing out, the spinning of the wood causes a far greater drying effect than anything else (with green wood). Also, sanding pieces while they turn probably creates more heat than cutting (although perhaps not as concentrated in one area.

Thomas......It's too late to disagree with me because I've changed my mind! Heh,heh,heh........:D

(See my subsequent posts.)

ooc
 
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Sometimes when your're right you can't stop the momentum. :)
 

Bill Boehme

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Problem solved!

Odie, my biggest "problem" with muscle lathes was not having one. :D

The problem has now been solved. Now, my new problem with muscle lathes is being able to find the time to do any turning. :(
 

odie

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Odie, my biggest "problem" with muscle lathes was not having one. :D

The problem has now been solved. Now, my new problem with muscle lathes is being able to find the time to do any turning. :(

I hear ya, Bill.........

For me, finding time was a problem, too.......but about 4-5years ago, I did something that changed my life as I had known it. I have absolutely no TV in my home.....none!

There is a relationship to not having that "boob tube" in the home, I believe, and that I'm probably spending at least 15-20 hours in the shop per week. Once I retire (very soon!), I expect to work in my home shop full time. It also helps that in the past few years, I've made some major advancements in my turning skills......this is also a great help with my motivation level, because there is nothing in the world that inspires.....than the satisfaction you feel from producing something you are proud of......(Not to mention the people you know and love, are noticing it, too!)

Sure do miss football, though.......:(

ooc
 
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muscle lathes

I'm with Odie, I don't watch TV, only videos on occasion (no commercials) but I'm lucky if I get 15-20 hours per month.
 
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