A question of stability........

[Odie]

Long bed vs short bed lathe.

When turning an out-of-balance piece, and since you can feel some vibration with your fingers on the bedways, it does mean that the entire lathe is flexing a minute amount.

When the centrifugal load thrust is at the 3 and 9 o'clock positions, it seems like the longer bed would vibrate less than a short bed lathe......given all else is equal. This is strictly because the "footprint" is better able to withstand pressure on a horizontal plane. Theoretically, a sliding head would be equal to a long bed, no matter which end the headstock is located......or, is it?

When the load thrust is at 6 and 12 o'clock positions, it would seem that the long bed vs short bed consideration would not apply.

Up for discussion.......

ko

 

[AlanZ]

Just to be clear, are you referring to the centripetal force?

 

[robo hippy]

Well, short bed lathes are primarily for bowl turners, as far as I am concerned. The vibration issues are like oak or maple, multiple varieties... The main difference seems to be 'fixed' headstock compared to sliding and pivoting headstocks, with the fixed/bolted down ones having less vibration issues, mostly due to a more secure mount, bolting compared to pressure plate. For me, the bigger the pressure plate, the more solid the mount is, and the less vibration there is going to be. If I was going to build my own, for some thing the size of my American Beauty, I would have a plate at least as long as the headstock bed, and maybe a handle on each end for maximum clamping pressure. The only pivoting headstock I have seen that looks really sound to me, is the one from Vicmark, which appears to be a tank, and I think would go back to dead on with no fiddling around. There many levers that play into vibration issues. Footprint, front to back, and side to side, lathe bed height, headstock height, spindle mount and how far the mounting point is away from the headstock, and how far the piece extends away from the headstock.. Then there is tool and cutting pressure... I don't really think bed length contributes a lot to the vibration issue as long as the bed is sufficient to span the distance. The steel vs cast iron difference is debatable. For sure they make different noises, but not sure how much actual difference there is or how to measure it. Then there is the VB36, which is a whole different beast.

robo hippy

 

[Odie]

Just to be clear, are you referring to the centripetal force?

Heh,heh,heh......heck, I don't know! Does the term used matter?

Whatever technical term correctly describes the forces that cause vibration from an out-of-balance condition. I suppose the only way to know for sure, is to have identical lathes, one short bed and one long bed......mounted with the same out-of-balance turning. See and feel the results for yourself. I've never had anything but a long bed lathe, so my thoughts on it are strictly theoretical?

I do know, for sure, there is a relationship to the vibration (by using fingers on the lathe bed, and watching the wiggle of a laser mount I have on the headstock), and the rpm. I can reduce, and sometimes eliminate vibration by adjusting the rpm this way. I don't know for sure whether the footprint of the lathe does, or could effect this......

ko

 

[AlanZ]

Well, centripetal is a force, and centrifugal is a lack of centripetal force (so there really is no such thing as centrifugal force)

Just trying to speak a common language... a fine thing for discussions.

<vbg>

 

[Doug Rasmussen]

The question may relate to the stiffness of the lathe beds. Whether they're designed to be stiffer vertically or horizontally. Not to get too complicated here, but the stiffness would be related to the moment of inertia of the bed design (the cross section of the bed and distribution of mass in it).

The overall deflection (or vibration) is then related to the length of the bed. For instance, if you have a given length of bed and double the length to maintain the same deflection under load the long bed would have to be eight times as stiff (that's not intuitive).

It seems lathes using large tubular beds like Oneway will have pretty much the same deflection under load vertically as horizontally (because the moment of inertia would be the same, or close, in both the vertical and horizontal axes).

 

[Sergio Villa]

Well, centripetal is a force, and centrifugal is a lack of centripetal force (so there really is no such thing as centrifugal force)

Just trying to speak a common language... a fine thing for discussions.

<vbg>

I'm not a physicist but centripetal is a force toward the center of rotation whereas centrifugal is a force directed away fron the center of rotation. From the old latin language.

 

[AlanZ]

Ah yes, but it's the physicists who know the difference, and why centrifugal is considered a "ficticious" force, regardless of the word origin.
A quick search (even on youtube) about centripetal vs centrifugal would be enlightening.

 

[hockenbery]

Ah yes, but it's the physicists who know the difference, and why centrifugal is considered a "ficticious" force, regardless of the word origin. A quick search (even on youtube) about centripetal vs centrifugal would be enlightening.

But fictitious forces are real or the clutch in your chainsaw would not work.
Fictitious forces balance newtons equations. 🙂 Al

 

[Bill Boehme]

Well, centripetal is a force, and centrifugal is a lack of centripetal force (so there really is no such thing as centrifugal force) .....

Words sometimes have different meanings for scientific disciplines that differs from the lay usage. Perhaps more familiar examples are the terms "impedance", "imaginary", and "complex". An electrical engineer uses the term "impedance" to describe in vector algebra a "complex" characteristic of resistance that has an "imaginary" component. The words "imaginary" and "complex" have somewhat different meanings than the way that a layman would use the words. Vector algebra really isn't "complex", but it uses vectors that represent "real" and "imaginary" axes in vector space. The imaginary axis isn't imaginary (as in, doesn't exist), but it represents that part of resistance that behaves as resistance except that energy isn't lost to "work" or dissipated as heat. BTW, "work" isn't quite the same as breaking-a-sweat kind of work ... but, ... getting back on track ...

In classical mechanics, forces are the result of the physical interaction of two objects. However, in non-inertial space (where our frame of reference is accelerating linearly or is rotating), then forces such as centrifugal or Coriolis doesn't require the interaction of two objects. The word "fictitious" describes a physical characteristic that is just as real as "imaginary" ... if you know what I mean ... er, even if you don't know what I mean ... got it? 🙄 🙄

And, no, engineers and physicists aren't smoking or snorting something when they are dealing with imaginary resistance and fictitious forces. Seriously. I think. Maybe. Who knows. 🙄

 

[Gerald Lawrence]

Odie in a way I agree with you. If skyscrapers can sway why can't a lathe flex. However I think the forces you are talking about are more like the legs of the lathe slightly lift and causing the vibration, even if it is bolted down

 

[olaf Vogel]

The question may relate to the stiffness of the lathe beds. Whether they're designed to be stiffer vertically or horizontally. Not to get too complicated here, but the stiffness would be related to the moment of inertia of the bed design (the cross section of the bed and distribution of mass in it).

The overall deflection (or vibration) is then related to the length of the bed. For instance, if you have a given length of bed and double the length to maintain the same deflection under load the long bed would have to be eight times as stiff (that's not intuitive).

It seems lathes using large tubular beds like Oneway will have pretty much the same deflection under load vertically as horizontally (because the moment of inertia would be the same, or close, in both the vertical and horizontal axes).

In my humble opnion, theres another factor. Since the head and tail stock extend up from the bed there more flexion induced: twist. Oneway and American had large tubular or box sections which resist that well.

Not sure if extra length makes much difference i that regard.
Mine is twin i-beams. Not great in that twist is evident.

 

[Odie]

I've always thought cast iron is more rigid, and less prone to flexing than steel. I understand that tubular and welded strut construction, and other methods of stabilizing the base material can influence rigidity.........

Opinions are welcome in this discussion......I'm full of opinions that are subject to change at any time. 🙄

ko

 

[robo hippy]

Well, I am thinking again.... I think cast is better at absorbing vibration as in deadening it but not sure about it being rigid, though it will crack with impact. Steel I think, tends to resonate more with vibration. On both, the bridge work can make huge differences in stability. Can't think of bridges without thinking about the bridge in the Tacoma Narrows that is part of every science class since I was young...

robo hippy

 

[Odie]

One thing I think is true.......is even a very small vibration, so small you probably won't even notice it, without a physical means of checking it, can and will cause your lathe tools to chatter an infinitesimal amount. Absolute stability is necessary for the best cut you can get. (Assuming all the other variables are at the maximum advantage......sharpness, presentation, feed rate, direction of cut, depth of cut, etc.) For me, a perfect cut, requiring an absolute minimum of sanding.....or, as close as I can get to it, is necessary for the goal of maintaining geometric integrity. Geometric integrity is what makes my style of turning possible.

ko

 

[olaf Vogel]

I've always thought cast iron is more rigid, and less prone to flexing than steel. I understand that tubular and welded strut construction, and other methods of stabilizing the base material can influence rigidity.........

ko

I'm not sure cast is necessarily more ridged. It depends hugely on design.
Oneway is tubular steel. From the pics, i'd guess the American is as well (never seen one in Canada) 🙂

Picture a large steel tube, on end. Now consider the force required to twist it. Those main tubes are about 10-12". The force would incredible. This is awesome torsional rigidity.

My i-beams suck at tortional rigidity. Thats bot what they are designed for.

Longitudinal rigidity is hugely increased as diameter increases. All other things being equal.

Mounting is important IMO. I raised my lathe up 12", cuz I'm tall. Then poured concrete pads right OVER the feet.
So its a 1000# lathe with 400# of concrete on each end, rebarred down into the floor. No vibrations now.

So design is critical. I suspect that these days its more cost effective to weld steel than pore cast. Theres tons of steel tubing commercially available, in almost any size, shape, guage. Its easy to weld and lighter when shipping, uses less material. Its possible to weld in most commercial shops, no foundry required.

Cast has the benefit of weight. And being able to make intricate shapes. Those shapes, gussets etc, can add rigidity as well. All commonly done on metal lathes. They Must be very rigid to get clean cuts at 1/1000 of an inch. But thats why my 16" metal lathe weighs 4000 lbs. 🙂. And sold for a lot of money, back in 1929...

 

[hockenbery]

I'm not sure cast is necessarily more ridged. It depends hugely on design. Oneway is tubular steel. From the pics, i'd guess the American is as well (never seen one in Canada) 🙂 Picture a large steel tube, on end. Now consider the force required to twist it. Those main tubes are about 10-12". The force would incredible. This is awesome torsional rigidity. ...

I agree with the tube structure being hard to beat
The ONEWAY has a an oval tube. The robust AB has a diamond shaped tube with a vertex pointing up
Both lathes are solid and have lots of vertical supports from the tube to the ways.

One lathe worth mentioning in this discussion is the NOVA DVR ( I don't like them at all for many reasons)

The DVR has a bed that is skimpier than some mini lathes
However I have roughed out of balance 14" bowls on a DVR and it has surprisingly little vibration.
This DVR wraps the motor around the spindle so the headstock has a lot of mass.
Looking at the bed you would think it would walk across the floor but that headstock mass and design keeps vibration to a minimum.

Al

 

[robo hippy]

Some what off the topic, but involving vibration from tool pressure. There always seems to be vibration or maybe oscillations caused by the differences of the cutting edge moving through the varying grain densities and orientation. "The bevel should rub the wood, but the wood should not know it." No idea who said that. Best example I ever saw was Ashley Harwood turning her delicate finials for her sea urchin ornaments. 6 inches long, less than 1/8 inch diameter at the end, with a gouge, and no support finger behind it as she does the final cuts..... Still trying to pick up that one. Really difficult because as soon as the tool starts to bounce a tiny bit, first reaction is to clamp down on it.

robo hippy

 

[olaf Vogel]

The DVR has a bed that is skimpier than some mini lathes
However I have roughed out of balance 14" bowls on a DVR and it has surprisingly little vibration.
This DVR wraps the motor around the spindle so the headstock has a lot of mass.
Looking at the bed you would think it would walk across the floor but that headstock mass and design keeps vibration to a minimum.

Al

There was a website, of a local guy, who liked to moved his lathe outdoors. But still rough out off balance pieces. So it had to be light but stable.
His solution was to mount a concrete filled bucket, high over the headstock. But firmly attached.

The reasoning (very solid, IMO), was that lots of weight, high up, will dampen oscillations. You figure it rocks back and forth with the legs being the pivot point.
Low weight, allows fast oscillations. High weight, high up, will dampen. Kinda like a metronome for teaching piano rhythm.

Never tried it. But I like concrete for damping as well. Here's some interesting reading: http://www.engineeringforchange.org...d-war-i-technology-meets-21st-century-design/

Olaf

 

[john lucas]

There were some good posts on adding an adjustable weight above the headstock many years ago. I played with it just a little but the weight needed to be moved as the wood lost weight. Although it worked it was more work than I was willing to put up with since I don't do a lot of heavy out of balance bowls. You might be able to do a search and find some of those posts. Don't remember what forum. Probably this one or Woodcentral.

 

[hockenbery]

There was a website, of a local guy, who liked to moved his lathe outdoors. But still rough out off balance pieces. So it had to be light but stable. His solution was to mount a concrete filled bucket, high over the headstock. But firmly attached. The reasoning (very solid, IMO), was that lots of weight, high up, will dampen oscillations. Olaf

I know at least two guys who had basement shops and basically held their lathes to the floor with their house.
They welded some thing like a jack post or lolly column to the top of the headstock and cranked it up until the house pushed it to the floor.
Pretty solid.

Al

 

[robo hippy]

Oh yea, and then there was the guy who made a solid concrete lathe. I can't remember when, but several years ago.... Heavy and solid. As a former concrete worker in high end residential work, the job done was exceptional.

robo hippy

 

[john lucas]

You know it could just be the wood. Wood often has different densities and when you try to make it spin it will vibrate. I've made quite a few spheres over the years and they almost always settle on one side due to different densities inside. I just turned this mirror and then handle was a nightmare of chatter. It had more flexibility than any wood I've tried before and fought me no matter what technique I used, and I've turned a lot of mirror handles, some a lot thinner than this one. A massive lathe would not have stopped this.