After reading the thoughtful suggestions about reducing spindle vibration, I tried two of them on exactly the same size balustrade, using the same wood. The first was to drive the spindle with a chuck at the head stock end instead of using a safe center. The second was to start turning at the tail stock end first instead of starting at the head stock end. The turning went MUCH better!
My big philosophical question is this: So it works in practice. but will it work in theory?
I happen to be a mathematician, so I know about the differential equations that govern the vibration of a spinning rod. So my joke actually had a bit of reality to it in the sense that I was interested in verifying that the modes of vibration will change as initial conditions change -- and a very important initial condition is the method of supporting the rod at its ends. In more practical terms, what is almost certainly going on by mounting the spindle in a chuck at the head stock end is that this practice allows the chuck to drive the spindle with relatively small pressure between centers. In other words the live center needs only enough pressure to keep the tail stock end of the spindle from flopping around; it's not required to apply much pressure thjrough the spindle to a safe center or spur center at the head stock end.
>I happen to be a mathematician, so I know about the
>differential equations that govern the vibration of a spinning rod.
I should connect you with some physicist and mathematician friends who made careers out of studying certain non-homogenious long rapidly spinning things to see if they can talk about it now. The traditional equations fell apart "suddenly" in this case.
From what I know about modeling physical properties, a perfect model or set of equations for the internal structure of a given piece of wood is not going to happen! In some cases very good approximations may not even get close.
Many times over the years I have observed an interesting thing about thin, rapidly rotating spindles made of wood fibers. The most recent was yesterday afternoon when turning a magic wand for my grandson.
Whether held between centers or, as I always do, with a turned morse taper jammed firmly into the headstock taper, certain actions can cause the perfectly spinning thin shaft to suddenly develop a bow in the middle, regardless of the pressure from the tailstock. It as if the fibers have suddenly "locked" into a new stable position. This bow can be significant in the worst cases, almost like setting up for an off-axis piece. Often the bow is only in the thinnest parts: the thicker handle next to the headstock can remain concentric to the axis while the thinner shaft is severely bowed.
At first when this happened, I usually turned the bow away to re-establish a perfect round shape. Later, I found I could always return the thin shaft to the original position while it was spinning, either by grabbing and flexing it with my hand, or better, by tapping it back into place. While spinning, I use the tool handle to tap lightly, near but away from the center of the bow. It usually takes several taps until I accidentally hit at just the right place on the circumference. When that happens, the shaft springs back and "locks" back into the exact original position and I continue with the turning.
This was always with magic wands so perhaps some of it was magic. :-) However, later, I saw the same thing with very thin conductor's batons. Of course, there is magic in music too...
Electrical engineering major, but loads of mechanical engineering courseware too...
When you look at a beam under load, if the two end points are not rigid attachments, like a log laying across a stream, the mid point of the log is the place where the maximum deflection will occur.
However, if you took the same log and bolted one side down, the midpoint is no longer where maximum deflection occurs. Rigid attachment actually makes the beam stiffer.
This is why driving it with a chuck reduces vibration.
I am in the middle of turning a 4 legged stool with stringers. One friend made the comment about whipping (how long spindles tend to wiggle a lot) that there is about a 10 to 1 ratio on whipping. 10 inch spindle, 1 inch in diameter will not whip. I have played around with that and found it to be pretty close to true. So, not being really well schooled in skew use and spindle turning, I am out there trying to get a good surface on some 24 inch spindles. It is an effort, but the same skills apply when making fine finish cuts on bowls. The trick for me now is making sure I have the proper edge, and being patient, and 'feeling' the cut....
You are right about feeling the cut. I taught a stool making class one time and gave them a choice of 16" or 28" blanks. I suggested the 16 because they would have much less problems with chatter. No one used them. They all picked the long spindles and fought it like crazy.
It took me quite a while to get the right feel for doing longer thin pieces with no chatter. I don't know what I did differently other than simply practicing. Kind of like Golf. You will be playing fine and then all the sudden the game goes to hell and you can't tell what your doing differently. Just one day I went to turn a long leg and had the right touch and the problem went away.
I did something similar on a Oneway lathe, where the programmed slow down actually twisted the tall thin goblet off the cone at the other end because they were spinning at different speeds.
Eliot, My comment was also a little tongue in cheek. I'm a surveyor so I'm not a theoreticist but more into the practical application of mathematics to understand and manipulate our world/environment. I do take a keen interest in some things like precession & nutation of the earth and spinning tops etc. However my theoretical knowledge is waning fast as I have very little requirement to use spherical trig, matrix theory & geodetic equations any more with push button technology.
The physics of a tippy top is still a contentious issue and has baffled some of the greatest minds for many years. So I'm never surprised about some of the mysteries of spinning wood on the lathe. I understand to a point how removing wood changes the internal stresses within a blank, from practical experience.
Wood is a non-homogeneous material of varying density and is effectively a bundle of fibers under varying tensions held together in a cellulose matrix. It has character and is full of surprises at times.
How we model the performance of the many species of wood on a lathe is a mystery and well beyond me. I have experienced spindle blanks from the same board performing quite differently, almost to the extremes of one being a joy to turn and another "unturnable" due to knots, cross grain etc.
I have no intention of modelling the centrifugal force and deflections but I do enjoy the practical experimentation and some of the surprising outcomes we can generate. Processes such as well controlled chatter to produce wavy surfaces on a curved upper surface of spin tops, something NZ turner Alby Hall can replicate time & time again. Or how we can turn a partly oval spindle on a single axis between centers. That one I can't control but I can replicate it with varying success, unfortunately.
I do love how the forced manipulation of parameters such as lathe speed, unsupported length (i.e. deflection), tool pressure, tail stock pressure (+ve & -ve), blank axes & even tool sharpness can generate controlled patterns and shapes to an extent.
So theory is great but real world practical solutions and investigations are much more fun for me.
>the programmed slow down actually twisted the tall thin goblet off
>the cone at the other end because they were spinning at different speeds.
Oh, new lathe design, everyone must have one, only twice the cost: two headstocks, one at at either end, synchronized electronically to turn at the same speed but opposite directions.
Feel free to patent this and send royalties to my retirement home.
I too have one of those adapters that allow exchanging a live center for a chuck at the tail stock. And I have been considering chucking a spindle at both ends. Again, because such an arrangement has fundamentally different initial conditions ( i.e. fundamentally different end point connections), the frequency and modes of spindle vibration will be different from between-centers holding. I suppose I could try solving the resulting differential equations of motion (not so easy). Fooling around with it would be easier and more fun. As I have said, using a chuck instead of a safe center at the head stock -- with everything else equal -- made vibration for the balustrades I just turned MUCH less. I mean .... there was a dramatic improvement!
>...using a chuck instead of a safe center at the head stock -- with
>everything else equal -- made vibration for the balustrades I just
>turned MUCH less. I mean .... there was a dramatic improvement!
I am glad that worked for you! I don't know if I mentioned it, but when I turn long thin spindles (think magic wands and conductor's batons) if I don't hold the blank tightly in a chuck at one end I simply can't get it done, at least not to suit me. I don't use a standard chuck but instead turn a small morse taper on the end while holding the blank between centers, then cram that taper into the lathe headstock spindle.
This "jam chuck" holds it just as well as a standard chuck but gives me a lot more room to work around the end while eliminating any knuckle busters. An added advantage: I can take the piece out of the lathe to work on something else and when replaced it always registers perfectly. Can't usually do that with a standard 4-jaw chuck!
Here is a wand nearly finished. I've made many dozens of these.
Thanks! I used to eyeball the morse taper which took a couple of tries. Then I made a little brass template which gave me two diameters a 1/2" apart. This works the first time! I do make a small relief in the middle of the taper so it fits tightly even if my taper isn't perfect:
I know some people just jam the square end of a thin blank into the headstock or whittle down the end until it fits, but I think making it like this is more secure and certainly registers better if the piece is removed and then replaced.
I press the taper into the headstock with pressure from the tail stock then back off on the pressure. BTW, some woods will split when applying pressure from centers so I always make a small hole in the exact center of each end first using a gimlet.
I'll usually start between centers and round it. I turn a small tenon about the same length as your morse taper, and 3/8" thick (sized with an open ended wrench. I throw a drill chuck in the head stock and just grab the small tenon. If the tenon is a little bigger or smaller, it still works just fine. I'm turning 10" long spindles that end up around 5/16" at the thickest, and less than 1/4" at the thinnest.
I made my gauge 2" long with the proper dimensions ground into each end as a no/go gauge. I use the gauge to mark the 2" distance and then use the parting tool to make the 2 dimensions. Then I just turn the taper to match those 2. Learned that from Mark StLeger a long time ago and use it frequently.
>I made my gauge 2" long with the proper dimensions ground into each end
>as a no/go gauge.
I thought it was KeepCutting.../StopNow! gauge. :-) Does yours look something like this?
If anyone wants to make one of these, I did this:
I first drilled a hole in the brass and reamed to get the right diameter then cut the straight parts on the bandsaw to make the U shape. Rounded the tips so they won't catch. The little extra wide spot at bottom left is just a bit too big so it gives me warning that I'm almost there with the parting tool.
At first I made longer tapers but later found that 1/2" - 5/8" was enough for the spindles I turn so I cut my two grooves 1/2" apart then extend them a little bit in both directions. The line is my 1/2" measure (more prominent on the other end) Making the taper short saves a bit of wood which is nice since I'm pretty stingy!
This is a whole lot quicker than using calipers.
I haven't had the pleasure of hanging out with Mark St Leger so I just made this up myself. However, I do have Mark's signature Thompson roughing gouge so I feel the connection! (My favorite roughing/shaping gouge for small spindles)
Both ends do with sizes for each end of the morse taper. Then the entire gauge is 2" long so I just hold it up to the wood and mark where both ends are and I have a full length morse taper. I keep thinking I'll make one with an inch or less so that I don't waste so much wood but haven't done it yet.
I'm having a Vooja Da moment. (Nothing this weird has ever happened before, George Carlin) I though I posted the suggestion to make a morse taper on the spindle several day ago. Wonder where it went?
On a small spindle like a finial I just grind a taper on the piece by eye and drive it into the headstock.
If I want to be more accurate, I have a gauge that I think I got from Mark St. Ledger. Its a brass "E". the spine of the E is the length and the slots on the side are the two extremes of the taper. I think I like the looks of the straight ones better. I'm a little afraid the one I have will get caught on the spindle.
On short spindles like Icicles you can pull the tail stock back out of the way and turn the tip without support. I usually turn the Icicle and the little hanger piece out of the same piece. Turn the icicle and part it off, leaving enough stock behind for the hanger.
>I though I posted the suggestion to make a morse taper on
>the spindle several day ago. Wonder where it went?
I remember reading that suggestion. It's still there, but on the other, initial thread.
I remember thinking at the time that while the morse jam chucking works will for me for small diameter spindles, I wondered if thinning a larger spindle to the headstock morse taper diameter would be as effective. Eliot's was 1-1/2" square by 33" long.
I've never tried one on a piece that big or long. When I have something larger than about 1" I've always grabbed it firmly with a scroll chuck. That would make an interesting experiment - turn both a tenon and morse taper on the end of a bigger stick and compare the response to the skew chisel.
Good. Here is something I did. Turned a nine foot long stick on a lathe with a two foot bed. Just for fun. Where this would work for you is to turn your wands. Take the jaws of a chuck and pass a stick through the chuck and into the shaft of the lathe. Turn about four inches and then advance the stick. You will probably need a little cup center on the tail stock to support the tip as it advances. or it will whip. Or you could remove the live center and pass the stick into and through the tail stock. Obviously the stick has to be small enough to pass through the shaft, but for a wand that should be just fine.
If you had two chucks and could mount one on the tail stock, you could actually put the piece in tension. Now you mathematician physics guys get busy on those calculations. Or, just try it.
>If you had two chucks and could mount one on the tail stock
Someone pointed out that the inertia of the wood plus the unpowered chuck at the tailstock would create a torque when accelerating/decelerating which might twist and possibly damage a spindle.
but more into the practical application of mathematics to understand and manipulate our world/environment. I do take a keen interest in some things like precession & nutation of the earth and spinning tops etc. However my theoretical knowledge is waning fast as I have very little requirement to use spherical trig, matrix theory & geodetic equations any more with push button technology.
