On Wed, 20 Apr 2005 14:36:27 -0700, the inscrutable "J" spake:
Seconded!
========================================================= Save the Whales +
Indeed I am. I could be a girl though.
Well, it's better than no answer at all.
I'm probably going to try number 4, but might also talk to a aquintance who is a professional turner, to see if he can do me a favour on his bosses expensive lathes.
Thanks for all the answers, my head is still spinning. It's almost elleptical now.
It may not be truly elliptical but it will look more like an ellipse than a circle.
No, this won't form an ellipse, but with proper layout it can form a reasonable approximation. I did a quick sketch in AutoCad to see what could be done. Start with a cylinder 4" in diameter and offset it ~1.3" at both ends and turn .75" of material from the highest spot. Have extra length and do NOT turn all the way to the ends , because the offsets are outside the finished ellipse. Now offset if again 1.3" from the original center but in the opposite direction as the first offset. Again turn .75" material from the highest spot. This will leave 4 lines the length of the "ellipse" where the original cylinder and the two offset turnings intersect. These lines are about 0.035" proud of the true ellipse and should be easier to hand sand to blend the curves. This is still not a true ellipse, as you go from the major diameter to the minor diameter the piece is a bit shy, with the maximum deviation being about 0.05". This might be good enough for his purposes. Hope this helps Martin
Consider a circular cylinder.
Consider what you get if you slice that cylinder at an angle.
Now, construct a cylinder perpendicular to that angled cross-section slice.
You have, obviously, and elliptical cylinder.
It should be "obvious to the casual observer" that you can take an angled slice of _that_ cylinder in a manner that will yield a circular cross- section. That cross-section is perpendicular to the minor axis, and at an angle to the major axis such that the 'diagonal' is the same length as the major axis. Mathematical "proof" gets a little messy, but is fairly straightforward -- take the equation of an ellipse, parameterized as a function of the major and minor axis lengths, set the two lengths equal, and "reduce".
"A quantitative answer is left as an exercise for the student." *GRIN*
Right, ... so far, so good.
You just broke my brain.
Or, he could just make a template, turn it close-enough, and fine tune it to fit.
Hm. Sometimes, "close enough" is, well, close enough.
Interesting idea. Seeing your note spurred me to do the same thing with another approach. Starting with a 2-1/2" x 4" rectangular blank, and with the table saw blade or band saw table set at 30 degrees, cut an elongated hexagon with sides tangent to the ellipse. The resulting points to plane off vary from slightly under 1/8" to slightly under
1/32" proud of the ellipse, with the worst ones being on the flat side. Cut four 15-degree bevels with the stock vertical before moving to the 30-degree bevel, and there is really very little planing to do.As someone else said, lots of good approaches to this one.
True, but irrelevant to what I understood to be suggested. I think the offset idea was to turn two circular arcs of relatively large diameter, which combine to form an elongated curved pointy-ended shape that then can be further worked with planes, scrapers, sanders, etc., to approximate an ellipse. As I understood it, both ends of the leg were going to be offset in the same direction, thus the concern for caution with inherent imbalance.
I think what you are talking about can be used to cut a circle in a single slanted plane, which will be useful as a gauge cut for the ellipse. But the ends of the leg will have to be offset pretty darned far from the lathe center if the leg is of any length.
He just makes it sound tough. Shine a flashlight straight down and it lights up a circle. On an angle it's an ellipse; has to do with trig if anyone insists. It still doesn't solve the probelm. Beside most layout is based on some hefty math, but the layout is to make it all easier. That's what got perspective geometry into the game, making tough to impossible jobs relatively simple. An ellipse can be drawn using two concentric circles and lines form the center ...etc..
But this doesn't in any way make "turning" a rectangle into an ellipse over a length of material any easier. There might be some giant industrial shaper around that would do it. The ellipse is symmetric ...the same in four sections... so only one section needs to be dealt with at a time, then times four. Baring an industrial machine, I'd go for approximate cuts on an oversize piece using the table saw, then hand plane then sand as was suggested earlier.
There are no simple solutions to turning an ellipse, or an oval for that matter. You might get some ideas from these websites. Dan
My oval turning lathe nearing completion
longer piece of wood? Say 75 cm (30")? Or would you need a similar contraption on the other end, linked/synced to the one closer to the lathe?
Piece of wood+drawknife+spokeshave=eliptical leg.
thanks! Yes, you'd need something on the other end for spindle turning unless you were OK with it gradually turning from an oval to a circle by using a standard dead center.
At some point carving it by hand is the cheaper way to go. Of course its a little difficult to make a bead or cove using only a drawknife, but your point is well taken nonetheless.
Besides carving...
a person could also fabricate this from gluing up multiple sections of split turnings...
or use a copy lathe like they do for oval axe and hammer handles...
or program a CNC lathe...
or turn a hollow tube and steam bend into an oval by squishing it in the middle...
all sorts of ways! Dan
If I were to do this, and I didn't want to hand carve it, I would likely do it on a CNC vertical mill. "Didn't suggest that as few people have that at their disposal.
It's not elliptical, really, and in any case needs some handwork at the end to blend it, but you certainly could get "close" with 3-center turning, the same way many "oval" things are turned on regular lathes.
You tun on center to the profile you want the "fat part" (long axis of the oval/ellipse) to be.
You go offcenter and turn one side down to (or as close as you want it to) the centerline. You turn the other side to match. You're a lot closer to where you want to be when you pull out the inevitable spokeshave, and the evitable or not rasps, files, and sandpaper.
Try some test pieces - there's a bit of a trick seeing/feeling when you've gotten the sides where you want them.
A lot more accessible to most mere mortals than anything that will cut a true ellipse leg shape.
Or the really easy (but slow) way - turn the leg round out of green wood, and wait for it to dry. Differential shrinkage will get you quite a ways along with this approach.
I happen to agree with the rough-cut and plane folks. Here's my contribution: Use the old fashioned moulding planes known as hollows and rounds. The sole and iron of the plane are rounded to cut convex and concave shapes. They come in sets with different radii, but you will probably have the most luck finding them at a flea market. You will probably need at least two hollows (with a concave sole...for cutting convex curves) with radii that approximate the major and minor radii.
Speaking of ovals. My oval turning lathe is nearing completion. I've been working on it for over three years. And no, it won't do oval spindles! Dan
I believe the problem can be solved throught the use of a spindle drum. construct a drum to proper specifications of the leg, length. place recesses in each end plate to hold the stock then add a second end plate. Mount the number of legs you wish to turn in the drum attaching with a screw through each end. The drum should contain spacers between each of the mounting points. you turn you turn down to the spacer prior to mounting the legs. You can use stock of any size as long as the recess is properly constructed. I am being simplistic in my description but I am sure there are people out there who could help..
To turn an ellipse turn the stock to the desired thickness and shape. then remount each leg by tuning it over once. 180 degrees. the circumference of the drum will dictate the curve. Old europe spindle turners used this technique often and keep a variety of drums and sample patterns on hand.
That's a elegant solution, especially if you need more than one spindle. You didn't say, but it should be noted, that the two end plates are firmly attached to each other by a central shaft. It will make a bobbin shape. This insures that the end plates travel in unison. The bobbin shaped drum is turned between the centers. Actually, I'd use a face plate attached to the drive side plate. Dan
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