Large Scale Turning.

[Woody1]

Hi

I make large wooden spheres (from 400 - 1800mm diameter) and other large single piece sculptures. Examples here: www.philipjameswalker.com

I currently make the spheres using a plywood former, pinned on each end and rotated around to form a sphere. I remove material with an Arbortech, it is a slow process as you can imagine.

I recently came across this video: http://holamonstudio.com/holavideo.html which made me start looking into the possibility of building a machine to turn large spheres on. Unfortunately I am not an engineer, nor do I know anything about turning, therefore I am seeking help.

The machine in the video is quite unique in that it runs very low RPM and has a powered cutter/milling head, which is the complete opposite to a standard lathe. I have no idea how to approach this so please feel free to throw in some ideas, comments, questions etc.

Thanks


Philip

 

[jurriaan]

I'll touch on some points for you.

1) wooden balls with 1800 mm diameter are well and truly huge (as would be the tree you'd need for that one). That'd be 3 m3 of wood, or 3 tons of ball (so the raw tree would weigh even more). Still, lathes that can make something like that do exist: http://www.hager-drechseln.de/conte...3,dsp,0,1,0,0,0,0,-kopfdrehbank khde1800.html has 900 mm center height. Do be prepared for a steep price, they call it 'especially manufactured at customers request', and the smaller model KHDE 1130 carries a price tag of EUR 7100, with a ball turning jig at another EUR 3500.

2) Whatever you do, be sure to invest in a crane near the lathe, to allow you to move these huge blocks of wood

3) Building something like that lathe yourself may not be that hard, provided you can do (or know someone that can do) the metalwork. You'd need a large spindle (at least 3" diameter, I'd guess). At one end, you'd need a pulley or pulleys, at the other a connection to the faceplate. Tradtionally, threads were used here. On a large spindle, you'd use large threads. The KHDE 1800 lathe above has M56, so about 2 1/4". Or, a bayonet mount, like on large metal lathes. Since you're building a one-off, it doesn't really matter as long as spindle and faceplate match. Bearings to match the spindle, for radial and axial load (so 2 simple bearing blocks may not suffice), perhaps connected to a very large block of concrete (say 3 times the weight of a 1800 mm tree).

The problem comes with the tailstock, that has to move quite a big when you're turning from 400-1800 mm - at 900 mm centre height, it's going to be very heavy to move, and has to be aligned quite well with the headstock. The video has no tailstock (probably because it's hard to do), which means the spindle/faceplate combination has to be even beefier.

As for the powered cutter, the video does show how that's done...

All in all, a lot of work. Is there really a market for balls that big (meaning you'd have to hire a crane to install them, prepare the lawn to prevent them from sinking, somehow be abloe to access 1800 mm diameter trees) ?

 

[dickm]

From my experience with an Arbortech (scary beasts!) with the aid of a jig, they would remove wood about as fast as you could hope to do with a lathe with a similar sort of powered cutter.
Does the spindle need to be powered? Couldn't you use some sort of between centres mounting device which allowed the blank to be rotated by hand while using Arbortech mounted on a swinging arm to cut the outer surface? The swinging arm would need to be set up so it could be shortened after each pass to produce a smaller diameter until you reach the desired dimension. That's basically how ball turning jigs made for, much smaller, metalworking lathes are arranged.

 

[chipmunk]

I used to work for a company making radar turntables and they were turned on a big vertical lathe.
I'd have thought that if you have the option, having a rotating horizontal platform with the ball's weight helping to hold it still would be much more sensible for such a big object than trying to support all that weight between centres.

HTH
Jon

 

[Retire2004]

Hi Philip,
Nice pieces on your website. I am guessing the balls are about 400mm dia. I have a home-built lathe which is 500mm centre height and used mainly for large wall hangings, hollow vessels etc. Theoretically, this machine could produce spheres up to 38" (which is the biggest size shown on Keith Holamon's website). This would depend largely on the "daring" of the operator. I have just completed the building of a slow speed spindle drive (0-5 rpm with VSD) this, combined with a router system or captive angle grinder with arbortech style cutter should be capable of producing large balls in comparative safety.
For anything bigger I would tend to agree with DickM and go for a "skeleton" style lathe built mainly from 100mm box section or similar, running at very slow speed (between centres). This would largely eliminate the out of balance forces associated with normal turning and you could get away with a smaller spindle and generally less rigid construction.
KInd regards,
Tudor

 

[dickm]

Good point about the vertical axis suggestion. If it's essential to have powered rotation, then that might work with relatively low power. But still not convinced about the need for powered rotation if using an Arbortech or similar - maybe vertical axis and something like a potter's kick wheel mechanism?

 

[finneyb]

Good point about the vertical axis suggestion. If it's essential to have powered rotation, then that might work with relatively low power. But still not convinced about the need for powered rotation if using an Arbortech or similar - maybe vertical axis and something like a potter's kick wheel mechanism?

+1

Plus - support the cutting device on two arms, one either side of the ball, pinned at the centre height to allow rotation in the vertical plane ensuring that you get the correct sphere radius in the vertical plane.

Brian

 

[Woody1]

Hi, thanks for the replys.

I put 1800mm down as that is the largest sphere I have made to date, it was from a very large Oak tree. But you're right, realistically requests for that size are going to be rare, and when they come in my current method works. If a little slow.

Perhaps I am better off concentrating on a machine that will produce a sphere say 400 - 1000mm. Tudor, I would be very interested to see your setup, I will drop you a pm.

I have pondered the idea of a vertical mill. As you say it makes supporting the piece much easier. However I couldn't think of a good way to mount and operate the cutting unit. Bearing in mind that the center height will vary with different sized spheres.

 

[jurriaan]

Make a sturdy pole to the back of the verticall mill. On that pole, a bar can slide up and down, and rotate on the sliding part. On that bar, make a sliding attachment with a powered cutter (and perhaps a weight on the other side for balance). If the rotating bar is balanced, you can slowly rotate that bar by hand and make a sphere while the vertical mill is turning the tree around. For spheres up to 1000 mm, the pole needs to be at least 500 mm high, and the rotating bar at least 500 from that pole.