how to cut a spherical cap in wood?

August 9, 2015 9:22 AM

For a sculpture I I want to cut a spherical cap in a piece of very hard old oak, with a base radius about three inches and half an inch deep (i.e. a =3 and h = 0.5 in Wolfram's diagram).

The surface of the cap will be polished very smooth and should be very accurately spherical, but I can't think of a way to do this without a lathe or a drill press, which I don't have. I have a basic handheld electric drill but few other power tools. I could buy and use a chisel but think it would be hard to get a true sphere that way.

So, I'm asking for clever ideas please.

posted by anadem to Sports, Hobbies, & Recreation (10 answers total)

It would be helpful if you describe the dimensions of the piece you're starting with.
posted by humboldt32 at 9:43 AM on August 9, 2015

That would be tough to do by hand without a lathe, if you are looking for precision. What about purchasing a wooden ball and slicing off the appropriate amount? unless the piece of wood you have is precisely the one you want to use. You can find wooden balls online pretty easily, but if you mean that the resulting piece will have a radius of 3" at the cut line and will be 1/2" high, you'd need to start with a pretty big ball.
posted by BillMcMurdo at 9:45 AM on August 9, 2015 [2 favorites]

Best answer: I've never been, but Google shows that Santa Cruz Hackerspace exists. You might try visiting someplace like this (and I'm sure there are others in your area, given that you're in the Bay Area) and seeing if you can purchase some time on a lathe, to make cutting this shape precisely easier.
posted by Alterscape at 10:01 AM on August 9, 2015 [2 favorites]

Best answer: Hire someone to turn it for you, or find a lathe you can use somewhere, get some basic instruction, practice on non-precious wood until you gain competence, and then make the part. No improvised solution that actually works reasonably well will be any easier than either of those two paths.

I could buy and use a chisel but think it would be hard to get a true sphere that way.

You wouldn't just be buying a chisel, you'd be buying sharpening stones and spending a lot of time learning to sharpen, and yes, it would still be very, very difficult to achieve your aim this way.
posted by jon1270 at 11:15 AM on August 9, 2015 [3 favorites]

If you have a turntable sculpting stand, you could build an adjustable jig to hold a wood rasp in contact with the wood while you spun it on the stand, but I don't even want to think about how many hours that would probably take. You could probably build your own springpole lathe in less time.
posted by bricoleur at 11:20 AM on August 9, 2015

The hole saw method listed here may be doable?

http://hackaday.com/2014/01/24/fun-with-wooden-balls/
posted by onya at 11:21 AM on August 9, 2015

To do the work without a lathe or drill press, you may have to build some jigs, or throwaway tools to help you get the job done. Cut the profile of the cap accurately in negative from a piece of thin, durable material. You will use this to fit over your work object, to show you where you are high or low.

You might need a miter box, to help you cut straight planes with controlled angles, and a back saw, to use with the miter box.

You might also need a hole drilled in the center axis of the work piece, so that you can insert a peg and block arrangement, so you can register the work object in the miter box. The hole can be discarded by cutting it off of the base when you are finished with the object.

The general approach is to approximate the spherical outline with planar cuts, leaving a faceted object that you will then smooth with fine sandpaper, using the aforementioned profile jig to guide your effort. The more planar cuts you make, the less material you will have to remove with abrasives.
posted by the Real Dan at 11:34 AM on August 9, 2015

Do you have a table saw and a lot of time (and perhaps a few too many fingers)?
posted by Poldo at 1:38 PM on August 9, 2015

Interesting problem.
Here's how I'd approach it. In this article, there is a jig for cutting dowels which is a router riding on a box with a hand crank to turn the dowel.

Imagine instead that you change the router bearing surface to the end opposite the crank and replace the crank with a drill. Then make the bearing surface a half circle and make a matching one for the router. You rough cut the oak into a half sphere, drill into the face center with a 3/8 bit, being careful to get it normal to the surface (maybe drill before you rough it) and glue in an 3/8 dowel so you can mount the whole thing into the chuck of a drill. Spin it on the drill in the guide at a slow rpm then use the router with a rounded flute bit to shape it into a half sphere. Sand it with in the jig then cut it free from the jig, by maybe cutting slits on the drill jig (when you make it - not after) that you could run a hacksaw blade through it to use as a parting tool.

Mind you, it will take about four times as long to make the jigs than it will to make the dome. The challenge will be to mount the drill solidly so that the axis of an object chucked in it will be normal to the jig surface. Use a plug in drill with speed control and a button to lock it turned on. The router bit should start well above the surface guide on the router side of the jig and you'll make multiple passes, lowering the bit, say, 1/8" at a time sneaking up on the final radius.
posted by plinth at 8:28 AM on August 10, 2015

Response by poster: Thanks for suggestions. My description forgot to say I'm aiming for a concave cap, not convex, so a lathe seems the most likely solution, though I'm not sure it will be possible without marring the rest of the piece (it's a sculpture).
posted by anadem at 7:26 PM on September 17, 2015