Perfectly spherical steel shells for wood bowl coring lathe

What's wrong with simply using a bar bent into a circle? Remove some metal at the appropriate edges to thin it while retaining the stiffness.

First drill/cut out a 2" or so conical plug inside the 3.25" bowl. Now, place a couple of bearings on a really, really thick bit of metal supporting a sickle-shaped cutter in the hole. The sickle shaped bar with cutter on the end is then advanced into the work to cut a hemisphere.

Any decent metal spinning shop can make them for you, but the spinning forms will be pricey.

"Ian Stirling" wrote: What's wrong with simply using a bar bent into a circle? (clip) ^^^^^^^^^^^ These are available on the market. Check with Craft Supplies, Packard, or any other vendor selling wood lathe tools. The cover a wide price range, depending on the diameter of the wood being handled, and the number of coring radii provided.

Of course, if you built your own lathe, you may also prefer to build your own coring system, but you do not need to re-invent the wheel. BTW, none of them use spherical shaped cutters--it ain't necessary, as Ian pointed out.

||Imagine an orange, with the peeling sliced top to bottom say, 8 times. Take ||one half of the peeling and that is what the cutter will look like.

Don't you mean 1/4 of the peeling (90 degrees of arc)? Texas Parts Guy

There was an article a number of years ago in Fine Woodworking mag about a machine that did this very thing, I believe the company was making wooden salad bowls. IIRC, the machine was quite old.

I remember thinking at the time the arms supporting the cutters were not as a beefy as you might expect they'd have to be.

Maybe someone has an index to back issues? A picture is worth a thousand or more words.

First of all, I have all copies of Fine Woodworking, including the issue with the coring lathe. Secondly, I own (and use) both the McNaughton System and a home made system for coring bowls. The problem (one of) with the commercially available systems is that you have to reduce the half log to a hemisphere, then reverse it on the lathe and core out the largest bowl. Then take the core, reverse it again, turn a tenon, take it off, reverse it back onto the lathe, cut the next core. Do this for each bowl. Takes a lot of time, and the cutters are kinda flimsy when they extend out over the support by 14-15" (measured along the curve).

The machine in FWW was over 100 years old, used !/4 arc cutters (same as I am looking for) and still works fine. But they have one guy whose full time job is to sharpen the cutters and maintain the machine. I don't anticipate making that many bowls, but I sure hate how long it takes me to make 3 or 4 bowls blanks currently.

Thanks for all the replies.

Regards, James

Here's a link that has a more efficient method of using the McNaughton System:

Automobile leaf springs would be a good cheap source of material. Regular hot rolled bar isn't very expensive either. If these are too narrow, just weld several together. You will need to anneal the springs first, by heating to red hot and allowing to cool slowly. Bend the bar to the radius desired, then use a hand held angle grinder to finish forming to the spherical shape. It shouldn't be too hard to hold the tolerance you are looking for.

I think 1/4 arc is overkill, but you can get this by welding in braces instead of using solid plate. Just bend the braces to the correct radius first.

p.s. You can also get 30% more bowls from each log by only allowing

10% thickness for each bowl. The 12" bowl thickness is about right, but each of the smaller bowls can be quite a bit thinner and still have enough meat to allow for shrinkage.