Hello, I have a large walnut bowl blank (actually two). They are around 18" in diameter and I suspect weigh about 45-50 lbs apiece. I have a PM
3520 and an 8" Oneway faceplate, and a SuperNova with the power jaws (3"). I am a little nervous about turning something this large, but I want to! I usually put the face plate on the top of the bowl and cut the outside and the fixing for the chuck and then turn it around and cut out the inner portion of the bowl. I will use the tail stock as much as possible. Any other considerations I am not thinking about? I have a new Sorby bowl gouge and a wolverine jig. I want this to be pretty, so any help you can offer I'd appreciate.
When I get these I also have two large Aromatic Cedar (17") and four
Consider which is more important, big or pretty. Be willing to make a pretty 14" bowl instead of wood-saving 18" bowl. You will be happier in the long run.
Big pieces that are out of balance can be miserable to get roughed into shape and balance starting at 120 RPM. Every minute spent with a chainsaw or bandsaw before the blank goes on the lathe will pay you back later.
I like to start between centers, making use of a pin chuck or pin jaws. Others claim they need the "artistic freedom" to reselect their axis after preliminary turning, and so use their spur center. A Google search shows a number of these to be the same who tell horror stories about blanks departing the lathe at speed. I have cut and split a few hundred cords of wood, so I can tell with reasonable certainty what is available in the log. When I make the preliminary cuts I reveal more, and can examine still more as I begin to rough on the bandsaw. Aside from modifying the original concept of the piece to avoid a bark pocket that went too deep, that suffices, and I've _never_ lost a piece held with pin and tailstock. Also, I have never made _the_ piece, and probably never will. Each is yet another, and if it's not perfect in my mind, the next will be better.
You can get pin jaws for your Nova, and they give you a few other creative options as well.
I don't know about variable speed, having only four on Ol' Blue, but if it bounces, I also believe slowing can help. Other options include removing some wood with a gouge, scrub plane or forstner bit to equalize the weight. It does, of course limit you, as wood so removed cannot be returned. If you have a real pin chuck
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youcan mount a plywood disk with holes to bolt counterweights into ahead ofyour piece. Be sure to use fiberlock nuts or add some CA to keep them fromunwinding. I suppose the Cole jaws and long bolts available for your Novawould work as well. Last advice is to say don't cut the edge of the bowl, cut the bottom up. If you have the nose of the gouge alternating between wood and air, you're much safer if the gouge is supported firmly and close to the piece on the toolrest, versus some mixed strong arm/hip rest method.
You've already got some good tips. I have a 3520 and use pieces like that regularly. I have had pieces that I already chainsawed round, that I had to take back off the lathe and remove minor corners that were hitting the lathe bed. The 3520 is up to the job.
Use decent screws for the faceplate, such as stainless. I now use a bunch of 1 1/4 screws. I used to use longer ones but it easy to snap the heads when you use a 1/2" drill to drive them in like me. Using the drill with a nut driver is a big time saver.
Like the others said, make it as round of a blank as possible with a chainsaw or bandsaw. Take light roughing cuts at a slow RPM until it is round. Use the tailstock, and regularly check it to ensure it is tight.
Even after it is round, keep the RPMs fairly low. Big bowls have a lot of kinetic energy. Have fun.
Hi RP, Your query elicited some good advice from two excellent turners, and you obviously are no slouch either. Beginners should heed their advice and follow your plan. David's caution re not letting the blank's size dictate the form should be emphasized. This advice is hard to follow with expensive or even 'road kill' wood. As with other turning admonitions like the bother of persistent resharpening of tools, it is important for turning 'pretty'. I've made a lot of ordinary NIP pieces by trying to get the largest possible instead of wasting (a misnomer) wood by adjusting to get a smaller but nicer piece.
Remember the potential toxicity for flora & fauna by the walnut shavings. Also, for security use a face plate instead of a scroll chuck. No doubt about Jim's advice that you should back off from vibration speed when roughing big blanks. I might mention that going a little beyond vibration speed on a _balanced blank can often safely smooth things out. Please share the story of your life with big bowls. Arch
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