I am wanting to prepare some logs for eventual hollow forming blanks. I have read discussions on various forums on taking out the pith, much as you would for prepping a bowl and for leaving the pith in or alternatively drilling it out, stopping just shy of the bottom. I have done a few pieces with leaving the pith in and have succeeded in getting a good hollow form, however I have not turned enough of them to go one way or another with the wood prep. The logs I have now are white oak and cherry and the diameters are such that I would like to leave the pith in so I can get a larger diameter hollow form. The diameters are now 8 to 10 inches, cutting the pith out would leave a small hollow form. Is there anyway to keep the pith in and not loose many blanks to cracking. Your thoughts, gentlemen?
My friend John Jordan used to drill out the pith. turn the hollow form. then after it drys, true up the hole and insert a matching plug in the bottom. If done carefully no one notices. I have done the same thing and it works well. If you part off part of the bottom and then save a thin slice you can put the knot or pith back in. If I'm going to do that I cut the plug about 3/8" thick and dry it very slowly by putting it in a plastic bag and turning the bag inside out every day. It takes several months to dry it this way but you get a pith with no checks.
Neither white oak or cherry are known for being easy drying species. Cherry is the worst of the two. I'd say you have to go pretty thin walls, and get to turning quickly if you leave in the pith. Letting it set around will almost certainly result in cracks.
I agree with John and Dick that cherry is best turned green, particularly if you leave the pith in. I have had some success with with pith-in pieces by turning them quite thin and dimple-ing the bottom in like a red wine bottle. I takes a bit of fuss, but it allows the pith part to bend instead of splitting.
Another trick for keeping the pith in wood is to make it cone shaped. Dave Barriger taught me that. If the wood is flat such as taking a slice off of a log when the inside dries and shrinks and moves the outside is still larger and isn't moving and something has to give, usually the wood checks. If the wood is thin and cone shaped when it starts drying and starts moving it will get narrower and taller and if you've turned it thin enough it has less chance of cracking because the wood can move.
Like Dennis, I have had success with dimpling the bottom like a red wine or champagne bottle. Once you get the idea its almost easier than turning it full depth. It actually shortens the inside. Turn it green.
2: Thin, or inverted cone bottom (which needs to be thin, too!)
3: If you want a flat'ish bottom, drill out the pith, and prepare a small plug from the adjacent chunk of wood, cut from right next to the eventual bottom.
In option 3, you dry the little plug, so you'd be best to dry the vessel before gluing in the plug. Otherwise, can you live with a hollow form turned green to finished thickness and leave it at that? I do this all the time, and usually don't ever turn them again. I typically wet sand these to 320, and slather 'em with Watco or Urethane Oil -- even though they are still damp and cool to the touch. You can blow some water out with a compressor, or use a hair dryer to dry the surface a bit (I've never done this), but surprisingly, unseasoned wood can absorb an oil finish (little trick I learned from Luke Mann).
Presentation pieces, like Funeral Urns, I do turn twice, but that process means you have to leave things thicker than you can get away with if the pith is still in there. If you feel the need to turn them twice, drill out the pith, no matter what you do at the other end of the process.
Some folks want everything to be perfectly round and precise.... I feel like wood is simply the wrong material for that
I haven't turned too many hollow forms, but they were usually maple. i found that if I turned them and then soaked the bottom with thin CA glue, they didn't check. Cherry and oak may behave differently.
Great question -- I think it depends on the species of wood, and how much it moves, as well as the size (diameter) of the piece. Probably 1/2" to 5/8" would do for a 6" diameter hollow form in most woods, but maybe up to an inch on Apple, Beech and other high-shrinkage species. Maybe John will weigh in on hole sizes, too.
From what I gather, no hole needed at all for Honey Mesquite, since it barely shrinks at all, but I don't have any pieces big enough for a H.F., and it doesn't grow anywhere near here
