I had more to say about blanks than I expected so they're getting their own post. I promise I'll get to turning next, hopefully later today.
The blank: You can make a sphere out of any wood you wish. I’d suggest starting with something that’s completely sound, but you can make spheres that have voids or other defects. A wood that turns nicely will allow you to focus on the process instead of fighting with a difficult wood. If I find small checks in the wood (common with exotics that are often not dried in a controlled way), I’ll wick in some thin cyanoacrylate glue (CA or “superglue”), which will usually find its way down into even pretty deep cracks. If the crack is more open at the top, I’ll start by wicking in thin CA, then follow it with some medium viscosity CA; it seems like wetting the crack with thin glue helps draw the medium viscosity in, but the medium is thick enough to bridge something wider than hairline cracks. Be advised that a squirt of CA accelerator will set the CA on the surface, but it doesn’t necessarily set the glue farther down as quickly. If you spin the piece too soon, you’ll be flinging CA glue all over. Don’t ask me how I know this (hint: I have a pair of very foggy safety glasses and for a time had some odd divots in my haircut). If getting hit in the eye with wood isn’t enough reason to make you wear eye protection, then a flying stream of uncured CA glue ought to convince you.
Bigger voids get filled with epoxy, as in this example of a piece of golden chain (Laburnum) that was dried in the round and split as a result.
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I typically start with blanks that are square(ish) in cross section, and 1/8” longer than the shorter of the side to side measurements (though I don't recommend this for your first go at it). I do this because it uses less wood and I’m often working with expensive wood that I’m trying to conserve. Especially for small spheres, leaving so little waste creates some minor challenges that I’ll mention later. If you’re just starting, or if you’re using a spur drive and/or cone shaped live center, I’d suggest leaving at least ½” of additional length, maybe a full inch if it’s not an issue to do so. Using blanks as short as I do requires that you make cuts very close metal drive and live centers, which may be uncomfortable, especially at first.
I just mount the blank square without knocking off the corners on the table saw or bandsaw, but no harm in doing that if you wish. Roughing to round just takes a minute or to so I don’t bother. . .though now that I think about it, when I did a larger (5.5”) Doug fir sphere, I guess I did cut the corners at 45 degrees first.
I developed a “zero waste” method where I make a sphere from a blank that is just as long as it is wide, no excess length. To do this, I use double stick tape to put a small (e.g., 1” square) piece of 1/16” thick hardboard over the center of both ends, and then the drive and live centers bite into that instead of into the blank (pic below). I thin the hardboard down to 1/16” so that my center finder still works over top of it; it doesn’t have to be that thin for any other reason. Using thicker pads on the ends would give more room to work the ends of the sphere. One thing I did learn the hard way is if you’re adding pads with double stick tape, if the ends aren’t cut square and parallel, the angular pressure when you squeeze with the tailstock can cause the double stick tape to creep a little, putting the blank off center. Using CA glue instead of double stick tape would fix this problem, but I’m often applying the pads to wood that has wax on the end grain, which I presume would keep CA from adhering. While I was proud of myself for achieving “zero waste”, unless I’m really trying to squeeze something out of a marginal piece, I usually do the width plus 1/8” approach because it’s faster to not have to deal with adding pads to the ends of the blank.
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Chunks of firewood are a common source of sphere blanks and these don’t come nicely square. Rather than going to the bother to square up the blank. I just cut a section of the appropriate length (if it’s firewood, I’m usually not concerned about being stingy with the length and will cut the blank comfortably longer than the expected sphere diameter) and mount it between centers without additional prep. If there are some really wonky parts sticking out I might rip those off with the bandsaw first. To mount it, I take a compass and draw the largest diameter circle that I can on the smaller end, then locate the same size circle where it should land on the larger end, and use those as center points. The circles also help confirm that you’ve got the blank mounted where you want it if they are still visible as circles once you start the lathe. Firewood often has loose bark; pry it off or take appropriate precautions. My PM 3520B has the big swing down wire cage guard, and I always lower it when turning something with bark on it.
Dry versus green wood: Using dry wood is nice because you get the finished product right away. However, a lot of sources of thicker wood for spheres are wood that is green, partially seasoned, or of unknown seasoning; buying exotic woods can be a real crapshoot in this regard. In my experience, a blank that is roughed out to sphere then coated with sealer and allowed to dry will dry faster than waiting for the intact blank to dry. For that reason, my general practice is to do as one does with twice-turned bowls made from green wood; rough out the sphere green, let it dry, then re-turn it using the same 3-axis method to restore it to spherical. Most woods will go really eggy when they dry because they shrink to different degrees on the three axes. You can see that in this sycamore sphere that was turned green and dried; I overlaid a true circle to emphasize how much it deformed in drying.
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While the resulting "drunken sphere" can be kind of entertaining as a novelty, the perfectly spherical shape is generally part of the appeal of doing this, so I re-turn them after drying. Because green turned spheres are going to go out of shape when they dry, I don’t put a lot of effort in getting perfectly spherical in the initial turning; the goal is just to remove the bulk excess wood so what remains dries more quickly/easily. Coat the entire sphere with Anchorseal or some other suitable sealer, then put it in a paper bag. This sounds kind of weird, but don't mix spheres in a drying bag that are wildly different in their moisture content. The point of the bag is to allow the spheres to create their own micro-environment; really wet spheres make the air in the bag moist compared to the ambient air outside the bag, and this helps prevent drying too fast. If you have other, dry spheres in the bag, then they take up some of this moisture, which does the opposite of what you want for both dryer and wetter spheres. I have three sphere drying bags, basically "really wet", "nearly dry", and "somewhere in between", and transfer spheres among them as it seems appropriate. This isn't an exact process, just something I do when I happen to think of it -- often when I add a new wet sphere to the wet bag I might notice one that's been in there for a while and move it on to the next bag. For a while I was weighing the spheres periodically to track their drying progress, which can be interesting. I more recently developed a way of actually measuring their drying status, but that's a long post in itself.
Spheres are thick and prone to checking if you don’t dry them slowly enough. Some dense exotics are tough to dry slowly enough to avoid checking (bocote, bloodwood, muirapixuna -- I’m talking to you); I put two coats of anchorseal on those, and also put them inside a smaller paper bag inside the larger paper bag to further limit drying rate.
If I get a wet blank that I don’t have the time to rough turn, but want to get going on drying, I’ll cut the appropriate length off for a sphere, coat both cut ends well with Anchorseal, then put the cube in a paper bag as I would a green-turned sphere. More endgrain means faster drying (but not too fast or it will check). I use nail polish to apply identifying codes because few markers will write on wax, and the oh so pretty pink color shows up well on both light and dark woods.
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I promise the next installment will actually involve turning. . .
Best,
Dave
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