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Thread: Turning spheres part 8: Re-turning green spheres after drying, sanding, finishing

  1. #1
    Join Date
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    Turning spheres part 8: Re-turning green spheres after drying, sanding, finishing

    Re-turning green spheres after drying: As I said in the “blanks” installment, I often turn spheres from green wood, allow them to dry, then true them up again. I’m pretty convinced that rough turning the spheres gets you to dry much faster than drying the blank, which makes sense not only because there’s less wood to dry, but the spherical shape brings end grain closer to most parts of the sphere than if it were a cube, and moisture loss is far faster through end grain than side or face grain. While I can’t offer data to prove it, drying in the spherical form should result in less shrinkage related cracking, as the degree of deformation required for drying is reduced relative to the cubical (or spindle blank) form.

    If you’re green turning spheres, don’t spend a bunch of time getting the green sphere perfect. The goal here is mostly to remove excess wood, not to get a nice sphere. I would be hard pressed to imagine any reason to turn a green sphere beyond the third axis; even if there’s some tearout or other defect in the surface, I would leave it, as you might not have to cut as much away after the sphere deforms in drying, and there’s no way you’ll have to cut more after drying than before to remove the defect. I coat the spheres with anchorseal and put them in a brown paper bag to dry. There’s more discussion of drying green spheres in Part 3.

    Because of differential shrinkage along the three axes of the sphere, the sphere will generally be out of round on all three axes after drying. I’m showing here a picture I showed before, of a sycamore sphere that was turned green and dried; I overlaid a perfect circle to show the deformation.

    Capture85.JPG

    Truing up the sphere is done by the same iterative turning along the three axes. In theory you could start truing it up along any of the three axes (or any other), but I believe the best option is to start with the end grain facing the compression chucks. This is the same orientation you started with when roughing the original shape, the traditional spindle orientation; if your equators are still visible, this would have the single line equator in the center between chucks, and the 2 and 3 line equators crossing under the chucks. The reason I choose this orientation is that the most important thing in restarting the process is having the sphere centered in the chuck, and the curvature around the end grain should be the one that is least distorted by drying shrinkage, and therefore easiest to get centered.

    Cut away material until you have a continuous cut around the sphere. The work is back in spindle orientation, so the downhill cut is from large diameter to small. If you positioned the sphere as I suggested, the longest axis of the sphere is between the centers; as a result, when you’re round in the area you’re cutting, you won’t be spherical, you’ll have more of ellipse if you look at it from above, even though it’s round. This is to be expected, and in the next rotation you’ll cut off the excess along the end grain and bring it more to spherical. When you’re round, remark the first equator with a single line, and rotate the piece so that the face grain is against the compression chucks and turn to round. This will take off the excess on the end grain parts of the sphere. Mark the second equator and move to the third axis and so on until you’re spherical enough to suit you.

    Sanding: As described, I like to start with the finest grit I can get away with. For me, that’s usually 150 or 220 – it depends on how hard the wood is and how good a surface I’ve left. One of the qualities I really like in my finished spheres is a glassy appearance, though how glassy is partly a function of the wood used. To get this, you really need to get “under” surface defects left by your tools. As I explained earlier, I prefer to cut out the defects to the maximum extent I can – this keeps your sphere rounder than if you fix defects through sanding, and generally reduces sanding time, which is always a good thing in my book. Because the resistance of the wood to sanding differs across face, side, and end grain, excessive sanding can take off more wood in some areas than others, bringing your sphere out of round.

    For sanding, I hold the work in the same compression chucks, but I don’t tighten down much at all, just enough to hold the sphere. Gripping too tightly can compress the wood of the sphere and leave a mark. For most woods, I can hold the sphere in the chucks directly, and avoid leaving a mark by using minimal tailstock pressure. However if you’re turning a really soft wood, you may want to pad the sphere (see pictures). I use some black foam rubber that I tape to the ends of the compression chucks. It’s harder to get the sphere sitting true in the chucks with the pads on, but if you don’t turn the work too fast while sanding it’s not a problem if it's a little off center; turn the work slowly enough that the sandpaper stays on the work.

    Capture112.JPGCapture113.JPG

    It may be overkill, but I sand each grit on all three axes. There’s a reason, but that doesn’t mean it’s necessary. I start with the face grain facing the compression chucks. The reason I start here is that the face grain is usually the softest, so I do most of the work with a new grit when I’m sanding end and side grain. Because I keep rotating the axis I’m sanding on, every time I sand the scratches from the previous sanding (whether the same grit or a coarser grit) are running 90 degrees to my current sanding, and it’s pretty easy to seen when you’ve sanded out the previous sanding marks. I do most of the work on a new grit with the face grain against the compression chucks, then I move to the putting the end grain against the compression chucks. There are only two small patches of face grain that haven’t been sanded at this point, so it usually doesn’t take very long to finish sanding on this axis. Then I rotate the sphere again to put the side grain against the compression chucks and sand briefly (usually) again. I finish with the side grain against the compression chucks because in this orientation, you are sanding parallel to the grain when you come over the face grain, just as you would finish sanding flat work. Then I rotate again to having the flat grain against the chuck, and start the process over with the next finest grit.

    Everyone has their preference for sanding sequences. As I said, I try really hard not to need anything coarser than 150, but if you’ve got tool marks that are hard to remove, I think you’re better off biting the bullet and starting with 100 than sanding a long time with 150 to get them out. From there I use 150, 220, 320, 400, 600, and finish with 800. I go this fine because I really like the spheres to shine once they’re buffed, and since there’s little finish on my spheres, sanding to fine grits is necessary to getting that shine. I have 1000 grit paper too, but I haven’t seen much benefit to going beyond 800 when you’re finishing up with buffing.

    If I can get on a soapbox for a moment -- sandpaper is a cutting tool, and it gets dull like any other cutting tool. Even at 800 grit, sandpaper should not make the work shiny, it should make it dusty/powdery looking. If your sandpaper is making the sphere look shiny, it’s either dull or clogged with sawdust, change it. While you’re sanding, keep the paper moving, not just across the sphere, but be constantly changing the part of the sandpaper that’s in contact with the wood. This avoids buildup of both dust and heat, neither of which are good. If your hand is getting hot, you’re either pushing too hard, turning too fast, or your sandpaper is dull. Dull sandpaper makes the surface shiny by burnishing rather than cutting, and the grain of a burnished surface will raise badly if it becomes moist later on.

    I change sandpaper very frequently. Except when using disc sanders, I use almost exclusively sticky back (PSA) paper in 4.5" rolls. For spheres, I cut off about an inch and double it over on itself so there’s grit on both sides (this makes the paper stouter and easier to hang onto because there’s grit against your fingers). Unless I sand a sphere of pretty soft wood, I usually use one face of that paper for only one sphere, and use a fresh face for the next. I think it gives you a better finish and is definitely faster to use fresh paper. A ten yard roll costs $15, so that’s about 4 cents per inch. Going through 6 grits I use the equivalent of 3 inches of paper or 12 cents worth of paper. Could I use it longer? Sure, though I’d have to sand for longer. For me it’s penny wise and pound foolish to try to stretch the life of sandpaper.

    Finishing: Finishing spheres has the challenge that there’s no surface to set it on while you finish the other side, and no break point where coats of finish on different parts can meet without being noticeable. For that reason, I don’t use film finishes on spheres. My usual is just one liberal coat of Danish oil (Watco is what I use, but that’s not because I necessarily think it’s any better than another). I coat the sphere well and keep it wet if it soaks it up. After 5 minutes or so I wipe off all the excess and let it dry. With this amount of oil applied, it doesn’t really build up on the surface of the sphere, it just soaks in, provides some optical depth, and brings out the chatoyance of woods that possess it.

    For open pored woods I have experimented with applying Danish oil on the lathe and using 800 grit paper to wet sand it in, creating a slurry to fill the pores. It worked to some extent, but it hasn't become a standard practice for me. In some ways, open pores are part of the character of the wood, as opposed to a defect to be eliminated.

    Danish oil is not a glossy finish so to make the spheres shine I buff them. I have the three wheel Beall system (8” wheels), with the tripoli, white diamond, and carnauba wheels. In the case of spheres with just a single coat of Danish oil, you’re not really buffing the finish to a shine as much as you’re buffing the surface of the wood.

    I have also used Shellawax cream (preceded by EEE shine, a companion product) to finish spheres, using the compression chucks to friction polish the Shellawax on. I have been able to achieve a seamless finish with Shellawax; you have to rotate the sphere in the chuck to cover all the sphere area, but I haven’t had problems with having multiple coats blend together. I’m not crazy about Shellawax cream because it seems easy to get too much on there and end up with gooey wax streaks that take a while to get back off, but that might be user error. I’ve thought about trying the liquid version of Shellawax but have not to date. Since Shellawax works, it seems likely that other friction polishes might as well. The issue is being able to blend coats applied to different parts of the sphere.

    That’s all, folks: That’s about all I can think of. I can’t imagine there could be any questions after all the yammering I’ve done, but if there are any, let me know. I might follow this up with a post that’s a gallery of spheres, but then again, I might not have the mo to pull that together. Thanks to those that persevered through the whole thing.

    I'll close with a recent sphere -- Russian olive, from a blank given to me by a generous turner I met through this bulletin board.

    Capture115.JPGCapture116.JPGCapture114.JPG

    Best,

    Dave
    Last edited by Dave Mount; 06-08-2021 at 10:39 AM. Reason: typo

  2. #2
    Dave,

    This series has been absolutely fascinating. Thanks for taking the time to put it all together. When I read the 1st step, I was thinking "Why would anyone turn a sphere?". Now I'm thinking I'd like to try it.

    Bryce

  3. #3
    Join Date
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    Thanks Bryce. Be forewarned, I thought I was doing a one-off myself. . .now my wife left me, I lost my job, the bank is foreclosing, and I just sit in the shop and make spheres. . .(JK)

    Dave

  4. #4
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    Quote Originally Posted by Dave Mount View Post
    Thanks Bryce. Be forewarned, I thought I was doing a one-off myself. . .now my wife left me, I lost my job, the bank is foreclosing, and I just sit in the shop and make spheres. . .(JK)

    Dave
    If you end up homeless you can put a cot in my shop. Wake up to lathes, tools, wood. The peacocks will make sure you don't sleep too much.

    JKJ

  5. #5
    Join Date
    Oct 2013
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    Northwest Indiana
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    Quote Originally Posted by John K Jordan View Post
    If you end up homeless you can put a cot in my shop. Wake up to lathes, tools, wood. The peacocks will make sure you don't sleep too much.

    JKJ
    I’m not likely to become homeless, but that’s a very tempting thought!!

    @ Dave Mount—awesome series, thanks for taking the time to do that!!
    earl

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