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Thread: Turning spheres part 6: Complete turning on second axis

  1. #1
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    Turning spheres part 6: Complete turning on second axis

    So as we begin cutting, we're spinning an irregular shape as shown here:

    Capture98.jpg

    Because of all the extra wood left on the ends of the workpiece in the prior step, the first cuts in this new orientation are pretty bumpy (very intermittent). I take small cuts using more of the tip if the bowl gouge to avoid engaging too much cutting edge. You can make this cut with or without bevel support. To do it with just the tip and bevel support you really have to swing the gouge to what for me is an awkward position, so I just make light unsupported cuts at first. If you go too hard, it you can cause the workpiece to slip in the compression chuck and possibly end up out of round. You learn over time how hard you can cut. It also depends on how soft the wood is, both in terms of the impact of the cut, but also in terms of how much the workpiece can deflect in the cut by compressing the wood around the chuck. If you bang into the sphere hard enough to cause it to move in the chuck, best to stop and reposition it as you may have moved it out of position with the equator in the cups, and you need that orientation to monitor your progress.

    You'll see in the ghost where most of the excess to be cut off is, generally nearest the middle. Start the first trimming cuts not too far from the middle and come back toward the center, starting farther from the middle as the ghost is reduced. Alternate sides to keep them both in generally similar stages of rounding. After most of the excess is removed and the ghost is becoming a thinner layer on the sphere, I change the cut orientation to ride the bevel on the sphere, making a shearing cut with the wing of the tool held at an upward angle. The shearing cut gives a cleaner cut and the greater (and flatter) bevel area helps keep the tool from bouncing so much. The picture I have of this tool orientation was posed with one hand on the tool and one on the camera, and the orientation of the cutting edge is too vertical; it should be at more of an angle (the tip of the tool farther right, handle farther left). I make the cuts by keeping the bevel rubbing and moving turning my whole body as I bring the tool around the face of the developing sphere. You’re cutting about 60 degrees of arc so you need to move a lot. I wait to use this orientation until some of the waste has been removed and a greater proportion of the rotation is supported by the wood. If I do this earlier, I usually end up engaging too much cutting edge and the gouge starts to bang against the work.

    Capture102.JPG

    I haven’t said anything about turning speed, in part because I don’t want to start a debate about what a “safe” turning speed is. I turn spheres pretty fast. The reason is that when doing interrupted cuts, the faster the work is turning, the less time there is for your tool to fall into the void when cutting, and therefore less bounce in the tool. That said, you are working on something that is not held terribly tightly; a bad catch could pop it loose, and if that happens, the faster it was spinning, the more kinetic energy has to be absorbed in ways that could well be unpleasant. I use sharp tools and light cuts as a hedge against this. Turn at a speed that is comfortable for you. The best woodworking advice I ever heard was, “if you’re doing something and it seems like it might be kinda dangerous, it probably is, don’t do it.”

    When the visible ghost is mostly removed, it’s a good time to stop the lathe and check the symmetry of your progress. On most woods you can see clearly where you’ve been cutting (across the end grain) and where you haven’t. In the pictures below, I drew a pencil line along the margin between cut and uncut wood (middle and right panels). What you want to see is that the cut area is approaching the equator more or less evenly from both sides, and that the distance from the equator to the cut is similar on both sides of the workpiece. In the example I show, things are looking pretty good. If things are not symmetrical either top to bottom or between the two sides (and this happens), then either the initial shaping (in the last step) was not symmetrical, or the workpiece is not centered in the compression chucks, or some combination of the two (actually, asymmetry in the first step will cause the workpiece to settle offline in the compression chucking). If something seems off I try reseating the sphere in the chuck and cut a little more (you can scribble pencil on the sphere to help you see where the cutting is taking place after adjustment). If adjusting the chucking improved things, great. If not, then I would shrug and continue cutting; the process will still produce a sphere, it will just be a tiny bit smaller than planned.

    Capture103.jpg

    Resume cutting material away, advancing the cut area toward the equator. I find that the ghost becomes invisible (to me) well before the piece is actually round. This is presumably because as you approach cutting evenly around the full circumference, the amount of time the wood “isn’t there” is so small that the image appears solid even though it's not entirely. As you approach fully round, you can switch from watching the ghost image to feeling the vibration in your gouge. You’ll feel the “rattle” of the gouge in the interrupted cut, which will decrease as you near completion. If you’re brave, you can also lay your finger on the spinning workpiece and feel the degree to which it is still out of round. Stop the lathe occasionally to assess progress. You may see that you’re closer to the equator in some areas than others; make a note of that and reduce the amount you cut in those areas to keep things even. In the pictures below you can see how the cut is approaching the equator on both sides of the sphere as turning continues. In the right panel, I’m really close, just a small flat spot just left of center. If you put your finger on the sphere while it was spinning, you would be able to feel the difference between the area where that little flat is and where it's completely round.

    Capture104.jpg

    Most sources of instruction I've read say you must leave the line. I guess in theory this is true, but in practice it’s often not how it works. The difference between leaving the line and leaving a flat spot is infinitesimal, and would also require absolute perfection in tool movement across the sphere to get the flats completely out but not take the line anywhere. This is not reality. The goal is to just kiss the line, which really means that you’re going to barely skim it off in places, and leave a little in others. This is fine. Just do it as evenly as you can. In the right hand panel of the picture above, I'd just take a very light cut over that flat spot in the middle of the left half and call it good. Your tolerance for flat spots generally decreases as you become more practiced at the process. A small flat like that would likely be almost, if not completely, undetectable by someone handling the sphere after sanding.

    If the workpiece was the slightest bit off center in the compression chucking (not uncommon even after considerable experience), you’ll cut the equator line on one side off completely before you reach the equator on the other. In this case you should keep cutting until you reach the equator on the second side; if you don’t do so now, you’ll have a persistent flat that you’ll have to cut off at some later point. Might as well do it now.

    I watched a Youtube video by a very talented turner that made these wonderfully creative segmented spheres. While his designs and finished products were terrific, he recommended in that video that you stop short of the equator line before re-orienting the workpiece, with the idea that you kind of incrementally come to round. This sounds reasonable, but I don’t recommend it. The reason is that if you leave a slight flat instead of a fully round surface, it increases the risk that when you go to the next rotation, the piece is not going to naturally settle into the chuck centered, because it’s not hitting the compression chuck evenly all around (because it’s not fully round). I recommend going to fully round before rotating the sphere again. But there are many paths to successful sphere-making.

    When you have gotten to round (as indicated by your cut reaching the equator, on the second side if one happened before the other), the last thing to do is mark a new equator (this would actually be the grand meridian if you’re thinking about the earth as a sphere and we marked the equator in the first step). To help track things, I make two lines on the second equator. Also, if I’ve cut away much of the first equator in this step, I may sketch it back in by hand. It’s not important that it be really accurate, in just helps align things for the next step.

    Capture105.JPG

    Having gotten this far, the work looks spherical, but it's not yet. Because the chucks limit how far toward the centerline you can cut, there are still imperfections in and around where the chuck contacts the sphere. In the next step, the sphere is rotated again to bring those around to where you can turn them off.

    Best,

    Dave
    Last edited by Dave Mount; 05-25-2021 at 11:43 AM. Reason: typo

  2. #2
    Dave, you need to do a video!

    robo hippy

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