I took a stab tonight at making a sphere using the method Soren Berger showed in his video, but without his caliper. I just measured and marked with standard calipers using the ratios Richard Kuivila gave in the Front Range Woodturners newsletter.
I picked a piece of figured white oak about 2.5" for this first try. Not finished, just wet with a a bit of watco oil.
Not perfect, but it was a good test and now I understand where the numbers came from and what to do with them. It was real easy to turn and pretty quick.
OK not being a math person it sort of resembles dividing 3rds from various points. I've always just eyeballed a sphere and then done the between cup centers thing to get it really right. I looked at his formula's but have no idea how to plug those into a calculator. I find it interesting but will probably stick with the method that works for me and doesn't require any math.
I remember in College I had a math class required for teachers. The first day the guy spent 20 minutes drawing formulas on the board to prove that 1+1 + 0. I knew I was in the wrong class. Dropped it and took a metals class on casting. Much more useful for me.
Actually, it "turned" out to be a lot simpler than I thought at first. The only math is multiplying the diameter by a number then multiplying the diameter by another number. No need to even know why!
The confusing Richard Kuivila diagram looks a lot worse than it is. It, and the Berger calipers basically just divide the outside of the sphere into 8 segments, then later divide it into 16 segments.
The only math needed is to multiply the diameter of the sphere by the magic number Richard gives! Then by the second magic number.
The Soren Berger video shows the procedure:
- measure the diameter of the cylinder
- make the first marks, what will be the 8ths after turning the first flats
- make the second marks, the 16ths, turn the flats
Make the finish cuts. Done!
After trying it once, I'm sure I could teach even a beginner in two minutes. (Not counting the actual turning.) This is a LOT easier than the other manual methods I've tried.
BTW, I love the PM3520 lathe, even after a tiny bit of use!
Don't turn alot of spheres and but have always found turning a rough sphere between centers and then using wood cups to rotate several times and turn away the tenons and the shadow. Pretty easy and don't have to refer to a book or calculate anything.
Sometimes though it can be a hassle. If you remove more than just the shadow when you rotate the ball there is another shadow to be removed. then you over do that one and create another one. This keeps on going until your tennis ball sized sphere is now a marble.
