Turning a sphere
This is a commercial job done with fir which is a terrible wood to turn because of the tear out. I drilled a 2" hole for mounting the chuck
Here I have turned multiple rough blanks
Turn the piece to desired diameter and then using the tool rest draw a horizontal line. Move the tail stock out of the way and draw a line from the horizontal line through the center of the piece to the other side. Re install the tail stock, rotate stock and draw a line using the tool rest connecting it the line thru the center. You now have 2 horizontal lines 180 degrees apart.
Mark three lines, the outside 2 are equal to the diameter of the stock and the other one is centered between them. Here I have mounted the faceplate with what I believe is called a carriage screw in a oak block. Since this ball is to mounted on a base I don't care that there is one bad point where the stock is mounted.
One of the 2 screw chucks I made for turning spheres.
For the points where you want a finished surface you have to keep the mounting points from reaching the wood that will be the finished surface. This is accomplished by using a plywood spacer. Drill a hole through it and in this case I used a scribe through the hole. Apply a big glob of very hot melt glue and quickly place the scribe point on intersection of the horizontal line and the center line. Press down hard and let cool before removing the scribe.
When you want a sphere without mounting points showing then you need to use spacers on both the head stock and tail stock. In that case I use the spur above. This spur is also great for spindle work because if you get a bad catch the stock will usually spin on the center.
Here the piece is mounted using the carriage screw and the plywood spacer. The next step is to paint a stripe between the centers. This is the secret of the whole process. Note the light aimed at the ways. This makes easier to see the shadow as the stock rotates. Start turning off the shadow. As the stock gets more balanced turn the speed up till you are getting clean cuts.
Here I have cut aligning marks. The distance between the two marks will be the same on both ides when the blank is properly aligned. Adjust the tail stock end till the distance is equal on both sides.
Here I have almost finished the headstock half. Stop often to check where and how much needs to be turned off.
Here I have turned off all the paint and done finish sanding. Be careful during sanding as the area between the growth rings is softer and will sand deeper than the growth ring.
Do not knock off the remaining spigot as most of the time it will break off below the surface you want. I use a angle grinder with 36 grit to sand off the plywood and then a palm sander to round the excess wood.
Here is the completed piece.