Due to only having a large enough piece of metal in square bar, I started by turning that in the 4 jaw chuck. As I was not too worried about centring the bar, I simply rotated the chuck by hand and used the small circles scribed on the end to adjust the bar so that it was close enough to being centred.
Once I'd turned one end of the bar down to a cylinder I swapped back to the three jaw chuck to finish the other end. I intentionally left the head a little large in case of problems and roughed out the dome shape by eye. I then filed the dome to shape. You need to be very careful with this technique, a handle is essential and an large file is also a good idea so that it is not ripped out of your hands but the lathe. This is not an appropriate technique for larger, more powerful lathes but you can get away with it on a smaller lathe. The last operation was the machine the base of the head to size. I've filled a flat on the side of the pin so that it can be brazed into place.
There are several different techniques for machining curves on a lathe.
- Cut the curve in steps, then smooth with a file. Good for purely cosmetic requirements or when the curve shape is complex.
- Grind a lathe tool to a curved shape. This is good for when you have to make many identical curves.
- Use a radius turning tool. This is good for turning balls and other accurate radii.