Stephan.wintner wrote:Jeremy, I was thinking about the turning technique below - using a steel rod with a hole to produce a radius. You state that a 4mm diameter hole will produce an 8mm diameter ball end, or 4mm radius.
I'm thinking the resulting surface is actually an elliptical shape, and the chord of the surface would be 4mm. Which works out to approximating a 2.8 mm radius (4 * sin45) - is there something I've overlooked? I assume you hold the steel tool such that the circular edge of the hole contacts the work all around.
Stephan
Hi Stephen
I never thought about the maths, but when the 90 degree angle on the cutting edge stops cutting, its before it reaches an equal angle on both sides of the cutting edge. It stops cutting and more polishes.
I have just spent the afternoon doing it to check it works and show how its done.
Drilling a piece of silver steel with a 4mm hole and squared off with approx. .75mm flat edge.
I have tapered the outer side so I can see where it is on the brass to be turned
A piece of 8mm Brass in the chuck. Always start by rounding the end with a file first.
Lathe spinning and hand tool pressed against brass. Keep rear of the toll moving in a cone shape style with the cutting edge pressed hard against the brass rod.
The black line across the rod is 4mm from the end.
Keep on going as before working to that line
Notice the marks on the cutter where I have fallen off the rod and hit the chuck as it needs some pressure.
when the cutting edge meets the bar all the way round the cutting edge you should have an equal radius.
I had to put the cutter back in the lathe and refaced the cutting edge. I didn't hardened the tool and should have. It definitely needs to be for brass.
Looking at the end you can see how much still needs removing so work the cutter for the centre to the outer edge.
Just finished and polished the length and ball of the rod with emery.
Putting the cutter back on the end to polish to a shine to show what's been cut.
Then measured from the end to where the curve starts. measuring the shiny end.
I have had to pose this picture as I used the pin on the other end of the callipers to measure.
With a bit of effort I could go most of the way round to form a sphere.
Measured across the diagonal not across.
The tool is a bit scratched from where I fell off the ball and hit the chuck I also had to reface it 4 times.
I have just cleaned up the lathe and thought I would just check again. 7.98mm