Creating a Sphere on the end of a shaft

[homebrewed]

For the life of me, I cannot figure out how I got the swivel to work. I went down to shoot a video for you and can't reproduce my results.
I know I turned the first few balls that way, and was proud of the fact that I did it without a tool.

I just gave up. And will think about it again.

Maybe you had a different type of tool post when you did it back then?

 

[woodchucker]

nope same tool post.
I think I cut it in two different arcs...
one from the front, and one from the back to get it to work.

 

[homebrewed]

In an interesting coincidence, I made a semi-spherical handle for a Weber BBQ "restoration" I'm currently doing. The bottom vent mechanism rusted out so I can't damp down the coals past a certain point. The Weber is _only_ about 30 YO so still has plenty of life left in it

I drilled/tapped a center hole in the starting length of rod, and to cut the back half-sphere I turned the piece around & screwed it into a length of all-thread, which was chucked in the lathe. The direction of rotation acted to tighten the piece against the chuck jaws so it was securely held in place during machining.

Here's a photo of the knob, along with some of the other pieces.

 

[Rick_B]

so how did you machine the knob?

Rick

 

[Inferno]

You need to have your cutting point behind the axis on the compound. If it's in front then it will cut concave.

 

[Shotgun]

You need to have your cutting point behind the axis on the compound. If it's in front then it will cut concave.

Unless you turn the tool upside down, and cut on the far side.
Which way is better is most likely VERY machine/cross-slide/compound dependent.

 

[Inferno]

Unless you turn the tool upside down, and cut on the far side.
Which way is better is most likely VERY machine/cross-slide/compound dependent.

It hurts my head to think of how that could work.

 

[Rick_B]

You need to have your cutting point behind the axis on the compound. If it's in front then it will cut concave.

I apologize for being such a machining idiot but I don't understand what you mean - can you possibly dumb it down some for me - maybe a sketch/picture?

Thanks
Rick

 

[RJSakowski]

It hurts my head to think of how that could work.

Cause it can't.

I apologize for being such a machining idiot but I don't understand what you mean - can you possibly dumb it down some for me - maybe a sketch/picture?

Thanks
Rick

The cutting edge of the tool swings through an arc, the radius of the arc determined by the distance from the pivot point. If the cutting edge faces the pivot point, you will cut a convex. If it faces away, you will cut a concave. The diagram below illustrates both possibles. The center lines depict the pivot points.

 

[Inferno]

I wanted to show a picture of what I was talking about but my compound slide won't go back far enough to get past the axis. It does look like I could modify and might do so one day. The limitation would be interference between the slide and the part but looks like I could turn a 4" sphere if I wanted to once the slide is modified.