Ball Turner Experience

[pontiac428]

Jim, how do you accomplish a concave sweep with a vertically set tool? Do you use a cutter with a dog leg or offset?

 

[Rex Walters]

I'll answer as Jim is about to depart for a trip:

how do you accomplish a concave sweep with a vertically set tool?

The amazing thing is that you do NOT need any relief for a tool on center when turning a cylindrical piece of stock on the lathe. You only need it when facing. This is because the cylindrical side of the stock already falls away from the cutter and provides relief.

You can plunge right into the side with a purely vertical (tangential) tool and it will cut just fine. This is why you don't need to pre-cut the "neck" when using Jim's ball turner to cut a convex sphere.

To cut a concavity into the side of the cylindrical stock, it's the same thing except the cutter is now in front of the central pivot of the swivel puck.

The pucks I'm currently making prototypes of and drawings for (with build notes) will have two locations for mounting the handle, and two cutters: one with the inner edge of the cutter a fixed distance from the pivot point, and another directly opposite with the far edge the same distance from the pivot. There will be two set screws in the side, and two locations for screwing in the handle. One puck for each radius you need, convex or concave.

 

[rwm]

He means concavity in the end.
I made this:

 

[pontiac428]

He means concavity in the end.
I made this:
View attachment 455190

That answers my question perfectly. The tangentially oriented cutter is limited to outside radii, so another solution is needed for internal- and yours is just as simple as the tangential with the benefit of being able to make both types of cut.

 

[Rex Walters]

He means concavity in the end.

Ah!

Yes, Jim mentioned to me that he'd also made something almost exactly like what you showed. The only way to get a bowl in the face of stock is if the tool projects into the cavity, of course.

He said the cutting tool he made was basically a radiused dowel pin, with half the diameter ground away. The pin projected horizontally from the pivot, not vertically/tangentially. The specific use case was making molds for miniature cannon balls!

 

[Rex Walters]

The tangentially oriented cutter is limited to outside radii,

Note, however, that you can also make (for lack of a better name) inverted fillets at the end of the part with a tangential tool (like if you were roughing out a piece of tool steel to make a rounding end mill).

But bowl shaped depressions in the end of the stock can't be done with a tangential cutter, nor can you face anything without at least a degree or two of relief.

 

[pontiac428]

Perhaps if you used something shaped like a boring bar you could make some inside cuts. I built a boring head ball cutter, which is similar to the simple tangential but the head of a boring bar is fatter than the neck clearance so it has reach.

To complete the thought, I don't recommend the boring head ball turner, at least not with chinesium parts. I used it a few times, tried a few different grinds, decided it was interesting but problematic, and put it away never to be seen again. I probably have pics, but it's not a great option, so who cares.

 

[Rex Walters]

Perhaps if you used something shaped like a boring bar

In my conversation with Jim, he said he used another cylindrical cutter (broken end mill or whatever) but radiused and split as I mentioned above.

He then made effectively a taller puck, but also split so the face of the bowl can get closer to the axis of rotation. He then put a drill or end mill in a collet in the spindle, and bored a hole through the puck the same diameter as the cutter (so exactly at center height). He drilled and tapped a hole in the top of the puck to secure the (now horizontal) horizontal cutter. Adjust the distance from the tip of the cutter to the split face of the puck to achieve the desired radius of the bowl.

Hope that makes sense. Hard to describe in words, but you end up with something almost exactly like Robert's photo, only instead of an insert, it uses a radiused and split piece of cylindrical HSS or carbide. The radius provides the relief, the split gives it a sharp edge.

 

[Jimsehr]

If you want to see how my super simple radius tools work search this ball turner by Jimsehr. Jan 21 2022 I of the threads shows tool cutting into od of a bar. Another thread shows turning a ball. And # 39 shows how I cut a hemispherical ball radius into the end of a bar.

jimsehr

there may be some earlier posts but this one shows 3 types of cuts with my simple radius turners. The vertical tool cutters have a preset radius . The horizontal cutter radius is set with a micrometer.

 

[Jimsehr]

Also I would recommend staying with a standard size round cutter when making the tool holding pucks. But any size will work. I use 1/4 inch.

jimsehr