Jim's truly simple ball turning tool

[Rex Walters]

{UPDATE: Here are the final drawings and here is the Fusion360 3D model. I've started a series of construction articles on my blog.]

Making or purchasing a ball-turning accessory seems to be a very popular activity among people with our interests.

Every time it comes up on these forums, my friend @Jimsehr mentions his super simple ball turner, but I've never seen anyone actually make one before. I think many are confused by the blurry photos and the terse replies that Jim has posted (sorry, Jim!).

I've been told I have the gift of gab, and I'm not one to shy away from explaining something I barely understand, so I wrote up my experience building Jim’s design on my blog. Note that I've included a downloadable 3D Model (Fusion 360) as well as a pdf of dimensioned drawings (for my lathe specifically, but they should be easily adapted to yours).

Here's the ORIGINAL design I settled on after several discussions with Jim:



Design notes:

• It mounts in the tee-slot on a lathe compound. Set screws hold it rigidly in place. Not all of us have "boring tables" or a place to mount tooling on the cross-slide, so mounting on the compound is quite convenient.
• The design is quite flexible. With 1/8" cutters and a 1.5" diameter by 1/2" thick puck on my 10" lathe, I can cut up to a little larger than 1" balls. The distance from the top of the puck to center height is the limiting factor. Larger diameter cutters might need a larger puck for the set screw to have enough threads engaged.
• The pucks have a boss on the bottom that registers into a bored and reamed hole in the tee base. The radii are fixed, not continuously adjustable, but it's easy to make multiple pucks at once for different radii (I'm making pucks 1", 3/4", 5/8", and 1/2" diameters -- smaller ones are easy enough to cut with a form tool).
• Cutters can be pretty much any diameter. The top is ground perfectly flat (sharpening is trivial).
• The pucks aren't even screwed into place, they just rest in the bored hole. Since all the cutting forces are directly tangential into the long axis of the cutter, and the cutter is directly supported below by the tee base, it's quite rigid with absolutely zero forces trying to lift it out of the hole.
• I've decided on one radius per puck. There are two holes: one is positioned so the distance from the center of the puck to the inner side of the cutter is the desired radius (for cutting balls). The other hole is for cutting concave radii with the distance from the center of the puck to the outer diameter of the cutter being the desired radius. Center height is set simply by loosening the set screw in the side, then tightening when you've got it spot on.
• I've decided to make two holes for the handle to screw into, 180 degrees apart from each other. I've made it angled for clearance, but whatever tickles your fancy is fine (I know: it needs a ball on the end). Note that you need almost a full 180° spin of the handle to turn a ball on a thin neck. The two locations are to allow a full 360 degrees of access in case something interferes for part of the cut.
• It uses a tangential cutter with zero rake and zero relief. Many don't think such a cutter will work, but I assure you it works fantastically well (see the blog article for details). It won't face, but it will turn fixed convex and concave radii into the side of a rod just fine.

I'm now going to start picking the brains of all my old-timer friends to see what I should build next!

 

[GeneT45]

The link that's ostensibly to your blog is incorrect (at least for anyone but you).

GsT

 

[Rex Walters]

The link that's ostensibly to your blog is incorrect

Oops. I accidentally linked to my dev server instance. Fixed.

The blog entry is at https://rexs.tools/blog/ball-turner

 

[GeneT45]

Thanks! I rather like that design. Low overhead. I'm putting it on the list of projects I'll never finish before I die, but it's probably up there high enough to get done as soon as the weather cools down a bit... I'm thinking I'll make a handful of blanks to be finished to specific radii as required.

Thanks again,

GsT

ETA: really enjoyed your blog - hope you keep it up!

 

[Rex Walters]

It's pretty quick and easy to make: just four parts to make: the tee base, the puck, the handle, and the cutter itself.

I also faced off the cup points on some set screws I had on hand, so my costs were $0.00 (everything was a cutoff or parts on hand).

It was one of the smoothest projects I've ever attempted — it's a pretty forgiving design. Even what I've shown in the drawings is fancier than necessary. You only need one vertical hole in the puck for the cutter, a set screw in the side, and you can glue in any old handle (or just use a large bolt).

Full disclosure: I've yet to build one to the drawing. I just made the first prototype shown in the video so far. The only major differences are the thinner outboard end of the base and the lack of a screw in the bottom to hold the puck captive. Neither are likely to cause ANY issue whatsoever, though.

 

[Shotgun]

Rather than making one for each size, I think it would be easier to mill a slot like on a fly-cutter, and then have a small bar with the hole for the cutter. The set screw would then hold the bar in place.
This does look like an easy yet useful project, though.

 

[Rex Walters]

mill a slot like on a fly-cutter, and then have a small bar with the hole for the cutter

That might work, but would increase part count, complexity, and might lose rigidity.

Everyone intuitively wants continuous adjustability, but realistically, how many differently sized radii will you really need to cut in your lifetime on a manual machine in a home shop?

Personally, I'd rather have a blank puck on hand without the cutter hole bored. Then it's only a moments work to position the hole at whatever oddball radius I need for some rare occasion. I can also drill multiple additional holes wherever I want in existing pucks, of course.

Most of the time I just want a ball on the end of nominally sized stock (3/4" and 1/2" being my most common sizes).

My 10" lathe has a small spindle bore, so I can only hold up to 1/2" stock in 3C collets. I recently discovered I can hold 3/4" stock in a 3MT collet, though, which is particularly great for this tool.

 

[RJSakowski]

I biggest concern is the way the slide is fixed to the compound. Similar to bottoming out a tee nut, it puts upward force on the two sides of the tee slot and potentially, one could crack them. A second piece from above could capture them by clamping.

 

[Rex Walters]

I biggest concern is the way the slide is fixed to the compound. Similar to bottoming out a tee nut, it puts upward force on the two sides of the tee slot and potentially, one could crack them. A second piece from above could capture them by clamping.

In theory, I suppose one could tighten the set screws enough to break off the lips of the slot. In practice, they are pretty thick and there is no need to gorilla the set screws. Just tighten them good and snug, then stop turning the screws.

I've got a "saw stop" feature on my handsaws, too.

In theory, tool pressure at the end of a boring bar also exerts leveraged lifting forces against the tops of the tee slot. They are made pretty stout for a reason.

If really concerned, one could use smaller set screws with soft tips (brass or plastic). Or make a clamp as you suggest. Personally, I don't find either necessary.

 

[Shotgun]

I was thinking the same thing, @RJSakowski .
I figured I'd put a couple threaded holes in my existing QCTP t-nut. Then I'd take the "extended" piece of the design, and screw it's base to the t-nut. Maybe that would make it even one less piece @Rex Walters ?

Ooh! Ooh! I know. How about I drill a hole in my existing t-nut to hold the cutter, and then use the compound to spin it? I'd need to add a short bolt to force the t-nut up like Rex demonstrates.