I don’t know whether this is a fail, a success, or, more likely, a sort of dubious success extracted from the snapping teeth of failure.
It does work, but it’s a fundamentally bad design, so in hindsight its functionality is a surprise.

I’m ambitiously attempting to get wordpress to embed a video.

Background: all the cool kids on my old guys writing about machine tools mailing lists (remember those?) are talking about tangential toolholders. Usually with a metal lathe you jam a tool in perpendicular to the workpiece. You grind the tool with top rake (the angle at which the tip touches the material, which somewhat determines how it removes material), back rake (the clearance beneath the tip, so non-cutting parts of the tool aren’t dragging on the material after cutting, adding heat and possibly work-hardening the material), and side rake (same as back rake but on the side that’s cutting into the material.) Then you jam it in, and it removes material, which smooshes against the top raked surface and disperses as chips.

Tangential toolholders put the tool tangent to the material rather than perpendicular. One reason to do this is that if you angle the tool, you automatically get your side and back rake without having to grind them, and you establish your top rake as just a flat surface ground across the tool, rather than a prism ground into the tool. It certainly simplifies resharpening tools. It has drawbacks. I’ll talk about those later.

I’ve wanted a tool that can turn spherical surfaces for a long time. Right now I have a vast array of tooling that uses janky stuff like carriage bolts as knobs and handles. I’ve made a spherical bit in the past using a DIY hand-driven rotary axis on my milling machine, with the mill spindle set at an angle and spinning a boring head that has an external boring bit in it. That was slow and had a terrible surface finish, although the surface is really beautiful.

That surface finish should have been foreshadowing, letting me know that my path was fundamentally flawed. But I didn’t think very much about it other than hey that’s interesting.

I was looking at my quick change tool post, specifically at one of the boring bar holders I have for it. It’s a big solid chunk of steel with a hole through it that the bar fits quite tightly into. I’d been thinking about the traditional spherical/ball turning attachment, which clamps into a lantern toolpost. Those suck because lantern tool posts suck. But, I was thinking, what if I instead mounted it in the boring bar holder? Waaaait a second, what if I used the boring bar holder as the pivot for the turning mechanism? Instead of a vertical axis ball turning arrangement, a horizontal axis one?

(Yes, I am aware other people have had this idea and in fact have turned it into a commercial product: I found it while researching my idea.)

Well, I have a dinky little lathe and my QCTP boring bars are only about 8mm in diameter and by the time I jam a bronze bushing in there and make a bearing shaft, there’s not much metal left. So my intent is to machine my own dovetail mount with a big fat hole in it, but because that takes time and I’m impatient, instead I grabbed a big chunk of steel, bored a 5/8″ hole through it, and pressed in a pair of 1/2″ ID bronze bushings, and then welded a chunk of steel to the side that’s the right size to clamp into one of my QCTP toolholders. It’s pretty solid.

I buy dumb stuff off ebay for the Mad Scientist Hut. Sometimes it comes in handy later. One thing I bought was an entire bag of absolutely lovely THK linear ways with recirculating bearings, tiny little things, with the ways about 5mm wide and each one maybe all of 50mm long, giving me maybe 40mm of travel. They are gorgeous tiny pieces of engineering.

Sooooo. I skimmed a piece of 1/2″ round steel so it would just barely fit in the bronze bushings. I bored a 1/2″ hole in a piece of steel strap and pressed it onto the round steel and now I have a nice base for the linear way. A piece of angle iron welded to the back, at the end of the way, provides a bearing for a 1/4″-20 screw, that’ll act as the leadscrew to drive the linear way.

The toolholder took more work. The basic form is a huge chunk of angle iron I cut off and bent to a slightly acute angle. This angle establishes the rake on the tool. It has four tiny holes drilled in one side to bolt it to the linear way carriage.

Here’s another janky bit. I welded an L-shaped extension onto the side of the angle iron, that bends parallel to the side of the way carriage. This has a hole in it. As a result, a piece of round brass passes through the hole and abuts the side of the way carriage. It has a perpendicular 1/4″x20 threaded hole in it. The aforementioned screw threads through this. Now, the whole works can be positioned along the way. This whole arrangement is super ghetto and I need to improve it. Like, the way I keep the screw placement fixed is by two jam nuts cranked against each other acting as a thrust washer against the L bracket. Ugh. (I do have two tiny oilite thrust washers in there. But still.)

And onto the fussy part. I hacksawed and machined a 45 degree right triangle prism-shaped chunk of metal, and then in the hypotenuse I cut a just slightly over 1/4″ wide and deep slot, normal to the hypotenuse, and then flatted one of the acute angles of the right triangle and drilled it for three screws. This is where the toolbit goes: snuggles down in the trough, and the screws keep it there, holding it at a 45 degree angle. When this is welded to that angle bracket, all the angles for presenting the tool to the workpiece are established. (Well, except for the top rake, but that’s a pass on the grinding wheel, a pass on some sandpaper, then the whetstone.)

I milled a flat on the other end of the round shaft that fits through the bushings, and made a janky handle with a setscrew that bears on the flat, and a carriage bolt as the actual handle that moves the whole thing back and forth, because I don’t have a ball turner to turn a nice spherical handle. This is traditionally the first thing you do with a ball turner: replace your squalid carriage bolt handle with something lovely. I’ll get to it.

ANYWAY. So I tried it out.
And technically it worked. It cuts a sphere. I can adjust the size of the sphere with the screw and linear ways.
The first thing I learned was that tangential toolholders load the tool in a very different way. It gets pried out of the clamp. This would be solved by broaching a square hole through the tool holder so it’s held solidly, with clamp bolts preferably pushing it against the outside faces.
The tool also gets shoved downwards: it slides backwards in the slot, away from the workpiece. I already knew this was going to happen so my design has a chunk of metal back there that when this gets annoying I’ll tap so I can put a setscrew in to back up the tool.
The second thing I learned is that it really wanted to cut lots and lots of very fine ridges. There was nothing even vaguely like a smooth spherical surface. I was making a spherical rasp. Which is really interesting, but not what I wanted.

Here, lemme zoom in on that.

I thought about that for a bit. The ways aren’t flexing measurably even with the long moment arm of the tool: they’re designed to handle exactly this kind of abuse. (They are NOT designed to handle you taking the thing apart and then mistakenly tilting it. The carriage runs right off the end and all the 0.2mm ball bearings fall out and it is really seriously impossible to reassemble. No really. I rebuild Shimano STI shifters for fun, but this was not fixable. But hey I bought a whole bag of the assemblies, so oh well.)
So, I cut out a curved bit that reinforced the angle iron that holds the tool, so it can’t flex. And, oh, so close to realizing my fundamental design flaw, I also slightly rounded the end of the tool so it wasn’t a razor sharp tip anymore.

That improved the action immensely. Which is hardly a surprise: the wider and smoother the cutting tip, the wider and smoother the trench that it cuts into the workpiece, so the finish improves.

And once I realized that, the lightbulb finally went on.
I’m cutting a spherical surface. I’ve never done this before, well, except when I tried with the boring head and made the same mistake.
I don’t WANT a sharp pointed cutting tool. I want a flat edge. In fact, I’d like a concave edge. The whole idea of the 45 degree presentation of the tool was bad. If I’d made it so it was tangential but flat to the surface, it would have worked better right from the beginning. This is a great design for turning cylinders. If I increased the angle of presentation, it could be a great threading tool as it would form the 60 degree flank angle required. But it’s not the right tool for turning spheres.

Nevertheless, it does work much better now than anything else I have for generating spherical surfaces.

Usage consists of orienting it to the end of the workpiece and adjusting the QCTP up and down until the tip is right at the lathe centerline, then backing the ways on the turner out until it’s swinging the diameter you want plus a bit, then using the compound to feed it into the workpiece a few thousandths at a time until you have a hemisphere. If I want more than a hemisphere, what I’ve been doing is first clearing out the other side with a standard turning tool, then roughing the tailstock end to shape, then opening the ball turning tool way up and working the headstock side down (without moving the carriage or compound slide, so the center of rotation is still aligned with the center of the hemisphere) until a single fine cut crosses the whole workpiece. Note that your knuckles and the chuck get awfully close when you’re doing a nearly complete sphere. You want to think carefully about where you position the handle. Mine is too long and hits the QCTP clamping handle, but again, it’s a stupid carriage bolt and the Right Answer is make a real handle.