One day, for no particular reason at all (well, there's a distant plan to make a Quorn tool an cutter grinder where turned balls are a part of the handle design, but that's likely several years off (starting) at least), I decided to make a ball turning attachment for my lathe. There are a few designs around, mostly catering to the creation of fairly small spherical features. Some use 'U' shaped arms that hold the cutter, some are based on off-the-shelf boring heads mounted in a lathe tool post. What I decided to do was to make basically the biggest and most rigid (hopefully) ball turner that I could using what I had lying around or could scavenge. My final design will be able to turn spherical features up to about a 50mm radius, though I doubt I'll ever need to actually go that big unless I end up making ball joints for a 3-point linkage or something. The cross slide of my lathe has a circular hole where the compound mounts, with a corresponding circular T slot to bolt it down to. This would form the basis of the base design. I happened to have the center of a large sprocket that I had trepanned out when fixing my lawnmower lying around. The center hub of this was, luckily, just a little bigger than the hole in the cross slide and the outer circumference about right for the overall size. So, this became the base for the design. On a trip back to the family farm, I scavenged some 3/4" x ~4" bar stock of mysterious origin and some other bits and pieces of old rusty steel. The large bar would form the base of the "turntable", and some other bits could be cobbled together to make the tool mount. https://pic8.co/sh/VGMO1G.jpg https://pic8.co/sh/SooIDo.jpg I sketched up a design (in SketchUp, one day I need to learn Free CAD or something) and began the build process. https://pic8.co/sh/MZB4ma.jpg https://pic8.co/sh/m77FOn.jpg First up was the base. I had bought some cheap tapered roller bearings off eBay or somewhere, so I bored out the pilot hole in the center to provide clearance for the shaft (20mm) and seats for top and bottom outer races of the bearings. I also turned down the diameter of the center boss and the overall length so that it would be a snug sliding fit in the cross slide. I tried to get the bearing seats as concentric and aligned as possible, as any misalignment here could cause the whole thing to bind and/or lose rigidity. Once the bearing seats were cut, I milled two small slots between them so that there was some way to remove the inner races of the bearings if the need ever arises (otherwise, they would have been pressed into blind ends and basically impossible to remove). https://pic8.co/sh/9kzoYa.jpg The "turntable was next. This is a fairly major part with lots of operations done to it that are fairly critical to the tool's success. To start with, I drilled then bored a shallow flat bottomed hole in the center of the underside of the base. This had a generous (~5mm) chamfer on it to allow for weld fillet and was oversized a little (~21mm). A shaft was rough turned down to a light press fit into this hole, then the two were welded together. Using the lower surface of the base plate as a reference, I dialed in the base plate and also made sure that the shaft could be turned down to final dimension then cleaned up the weld, skimmed the back surface (which left a horrible, though smooth to the touch, finish) and turned the shaft to final dimension in one setup, ensuring it would be perpendicular to the base. A flat bottomed counterbore was also made in the shaft and an M6 thread put in it to accept the bearing stack washer and bolt. The "washer" would be machined then hand tuned to achieve my desired preload on the bearings. https://pic8.co/sh/JLrIRy.jpg With the base of the turntable completed, it was time to add the top features. This involved first flattening the top with the fly cutter which left a much nicer finish than the bottom. Then the sides were milled (again with the fly cutter) so that the shaft was as close to center as I could get it, and the edges parallel. The parallelism of the edges wasn't critically important though, as long as I always used one edge as the reference datum. I set about making the tool post. This was done by roughly forging the curved shape of the upper portion into a chunk of steel, then cutting the required bit off and welding it to a chunk of the 3/4" bar to form the base that the dovetail will be cut into. The forged tool post was machined until the sides were flat and parallel, so that I could use that as a reference surface to machine the base parallel/perpendicular to that surface. This ensured that the top of the tool post would be centered when it is registered on the base. After the general shape of the tool post has been machined, the male dovetails were cut into each side. Care was taken to get the depth of the dovetails the same and the height of the dovetails just over that of the female dovetail (the shoulder has about a 0.1mm clearance, guaranteeing that the tool post registers on the base of the dovetail slot). https://pic8.co/sh/5lGA22.jpg https://pic8.co/sh/xG5k02.jpg https://pic8.co/sh/HFI7qH.jpg https://pic8.co/sh/Zx03D6.jpg A trapezoidal wedge was cut from a strip of 5mm thick steel, fly cut flat top and bottom to a final thickness of about 4.7mm. This piece had slightly elongated (1mm longer than they were wide) mounting holes milled in it and was clamped to a sacrificial aluminum carrier plate (fly cut flat) to machine the side angles. This was done with the dovetail cutter in one setup ensuring that the resulting sides were aligned along their length. The trapezoidal wedge could then be clamped with the tool post into the dovetail slot on the base securely. https://pic8.co/sh/VjWmbs.jpg After carefully aligning the part and triple checking everything, the channel for the dovetail slideway was cut. The design was a little unorthodox as the material I had was fairly thin and my only dovetail cutter very small, I didn't have enough room to make a "normal" dovetail setup, where locking screws would apply force from the side using a flexure joint on one side of the dovetail. Instead, my design is based on the bottom of the dovetail slot being the main load bearing surface. This is because in use, the cutting forces are transmitted down through the tool primarily. So, I designed a method of locking the cutting head down and into a single female dovetail slot on the turntable by using a trapezoidal locking plate. Basically, one side would have a traditional female dovetail undercut an the other would have additional space then a 60deg slope. The trapezoidal plate would fit between the second side of the male dovetail and the slope and be tightened down with screws, applying force downward and towards the reference dovetail. I'm not sure if this is a good idea, or a stupid idea. I've never seen anyone else do it, so it's probably a stupid idea for some reason I'm not yet aware of. Anyway, the slot was roughed out to within about 0.1mm of final dimensions (depth being the main one). Then it was on to the very slow process of very carefully cutting the dovetail with my only small dovetail cutter. In the end I took 4 passes to complete the cut, starting at 1mm depth, then 1.7mm, 2.1mm and finally 2.4mm. Each pass was shallower as the cutter would engage more material vertically. This was done basically as slow as my mill auto feed would go, with generous amounts of cutting oil. The finish was actually really good and I ended up using the dovetail cutter to skim the final depth of the slot too. The 60deg slope was cut with an end mill by setting the entire base up on a 60deg angle. The precision of this surface wasn't as critical as the main dovetail though. https://pic8.co/sh/sit9vY.jpg https://pic8.co/sh/brV6NY.jpg https://pic8.co/sh/brV6NY.jpg https://pic8.co/sh/PJPe8P.jpg https://pic8.co/sh/C2s3iV.jpg Matching holes were drilled and tapped on the base to accept the clamping screws. https://pic8.co/sh/gkyJLA.jpg Once the basic tool post was constructed, I could put everything into the lathe to mark out the tool height. This lead to the discovery of a BIG problem. The cross slide of my lathe did not travel far enough for the center of the mounting hole to get to the center line of the lathe. It was short by about 1/4". After having a look, I determined that this was caused by the cross slide nut running hard up against the back of the apron of the lathe. I couldn't do much about the position of the nut or its size, so the only thing left to do was to modify the lathe apron to allow a little more travel. The lathe was disassembled and the apron mounted on the mill. I milled out a pocket to fit the nut and two small pockets for the backlash adjust screw heads. This allowed an additional ~7mm of travel which meant I could position the ball turner right on the lathe centerline. https://pic8.co/sh/Ylcxqf.jpg https://pic8.co/sh/7WfMUR.jpg https://pic8.co/sh/MALXfe.jpg Once the tool post dovetail was machined, the platforms for the inserts could be machined so that they would end up on center height of the lathe (hopefully). Then the tool post was mounted on the base in the lathe to mark the hole locations for the insert mounting holes. This was probably the most stressful step so far, tapping a deep, blind M2.5 hole in steel. If the tap broke, that whole part would be scrapped. But, it worked. https://pic8.co/sh/mp6Vgo.jpg https://pic8.co/sh/XQsuJF.jpg https://pic8.co/sh/u8JqvQ.jpg Excess material from the tool post was removed by first fly cutting the sides to remove some material, then a combination of filing, grinding and linishing to remove the corners. https://pic8.co/sh/8K084N.jpg The ends of the turntable base were marked out by putting them in the lathe and scoring a line, then the bulk of the corners was cut off in the bandsaw before turning them to roughly the same diameter as the base. Additionally, two small scallops were cut in the sides to allow the base locking screws to be installed and removed and oil channels were cut into the underside. I haven't yet installed an oil port in the top, but will do that when I get around to buying some button oilers. https://pic8.co/sh/r4v6ku.jpg https://pic8.co/sh/cHUEDO.jpg https://pic8.co/sh/O2LRLR.jpg https://pic8.co/sh/r8Blmw.jpg With the ends rounded over, it was possible to drill and tap the handle mounting holes. These were tapped M10 and a short length of stainless steel rod I had lying around had an M10 thread cut on each end. I also made an aluminum locking nut that would by used to lock the handle in the desired position. The handle was then bent to the desired angle and an old aluminum knob that I had lying around added. https://pic8.co/sh/hvsg18.jpg I wasn't happy with the amount that the socked head screws that were holding down the tool post were sticking up, so I decided to use some countersunk ones. To do this, I made some small washers which were countersunk on one side to allow them to clamp firmly while maintaining minimal (~3mm) height. https://pic8.co/sh/0xQ5XA.jpg https://pic8.co/sh/FsCAoz.jpg The last bit was to add a bracket for holding an indicator so that fine adjustments could be made. A series of M4 holes were drilled and tapped into each side of the base of the tool post at 10mm spacings. Then a T shaped bracket with 4mm slots was made to bolt to it. The combination of holes and slots allows the bracket to be positioned anywhere along the tool post base to accommodate for the limited travel of the indicator. The indicator was held in a split clamp that bolts to one side of the base plate. https://pic8.co/sh/1KvEWs.jpg https://pic8.co/sh/pn7ki4.jpg https://pic8.co/sh/qvGWEx.jpg My next big problem that I ran into was clearance. Despite making the base as low profile as I could, and mounting it directly on (and in) the cross slide of the lathe, it was going to hit the chuck jaws pretty easily. To get around this, I ended up buying a smaller (125mm) three jaw chuck that I can hold in the main lathe chuck to get some extra clearance. This was JUST enough. It cuts balls though. Surface finish on the mystery steel was a little crappy, but some sanding with 80/120/400 grit helped a lot. https://pic8.co/sh/TslC37.jpg https://pic8.co/sh/2eOIvM.jpg Future improvements/changes are likely to include a conical tool post design to allow the use of a button insert and allow for tighter clearances (cutting balls closer to the chuck). Additionally, I could make tool holders that would convert the turntable into a fly cutter or boring head, by extracting it from the bearing and mounting it in an ER32 collet in the mill. Various different bits could be added for the fly cutter mod, including tangential cutters to get a very nice surface finish with minimal tool load. The high rotational mass should also help, but I think I would need to bolt the fly cutter heads to the turntable rather than just relying on the trapezoidal plate because there would be greater side loads in this configuration, although if the fly cutting attachment was full width, the use of 4 M6 bolts to hold it down should be sufficient. Album of all photos (some weren't used here): https://pic8.co/a/69e2c91a-65d5-4948-87ab-734f4484991a