My ball turner is not ideal - it has several shortcomings - but it does a nice solid job for me.
The construction caused me to backtrack a few times as I made various tools that I needed to make the ball turner, as is often the case.
The deepest level of nesting was : 1) I needed an unobtainable Acme (trapezoidal) thread, so cut tap... then 2) I needed to cut relief on tap, so, make tailstock for rotary table... then, and so on...
But, back to the chase. I wanted a good beefy ball turning tool, to go on my spindly plain Lorch lathe. Now, I have only 64mm centre height above my cross slide, which with my normal top slide gives me a paltry 11mm of tool height, and I can't use the cross slide as an anchor for rotation as there's no space below decks before the feedscrew gets in the way.
I wanted to match that 11mm with the ball turner toolpost. That gave me 53mm into which I had to fit a full swivelling dovetailed slide a tool post mounted on top. And I wanted to turn big balls - my target was 73mm in diameter - so I wanted the slide and swivel as low as possible.
I ended up with 5mm of swivel plate, 22mm of slide, a 5mm adaptor plate and a toolpost of 20mm. Yes, that made 52mm which I'm still unhappy about. It means I've got to add a 1mm shim, or shim up every one of my tools. The final version has the slide at 75mm wide, 22mm high, and with 45mm of travel.
What I couldn't find a way to squeeze in was a cover for the feed screw. You copy what you know, and the GA for my slide is pretty much a half-scale version of the top slide of my Lorch. (I have no experience of the Lorch and Boley top slide for watchmakers' lathes). Lorch make the circular base (top) of the nut (which I made out of brass, not bronze - tut tut) wider than the channel, so a thin steel 'roof' covers the channel, looping itself over the nut, between the nut and the slide the nut is attached to. Well, that level of detailed design was beyond me first time round - maybe next time.
When I started to use the tool I was quite confused about what was happening until I realised a number of things (I use o'clock to mean from the user's point of view - the tailstock is three o'clock):
1) Setting the cross slide so the ballturner's pivot is directly under the lathe axis is crucial, and I must make measurements so I can reproduce this at will. Preset stops on the cross slide travel ?
2) Where the tool tip is in relation to the pivot does not really matter - I'll still get a ball. But the further to the left it is, the fuller a ball I can turn. To put this another way, if the tool is pointing down the centre line of the toolpost and the slide is at the twelve o'clock position, there'll be more things getting in the way of each other than when the tool tip is over towards the headstock. A little here helps a lot. Moving the tool tip to the left 5mm, say 15 degrees, makes the difference between say getting to seven o'clock and getting to eight o'clock. And that's the difference between a round lump on the end of a rod, and a sphere that looks as if it's about to part itself off.
3) The tool tip needs some thinking about. Pointing about 15 degrees left, I get a reasonable ball with an tool shaped like an acme thread cutter, but it isn't much good between three and four o'clock. If I point the tool to the left, then when I'm cutting near the lathe centre line, at three o'clock, the tool is pointing back at me, so it needs relief on it's right flank. Once we're round at seven o'clock, the tool is cutting like a parting tool, so it needs a more normal front rake.
4) All bets about tool positioning will be off as soon as I try to turn a ball halfway down a rod.
|The parts of the ball turner laid out ready for assembly. The allen key isn't a permanent attachment - it just comes in handy from time to time.
|The only three components whose undersides are of interest.|
|The two Acme 8mmx2mm taps before hardening and tempering and a final sharpen.|
|The pivot is attached to the mounting plate, the nut is screwed into the slide, and the gib and lock nut are fitted.|
|The swivel is bolted to the mounting plate and the LH feedscrew is fitted.|
|The slide is fitted to the swivel, the collar is screwed on and the toolpost is screwed to the adaptor plate.|
|The knob is screwed on to take up feed screw backlash, the adaptor plate and the handle go on, and there we have it. At the time my young daughter thought it only needed wheels to become a toy tank !|
|Only 5mm of tool offset results in a considerable change in cutting geometry, but this allows the tool to get further "round the corner" despite obstacles.|
|Lots of chips up front, but none in the feedscrew channel !|