In which I make a tool, curse brass, do a bit of boring, create an internal metric thread and eventually retire the faithful cork in the end of the spindle. My second thing! (Well, third if you count the tooling).
A common issue with these lathes is that the end of the spindle is inside the gear cover, so swarf which travels along the spindle will fall down into the gears. The cork I’d put in the end was doing OK, but I’d found something very cheap which I figured could be re-purposed to make an extension.
These were in the “discontinued” bin at B&Q, two for 50p. They’re plumbing fittings, for some kind of push fit DIY system. The threaded end is a 3/4 BSP thread (which is a 14tpi thread with a 1.041″ major diameter). The internal diameter throughout is 22mm, the outside diameter of the biggest bit is about 32.6mm and the length overall 17.3mm. I’ve found it’s handy to carry a digital caliper when perusing discount bins (doesn’t everyone?). Inside the un-threaded end, about 3mm back, is a roughly 2mm deep groove for an ‘O’ ring. These also came with free ‘O’ rings :-).
I spent ages browsing mini-lathe sites when I first got the lathe. One of them, www.mikesworkshop.weebly.com, has a lot of beautifully clear descriptions of some of his projects, with very nice photos, which I found quite inspirational. His spindle extension replaces one of the spindle locking nuts and incidentally houses a magnet for his digital rpm readout.
I didn’t have a suitably chunky bit of steel to hand for a direct copy of his design, but my spindle has a useful few turns of spare thread sticking out from behind the rearmost nut, and I figured a spindle extension needn’t be all that heavy-duty. My bargain brass thing was almost the perfect size. Also, if I messed it up, I had a spare. And some free ‘O’ rings.
The spindle is 27mm OD, and the threads are 1.5mm pitch, so I needed to bore out the brass thing to 25.5mm for the thread. I didn’t have an internal threading tool, or a boring bar. With such a short section, I thought I could bore it with one of my other tools, but none of the tools I had would get inside to turn the thread.
Tool shops around here don’t stock silver steel (drill rod) or any such, and while good old Machine Mart will sell you a carbide insert boring bar, it’s rather expensive and only comes in a set with several other rather expensive tools. Passing through Tesco, though, I noticed that their own-brand “value” screwdrivers were cheap and stamped “CHROME VANADIUM”. At a couple of quid for an 8mm diameter, 10″ long section of what might be half-decent steel, I took a chance.
Gratifyingly resistant to files and hacksaws at the business end, the Tesco screwdriver looked promising. I cut the plastic handle off, then heated the middle with a small blowtorch to temper it to the point where a hacksaw no longer lost its teeth on it, then cut it in half. Then I faced off the ends in the lathe and attacked the tang end with an assortment of very dubious grinding arrangements – I still haven’t got a bench grinder.
I decided early on that there was no way I was going to try putting a short, coarse-ish thread into a blind hole working towards the headstock, so I made the tool suitable for cutting on the back side, working outwards, with the lathe running in reverse. I also put what I fondly hoped might be a useful boring edge on the other side, with a quite rounded corner, but this turned out to have too obtuse an angle, and the rounded corner encouraged flexing, so in the event I also used the threading tool side for boring, again working on the back side with the lathe running in reverse.
I needed a way to clamp this in the toolpost, and after thinking about things like vee-blocks or purpose-made bar holders with long holes through them, I came up with a quicker and dirtier solution – I smooth-bored three stainless M8 nuts by drilling the threads out, then slit one side of each, making three individual small clamps, one per toolpost screw. This worked far better than I had any right to expect, and also made it easy to clamp the tool in a vice while carving the tip.
I was expecting to have to shim this to get it at the right height, but by pure serendipity it came out pretty much bang on – the tiniest of rotations of the shaft got the point lined up exactly with the tailstock centre.
Hardening this with my tiny blowtorch took a while. I tried oil quenching it at first, but a scrape with an HSS hacksaw blade left marks, so I tried again with water – that seemed to leave it dead hard. As I’d only managed to get the tip itself to red heat, and the steel behind it is pretty substantial, I didn’t bother trying to temper it, it “seemed” about the right hardness when honing the edges, and it worked fine in practice, neither bluntening nor shattering.
I’d read that I needed a zero or negative top rake for brass, so I made it that way, and it cut really well. (Although the obtuse angled edge I put on the side opposite the point didn’t cut so well – not enough relief angle immediately below the rounded corner, I think.) Techincally, because of the shape of the tang I started with, tapering gently thinner towards the end, there’s a slight postive rake relative to the cutting edge when the tool’s cutting out from the headstock (backwards) in the way I intended to use it, but I don’t think this hurt.
The point angle (hence thread trough profile) probably isn’t exactly 60 degrees, I just did it by eye, erring on the side of too obtuse rather than too acute, I didn’t want the thread to jam.
One disadvantage with this particular screwdriver steel for lathe tool purposes is that when ground or cut, it seems to get steadily magnetized. Heating it to red heat and and quenching to harden gets rid of this, but meanwhile it’s a total pain around other steel and steel swarf.
Thread charts say a 25.5mm hole for an M27x1.5 thread, so I bored the 3mm long inner section of the big end to that – the ‘O’ ring groove was a shade larger in ID, though I later realised I should have cut at least part of the ‘O’ ring groove deeper again as a start zone for the thread.
When turning the thread, I set up the spare end of the screwdriver attached with double sided sticky foam to the topslide as a visual depth cue for the inside end of the thread. It’s a metric thread, so the halfnuts had to stay engaged throughout. I got the gear settings from my change gear calculator thingy. I used A=40, B=50, C=65, D=55. I angled the topslide at around 30 degrees for “American style” tool advance, with the handle end closer to the headstock, and paid careful attention to the dial on the slide advance as I didn’t think I was going to be able to re-centre and re-sync it for a second try.
Naturally, having set up carefully to cut the thread, depth gauge set up, tool inside at the innermost end… I started the lathe going very, very, slowly.. forwards. I can’t believe I did that. Crunch into the step behind the bored out bit. Still, as I said, very, very slowly, so no real harm done, just a nick and embrassment. Didn’t even blow the fuse. Ahem. Resuming after backing out and resetting everything, things went fine ’til about the fourth pass – not even any chatter from the tool (I set it up with less protruding than in the photo above).
After a while, it became apparent that I’d not thought the start point through too well, though, I was effectively starting from cold each time, once beyond the minimal extra depth of the ‘O’ ring groove, with the tool nudged into the surface of the brass. The spring in the tool and the fact that it’s pretty sharp and pointy (and cutting only one side because of the angled topslide approach) made this kind of work – the lathe is quite good at starting under moderate load even at very low speed. In retrospect, though, a more elegant work-around for my oversight in failing to cut a proper starting zone would have been to start the lathe on the previous depth of cut and then advance the tool a division or two during the first turn.
Despite the hole being slightly oversize, mindful of my first attempt at threading, I cut a full 1.5mm depth – this needn’t be a super tight fit, but I didn’t want to find it’d not go on.
Having finished the thread, all that remained was turning the unwanted BSP threads off the other end. Initially, I used one of my inherited tools for this, the sharp pointy ones with chip-breaker grooves, but the finish was astonishingly bad compared with what my home-made threading tool was managing. Unfortunately, the threading tool wouldn’t get close enough in to the shoulder because of the angle of it. I switched to a parting tool used at an angle, to skim the last couple of thou off, and that gave a better (but still not great) finish, though I neglected to pay attention to its depth, so there’s a slight groove now where the backside of it touched the annular protrusion on the outer edge of the shoulder (which was there to retain another ‘O’ ring) and biased the point in a shade. Ah well, y’live and learn. The finish was still a bit rubbish, but the dimension not critical so I finished off with some 400 grit wet-or-dry.
I hand cut a slot in the annular projection on the shoulder, the idea being that I could slot in a small round neodymium magnet glued in place with epoxy, for the rev counter. The bottom of the slot is deliberately scuffed up to provide a key for the glue. The magnet I have is a shade too thick, though, so when installed it just rubs on the casing – I’ll cut an old 6mm drill bit off square and use it as a mill to take about 1mm more out of the bottom of the slot, with the piece secured to the toolpost using a drawbar type arrangement, I think, before gluing the magnet in.
I am very slow at this – it took far longer to make than I imagined possible, not just the messing around with making tools out of bits of screwdrivers without a proper grinder, but also the turning operations, particularly the thread cutting. Still, I’m learning stuff, and can only get quicker.