I hesitate to call it “milling”. I made a cutter to deepen the magnet hole on my spindle extension, trying it out on a spare bit of brass first. Could have been better, but given that I wanted roughness for the epoxy to key into, it did the job.
I have some drill bits made of cheese. Bought from Wilkinsons, labelled “Am-Tech F1140,” they seemed like too good a price to be true, and were. They’re in a metal box (which is OK, and worth keeping). It says “coated with Titanium Nitride” and various other claims which broadly attempt to give the impression that they’re worth having. It’s mostly lies, as far as I can tell – the “special anti-wander profile”, for example, produces the most wandery drill bits I’ve ever experienced. At base, though, I think they simply forgot to harden them. The first one of these I tried was a 10mm one, which went blunt the instant I showed it a bit of mild steel. Next, I tried the 2mm one, which I was abusing with a hand drill when there was an ominous bang and and lurch. Now I’ve abused a few twist drills in my time, and that always means one thing: the splintered remains of the drill bit might or might not be stuck in one’s person somewhere. On this occasion, not a bit of it – this one had bent through 90 degrees! Moreover, I straightened it out and it still didn’t snap! It was also completely blunt, obviously. I filed some new edges on it, hardened it with water quenching and tried again. Then it shattered.
So anyway, the point of this digression is that I have a stock of useless un-hardened drill bits which appear to be harden-able, and I wanted to make a thing like an end mill to cut the little magnet hole in my spindle extension a bit deeper. I filed the pointy end of the 6.5mm one flat (it didn’t need softening first, and the file cut it really easily) then I gently trued up the flat in the lathe, filed what I hoped was a suitable cutter-ish sort of profile on it, then heated it to red heat and oil quenched it (mindful of how brittle the water quenched one had been, but disinclined to attempt to temper it given that it was a funny colour from the TiN coating.)
I should point out that I’m aware that this isn’t an end mill per-se on several counts – it’s far too long, so won’t take any side loads at all, the flute profile is inappropriate, the cutter edges are dubious, etc. I really just wanted a flat-bottomed drill.
Tentatively skidded a file over the tip – seemed hard. Good. Honed it a bit and chucked it up, then clamped the only spare bit of brass I had to hand – a little pipe end-stop/washer thing – into the toolpost using a nut for a spacer. Filed a flat on the edge, so I had a not-full-width flat target – the intended use called for it to cut over the edge of the workpeice.
Anyway, the test piece seemed to go OK, so I concocted a clamp for the spindle extension using an 8mm coach bolt, a big washer, a socket spanner and a certain amount of artistic licence (see pics). Then I crossed my fingers and used the saddle handwheel to gently advance the workpiece against my “mill”. I think the spindle extension brass is harder, and the cutter a bit worn from the (not exactly demanding) test, but it still managed to produce some little curls of brass turnings rather than just wearing it away, though the finish isn’t very even.
I think the grooves are partly down to the relief angles I ground on the cutter being too shallow (so it skids, and/or chips get trapped?) and partly ‘cos the cutter itself has worn – either bluntening because it’s not very good steel, or perhaps chipping because I made it too hard, I haven’t examined it under enough magnification to figure that out.
I might re-grind the Am-Tech drill bit into a drill bit again and try re-hardening and tempering it, just to see if working bits can be made from these things.
A final observation about brass, perhaps it’s just poor technique and inappropriate cutters, but the damned stuff seems to get everywhere – I swear it took as long to clean the lathe down afterwards as to do any of the above. While the cutter I used to make the threads on this thing produced continuous curls of swarf, even they seemed to break up the moment they landed, and various other operations produced loads of nasty fine chips and filings and dust which stuck to everything.