Square-ended drilling

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.

Still, it basically worked out. The picture here seems to emphasise the scored finish, but it looks quite tidy to casual examination in the hand, honest!

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.

About lathenovice

Turning bits of metal into differently-shaped bits of metal.
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4 Responses to Square-ended drilling

  1. Wait til you try machining cast iron 🙂

    If you look at “bought it in a shop” end mills, they look uncommonly like stub drills, only with a flat end. Istr from doing this that the biggest problem is the width of the bit in the middle of the drill (technical name, that) which tends to be too thick up near the shank end of the drill.

    The next thing you need to look up is hardening and tempering – you’ve got the hardening part sussed, it would seem… but tempering is the key to gettin’ it right.

  2. lathenovice says:

    Ahah, yeah, I had a close look at the cutter under magnification last night after I wrote this and concluded that the bit in the middle of the drill (BITMOTD) was indeed one of the issues, it’s devilishly tricky to make the middle bit cutter-y, which Bought-It-In-A-Shop (BIIAS) ones seem to address often by having offset cutters and a gap for the BITMOTD, IYSWIM. Essentially, my BITMOTD wasn’t really cutting, and standing slightly proud moreover, at least after a bit of wear, so basically propping the cutter arms away from the workpiece. I attacked it with a diamond stone under a magnifying glass and re-shaped it, paying particular attention to overall flatness and making the place where the two cutters meet as narrow as possible, then tried it just by hand on a piece of aluminium (passing it through a hole in a bit of wood, which it chewed through very nicely, to keep it in place) – it seems to cut quite cleanly now, so a nice enough drill bit for square bottomed holes but still too long and bendy for use as a milling cutter. Useful proof of concept, mind, milling cutters are expensive 🙂

    As you say, if I’m gonna make reliable tools then I need to get the hang of this tempering business – mebbe next time I’m round your way y’can give me some pointers and we can have a go at some slightly bigger stuff? Fiddly tiny little tools are, well, fiddly and little, but I’d quite like to make (say) a 10mm or 15mm endmill and mebbe a side mill ‘cos I want to be able to make Flat Things using the lathe as a mill.

    I have *read* about hardening and tempering, a bit, and with these little things I’ve this far made, I have a hunch they’ve self-tempered, largely by luck, because of my limited resources – the drill bit I oil-quenched rather than water quenching, which has left it hard but not apparently brittle (no evidence of chipping or flaking at the cutting edge) – I rather suspect that this may be because I quenched just the end, peered at it for while, went to take it out of the pliers and noted it was still rather hot, then cooled it in water. One of the workshop manuals I’ve looked at gives essentially that process for hardening and tempering chisels etc. – quench just the point and then let the residual heat in the rest of the workpiece re-heat it ’til you see the desired colour, then cool the whole piece. Or perhaps it’s just that oil quenching doesn’t introduce so much stress as water quenching, and hard-as-it-gets-with-oil is OK for this steel and this purpose?

    The sort of temper I’m after for these things is just enough to remove the internal stresses, so a “light straw” or so I suppose? With the drill bit, I couldn’t be bothered tio attempt to clean off enough of the TiN coating to be able to see the colour of the metal, I was gonna try that if the “dunk in oil and hope” method hadn’t worked out.

    The screwdriver material didn’t harden much with oil quenching, and I was only able to get the end to red heat as I was using a truly microscopic blowtorch. Quenching the end first, albeit in water, I have a suspicion that perhaps residual heat in the very much bulkier shaft behind it has slightly tempered it, because again, it seems to be good and hard but not fragile. I also noticed when annealing the middle of it (to saw it in half) that the series of oxide colours seemed to come along after a lot more heating than I expected – I have no way of knowing, but I have a strong feeling that the pale straw colour didn’t appear ’til well beyond the couple of hundred degrees they reckon for regular carbon steel – summat to do with the CrV alloy? Or mebbe it’s nickel plated or something, it seems quite shiny.

    I’ll bring ’em along for you to peer at critically next time I’m ’round…

  3. Septic says:

    When machining flat bottomed recesses such as the one in the photos I take a different approach altogether.

    If the centre of the “hole” is within the material to be cut, I first use a small drill, ground to a shallow angle as a pilot to the full depth which removes the awkward centre part and provides an easy path to prevent the larger drill from wandering, especially if part of the cut is (off the edge)

    Then I use the correct diameter drill (once again ground to a shallow angle) until the tip reaches full depth. This prevents “wandering” on the final cut.

    Lastly, I grind the opposite end of the drill into a short, “spade” and put just a tiny clearance angle on either cutting edge, temper it and flip it over in the chuck, using short, light cuts to prevent the swarf building up.

  4. lathenovice says:

    That sounds like an altogether more sensible approach, thanks for the suggestion!

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