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18th Nov 2021, 04:10 PM #1
? new drill bit won't cut on crossdrilling
I don't understand. I've got a 10mm ms rod that I'm trying to cross-drill with a 2.5mm drill bit. I've got it on a pedestal drill, in v-blocks, brand new cobalt drill bit (coz I had no luck with the previous new hss bit, or the previous bit before that), using spray lubricant, going at about 680rpm -- just won't cut. It's got down about 0.5mm, that's all. Why?
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18th Nov 2021, 04:29 PM #2Most Valued Member
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Three drills not drilling the same rod, I’d suggest the rod isn’t MS or is hardened. Is it new stock or is it recycled?
A drill that size also needs to be run much faster than 680rpm.
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18th Nov 2021, 04:40 PM #3
Hmm, I don't know too - it's the original axle of my old Champion buzzbox welder, mfd c.1980 in Australia (I'm crossdrilling it for a split pin instead of cheap axle caps). I just assume it wouldn't be any fancier than plain mild steel.
My first attempt was at 1500 rpm -- I reduced the speed when I realised it wasn't cutting, to try stop the bit overheating. Maybe I've work-hardened it?
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18th Nov 2021, 05:07 PM #4Most Valued Member
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That is not beyond the realms. Can you drill the hole in a different position to test that theory?
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18th Nov 2021, 05:09 PM #5Member: Blue and white apron brigade
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That size needs about 3000rpm. Close up photo of the drill bit tip might be worth seeing
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18th Nov 2021, 05:09 PM #6Most Valued Member
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I had similar problem in the past with stainless, trying to drill with a blunt drill bit and being lazy. The blunt drill work hardens the stainless and makes it very difficult to finish the hole, even after Sharpening the drill.
Not sure if that's what's happened here. Could be a hardened steel shaft?
Seems strange that all three attempts had the same result. Cant imagine you work hardened it. That would mean it's a hadenable steel shaft originally in its annealed state which I'm not sure that would be the case.
I'm going for it being hardened all along.
Simon
Sent from a galaxy far far awayGirl, I don't wanna know about your mild-mannered alter ego or anything like that." I mean, you tell me you're, uh, super-mega-ultra-lightning babe? That's all right with me. I'm good. I'm good.
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18th Nov 2021, 06:29 PM #7
Thanks for the replies! Well I don't know what the problem was. The problem solved itself -- that is, I can't say I had any skilled input to the result.
I got obsessive, tried a whole heap of drill bits and overworked the Drill Doctor -- hit the hole with one sharpened bit after another. They all just skated on the metal at the bottom of that 0.5mm deep hole. But, luckily I gave it another go with the cobalt drill (which I thought I'd munted, so had no real hope -- as I say I was merely obsessive by this stage) and lo and behold I got an actual chip, then another and I was through whatever that hard surface was -- after which, the hole drilled as normal (at 640 rpm!) and no problem exiting from the other side.
Weird. Open to anyone's theories.
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18th Nov 2021, 06:51 PM #8Most Valued Member
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Was the job centred correctly under the drill?
Was the drill bit starting in a pop mark?
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18th Nov 2021, 07:04 PM #9
Ah, could this have been my undoing? The start was by a dimple I ground with a Dremel engraving bit. The centering -- mmm, can't be sure it was absolutely centred -- the Dremel dimple does allow misalignment and the small bit can indeed bend a bit while still remaining engaged.
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18th Nov 2021, 08:44 PM #10Most Valued Member
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There is the possibility that the drill was running off and not allowing it to cut on its edges, you probably after numerous attempts got the drill cutting on a full diameter.
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18th Nov 2021, 09:06 PM #11
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18th Nov 2021, 09:13 PM #12
Hi Guys,
Small drills are notoriously difficult to sharpen properly ! I tend to use carbide PCB drills for holes less than 3 mm in diameter, but they do need a very high rotational speed compared to a drill double the diameter.
I have a small hand held jig that I use for drills 0.75 mm up to 2.5 mm, but you do need an eye glass in order to see to get the facets in the right place. Once correctly aligned in the jig it only takes a couple of swipes on a 1000 grit diamond stone and its sharp again ! Unless you have broken it In which case its easier to bin it...Best Regards:
Baron J.
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18th Nov 2021, 09:24 PM #13Most Valued Member
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Glad you got it sorted. I can’t offer any idea as to why you had so much trouble. I would only add that the comments about your RPM being too slow are correct for mild steel, but not stainless steel. For SS you want a cobalt bit, low RPM, lube and a high feed rate (i.e. lots of pressure on the drill press handle). I’m not saying your axle was SS, just that if it was, a high RPM is a recipe for a blunt drill bit in quick time.
You didn’t have the depth stop set on the PD did you?Chris
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18th Nov 2021, 09:26 PM #14Most Valued Member
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18th Nov 2021, 10:48 PM #15
I'm no expert, but I wouldn't have thought so -- it cuts its hollow pretty quickly, and it's done. Maybe its true the problem was (or exacerbated by) too low rpm, but 1500 rpm on that pedestal drill (as I used initially) already makes it rattle -- just a standard Asian drill press I bought from Hare & Forbes about 20 years ago. The only refined thing about it is the Jacobs chuck I put on it. Other than that, its a noisy, shaky thing with probably too much runout. Thing is, this problem doesn't happen always -- I only ever had one other time on that drill press that I couldn't drill through a spindle, and that was some weird automobile steel part, and I'll put the difficulties down to the hardness of that steel and that I was using number drill -- it was a tiny part. And it always gets through bar, plate and sheet. I think I'll pay more attention to lining up the work next time I'm cross-drilling, in case work hardening occurred due to the bit coming onto the work in an unbalanced way.
I kinda like that idea of carbide pcb drills for small diameters, as per BaronJ -- I might try that out too next time.
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