Topics include, Machine Tools & Tooling, Precision Measuring, Materials and their Properties, Electrical discussions related to machine tools, setups, fixtures and jigs and other general discussion related to amateur machining.
grip a (damaged) drill by the fluted end, and take a light pass
Building on that idea. I am not keen on holding a drill bit in a collet I may not even have and running my cheap carbide tool into hardened HSS but a quick and easy alternative may be to clamp the bit in the wise and draw a file.
I knew I saved all those broken bits until the day I came into a method to make good use of them.
If the shank was hardened it wouldn't get damaged, and even if it was carbide could handle it. A file won't give you concentricity. I'd be worried about damage to the collet, though.
It depends on how many you'd want to try machining in a lathe of course. A short piece of round stock held in the lathes 3 jaw, faced and drilled, bored for your chosen drill size and then cross drilled and tapped for a set screw would lock it down by one of the drill flutes and keep it concentric Mauro. Probably with decent collets there hard enough there wouldn't be any damage, but I still refuse to use double ended end mills in mine for the same reason.
My comment related to finding the transition between the hardened and soft segments. Not any kind of machining. Concentricity here is not a concern . A swipe with file will immediately let you feel the difference. Mark the spot with a magic marker, cut off most of the hard flutes and what is left is a soft shank with a short hardened portion on the end. If you can grind the tool you need inside the hardened portion you got it made.
A couple thoughts.
The hard portion has been, in my experiences, well beyond the end of the flute. Far enough that you can grip a short portion of the shank with a collet and still have part of the hardened end exposed.
Interesting, although misguided thoughts about not using double ended end mills in a collet. There's absolutely no reason why one shouldn't use them, as the gripping portion of a collet is short---far shorter than the length of the end mill shank. There's a relieved area beyond that portion (of the collet), so the cutting edges don't make contact with the body of the collet. And, in the R8 example, the collet is extra long, to accommodate the long end mills. Just don't extend the end mill unnecessarily, so you risk gripping by the flutes. That would damage the end mill, anyway, so it's just not smart to do that.
One more thing. In the thread that revolves around the commercial build of a UP Northern, youtube presentations are posted. In them I have witnessed the use of die sink end mills when they are not really needed. There's an endless cacophony emanating from their use. Keep end mills as short as possible, and grip them as closely to the flutes as is possible. They'll perform so much better, and usually silently, yielding far straighter cuts. Reserve the use of die sinks for when they are really needed.
H
edited to add content
Wise people talk because they have something to say. Fools talk because they have to say something.
Yes your obviously correct about the reduced cross section with double ended end mills and since I've never bothered to buy any I'd forgotten about that. For the most part they still wouldn't work for me anyway. With my ER-40's as an example I have approximately 1.8" in total holding length available and roughly 1" in maximum diameter for the largest collet. Minimum suggested shank length due to the collet collapsing from both ends is approximately 2/3rds the collet depth. For end mills of even 1/2" diameter or more you'd need very short end mill flute lengths verses the plain shanks middle section to even work. It could be done with much smaller diameter double ended end mills in a ER-40 sized collet. Since any of the double ended end mills I've seen all seem to come with the Weldon shank design then my conclusion is there generally meant to be used with either an R8 collet or in an end mill holder and not with ER collets.
