Cobalt Drill Bits

They're pretty much obsolete. They combine the hardness of low-grade HSS with less thermal tolerance than carbide, and with most of the brittleness of carbide. Micrograin carbides are better in almost every way, especially when you aren't using them in modern, rigid machine tools. For use in modern machine tools, other carbides may be better. For use in hand-held drill motors...I'll still stand back and watch.

manufacturing environment. Once had to remove a fastener made of very hard material (set screw as I recall). Went through several HSS bits and barely made a divet. One of the other techs provided me with a cobalt bit that went through the piece like butter.

By hand? I'd be surprised if that was the case. What cobalt offers is hardness at elevated temperatures. In the case of the set screw, money says that you heated the screw to the point of hardness collapse. It doesn't take much if it is hardened by the carbon cycle.

several rates from slow to max.

You've already been wished good fortune. I'm going to suggest that it probably won't happen. Cobalt drills have a much heavier web than plain HSS drills, so the pressure required to drill is much greater. I doubt you can keep the needed pressure on the hardened piece long enough to raise the temperature to the point where it will drill well. Any chance you can tip the building on its side and get it under a spindle with a lever feed?

Harold

1. These fasteners are used to hold storm shutters onto window frames, have been in place for a while, and are apparently made of nuclear-grade steel. I can't imagine why such a hard fastener was used in the 1st place; I've had less trouble removing a broken head bolt from a 302 with the engine in the car......

Lopping off the heads with a Dremel leaves the remainder of the screw in place (obviously) which pins the screen frame to the window frame. Trades one problem for another.

I have nine shutters to go.

Ernst

Hmm ... I've never heard about *drill* bits made of Stellite, but I have a couple of *lathe* bits (thanks to Harold Vrodos, from whom I learned about them.

Making drill bits from that is probably as difficult as making them from solid carbide -- but they would be nice to have.

Agreed -- but only if the shank of the screw is large enough so a reasonable strength drill bit could be used.

Next time! Too late now. (And he probably did not have any control over how they were originally installed. :-)

Good Luck, DoN.

Begs some questions: What is the proper technique? Lube? no lube? high speed or low?

Well, I had considered sinking bolts into the structure in order to mount a rectangular steel framework, to provide support for a chainsaw- motor-powered drill head.... ;-)

Ernst

Haven't done any chemical, spectrometric, or rockwell analysis on the bits, so the short answer would be "no, I'm not sure". Package says "Cobalt" although there are no MIL-SPECs or other grading standards listed. From what I gather above, cobalt is by degree, so I suspect these are on the fringe.

Seems they are inappropriate for the task anyway. I'm going to try the bits with a grade-8 fastener and see how they fare.

Thanks. I'm going to use valve-lapping compound on the screwdriver for more bite and hopefully avoid too many more screws that require special attention. The Dremel is the silver bullet this time.

Ernst

My limited experience when altering some existing defense parts was to run (reasonably) fast, with no lube. Bear down, then when it's hot enough, the drill starts cutting. Heat, on heat treated parts, is very much part of the reason these drills work. They tend to anneal the hard material at the point of contact. The cobalt content allows them to retain the necessary hardness at elevated temperatures.

Harold

Maybe an easier approach would be cutting away the broken off screws, Ernst.

There are some small hole saws for use in drills or die grinders with 1/4" collets that could be used to cut out the base metal around the frozen screw. I saw some yesterday that had diamond grit on them, but carbon steel saw teeth or carbide grit would likely be better.

Another method would be to use a piece of tubing in a drill chuck and dip it in a grit paste like valve grinding compound. May get a bit messy, but a good solvent on a rag should be all that's required for cleanup, just avoid too much rubbing.

I think you mentioned the screws were in a window frame of some sort. If there is a feature (channel or edge) that would position a guide over the screw stub (so the hole saw can't run away), make up a guide for a small hole saw that could be clamped into position for cutting out a broken screw, then repositioned over another, etc.

It shouldn't be too difficult to find something to fill the hole (push-in plastic gizzy, etc) for a new screw to anchor in.. or just fill the holes with some "miracle hole filler in a tube" and screw into that.

When I had a screw in the corner of a shower door panel with the head broken off, I was able to go through the slot in the extrusion with a Dremel cut off wheel and cut a slot through the axis of the remainder of the screw whereupon the two slivers of screw were easily persuaded to fall out. Gerry :-)} London, Canada

Yep, a Dremel tool or similar rotary tool can save a lot of grief sometimes. A very valuable tool when they are needed.

Coblat isn't that much harder than HSS, they just have better high temperature hardness. You might try repointing a masonery bit. Not guaranteed, but it's a lot cheaper than full blown carbide drills.