On Mon, Jul 11, 2022 at 09:09 AM, Owen Thorne - owen at udel.edu wrote:

So, while choosing high-speed bits for hand-drilling plastic and resin seems counterintuitive, the commonly available carbide bits are specifically NOT recommended, as per above. Does anyone have experience with cobalt or carbon-tipped bits? Or is there another material, tip shape, angle, or coating we should be using for resin or plastic?

Most of the information you quoted is aimed at users who use the drills in machine tools. The material is typically chosen for its wear characteristics... But modelers don't wear drills out, they break them because they cannot keep them lined up with the portion that is buried in the work, which snaps the drill off. Remember, twist drills (the proper name for what you are calling "bits") unlike end mills, are not intended to cut on the side of the tool, so whatever direction the drill starts cutting, the tool has to be held in line, not allowed to bend. This would indicate that the toughest, most flexible drill would have the best chances of survival, and that would be high speed steel. Cobalt steel would be right up there also, but I doubt you will ever find that material in the sizes we modelers need. Solid tungsten carbide (carbide for short) is the exact opposite; extremely hard, but also exceedingly brittle. Carbide tipped drills aren't made in the small sizes, that is indeed a strategy to get hard cutting edges on a tough drill, but there is no commercial need in the sizes we use.

There seems to be a bit of confusion about carbide vs. carbon. Carbide is, as I said, short for tungsten carbide. Carbon is short for high carbon steel, which drills USED to be made of before high speed steel was developed decades ago. Some of the cheap import drills are still high carbon steel. Why? Because it's cheap, its basically high speed steel without all the alloy elements that give HSS its toughness. If you are snapping off drills all the time, they are likely high carbon steel.

As to grind (tip shape) years ago, when I still had eyesight and steady hands, I used to be able to resharpen hobby drills under magnification with a fine stone. In fact, when I snapped all the flutes off the drill, I found I could make an 'emergency' drill by stoning the remaining round rod off at a flat angle, so the result was a flat ovaloid face. These took a LOT more turning, but did get the job done. I wonder if a piece of phosphor bronze wire, like Tichy sells, would work, sharpened like this and driven at slow speed by a battery motor tool? They would certainly be cheap enough.

Dennis Storzek