Post #4480015

A brief brain dump about drilling in metals: The short version: modern high speed steel twist drill bits are ok to use for drilling in wood, aluminium, brass and mild steel. People will say use lube on steel but I rarely bother with lube below 10mm diameter, a good rule of thumb is that they can be run at 2000rpm up to 5mm, 1000rpm up to 10mm and 500rpm or less above. Faster in things other than steel. They will be "grabby" in copper-based alloys like brass, more on that downthread. The long version: Iron, as found in chunks near that crater made when the star fell from the sky during your grandmother's time, is superior to copper and bronze for both building pyramids and for defending the village from Thulsa Doom's horde. But it's kind of soft and never stays sharp as long as that bronze knife you bought from a traveling trader. Now STEEL on the other hand, is good stuff. Steel, as was known since the time before the oceans drank Atlantis, you can trust. Adding a small amount of carbon from charcoal or coal makes it both harder and more ductile (able to bend without breaking). Ordinary, or "mild" steel is iron with about 0.2 percent carbon, and is used for most structural purposes. Medium carbon (steels around 0.5%) are used for high-wear applications like bearings and axles. For cutting tools (including cutting and drilling of iron and those lesser steels) we use high-carbon steels, containg around 1--2% carbon. High carbon steel (unlike mild steel) has the property that it quench-hardens. Heat it orange hot and cool it rapidly in water, oil or the body of a handy vanquished enemy, and the crystal structure of the metal changes; it becomes harder, tougher, but also more brittle. The brittleness can be reduced by "tempering", heating it to a few hundred degrees changes the crystal structure again to retain much of the strength but gain flexibility. The degree of tempering depends on use - a file has been minimally tempered to retain maximum hardness but is so brittle it could snap or shatter if mistreated. A drill bit receives a medium temper and could still snap if abused. A knife, sword or axe is generally tempered to achieve the ability to survive impacts, sacrificing maximum hardness for reliability. A broken sword on the battlefield often disappoints. If a hardened carbon steel is heated above its critical temperature, it loses its temper. That is, the crystal structure begins to revert to its pre hardened state. Heat it red hot and let it cool gradually (called annealing) and it regains maximum softness. For tools, this means you must be careful to preserve the temper. Drill too fast, or overheat the metal when sharpening at the grindstone, and the blade is ruined and will no longer stay sharp, requiring re-hardening. This is why we use coolant when drilling, and quench blades when grinding, in order to avoid inadvertently annealing the hardened steel, ruining the temper. (ctd.)