If you haven't already, I'll suggest getting some resources on cutting speed for various drill bit sizes and materials. In the most recent Hot Iron News (the quarterly newsletter of the NWBA) there was an article by a machinist called "Drilling Holes: A Few Pointers form a Machinist" and here's some info that might be useful:
- "When drilling a small hole, the initial tendency is to have your RPM too slow. This often results in the drill bit breaking, because the pressure being applied (the “feed rate”) at low speed results in too much “chip load,” meaning it’s trying to cut too much in one revolution.
- Here is the simplest formula to come up with an RPM that will work with a wide variety of low-carbon steels: 240/drill bit dia = RPM
- What do you do when using a hand drill and you don’t know what RPM you are running? Start with guessing. We all can tell if the drill is running at its full speed or barely turning. For small drills, pay attention to your chip. If you start seeing your chip get very thick, you need more RPM and less pressure. For larger drills, if your chips are blue you are starting to run too fast.
- When drilling larger holes, a common practice is to drill a small hole in your material, and then work your way up. The reasoning is that it takes less force or pressure to move the drill through the material. The problem is that the drill bit tends to grab, which can break the bit . . .try using a pilot hole just a bit larger than the chisel point on your final drill bit, and drill the hole in two steps.
- a drill bit with a split point will take less pressure than a drill bit with a chisel point.
- Some drill bits have a gold coating and it’s probably a very good choice. This gold coating is harder and more slippery and works better than the substrate that the drill is made of. That doesn’t mean you can’t sharpen one of these drills - you just lose the benefit of the coating.
- another common option you may have at the hardware store will be a cobalt drill bit and this is usually the best drill bit you can find there for drilling steels.
- Cobalt increases what is called “red hardness.” and the higher the % Cobalt, the better the bit holds up to heat.
- When drilling stainless steel, at least 5% cobalt holds up well and you’ll need to slow your cutting speed
- For stainless, you need a sharp drill bit (Will the cutting edge of the drill scratch your fingernail? If not, it’s not sharp enough for stainless
- You do not want your drill bit to rub. This can cause “work hardening.” To keep your drill bit from rubbing you need a little higher feed rate or more pressure. If you’re not making a chip you’re rubbing.
- use oil, or coolant with a higher percent of cutting fluid, not water. Stainless needs the lubrication."
here's some of what he had to say about stainless:
- When drilling stainless steel, I have found at least 5% cobalt holds up well.
- Stainless is tougher than low carbon steel but it is not necessarily “harder” so it’s more like drilling rubber than glass.
- For stainless, you need a sharp drill bit (Will the cutting edge of the drill scratch your fingernail? If not, it’s not sharp enough for stainless).
- You’ll need to slow your cutting speed down to somewhere close to 35 to calculate your RPM. (what this means is that the equation above changes to 210/drill bit dia= RPM)
- You do not want your drill bit to rub. This can cause “work hardening.” To keep your drill bit from rubbing you need a little higher feed rate or more pressure.
- use oil, or coolant with a higher percent of cutting fluid, not water. Stainless needs the lubrication.
- You need bit with increased web thickness if you are being aggressive and drilling through a tough material like stainless.
