There's no universal drill bit. The geometry, material, and coating that work perfectly in aluminum will cause problems in hardened steel or cast iron. Here's how to match the bit to the job — without overcomplicating it.
Before picking a drill bit, you need to know:
Most shops have a good collection of HSS bits and know how to use them in mild steel. The mistakes happen when the material changes and the bit doesn't.
Bit material: HSS (standard) | Point angle: 118° conventional, or 135° split-point | Cutting fluid: Cutting oil or sulfurized oil
The baseline application for HSS. A properly resharpened HSS bit will cut mild steel cleanly at correct feeds and speeds. Cobalt isn't required here — save it for harder materials. 118° works fine for general work. 135° split-point reduces walking at entry and cuts easier with less feed force, which makes it better for handheld drilling or drill presses without great downfeed control.
Bit material: Cobalt HSS (M35 or M42) — standard HSS works but wears faster | Point angle: 135° split-point strongly preferred | Cutting fluid: Sulfurized cutting oil — don't skip this
Stainless work-hardens when rubbed instead of cut. This means two things: use high enough feed to keep the bit cutting aggressively (counterintuitive — slow feed is what causes work hardening), and use consistent pressure without dwelling. RPM should be slower than for mild steel — roughly 50–60% of your mild steel speed for the same diameter. Heat is the enemy. Keep the bit and work cool.
Cobalt's advantage here is that it retains hardness at higher temperatures than standard HSS, giving you more margin when the cut gets hot.
Bit material: HSS (standard) | Point angle: 118° conventional or 90° flat bottom for shallow holes | Cutting fluid: Dry, or compressed air for chip clearing
Cast iron is abrasive but not tough. It cuts easily and produces a powder rather than chips. This is one of the few materials where you drill dry — cutting oil makes the graphite in cast iron into a lapping compound that accelerates bit wear. The main issue with cast iron is that it's hard on cutting edges due to abrasion. Bits used heavily in cast iron need resharpening more frequently even if they don't look dull.
Bit material: HSS (standard) — bright/uncoated preferred | Point angle: 118° to 130°, high helix angle preferred | Cutting fluid: WD-40, kerosene, or light cutting oil
Aluminum's challenge isn't hardness — it's built-up edge (BUE). Aluminum is soft enough that it can weld onto the cutting edge, turning your drill bit into a scraper. High helix angles (faster spiral in the flute) evacuate chips faster and reduce BUE. A 40° helix is noticeably better in aluminum than a standard 30° helix. TiN-coated bits (gold-colored) perform worse in aluminum — the titanium nitride has an affinity for aluminum that promotes BUE. Stick with uncoated bright HSS.
Bit material: Carbide or solid cobalt — HSS will not cut hardened steel without premature failure | Point angle: 135° split-point, full relief angles | Cutting fluid: Heavy cutting oil or soluble coolant
If you're drilling into hardened steel with an HSS bit, you're going to burn up the bit before you finish the hole. HSS simply doesn't have the hardness to cut material that approaches its own hardness rating. Carbide drill bits are brittle — they need rigid setups, no flex, no wobble. They're not for handheld drilling. If you're in a drill press with a rigid setup, carbide is the answer.
Bit material: HSS (standard) — modified geometry helps | Point angle: Steep relief, 60–90° included angle works well | Cutting fluid: Dry or light air cooling
Standard twist drills crack and chip plastics because they grab aggressively. A bit that's been modified to scrape rather than grab makes cleaner holes. You can modify a standard bit by grinding a small negative rake into the cutting lip. Acrylic and polycarbonate crack most easily — slow feed, low RPM, let the bit scrape rather than cut.
| Material | Bit Material | Point Angle | Fluid |
|---|---|---|---|
| Mild steel | HSS | 118° or 135° | Cutting oil |
| Stainless steel | Cobalt HSS | 135° split | Sulfurized oil |
| Cast iron | HSS | 118° | Dry / air |
| Aluminum | HSS (bright) | 118–130° | WD-40 / kerosene |
| Hardened steel | Carbide | 135° | Heavy coolant |
| Plastics | HSS (modified) | 60–90° | Dry / air |
Most of these materials benefit from the same bit — a properly sharpened HSS bit — with adjustments to speed, feed, and fluid. The bit geometry matters, but it's the cutting edge condition that makes the difference at the hole. A sharp bit cuts where a dull bit rubs. A dull bit in the wrong material doesn't just make a bad hole — it makes a damaged bit that's harder to recover.
Send in your dull bits before they get to the point where they can't be resharpened. Flat-rate pricing by size, return in 5–7 business days.
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