Brass is soft. It should be easy to drill. And then you try it with a standard HSS bit and the drill grabs, the workpiece tries to spin, and you get a hole that looks like someone drilled it with a corkscrew. Experienced machinists learn fast that "soft metal" and "easy to drill" are not the same thing.
The problem isn't the material's hardness — it's the cutting geometry. Standard HSS drill geometry is designed for steel. When you put that geometry into brass or bronze, you get behavior that ranges from annoying to dangerous. Understanding why, and what to do about it, makes brass and bronze work straightforward.
Why Brass Grabs: The Rake Angle Problem
A standard HSS jobber drill has a positive rake angle — the cutting edge is inclined in a direction that pulls the bit into the cut. In steel, that positive rake is controlled by chip formation. Steel work-hardens slightly as the chip forms, and the material's resistance gives the cutting edge something to push against. The rake helps, the chip breaks, and everything runs smoothly.
Brass behaves differently. It's ductile and cuts cleanly, but it doesn't work-harden. When a positive-rake drill enters brass, there's nothing to resist the rake angle pulling the bit forward. The drill self-feeds — it threads itself into the workpiece instead of cutting at a controlled rate. On a drill press with the workpiece hand-held or lightly clamped, that self-feed grabs the work and yanks it. On a lathe, the bit will pull itself into the cut faster than the carriage is feeding and snap.
This is called "grabbing" or "self-feeding," and it's the defining hazard of drilling brass with standard geometry.
The Fix: Reduce Rake to Zero or Slightly Negative
The standard correction for brass drilling is to reduce the cutting edge rake to near-zero or slightly negative. This removes the forward-pulling tendency and forces the drill to cut at the rate you're feeding it rather than the rate it wants to feed itself.
On a resharpened or reground bit, this is done by relieving the cutting face slightly — taking a small flat on the edge that reduces or reverses the rake angle. The result is a drill that requires you to push it into the cut rather than one that pulls itself in. It feels different to run, but the hole quality improves dramatically and the workpiece stops trying to grab.
You can do this modification on a bench grinder with a soft touch, but it's easy to get wrong. A machine regrind that incorporates the rake reduction produces consistent results across both lips. Inconsistent rake — one lip reduced, one lip not — creates a worse situation than standard geometry because now the two lips are pulling differently.
Chatter and What Causes It
Chatter in brass drilling is usually a geometry or setup problem. The most common causes:
- Positive rake on one lip only: As described above, asymmetric rake causes the drill to pull unevenly and walk, producing chatter and oversize holes.
- Insufficient spindle speed: Brass likes higher surface footage than steel. Running too slow produces a poor chip and causes the edge to scrape rather than cut.
- Excessive relief angle: Too much relief behind the cutting edge makes the bit chattery in soft materials because the edge is too aggressive. Moderate relief (8–10 degrees) works better in brass than the higher relief angles used for aluminum.
- Loose workpiece or fixture: Brass's tendency to self-feed means any looseness in the setup will cause a sudden grab and chatter event. Work needs to be positively secured.
Bronze Variants and How They Differ
Bronze isn't one material — it's a family, and the drilling behavior varies more than most machinists expect.
Phosphor bronze (C510, C544) is harder and tougher than brass. It doesn't self-feed as aggressively, and it can tolerate slightly more positive rake. It also work-hardens more than brass, which means dull tools cause real problems — a dull drill in phosphor bronze will rub, generate heat, and harden the surface of the hole, making the next pass worse. Sharp tooling matters more here than in free-cutting brass.
Aluminum bronze (C954, C955) is significantly harder — some grades approach mild steel in hardness. It generates more heat and is more abrasive than standard brass or phosphor bronze. HSS works, but cobalt HSS performs better in high-volume applications because it holds its edge at higher temperatures. The rake angle concerns still apply, though the severity of self-feeding is reduced compared to free-cutting brass.
Silicon bronze and naval brass tend to fall between the extremes — tougher than free-cutting brass but not as hard as aluminum bronze. Moderate positive or near-zero rake usually works fine for both.
Lubricant vs. Dry
Free-cutting brass (360, 385) is typically drilled dry. The material cuts cleanly, generates minimal heat at correct speeds, and coolant is more nuisance than help. Cutting fluid can actually cause chip welding issues in very free-cutting alloys where the chip breaks so cleanly that there's nothing for the fluid to flush.
Phosphor bronze and aluminum bronze benefit from cutting fluid, particularly at depth. These alloys generate more heat and the chips are longer and tougher. A light sulfurized cutting oil or a general-purpose coolant helps with chip evacuation and keeps the tool from overheating on through-holes. Dry drilling is workable for shallow holes; go wet for anything over 2–3 diameters deep.
How Resharpening Affects Brass Performance
A brass-specific regrind (near-zero rake, moderate relief) transforms how a standard HSS drill behaves in these materials. If you run a significant amount of brass or bronze, it's worth keeping a dedicated set of bits ground specifically for non-ferrous work rather than trying to run the same geometry on everything.
When you send brass-specific bits for resharpening, note that on the order. A resharpener who knows what the bits are being used for can maintain the appropriate rake geometry rather than restoring them to standard steel-cutting geometry. MachinistPost handles this — just label your envelope or include a note with the order.
MachinistPost resharpens HSS drill bits by mail from anywhere in the US. If you run brass, bronze, or other non-ferrous work and want geometry optimized for it, note that when you ship — we'll regrind accordingly. Learn more at machinistpost.com.