Drilling Copper and Brass: Why These 'Soft' Metals Cause More Problems Than You Expect

The textbook says brass and copper are easy materials. Low hardness, low melting point, machines cleanly. A beginner can work with them. Then a machinist drills their first piece of copper with a standard jobber drill and spends the next ten minutes trying to extract a broken bit from a part that cost $40 in raw material. The textbook skipped some important details.

Brass and copper are genuinely easy to machine — with the right geometry, the right technique, and an understanding of what these materials actually do under a drill. Without that understanding, they're responsible for some of the most frustrating failures in the shop.

Brass: The Self-Feeding Problem

Brass's primary issue with drilling is self-feeding — the tendency of the drill to advance faster than the operator is feeding it, digging in and potentially spinning the workpiece or snapping the bit. This behavior comes directly from drill geometry.

Why It Happens

Standard jobber drills are ground with positive rake angles on the cutting lips. This geometry is designed for steel: the positive rake lets the edge bite aggressively into material that has enough shear strength to resist being pulled down by the bit. Brass has very low shear strength and does not work-harden. When a positively-raked edge enters brass, there's nothing to resist the cutting force except the operator's hands on the feed. The drill wants to corkscrew in, and on thin sheet or in an unsupported setup, it will.

The Fix: Zero or Negative Rake

Brass requires drill geometry with zero or slightly negative rake on the cutting lips. This is achieved by stoning or grinding a small flat on the rake face of each lip — reducing the cutting angle so the drill scrapes rather than bites. The result is complete elimination of self-feeding behavior. The drill removes material at exactly the rate you feed it, no more.

This modification takes about two minutes with a hand stone on each cutting lip. Alternatively, buy drills specifically ground for brass and non-ferrous metals — several manufacturers offer these in their standard catalogs under "brass drills" or "non-ferrous drills."

Speeds for Brass

Brass drills best at high surface speed — 150–250 SFM for HSS. Higher than mild steel for the same diameter. Chip formation in well-set-up brass drilling should be short and almost powdery at higher speeds. Long curling chips indicate the rake is still positive and the dig-in risk is still present.

Copper: The Galling Problem

Copper shares the positive-rake grab issue with brass, but adds a second challenge: it is one of the most adhesive metals under a cutting tool. Copper atoms have a strong tendency to weld to HSS at elevated temperatures — this is called built-up edge (BUE), and in copper it happens aggressively.

Built-Up Edge in Copper

Built-up edge means copper material welds to the cutting edge and changes the geometry of the drill. The deposited copper acts as a new cutting edge, usually with worse geometry than the original. This built-up material eventually shears off and re-welds, producing a cyclical process that rapidly degrades the hole quality and accelerates drill wear. In copper sheet, BUE is often the reason holes come out rough, oversize, and with torn edges rather than clean cuts.

Fixes for Copper

The same zero-rake modification that fixes brass self-feeding also helps with BUE in copper by reducing the area of contact between the cutting edge and the material. Additionally:

• Cutting oil: A light cutting oil or even kerosene reduces adhesion at the cutting zone. Water-based coolant doesn't provide the same adhesion barrier. For decorative copper where staining matters, use a very light, clean oil.
• Higher speeds: Counter-intuitively, higher surface speeds in copper reduce BUE tendency by maintaining higher cutting temperatures that prevent chip welding in some conditions. Experiment in the 200–300 SFM range.
• Sharp, fresh tools: BUE is more severe on worn edges. Fresh or recently resharpened drills with clean geometry show less BUE development.
• Polished flutes: Tools with high-helix, polished flutes are less prone to chip adhesion. If you're doing regular copper work, dedicated high-helix polished drills pay for themselves.

Thin Sheet Work

Both brass and copper sheet — anything under about 1/8" — presents additional challenges. Thin sheet wants to flex and be pulled through by the drill at breakthrough. Use a backing board of MDF or hardwood directly under the sheet, and clamp both the sheet and the backing board firmly. The backing board gives the drill somewhere to go other than through the part and into the workpiece table, and it supports the sheet against flexing.

For clean, burr-free holes in thin brass or copper sheet, a step drill (unibits) often produces better results than a standard twist drill for hole diameters under about 1/2". The stepped geometry reduces the violent breakthrough event that tears thin sheet.

Carbide vs. HSS in These Materials

Standard HSS with correct geometry is the right choice for brass and copper in most shop applications. Carbide doesn't solve the positive-rake grab problem — the self-feeding behavior is a geometry issue, not a material hardness issue. Carbide's brittleness is actually a liability when a drill grabs and torques: the shock load that HSS flexes through will snap a carbide bit. Save carbide for abrasive materials and high-volume production applications where wear life is the limiting factor.

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MachinistPost can modify drill geometry for specific applications including brass and copper work — zero-rake geometry for non-ferrous metals restored alongside standard HSS resharpening. Mail us your worn bits from anywhere in the US and we'll have them back set up for your material.