Drilling Brass: Why the Drill Grabs and How to Stop It

Drilling brass goes fine right up until the bit screws itself into the part. If you've felt a tailstock or quill lurch forward, you know the feeling.

Why Does a Drill Self-Feed in Brass?

A standard twist drill has positive rake on its cutting lips, and free-cutting brass offers little resistance, so the drill acts like a screw and pulls itself in. It's worst when you're opening up an existing hole.

That's what happened to an r/Machinists user. He was opening a 3/16 hole up to 7/16 on his lathe, and the drill pulled the tailstock in and fed itself about half an inch. Replies said to hone a little flat on the cutting edge and to look up dubbing. One said to lock the tailstock and use the feed wheel instead of pushing an unsecured one.

Drilling brass with a dubbed drill

Dubbing means stoning a small flat on each cutting lip so the rake goes to zero or slightly negative. Robert Bastow's notes on the yarchive brass drilling page describe the same fix, done equally on both lips. For a 3/4" drill he suggests a flat somewhat less than 1/16", and on small drills a few wipes with a slip stone. It's a newsgroup archive, not a handbook, but it matches the thread.

If you'd rather not grind, one reply suggested Type H drills, which have a steeper spiral and less sharp edges that limit self-feeding.

Keep any dubbed drills separate and labeled. The original poster's worry was that most of his work is steel and he wanted sharp edges for it, which is why people keep a dedicated brass set.

What C360 actually handles

C360 is the benchmark for free-cutting metal. The Copper Development Association's high-speed summary calls C36000 the standard other materials are measured against and says it's about five times more machinable than leaded steel. In tests on five brass alloys, drilling ran up to 2,000 SFPM with an uncoated 0.50 in carbide drill. Those were lab runs on a Makino machining center, so don't copy them onto a drill press.

On a CNC the feed is programmed, so the drill can't drag the axis forward the way it dragged that tailstock. The grab is mostly a hand-feed problem.

For other hole problems, see carbide drills chipping out and tap drill percentage. Cross holes on a lathe part raise the live tooling vs separate mill op question.

At DigiForge

Brass C360 is on our Materials page for fittings, electrical and decorative parts. Our CNC service lists ISO 2768-m as the standard tolerance. If a hole needs to be tighter than that, put it on the drawing and ask us when you quote.