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Copper pipe is easy to dent, and a drill bit can grab the curved wall hard enough to twist or flatten it. The right bit helps, but setup matters just as much: support the pipe, keep the hole small, and use low speed with light pressure. For a clean hole in thin plumbing tube, a sharp twist bit is usually the practical choice; for a larger opening, a step bit can reduce grabbing if the pipe is firmly supported.
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Choose the bit for the hole
For small holes—roughly 1/8 to 3/8 inch—a sharp high-speed-steel (HSS) twist drill is inexpensive and works well. A split-point tip, often listed as 135 degrees, is less likely to wander than a standard 118-degree point. Cobalt bits are tougher and hold up better in repeated work or harder metals, but they cost more and are unnecessary for an occasional hole in soft copper.
For holes larger than about 3/8 inch, consider a step bit. Its staged cutting edges enlarge the opening gradually, which can be easier to control than a large twist bit that catches at once. Check the bit’s stated range and use only the step that matches your target diameter. Step bits are not magic: on unsupported, thin-wall pipe they can still pull the tube out of position or distort it.
A hole saw is generally a poor first choice for thin copper pipe. Its broad teeth can snag, chatter, or crush the tube unless the work is held securely. It makes more sense for larger holes in supported sheet or thicker material than for a small opening in plumbing tube.
| Bit type | Best use on copper | Main trade-off |
|---|---|---|
| HSS twist bit | Small holes, occasional jobs | Low cost; larger sizes can grab |
| Cobalt twist bit | Repeated drilling or tougher metals | More expensive; little benefit for one soft-copper hole |
| Step bit | Gradually enlarging a supported hole | Useful control, but still needs firm support |
| Hole saw | Large openings in well-supported material | High snagging and crushing risk on loose pipe |
What to buy
For a one-off small hole, a basic, sharp HSS split-point drill bit is usually enough. Buy the diameter you need rather than forcing a much larger bit through in one pass. If you routinely drill metal or need many holes, a cobalt drill bit set can be worth the added cost for wear resistance, though it will not prevent a pipe from collapsing.
If the job calls for a larger hole, a step drill bit for metal is a sensible option when you can clamp or otherwise support the pipe. Choose a bit with a cutting range that includes the target size; there is no need to buy a large set of steps you will not use.
Support the pipe before drilling
Drain and disconnect the pipe before drilling. Do not drill a live water line, and do not drill pipe that may contain gas or another hazardous material. If you are unsure what a line carries, stop and identify it first. Wear eye protection, and deburr the hole afterward; a sharp burr can cut hands or damage anything routed through the opening.
Clamp the pipe close to the drilling point, but do not crush it with the clamp. A vise with soft jaws, or padded blocks secured around the tube, can hold it without concentrated pressure. If possible, place a snug-fitting wood dowel inside the pipe behind the hole to support the wall. The support should fit closely enough to resist the bit’s push without being forced in so tightly that it expands the pipe.
For a small hole, mark the spot with a fine-point marker or a light center punch. Avoid a hard punch blow: copper is soft, and denting it before drilling defeats the purpose. If the bit wanders on the curved surface, a shallow starter mark or a small pilot hole can help.
Drill slowly and let the bit cut
Use a cordless drill in its low gear, not an impact driver or hammer mode. Start around 500–1,000 rpm for small twist bits, using the lower end for larger diameters. Keep the drill square to the pipe and apply steady, modest pressure. A dull bit, high speed, or leaning hard on the tool can cause heat, a ragged hole, and sudden grabbing.
For a hole larger than about 1/4 inch, drill a small pilot hole first—around 1/8 inch is a useful starting size. Then enlarge it with the final bit. With a step bit, increase the hole one step at a time and stop as soon as the desired diameter is reached. Back the bit out occasionally to clear copper chips; do not keep forcing it when the flutes are packed.
As the bit breaks through the far wall, ease off the pressure. Breakthrough is when a bit is most likely to catch and wrench the pipe. If the tube starts to rotate, stop immediately and improve the clamping rather than gripping it by hand. Never wear gloves near a rotating bit; loose material can wind around the tool.
Common problems and fixes
The pipe dents before the hole is made: The clamp may be too tight, or the tube lacks internal support. Add a close-fitting backing dowel and spread the clamping force with padded jaws.
The bit skates across the curve: Use a split-point bit, make a restrained starter mark, or create a small pilot hole. Keep the drill perpendicular rather than trying to steer a spinning bit sideways.
The bit grabs or twists the pipe: Stop, secure the tube more firmly, and reduce pressure and speed. A larger bit may need a pilot hole or a step bit. Do not solve grabbing by holding the pipe harder with your hand.
The hole has a rough edge: Use a sharp bit and steady feed, then remove the burr with a deburring tool or a hand-held countersink turned gently. Avoid spinning a countersink aggressively; it can enlarge the hole quickly in soft copper.