Why bits fail in metal
Most ruined bits die from heat. Drilling metal creates friction, and a dull or overheated bit loses its hardness and stops cutting. At that point it rubs rather than cuts, which makes more heat, and the bit may turn blue or black at the tip. A bit can also snap if the work shifts, if the bit binds as it breaks through, or if too much side pressure is applied.
Four habits prevent most of this: the right speed, enough lubrication, firm clamping, and a sensible feed.
Match the bit to the job
Bit material matters. Plain high-speed steel bits handle mild steel, aluminum and softer metals. Bits with cobalt content or special coatings generally cope better with harder or tougher metals such as stainless steel, though they cost more and can be more brittle. The overview of drill bits for metal and the comparison of bit material cover these differences in more detail.
Sharpness matters as much as material. A dull bit generates heat and forces you to push harder, which in turn dulls it further. If a bit produces dust or smoke instead of curled chips, it is probably time to replace or resharpen it.
Speed: slower than you may expect
As a general rule, larger bits and harder metals call for slower speeds, while small bits in softer metals can run faster. Running too fast is one of the most common causes of burned bits. Charts of recommended speeds for different bit sizes and materials are widely available, and following one is better than guessing. The idea behind it is explained in the article on tool RPM.
A drill press or a drill with variable speed control makes it easier to hold a suitable speed. With a hand drill, a gentle trigger squeeze and a steady feed are better than running at full speed.
Use lubrication
Cutting fluid or cutting oil reduces friction, carries heat away and helps chips clear. Apply a little at the start and add more as you go, especially for steel and stainless. Aluminum often drills well with a light lubricant, and some people use a special cutting fluid for it. Cast iron is traditionally drilled dry. Check what the fluid manufacturer recommends for each metal.
Wipe up spilled oil. It makes floors slippery and workpieces slide.
Clamp the work
A hand-held workpiece can spin with the bit, especially at breakthrough. Thin sheet is the worst offender, since the bit can grab and lift the sheet, spinning it into your hand. To prevent this:
- Clamp the piece to a bench or hold it in a vise or drill press vise.
- Back it with a scrap of wood so the bit has support at breakthrough.
- Keep your hands clear of the area around the bit.
Clamps and fixtures that you build or buy come under what are called workholding systems, and they are worth setting up before any serious drilling.
Start with a punch and a pilot hole
Bits tend to wander on smooth metal. Make a small dimple with a center punch at the mark so the tip has somewhere to sit. For holes larger than about a quarter inch (6 mm), many people drill a smaller pilot hole first and then step up to the final size. The larger bit then has less material to remove at its center, which reduces the force needed.
For thin sheet, a step drill produces round holes with little burr. It works in stages rather than in one cut.
Feed pressure and chips
Steady, moderate pressure is the aim. Look at the chips: ribbons or curls generally mean the bit is cutting properly. Fine powder suggests the bit is dull or the pressure too low, and blue smoke suggests too much speed or heat.
For deeper holes, back the bit out from time to time to clear chips and add lubricant. Ease off as the bit nears the exit, since that is when it is most likely to grab.
Safety
- Wear safety glasses. Chips can be hot and sharp.
- Do not wear gloves or loose sleeves near a spinning drill, and tie back long hair.
- Do not clear chips with your fingers. Use a brush or pliers when the drill has fully stopped.
- Unplug the drill or remove the battery when changing bits.
- Expect sharp edges and hot metal after drilling. Let the part cool and clean the hole with a deburring step.
A quick routine
- Mark the spot and center punch it.
- Clamp the work and back it up.
- Choose a sharp bit suited to the metal.
- Set a suitable slow speed.
- Add cutting fluid.
- Drill a pilot hole if the final hole is large.
- Feed steadily, clear chips, and ease off at breakthrough.
- Deburr the hole.
This article is general information. Follow the guidance in your drill and bit manuals.