How to tap a hole

Tapping a hole means cutting a thread into it so a bolt or machine screw can screw straight in. It takes a drill, the right size drill bit, a tap, a tap wrench and some oil. Done carefully, a tapped hole in steel or aluminum is as strong as a nut, and often stronger.

What you need

• A tap for the thread you want, such as 1/4-20 or M6 x 1. Most hardware stores sell plug taps, which have a few tapered threads at the tip and suit most work.
• The tap drill for that thread. Find it on the tap drill size page for your thread.
• A tap wrench: a T-handle for small taps and a two-handled bar wrench for larger ones.
• Cutting fluid: tapping oil for steel and stainless; a light oil, WD-40 or kerosene for aluminum. Cast iron and brass tap dry.
• A center punch, a countersink and, ideally, a drill press or a tapping guide to keep everything square.

1. Find the right drill size

The tap drill is smaller than the screw, because the tap cuts the thread into the wall of the hole. The standard sizes leave about 75% of a full thread, which is plenty: a 75% thread is nearly as strong as a full one and far easier to tap. In stainless or other tough metal, go one size larger. Each thread page on this site lists the drills that give anywhere from about 50% to 85% thread, with what each is good for.

2. Mark and drill the hole

Center-punch the spot so the drill doesn't wander. For holes over about 1/4 in in steel, drill a small pilot hole first. Then drill the tap drill size, square to the surface, with cutting oil in steel. For a blind hole, one that doesn't go all the way through, drill deeper than the thread you need so the tap's tapered tip and the chips have somewhere to go. Each thread page gives drill depths for steel, aluminum and plastic.

3. Chamfer the top

Run a countersink lightly into the top of the hole until the chamfer is about as wide as the screw. It helps the tap start straight and stops the first thread from raising a burr.

4. Start the tap square

This is the step that decides whether the thread comes out straight. Put a drop of oil on the tap, set it in the hole and press down gently while turning clockwise. Check from two sides that it's vertical after the first couple of turns. A drill press helps: put the tap's shank in the chuck, turn the chuck by hand with the power off, and use the quill to keep it in line. A tap guide block does the same job without a drill press.

5. Cut the thread

Turn the tap a half to one full turn clockwise, then back it off about a quarter turn. You'll feel a small click as the chip breaks. Keep going in that rhythm, adding oil every few turns. On small taps, #10 and under or M5 and under, go gently and back off more often: they break with surprisingly little force.

In a through hole, keep going until the full thread comes out the bottom. In a blind hole, back the tap all the way out every few turns to clear chips, and stop before it reaches the bottom. If you need full thread close to the bottom, finish with a bottoming tap, which has almost no taper.

6. Clean and test

Back the tap out, blow or rinse out the chips, and run in a screw by hand. It should turn smoothly all the way down.

If a tap gets stuck

Stop. Don't force it, and don't rock it side to side. Back it out a quarter turn, add oil and try again in small movements. If it's still stuck, the hole may be too small or packed with chips. A broken tap is hard steel and can't be drilled out with an ordinary bit; it usually has to be removed with a tap extractor or by a machine shop using EDM, which is why patience at this point pays off.

Common mistakes

• Wrong drill. Too small and the tap binds or breaks; too large and the thread is weak. Check the thread page.
• Tap not square. The screw goes in crooked and the thread is weak on one side.
• No oil in steel. The tap tears the thread and wears out fast.
• Not clearing chips in a blind hole. The chips jam the tap at the bottom and snap it.
• Using a power drill to turn the tap. Unless you have a tapping attachment, turn taps by hand.

Find your thread: tap drill sizes, inch tap drill chart, metric tap drill chart, NPT chart.