Auto & Shop InformationLesson 13 of 17
Drilling and Boring Tools
Drills, bit types and what each is for, countersinks and reamers.
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Drilling is the one shop operation where getting the setup wrong ruins the work before you notice. The bit, the speed and what happens before the bit touches the metal are all asked.
The drills
| Tool | Notes |
|---|---|
| Hand drill or brace | no power; a brace gives high torque at low speed |
| Power drill | corded or cordless, with a chuck to hold the bit |
| Hammer drill | adds a rapid forward tapping for masonry |
| Impact driver | rotational hammering for driving fasteners, not for precise holes |
| Drill press | fixed, accurate, perpendicular; the work is clamped |
A drill press is the answer whenever a question mentions accuracy or a hole that must be square to the surface. A hand-held drill cannot be held perpendicular reliably, and the press also lets the work be clamped, which is a safety point as much as an accuracy one.
A chuck holds the bit, and a keyless chuck is tightened by hand while a keyed chuck uses a removable key. The key is removed before starting, always, which is asked as a safety item.
The bits
| Bit | For | Recognized by |
|---|---|---|
| Twist bit | general purpose, wood, metal, plastic | spiral flutes |
| Brad point bit | clean, accurately placed holes in wood | a sharp center spur |
| Spade or paddle bit | fast, rough, large holes in wood | a flat blade with a point |
| Auger bit | deep holes in wood | a screw tip that pulls it in |
| Forstner bit | flat-bottomed holes, overlapping holes | a rim that cuts the circle |
| Hole saw | large holes through thin material | a toothed cylinder |
| Masonry bit | brick, block and concrete | a carbide tip wider than the shank |
| Countersink | a cone so a flat-head screw sits flush | a tapered cutter |
| Counterbore | a flat recess so a bolt head sits below the surface | a flat-bottomed cutter |
| Reamer | enlarging and finishing an existing hole precisely | many straight or spiral flutes |
| Step bit | several sizes in one, for thin sheet | a cone of steps |
Countersink against counterbore is the pair to keep straight. A countersink cuts a cone, which matches the tapered underside of a flat-head screw so it sits flush with the surface. A counterbore cuts a flat-bottomed recess, which lets a round bolt head or a socket head cap screw drop below the surface.
The shape of the fastener head decides which you need, and that is the fastest way to answer the question: a cone-shaped underside wants a countersink, and a head with a flat underside wants a counterbore.
A reamer is not a drilling tool. It removes a very small amount from an existing hole to bring it to an exact size and a smooth finish. If a question mentions a hole that must be precisely sized, the answer is a reamer, and if it mentions making the hole at all, it is not.
A Forstner bit is the one that cuts a flat bottom. Nothing else in the table does, which makes it the answer whenever a flat-bottomed or partly overlapping hole is described.
A center punch before drilling metal is not optional, and neither is clamping the work. An unclamped piece of sheet metal caught by a bit becomes a spinning blade at the speed of the drill, and it is one of the most common serious shop injuries. Clamp the work, punch the center, start slowly, and let go of the trigger if the bit binds. Gloves are the wrong answer here: loose fabric near rotating machinery is a hazard, not protection.
Speed and feed
Large bits turn slowly; small bits turn quickly. The bit's edge travels farther per revolution on a large diameter, so the same speed at the rim needs fewer revolutions.
Hard material means slower. Steel is drilled slower than aluminum, and aluminum slower than wood.
Too fast burns the bit's edge. The blue color on a ruined bit is the steel losing its temper, after which it will not hold an edge at all.
Cutting fluid cools and lubricates when drilling metal, and it lengthens bit life considerably.
A pilot hole is drilled first when the final hole is large, so that the large bit's web is not trying to cut at its own dead center, where its speed is zero.
Taps and dies
| Tool | Job |
|---|---|
| Tap | cuts internal threads, inside a hole |
| Die | cuts external threads, on a rod |
| Tap wrench | turns the tap squarely |
| Die stock | holds and turns the die |
Tap is inside, die is outside. That is the distinction, and it is asked directly.
A tap needs a hole of the right size first, called the tap drill size. Too large and there is not enough metal left to form a thread; too small and the tap binds and snaps off in the hole, which is an unpleasant repair.
Taps are turned forward a partial turn and then backward to break the chip off, which is the practice question that comes up.
Know it by sight
| Item | How to recognize it |
|---|---|
| Hand drill | a crank handle turning a gear wheel |
| Power drill | a pistol-grip motor with a chuck |
| Drill press | a drill head on a column above an adjustable table |
| Twist drill bits | spiral flutes running up a round shank |
| Spade bits | a flat paddle with a center point |
| Forstner bits | a flat-bottomed cutter with a rim, for clean flat-bottomed holes |
| Masonry bits | a wider, flattened carbide tip on a twist bit |
| Countersink bit | a short cone with cutting edges |
| Reamer | straight or slightly spiral flutes along a long body, for finishing a hole to exact size |
| Extractors | a tapered bit with a reverse spiral, for backing out a broken bolt |
What you can skip
Across the 59 questions on this topic in our bank:
- Hand-powered boring tools. The brace never appears and the auger once.
- Step bits and bit metallurgy. Step bits never appear, cobalt comes up once and carbide three times, as the masonry tip.
- Speed charts. Know the rule - larger bit and harder material, slower speed - rather than any table of numbers; a speed in rpm is asked once.
Where people lose points
Swapping countersink and counterbore. Cone against flat recess.
Thinking a reamer makes holes. It finishes them.
Running a large bit at high speed. Large means slow.
Skipping the center punch on metal. The bit wanders.
Swapping tap and die. Tap is internal.
Wearing gloves at a drill press. Loose material and rotating tools do not mix.
Work one in under a minute
A machine screw with a tapered head must finish flush with the surface of a steel plate. Which tool prepares the hole, and what would the other choice have given?
A countersink.
The head's underside is a cone, so the hole must be given a matching cone for the head to seat evenly and end level with the surface.
A counterbore cuts a flat-bottomed recess of one diameter. A tapered head dropped into that would touch only at the rim of the hole, would not be supported underneath, and would not sit flush.
Match the recess to the shape of the head, which answers every question of this kind without memorizing the tool names against fastener names.
Where this leads
The bit is a rotating wedge, the tap is a screw cutting its own thread, and the fasteners these holes receive are the next lesson but one.
Related lessonsReference
- Measuring and Marking Tools - the center punch that starts the hole
- Fasteners and Joining - the threads a tap cuts
- Inclined Planes, Wedges and Screws - the screw a tap follows
- Shop Safety and Practice - clamping, chuck keys and gloves
Practice this topic
Check that this lesson stuck. Answer questions on drilling and boring tools only, and see the right answer and why after each one.
Practice Drilling and Boring Tools questions