The Toolbox — Charts

Drill Types

What each kind of drill is for, from the standard twist drill to double-margin aircraft drills, gun drills, reamers and countersinks. For sizes and decimal equivalents, see the drill size and tap drill chart.

Flutes and margins

The margin is the narrow, full-diameter strip running along the edge of each flute. It rubs the hole wall and keeps the drill centered. How many margins a drill has, and how its flutes are shaped, decide how straight and round the hole comes out.

Single margin

Standard twist drill

How to spot it: One margin along the leading edge of each land. The most common drill in any index.
Use it for: General-purpose drilling in metal, plastic and wood. Fine for most single-layer work.

Can wander or drill slightly out of round in thin sheet, especially by hand.

Double margin

How to spot it: Two margins on each land, one at the leading edge and one at the trailing edge, so four contact points around the hole.
Use it for: Aircraft sheet metal and stacked layers. The extra margins guide the drill, giving rounder, straighter, better-sized holes, and they help it ride through drill bushings and templates.

The usual choice for rivet and fastener holes in structure.

Parabolic flute

How to spot it: Wide, open flutes with a deep, rounded profile.
Use it for: Deeper holes, roughly 3 to 10 times the diameter, and gummy materials like aluminum. The open flutes clear chips so you peck less.

Gun drill

Sometimes called a gun barrel drill

How to spot it: A long, straight drill with a single V-shaped flute, one cutting edge and a coolant hole through the body.
Use it for: Very deep, very straight holes, like rifle barrels, hydraulic manifolds and shafts. It's a machining tool, run on dedicated machines with high-pressure coolant.

Not a hand-drill tool. You'll meet the parts it makes more often than the drill itself.

Points

The point angle is the included angle across the tip. The point shape decides how well the drill starts without walking.

118° point

Conventional point

How to spot it: The standard general-purpose tip, with a straight chisel edge at the center.
Use it for: Mild steel, aluminum, plastic and wood. Center-punch or spot first, because the chisel edge tends to walk.

135° split point

Self-centering

How to spot it: A flatter tip with the chisel edge split into small cutting lips.
Use it for: Starts without a center punch and drills with less push. The better choice for stainless steel, titanium, hard alloys and curved surfaces.

A good default for hand drilling on aircraft.

Lengths and shanks

Use the shortest drill that reaches. Shorter drills flex less, so holes come out straighter and drills break less.

Stub length

Screw machine length

How to spot it: Short overall length with short flutes.
Use it for: Rigid and accurate. Good for hand drilling, tight spots and angle drill motors.

Jobber length

How to spot it: The standard length found in most drill indexes.
Use it for: Everyday drilling where you don't need extra reach.

Taper length

How to spot it: Longer than jobber, with longer flutes.
Use it for: Deeper holes and thicker material.

Extension drill

How to spot it: A normal-size cutting end on a long shank, often 6 or 12 inches.
Use it for: Reaching past structure, like through lightening holes or between ribs. Hold the shank in a guide or your free hand to stop it whipping.

The flexible shank wanders easily, so start slowly.

Threaded shank

How to spot it: A short drill with a threaded shank instead of a plain round one.
Use it for: Screws straight into 45° and 90° angle drill heads, which have no chuck, so they fit into tight spaces between structure.

Special purpose

Tools for jobs a standard twist drill doesn't do well.

Spotting drill

Not the same as a center drill

How to spot it: A short, stiff drill with a 90° or 120° point.
Use it for: Spotting an exact starting point so the real drill doesn't walk. Mostly used on drill presses and machines.

A center drill has a small pilot and a 60° cone. It's made for lathe centers, not for starting holes.

Step drill

Unibit

How to spot it: A cone of stepped diameters, each step marked with its size.
Use it for: Clean, round holes in thin sheet, several sizes with one tool. Common for electrical connectors and panel cutouts.

Composite drill

Dagger or brad-point style

How to spot it: A sharp, specialized point geometry, usually solid carbide or diamond-coated.
Use it for: Carbon fiber and fiberglass. It shears the fibers cleanly instead of splintering the back face or delaminating the layers.

Reamer

How to spot it: Straight or spiral flutes with no real point. It cuts only slightly.
Use it for: Bringing a drilled hole to an exact final size and a smooth finish, like close-tolerance bolt holes. Drill slightly undersize first, then ream.

Countersink cutter

Used in a microstop cage

How to spot it: A cone-shaped cutter with a pilot that rides in the hole.
Use it for: Cutting the recess for flush-head rivets and screws. Aircraft fasteners use a 100° countersink. A microstop cage sets the exact depth.

Point Angle Guide

Point angle	Best for	Why
90°	Soft plastics, and spotting or centering	A sharp point that starts precisely
118°	General purpose: mild steel, aluminum, wood	The all-round standard
135° (split point)	Stainless, titanium and hard alloys; hand drilling	Flatter and stronger, and starts without walking

Drill materials, in brief

High-speed steel (HSS) is the everyday choice for aluminum and mild steel. Cobalt (often marked M35 or M42) holds its edge in stainless steel and titanium. Solid carbide is very hard but brittle, and it's best in rigid setups and composites. Coatings like black oxide or TiN add some wear resistance but don't change the drill's basic type.

Always follow the maintenance manual or structural repair manual for hole sizes, drill types and procedures on aircraft structure.