Cutting Bits for Drills: Types, Materials & How to Choose

The Relationship Between Drill and Cutting Bit

A cutting bit for a drill is the interchangeable tip that performs the actual material removal; the drill itself only supplies rotation and axial feed force. This separation between tool and drive is one of the most productive design decisions in hand tool history — a single drill body can accept hundreds of different bit types and sizes, each optimised for a specific material or hole geometry. Matching the correct cutting bit to the material being drilled is the single most important factor in drilling performance, tool life, and hole quality. Using the wrong bit type not only produces poor results but can damage both the bit and the workpiece.

Twist Drill Bits: The Universal Standard

The helical twist drill bit is the most widely used cutting bit across all drilling applications. Available from under 1 mm to over 100 mm diameter, twist bits are manufactured in three primary material grades that define their application range:

• Carbon steel twist bits: Suitable only for wood and soft plastics. Lose their edge rapidly in metal. Identifiable by their dull grey finish. Suitable for occasional light-duty use where cost is the overriding factor.
• HSS (high-speed steel) twist bits: The standard for metal drilling. Maintain cutting edge hardness up to approximately 600 °C, enabling sustained drilling in steel, aluminium, brass, and copper. HSS bits coated with titanium nitride (gold-coloured) last 3–5× longer than uncoated equivalents in production drilling.
• Cobalt HSS bits (M35/M42): Alloyed with 5–8% cobalt, these bits handle stainless steel, hardened alloys, and cast iron that rapidly dull standard HSS. M42 cobalt bits retain hardness to approximately 700 °C and are the professional standard for stainless steel fabrication and maintenance drilling.

Point angle is a frequently overlooked specification. The standard 118° included angle suits general-purpose drilling in mild steel and wood. A 135° split-point geometry self-centres on smooth surfaces without a centre punch, reduces thrust force, and is better suited to hard materials and thin sheet metal where a 118° bit tends to walk and grab.

Masonry Bits: Carbide Tips for Concrete, Brick, and Stone

Masonry cutting bits use a tungsten carbide tip brazed to a steel body. The carbide — far harder than the steel or aggregate particles in concrete — pulverises the material at the hole base rather than cutting it in the conventional sense. The rotary hammer action of a hammer drill or SDS drill delivers repeated impact blows that fracture the material ahead of the tip while rotation clears debris through the flutes.

SDS-Plus and SDS-Max shank systems, developed by Bosch and now industry-standard, allow the bit to slide axially within the chuck during hammering while remaining rotationally engaged. This transmits hammer energy efficiently to the tip without the energy losses of older keyed-chuck hammer systems. SDS-Plus is standard for bits up to 26 mm diameter; SDS-Max handles larger diameters and heavy-duty breaking work.

For larger holes in concrete (above 50 mm), diamond core bits replace solid masonry bits. A steel tube with diamond segments on the cutting face rotates without hammer action, using water or dry cutting with diamond abrasion to produce clean-walled holes for plumbing, electrical conduit, and structural penetrations.

Wood Drilling Bits: Geometry Matched to Grain

Wood requires different cutting geometry from metal because the fibrous grain structure tears rather than shears if the bit geometry is poorly matched to the application:

• Brad-point bits: Feature a central spur that registers the bit precisely before the outer spurs and cutting lips engage. Produce clean-entry, splinter-free holes in solid timber and sheet materials. The standard choice for furniture making and joinery.
• Spade bits: Flat paddle-shaped bits for fast boring of large-diameter holes (typically 16–50 mm) in structural timber. Aggressive and fast but leave a rough hole wall. Suitable for rough framing where a cable or pipe will pass through.
• Auger bits: Long helical bits with a coarse screw thread lead point that pulls the bit through the wood. Used for deep holes in beams and joists. The aggressive thread means the operator must control feed rate carefully to prevent the bit pulling through faster than the drill can manage.
• Forstner bits: Produce flat-bottomed, clean-walled holes with minimal tearout on the entry face. The choice for hinge recesses, dowel holes, and any application where hole quality matters more than drilling speed. Require a drill press or very steady hand to maintain perpendicularity.

Specialised Cutting Bits for Specific Applications

How to Match Cutting Bit to Material: A Practical Summary

The correct bit selection follows directly from the workpiece material and the required hole quality:

• Mild steel and aluminium: Standard HSS twist bit, 118° point. Add TiN coating for extended life in repetitive production work.
• Stainless steel and hardened alloys: Cobalt HSS (M35 or M42), slow speed, consistent cutting fluid application. Never use standard HSS — work hardening will dull the bit within seconds.
• Concrete, brick, block: SDS-Plus carbide masonry bit with hammer drill. For holes above 50 mm, use a diamond core bit with rotary-only mode.
• Hardwood and softwood (precision): Brad-point bit for clean entry. Forstner bit for flat-bottomed or overlapping holes.
• Ceramic tile and glass: Carbide spear-point tile bit, slow speed, no hammer, water cooling or masking tape to prevent bit walking on smooth surfaces.