The Impact Drill — What It Does, When to Use It, and When You've Outgrown It

The impact drill — sold in Kenya as a hammer drill, percussion drill, or combi drill with hammer function — is the most widely owned power drill on the market. It sits between a standard drill and a rotary hammer in capability, and for many tradespeople and serious DIY users, it is the one drill that handles the broadest range of everyday tasks.

But it has limits. Knowing exactly where those limits are is what separates a professional who uses the right tool from one who wears out equipment doing work it was never designed for.

How an Impact Drill Works — The Cam Mechanism

Unlike a rotary hammer — which uses a pneumatic piston to deliver independent, high-energy blows — an impact drill creates its hammering action through a purely mechanical system involving two corrugated metal discs called cams.

Here is what happens inside the tool:

• Two hardened metal discs sit face to face inside the chuck assembly.
• The front disc is connected to the chuck; the rear disc is fixed to the body.
• As the motor drives the chuck forward, the ridges of the two discs ride over each other.
• Each time a ridge passes the opposing surface, the chuck jumps forward and snaps back in a rapid, shallow pulse.
• This produces between 25,000 and 50,000 impacts per minute — far more frequent than a rotary hammer, but with far less energy per blow.

The key distinction: An impact drill’s hammering action is mechanical vibration under forward pressure. It is not a true hammering blow. The energy per impact is typically 0.05 to 0.2 joules — compared to 1 to 3 joules in a professional SDS-Plus rotary hammer. Frequency is high; force per blow is low.

When you press the bit against a masonry surface and apply forward pressure, the cam ridges engage and the vibration helps the carbide tip chip at the material as it rotates. Release the pressure and the vibration stops. This pressure-dependency is one of the key differences from a rotary hammer, whose piston fires independently of how hard you press.

What an Impact Drill Does Well

The impact drill is genuinely excellent within its design parameters. The mistake is not owning one — it is using it for work that exceeds those parameters.

Brick and mortar

Soft brick, reclaimed brick, mortar joints, and lightweight concrete block (AAC / Aircrete) are the natural home of the impact drill. The cam mechanism generates enough energy to chip through these materials efficiently, and for occasional work or light-volume drilling, it is a practical and affordable solution.

Ceramic tiles — with care

Drilling through glazed ceramic tiles requires the hammer function to be switched OFF. Use a carbide or diamond-tipped tile bit at slow speed with light pressure. The vibration of hammer mode cracks tiles. This is a rotation-only task, and the impact drill handles it well in that mode.

Multi-material versatility

The impact drill’s real strength is versatility. Switch the hammer function off and it performs as a standard drill — wood, metal, plastic, screw driving. Switch it on and it handles light masonry. One tool, multiple applications. For a tradesperson or contractor who works across a variety of materials and does not drill concrete daily, this flexibility is genuinely valuable.

Lighter-duty site work

For a plumber or electrician drilling occasional anchor holes through lightweight block walls, or a carpenter who occasionally needs to fix into soft brick — the impact drill is a proportionate and cost-effective choice. The job gets done; the investment is reasonable.

Where an Impact Drill Reaches Its Limits

Structural and reinforced concrete: Dense concrete — C25 slabs, foundations, columns, and any concrete with reinforcing bar — is genuinely beyond the impact drill’s capability for professional use. The cam mechanism lacks the energy to chip dense aggregate efficiently. The tool will overheat before the job is done, and the bit will wear rapidly.

The honest markers that tell you an impact drill is the wrong tool:

• The drill is hot after drilling a small number of holes — the motor is working far harder than the result justifies.
• The bit tip is visibly worn after a short time — the material is harder than the mechanism can efficiently handle.
• Progress stops or slows dramatically as you go deeper — you have hit aggregate or rebar.
• You are drilling holes above 16mm diameter in any hard masonry — the cam mechanism does not generate enough force for efficient large-diameter work.
• You need to drill more than 20–30 holes in a session in any masonry — sustained use in hammer mode generates heat that accumulates in the cam discs and motor windings.

Any of these signals means the work calls for a rotary hammer, not an impact drill.

The Bits That Belong in an Impact Drill

An impact drill uses a standard keyed or keyless chuck that clamps onto round-shank bits. This is the same chuck as a standard drill — which means the same bit range applies, with the addition of impact-rated masonry bits when the hammer function is in use.

Carbide-tipped masonry bits — the primary bit in hammer mode

The essential bit for any masonry work in an impact drill. The carbide tip is brazed or mechanically fixed to the steel shank and must be rated for percussive use — not all masonry bits are.

• Look for: full-ground carbide tip (not just a brazed plug), which provides more cutting edges and better heat dissipation
• Sizes: 4mm to 16mm for most impact drill applications — above 16mm, a rotary hammer is more appropriate
• Examples: Bosch Expert Masonry bits (impact-rated), Makita B-45354 masonry bit sets, Hilti TE-C bits for hammer drills

Do not use: Standard masonry bits not rated for percussion use — the tip will loosen from the shank under repeated vibration, which is both ineffective and potentially dangerous.

HSS (High Speed Steel) bits — for metal, hammer mode OFF

For drilling mild steel, aluminium, copper pipe, and sheet metal. The hammer function must be switched off — percussion action in metal damages the hole, the bit, and the workpiece.

• Standard HSS-G: for mild steel up to 10mm in most impact drills
• Cobalt HSS: for harder metals, stainless steel, and cast iron — more heat resistant
• Examples: Bosch HSS-G metal drill sets, Makita B-65399, DeWalt HSS metal bits

Brad point wood bits — for clean holes in timber, hammer mode OFF

The central point of a brad point bit prevents walking on the wood surface and produces a clean entry hole with minimal tear-out on the exit side. Use at medium speed with no hammer function.

• For softwood, hardwood, plywood, MDF, and chipboard
• Examples: Makita B-49000 wood bit sets, Bosch wood brad point sets

Spade (flat) bits — for large diameter holes in wood, hammer mode OFF

For boring large holes in timber framing, joists, and boards — commonly used by plumbers and electricians running pipes and cables through structural timbers. Fast and aggressive; not suitable for finished surfaces where a clean hole is required.

• Available from 16mm to 40mm+
• Examples: Bosch spade bit sets, Makita flat bit sets

Hole saws — for large circular holes, hammer mode OFF

Cylindrical cutting tools for producing large, clean holes in wood, plasterboard (drywall), thin sheet metal, and plastic. Essential for back-boxes (electrical), downlighter cutouts, and cable entry holes.

• Bi-metal hole saws: for wood and thin metal
• Carbide-grit hole saws: for ceramic tiles and fibre cement
• Examples: Bosch Progressor hole saw sets, Makita bi-metal hole saw sets

Screwdriver bits — for driving fasteners, hammer mode OFF

An impact drill in drill/driver mode is an effective screw driver for wood screws, self-drilling screws, and machine screws. Use a bit holder with PH2, PZ2, TX25, or flat bits as required. Note: for high-volume, high-torque screw driving, a dedicated impact driver is more efficient — but for general use, the impact drill handles the task well.

Settings and Technique — Getting the Most from an Impact Drill

• Always switch hammer mode OFF when drilling wood, metal, or plastic, and when driving screws. The cam mechanism adds no benefit and causes damage in non-masonry materials.
• Use the torque clutch (the numbered ring behind the chuck) for screw driving — set it low for small screws in soft materials, higher for larger fasteners. This prevents overdriving and stripped heads.
• For masonry work, apply firm, steady forward pressure — the cam mechanism only engages when the bit is pressed against the surface. Intermittent pressure produces intermittent action.
• Let the tool and bit do the work. Excessive force does not improve drilling speed in masonry and accelerates wear on both the bit and the cam discs.
• Allow the tool to cool between extended masonry sessions. Impact drills are not rated for continuous masonry use — build in short rest periods.

When to Upgrade to a Rotary Hammer

The impact drill is the right tool until it is not. The clearest signals:

• You are regularly drilling into dense concrete, reinforced concrete, or structural slabs
• You need holes above 16mm diameter in any masonry
• You are drilling 30 or more holes per session in masonry
• Your impact drill is running hot, wearing bits fast, or struggling to make progress
• You need to chisel, channel-cut, or do any demolition work

Any of these conditions means a rotary hammer is the proportionate tool. The investment is higher upfront — but as the maths in our cost-of-ownership guide shows, it is almost always the cheaper choice over 18 months of professional use.

Recommended Models — Available Through BOLD Industrial Kenya

The best impact drill for your situation depends primarily on duty cycle, torque, and whether you need a single-speed or variable-speed machine. These are the models we stock and recommend:

• Bosch GSB 16 RE — 750W, variable speed, hammer and drill modes. The practical choice for site work and serious DIY.
• Bosch GSB 162-2 RE — 800W, 2-speed gearbox, impact and drill modes. For tradespeople who need more power and a wider speed range.
• Makita HP1631K — 710W, variable speed, reversible. Reliable and well-supported in Kenya.
• Makita HP2071F — 1,010W, 2-speed. For heavier-duty work in brick and block where a full rotary hammer is not yet warranted.
• DeWalt DWD024KS — 650W, variable speed. Compact and durable for mixed-material work.

Not sure whether an impact drill or a rotary hammer is right for your work? Describe what you are drilling, how often, and the material — our team will give you a straight answer.