Rotary Hammer or Impact Drill? What Each One Is For

These two tools look similar, are shelved beside each other, and both say “hammer”. They are not the same machine and the difference is not a matter of degree.

How each one actually works

An impact drill — a hammer drill, in older language — has two ridged plates behind the chuck. As the chuck turns, the ridges ride over each other and shake the bit forwards thousands of times a minute. The blows are small and fast, and they depend on you leaning on the tool. Take the pressure off and the hammering stops.

A rotary hammer has a piston driven by the motor, firing a striker at the back of the bit. The blow is delivered by the tool, not by your weight, and it is perhaps ten times the energy. That is why a rotary hammer goes through concrete with light pressure while an impact drill needs you to push and still polishes the hole instead of cutting it.

Which one for which job

• Brick, block, plaster, a dozen holes. Impact drill. Cheaper, lighter, and it does wood and steel too.
• Concrete, RCC, stone. Rotary hammer. An impact drill will do it once, badly, and blunt the bit doing so.
• Anchors above 10mm. Rotary hammer, regardless of material.
• Chiselling, breaking tile, chasing a wall. Rotary hammer, and only one with a hammer-only mode.
• Drilling all day, whatever the material. Rotary hammer. This is about your wrists as much as the tool.

SDS-plus, and why the bits are not interchangeable

A rotary hammer does not have a normal chuck. It uses SDS-plus, a slotted shank the bit slides into and locks against — and the bit is meant to move a few millimetres back and forth inside it. That freedom is what lets the piston strike the bit rather than the tool.

So an ordinary round-shank masonry bit does not fit a rotary hammer, and an SDS bit does not fit a normal drill. Adapters exist in both directions and both are a poor idea: a chuck adapter on a hammer puts the hammering through a joint never designed for it.

SDS-plus is the size for everything up to about 26mm, which covers virtually all site work. SDS-max is the bigger system, for breakers and heavy structural drilling.

Reading the specification

Ignore the wattage. On a rotary hammer the number that decides what the tool can do is the impact energy in joules — how hard each blow lands.

• Around 2 J — light: anchors up to 10mm, electrical boxes, a tool you can hold up all day.
• 2.5 to 3.5 J — the general site machine. Handles most anchor work and light chiselling.
• 5 J and above — heavier, for structural anchors and real breaking work. Also considerably heavier to hold.

The second thing to check is the modes. Three-mode machines do rotation only, rotation with hammer, and hammer only. The third is what lets the tool chisel; a two-mode machine cannot, whatever its joules.

If an impact drill is the right answer

For most people buying their first serious drill, it is. It does wood, steel, brick and plaster, it costs a fraction of a rotary hammer, and it is the tool you will actually pick up for the small job.

Corded or cordless

A rotary hammer is one of the hardest things you can ask a battery to do, and it is where ProCORE batteries earn their price most clearly. A cordless hammer on a 2.0Ah battery will disappoint you within twenty holes.

If the tool lives on a site with power available, corded is lighter, cheaper and never runs out mid-hole.

Things that shorten a hammer's life

• Leaning on it. A rotary hammer needs guiding, not pushing. Extra pressure does not drill faster; it stops the piston travelling properly.
• A dry SDS shank. A smear of grease on the shank before it goes in. Dry shanks wear the chuck, and the chuck is not the cheap part.
• Blunt bits. A worn carbide tip makes the tool work harder for a worse hole. Bits are consumables; hammers are not.
• Rebar. Hitting steel in RCC at full hammer is how bits snap and wrists get hurt. Slow down, or move the hole.

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