Knowledge · Core drilling
A hole in concrete is drilled with a diamond core bit chosen for the diameter, the core drill's thread and the material. Reinforced concrete is drilled wet, masonry usually dry. The hole diameter is chosen with clearance for insulation or sealing, and the speed so that the bit's peripheral speed is about 3–5 m/s.
The key points in 30 seconds
Most wet core drilling rigs have a 1 1/4" UNC spindle. Small core bits, usually up to Ø 62 mm, have 1/2" BSP. Dry core bits for masonry – M16, for porcelain on an angle grinder – M14. How to check your core drill's thread and when an adapter is enough: core bit thread chart. When to drill dry and when wet: dry or wet core bit.
A Ø 68 mm core bit for a Ø 60 box and Ø 82 mm for a Ø 80 box – dry, M16 with an SDS-plus adapter and a centre drill. Diameters for drainage, ventilation and air conditioning: hole diameters for building services.
Usually because the debris is not flushed out, the segments have too little side clearance or the core drill is tilted. A constant water flow and a stand anchored on the hole axis help.
It depends on the diameter – in concrete a peripheral speed of about 3–5 m/s is assumed. The larger the bit, the lower the speed and the lower the drill's gear. An rpm table for 25–500 mm and a gear calculator: core drill speed and bit diameter.
Wet, with less feed pressure on contact with steel and a constant speed, using a core bit with impact-resistant grit.
Small holes for anchors and bars are drilled with a rotary hammer and a carbide SDS bit – with rebar, ideally a four-cutter bit that does not jam on the bar. When the hole is larger or the bit hits rebar, a wet diamond core bit is better: it cuts concrete and steel without hammering and without micro-cracks around the hole.
UNC, BSP, M16, M14 and adapters between systems.
How much clearance to leave for insulation and sealing.
Peripheral speed and gear selection.
How to drill a hole deeper than the core bit.
Material, dust and segment life.
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