Speed drops as the hole grows
A Ø50 mm core bit runs 760–1,150 rpm; a Ø150 mm bit only 250–390 rpm. Same peripheral speed, half the rpm — because it is the line speed at the rim that matters, not the spindle.
02
300–600 rpm
Standard concrete
300–400 rpm
Heavily reinforced
2–4 L/min
Water flow
4–6 L/min
Into rebar
01 — At a glance
The three things that matter most
Concrete is not one material. Standard C20–C40, heavily reinforced slabs and soft brick each want a different speed and a different tool. Getting the water flow right matters as much as the bit.
The slurry should look like milk
Clear water means too much water and the diamonds polish instead of grind. Thick paste means too little and the bit overheats and binds. Milky is the target.
Reinforced concrete is a different job
Rebar needs lower rpm (300–400) and more water (4–6 L/min). When you hit steel, never add pressure. Laser-welded segments survive the torque; forcing the drill snaps segments off.
02 — Tool matching
Match the tool to the material
Pick the row for your material. If two rows look similar, start with the slower option — it is almost always the safer and cheaper mistake.
| Material | Recommended tool | Speed | Cooling |
|---|---|---|---|
| Standard concrete (C20–C40) | Wet diamond core bit | 300–600 rpm | 2–4 L/min |
| Heavily reinforced concrete | Wet core bit, laser-welded segments | 300–400 rpm | 4–6 L/min |
| Brick, block, soft masonry | T.C.T. hollow drill bit | 600–900 rpm | Dry + dust extraction |
| Asphalt | Segmented diamond blade / core bit | 500–1,000 rpm | Water |
| Slab / wall cutting | Segmented or laser diamond blade | 30–40 m/s | Wet preferred |
| Mortar & tuckpointing | Tuck point blade | Per saw rating | Dry + shroud |
| Steel studs, pipe, conduit | Bi-metal hole saw | 150–350 rpm | Cutting fluid |
03 — Step by step
Core drilling in seven steps
In order. Skipping a step usually costs more time than it saves.
01
Survey before you mark
Ground-penetrating radar locates rebar, post-tension cables and live conduit to roughly 50 mm accuracy. A 15-minute scan prevents hitting a tensioned cable — which is the kind of mistake that stops a whole site.
02
Mark and pilot the centre
Chalk or a permanent marker for position, then a centre-punch dent so the bit cannot walk when you apply pressure. A wandering bit wastes diamond on an abandoned hole.
03
Anchor the rig properly
Place anchors within 150 mm of the hole centre and test stability by pressing down without spinning. If the rig rocks, re-position. Vacuum anchors hold around 1,500 kg on smooth concrete but fail on textured surfaces — use mechanical anchors there.
04
Start slow, then load
Begin at a lower speed with light pressure for the first 25 mm to establish a clean entry ring. Once the bit is seated, go to full force and the working rpm for that diameter.
05
Clear the hole every 75 mm
Pause, lift the bit, clear the debris. A clogged cutting face generates heat that destroys diamonds within seconds. Check hole straightness every 150 mm with a straight edge.
06
Reduce pressure at breakthrough
As you approach the far face, ease off to avoid blowing out the exit. Then withdraw the bit while it is still rotating — this prevents the bit jamming in the hole.
07
Clean the slurry while it is wet
Shovel or wet-vac the slurry immediately. Dried slurry becomes airborne silica dust — the one thing wet drilling was supposed to prevent.
04 — Feeds & speeds
Numbers to start from
Use these as a starting point and adjust to what the cut is telling you. Start slow and work up.
| Hole Ø | Concrete 2–3 m/s | Reinforced 1.2–1.8 m/s | Water flow (L/min) |
|---|---|---|---|
| 20 mm | 1,910–2,865 | 1,146–1,719 | ≈1 (handheld) |
| 32 mm | 1,194–1,790 | 716–1,074 | 1–2 |
| 50 mm | 764–1,146 | 458–688 | 2–3 |
| 82 mm | 466–699 | 279–419 | 2–3 |
| 107 mm | 357–535 | 214–321 | 3–4 |
| 152 mm | 251–377 | 151–226 | 3–4 |
| 200 mm | 191–286 | 115–172 | 4–6 |
| 300 mm | 127–191 | 76–115 | 6 |
Overhead work needs 6 L/min to manage both cooling and debris, and a rig that holds pressure without drifting. Start every hole 20–30 % below the table value and ramp up.
05 — Troubleshooting
If it goes wrong
The symptom is rarely the problem. Read the root-cause column before you change anything.
| Symptom | Root cause | Fix |
|---|---|---|
| Motor stalls or overloads | Too much pressure, or speed too high for reinforced concrete | Drop to 300–400 rpm, ease off the feed, keep water flowing — let the diamonds cut |
| Segments glazed, no progress | Speed too high, water too low, or the bond polished over | Re-dress the bit in an abrasive block, correct rpm and water, then resume |
| Diamonds polish instead of grinding | Too much water — slurry runs clear | Cut water back until the slurry is milky |
| Hole drifts off the mark | Rig not anchored, or wander in the first 50 mm | Re-anchor within 150 mm of centre, correct within the first 50 mm, check every 150 mm |
| Sparking inside the hole | You are into the rebar | Slow to 300–400 rpm, raise water to 4–6 L/min, keep steady — do not force |
| Core stuck in the barrel | Compacted slurry or vacuum seal | Tap the barrel to break the seal; never lever the bit out while it is spinning |
| Dust everywhere despite wet drilling | Hose pressure dropped or nozzle mis-aimed | Check water pressure weekly; aim nozzles so the bit and the cutting area are wetted |
06 — Recommended tools
Tools we make for concrete and masonry
Every item below is made by EXTRACUT and links straight to the product page.
Diamond Core Bits
Wet diamond core bits for 20–300 mm penetrations in concrete and stone.
View product
Vacuum Brazed Diamond Core Drill Bit
Vacuum-brazed core bits: one solid layer of diamond for the hardest reinforced pours.
View product
T.C.T. Hollow Drill Bit for Wall
T.C.T. hollow drill bit for fast dry holes in brick, block and aerated concrete.
View product
Bi-Metal Hole Saw
Bi-metal hole saw for studs, pipe, conduit and sheet work.
View product
Segmented Diamond Blades
Segmented blade for cutting slabs, kerbs and openings.
View product
Tuck Point Blades
Tuck point blade for grinding out mortar joints for repointing.
View product
T.C.T. Annular Cutter
T.C.T. annular cutter for clean, fast holes in structural steel plate.
View product
Laser Diamond Blades
Laser diamond blade rated for dry cutting where water is not an option.
View productLaser-welded segments and hardened bodies — specified for reinforced concrete and site abuse.
07 — Safety
Before you switch anything on
Short list, long consequences. These apply to every job on this page.
Respirable crystalline silica is the main hazard. Wet drilling is the primary engineering control. Dry drilling requires HEPA-filtered extraction and a P2/N95 or better respirator.
Scan before you drill. Rebar, post-tension cables and live conduit are hidden; GPR accuracy is roughly 50 mm.
Anchor the rig and test it. Place anchors within 150 mm of the hole centre and press down without spinning before you start.
Protect what is below. Cover surfaces, block doorways and contain the slurry — it stains and it is slippery.
Clean up wet, not dry. Sweeping dried slurry puts silica back into the air.
Stop on sparks. Sparking means steel; slow down and increase water rather than pushing harder.
08 — FAQ
Frequently asked questions
The questions our technical team answers most often.
What rpm should I drill a 100 mm hole in concrete?
In standard concrete, 350–500 rpm. In heavily reinforced sections, drop to 300–400 rpm and raise water flow — the bit will survive, the schedule may slip slightly, and the motor will thank you.
Do I really need to scan for rebar?
If the structure is reinforced or post-tensioned, yes. Hitting a post-tension cable can release enormous stored energy and is a life-safety event, not just a delay. Scan, then mark.
How do I know the water flow is right?
Look at the slurry. Milky is correct. Clear means too much water and the diamonds polish; thick paste means too little and the bit overheats. Standard drilling takes 2–4 L/min, overhead work 6 L/min.
Can I dry drill concrete?
Only with a shroud and HEPA extraction plus respiratory protection. Wet drilling is the standard method and the only one that controls silica at the source for long or deep holes.
Why does my core bit stop cutting halfway through?
Almost always glazing — the metal bond has polished over the diamonds. Re-dress the bit in an abrasive block or cinder block for a few seconds to expose fresh diamond, then correct your rpm and water before you continue.
Still not sure which tool fits the job?
Send us the material, the machine and the finish you need. Our engineers will specify the blade, bond or bit — and quote it with a sample if you want to test before you commit.