Diamond core drilling cuts rather than hammers. Instead of shattering material, industrial diamonds bonded into the rim of a hollow bit grind a clean annular slot and leave a core behind. The result is a precise hole with no cracking around the edge — which is why it is the method used for plumbing and electrical penetrations through structural concrete.
The two machines
| RM80 | RM205 | |
|---|---|---|
| Input power | 1500 W | 2400 W |
| Speed | 2 speeds — 950 / 2100 rpm | 750 rpm |
| Max diameter, concrete | 80 mm | 205 mm |
| Max diameter, brick | 200 mm | 205 mm |
| Use | Handheld or on stand | Stand-mounted |
| Supply | 240 V ~ 50 Hz | 240 V ~ 50 Hz, 15 A plug |
| Angled drilling | Via stand | Up to 45° |
| Notable features | 2 speeds, spirit level, earth leakage protection | Suction base, wheels, spirit level, earth leakage protection |
Choosing between them
The RM80 is the versatile option. Two speeds let it work efficiently across a wide range of bit sizes, and it can be used handheld for smaller holes or mounted on the RM-DSD1 stand for accuracy. Its concrete limit is 80 mm — that is the manufacturer’s rating, and exceeding it will stall the machine and overheat it.
Note the difference between the concrete and brick ratings. The RM80 handles up to 200 mm in brick, where cutting resistance is far lower, but only 80 mm in concrete. A 202 mm RM-DC200 bit on an RM80 is a brick-only proposition.
The RM205 is the machine for large-diameter work in any construction material, up to 205 mm. Its single low speed of 750 rpm is right for big bits, and the stand, wheels and suction base make it a practical site tool. It supports angled drilling to 45°.
The RM205 requires a 15 A outlet. A standard 10 A domestic power point will not run it. Check site power before hiring or buying, and never modify a plug to fit — have a licensed electrician install a proper 15 A outlet.
The bits
Rural Max diamond core bits are laser-welded using German technology, with standard segments suitable for general-purpose drilling. All share a 1-1/4” UNC thread — the industry standard — and a 400 mm working length.
Fifteen diameters are available from 25 mm to 202 mm:
| Diameter | Common use |
|---|---|
| 25 mm | Small conduit, anchor holes, resin fixings |
| 32 mm | Conduit, cable penetrations |
| 38 mm | Conduit, small pipe |
| 42 mm | Pipe penetrations |
| 50 mm | Waste pipe, general services |
| 57 mm | Pipe and cable bundles |
| 66 mm | Larger services |
| 76 mm | Waste and vent pipe |
| 95 mm | Ducting, large services |
| 102 mm | 100 mm waste pipe |
| 110 mm | 100 mm PVC with clearance |
| 127 mm | Ducting, ventilation |
| 152 mm | Large ventilation, structural coring |
| 178 mm | Large penetrations |
| 202 mm | Maximum diameter — RM205, or brick only on the RM80 |
Size the bit for the pipe plus clearance, not the pipe’s nominal size. A 100 mm PVC waste pipe has an outside diameter above 100 mm, so a 110 mm bit gives room to work and to seal afterwards. This is why 102 mm and 110 mm both exist.
Matching speed to diameter
The governing principle: large bits turn slowly, small bits turn quickly. The rim of a large bit travels much further per revolution, so the same rpm gives a far higher cutting speed. Too fast and the diamonds polish over instead of cutting; too slow and progress stalls.
Using the speeds the machines actually offer:
| Bit diameter | Machine and speed |
|---|---|
| 25–42 mm | RM80 high speed (2100 rpm) |
| 50–80 mm | RM80 low speed (950 rpm) |
| 80–200 mm in brick | RM80 low speed (950 rpm) |
| 80–205 mm in concrete | RM205 (750 rpm) |
If the bit stops cutting, the answer is rarely more pressure. Check the speed first.
Wet versus dry drilling
Wet drilling is standard for concrete. Water cools the segments, flushes slurry from the kerf, and suppresses dust.
- Cooler bits last significantly longer.
- Cutting is faster and cleaner.
- Silica dust — a serious respiratory hazard — is largely controlled.
- Needs a water feed and a way to manage the slurry.
Dry drilling suits brick, block and soft masonry, and situations where water cannot be used.
- Requires dust extraction. A wet and dry vacuum such as the 70 L or 100 L stainless models in the Rural Max range handles both the dry dust and the wet slurry.
- Bits run much hotter, so withdraw regularly to let them cool.
- Never dry-drill reinforced concrete with a standard wet bit — it will overheat and lose segments.
Water and electricity. Wet drilling with a mains tool demands proper RCD protection. Both machines include earth leakage protection for construction site use, but the supply must be RCD protected as well. Never bypass either.
Technique
Set up properly. For anything above about 50 mm, use a stand. The RM-DSD1 works with the RM80 and gives the control that produces a straight, clean hole; it supports angled drilling to 45° and has a suction base for use with a vacuum pump. Handheld drilling of large bits is how wrists get hurt and bits get jammed.
Secure the machine. Suction base on smooth sound surfaces, anchor bolts on rough or unreliable ones. Check the vacuum holds before applying any load.
Start the hole. Begin at reduced speed and light pressure until the bit has cut a groove deep enough to guide itself, then increase to working speed. Starting at full speed lets the bit wander and skate.
Maintain steady pressure. Firm and consistent. Too little and the diamonds polish rather than cut; too much and the machine bogs, the segments overheat, and the motor may stall.
Keep water flowing on wet cuts. Slurry coming out of the hole should be a steady flow. If it stops, so should you.
Withdraw periodically to clear the kerf, especially on deep holes.
Ease off at breakthrough. As the bit exits the far face, reduce pressure. This prevents spalling on the back face and stops the core dropping unexpectedly.
Deal with the core. It usually comes away inside the bit. Tap it out or lever it free — never reach into a bit that is still turning.
Hitting reinforcement
Rebar is normal in structural concrete and diamond bits will cut through it, but:
- Slow down when you feel the change in resistance and sound.
- Ease the pressure. Forcing through rebar damages segments.
- Progress becomes noticeably slower. That is expected.
- Keep water flowing — steel generates more heat than concrete.
- Scan first where it matters. Cutting structural reinforcement can compromise a slab or beam. Use a rebar scanner and get engineering approval before coring structural elements.
Bit care
Glazing. The commonest problem: the diamond segments become smooth and shiny and stop cutting. It comes from running too fast, too little pressure, or cutting material that is too soft and abrasive-free.
The fix is dressing — drill a few holes into an abrasive material such as a soft sandstone block or a purpose-made dressing block. This wears back the bond and exposes fresh diamond. A glazed bit is not a worn-out bit.
Other care:
- Rinse after use, especially after wet drilling. Dried slurry is difficult to remove.
- Check the segments for cracks and missing pieces before each use.
- Store dry to protect the thread.
- Grease the 1-1/4” UNC thread lightly so the bit does not seize onto the machine.
- Never straighten a bent bit. Retire it.
Safety
- Eye protection, hearing protection and gloves as a minimum.
- Respiratory protection when dry drilling. Silica dust causes irreversible lung disease.
- Check for services before drilling — electrical, water, gas, data. Scan, and consult drawings.
- Secure the work area, especially below a ceiling or wall penetration where a core may fall through.
- Never leave a running machine unattended.
- Confirm RCD protection at the supply before starting.
- Have the drilling of structural elements approved by an engineer.
