Quick Answer

For holes larger than about 20mm in concrete, use a diamond core bit on a core drilling rig, not a hammer drill. Anchor the rig firmly, run the bit at the recommended RPM for its diameter, keep water flowing, and remove the core cleanly. Large-diameter holes need a stable rig and patient technique; forcing the bit is what burns cores and breaks anchors.

Choose the Right Core Bit and Rig

Match the bit diameter to the job and the rig to the bit. A hand-held drill works for 25-50mm holes in thin sections; anything 75mm and up needs a stand-mounted or rig-mounted core drill so the bit stays true. Wet core bits, such as diamond core drill bits, stay cool and cut faster in reinforced concrete. Dry bits are a slower option for smaller holes in masonry and thin slabs.

Set Up the Rig Correctly

Anchor the rig with a bolt through the base into a pilot hole, or use a vacuum base on smooth cured concrete. Check the rig is level in both directions; a tilted bit binds and ovalizes the hole. Mark the hole center, fix the pilot drill, and start the core with light pressure until the segments have seated, then increase to full feed.

RPM, Water, and Feed for Large Holes

Large bits spin slower: a 100mm bit runs around 300-500 RPM and a 200mm bit around 150-250 RPM. Water flow should be steady enough to carry the slurry away without flooding; the slurry should look gray, not thick paste. Feed pressure should be firm but not heavy; if you hear the bit grinding down, ease off and let the diamonds cut.

Remove the Core and Finish the Hole

When the core drill breaks through, stop the spindle, then lift the bit while it still turns slowly so the core stays in the barrel. Break off the core at the base, or tap it out from the top. Clean the hole with water and a brush. For through-holes, a vacuum anchor or core removal tool keeps the core from dropping into the void.

Frequently Asked Questions

Q: Can I drill a 100mm hole with a hammer drill?

A: No. Hammer drills chip and crack concrete at large diameters and take hours. A diamond core drill is the correct method.

Q: Why does my core bit overheat on deep holes?

A> Usually insufficient water or too high RPM. Reduce speed, increase water, and withdraw the bit slightly to let the barrel clear.

Q: Do I need a pilot bit for large core holes?

A: Yes, for most rigs. The pilot holds the center so the core starts true. On a vacuum-anchored rig you can skip it on small holes.

Conclusion

Large holes are a core drilling job: a solid rig, the right bit for the diameter and concrete, steady water, and controlled feed. Get the setup right and even 200mm holes come out clean, round, and fast.