Quick Answer

A core bit stops cutting for three reasons: the diamonds are glazed (rubbing instead of cutting), the bond is too hard for the concrete, or the feed pressure is wrong. Most of the time you can fix it by reconditioning the bit on an abrasive block or changing your speed and feed, not by buying a new bit.

Cause 1: Glazed Diamonds

When a bit rubs without cutting, the diamond surface polishes smooth and stops biting. This happens most often when the bit spins too slow or presses too lightly against hard concrete. The fix is to dress the bit on a silicon carbide block or a piece of old abrasive pad for 30-60 seconds to expose fresh diamonds, then resume cutting with firm, steady feed.

Cause 2: Wrong Bond for the Concrete

Soft concrete releases diamond particles too slowly and the bond never wears, so the bit glazes; very hard concrete wears the bond faster than the diamonds cut. If your bit repeatedly stops cutting on the same floor, the bond hardness does not match the concrete. Match a hard bond to soft concrete and a soft bond to hard concrete.

Cause 3: Feed and Speed Issues

Too little pressure lets the bit polish; too much pressure overheats the segments and cracks them. Use the recommended RPM for the bit diameter and push hard enough that you feel steady cutting, not spinning. With water-cooled bits, check the water flow: a blocked or weak flow lets the bit overheat and dull instantly.

How to Recondition a Bit on Site

Mount a silicon carbide dressing stone on the drill rig, run the bit at low speed against it for a minute, then try the concrete again. Alternatively, drill briefly into a soft abrasive block or a chunk of masonry. Reconditioning restores cutting on glazed bits in most cases and is the cheapest fix to try before replacing the core.

When to Replace Instead

Replace the bit when the segments are worn below about half their height, when segments are missing or cracked, or when reconditioning no longer restores cutting. Also replace if the barrel is bent, since a bent bit wobbles and cannot cut true regardless of dressing.

Frequently Asked Questions

Q: How do I know if my bit is glazed or actually worn?

A> Look at the segment surface: shiny and smooth means glazed (recondition it); short and rounded with visible voids means worn (replace it).

Q: Can I cut with a dry bit wet?

A: No. Dry bits are not designed for water; wetting them can crack the segments. Use a wet bit with water.

Q: Why does my bit only cut on the edge and stall in the middle?

A: The center of the segments is glazed or the feed is uneven. Dress the bit and keep the pressure centered on the bit axis.

Conclusion

When a core bit stops cutting, diagnose before replacing: check for glaze, match the bond to the concrete, and correct feed and speed. Reconditioning on an abrasive block restores most bits, so a stopped bit is usually a fixable problem, not the end of the tool.