One broken bit. Four different diagnoses.
Inspect what remained attached before blaming carbide, brazing or straightening.
Fracture inside the carbide
Primary suspects: Toughness, sintering, microcracks, edge support, geometry, rebar or hard aggregate.
Do not conclude: Not automatically a brazing failure.
Joint system failed
Primary suspects: Face cleanliness, slot fit, wetting, voids, brazing cycle or welding parameters.
Do not conclude: Not the same as an edge chip.
Axis or symmetry failed
Primary suspects: Off-centre head, poor symmetry, circular run-out or no measurement after straightening.
Do not conclude: Not a carbide-grade diagnosis.
Steel or heat treatment failed
Primary suspects: Body steel, hardening and tempering, unsafe length-to-diameter ratio or machine mismatch.
Do not conclude: Not a carbide-joint diagnosis.
Failure location first. Process conclusion second.
Problem 1: Blue or Black Tip (Overheating)
Symptom: The cutting tip changes colour — straw yellow → blue → black.
Cause: RPM too high with insufficient cooling. HSS loses hardness above 600°C; once blue, the steel is permanently soft and cannot be recovered by cooling.
| Colour | Temperature | Action |
|---|---|---|
| Straw / gold | ~250°C | Warning — reduce RPM and add coolant immediately |
| Blue / purple | ~350°C | HSS hardness already reduced — bit is degraded |
| Black | 600°C+ | Discard — bit is fully annealed |
Fix: Check your RPM chart for the material and diameter. Use cutting fluid on metal. Peck every 5–10 mm to allow the bit to air-cool.
Problem 2: Chipped or Broken Carbide Corners
Symptom: One or more carbide cutting corners are missing while the insert or head remains attached.
Likely causes: a carbide grade that is too brittle for the impact, inconsistent sintering or latent cracks, insufficient steel/carbide support behind the edge, an overly thin cutting section, or a rebar/hard-aggregate strike the head geometry cannot distribute.
Do not diagnose this as brazing by default. Brazing joins the carbide to the body. It does not determine every fracture that occurs inside the carbide itself. A full-carbide head can also chip if its material or geometry is wrong.
Action: preserve the failed bit, photograph the fracture, record substrate and machine, and compare several samples under the same conditions. Repeated edge-only fractures point to material/geometry control; one isolated rebar strike may be an application event.
Problem 3: Shank Deformation or Slipping
Symptom: The SDS-Plus shank grooves are worn smooth; the bit slips in the chuck or does not lock. Alternatively, the round shank has visible score marks from chuck slipping.
Cause:
- SDS shank wear: Normal wear accelerated by using an oversized bit for the machine class (e.g. 25 mm bit on a 2 kg light rotary hammer), or using the drill outside its rated BPM range.
- Round shank slipping: Chuck is worn or undertightened. Heavy drilling in metal requires a keyed chuck and a properly torqued grip — keyless chucks slip under sustained lateral load.
Fix: Check the chuck — tighten with a chuck key if present. For SDS shanks, inspect the locking slot edges: if the edges are rounded rather than sharp, the bit must be replaced. Match bit size to machine weight class.
Problem 4: Carbide Insert or Head Detachment
Symptom: The whole insert or head becomes loose or separates from the steel body.
Likely causes: contaminated joining faces, poor slot fit, incomplete filler wetting, porosity, incorrect brazing temperature/time, or uncontrolled welding force/current. Severe thermal or impact overload may contribute, but detachment and edge chipping are not the same failure.
Action: inspect the steel seat and the separated carbide face. A clean separation with poor filler coverage points to the joint. Ask for the joining-cycle record, symmetry check and a polished cross-section or validated joint test. “Controlled-atmosphere brazed” is a process label, not proof by itself.
Problem 5: Bit Walking Off-Centre or Drilling an Oversized Hole
Symptom: The hole is in the wrong position, or significantly larger than the bit diameter. The drill wanders during starting.
Cause:
- Walking on startup: No centre punch or pilot hole on metal. Tile / glass without tape. Bit started at high speed without establishing a starting groove.
- Oversized hole: Unequal lip heights on HSS bits (one lip doing all the work, causing the bit to oscillate). On SDS bits, a chipped or asymmetrical carbide tip causes the same effect.
Fix: On metal, always centre-punch first. On tile, use masking tape or a diamond starter dimple. Inspect HSS bits with a drill point gauge to check lip symmetry. Replace bits with asymmetrical tip wear.
Problem 6: Slow Progress or Complete Stall in Deep Holes
Symptom: Drilling rate drops significantly as the hole deepens. Eventually the bit stops advancing despite full motor speed.
Cause: Flute packing — the helical grooves that carry dust out of the hole are completely filled. The bit is spinning in a slug of compacted dust with no contact between carbide and fresh concrete.
Fix: Withdraw the bit fully every 15–20 mm and allow the dust to clear. For deep holes (100 mm+), use a vacuum extraction bit (hollow centre with dust extraction port) or blow compressed air into the hole between peck cycles.
Failure Mode Summary
| Failure | Primary Cause | Preventable? |
|---|---|---|
| Blue tip | Excessive RPM / no coolant | ✅ Yes — check RPM chart |
| Chipped carbide | Carbide toughness / cracks / weak edge support / rebar strike | ✅ Partially — match grade and geometry; verify the application |
| Shank wear | Machine/bit size mismatch | ✅ Yes — match bit to machine class |
| Tip detachment | Brazing failure / thermal shock | ✅ Partially — air-cool; buy controlled-atmosphere brazed bits |
| Bit walking | No centre punch / worn tip | ✅ Yes — always centre-punch metal |
| Flute packing | No pecking technique | ✅ Yes — peck every 15–20 mm |
Failure Prevention Summary
The majority of drill bit failures fall into three root causes: wrong bit for the material, incorrect speed or pressure, and insufficient cooling. A bit that runs at the right speed with the right feed pressure in the right mode will reach its rated hole count. Skipping any one of these three conditions — even briefly — can shorten life by 50% or more. Keep a bit condition log for large drilling contracts: recording holes drilled per bit provides the benchmark data to catch early decline before it causes a mid-hole failure.
