Chips are the "thermometer" and "stethoscope" of deep hole drilling. Shape reveals mechanical conditions (feed, sharpness, geometry). Color reveals thermal conditions (speed, coolant, heat). Together they provide real-time diagnosis of the hidden cutting zone.
| Bore Diameter | Ideal Length | Ideal Width | Ideal Curl |
|---|---|---|---|
| <5mm | 2–4mm | 0.5–1.0mm | Tight C-shape |
| 5–15mm | 4–8mm | 1.0–2.5mm | C-shape or 6-shape |
| 15–30mm | 6–12mm | 2.0–4.0mm | C-shape or short spiral |
| >30mm (BTA) | 8–15mm | 3.0–6.0mm | Tight C, figure-6, short conical |
| Color | Temp | Diagnosis | Action |
|---|---|---|---|
| Bright / Silver | <200°C | Normal; may be conservative | No action; headroom if needed |
| Golden / Straw | ~230°C | Sweet spot | Process under control |
| Brown to Purple | ~255–280°C | Upper edge; heat building | Check coolant; reduce Vc 10–15% |
| Dark Blue | ~300°C+ | Warning — excessive heat | Reduce Vc 20–30%; check coolant passages |
| Blue-black / Grey | >400°C | Critical — thermal damage | Stop; inspect edge; resolve coolant issue |
| Grey powder (no chips) | — | Edge rubbing, not cutting | Replace or resharpen; increase feed |
| Chip Shape | Diagnosis | Action |
|---|---|---|
| C-shaped / Short spiral | Ideal condition | Maintain parameters |
| Long ribbon (>20mm) | Insufficient chip breaking; feed too low | Increase feed; check chip breaker |
| Figure-6 / tight conical | Good chip breaking (BTA target) | Maintain |
| Powder / Dust (steel) | Feed too low; Vc too high; dull edge | Reduce Vc or increase feed; replace tool |
| Bird's nest (tangled) | Ductile material; inadequate coolant flow | Increase feed; add peck; check flow |
| Splintered / Fragmented | Brittle material; excessive feed; chipped edge | Reduce feed; inspect edge |
| Welded / BUE on edge | Insufficient cooling; coating failure; gummy material | Increase coolant; change coating; increase Vc |
| Tapered spiral (tight cone) | Poor evacuation; chip entry restriction | Increase flow; check chip opening |
| Two unequal streams | Asymmetric point grind | Regrind symmetrically; check for oversize |
Inspect chips from the first few holes under known-good parameters — you cannot judge a deviation without a reference.
A gradual shift in shape or color indicates progressive tool wear; a sudden change indicates a process event (coolant loss, chip jam, hard spot).
The same chip shape can be fine in one material and alarming in another — powdery chips are normal for cast iron but a serious warning in steel.
Most chip problems are speed/feed issues — exhaust feed, speed, and coolant adjustments first.
If chips keep degrading and parameter changes don’t restore normal form, the cutting edge is worn or damaged — stop and inspect.
| Symptom | First Move | Second Move |
|---|---|---|
| Blue chips, hole on size | Reduce Vc 20–30% | Check coolant flow at tool tip |
| Silver wire wrapping drill | Increase feed | Add peck cycle; check point geometry |
| Dust in steel | Replace or resharpen | Increase feed so edge cuts |
| Brown dust + squeal in SS | Stop dwelling — constant feed | Sharp M35+ drill; lower RPM |
| Unequal chip streams | Regrind point symmetrically | Check for oversize condition |
| Shiny welded flakes (BUE) | Increase Vc + improve lube | Use coated or polished-flute tool |
| Bird's nest in deep hole | Add peck cycle | Increase feed; parabolic flute |
| Long ribbons in BTA | Increase feed | Check chip breaker geometry |