Appearance
Research at the University of Stuttgart (Klocke et al., 2023) characterised chips from BTA drilling of AISI 4140+QT (Ø50 mm bore, L/D 30:1, Vc = 40–100 m/min, f = 0.06–0.20 mm/rev) and classified four morphological types: Type I — tightly coiled short chips (low speed/low feed, tempered martensite, no phase transformation); Type II — loosely coiled medium chips (moderate parameters, partial grain refinement, occasional white etching bands); Type III — long ribbon chips (high speed/high feed, continuous adiabatic shear bands with white etching microstructure); and Type IV — segmented sawtooth chips (high speed/low feed, periodic shear band spacing). The Type I→III transition occurred at ~650°C — the accelerated crater wear threshold for carbide tools. A hydraulic valve spool manufacturer (AISI 4140, Ø25 mm × 400 mm gun-drilled) implemented daily chip inspection: Type III/IV triggered 10% speed reduction to return to Type II. Tool changes reduced from 2 per shift to 1 per 2 shifts; scrap reduced from 4.2% to 1.1%.
Chip Formation Mechanics and Shear Zone Metallurgy
Shear Zone Characteristics in Deep Hole Drilling
| Shear Zone | Location | Deformation Mode | Strain Range | Strain Rate Range (s⁻¹) | Temperature Range (°C) | Microstructural Features | Thickness (µm) | Energy Dissipation |
|---|---|---|---|---|---|---|---|---|
| Primary shear zone (PSZ) | Between workpiece and chip (shear plane) | Simple shear | 2–8 (shear strain) | 10⁴–10⁵ | 400–900 (steel) | Elongated grains → subgrain formation → dynamic recovery → recrystallisation | 5–50 | 55–70% of total cutting energy |
| Secondary shear zone (SSZ) | Between chip and tool rake face | Severe shear + friction | 5–30 (equivalent strain) | 10⁵–10⁶ | 500–1,000 (steel) | Nanocrystalline layer (10–200 nm grain size); possible white etching layer on chip underside | 10–100 | 20–35% of total cutting energy |
| Tertiary shear zone (TSZ) | Between tool flank and new workpiece surface | Elastic recovery + friction | 0.5–3 | 10³–10⁴ | 300–700 (steel) | Deformed surface layer on bore; grain refinement near surface | 10–50 | 5–10% of total cutting energy |
| Guide pad contact zone | Between guide pad and bore surface | Sliding + burnishing | 0.1–1 (compressive + shear) | 10²–10³ | 150–400 (steel) | Compressive plastic deformation; grain elongation in circumferential direction | 20–100 | 2–8% of total cutting energy |
Chip Morphology Classification for Deep Hole Drilling
| Chip Type | Morphology Description | L/D Ratio | Typical Cutting Speed Vc (m/min) | Typical Feed f (mm/rev) | Material | Shear Banding | Microstructural Features | Chip Underside Appearance | Coolant Evacuation Performance | Process Indicator |
|---|---|---|---|---|---|---|---|---|---|---|
| Type Ia — Tight coil, short | Tight helical spiral, 1–3 turns, easily breaks | < 3:1 | < 50 | < 0.08 | Carbon steel, alloy steel | None (homogeneous deformation) | Tempered martensite; no phase transformation | Smooth, silver-grey | Excellent — flushes easily | Conservative parameters — safe |
| Type Ib — Tight coil, medium | Tight helical spiral, 3–8 turns | 3–8:1 | 50–70 | 0.08–0.12 | Carbon steel, alloy steel | Infrequent — narrow shear bands (< 5 µm) | Some grain elongation in PSZ | Smooth, light grey | Good | Moderate parameters — acceptable |
| Type II — Loose coil | Open helical spiral, 5–15 turns | 5–15:1 | 60–90 | 0.10–0.16 | Carbon steel, alloy steel | Occasional — shear bands 5–15 µm thick | Partial grain refinement; equiaxed grains 0.5–2 µm in shear bands | Moderate roughness, grey | Fair — may pack in flute | Optimised parameters — target for production |
| Type IIIa — Long ribbon, continuous | Straight or curved ribbon without coiling | > 15:1 | > 80 | > 0.16 | Carbon steel, alloy steel | Continuous — shear bands 15–30 µm thick; possible white etching bands | Continuous adiabatic shear bands with transformed microstructure; nanocrystalline grains 50–200 nm | Rough, silvery-white bands | Poor — risk of flute blockage | Aggressive parameters — caution (approaching tool wear limit) |
| Type IIIb — Long ribbon, segmented | Straight ribbon with periodic variations | > 15:1 | > 90 | 0.10–0.16 | Carbon steel, alloy steel | Periodic adiabatic shear bands at 100–500 µm spacing | Periodic microstructure: deformed band + transformed band | Rough, periodic surface texture | Poor — similar to IIIa | High-temperature regime — tool wear accelerated |
| Type IVa — Segmented (sawtooth) | Distinct sawtooth segments, partially connected | 2–10:1 (segment length) | > 80 | < 0.08 | Titanium, stainless steel, superalloys | Highly localised — shear bands 5–20 µm between segments | Narrow transformed bands separating deformed segments; grain refinement to 100–500 nm | Irregular sawtooth surface | Fair — segmented chips clear well but high forces | Characteristic of difficult-to-machine materials |
| Type IVb — Segmented (discrete) | Fully separated segments, individual chip pieces | 1–3:1 | > 50 | 0.05–0.15 | Titanium, hardened steel | Extreme localisation — bands < 5 µm | Complete transformation within shear bands; very fine grains 20–100 nm | Highly irregular | Good (small pieces) but high frequency | High speed, high hardness regime |
Chip Microstructural Features by Material
| Material | Chip Type (Typical) | Shear Band Microstructure | Adiabatic Shear Band Width (µm) | Grain Size Inside Band (nm) | Grain Size Outside Band (µm) | Phase Transformation in Shear Band | Hardness Inside Band (HV0.01) | Hardness Outside Band (HV0.01) | Characteristic Feature |
|---|---|---|---|---|---|---|---|---|---|
| AISI 4140 (28 HRC) | Ib → II → IIIa | Dynamic recovery + recrystallisation | 10–30 | 50–200 | 1–5 (ferrite) + 0.5–2 (carbides) | Tempered martensite → nanocrystalline ferrite + spheroidised carbides | 650–800 | 350–450 | Narrow, well-defined bands at high speed |
| AISI 4140 (48 HRC) | II → IIIa → IVb | Dynamic recrystallisation | 5–20 | 20–100 | 0.5–2 | Tempered martensite → austenitisation → rapid quench → untempered martensite (white etching) | 850–1,050 | 480–520 | Very hard white etching bands at high speed |
| 316L stainless | IIIb → IVa | Dynamic recovery (no recrystallisation) | 15–50 | 100–500 | 5–20 | No phase transformation; grain refinement only | 450–550 | 200–250 | Wide diffuse bands; no sharp transition |
| Inconel 718 (42 HRC) | IVa | Dynamic recovery + partial recrystallisation | 5–15 | 50–300 | 0.5–3 | γ phase dissolution in shear band; grain refinement | 550–700 | 420–500 | Very narrow bands; severe segmentation |
| Ti-6Al-4V | IVa → IVb | Dynamic recrystallisation | 1–10 | 20–100 | 1–10 | α → β transformation on heating → α' martensite on rapid cooling | 450–550 | 300–380 | Extremely narrow bands; catastrophic shear localisation |
| SAE 1026 (180 HB) | Ia → Ib → II | Dynamic recovery | 15–40 | 200–500 | 5–15 | No transformation; ferrite grain refinement + pearlite spheroidisation | 300–400 | 180–220 | Gradual transition; wide bands |
Chip-Based Process Monitoring and Optimisation
Chip Morphology as Process Diagnostic
| Chip Observation | Visual Characteristic | Probable Process Condition | Recommended Action | Urgency | Confirmation Test |
|---|---|---|---|---|---|
| Tight coil, short (Type Ia) | Small helices, 1–3 turns, silver-grey colour | Low speed, low feed — conservative parameters, long tool life | Can increase productivity (increase Vc or f) if cycle time is critical | Low | Check surface finish (Ra target) |
| Loose coil, medium (Type II) | Open helices, 5–10 turns, uniform light grey | Moderate parameters — in target process window | Maintain current parameters | None | Normal SPC monitoring |
| Long ribbon, continuous (Type IIIa) | Straight, curved ribbons, > 15× length-to-diameter | High speed, high feed — approaching thermal limit of tool | Reduce cutting speed by 10–15% or increase coolant pressure | Medium | Check tool flank wear; measure bore surface finish |
| Long ribbon with white bands (Type IIIa with white etching) | Ribbon with intermittent bright bands visible to naked eye | Cutting edge temperature > 650–700°C — accelerated crater wear regime | Reduce cutting speed by 15–20% immediately | High | Inspect cutting edge for crater wear; measure chip colour (blue/black = > 600°C) |
| Segmented/sawtooth (Type IV) | Distinct segments, jagged edges, periodic structure | Severe thermomechanical loading — material-specific instability | Reduce speed 10–15% and check feed; may need material-specific geometry | High | Check for chatter marks on bore surface; measure spindle power fluctuation |
| Blue or black chip colour | Dark blue, purple, or black chip surface | Oxidation temperature exceeded: blue = 300–350°C, black = > 400°C (steel) | Reduce speed or increase coolant flow; chip colour indicates cutting temperature | Medium | IR temperature measurement if available |
| Chip packing in coolant return | Intermittent chip flow; chips accumulating in bore | Inadequate chip evacuation — risk of flute blockage and tool breakage | Increase coolant pressure or flow rate; check chip breaker geometry | Critical — immediate | Stop feed immediately; retract tool and clear chips |
| Chip with built-up edge fragments | Chunks of workpiece material adhered to chip surface | Built-up edge forming on tool — degrades surface finish and bore tolerance | Adjust cutting speed (increase or decrease 20% to exit BUE range) | Medium | Check bore surface for smearing; measure surface finish |
| Fused chip balls | Spherical agglomerations of fine chips welded together | Excessive temperature in chip evacuation path; inadequate coolant flow | Increase coolant flow; check for chip congestion in drill head | Critical | Inspect drill head for chip packing damage |
| Excessive dust or fine chips | Fine metallic powder instead of coiled chips | Tool edge breakdown (chipping or fracture) or severe rubbing (feed too low) | Stop immediately; inspect cutting edge | Critical — immediate | Replace or recondition tool; verify feed rate |
Chip Morphology vs. Cutting Parameters and Tool Wear
| Cutting Speed Vc (m/min) | Feed f (mm/rev) | Dominant Chip Type | Chip Temperature at Formation (°C) | Shear Band Width (µm) | Flank Wear Rate (µm/km) | Surface Finish Ra (µm) | Recommended Action |
|---|---|---|---|---|---|---|---|
| 40 | 0.08 | Ia (tight coil) | 400–480 | None | 2–4 | 0.6–1.0 | Increase speed for productivity if acceptable |
| 60 | 0.08 | Ib (tight coil) | 480–550 | None | 3–6 | 0.5–0.8 | Target for finishing passes |
| 60 | 0.12 | Ib → II (mixed) | 520–600 | 5–10 (occasional) | 4–8 | 0.6–1.0 | Acceptable production range |
| 80 | 0.12 | II (loose coil) | 600–680 | 10–20 | 8–15 | 0.5–0.9 | Optimal production parameters |
| 80 | 0.20 | IIIa (ribbon) | 680–780 | 15–25 | 20–40 | 0.8–1.4 | Reduce speed — approaching wear threshold |
| 100 | 0.12 | IIIa → IIIb (ribbon, intermittent white bands) | 750–850 | 20–30 | 40–80 | 0.8–1.6 | Reduce speed immediately |
| 100 | 0.20 | IIIb (ribbon with white bands) | 820–920 | 25–40 | 80–150 | 1.2–2.0 | Excessive wear — not recommended |
| 120 | 0.08 | IIIb → IVa (segmented onset) | 850–950 | 30–50 | 150–300 | 1.5–3.0 | Reduce speed; check tool condition |
Chip Evacuation Performance by Chip Type and Bore Geometry
| Chip Type | Bore Diameter (mm) | L/D Ratio | Minimum Coolant Pressure for Evacuation (bar) | Minimum Coolant Flow Rate (L/min) | Maximum Chip Concentration in Coolant (vol%) | Chip Transport Velocity in Annulus (m/s) | Risk of Chip Blockage | Recommended Coolant Velocity (m/s) |
|---|---|---|---|---|---|---|---|---|
| Type Ia (tight coil, short) | 10–30 | 30:1 | 30–50 | 10–25 | 2–5 | 2–5 | Very low | 3–5 |
| Type Ia (tight coil, short) | 30–80 | 30:1 | 20–40 | 25–60 | 3–8 | 1.5–4 | Very low | 2–4 |
| Type Ib (tight coil, medium) | 10–30 | 30:1 | 40–60 | 15–30 | 1–3 | 1.5–3 | Low | 3–5 |
| Type Ib (tight coil, medium) | 30–80 | 30:1 | 30–50 | 30–70 | 2–5 | 1–2.5 | Low | 2–4 |
| Type II (loose coil) | 10–30 | 30:1 | 50–80 | 20–40 | 1–2 | 1–2.5 | Moderate | 4–6 |
| Type II (loose coil) | 30–80 | 30:1 | 40–70 | 40–90 | 1–3 | 0.8–2 | Moderate | 3–5 |
| Type IIIa (ribbon, continuous) | 10–30 | 30:1 | 80–120 | 25–50 | 0.5–1 | 0.5–1.5 | High | 6–8 |
| Type IIIa (ribbon, continuous) | 30–80 | 30:1 | 60–100 | 50–120 | 0.5–1.5 | 0.3–1 | High | 5–7 |
| Type IIIb (ribbon, segmented) | 10–30 | 30:1 | 60–90 | 20–40 | 1–2 | 0.8–2 | Moderate | 5–7 |
| Type IV (segmented/sawtooth) | 10–30 | 30:1 | 40–70 | 15–30 | 2–4 | 1–3 | Low-moderate | 4–6 |
FAQ
How does chip morphology change with cutting parameters in deep hole drilling?
Chip morphology in deep hole drilling follows systematic transitions with cutting speed and feed rate, and these transitions directly reflect the thermomechanical conditions at the cutting edge. The chip morphology map for steel drilling (based on the AISI 4140 research) shows four regimes: (1) Low speed, low feed (Vc < 50 m/min, f < 0.08 mm/rev) — Type Ia chips (tightly coiled, short helices). The cutting temperature is below 500°C, and chip formation occurs by homogeneous plastic deformation in the primary shear zone without localisation. The chip microstructure is unchanged from the workpiece material (tempered martensite in 4140 steel). This regime is characterised by long tool life (flank wear rate 2–4 µm/km) but low productivity. (2) Moderate speed, moderate feed (Vc = 50–80 m/min, f = 0.08–0.16 mm/rev) — Type Ib and Type II chips (tight coils to loose coils). The cutting temperature ranges from 500–680°C. The primary shear zone begins to show localisation: narrow adiabatic shear bands (5–15 µm wide) form intermittently, visible as regions of grain refinement (grain size 0.5–2 µm) within the chip microstructure. This is the target regime for production deep hole drilling because it balances tool life and productivity. (3) High speed, high feed (Vc = 80–120 m/min, f = 0.12–0.20 mm/rev) — Type III chips (long ribbon chips). The cutting temperature exceeds 650–750°C, and continuous adiabatic shear bands form along the entire chip length. The shear band microstructure shows complete transformation: nanocrystalline ferrite (50–200 nm grains) with spheroidised carbides, sometimes with white etching bands from austenitisation and rapid quenching. This regime produces accelerated tool wear (flank wear rate 20–150 µm/km) due to the cutting edge temperature exceeding the carbide crater wear threshold. (4) Very high speed (Vc > 100 m/min) — Type IIIb to Type IV chips (segmented chips with periodic adiabatic shear bands). At these speeds, the chip formation becomes cyclic: the shear strain localises catastrophically in a narrow band (5–15 µm), causing a sudden slip event, followed by re-stressing of the workpiece material and another localisation event. The segmentation frequency (typically 2–10 kHz for steel) is determined by the material's thermal diffusivity and the strain rate sensitivity. The practical significance of the chip morphology map is that chip inspection provides an immediate diagnostic of the cutting regime: if operators see Type III ribbon chips (particularly with white etching bands visible to the naked eye), the cutting speed should be reduced by 10–20% to return to the Type II regime, preventing accelerated tool wear and potential tool failure.
What is an adiabatic shear band and how does it form in deep hole drilling chips?
An adiabatic shear band (ASB) is a narrow region of intense localised shear deformation that forms in the chip during high-strain-rate machining when the thermal softening of the material exceeds its strain and strain-rate hardening capability. In deep hole drilling, ASBs form in the primary shear zone under conditions of high cutting speed and/or low thermal conductivity. The formation mechanism follows four stages: (1) Homogeneous deformation — in the primary shear zone, the material undergoes shear at strain rates of 10⁴–10⁵ s⁻¹. The plastic deformation work is converted to heat, causing a temperature rise in the shear zone. (2) Thermal softening onset — when the rate of temperature rise exceeds the material's strain-hardening rate (which decreases with increasing strain), the material in a localised region begins to soften. For steel, this typically occurs at local temperatures above 500–600°C, where the flow stress begins to decrease more rapidly than it increases through work-hardening. (3) Strain localisation — the initial softening causes further deformation to concentrate in the softening region, generating more heat and accelerating the softening process. This positive feedback loop causes the deformation to collapse from a wide zone (50–200 µm initially) into a narrow band (5–30 µm). Within the band, the shear strain can reach 10–50 (compared to 2–8 in homogeneous deformation). (4) Microstructural transformation — the combination of high strain and high temperature (700–1,000°C) within the shear band causes microstructural changes: dynamic recovery (formation of subgrains), dynamic recrystallisation (equiaxed grains 50–500 nm), and in some cases, phase transformation (austenitisation followed by rapid quenching to form untempered martensite — the white etching layer observed in shear bands). The band then cools rapidly (10⁴–10⁶°C/s) through heat conduction to the surrounding chip material and convective cooling by the coolant. The width of the adiabatic shear band depends on the material's thermal diffusivity (α = k/ρCp) and the strain rate sensitivity (m): band width ∝ √(α/ε̇) × f(m). Materials with low thermal diffusivity (titanium: α = 3.2 × 10⁻⁶ m²/s, Inconel 718: α = 3.3 × 10⁻⁶, steel: α = 8–12 × 10⁻⁶) form narrower bands because the heat cannot diffuse away from the localisation zone. This is why titanium and nickel alloys form very narrow bands (1–15 µm wide) and are prone to segmented chip formation even at moderate cutting speeds. The presence and characteristics of adiabatic shear bands in deep hole drilling chips are diagnostic of the cutting temperature: narrow, well-defined bands indicate high temperature and potential tool wear acceleration; wide, diffuse bands indicate moderate temperature with less risk.
How can chip analysis be used for production monitoring of deep hole drilling operations?
Chip analysis is one of the most practical and cost-effective production monitoring tools for deep hole drilling because it requires no additional sensors or data acquisition equipment — only visual inspection of the chips produced during the drilling cycle. The implementation follows these steps: (1) Chip sampling — collect a representative chip sample from the coolant return or chip conveyor at regular intervals (every 5–50 bores depending on production volume and process stability). The sample should be taken during steady-state drilling (avoiding the entry and exit phases where chip formation may differ). (2) Visual classification — classify the chips by morphology using a reference chart showing the four chip types (Ia, Ib, II, IIIa/b, IVa/b) with photographs and descriptions. The classification should note: chip type (coil, ribbon, segmented); chip dimensions (length, diameter, coil pitch); chip colour (silver, grey, blue, black); presence of shear bands or white etching bands; and any anomalies (BUE fragments, fused chips, dust). (3) Comparison with baseline — compare the current chip characteristics with the baseline established during process qualification. The baseline should be the chip type produced at the validated parameters that meet the bore quality and tool life requirements. (4) Trend monitoring — track chip type on a control chart (p-chart: proportion of chips in each type category). A shift from the baseline chip type (e.g., from Type II to Type IIIa) indicates a process change that may require corrective action, even if bore quality measurements are still within specification. (5) Decision criteria — establish clear criteria for corrective action based on chip observation. For example: if Type IIIa chips are observed, reduce cutting speed by 10% and verify return to Type II; if Type IIIb or white etching bands are observed, reduce speed by 15–20% and inspect tool condition; if Type Ia chips are observed consistently, consider whether speed can be increased to improve productivity. (6) Correlation with quality data — periodically correlate chip type with measured bore quality (surface finish, diameter tolerance, tool wear) to validate the chip-based decision criteria. The correlation between chip type and tool wear is particularly important: Type III chips with white etching bands reliably indicate cutting edge temperatures > 650–700°C, at which carbide tool wear accelerates. The advantages of chip monitoring over instrumented process monitoring (spindle power, acoustic emission, vibration) are: zero capital cost (no sensors required); direct physical evidence of cutting conditions (not an indirect measurement); and immediate availability (chips are visible as they exit the bore). The limitations are: it requires operator training and discipline; it is a discrete sampling method (not continuous); and it provides only a qualitative or semi-quantitative assessment. Despite these limitations, chip monitoring is recommended as a first-line process monitoring method for all deep hole drilling operations, supplemented by instrumented monitoring for high-value or safety-critical applications.
What is the relationship between chip microstructure and cutting edge temperature in deep hole drilling?
The chip microstructure records the thermal history of the material as it passes through the primary and secondary shear zones, and systematic microstructural analysis can be used to estimate the peak temperature reached at the cutting edge. The relationships between microstructure and temperature for steel chips are: (1) No visible microstructural change (tempered martensite or ferrite-pearlite identical to workpiece) — peak temperature < 500°C. This corresponds to Type Ia chips at low cutting speed. The tool-chip interface temperature is below the onset of significant diffusion wear for carbide tools. (2) Grain refinement in shear bands (0.5–2 µm equiaxed grains) — peak temperature 500–650°C. Dynamic recrystallisation has occurred in the shear band, indicating temperatures above 0.5× the melting point (Tmelt ~ 1,500°C for steel) = 750 K = 480°C, consistent with the recrystallisation temperature for deformed steel. This corresponds to Type Ib and Type II chips at moderate parameters. (3) Spheroidised carbides in shear bands — peak temperature 650–750°C. The cementite (Fe₃C) lamellae in pearlite have spheroidised, indicating sufficient time at temperature for diffusion-controlled spheroidisation. This corresponds to Type II chips transitioning to Type IIIa. (4) White etching bands in shear bands — peak temperature > 750–800°C (above the A1 temperature for steel, approximately 727°C for eutectoid composition). The material has been austenitised and then rapidly quenched (by the coolant or by heat conduction to the surrounding chip) to form untempered martensite, which appears white under optical microscopy after etching. The hardness of the white etching bands is 850–1,050 HV0.01 compared to 350–450 HV0.01 for the surrounding chip material. This corresponds to Type IIIa/IIIb chips at high parameters. (5) Complete transformation of the chip (fully white etching chip) — peak temperature > 850–1,100°C (well above A3 temperature). The entire chip cross-section has been austenitised. This indicates extreme cutting conditions and imminent tool failure. The metallographic method for temperature estimation is more accurate than chip colour (which depends on oxide layer thickness and is influenced by coolant chemistry) but requires laboratory preparation (sectioning, mounting, polishing, etching) and is not suitable for real-time monitoring. For production applications, the practical approach is: establish the correlation between chip type (visual classification) and cutting edge temperature (measured by embedded thermocouple or estimated from FEM) during process development; then use chip visual classification as the production monitoring method, with the knowledge that Type III chips with visible white etching bands correspond to cutting edge temperatures exceeding 700°C — the threshold for accelerated crater wear in carbide tools.
How do different workpiece materials produce different chip types in deep hole drilling?
Different workpiece materials produce fundamentally different chip types in deep hole drilling because of differences in their mechanical and thermal properties — particularly the work-hardening exponent, thermal conductivity, and strain rate sensitivity. The material-specific chip formation characteristics are: (1) Carbon and alloy steels (SAE 1018–1045, AISI 4140/4340) — produce the full range of chip types from Type Ia (tight coil at low speed) through Type IV (segmented at very high speed). Steel has moderate thermal conductivity (40–55 W/m·K for low-carbon, 30–45 W/m·K for low-alloy) and moderate work-hardening (n = 0.15–0.25). The chip transition from coil to ribbon to segmented occurs at cutting speeds of 50–120 m/min depending on hardness. The chip colour provides a useful temperature indicator: silver (< 250°C), straw/yellow (250–300°C), blue (300–350°C), grey/black (> 350°C chip exit temperature). (2) Stainless steel (304/316, 17-4PH) — predominantly produces Type IIIb and Type IVa chips even at moderate speeds (Vc > 50–60 m/min) due to low thermal conductivity (15–20 W/m·K) and high work-hardening rate (n = 0.40–0.50). Stainless chips are characteristically long, stringy, and difficult to break. The chips often have a dark grey or black surface due to the high temperature at the tool-chip interface. The high work-hardening rate causes the chip to be significantly harder than the workpiece (chip hardness 350–500 HV vs workpiece 180–250 HV for 304L). (3) Titanium alloys (Ti-6Al-4V) — exclusively produce Type IV segmented or sawtooth chips across the practical drilling speed range (Vc > 20–30 m/min). The low thermal conductivity (7.5 W/m·K) and low strain rate sensitivity (m = 0.02–0.05) cause extreme adiabatic shear localisation. The shear bands in titanium chips are extremely narrow (1–10 µm) and show the α → β → α' (martensite) transformation sequence. The chip segmentation frequency (5–20 kHz) is diagnostic of the cutting speed: higher speeds produce higher segmentation frequencies and thinner segments. (4) Nickel-based superalloys (Inconel 718, Waspaloy) — produce Type IVa chips with narrow shear bands (5–15 µm). The chips are characterised by severe serration and high hardness (500–700 HV in the chip body, 550–700 HV in the shear band). The chips often have a characteristic blue-grey colour and tend to be abrasive to the tool. The chip thickness ratio (undeformed chip thickness / actual chip thickness) is high (3–5) compared to steel (1.5–2.5), indicating higher cutting energy and higher specific cutting forces. (5) Aluminium alloys (6061-T6, 7075-T6) — produce Type Ia and Type Ib chips at all practical speeds (Vc = 100–600 m/min). Aluminium's high thermal conductivity (160–180 W/m·K) prevents thermal localisation and adiabatic shear band formation. The chips are characteristically tight coils or small helices with a bright, clean appearance. Built-up edge formation on the chip underside is a common problem in aluminium deep hole drilling, appearing as smeared aluminium fragments adhered to the chip surface. (6) Cast iron (grey, ductile) — produces fragmented, discontinuous chips (Type IVb-like) due to the inherent brittleness of the material. Cast iron chips range from fine dust (at very low feeds) to small, irregular fragments (at higher feeds). The chip colour is typically dark grey (graphite content). Cast iron chips are the easiest to evacuate but generate the most abrasive wear on the tool and guide pads.
This article provides an overview of chip metallurgy and shear zone phenomena in deep hole drilling. Chip morphology, microstructural evolution, and shear band formation depend on the specific workpiece material, tool geometry, and cutting parameters. Chip-based process monitoring is a practical and cost-effective method for production quality control, but it should be supplemented with instrumented monitoring for high-value or safety-critical applications. The technical data presented here reflects published research and documented case studies as of 2026.