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A manufacturer of hydraulic cylinder rods (AISI 4140, 28 HRC, Ø20 mm × 1000 mm deep, BTA drilling) developed sudden chip packing causing three drill breakages in one shift. Increasing coolant pressure from 40 to 60 bar did not help. Following a structured sequence — (1) verify pressure at drill head (found 40 bar at pump, 18 bar at head — clogged filter), (2) inspect coolant filter (collapsed element bypassing chips), (3) inspect chip form (inconsistent 2–15 mm — worn chip breaker + coolant flow variation) — the filter was replaced, the worn BTA head was changed, and breakages stopped. Root cause: not coolant pressure, but coolant cleanliness — recirculating chips blocked the annulus.
Primary Failure Modes: Master Troubleshooting Matrix
Master Problem-Cause-Solution Matrix for Deep Hole Drilling
| Failure Mode | Common Causes (ranked by frequency) | Immediate Corrective Actions | Solution Category | Verification Method | Priority |
|---|---|---|---|---|---|
| Poor surface finish (Ra > specification, visible tool marks, scoring lines, roughness) | 1. Worn tool (flank wear VB > 0.2 mm) — most common cause | Replace tool (regrind or new); verify tool geometry after resharpening | Tooling | Measure flank wear (VB) with toolmaker's microscope at 20× | Immediate — affects bore function |
| 2. Inadequate coolant EP additive concentration — guide pads are not lubricated, causing scoring | Verify coolant EP concentration (refractometer for oil; titration for EP additives); add EP concentrate as needed | Coolant | Refractometer reading; EP additive test kit | High — prevents scoring | |
| 3. Coolant pressure too low — chips recirculate and score the bore wall | Measure coolant pressure at the drill head (not at the pump); target > 20 bar for gun drilling, > 30 bar for BTA; clean filter | Coolant | Pressure gauge at drill head (not pump) | High | |
| 4. Chip packing in the flute — chips recirculate and scratch the bore surface | Reduce peck depth; increase coolant pressure; check chip form (long chips indicate chip breaker failure) | Chip control | Inspect chips from the coolant filter | Medium | |
| 5. Built-up edge (BUE) on the cutting edge | Increase cutting speed by 10–20% (to increase temperature and reduce BUE); verify coolant EP additive concentration | Process parameter | Inspect cutting edge under microscope (10×) for adhered material | Medium | |
| 6. Machine vibration / chatter | Check spindle runout (< 0.005 mm); check bushing wear; verify workpiece clamping rigidity | Machine condition | Dial indicator on drill near bushing; accelerometer on workpiece fixture | Medium | |
| Bore deviation (straightness out of tolerance, bore position error, taper) | 1. Worn or incorrect drill bushing — bushing bore diameter > G6 tolerance; bushing-to-drill clearance > 0.010 mm | Replace bushing; verify bushing bore diameter with air gauge; clearance should be 0.003–0.008 mm for gun drilling | Bushing | Air gauge measurement of bushing bore and drill OD | Immediate |
| 2. Spindle-to-bushing misalignment — axial misalignment > 0.01 mm | Realign spindle to bushing using laser alignment or test bar + indicator | Machine alignment | Laser alignment fixture or dial indicator on test bar | High | |
| 3. Inconsistent feed — feed variation > 10% caused by servo tuning, stick-slip guideways, or coolant pressure fluctuation | Check feed servo tuning (gain, velocity loop); inspect guideways for stick-slip; check coolant pump pressure stability | Feed system | Feed rate trend from CNC (1 s sampling); coolant pressure trend | High | |
| 4. Entry chipping of drill cutting edge — chipped edge causes asymmetric cutting forces | Verify entry sequence (reduce feed for first 2–3 mm of penetration); check workpiece hardness at entry; inspect edge under microscope | Tooling | Visual inspection of drill edge at 20× | Medium | |
| 5. Incorrect guide pad height differential — leading pad worn or incorrectly ground | Measure pad height (leading pad should be 0.02–0.05 mm proud of trailing pad for gun drilling) | Tooling | Micrometer or toolmaker's microscope on pad heights | Medium | |
| 6. Hard inclusions or inconsistent material hardness | Verify material certification (hardness survey if suspect); adjust feed rate by −10% if hardness > specification | Material | Hardness test on workpiece cross-section | Low | |
| Tool breakage (drill snaps in the bore) | 1. Chip packing — the most common cause of tool breakage; chips block the flute, torque spikes to 3–5× normal | Stop immediately when torque spike detected (spindle power monitoring); retract drill; inspect chip form; reduce peck depth; increase coolant pressure | Chip control | Spindle power trend (continuous monitoring); chip inspection | Immediate (catastrophic) |
| 2. Feed overload — feed rate exceeds the chip-breaking capacity; chip thickness > flute capacity | Reduce feed; verify chip breaker geometry (if mechanical chip breaker, groove width matched to feed range) | Process parameter | Feed rate trend; chip thickness measurement | Immediate | |
| 3. Coolant starvation — coolant flow interrupted (pump failure, filter blockage, hose kinked, rotary union seal failure) causing chip packing and heat buildup | Check coolant flow at drill exit (visual); check filter pressure differential; inspect rotary union seal | Coolant | Coolant pressure gauge at drill head; flow meter | Immediate | |
| 4. Workpiece movement — clamping failure or thermal movement shifts the workpiece during drilling | Verify clamping force; check workpiece temperature rise during drilling; use hydraulic or pneumatic clamping with pressure switch monitoring | Fixture | Clamping pressure gauge; workpiece position sensor | High | |
| 5. Excessive tool wear — flank wear VB > 0.3 mm increases thrust force beyond the drill's column strength | Replace tool at VB = 0.2 mm (preventive); implement tool life tracking (bores per tool) | Tooling | Flank wear measurement (VB); cumulative bore count | High | |
| 6. Material defect — hard inclusion (carbide, slag, or hard spot) in the workpiece | Inspect chip form during drilling (sudden long chips or powder indicate inclusion); verify material quality (ultrasonic testing on input material) | Material | Ultrasonic inspection of workpiece (if recurrent) | Low (rare) | |
| Chip packing (chips accumulate in the flute, coolant flow restricted, torque increases) | 1. Chip breaker failure — chip breaker groove worn or incorrect for the feed range | Replace tool or regrind chip breaker; verify chip breaker groove width matches feed per revolution | Tooling | Chip form inspection (long chips > 5× ideal length indicate chip breaker failure) | Immediate |
| 2. Coolant flow rate insufficient — annular velocity < 8 m/s for gun drilling, < 10 m/s for BTA | Increase coolant pressure; check for coolant restrictions (filters, hoses, rotary union); verify pump capacity | Coolant | Calculate annular velocity: V_ann = Q / (60 × A_ann) where Q = flow rate (L/min), A_ann = annular area (mm²) | Immediate | |
| 3. Incorrect chip form — too long or too wide for the flute | Adjust feed (increase to thicken chip and improve breakage); adjust chip breaker groove width; consider MAM (modulated feed) | Process parameter | Chip length measurement; comparison to chip form chart | High | |
| 4. Annular clearance too small — drill OD too close to bore diameter | Verify drill OD and bushing ID; annular clearance should be 0.1–0.3 mm per side for gun drilling | Tooling | Drill OD measurement (micrometer); bore diameter measurement (air gauge) | Medium | |
| 5. Material excessively ductile (copper, stainless steel, titanium at low speed) | Increase feed to produce thicker, more breakable chips; consider MAM or CNC oscillating feed | Process parameter | Observe chip form before/after parameter change | Medium | |
| Chatter (visible chatter marks on bore surface, audible squealing) | 1. Drill tube resonance — cutting frequency (tooth pass frequency) near the drill tube's natural bending frequency | Change spindle speed by ±10–20% to move tooth pass frequency away from resonance; calculate tooth pass frequency = N/60 (Hz) for single-flute gun drill | Process parameter | Chatter frequency analysis (FFT of accelerometer signal or acoustic measurement); compare to drill tube natural frequency | Immediate |
| 2. Low damping in the drill tube — the drill tube has insufficient vibration damping | Check drill tube support (steady rests properly positioned?); add external damping (tuned mass damper, viscoelastic wrap on the tube) | Machine | Coherence spectrum (modal hammer test of drill tube); damping ratio < 3% indicates inadequate damping | High | |
| 3. Excessive feed or chip width — feed rate places the process in an unstable zone of the stability lobe diagram | Reduce feed by 20%; if stable, increase speed to the nearest stable lobe centre (per stability lobe diagram) | Process parameter | Stability lobe diagram (if available); otherwise trial speed increases/decreases in 10% steps | High | |
| 4. Worn guide pads — pad clearance has increased; the drill is not properly supported in the bore | Replace tool or re-hone guide pads; verify pad height differential (leading vs trailing pad) | Tooling | Pad height measurement; bore diameter trend increasing by > 0.01 mm from new tool indicates pad wear | Medium | |
| 5. Workpiece vibration — thin-wall workpiece vibrates at the chatter frequency | Improve workpiece clamping; add vibration damping supports on the workpiece; increase feed to reduce specific cutting energy (thicker chip = lower specific energy) | Fixture | Accelerometer on workpiece; vibration amplitude > 0.02 mm indicates vibration problem | Medium | |
| Edge chipping (small fractures on the cutting edge, visible at 10–20×) | 1. Interrupted cut — cross-hole, keyway, or slot in the bore path that the drill encounters | Feed reduction by 50% 2 mm before the interruption and for 2 mm after; increase point angle to 140° for stronger edge; use PCD tooling for best edge toughness | Tooling + Process | Visual inspection of cutting edge at 20×; counting of impact cycles until chip | Immediate |
| 2. Entry or exit impact — feed rate too high at start or breakthrough | Reduce feed by 50% for first 2 mm of penetration and last 3 mm of breakthrough; verify bush/support at entry | Process parameter | Feed rate profile at entry and exit from CNC trend data | High | |
| 3. Hard inclusion in workpiece — localised hard spot (carbide, slag) impacts the cutting edge | Reduce cutting speed by 20% for the affected part; if recurrent, implement ultrasonic inspection of incoming material | Material | Check chip form during drilling (sudden change indicates inclusion) | Low (rare) | |
| 4. Edge preparation inadequate — edge radius too small (< 2 µm) for the cutting forces | Increase edge hone to 5–10 µm; verify edge preparation after resharpening (edge radius gauge or silicone replica) | Tooling | Edge radius measurement at 50–100× | Medium | |
| 5. Excessive feed for the edge strength — chip thickness exceeds the edge's load capacity | Reduce feed to bring chip thickness below the edge's load limit; calculate chip thickness = f × sin(k_r) where k_r = cutting edge angle | Process parameter | Chip thickness measurement; edge radius comparison to chip thickness (edge radius should be < 0.3 × chip thickness) | High |
Material-Specific Troubleshooting
Common Problems and Solutions by Workpiece Material
| Material | Most Common Problem | Root Cause | Recommended Corrective Action | Secondary Problem | Secondary Action |
|---|---|---|---|---|---|
| Low-carbon steel (1018, 1020) | Chip packing — long, continuous chips that tangle in the flute | High ductility; chip breaker not aggressive enough for low-carbon steel's elongation | Increase feed to 0.08–0.12 mm/rev (thicker chip breaks more readily); use chip breaker with narrower groove width (2.0–2.5 mm for Ø10 mm) | Burr at bore exit | Reduce feed by 50% for final 3 mm; use backup plate |
| Alloy steel (4140, 4340, 8620) — annealed | Poor surface finish — Ra > 1.5 µm | Built-up edge from insufficient EP additive activity in coolant | Increase coolant EP concentration (sulphur content to 1.5–2.0%); increase cutting speed to 80–120 m/min to reduce BUE | Edge chipping at interrupted cuts | Reduce feed by 50% at cross-hole intersections |
| Alloy steel (4140, 4340) — hardened (35–48 HRC) | Tool wear rapid — VB > 0.2 mm in < 10 m | Hardness > 35 HRC accelerates flank wear; carbide tool is marginal | Reduce cutting speed by 20–30%; use AlCrN-coated carbide; consider PCD tooling for production | Chatter at high feed | Reduce feed to 0.04–0.08 mm/rev; check stability lobe diagram |
| Stainless steel (304, 316, 316L) — austenitic | Chip packing + work hardening — chips weld to tool and pack in the flute | High work hardening rate; low thermal conductivity (15 W/m·K); chip adhesion to tool | Increase coolant pressure to > 60 bar; use chip breaker with polished rake face (Ra < 0.1 µm); MAM or CNC oscillating feed recommended | Built-up edge | Increase cutting speed to 60–80 m/min; use AlCrN-coated tool; verify coolant EP at 2.0% S |
| Stainless steel (17-4 PH) — H900/H1025 | Bore deviation — drill wanders in the bore | Hardness 35–45 HRC combined with high modulus (200 GPa) creates asymmetric cutting forces | Increase bushing support length to 2.5× drill diameter; verify drill centring at entry with pilot drill | Tool wear (notch wear at outer corner) | Reduce feed by 15%; use TiAlN-coated carbide |
| Titanium (Ti-6Al-4V) — annealed | Rapid tool wear — flank wear + notch wear at outer corner | Low thermal conductivity (7 W/m·K) causes high cutting temperature; chemical reactivity with tool | Use PCD or diamond-coated tool; cutting speed < 45 m/min; high coolant pressure 60–80 bar; cryogenic LCO₂ for production | Chip packing (gummy chips) | Short peck cycles (2–3× diameter); MAM at 5–10 Hz; do not stop feed advance (dwell causes work hardening) |
| Titanium (Ti-6Al-4V) — aged (> 35 HRC) | Tool breakage — catastrophic edge failure | High hardness + low thermal conductivity = thermal overload of cutting edge | PCD tooling mandatory; Vc < 25 m/min; cryogenic LCO₂ coolant essential; reduce feed to 0.02–0.035 mm/rev | Surface microcracking | Verify surface integrity by SEM; reduce Vc to prevent thermal damage |
| Inconel 718 — annealed | Very rapid tool wear — VB > 0.2 mm in < 5 m | High work hardening + abrasive carbides + low thermal conductivity | PCD or PCBN tool mandatory; Vc 15–25 m/min; high coolant 70–100 bar; short peck 2–3 mm; cryogenic or hybrid MQL-cryogenic | Surface white layer | Reduce Vc to < 20 m/min; verify surface by metallography (Nital etch — white layer > 2 µm = reject) |
| Aluminium (6061, 7075) | Burr at bore exit | High ductility; built-up edge on tool | Increase cutting speed to 200–300 m/min; use PCD tooling; reduce feed by 50% at breakthrough | Chip packing (long chips at low feed) | Increase feed to > 0.06 mm/rev; use chip breaker geometry specific to Al |
| Copper (C110, C102) | Chip packing — extreme ductility causes continuous ribbon chips | Elongation 40–50%; high thermal conductivity keeps chip hot and ductile | MAM at 15–25 Hz + 0.08–0.15 mm amplitude essential; use PCD tooling; increase coolant pressure to 60–80 bar | Burr at entry and exit | Use chamfered entry bushing; reduce feed by 50% at breakthrough; backup plate mandatory |
| Cast iron (grey, ductile) | Fine abrasive dust in coolant accelerates bushing wear | Graphite particles + recirculating metal fines | Improve coolant filtration to < 10 µm; use carbide bushings (wear life 10 000+ bores) | Tool wear (micro-chipping from graphite) | Use uncoated K10 carbide; cutting speed > 80 m/min |
FAQ
What is the most common root cause of drill breakage in deep hole drilling, and how can it be prevented?
The most common root cause of drill breakage in deep hole drilling is chip packing — the accumulation of chips in the drill flute or BTA drill tube that blocks the chip evacuation path, causing a sudden increase in torque (3–5× normal) that exceeds the torsional strength of the drill. Chip packing accounts for approximately 60–70% of all drill breakages in gun drilling and 40–50% in BTA drilling (BTA is more tolerant of chip packing because the chip path is through the centre of the drill tube, which has a larger cross-section than the gun drill flute). The chip packing event typically progresses as follows: a chip that is longer than the flute or tube segment cannot pass through the chip evacuation path; it lodges at the flute entrance; the next chip impacts the lodged chip, compressing it; the compressed chip mass blocks the coolant flow; the coolant pressure drop at the cutting edge reduces chip flushing, causing more chips to accumulate; the torque rises to 3–5× normal; and the drill breaks at the cross-section of maximum torsional stress (typically at the flute-crossover point in gun drills or at the head-tube joint in BTA). The entire sequence from initial chip lodging to breakage takes 0.5–3 seconds — too fast for manual intervention but detectable by a spindle power monitoring system with a response time of < 0.1 seconds.
The prevention of chip packing requires a multi-level approach. Level 1 (design): select the correct chip breaker geometry for the material and feed range — the chip breaker groove width should be 1.0–1.5× the feed per revolution for most materials. If the chip breaker groove is too wide for the feed, the chip does not contact the groove at all and forms a continuous ribbon. Level 2 (process): maintain a minimum coolant annular velocity of 8 m/s for gun drilling and 10 m/s for BTA, calculated as V_ann = Q / (60 × A_ann) where Q is the coolant flow rate in L/min and A_ann is the annular area in mm² (for gun drilling: the area between the drill OD and the bore ID; for BTA: the area between the drill tube OD and the bore ID, minus the area of the coolant tube). The coolant flow should be measured by a flow meter, not inferred from pump pressure, because a clogged filter reduces flow by 50–70% while the pump pressure gauge still reads the pump's rated pressure. Level 3 (monitoring): implement spindle power or torque monitoring with an automatic feed hold or spindle stop when the torque exceeds 150% of baseline. The monitoring threshold should be set based on the stable torque range measured over 10 bores at the operating parameters. Level 4 (chip inspection): inspect the chip form from every production batch — chips should be consistent in length (±20% of the target length, typically 3–8 mm for gun drilling), consistent in curl radius, and free-flowing through the coolant filter. A change in chip form (longer chips, irregular shape, or powder chips) is the earliest warning of chip packing risk and should trigger a tool inspection before the next bore.
How do I diagnose the cause of poor surface finish in a deep hole drilled bore?
Poor surface finish in deep hole drilling — characterised by Ra above specification, visible tool marks, scoring lines, or roughness — has six possible root causes that can be diagnosed by examining the surface finish pattern and the chip form. Diagnostic step 1: examine the surface finish pattern. If the surface has regular, evenly spaced marks at intervals equal to the feed per revolution (visible at 10–20×), the cause is excessive feed rate — the tool marks are the feed marks of the cutting edge. Reduce feed by 20–30% and measure Ra again. If the surface has irregular, randomly spaced deep scratches or scoring lines, the cause is chip recirculation — chips that have not been evacuated are recirculating in the coolant flow and scraping against the bore wall. Check the coolant filter for chips, measure coolant flow at the drill head, and calculate the annular velocity (minimum 8 m/s for gun drilling). If the surface has a matte, torn, or smeared appearance with no distinct pattern, the cause is built-up edge (BUE) — material from the workpiece has adhered to the cutting edge, altering the effective geometry and causing ploughing rather than shearing. Inspect the cutting edge under a microscope at 10–20× for adhered material. BUE is treated by increasing cutting speed by 10–20% (to increase the edge temperature above the BUE stabilisation temperature) and verifying that the coolant EP additive concentration is adequate (sulphur content 1.5–2.0% for steels).
Diagnostic step 2: examine the cutting edge condition. If the edge has a visible wear band (VB > 0.15 mm), the flank wear has increased the edge radius, causing ploughing rather than shearing. Replace or regrind the tool. If the edge has small chips or notches (0.05–0.2 mm), the edge chipping has created localised cutting geometry changes that produce corresponding grooves in the bore surface. Reduce feed, increase edge hone radius, or change to a tougher tool material. Diagnostic step 3: measure the coolant EP additive concentration (refractometer reading for water-miscible; additive test kit or oil analysis for oil-based). Low EP concentration (< 1.0% S for steel drilling) causes the guide pads to lose lubrication, creating scoring marks from pad contact. Diagnostic step 4: inspect the machine spindle runout — a runout > 0.005 mm at the drill bushing exit creates an orbital motion of the drill tip that produces oversize and a rough, wavy surface. Measure runout with a dial indicator (±0.001 mm resolution) on the drill shank at the bushing exit with the spindle rotating at operating speed. Diagnostic step 5: listen for chatter — an audible squealing or roaring sound indicates chatter vibration, which produces a distinctive wavy surface with a wavelength determined by the chatter frequency and cutting speed. The most common fix for chatter is changing the spindle speed by ±10–20% to move the tooth pass frequency away from the structural natural frequency. All six diagnostic steps can be performed in 5–10 minutes by an experienced operator and will identify the root cause of 95% of surface finish problems.
What is the correct response when a drill breaks in the bore, and how is the broken tool removed?
When a drill breaks in the bore, the immediate priority is to prevent further damage to the component and the machine tool. Step 1 (immediate stop): the machine should be stopped as soon as the breakage is detected (spindle power spike > 200% of baseline triggers automatic stop). Do not attempt to retract the machine spindle — the broken drill may be wedged in the bore, and retraction force can damage the spindle bearings or the machine feed system. Step 2 (assess the break location): determine how much of the drill is in the bore and where the break occurred. If the break is within 50 mm of the bore entry and the broken piece is visible from the entry, mechanical extraction with a specially designed extractor tool (a carbide-tipped, reverse-taper tool that threads into the coolant hole of the broken gun drill) is possible. If the break is deep in the bore (more than 100 mm from entry), the extraction is more complex and depends on the access from both ends of the bore (if the bore is through-hole, access from the exit side may be possible). Step 3 (select extraction method): the most common extraction methods for broken gun drills are: EDM drilling — a small-hole EDM (EDM drill, Ø1–3 mm electrode) is used to drill through the centre of the broken drill, and the remaining shell collapses inward and is removed by flushing. This is the preferred method because it does not damage the bore surface. The EDM time is 10–60 minutes depending on the drill diameter and depth. Chemical dissolution (for HSS drills only) — nitric acid (30–40% concentration) dissolves HSS without attacking the workpiece steel. The acid is introduced through the coolant hole or through the annular space. This takes 4–24 hours depending on the drill cross-section. Mechanical extraction (for shallow breaks) — a threaded extractor is inserted into the coolant hole of the broken drill and tightened until the drill loosens. This works only if the break is clean and the drill is not wedged.
If the broken drill cannot be removed without damaging the bore surface, the component must be scrapped or the bore must be repaired by machining a larger-diameter bore and pressing in a bush (for components where the bore diameter can be increased). The cost of a broken drill extraction (machine downtime 2–8 hours, extraction tooling cost $200–2000, potential component scrapping cost $500–5000) is the economic driver for preventing breakages through chip packing prevention, tool life management, and process monitoring. For high-value components (aerospace, nuclear, medical), the recommended practice is to implement real-time spindle power monitoring with automatic feed hold (not stop — the drill continues rotating at low speed to prevent chip welding during the pause) when torque exceeds 150% of baseline. This pause gives the operator time to retract the drill while it is still free, before it packs and breaks. The spindle power monitoring system should be tested weekly by running a blocked-chip simulation (inserting a piece of adhesive tape into the flute at the entry — the system must detect the resulting torque increase within 0.5 seconds and activate the feed hold). A working spindle power monitoring system typically prevents 80–90% of drill breakages.
How do I determine the correct feed rate for a new material without published reference data?
When drilling a new material without published reference data, the feed rate can be determined by a systematic five-step method that establishes a process window in 10–20 bores without risking tool breakage. Step 1 (estimate the machinability rating): compare the new material's hardness and elongation to a known reference. For steels, the relative machinability is approximately proportional to (100 − HRC)/100 for the speed, and to (100 − 15 × elongation) for the feed. For a new steel at 25 HRC with 25% elongation: speed factor = (100 − 25)/100 = 0.75 (75% of the speed for AISI 1112, which is the 100% standard); feed factor = (100 − 15 × 0.25)/100 = 0.96 (96% of the feed for 1112). For non-ferrous materials, compare the new material's thermal conductivity and work-hardening rate to a known reference (aluminium 6061 for low-hardening, soft materials; titanium 6Al-4V for high-hardening, low-conductivity materials). Material-specific heuristic: if the material has thermal conductivity below 20 W/m·K and elongation above 20% (stainless steel, titanium, Inconel), start with the lowest feed in the tool manufacturer's range and increase cautiously.
Step 2 (set the starting parameters): for gun drilling, select a starting feed of 0.015–0.025 mm/rev for difficult materials (low thermal conductivity, high work hardening, high ductility) and 0.04–0.08 mm/rev for easy materials (high thermal conductivity, low ductility). Set cutting speed based on the tool material: for carbide: Vc = 30 m/min for hard/high-temp materials (nickel, titanium), 60–100 m/min for steels, 150–250 m/min for aluminium. Step 3 (drill one bore at the starting feed): after the first bore, inspect the chip form — the chip should be short (3–10 mm), consistent in length, and free-flowing. If the chip is a continuous ribbon (longer than 50 mm), increase feed by 50% and drill another bore. If the chip is fine powder or dust, decrease feed by 30%. Step 4 (inspect the bore surface): after acceptable chip form is achieved, measure the bore surface finish Ra. If Ra > 1.5 µm and the chip form is good, the surface finish is limited by the feed rate — increase cutting speed by 20% (to reduce BUE and improve surface finish). If Ra > 1.5 µm and the chip form is marginal (irregular length, some long segments), reduce feed by 20% to improve chip breaking and surface finish. Step 5 (tool life verification): drill 10 bores at the candidate parameters and measure the flank wear VB on the tool. If VB > 0.2 mm after 10 bores, reduce cutting speed by 20% and repeat. The objective is to find a feed rate that produces consistent short chips (the feed is the primary chip-control parameter) and a cutting speed that keeps VB < 0.2 mm for at least 10 bores (the speed is the primary tool-life parameter). The feed does not need to be reduced further once chip control is achieved — any reduction in feed below the chip-breaking threshold will actually increase the risk of chip packing, even if it reduces tool wear slightly. This five-step process typically requires 5–15 bores to establish a stable process window and is applicable to any new material, regardless of the availability of published reference data.
The information provided in this article is for general informational purposes only and does not constitute professional engineering advice. Always consult qualified manufacturing engineers, tooling suppliers, and equipment manufacturers for specific deep hole drilling troubleshooting applications. Data and recommendations are based on published research and industry experience as of 2026.