Diagnose by Symptom This quick-reference guide organizes deep hole drilling problems by observable symptom. Find the symptom that matches what you’re seeing, then follow the corrective actions.
Surface Finish Problems Rough Bore Surface (Ra above specification) Likely Cause Check Corrective Action Tool wear Chip color (blue/burnt), torque increase Replace or regrind tool Chatter Vibration at spindle, chatter marks on bore Adjust speed ±10% (avoid resonant frequency); check rigidity Coolant pressure too low Pressure gauge reading Increase pressure 10–20% Guide pad wear (gun drilling) Pad OD measurement Replace guide pads Wrong tool geometry Check tool spec against material Select appropriate tool geometry Feed too high Check programmed feed Reduce feed 10–15% Coolant contamination Filter condition, chip content in coolant Change filters; clean coolant system
Spiral Marks on Bore Wall Likely Cause Check Corrective Action Chatter Vibration reading, chip morphology Change speed resonant frequency Feed too high (gun drilling) Feed rate vs. recommended Reduce feed Guide pad condition Pad surface for galling Replace or regrind pads Coolant flow intermittent Pressure stability at gauge Check pump, filter, rotary union
Burnished / Glazed Bore Surface Likely Cause Check Corrective Action Guide pads rubbing excessively Pad wear pattern Check tool alignment; reduce guide pad width Coolant insufficient Flow rate, temperature Increase flow or pressure Speed too high Chip color (blue/burnt) Reduce speed 10–15% Tool misalignment Runout at tool holder Realign; check guide bushing condition
Dimensional Problems Oversized Hole (Diameter too large) Likely Cause Check Corrective Action Drill diameter incorrect Tool OD measurement Use correct diameter tool Tool runout excessive Runout at tool tip (TIR) Correct spindle/collet; TIR < 0.005 mm Feed too high Programmed feed vs. recommended Reduce feed Guide bushing worn (gun drilling) Bushing ID measurement Replace guide bushing Speed too high Chip characteristics Reduce speed Tool holder misalignment Alignment of machine spindle to workpiece Realign per machine specification
Undersized Hole (Diameter too small) Likely Cause Check Corrective Action Tool wear (diameter reduction) Tool OD; cumulative hole count Regrind or replace tool Material elastic recovery Material type (Ti, stainless, polymers) Use oversize drill 0.02–0.05 mm Coolant pressure too high Pressure gauge Reduce pressure (for thin-wall parts) Feed too low Programmed feed Increase feed 10–15% Thermal contraction after drilling Part temperature at measurement Allow part to stabilize at 20°C before measurement Built-up edge Chip morphology, tool inspection Adjust speed; increase coolant pressure
Tapered Hole (Entry larger than exit, or vice versa) Likely Cause Check Corrective Action Entry taper (entry larger): tool misalignmentRunout at start of cut Realign tool to workpiece centerline Exit taper (exit larger): tool deflectionVibration at depth, rigidity Reduce feed at depth; use whip guide for long drills Guide pads worn unevenly (gun drilling) Pad thickness variance Replace pads Coolant pressure variation with depth Pressure reading at start vs. full depth Check coolant delivery system Workpiece thermal growth Temperature during drilling Use coolant chiller; allow warm-up cycles
Straightness and Position Problems Poor Hole Straightness (Deviation from axis) Likely Cause Check Corrective Action Tool misalignment Machine spindle-to-workpiece alignment Realign; check tailstock center Guide bushing worn (gun drilling) Bushing clearance Replace bushing Coolant pressure variation Pressure stability during cut Check pump, filter, rotary union Material hard spots Material certification, hardness test Use annealed material; adjust parameters for material Feed too high Feed rate Reduce feed Insufficient pilot hole depth Pilot hole depth Extend pilot hole to 1.5–2× diameter Chip packing (deflects tool) Coolant pressure oscillation, chip flow Check chip evacuation; increase coolant
Hole Position Shift (Location out of tolerance) Likely Cause Check Corrective Action Pilot hole misaligned Pilot hole position Redrill pilot hole correctly Workpiece movement (clamping) Fixture rigidity Improve clamping force or fixture design Tool wander at entry Entry location Use starter bushing; improve pilot hole quality Material residual stress Material history (heat treat, forming) Stress relieve before drilling
Tool Breaks Immediately (at entry) Likely Cause Check Corrective Action Feed too high at entry Feed rate at start Reduce starting feed to 1/3 for first 2–3 mm Pilot hole missing or too small Pilot hole diameter and depth Correct pilot hole Tool hitting workpiece off-center Alignment Realign; use guide bushing Coolant not flowing before cut Coolant interlock Verify coolant-on before feed-start sequence Workpiece surface irregular Entry face condition Face flat before drilling; spot face if needed
Likely Cause Check Corrective Action Chip packing Coolant pressure oscillation Increase coolant pressure; reduce feed; add peck cycle Coolant blockage Coolant pressure loss at tool tip Check rotary union, coolant channel in drill Material hard spot or void Material certification Material inspection before drilling Excessive tool wear Cumulative hole count Replace tool at recommended interval Chatter (fatigue failure) Vibration, chatter marks Change speed to avoid resonant frequency Feed too high at depth Feed rate vs. L/D derating Apply depth-dependent derating
Likely Cause Check Corrective Action Exit breakthrough too aggressive Feed rate at end of cut Reduce feed by 50% for last 2–3 mm Thin wall breakout Wall thickness at exit Support exit face; reduce parameters Inconsistent material at exit Material condition Check for through-hardening issues
Coolant System Problems Symptom Likely Cause Corrective Action Coolant pressure too low Pump issue, filter clogged, rotary union leak, relief valve set too low Check pump; replace filter; service rotary union; adjust relief valve Coolant pressure too high Flow restriction, drill coolant hole clogged Back-flush drill; check coolant hose for kinks Pressure fluctuating > ±5% Worn pump, air in system, chip packing Bleed air; check for cavitation; retract drill to clear chips Coolant temperature rising Chiller fault, coolant volume low, excessive cutting heat Service chiller; top off coolant; reduce speed Foaming coolant Wrong concentration, soft water, detergent contamination Check concentration; use anti-foam additive Dirty coolant (particles visible) Filtration bypass, filter worn Change filters; consider upgrading filtration
Spindle Load / Torque Problems Symptom Likely Cause Corrective Action Torque increasing gradually Normal tool wear Monitor; schedule tool change at threshold Torque increasing rapidly Built-up edge, workpiece hard spot, coolant starvation Inspect tool; check coolant; adjust speed Torque fluctuating Chip packing, varying material condition Check chip evacuation; verify material batch Torque suddenly drops Tool breakage Immediate stop; withdraw tool Torque high at entry Feed too high at start, pilot hole undersize Reduce start feed; check pilot hole Torque high at exit Exit breakout, wall thickness change Reduce feed at exit
Sound and Vibration Symptom Likely Cause Corrective Action High-pitched squeal Speed too high, inadequate coolant Reduce speed; increase coolant Rattling / knocking Tool hitting bore wall, loose fixturing Check alignment; tighten fixture; inspect guide bushings Low-frequency rumble Chatter onset Adjust speed ±10% Intermittent grinding Chip packing breaking loose Check chip evacuation; increase coolant Sudden loud crack Tool breakage Stop immediately; retract
Quick-Action Checklist When starting a troubleshooting session:
Stop cutting and retract the tool if still in the holeInspect the tool — compare wear pattern to known normsExamine the chips — refer to the chip morphology guideCheck coolant — pressure, temperature, cleanlinessReview parameters — compare speed, feed, and coolant to recommendationsCheck alignment — spindle-to-workpiece runoutVerify material — grade, hardness, heat treatment conditionCheck historical data — compare current readings to first-article baselineFor detailed troubleshooting by deep hole drilling method, see the gun drilling problems and solutions guide and the BTA troubleshooting guide . For chip analysis in detail, refer to the chip morphology reference guide .
DH
Deep Hole Drilling Technical Team
Practical engineering knowledge for deep hole drilling — gun drilling, BTA drilling, ejector systems, troubleshooting, and process optimization.
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Last updated July 4, 2026