Deep Hole Drilling Strategies

Compiled 2026-04-04 · 50 chunks, 15 posts · drilling · deep-holes · gun-drilling · carbide-drills · peck-drilling · chip-evacuation

Summary

Deep hole drilling is defined by depth-to-diameter ratios, requiring specialized techniques and tooling to manage chip evacuation, maintain hole straightness, and prevent drill breakage. Success depends on matching the right drilling method to the depth ratio: standard twist drills work to 3-5xD, solid carbide through-coolant drills handle 8-20xD, and gun drills tackle extreme ratios beyond 25xD. The critical factors are aggressive chip evacuation, proper speeds and feeds, and choosing between pecking cycles versus continuous drilling based on tool geometry and coolant delivery.

Drilling Methods by Depth Ratio

Standard Twist Drills (3-5xD)

  • HSS/Cobalt drills: 50-100 SFM in steel, 0.001-0.003" per rev feed
  • Carbide drills: 150-300 SFM, 0.002-0.005" per rev
  • Peck depth: Start at 3xD for first plunge, reduce by 1xD each cycle to minimum 1xD
  • Retract: Full retract for chip clearing, rapid return

Solid Carbide Through-Coolant (8-20xD)

  • Steel: 80-130 SFM, 0.0008-0.002" per rev
  • [[aluminum-6061]]: 250-400 SFM, 0.003-0.005" per rev
  • [[304-stainless]]: 75 SFM, 0.0008" per rev, 0.2" peck with 0.005" partial retract
  • Coolant pressure: 900-1000 PSI minimum
  • Key advantage: Continuous drilling without pecking when chips break properly

Gun Drilling (25xD+)

  • Entry cycle: 60 RPM feed to engagement, then ramp to full parameters
  • Running parameters: 80-120 SFM, 0.0015-0.003" per rev
  • Exit cycle: Return to 60 RPM, coolant off, slow retract
  • Pilot hole required: Use 8xD carbide drill first, then gun drill to depth

Material-Specific Parameters

Steel ([[4140-steel]], 1018-1045)

  • Carbide drills: 100-130 SFM, 0.0015" per rev
  • Gun drills: 80 SFM, 0.0015" per rev
  • Chip formation: Target "6's and 9's" - small broken chips

Stainless Steel ([[304-stainless]])

  • Conservative approach: 75 SFM, 0.0008" per rev
  • Peck: 0.2" with 0.005" partial retract to break chips
  • Critical: Avoid [[work-hardening]] with consistent feed rates
  • Tool material: Carbide preferred over HSS for heat resistance

Aluminum ([[aluminum-6061]], [[7075-aluminum]])

  • High-speed approach: 250-400 SFM, 0.003-0.005" per rev
  • Chip evacuation: Critical due to welding tendency
  • Coolant: Flood recommended, avoid through-coolant in some alloys

Soft Tool Steels (H-13 pre-heat treat)

  • Parameters: 185 SFM, 0.0015" per rev starting point
  • Walter charts suggest: 80 SFM, higher feed per rev
  • Shop experience: Start conservative, increase based on chip formation

Tooling Selection and Setup

Through-Coolant Carbide Drills

  • Titex 8xD pilots: For hole starting and straightness
  • Titex 25xD: For deep drilling to full depth
  • Uncoated vs. Coated: Uncoated often preferred for better chip flow
  • Shank selection: Minimize runout with precision toolholders

Gun Drills

  • Single-lip design: Self-guiding, requires coolant for chip evacuation
  • Coolant delivery: Through drill body, 900+ PSI pressure
  • Workholding: Rigid setup essential, contra-rotation ideal

Parabolic Flute Drills

  • Geometry advantage: Improved chip evacuation over standard twist
  • Application: Bridge between standard and gun drilling
  • Depth capability: 8-12xD typically

Chip Management Strategies

Peck Drilling Guidelines

  • Initial peck: 3xD or 0.100" minimum for small holes
  • Progressive reduction: Decrease peck by 1xD each cycle
  • Minimum peck: 1xD or 0.050", whichever is larger
  • Retract strategy: Full retract vs. partial (0.005-0.010") for chip breaking

Through-Coolant Advantages

  • Continuous drilling: No pecking required when working properly
  • Chip size indicator: Look for small, broken chips - not long strings
  • Pressure requirements: 900-1000 PSI for effective evacuation
  • Flow rate: Match to hole diameter and depth

Manual Techniques

  • Bridgeport drilling: Cobalt drills often better than carbide for feel
  • Chip monitoring: Stop frequently to check chip formation
  • Feed pressure: Consistent hand pressure prevents work hardening

Common Problems and Solutions

Drill Breakage Mid-Hole

  • Cause: Chip packing, insufficient coolant, too aggressive feed
  • Solution: Reduce feed 50%, increase peck frequency, verify coolant pressure
  • Prevention: Test chip formation on shallow test holes first

Hole Wandering

  • Cause: Inadequate pilot hole, drill deflection, uneven entry
  • Solution: Use shorter, rigid pilot drill; spot with proper included angle (140°+)
  • Gun drill advantage: Self-guiding geometry maintains straightness

Poor Chip Evacuation

  • HSS/Standard drills: Increase peck frequency, ensure flood coolant
  • Carbide through-coolant: Verify 900+ PSI pressure, check for clogs
  • Feed rate: Often need MORE feed to break chips properly

Tool Wear Patterns

  • Corner wear: Reduce speed 20-30%
  • Flank wear: Increase feed, ensure adequate coolant
  • Chipping: Reduce feed, check for interrupted cuts or hard spots

Shop Floor Tips

Setup Secrets

  • Spot drill angle: Use 140° or drill point angle + 10° - never 90°
  • Pilot hole depth: Go 0.500" past pilot drill engagement to verify chip formation
  • Coolant test: Run shallow test hole to verify chip size before committing to full depth

Programming Tricks

  • Gun drill cycles: Custom macros for spindle speed ramping and coolant sequencing
  • Partial retract: 0.005-0.010" moves to break chips without full cycle time
  • Speed ramping: Start at 60 RPM, ramp to full speed over 0.100" engagement

Material-Specific Hacks

  • Stainless: Use cobalt drills on manual machines for better control
  • Aluminum: Skip through-coolant on some alloys - chips weld to coolant holes
  • Tool steel: Pre-drill undersize, ream to final - straightens intersecting holes

Economics

  • Gun drilling: Slow but extremely consistent - 5 minute cycle times normal
  • Carbide through-coolant: Faster but requires more skill and setup
  • Cost analysis: Factor tool cost, cycle time, and success rate together
  • [[drilling]] — Basic drilling fundamentals and standard practices
  • [[carbide-drills]] — Tool selection and grades for deep hole applications
  • [[coolant-management]] — Coolant pressure, flow, and concentration requirements
  • [[chip-control]] — Understanding chip formation and evacuation strategies
  • [[workholding]] — Rigid setups essential for deep hole accuracy
  • [[tool-wear-diagnosis]] — Identifying and correcting wear patterns in deep drilling
  • [[boring]] — Alternative approach for large diameter deep holes
  • [[speeds-feeds-fundamentals]] — Base knowledge for parameter selection