Through-Hole vs Blind-Hole Deep Hole Drilling

The distinction between through-holes (drill exits the workpiece) and blind-holes (drill stops inside the workpiece) fundamentally affects deep hole drilling method selection, chip evacuation strategy, tool design, and parameter selection. What works well for a through-hole may fail completely for a blind-hole — and vice versa.

This guide covers the differences and how to adjust your approach for each hole type.

Fundamental Differences

FactorThrough-HoleBlind-Hole
Chip exitOut the far side of the workpieceMust return past the cutting zone
Coolant flowFull flow-through (flushes chips)Must overcome back-pressure
Tool entry at exitDrill breaks through far surfaceDrill stops before far end
Chip accumulationNone — chips exit continuouslyChips accumulate at bottom
Depth accuracyLess critical (exits anyway)Critical — must stop at exact depth
Burr at exitExists — may need deburringNone (unless bottom surface matters)

Method Suitability

MethodThrough-HoleBlind-HoleBest For
Gun drillingExcellentGoodThrough: external chip evacuation works well; Blind: V-flute evacuation remains effective even without through-flow
BTA drillingExcellentPoorThrough: internal chip evacuation excels with flow-through; Blind: chips must fight coolant pressure to enter tube
Ejector (DTS)GoodExcellentThrough: Venturi works well; Blind: Venturi suction actively pulls chips, making it the best blind-hole method
Hole TypePrimary RecommendationSecondaryWhy
Through, < 50 mm diaGun drillingBTASimple setup, single-pass precision
Through, > 25 mm diaBTAGun drillingBTA’s internal chip evacuation excels with flow-through
Blind, < 50 mm diaGun drillingEjectorV-flute evacuation independent of flow direction
Blind, 18–200 mm diaEjector (DTS)—Venturi suction is the most reliable blind-hole chip evacuation

Chip Evacuation Differences

Through-Hole Chip Evacuation

In a through-hole, coolant and chips flow through the bore and out the far end:

Gun drilling:
  Coolant → through tool → out V-flute → exits far side ✅

BTA drilling:
  Coolant → through annulus → chips through tube center → exits far side ✅
  (Coolant flows past the drill head and out the open end)

Ejector drilling:
  Coolant → through outer tube → Venturi → inner tube → exits far side ✅

Key advantage: Gravity and coolant flow direction both help chip evacuation. Chips and coolant naturally want to exit the hole.

Blind-Hole Chip Evacuation

In a blind-hole, chips and coolant must flow back past the cutting zone:

Gun drilling:
  Coolant → through tool → chips up V-flute → returns to entry ✅
  (V-flute remains effective; chips flow against incoming coolant)

BTA drilling:
  Coolant through annulus → chips tube center ⚠️
  (At hole bottom, coolant must turn 180° with chips — less efficient)

Ejector drilling:
  Coolant through outer tube → Venturi suction → inner tube ✅✅
  (Venturi actively pulls chips — most reliable for blind holes)

Tool Geometry Adjustments

Gun Drill Adjustments for Blind Holes

FeatureThrough-HoleBlind-HoleAdjustment
Nose grindStandardOften sharper to reduce thrust at bottomReduce thrust 10–15%
Guide pad clearanceStandardSlightly tighter — no exit to relieve pressureReduce 0.01–0.02 mm
V-flute widthStandardSame—
Entry angleStandardSame—

BTA Drill Adjustments for Blind Holes

BTA drilling of blind holes is not recommended. If unavoidable:

AdjustmentReasonRecommendation
Reduce feed 20–30%Chips struggle to enter tube against coolantIncrease chip breaking
Increase coolant pressure 15–20%Overcome back-pressure at bottomHelps chip entry into tube
Install chip breaker grooves in headForce chip breaking at bottomReduces chip size for evacuation
Reduce depth ratio to < 30:1Longer holes worsen chip accumulationPractical limit for blind BTA

Ejector Drill Adjustments for Blind Holes

Ejector drilling is naturally suited to blind holes. No major adjustments needed beyond:

AdjustmentReasonRecommendation
Monitor Venturi flowBlind holes test the suction systemVerify minimum flow per diameter
Maintain coolant temperatureBlind holes have less heat dissipationTarget 30–40°C
Reduce feed 10% at last 10 mmPrevent bottom impactProgrammed feed reduction

Parameter Differences

Coolant Pressure

Hole TypeGun DrillingBTAEjector
Through50–150 bar20–50 bar20–35 bar
Blind50–150 bar (same)25–60 bar (+20%)20–35 bar (same)

Feed Rate

Hole TypeGun DrillingBTAEjector
Through100%100%100%
Blind95–100%70–80%95–100% (reduce 10% at bottom)

Peck Strategy

Hole TypeGun DrillingBTAEjector
ThroughNo pecking neededNo pecking neededNo pecking needed
BlindOccasional clearing peck at > 50:1Peeking reduces chip accumulationNo pecking needed (Venturi clears)

Blind-Hole Specific Problems

ProblemCauseSolution
Chip packing at bottomBTA drilling blind holeSwitch to ejector or gun drilling
Hydraulic lock (coolant pressure prevents chip entry)BTA blind holeReduce pressure at bottom; ejector avoids this
Bottom surface damageDrill contacts bottomProgram controlled deceleration at last 5 mm
Chip re-cuttingChips not evacuating from bottomIncrease coolant flow; peck if necessary
Back-pressure buildupBlind hole coolant cannot exitEnsure adequate ID for return flow; verify Venturi function

Summary

The distinction between through-holes and blind-holes is fundamental to deep hole drilling method selection. BTA drilling excels at through-holes but is poor for blind-holes — chips struggle to enter the tube against coolant flow at the hole bottom. Ejector (DTS) drilling is the best method for blind-holes due to its Venturi suction, which actively pulls chips regardless of hole termination. Gun drilling works well for both types in small diameters. For detailed method selection, see how to choose the right deep hole drilling method. For the full methods overview, see deep hole drilling methods guide.