Deep Hole Drilling Chip Morphology Reference Guide

Chip shape is the most immediate and accessible indicator of process health in deep hole drilling. An experienced operator can read chip morphology to diagnose tool wear, coolant problems, parameter mismatch, and material issues — often before the hole is even complete.

This guide provides a reference for chip types across deep hole drilling methods, what each chip shape indicates, and corrective actions.

The Role of Chip Formation

Why Chip Morphology Matters

Aspect	Importance in Deep Hole Drilling
Chip evacuation	Chips must travel 50–300× drill diameter through a narrow flute — shape determines whether they clear or pack
Heat removal	Chips carry 25–50% of cutting heat — efficient chip formation = efficient heat removal
Surface finish	Chip shape directly reflects cutting edge condition at the tool-workpiece interface
Tool wear indication	Chip shape changes measurably before diameter or finish degrade
Fault prediction	Abnormal chips precede catastrophic failures by 5–50 holes

Chip Formation Physics in Deep Hole Drilling

The confined chip evacuation space in deep hole drilling imposes unique requirements:

Gun drilling chip path:
Cutting tip → coolant pushes chip up V-flute → flute exit → chip bin
              └── 10–300× diameter distance ──┘

BTA drilling chip path:
Cutting edges → center of drill tube → tube center → chip separator
                  └── 10–100× diameter distance ──┘

Ideal chip properties:

Short enough to clear the flute or tube without bridging
Curled or C-shaped to minimize contact with the bore wall
Consistent in shape — indicates stable cutting conditions
Free-flowing under coolant pressure

Chip Types by Method

Gun Drilling Chip Types

Chip Type	Image Description	Significance	Assessment
Fan-shaped / C-curled	Curled, ribbon-like, 5–15 mm long	Ideal for gun drilling — stable chip formation, proper chip breakage	✅ Excellent
Short C-chips	Tightly curled, 2–5 mm segments	Good — indicates proper feed rate; slightly conservative parameters	✅ Good
Long stringy ribbons	Continuous chip 50–300+ mm long	Feed too low — chip not breaking; risk of chip packing in flute	⚠️ Adjust
Needle / splinter chips	Thin, sharp, needle-like fragments	Speed too high or feed too low — edge chipping risk	⚠️ Adjust
Dust / powder	Fine metallic dust	Severe tool wear or wrong tool geometry	❌ Stop
Built-up edge fragments	Irregular lumps of welded material	Coolant pressure too low or speed too low for material	⚠️ Adjust
Blue / burnt chips	Heat-colored (straw to blue)	Speed too high, feed too low, or coolant insufficient	⚠️ Reduce speed
Segmented / serrated chips	Sawtooth edge along chip length	Chatter or incipient tool failure	❌ Inspect

BTA Drilling Chip Types

Chip Type	Image Description	Significance	Assessment
Compact C-chips	Curled, 3–10 mm, compact	Ideal for BTA — good evacuation through tube center	✅ Excellent
Figure-8 / spiral chips	Twisted, spiral shape	Good — typical for BTA at optimal parameters	✅ Good
Long helical chips	Continuous spiral, 20–50+ mm	Feed too low for BTA — risk of tube blockage	⚠️ Increase feed
Wedge / half-moon chips	Thick, wedge-shaped segments	Feed too high or insert geometry wrong for material	⚠️ Reduce feed
Discolored (blue/purple)	Heat-colored throughout	Speed too high or coolant flow insufficient	⚠️ Reduce speed
Broken insert fragments	Tool edge fragments in chip pile	Insert chipped or broken	❌ Stop and inspect
Excessively fine / dust	Powder-like	Insert severely worn or material is too hard	❌ Change tool

Ejector Drilling (DTS) Chip Types

Similar to BTA, but with additional constraints due to the venturi effect:

Chip Type	Significance
Short, broken chips (3–8 mm)	Ideal — venturi system moves these efficiently
Long strings (> 20 mm)	Feed too low — chips can bridge in the venturi section
Extremely fine / dust	Tool wear or wrong cutting edge geometry

Chip Diagnosis by Material

Steel (Alloy / Carbon)

Material	Ideal Chip	Problem Chip	Likely Cause
4140 / 4340	Short C-curled, 3–8 mm	Long ribbons	Feed too low
300M	Compact C-chips, 5–10 mm	Blue-discolored	Speed too high
20MnCr5 (case-hardened)	Fan-shaped, 5–12 mm	Segmented	Chatter
Stainless 304	Tight C-chips, 2–5 mm	Gummy, stringy	Feed too low + speed too low
Stainless 316	Curled segments, 3–6 mm	Built-up edge fragments	Coolant pressure insufficient

Superalloys

Material	Ideal Chip	Problem Chip	Likely Cause
Inconel 718	Segmented, 2–4 mm (normal — Inconel produces segmented chips)	Blue/purple, continuous	Severe thermal overload
Waspaloy	Segmented, 3–5 mm	Glazed, smeared	Cutting speed too high
Hastelloy X	Short, fan-shaped, 2–4 mm	Needle splinters	Edge chipping

Aluminum

Material	Ideal Chip	Problem Chip	Likely Cause
6061-T6	Short, curved, 3–8 mm	Long, stringy	Feed too low
7075-T73	Fan-shaped, 5–15 mm	Built-up edge on chip	Coolant pressure low
Cast aluminum	Fine, broken, 1–3 mm	Dust	Tool wear

Parameter-Chip Relationship

Adjusting Parameters Based on Chip Shape

Observed Chip	Parameter Diagnosis	Corrective Action
Long, stringy, continuous	Feed too low	Increase feed 10–20%
Blue / burnt	Speed too high or coolant insufficient	Reduce speed 10–15% or increase coolant pressure
Dust / powder	Tool worn	Replace or regrind tool
Segmented / serrated	Chatter or incipient failure	Adjust speed ±10% (change frequency), check rigidity
Needle / splinter	Speed too high	Reduce speed 10–15%
Built-up edge fragments	Speed too low or coolant insufficient	Increase speed 10–15%; check coolant pressure
Excessively short (powder-like)	Tool geometry wrong for material	Check tool specifications
Inconsistent shape (mix of good and bad)	Tool chipped on one edge	Inspect and replace

Feed Rate Effect on Chip Shape

Feed rate → Low: Long strings, high chip flow friction
         → Optimal: C-curled or fan-shaped chips
         → High: Thicker, harder-to-form chips, increased torque

Optimal chip formation generally occurs when:

Chip thickness per revolution = 0.02–0.06 mm (steel)
Chip thickness per revolution = 0.04–0.12 mm (aluminum)
Chip thickness per revolution = 0.01–0.03 mm (superalloys)

Chip Packing and Evacuation Problems

Root Causes of Chip Packing

Cause	Mechanism	Prevention
Chips too long	Ribbons bridge across the flute of a gun drill	Increase feed rate
Chips too thick or wedge-shaped	Overload the flute or tube cross-section	Reduce feed or change chip breaker geometry
Coolant pressure too low	Insufficient hydraulic force to push chips out	Increase coolant pressure 10–20%
Coolant flow interrupted	Blockage in coolant channel or rotary union	Inspect coolant path
Flute or tube clogged	Accumulated chips from previous cycle	Clean tool or tube before use
Wrong chip breaker design	Tool geometry does not produce chip breaking for specific material	Regrind with appropriate chip breaker

Signs of Chip Packing

Early Warning	Confirmation	Action
Coolant pressure oscillation > 10%	Check coolant return flow	Peck retract to clear chips
Spindle torque fluctuation	Listen for change in cutting sound	Retract and inspect
Reduced penetration rate	Compare cycle time to baseline	Stop and withdraw tool
Chip flow from flute/tube stops	Visual check at tool entry	Immediate retraction

Chip Packing by Method — Immediate Response

Method	Response Sequence
Gun drilling	1. Stop feed immediately. 2. Withdraw tool while maintaining rotation. 3. Clear flute manually or with coolant flush. 4. Inspect tool for damage before resuming.
BTA drilling	1. Stop feed. 2. Maintain rotation and coolant flow. 3. Withdraw tube slightly (50–100 mm). 4. Increase coolant flow to flush chips. 5. Resume at reduced parameters.
Ejector drilling	1. Stop feed. 2. Maintain coolant flow (venturi may clear chips automatically). 3. If pressure does not normalize, withdraw and inspect.

Chip Monitoring

Manual Monitoring (Every Cycle)

Check chip pile after each hole — color, shape, size distribution
Compare to baseline established at first-article approval
Log chip quality (Good / Fair / Poor) on the production record

Automated Monitoring

Method	Sensor	What It Detects
Coolant pressure trend	Pressure transducer	Chip packing (pressure oscillation)
Coolant temperature rise	Temperature probe	Excessive heat from chip friction
Chip presence sensor	Capacitive or inductive sensor at exit	No chip flow = blockage
Chip weight / volume	Scale or laser volume sensor	Irregular chip production rate

Summary

Chip morphology is the most accessible real-time indicator of deep hole drilling process health. Ideal chips — C-shaped or fan-shaped, 3–15 mm long, free-flowing — indicate stable cutting conditions with correct parameters. Deviations from the ideal chip form point to specific parameter, tool, or coolant problems: long ribbons (feed too low), blue discoloration (speed too high), dust (tool worn), or built-up edge fragments (coolant inadequate). Regular chip inspection, ideally every cycle, enables early intervention before chip packing or tool failure occurs.

For a quick-reference diagnostic approach by symptom, see the troubleshooting by symptom guide. For method-specific troubleshooting guides, refer to the gun drilling problem guide and the BTA troubleshooting guide.