Appearance
Chip evacuation is the single most common cause of process failure in deep hole drilling. If the chips are not coming out correctly, nothing else matters — the tool will fail within seconds to minutes.
Overview
Chip evacuation is the bottleneck operation in deep hole drilling. The tool can only remove material as fast as the coolant can flush the chips away. When chip breakage and evacuation are working correctly, the process runs smoothly for hours. When they break down, tool failure is rapid and often catastrophic.
Approximately 40% of premature tool failures in deep hole drilling are chip-related. The root cause is rarely a single factor — it is typically an interaction between chip formation (controlled by feed and geometry) and chip transport (controlled by coolant and evacuation channel design).
Ideal Chip Shapes
The target chip shape for reliable evacuation depends on the drilling method:
Gun Drilling
- Target: C-shaped or 6-shaped chips, 2–5 mm in length
- C-shaped — well-broken, compact, easily flushed through the V-flute
- 6-shaped / spiral — short, tight spirals that roll compactly
- Needle chips — acceptable in small quantities, but indicate edge wear if persistent
BTA Drilling
- Target: Small, broken segments, 3–10 mm
- C-shaped — ideal for internal chip evacuation through the tube center
- Granular — very small segments, acceptable from cast iron and brittle materials
Problematic Chip Shapes
| Chip Shape | Problem | Likely Cause |
|---|---|---|
| Long continuous spirals | Tangle in flute, block evacuation | Feed too low |
| Loose string / ribbon | Cannot be flushed; wrap around tool | Feed too low, material too ductile |
| Thick, heavy arc | May not fit through evacuation channel | Feed too high |
| Powder / dust | Indicates edge wear or excessive rubbing | Worn tool, insufficient relief |
| Needle / fine | Pack in filter system, indicate chipping | Edge chipping, vibration |
Diagnostic Process
When chip evacuation problems occur, follow this systematic approach:
- Stop immediately — continuing with blocked evacuation will break the tool
- Retract the tool and inspect chip formation at the cutting edge
- Examine chip shape — compare against the ideal shapes above
- Check coolant pressure — compare actual vs. expected at the cutting zone
- Inspect the cutting edge — look for chipping, wear, or built-up edge
- Verify the pilot hole — correct diameter, alignment, and entry condition
Feed and Speed Adjustments
Feed rate is the primary control parameter for chip breakage.
Feed Too Low
Insufficient feed produces thin, stringy chips that cannot break under their own weight. This is the most common chip evacuation problem.
Symptoms:
- Long, continuous spiral chips
- Chips accumulating at the tool entry point
- Rising coolant back pressure
- Chips appearing as tangled masses
Fix: Increase feed in 10–15% increments while monitoring chip shape. Target feed should produce chips that break every 2–5 mm of travel.
Feed Too High
Excessive feed produces thick, heavy chips that may not fit through the evacuation passage.
Symptoms:
- Thick, heavy arc segments
- Spindle load at or above maximum
- Coolant pressure spikes (chips momentarily blocking passage)
- Rough surface finish
Fix: Reduce feed in 10% increments until chips are consistently well-broken and coolant pressure is stable.
Speed Adjustments
Cutting speed plays a secondary role in chip breakage but affects chip shape through temperature:
- Higher speed — increases cutting temperature, making chips more brittle and easier to break. Chip color shifts from silver to blue as temperature rises.
- Lower speed — produces longer chips with less tendency to break. Useful for managing chip formation in certain materials.
Speed vs. feed for chip control
Always adjust feed first for chip breakage, then use speed to fine-tune chip shape and control tool wear. Speed changes alone rarely fix chip evacuation problems.
Chip Breaker Geometry
When feed adjustments alone cannot achieve adequate chip breakage, chip breaker geometry modifications are needed.
Types of Chip Breakers
| Type | Description | Best For |
|---|---|---|
| Groove-type | Ground-in depression on rake face | Gun drills, consistent chip formation |
| Step-type | Stepped geometry at cutting edge | BTA inserts, multi-edge heads |
| Notch / splitter | Narrow notches along cutting edge | Breaking wide chips into narrow strands |
| Curved rake face | Modified rake surface geometry | Difficult materials, stainless steel |
Chip Breaker Design Parameters
- Width — narrower chip breakers increase chip strain and improve breakage, but reduce edge strength
- Depth — deeper breakers produce tighter chip curl and more consistent breakage
- Distance from edge — closer to the cutting edge engages the chip sooner; 0.5–1.5 mm is typical
Material-Specific Breaker Adjustments
| Material | Breaker Strategy |
|---|---|
| Low-carbon steel | Wider, deeper breaker; aggressive breakage needed |
| Stainless steel | Sharp breaker edge; shallow to moderate depth |
| Aluminum | Wide, shallow breaker to prevent chip packing |
| Titanium | Moderate width, close to cutting edge |
| Cast iron | Minimal breaker — chips are naturally brittle |
Coolant-Related Evacuation Issues
Coolant problems are the second most common cause of chip evacuation failure.
Low Coolant Pressure
Insufficient pressure fails to overcome the hydraulic resistance of the evacuation path. Chips settle in the flute or tube instead of being flushed out.
Diagnosis: Pressure gauge reading below the minimum for the diameter and depth. Pressure drops as chips accumulate.
Fixes:
- Increase pump pressure setting
- Check for leaks in the coolant delivery system
- Verify coolant lines are not restricted or blocked
- Inspect the coolant orifice in the tool for blockage
Low Coolant Flow
Even with adequate pressure, insufficient flow means low coolant velocity at the cutting zone. Chips are not entrained in the flow.
Diagnosis: Flow meter reading below specification. Chips appear at the exit intermittently rather than continuously.
Fixes:
- Verify pump is delivering rated flow
- Check filter condition — blocked filters reduce flow
- Inspect coolant lines for kinks or undersized fittings
- Consider larger-diameter coolant lines
Coolant Quality Issues
| Issue | Effect on Evacuation | Fix |
|---|---|---|
| Low concentration | Reduced lubricity, chips stick to flute | Adjust to 5–10% emulsion |
| Contaminated coolant | Chip packing in filter, reduced flow | Replace coolant, clean system |
| High temperature | Reduced viscosity, chips settle out | Service chiller, increase cooling |
| Wrong coolant type | Insufficient chip transport | Switch to deep-hole-drilling-grade coolant |
Material-Specific Strategies
Low-Carbon and Mild Steel
- Problem: Ductile — produces long, stringy chips
- Feed: Aggressive feed (upper third of recommended range)
- Geometry: Sharper chip breaker, larger outer angle
- Coolant: 50–80 bar, high flow
Stainless Steel
- Problem: Gummy, work-hardens, produces tough continuous chips
- Feed: Moderate feed (lower third of range)
- Geometry: Deep chip breaker, sharp cutting edge, polish rake face
- Coolant: 70–120 bar, oil-based coolant preferred
- Note: Built-up edge on the cutting edge rapidly degrades chip formation; change tool at first sign of edge breakdown
Aluminum
- Problem: Pure aluminum smears and packs in the flute
- Feed: Low to moderate (0.02–0.05 mm/rev for pure Al)
- Geometry: N4 nose grind (15° outer angle), polished flute surface
- Coolant: 30–50 bar, high flow; oil-based or high-lubricity emulsion
- Note: Alloyed aluminum (6061, 7075) machines much more easily than pure aluminum
Titanium Alloys
- Problem: Low thermal conductivity concentrates heat; chips are tough and can weld to the cutting edge
- Feed: Moderate, consistent feed (never let feed drop to zero)
- Geometry: Sharp edge, adequate chip breaker, large coolant orifice
- Coolant: 70+ bar, high flow; oil-based preferred
Cast Iron
- Problem: Minimal — naturally broken chips
- Feed: Standard
- Geometry: Standard
- Coolant: 30–50 bar — primarily for cooling and dust control
Troubleshooting Reference Table
| Symptom | Most Likely Cause | First Action |
|---|---|---|
| Long spiral chips | Feed too low | Increase feed 15% |
| Thick chips, pressure spikes | Feed too high | Decrease feed 10% |
| Chips not exiting hole | Coolant pressure too low | Increase pressure, check for leaks |
| Fine needle chips | Cutting edge chipped | Inspect and replace tool |
| Chip packing at tool entry | Insufficient pilot hole depth | Increase pilot depth |
| Intermittent chip flow | Filter blockage or pump cavitation | Check and clean filters |
| Chip discoloration (blue) | Speed too high for material | Reduce speed |
| Chips welded to cutting edge | Insufficient coolant at cutting zone | Check coolant orifice alignment |
| Built-up edge on aluminum | Wrong nose grind, feed too low | Switch to N4 grind, adjust feed |
| Vibration marks on chips | Machine or workholding instability | Check rigidity, reduce parameters |
Summary
| Tool | Ideal Chip Shape | Primary Control | Coolant Priority |
|---|---|---|---|
| Gun drilling | C-shaped / 6-shaped, 2–5 mm | Feed rate | Pressure (high) |
| BTA drilling | Broken segments, 3–10 mm | Feed + chip breaker geometry | Flow (high volume) |
| Both | Consistent, compact, non-tangling | Start with feed, then geometry | Both pressure and flow |
FAQ
What is the most common cause of chip evacuation failure?
The most common cause is feed rate that is too low for the material, producing thin, stringy chips that cannot break and tangle in the evacuation passage. Increasing feed within the recommended range resolves the majority of chip evacuation problems.
How do I know if my chips are the right shape?
Ideal chips for deep hole drilling are C-shaped or tightly curled 6-shaped segments, 2–10 mm in length, that flow freely with the coolant stream. They should not tangle with each other and should exit the hole continuously. Collect and examine chips from the first production hole before committing to full production.
Can coolant alone fix chip evacuation problems?
No — coolant cannot compensate for incorrect chip formation. If the chips are the wrong shape (long spirals, thick arcs, or needles), no amount of coolant pressure or flow will evacuate them reliably. Chip formation must be corrected first through feed, geometry, or chip breaker adjustments, then coolant parameters optimized for transport.
Why do my chips look fine but the tool still fails from chip packing?
Intermittent chip packing can occur even when chips appear well-formed. Possible causes include: coolant flow interruptions (filter cycling, pump cavitation), variations in material hardness, gradual edge wear that changes chip formation, or coolant temperature rise that reduces transport capacity. Monitor coolant pressure continuously — a slowly rising trend indicates gradual chip buildup before catastrophic packing occurs.
How does tool wear affect chip formation?
As the cutting edge wears, the edge geometry changes — the cutting edge radius increases, and the effective rake angle becomes more negative. This produces thicker, less consistent chips that resist breaking. A tool that produced good chips at the start of its life may begin producing problematic chips as it wears. This is a primary reason to track tool life and change tools proactively.
What should I do if chips have welded to the cutting edge?
Chip welding (built-up edge) indicates insufficient coolant flow at the cutting zone, incorrect cutting speed (too low for the material), or an unfavorable chip breaker geometry. Stop immediately and replace the tool — welding indicates the tool surface is already damaged. Adjust coolant flow to ensure adequate volume reaches the cutting edge, increase speed to raise cutting temperature above the material's adhesion threshold, and consider a coated tool to reduce friction.
Chip evacuation solutions depend on specific workpiece material, tool geometry, and machine configuration. The adjustments in this article are starting points — fine-tune based on observation of actual chip formation. Consult your tool supplier for material-specific chip breaker recommendations. This article reflects industry knowledge as of 2026.