Appearance
Every drilling operation produces burrs — the question is whether they are small enough to ignore or large enough to cause functional problems. In deep hole drilling, burrs at the bore entry, exit, and internal intersections can lead to seal failure, hydraulic contamination, assembly interference, and fatigue crack initiation. The depth of the bore makes these burrs difficult to inspect and expensive to remove, placing a premium on burr prevention through process design rather than post-process removal.
Overview
A burr is an unwanted projection of material beyond the theoretical edge of a machined surface, formed by plastic deformation during cutting. In drilling, burrs appear at three locations:
| Location | Typical Severity | Functional Risk |
|---|---|---|
| Entry burr (drill entrance) | Small | O-ring damage, assembly interference |
| Exit burr (drill breakthrough) | Large to very large | Seal failure, loose debris, safety hazard |
| Internal burr (cross-hole intersection) | Moderate to large | Hydraulic contamination, flow restriction |
Research shows that drilling burrs can reduce the fatigue life of assemblies by up to 70%, making burr control a functional requirement rather than a cosmetic one in many deep hole drilling applications.
Burr Classification and Formation Mechanisms
Gillespie and Blotter's Four Basic Types
The foundational classification system for drilling burrs identifies four fundamental types based on the formation mechanism:
| Burr Type | Formation Mechanism | Typical Location |
|---|---|---|
| Poisson burr | Material bulges sideways due to hydrostatic pressure at the cutting edge margins | Entry side, along bore wall |
| Rollover burr | Material bends and flows in the cutting direction rather than shearing cleanly | Exit side |
| Tear burr | Material tears loose rather than shearing | Entry or exit, more common in ductile materials |
| Cut-off burr | Final separation of the workpiece leaves a protruding edge | Exit side |
In drilling, the most common and problematic burr types are Poisson burrs at the entry and rollover burrs at the exit.
Exit Burr Classification
Exit burrs in drilling have been extensively classified. The most widely used system, developed by Dornfeld and Ko, categorises exit burrs by morphology:
| Exit Burr Type | Characteristics | Formation Conditions |
|---|---|---|
| Uniform burr | Forms uniformly around the hole circumference | Low feed, sharp tool, good support |
| Uniform burr with drill cap | Residual cap of material remains attached to burr | Intermediate conditions |
| Crown burr | Irregular, crown-like shape | Higher feed rates, tool wear |
| Transient burr | Small, irregular burr | Lower cutting speeds and feeds |
| Roll-back burr | Burr rolls back toward the bore (Ti alloys, dry cutting) | Thermal effects in difficult materials |
The crown burr is the most undesirable type — it is large, irregular, and difficult to remove. The uniform burr is the preferred outcome because it is predictable in size and can be controlled through parameter adjustment.
Stages of Exit Burr Formation
The formation of an exit burr proceeds through four stages:
- Initial plastic deformation — as the drill point approaches the exit surface, the remaining material thins and begins to deform plastically under the thrust force
- Material extrusion — the drill pushes the thinned material outward, forming a bulge on the exit surface
- Stretching and bending — the bulge stretches and bends as the drill breaks through, with the peripheral cutting edge rolling the material outward
- Fracture and separation — the stretched material fractures, leaving a burr around the hole periphery
The size and shape of the final burr depend on how far the process progresses through stage 3 before fracture occurs in stage 4.
Drill Geometry Effects on Burr Formation
Drill geometry has been identified as the single most influential factor in burr formation — in some studies, more important than cutting parameters.
Point Angle
Point angle is the most studied geometric parameter for burr control:
| Point Angle | Effect on Burr | Recommended Application |
|---|---|---|
| 118° (standard) | Moderate burr height | General purpose, softer materials |
| 135° (larger) | 30–50% smaller exit burr | Stainless steel, hard materials |
| 90° (smaller) | Larger burr, more rollover | Not recommended for burr control |
| Step drill (multi-angle) | Smallest burr, best control | Production, high-value parts |
Research consistently shows that 135° point angle significantly reduces exit burr height compared to the standard 118°. In drilling AISI 316L stainless steel, a 135° point angle combined with optimized feed (0.1 mm/rev) produced the minimum burr height in genetic algorithm optimization studies.
Helix Angle
Larger helix angles (30–40°) reduce burr height compared to standard (20–25°) helix angles. The increased rake angle improves cutting action, reducing the plastic deformation that drives burr formation.
Step Drill Geometry
The step drill is one of the most effective tool designs for burr minimization:
- How it works: The front cutting edge performs the initial cut; the step edge (larger diameter) follows and removes the burr created by the front edge, producing a significantly smaller final burr
- Burr reduction: Step drills produce burr heights of 0.07–0.21 mm compared to 0.14–0.31 mm for conventional drills (Ko et al., 2003)
- Critical application: Oil pass holes in automotive steering shafts where internal burrs can cause sealing failure
Edge Preparation
| Edge Condition | Effect on Burr |
|---|---|
| Sharp (honed < 0.02 mm) | Minimum burr, clean shear |
| Moderate hone (0.03–0.08 mm) | Acceptable, improved edge strength |
| Excessive hone (> 0.10 mm) | Increased burr due to wiping/ploughing |
| Worn / chipped edge | Large irregular burr — replace immediately |
Even a 0.1 mm edge chip doubles the burr risk. Tool condition monitoring is essential for burr control in production.
Cutting Parameter Optimization
Feed rate is the most influential cutting parameter for burr size, followed by cutting speed.
Feed Rate Effects
| Feed Rate | Burr Height | Burr Type | Trade-off |
|---|---|---|---|
| Very low (< 0.05 mm/rev) | Small | Uniform | Reduced productivity, heat buildup |
| Moderate (0.08–0.15 mm/rev) | Moderate | Uniform or uniform with cap | Optimal balance |
| High (> 0.20 mm/rev) | Large | Crown | High productivity but poor burr |
The relationship between feed rate and burr size is monotonic: higher feed rates produce larger burrs. However, excessively low feed rates can cause work-hardening and increase burr formation in some materials. The optimal feed rate for burr minimization typically falls at the lower end of the tool manufacturer's recommended range.
Cutting Speed Effects
Cutting speed has a less dominant effect than feed rate, but lower speeds generally produce smaller burrs:
| Material | Optimal Cutting Speed for Burr Control | Optimal Feed | Source |
|---|---|---|---|
| AISI 316L stainless steel | 4–15 m/min | 0.10 mm/rev | Kilickap & Huseyinoglu (GA/RSM) |
| 42CrMo4 quenched and tempered | 80–120 m/min (3183 rpm) | 0.12 mm/rev | Springer (2025 deep drilling study) |
| Aluminium alloys | Higher speeds (100–200 m/min) with low feed | 0.05–0.08 mm/rev | Pilny et al. (2012) |
Feed Rate Management at Exit
The most effective single technique for exit burr reduction is to reduce feed rate as the drill approaches breakthrough:
| Phase | Feed Reduction | Duration |
|---|---|---|
| Approach (1–2 mm before exit) | 25–50% reduction | Until breakthrough |
| Breakthrough | Maintain reduced feed | Complete hole |
| After exit | Return to normal | Next hole |
This technique reduces the thrust force at the critical moment when the remaining material is thinnest and most prone to plastic deformation.
Internal Burrs in Deep Hole Drilling
Internal burrs in deep hole drilling present unique challenges not encountered in conventional drilling:
- Location: Burrs form at the intersection of cross-holes with the main bore, often metres from the bore entrance
- Access: The depth of the bore limits tool access for deburring
- Inspection: Borescope inspection is required — internal burrs cannot be seen or felt directly
- Functional risk: Loose burrs can contaminate hydraulic systems, block oil passages, or damage seals
Sources of Internal Burrs
In deep hole drilling, internal burrs arise from:
- Cross-hole intersections — when a radial hole is drilled into an existing bore, the interrupted cut generates burrs on both the entry and exit sides of the intersection
- Chip re-welding — hot chips (400–900°C) can weld to the bore surface under high pressure
- Guide pad deposits — built-up edge material transferred from guide pads can form raised ridges
- Tool change marks — step marks at tool change points can act as burr-like projections
Burr Control for Internal Features
For deep hole drilling, burr control at internal features relies on prevention rather than removal:
| Strategy | Implementation | Effectiveness |
|---|---|---|
| Reduce feed before intersection | 25–50% reduction 1 mm before cross-hole | High |
| High coolant pressure during cross-hole drilling | 7–10 MPa minimum | High — flushes burrs before they burnish |
| Port entry chamfer | Break edge before cross-hole drilling | Moderate |
| Drill sequence planning | Drill main bore first, then cross-holes | Essential |
| Borescope inspection between operations | Visual check after each critical step | Required for critical parts |
Deburring Methods for Deep Holes
When prevention is insufficient — which is the case for most production applications — deburring is required. The depth of deep holes limits the available methods.
Abrasive Flow Machining
Abrasive flow machining (AFM) is the most effective method for deburring deep holes with complex internal features:
| Parameter | Typical Range |
|---|---|
| Media viscosity | 50,000–500,000 Poise |
| Extrusion pressure | 0.5–10 MPa |
| Surface finish improvement | Up to two ISO classes |
| Access | All internal passages simultaneously |
| Cycle time | 5–20 minutes per cycle |
AFM uses a viscoelastic abrasive-laden polymer that is extruded through the bore. The abrasive media removes burrs and radiusing edges uniformly across all internal surfaces, including cross-hole intersections that are inaccessible to mechanical tools. Mollart Engineering received a SMART award for advancing AFM technology for deep hole drilling applications.
Flexible Hone (Ball Hone)
The flexible hone — a ball-shaped tool with abrasive globules on flexible filaments — is effective for deburring the main bore, including cross-hole intersections:
- Self-centering and self-aligning
- Available from 4 mm diameter upward
- Rotate in both CW and CCW directions for symmetrical deburring
- Can be used in CNC machines or with a cordless drill
- Effective for burr root thickness ≤ 0.1 mm
Ceramic Abrasive Brushes
XEBEC-style ceramic abrasive brushes use high-density ceramic fibre filaments (80% ceramic content) rather than abrasive grains embedded in nylon:
| Parameter | Recommendation |
|---|---|
| Rotational speed | 8,000–11,000 min⁻¹ |
| Action direction | Pull brush past cross-hole (do not push) |
| Max depth | Up to 400 mm with extra-long versions |
| Target burr size | Root thickness ≤ 0.1 mm |
The pulling action is critical — pushing the brush can lay burrs flat against the bore surface rather than removing them.
Mechanical Deburring Tools
For accessible intersections, spring-loaded mechanical deburring tools provide reliable results:
| Tool | Manufacturer | Application |
|---|---|---|
| COFA-X | Heule | Single cross-bore intersection deburring |
| Burraway | Cogsdill | General production deburring through main bore |
| Flex-hone | Brush Research Manufacturing | Internal bore edge blending |
| ORBITOOL | J.W. Done | Flexible shaft deburring for offset intersections |
Skive Burnishing
For BTA-drilled bores, skive burnishing can remove burrs while simultaneously improving surface finish:
- Single-pass operation removes burrs and burnishes in one step
- Achieves Ra < 0.1–0.8 μm surface finish
- Effective for bores 38–400 mm diameter
- Feed rates of 3–6 mm/rev (much faster than honing)
Electrochemical and Thermal Deburring
For internal burrs that are completely inaccessible:
| Method | Principle | Best For |
|---|---|---|
| Electrochemical deburring (ECD) | Anodic dissolution of burrs | Precision parts, no mechanical force |
| Thermal energy method (TEM) | Combustion at 3000°C oxidises burrs | High-volume, many intersections |
| Ultrasonic cleaning | Cavitation removes loose micro-burrs | Final cleaning step |
Process Planning for Burr Control
A systematic approach to burr control in deep hole drilling follows these steps:
- Select drill geometry for minimum burr — 135° point angle, step drill if possible, sharp edge preparation
- Optimise cutting parameters — moderate-to-low feed rate, appropriate cutting speed for the material
- Plan feed reduction at exits and intersections — 25–75% reduction through critical zones
- Use high coolant pressure at internal intersections — 7–10 MPa to flush burrs
- Inspect with borescope after each cross-hole drilling operation
- Deburr using AFM, flexible hone, ceramic brush, or mechanical tool as appropriate
- Final flush — high-pressure wash to remove any remaining loose debris
- Final inspection — verify all burrs removed
Summary
| Aspect | Key Consideration |
|---|---|
| Burr types | Poisson (entry), rollover (exit), tear, and cut-off |
| Exit burr classification | Uniform, with cap, crown, transient, roll-back — uniform is preferred |
| Most influential factor | Drill geometry — especially point angle (135° preferred) |
| Most influential parameter | Feed rate — lower feed reduces burr size |
| Best tool design | Step drill — removes burr from front edge with following step |
| Feed management at exit | Reduce 25–75% before breakthrough |
| Internal burr risk | Cross-hole intersections — require prevention + borescope inspection |
| Primary deburring methods | AFM for complex internals, flexible hone for bores, ceramic brushes for accessible intersections |
| Coolant pressure for deburring | 7–10 MPa minimum for flushing internal burrs |
| Edge condition | Sharp tools produce smaller burrs — monitor for wear and chipping |
FAQ
What is the difference between entry burr and exit burr in drilling?
Entry burrs form at the drill entrance and are typically small Poisson burrs caused by material side-flow at the drill margins. Exit burrs form at breakthrough and are typically larger rollover burrs caused by material bending and stretching. Exit burrs are generally more problematic and receive more attention in burr control research.
What drill point angle produces the smallest burr?
A 135° point angle produces the smallest burr for most materials, particularly stainless steels and hard alloys. Research using genetic algorithm optimization on AISI 316L identified 135° as optimal, reducing burr height by 30–50% compared to standard 118° point angle drills.
How does feed rate affect burr size?
Feed rate has a direct and monotonic relationship with burr size: higher feed rates produce larger burrs, especially at the exit. Reducing feed rate before breakthrough is the single most effective in-process technique for minimizing exit burrs. However, excessively low feed rates can cause work-hardening in some materials.
What is a step drill and why does it reduce burrs?
A step drill has two diameters — a front cutting edge and a larger step edge behind it. The front edge performs the initial cut, and the step edge follows to remove the burr created by the front edge, producing a significantly smaller final burr. Step drills can reduce burr height from 0.14–0.31 mm (conventional) to 0.07–0.21 mm.
How do I remove burrs from inside a deep hole?
The most effective methods are abrasive flow machining (AFM) for complex internal features, flexible hones (ball hones) for general bore deburring, and ceramic abrasive brushes for accessible cross-hole intersections. For BTA-drilled bores, skive burnishing removes burrs while improving surface finish in a single pass.
What causes crown burrs and how can they be avoided?
Crown burrs (irregular, crown-shaped exit burrs) form at higher feed rates and with worn tools. They are the most undesirable burr type because they are large, irregular, and difficult to remove. To avoid crown burrs: reduce feed rate, keep tools sharp, and use a 135° point angle.
Do coated drills produce smaller burrs than uncoated drills?
Coated carbide drills generally produce smaller and more consistent burrs than uncoated HSS or carbide drills, especially in abrasive materials. TiAlN and TiN coatings reduce friction and built-up edge formation, both of which contribute to burr formation. Diamond coatings are particularly effective for aluminium and non-ferrous materials.
What is the best deburring method for cross-holes in deep bores?
For cross-hole intersections accessible through the main bore, ceramic abrasive brushes (XEBEC-style) or mechanical tools (Heule COFA-X, Cogsdill Burraway) provide reliable results. For inaccessible intersections, abrasive flow machining is the most effective method. A flexible hone is a good general-purpose solution for moderate burr sizes.
Can burrs be completely eliminated in deep hole drilling?
Complete elimination of burrs is not possible in practical drilling operations. The goal of burr minimization is to reduce burr size to an acceptable level (typically < 0.1 mm for critical applications) where they do not affect function and can be removed by a final deburring pass or cleaning operation.
How do I inspect for burrs inside deep holes?
Borescope inspection is the primary method for detecting internal burrs in deep holes. A flexible or rigid borescope with articulating tip allows visual inspection of cross-hole intersections and the full length of the bore. For critical applications, mouldable impression materials can capture the geometry of internal features for off-line measurement.