A deep hole's internal surface is invisible to conventional measurement instruments. Calipers, micrometers, and CMM probes measure the entry and exit — but the entire length between them remains unseen. Borescope inspection fills this gap, providing visual confirmation that the hole's internal surface meets quality requirements.
Borescope Types for Deep Hole Inspection
Comparison of Borescope Types
| Type | Diameter Range | Max Length | Image Quality | Cost | Best For |
|---|
| Rigid borescope | 2–20 mm | 0.5–3 m | Excellent | $2,000–$10,000 | Straight holes, small diameters |
| Flexible fiberscope | 3–12 mm | 1–10 m | Good (grainy) | $3,000–$15,000 | Curved access, long holes |
| Video borescope | 4–20 mm | 1–30 m | Excellent (digital) | $5,000–$40,000 | Production inspection, documentation |
| Rigid video borescope | 6–20 mm | 0.5–3 m | Excellent | $8,000–$25,000 | High-volume production, automated |
Selection by Hole Parameters
| Hole Diameter (mm) | Recommended Borescope Type | Minimum Outer Diameter | Maximum Working Length |
|---|
| 3–6 | Rigid borescope | 2.7 mm (for 3 mm hole) | 300 mm (typical) |
| 6–15 | Rigid or video borescope | 4–6 mm | 1,000–2,000 mm |
| 15–30 | Video borescope or fiberscope | 6–10 mm | 2,000–5,000 mm |
| 30–50 | Video borescope | 8–12 mm | 5,000–15,000 mm |
| > 50 | Video borescope (larger lens for better image) | 10–20 mm | Up to 30 m |
Selection Criteria
Key Specifications
| Specification | What It Determines | Minimum for Deep Hole QC | Recommended for Production |
|---|
| Working length | Maximum hole depth inspectable | Match to longest hole + 20% | Same + margin |
| Outer diameter | Smallest hole inspectable | 0.5–1.0 mm less than hole ID | 2 mm less than hole ID |
| Resolution | Image clarity for defect detection | 640 × 480 | 1280 × 720 (HD) |
| Field of view | Area visible in one image | 50° | 70–90° |
| Depth of field | Focus range | 10 mm to infinity | 5 mm to infinity |
| Articulation | Ability to steer tip | N/A (rigid) or 2-way | 4-way (for flexible) |
| Image capture | Documentation capability | Still image | Video + still |
Lighting Considerations
| Lighting Type | Best For | Limitation |
|---|
| Integrated LED (distal tip) | General inspection | Shadows in rough surfaces |
| Ring light (proximal) | Even illumination | Less light at depth |
| Dedicated light source (fiber optic) | Maximum brightness | Bulky, tethered |
| UV light source | Fluorescent penetrant inspection | Requires dye application |
Inspection Procedure
Preparation
| Step | Action | Detail |
|---|
| 1 | Clean the hole thoroughly | Remove coolant, chips, oil with solvent |
| 2 | Dry the hole | Compressed air or lint-free swabs |
| 3 | Set up borescope | Connect light source, camera, monitor |
| 4 | Calibrate white balance | If using video borescope |
| 5 | Verify focus | Test on known reference surface |
| 6 | Set insertion speed guide | Maximum 25 mm/s to avoid damage |
Inspection Sequence
| Step | Action | What to Look For |
|---|
| 1 | Inspect hole entry | Bell mouth, tool marks, edge condition |
| 2 | Insert borescope to first 10 mm | Surface finish, circumferential marks |
| 3 | Advance in 50 mm increments | Scan entire circumference at each stop |
| 4 | Rotate probe 360° at each stop | Check for localized defects |
| 5 | Record images at key positions | Entry, suspected defects, exit |
| 6 | Advance to full depth | Hole straightness, spiral marks |
| 7 | Inspect hole exit | Breakout condition, burrs |
| 8 | Retract slowly | Second pass verification |
| 9 | Clean probe after removal | Wipe with lint-free cloth |
Defect Identification
Common Internal Defects
| Defect | Appearance | Typical Cause | Severity |
|---|
| Tool marks / feed marks | Continuous spiral lines | Feed too high, worn drill | Minor to moderate |
| Wash-out marks | Smooth, wavy surface variation | Coolant erosion, incorrect pressure | Moderate |
| Drill walking marks | Wavy entry section | Drill deflection at start | Moderate |
| Spiral marks / helical grooves | Deep spiral pattern on bore wall | Guide pad wear, chip packing | Severe |
| Scale / discoloration | Dark patches | Overheating, coolant breakdown | Moderate to severe |
| Surface tearing | Rough, torn appearance | Built-up edge, insufficient lubricity | Severe |
| Reaming chatter | Fine circumferential lines | Reamer vibration, speed mismatch | Minor to moderate |
| Step / ledge | Abrupt diameter change | Tool change mark, misalignment | Moderate |
| Scratches (longitudinal) | Axial lines along bore | Debris in coolant, guide pad damage | Minor to moderate |
| Pitting | Small pits in surface | Corrosion, material defect | Moderate to severe |
Defect Measurement Using Borescope
| Measurement | Method | Typical Accuracy | Limitation |
|---|
| Defect length | Scale bar in image | ±0.5 mm | Requires calibration |
| Defect width | Comparison to probe OD | ±0.1 mm | Subjective |
| Depth of defect | Shadow measurement | ±0.05 mm | Requires practice |
| Surface finish comparison | Visual comparison to reference | Ra ±0.2 µm (rough estimate) | Subjective, not quantitative |
| Hole straightness (qualitative) | Probe insertion feel | Pass/fail | Operator-dependent |
Tip: For quantitative surface finish measurement, use a surface roughness comparator or profilometer replica. Borescope visual inspection detects defects but cannot measure Ra or Rz with precision. Combine borescope inspection with surface roughness measurement for complete quality data.
Documentation Standards
Image Capture Requirements
| Requirement | Minimum Standard | Recommended Standard |
|---|
| Resolution | 640 × 480 | 1920 × 1080 |
| File format | JPEG | JPEG + RAW |
| Image labeling | Date, part ID, hole ID | Date, part ID, hole ID, depth, orientation |
| Lighting reference | Auto | Manual (consistent across inspections) |
| Scale reference | None (probe diameter known) | Scale bar in image |
| Field | Content | Example |
|---|
| Part number | Customer part identifier | PN-4140-CYL-01 |
| Hole ID | Specific hole identifier | Hole #3 |
| Inspection date | Date of inspection | 2026-06-04 |
| Inspector | Who performed inspection | J. Smith |
| Borescope used | Equipment identification | Olympus IPLEX GX-8 |
| Inspection length | Full depth or partial | Full depth (1,200 mm) |
| Defects found | Type, location, severity | Light spiral marks at 200–400 mm |
| Measurements | Key dimensions at defects | Line width approx 0.15 mm |
| Acceptance | Pass / fail / rework | Pass |
| Image references | File names of captured images | IMG_20260604_001–012 |
Integration with QC Systems
Borescope in the Quality Workflow
| Stage | Action | Documentation |
|---|
| First article | Full borescope inspection of first hole | Complete report + image set |
| Production monitoring | Reduced inspection every Nth part | Checklist + exception images |
| Process change | Full inspection before and after change | Comparative report |
| Final inspection | 100% or AQL sampling per specification | Certificate of conformance |
| Rework verification | Re-inspect after rework | Rework report + new images |
Automated Inspection Systems
| System Type | Capability | Investment | Throughput |
|---|
| Manual borescope with camera | Operator-controlled, image capture | $5,000–$15,000 | 5–10 holes/hour |
| Semi-automated (motorized insertion) | Programmed scan path, video recording | $20,000–$50,000 | 15–30 holes/hour |
| Fully automated (robotic) | Programmed inspection, auto-defect detection | $100,000–$300,000 | 30–60 holes/hour |
| In-line borescope (integrated in machine) | Post-drill inspection without unload | $50,000–$150,000 | 60+ holes/hour |
FAQ
What type of borescope is best for deep hole drilling inspection?
A video borescope with articulation is best for most deep hole drilling applications. It combines digital image quality, long working length (up to 30 m), and the ability to steer the tip around corners. For small-diameter holes (< 6 mm), a rigid borescope provides better image quality at a lower cost. For production inspection requiring documentation, a video borescope with built-in recording is essential.
How do I identify spiral marks on a borescope image?
Spiral marks appear as continuous helical lines following the drill's rotation pattern. They are typically caused by guide pad wear or chip packing that forces the drill off-center. The marks have a consistent pitch matching the feed per revolution. Compare the pitch to the programmed feed rate — if the pitch matches, the marks are tool feed marks. If deeper and irregular, they indicate a vibration or deflection problem.
Can a borescope measure surface roughness quantitatively?
No — borescope inspection is qualitative, not quantitative for surface roughness. Visual inspection can detect defects, compare surface texture to reference images, and identify anomalies, but it cannot measure Ra, Rz, or other roughness parameters with accuracy. For quantitative roughness measurement, use a surface profilometer, a roughness comparator, or make a silicone replica of the bore surface for lab measurement.
How often should I calibrate a borescope?
Calibrate the borescope image measurement system (scale bars, measurement software) every 12 months or per manufacturer recommendation. White balance should be checked before each inspection session. Focus should be verified at the start of each shift. After any impact or damage, the borescope must be recalibrated before use — a bent probe gives inaccurate images.
What is the difference between a fiberscope and a video borescope?
A fiberscope uses a coherent fiber-optic bundle to transmit the image from the distal tip to the eyepiece or camera. The image has a visible pixel pattern (honeycomb or grain). A video borescope has a miniature CCD or CMOS camera chip at the distal tip, producing a direct digital image without fiber-optic grain. Video borescopes offer significantly better image quality, higher resolution, and built-in recording capabilities, at a higher cost.
Borescope inspection reveals what external measurements cannot — the internal surface quality of a deep drilled hole. Combined with diameter and straightness measurements, it provides complete quality assurance for deep hole drilling operations. This article reflects industry practice as of 2026.
