Appearance
Surface finish is one of the most frequently specified requirements in deep hole drilling, yet one of the most misunderstood. The achievable finish depends on the drilling process, workpiece material, tool condition, and operating parameters. This reference provides comprehensive data on achievable Ra, Rz, and Rmax values for gun drilling, BTA drilling, and finishing operations — helping designers specify realistic finishes and shops select the right process.
Surface Finish Parameters
Three parameters are commonly used to specify bore surface finish:
Parameter Definitions
| Parameter | Full Name | Definition | Typical Use |
|---|---|---|---|
| Ra | Arithmetic Average Roughness | Average deviation of the surface profile from the mean line | General-purpose specification |
| Rz | Average Maximum Height | Average of the five highest peaks minus five lowest valleys | Sealing surfaces, where peak height matters |
| Rmax | Maximum Roughness Depth | Single highest peak-to-valley within the evaluation length | Critical sealing, dynamic contact surfaces |
| RMS / Rq | Root Mean Square | RMS of the surface profile deviations | Research, legacy specifications (approx 1.11 × Ra) |
Conversion Between Parameters
| Conversion | Factor | Example (Ra 1.6 →) |
|---|---|---|
| Ra → RMS (μm) | × 1.11 | RMS 1.78 μm |
| Ra → Rz (approximate) | × 4–5 | Rz 6.4–8.0 μm |
| Ra → Rmax (approximate) | × 5–8 | Rmax 8.0–12.8 μm |
| Ra (μm) → Ra (μin) | × 39.37 | 63 μin |
| Ra (μin) → Ra (μm) | × 0.0254 | 1.6 μm |
WARNING
Conversion factors between Ra and Rz are approximate. The actual ratio depends on the surface profile shape. Ground surfaces typically have Ra-to-Rz ratios of 4–5:1, while turned or drilled surfaces can range from 5–8:1. For critical applications, specify the required parameter directly rather than relying on conversions.
Achievable Surface Finish by Drilling Process
Gun Drilling (External Chip Removal)
| Condition | Ra (μm) | Ra (μin) | Rz (μm) | IT Grade |
|---|---|---|---|---|
| Standard production, steel | 0.8–1.6 | 32–63 | 4–8 | IT8–IT10 |
| Optimized, steel | 0.4–0.8 | 16–32 | 2–4 | IT7–IT9 |
| Precision, favorable material | 0.2–0.4 | 8–16 | 1–2 | IT7–IT8 |
| With post-drill honing | 0.1–0.2 | 4–8 | 0.5–1 | IT6–IT7 |
Typical diameter range: 1–50 mm Maximum L/D ratio: Up to 100:1 (300:1 with special setups) Straightness: < 0.05 mm per 300 mm depth
Gun drilling produces a distinct surface characterized by:
- Visible feed lines from the single cutting edge
- Burnished zones where guide pads contact the bore wall
- Slightly higher Ra at entry and exit than mid-hole
- A "chatter" pattern if parameters are not optimized
BTA Drilling (Internal Chip Removal)
| Condition | Ra (μm) | Ra (μin) | Rz (μm) | IT Grade |
|---|---|---|---|---|
| Standard BTA drilling | 3.2–6.3 | 125–250 | 16–32 | IT9–IT11 |
| Fine BTA (optimized) | 1.6–3.2 | 63–125 | 8–16 | IT8–IT9 |
| Precision BTA | 0.8–1.6 | 32–63 | 4–8 | IT7–IT9 |
| With roller burnishing | 0.2–0.4 | 8–16 | 1–2 | IT8–IT9 |
| With post-drill honing | 0.1–0.4 | 4–16 | 0.5–2 | IT6–IT7 |
Typical diameter range: 16–200 mm (up to 630+ mm) Maximum L/D ratio: Up to 100:1 (200:1 with special setups) Straightness: < 0.1 mm per 300 mm depth
BTA drilling produces a surface influenced by:
- Guide pad burnishing action (can improve finish by 30–50% compared to the cutting edge alone)
- Internal chip evacuation (chips do not contact the bore wall)
- Characteristic scalloped pattern from the cutting edge geometry
- Potential for "wavy" surface at non-optimal parameters
Ejector Drilling (Jet Suction)
| Condition | Ra (μm) | Ra (μin) | Rz (μm) | IT Grade |
|---|---|---|---|---|
| Standard ejector drilling | 1.6–3.2 | 63–125 | 8–16 | IT9–IT11 |
| Optimized | 0.8–1.6 | 32–63 | 4–8 | IT8–IT10 |
Typical diameter range: 18–65 mm Maximum L/D ratio: Up to 100:1
Finish Comparison Summary
| Process | Ra Range (μm) | Cost per Hole | Relative Finish Quality |
|---|---|---|---|
| Gun drilling | 0.4–1.6 | High | Best for small diameters |
| BTA drilling | 1.6–6.3 | Medium | Best for large diameters |
| Ejector drilling | 0.8–3.2 | Medium | Good mid-range option |
| Gun drilling + hone | 0.1–0.2 | Highest | Mirror finish |
| BTA + roller burnish | 0.2–0.4 | Medium-High | Excellent finish, low cost increase |
| BTA + hone | 0.1–0.4 | Highest | Precision finish |
Surface Finish by Workpiece Material
Steel Alloys
| Material | Standard Drilling Ra (μm) | Optimized Ra (μm) | Notes |
|---|---|---|---|
| Carbon steel (1018, 1045) | 0.8–1.6 | 0.4–0.8 | Stable chip formation, good finish |
| Alloy steel (4140, 4340) | 0.8–1.6 | 0.4–0.8 | Excellent finish potential |
| Tool steel (A2, D2) | 0.8–1.6 | 0.4–0.8 | Annealed: good; hardened: more challenging |
| Case-hardened steel | 0.8–1.6 | 0.4–0.8 | Surface hardness helps burnishing |
| Nitrided steel | 0.8–1.6 | 0.4–1.0 | Very hard surface can wear tool |
Stainless Steel Grades
| Material | Standard Drilling Ra (μm) | Optimized Ra (μm) | Notes |
|---|---|---|---|
| 303 stainless | 0.8–1.6 | 0.4–0.8 | Best machinability of stainless grades |
| 304/316 stainless | 1.0–2.0 | 0.4–1.0 | Work-hardening tendency affects finish |
| 17-4 PH (H900) | 0.8–1.6 | 0.4–0.8 | Good finish in hardened condition |
| Duplex 2205 | 1.0–2.0 | 0.8–1.6 | Tough, can produce smearing |
| Super duplex 2507 | 1.6–3.2 | 0.8–1.6 | High strength, challenging |
Nickel-Based Superalloys
| Material | Standard Drilling Ra (μm) | Optimized Ra (μm) | Notes |
|---|---|---|---|
| Inconel 625 | 1.6–3.2 | 0.8–1.6 | Work-hardens rapidly, heat concentrates |
| Inconel 718 | 1.6–3.2 | 0.8–1.6 | Requires sharp tooling |
| Hastelloy X | 1.6–3.2 | 0.8–1.6 | Similar to Inconel |
| Monel 400 | 1.0–2.0 | 0.4–1.0 | Moderate machinability |
Titanium Alloys
| Material | Standard Drilling Ra (μm) | Optimized Ra (μm) | Notes |
|---|---|---|---|
| Ti-6Al-4V (Grade 5) | 0.8–1.6 | 0.4–0.8 | Low thermal conductivity — coolant critical |
| Ti-6Al-4V ELI (Grade 23) | 0.8–1.6 | 0.4–0.8 | Medical grade, similar behavior |
| CP Titanium (Grades 1–4) | 0.8–1.6 | 0.4–0.8 | Softer, can produce smearing |
Other Materials
| Material | Standard Drilling Ra (μm) | Optimized Ra (μm) | Notes |
|---|---|---|---|
| Cast iron (gray) | 1.6–3.2 | 0.8–1.6 | Graphite acts as lubricant |
| Ductile iron | 1.6–3.2 | 0.8–1.6 | Nodular graphite |
| Aluminum (6061, 7075) | 0.8–1.6 | 0.4–0.8 | Built-up edge risk at low speed |
| Brass / Bronze | 0.4–1.0 | 0.2–0.8 | Excellent finish potential |
| Copper | 0.8–1.6 | 0.4–1.0 | Soft, can produce smearing |
| Plastics (nylon, acetal) | 1.6–3.2 | 0.8–1.6 | Melting risk, requires sharp tooling |
| Fiber-reinforced composites | 1.6–3.2 | 0.8–1.6 | Abrasive, tool wear degrades finish |
Process Parameter Effects on Surface Finish
Feed Rate
Feed rate has the most direct effect on surface finish in deep hole drilling:
| Feed Rate vs. Optimal | Effect on Ra | Mechanism |
|---|---|---|
| Too low | Higher Ra | Insufficient burnishing force from guide pads |
| Optimal (typically 0.05–0.15 mm/rev) | Lowest Ra | Maximum burnishing effect, stable cutting |
| Too high | Higher Ra | Excessive pad load, chatter, chip evacuation problems |
Research on BTA drilling of steel shows that intermediate feed rates (approximately 0.08–0.10 mm/rev) produce the lowest surface roughness due to the guide pad burnishing effect. Below this range, the burnishing force is insufficient; above it, pad overload degrades the surface.
Cutting Speed
| Speed Range | Effect on Ra | Mechanism |
|---|---|---|
| Low (below 50 m/min) | Higher Ra | Built-up edge formation, material tearing |
| Optimal (60–80 m/min for steel) | Lowest Ra | Clean cutting, effective burnishing |
| High (above 100 m/min) | Variable | Vibration, bar whip at high speeds |
Coolant Pressure and Filtration
| Factor | Effect on Surface Finish |
|---|---|
| Adequate pressure (> 80 bar for gun drilling) | Effective chip evacuation, stable process |
| Insufficient pressure | Chip packing, surface scoring |
| Clean coolant (10 μm or better filtration) | No particle scoring of bore wall |
| Contaminated coolant | Grooves, scratches in bore surface |
Tool Condition
| Condition | Effect on Ra |
|---|---|
| Sharp, properly set tool | Minimum Ra achievable for the process |
| Worn cutting edge | Increasing Ra, visible degradation |
| Worn guide pads | Loss of burnishing effect, higher Ra |
| Incorrect tip offset | Oversized or undersized hole, irregular surface |
Surface Finish Specifications for Common Applications
Recommended Finish by Application
| Application | Recommended Ra (μm) | Process | Notes |
|---|---|---|---|
| Hydraulic cylinder tubes | 0.4–0.8 | Gun drill + hone or BTA + roller burnish | Seal surface |
| Pneumatic cylinder tubes | 0.8–1.6 | Gun drill or BTA | Lower seal pressure |
| Fuel injector bores | 0.2–0.4 | Precision gun drill + hone | Metering surface |
| Hydraulic valve bores | 0.4–0.8 | Gun drill or fine BTA | Spool valve clearance |
| Oil and gas valve bores | 0.8–1.6 | BTA or gun drill | Seal and flow surface |
| Aerospace structural bores | 1.6–3.2 | Gun drill | Fastener holes |
| Medical implant bores | 0.4–0.8 | Precision gun drill | Bone/implant interface |
| Mold cooling channels | 1.6–3.2 | Gun drill | Heat transfer surface |
| Automotive transmission shafts | 0.8–1.6 | Gun drill | Oil passage bores |
| Heat exchanger tubes | 1.6–3.2 | BTA or ejector | Flow surface |
Cost Impact of Surface Finish
| Ra Requirement | Relative Cost | Cycle Time Impact |
|---|---|---|
| ≤ 3.2 μm | 1× (baseline) | Standard parameters |
| ≤ 1.6 μm | 1.2–1.5× | Reduced feed rate, finishing pass |
| ≤ 0.8 μm | 1.5–2.5× | Low feed, optimized parameters, tool condition critical |
| ≤ 0.4 μm | 2.5–5× | Secondary operation (hone or burnish) required |
| ≤ 0.2 μm | 5–10× | Precision honing or superfinishing required |
TIP
The single most cost-effective way to improve surface finish in deep hole drilling is to ensure the coolant filtration is adequate. Improving from 40 μm to 10 μm filtration can reduce bore surface roughness by 20–40% without any change to speeds, feeds, or tooling. This is because fine particles in the coolant act as abrasives, scoring the bore wall as they recirculate through the cutting zone.
Surface Texture and Measurement
Measurement Methods
| Method | Typical Application | Advantages | Limitations |
|---|---|---|---|
| Stylus profilometer | Laboratory, quality control | High accuracy, standardized | Contact method, line measurement |
| Optical (laser) profilometer | Laboratory, R&D | Non-contact, fast | Surface reflectivity sensitive |
| Contact roughness comparator | Shop floor | Quick comparison, low cost | Subjective, limited accuracy |
| Replica tape | In-situ measurement | Access tight locations | Lower accuracy |
Measurement Parameters
| Parameter | Cut-off Length | Evaluation Length | Typical Filter |
|---|---|---|---|
| Ra ≤ 0.1 μm | 0.25 mm | 1.25 mm | ISO 11562 |
| Ra 0.1–2.0 μm | 0.8 mm | 4.0 mm | ISO 11562 |
| Ra 2.0–10.0 μm | 2.5 mm | 12.5 mm | ISO 11562 |
Measurement Guidelines for Deep Hole Bores
- Measure in the axial direction (along the bore axis) to capture feed marks
- Take measurements at multiple depths (entry, middle, exit) — finish often varies along the hole
- For hydraulic applications, measure the circumferential direction as well (for seal compatibility)
- Average 3–5 measurements at each location
- Document both Ra and Rz for critical sealing surfaces
FAQ
Q: What surface finish can gun drilling achieve? Standard gun drilling in steel produces Ra 0.8–1.6 μm. With optimized parameters, sharp tooling, and adequate coolant, Ra 0.4–0.8 μm is achievable. For Ra below 0.4 μm, secondary operations such as honing or roller burnishing are required.
Q: What surface finish can BTA drilling achieve? Standard BTA drilling produces Ra 3.2–6.3 μm. Fine BTA with optimized parameters achieves Ra 1.6–3.2 μm. Precision BTA can reach Ra 0.8–1.6 μm. BTA combined with roller burnishing achieves Ra 0.2–0.4 μm.
Q: How does material affect achievable surface finish in deep hole drilling? Material significantly affects finish. Carbon and alloy steels produce the best finishes (Ra 0.4–0.8 μm). Stainless steels and titanium are more challenging (Ra 0.8–1.6 μm). Nickel-based superalloys such as Inconel are the most difficult (Ra 1.6–3.2 μm). Brass and bronze produce excellent finishes (Ra 0.2–0.8 μm).
Q: What is the relationship between Ra and Rz? For machined surfaces, Rz is approximately 4–8 × Ra. The exact ratio depends on the surface profile. Ground surfaces have lower ratios (4–5:1), while drilled or turned surfaces have higher ratios (5–8:1). For critical applications, specify the parameter directly rather than converting.
Q: Can surface finish be improved by adjusting feed rate? Yes. In BTA drilling, there is an optimal feed rate (typically 0.08–0.12 mm/rev for steel) that produces the lowest Ra due to the guide pad burnishing effect. Both too-low and too-high feed rates increase surface roughness. Feed rate has the most direct effect on finish of any adjustable parameter.
Q: What is the cost impact of specifying a tighter surface finish? Specifying Ra 1.6 μm instead of Ra 3.2 μm increases cost approximately 20–50%. Specifying Ra 0.8 μm instead of Ra 1.6 μm doubles or triples the cost. Achieving Ra 0.4 μm or better typically requires secondary operations, increasing cost 2–5×.
Q: Does coolant filtration affect surface finish? Yes, significantly. Contaminated coolant with particles larger than 10–20 μm will score the bore wall as particles recirculate through the cutting zone. Improving filtration from 40 μm to 10 μm can reduce surface roughness by 20–40% without changing any machining parameters.
Q: How should surface finish be measured in deep holes? Use a stylus profilometer with the appropriate cut-off length (0.8 mm for typical finishes). Measure axially along the bore to capture feed marks. Take readings at entry, middle, and exit depths — finish often varies along the hole. Average 3–5 measurements per location.
Q: What is the role of guide pads in surface finish? Guide pads in gun drills and BTA heads burnish the bore wall as the tool rotates. This plastic deformation of surface peaks reduces roughness significantly — effectively a secondary finishing operation integrated into the drilling process. The burnishing effect depends on sufficient radial force from the feed rate.
Q: What secondary operations can improve deep hole surface finish? Roller burnishing (BTA only, Ra 0.2–0.4 μm), honing (both processes, Ra 0.1–0.4 μm), fine boring (BTA, Ra 1.6–3.2 μm), and reaming (gun drilling, Ra 0.8–1.6 μm). Roller burnishing is the most cost-effective for BTA-drilled holes, adding minimal cycle time.