Appearance
Before skiving and roller burnishing became the standard, a hydraulic cylinder bore was roughed by BTA drilling, then finish-bored, then honed for hours to achieve the required surface finish. The process was slow, labour-intensive, and produced abrasive sludge that required disposal. Skiving and roller burnishing replaced all of that with a single combined tool that cuts and cold-forms the bore in one pass at 200 m/min. The skiving blades remove the rough BTA surface in a clean cutting action. The roller burnishing elements that follow immediately press the surface peaks into the valleys, creating a mirror-like finish with compressive residual stress that improves fatigue life. The process is 8–20× faster than honing, achieves better seal performance, and leaves no abrasive residue. It is one of the most significant productivity improvements in the history of deep hole finishing.
The SRB Process
Skiving and roller burnishing (SRB) is a combined single-pass finishing process applied to pre-drilled or pre-bored holes:
| Stage | Tool Element | Action | What It Achieves |
|---|---|---|---|
| 1 | Skiving blades (carbide) | Cutting | Removes 0.2–0.5 mm stock, corrects ovality and straightness |
| 2 | Roller burnishing elements | Cold forming | Plastic deformation of surface peaks into valleys |
| 3 | (Simultaneous) | Combined | Final bore with finished surface in one pass |
Process Sequence
The SRB tool is mounted on a boring bar and pushed or pulled through the pre-drilled bore:
- Tool enters bore — skiving blades expanded hydraulically to programmed diameter
- Skiving stage — multi-blade carbide cutters remove the machined surface from the rough bore
- Roller burnishing stage — hardened rollers (typically 6–12 rollers) press the surface at controlled force
- Tool exits — bore completed with finished diameter and surface
The entire process is typically performed in a single pass on a dedicated skiving machine (horizontal or vertical).
Skiving Parameters
Cutting Parameters
| Parameter | Typical Range | Notes |
|---|---|---|
| Depth of cut (radial) | 0.2–0.5 mm per side | Minimum 0.15 mm to ensure clean cut |
| Cutting speed | 150–300 m/min | Higher speeds improve surface finish |
| Feed rate (per blade) | 1–6 mm/rev | Multi-blade tools multiply effective feed |
| Number of blades | 2–6 (typical) | More blades = higher effective feed |
| Insert geometry | Positive rake, wiper edge | Wiper edge essential for surface finish |
Skiving Tool Design
| Feature | Standard | High-Performance |
|---|---|---|
| Insert type | Carbide, TiAlN coated | PCBN or PCD (for non-ferrous) |
| Blade mounting | Floating (self-centring) | Hydraulic expansion |
| Coolant delivery | Through-tool, high pressure | Internal + external nozzles |
| Insert life (steel) | 500–2,000 m of bore | Depends on material and parameters |
Skiving Quality Achievable
| Quality Metric | After Skiving (before burnishing) |
|---|---|
| Surface finish (Ra) | 1.0–3.2 µm |
| Diameter tolerance | IT8–IT9 |
| Roundness | ≤ 0.036 mm / 200 mm diameter |
| Cylindricity | ≤ 0.05 mm / 500 mm length |
Roller Burnishing Parameters
Burnishing Parameters
| Parameter | Typical Range |
|---|---|
| Burnishing speed | 150–300 m/min (same as skiving speed) |
| Feed rate | 1–6 mm/rev (same as skiving feed) |
| Roller burnishing pressure | 10–30 kN (tool-size dependent) |
| Penetration depth (plastic deformation) | 0.01–0.03 mm |
| Number of rollers | 6–12 (depending on diameter) |
| Roller hardness | ≥ 62 HRC |
Surface Improvement from Burnishing
| Material | As-Skived Ra | After Burnishing Ra | Improvement |
|---|---|---|---|
| Carbon steel (ST52) | 1.5–3.0 µm | 0.08–0.20 µm | 10–20× |
| Alloy steel (4140, 4340) | 1.0–2.5 µm | 0.08–0.20 µm | 10–15× |
| Stainless steel | 1.5–3.5 µm | 0.10–0.30 µm | 10–15× |
| Aluminium | 1.0–2.0 µm | 0.05–0.15 µm | 15–20× |
Metallurgical Effects
| Effect | Magnitude | Benefit |
|---|---|---|
| Surface hardness increase | Up to 50% (steel) | Improved wear resistance |
| Compressive residual stress | −200 to −800 MPa | Improved fatigue life |
| Micro-crack closure | Surface cracks sealed | Improved corrosion resistance |
| Grain refinement | Near-surface grain deformation | Improved surface integrity |
Combined SRB Tooling
Tool Types
| Tool Design | Diameter Range | Configuration | Supplier |
|---|---|---|---|
| Floating skiving + fixed rollers | 30–150 mm | Skiving head followed by roller cage | ECOROLL |
| Hydraulic expansion + rollers | 50–300 mm | Expandable skiving + burnishing | UNISIG, SUGINO |
| Modular (separate skive and burnish) | 15–1,000 mm | Interchangeable heads | Various |
| Single-pass combined (push type) | 25–200 mm | Pulled through bore on boring bar | SUGINO SV series |
Key Design Features
| Feature | Purpose |
|---|---|
| Floating blade mounting | Self-centring in bore, compensates for bore wander |
| Hydraulic blade expansion | Precise diameter control during skiving |
| Roller feed adjustability | Controls burnishing force independent of cutting force |
| Through-tool coolant | Cools cutting edge, lubricates rollers, evacuates chips |
| Wiper inserts | Cleans bore surface before burnishing |
SRB vs. Honing
| Factor | Skiving + Roller Burnishing | Honing |
|---|---|---|
| Cycle time (2 m bore) | 15–30 minutes | 2–6 hours |
| Surface finish (Ra) | 0.05–0.20 µm | 0.05–0.20 µm |
| Surface pattern | Smooth, plateau | Crosshatch |
| Oil retention | Moderate (low porosity) | Excellent (crosshatch retains oil) |
| Surface hardness | Increased (50% harder) | Unchanged |
| Residual stress | Compressive (−200 to −800 MPa) | Usually tensile or neutral |
| Diameter tolerance | H8–H9 | H7–H8 |
| Geometric correction capability | Good (straightness, roundness) | Excellent |
| Abrasive waste | None (cutting chips only) | Abrasive sludge |
| Per-part cost (high volume) | Low | High |
| Tooling cost | Moderate | Moderate |
| Machine cost | High | Moderate |
When to Choose Each
| Application | Recommended | Reason |
|---|---|---|
| Hydraulic cylinder, high volume | SRB | Speed, cost, compressive stress |
| Landing gear, aerospace | SRB | Fatigue life, surface hardness |
| Low-volume custom cylinder | Honing | Flexibility, lower setup cost |
| Repair / refurbishment | Honing | Removes minimal material |
| High-pressure sealing surface | SRB | Plateau surface improves seal life |
| Bearing surface (oil retention needed) | Honing | Crosshatch retains lubricant better |
Materials and Achievable Quality
Recommended Parameters by Material
| Material | Cutting Speed (m/min) | Feed (mm/rev) | Depth of Cut (mm) | Burnishing Pressure |
|---|---|---|---|---|
| Carbon steel (ST52, C45) | 200–300 | 2–5 | 0.3–0.5 | Moderate |
| Alloy steel (4140, 4340, 300M) | 150–250 | 2–4 | 0.2–0.4 | Moderate-high |
| Stainless steel (304, 316) | 150–200 | 2–4 | 0.2–0.4 | Moderate |
| Aluminium (6061, 7075) | 300–500 | 3–6 | 0.3–0.5 | Low-moderate |
| Copper alloys | 200–350 | 2–5 | 0.2–0.4 | Low-moderate |
| Titanium (Ti-6Al-4V) | 60–100 | 1–3 | 0.15–0.25 | Moderate |
Achievable Quality Summary
| Parameter | Typical | Best (with optimal parameters) |
|---|---|---|
| Surface finish (Ra) | 0.1–0.3 µm | 0.05–0.08 µm |
| Diameter tolerance | H8–H9 | H7 |
| Roundness | 0.02–0.04 mm | 0.01 mm |
| Cylindricity | 0.05 mm / 500 mm | 0.02 mm / 500 mm |
| Surface hardness increase | 30–50% | 60% |
| Compressive residual stress | −300 to −600 MPa | −800 MPa |
Surface Finish Standards
| Quality Level | Ra (µm) | Rz (µm) | Application |
|---|---|---|---|
| Standard hydraulic | 0.3–0.4 | 2.0–3.0 | General hydraulic cylinders |
| Precision hydraulic | 0.15–0.25 | 1.0–2.0 | High-cycle actuators |
| Aerospace / landing gear | 0.1–0.2 | 0.8–1.5 | Landing gear struts |
| Mirror finish | 0.05–0.1 | 0.4–0.8 | High-speed seal surfaces |
Common Process Issues
| Problem | Cause | Solution |
|---|---|---|
| Spiral marks on bore | Feed too high, chip packing | Reduce feed, check chip breaker geometry |
| Inconsistent surface finish | Worn skiving inserts | Replace inserts at scheduled interval |
| Burnishing rollers marking bore | Contamination (chips on roller path) | Improve coolant filtration, clean bore before SRB |
| Diameter oversize | Skiving depth incorrect | Check hydraulic expansion calibration |
| Chatter marks | Tool vibration, insufficient damping | Reduce speed, check boring bar support |
| Roller burnishing not improving finish | Insufficient stock for skiving | Increase depth of cut to minimum 0.2 mm |
| Flaking / surface peel | Excessive burnishing pressure | Reduce roller feed or pressure |
FAQ
Q: What is skiving and roller burnishing (SRB)? SRB is a combined single-pass finishing process for pre-drilled bores. A tool with carbide skiving blades and roller burnishing elements is pushed or pulled through the bore, simultaneously cutting to size and cold-forming the surface to a mirror finish.
Q: How does SRB compare to honing in cycle time? SRB is 8–20× faster than honing. A 2-metre hydraulic cylinder bore that takes 15–30 minutes with SRB requires 2–6 hours with honing. The speed advantage comes from the single-pass combined tool and the much higher material removal rate of the skiving stage.
Q: What surface finish can SRB achieve? SRB achieves Ra 0.05–0.30 µm depending on the material and parameters. For standard hydraulic cylinders, Ra 0.2–0.4 µm is typical. With optimal parameters on steel, Ra 0.08–0.15 µm is achievable.
Q: Does roller burnishing increase surface hardness? Yes. The cold working from roller burnishing increases surface hardness by up to 50% in steel. The plastic deformation creates a work-hardened layer typically 0.1–0.3 mm deep.
Q: What standard governs SRB parameters? VDI 3209 Blatt 2 (Deep hole boring — Approximate values for skiving and roller burnishing of bores) is the governing standard. It provides recommended speeds, feeds, depths of cut, and burnishing pressures for a range of materials and diameters (15–1,000 mm).
Q: What is the difference between combined SRB and separate skiving and burnishing? Combined SRB uses a single tool with both skiving blades and roller burnishing elements, completing both operations in one pass. Separate operations use two tools and two passes. Combined SRB is faster and more common in production.
Q: Can SRB be applied to blind holes? SRB is designed for through-holes. The tool enters one end and exits the other. For blind holes, alternative processes such as honing or internal grinding must be used.
Q: What types of cylinders use SRB finishing? Hydraulic cylinders (all types), pneumatic actuators, landing gear struts, telescopic booms, pump housings, and any precision bore that requires a smooth surface with good seal performance. SRB is particularly valued for aerospace landing gear where the compressive residual stress improves fatigue life.
Q: Does SRB remove or correct bore geometry errors? SRB corrects ovality and improves straightness (the skiving stage removes 0.2–0.5 mm stock evenly). However, it cannot correct severe bore wander or eccentricity from the previous drilling operation — those must be addressed before SRB.
Q: What is the maximum bore length for SRB? Dedicated skiving machines can process bores up to 18,000 mm (18 m) in length. Diameter range is 15–1,000 mm. The limiting factor is the boring bar rigidity and the machine's stroke length.