Skiving and Roller Burnishing (SRB) is the standard finishing process for hydraulic cylinder tubes and other precision bores that require mirror-like surface finishes combined with high geometric accuracy. A single SRB pass replaces multiple secondary operations — rough boring, finish boring, and honing — while producing a surface that is functionally superior to honed surfaces in many applications.
The process combines two operations in one tool pass:
- Skiving — a floating reamer with multiple carbide blades removes a controlled amount of stock (0.2–1.6 mm radial depth)
- Roller burnishing — hardened rollers cold-form the surface, compressing micro-peaks into valleys
The result is a bore with surface finish down to Ra 0.05 µm, tolerance IT7–IT8, and a work-hardened surface layer with improved wear resistance.
How SRB Works
The Combined Tool
An SRB tool consists of two sections mounted on a single boring bar:
Skiving section (forward):
- 2–6 carbide cutting blades positioned around the tool circumference
- Blades are expandable — retracted during tool insertion, expanded hydraulically or pneumatically for cutting, retracted for withdrawal
- Each blade removes a thin layer of material (typically 0.2 mm radial depth per blade)
Roller burnishing section (rearward):
- 8–20 hardened rollers (number depends on tool diameter)
- Rollers are spring-loaded or hydraulically loaded against the bore wall
- They cold-form the surface through plastic deformation — peaks are compressed into valleys rather than being cut away
The tool typically operates in forward boring mode: the skiving blades cut on the forward stroke, and the burnishing rollers follow immediately behind, processing the freshly cut surface. On the return stroke, both blades and rollers retract to prevent damage.
Material Flow During Burnishing
The burnishing process is fundamentally different from cutting. The rollers apply pressure exceeding the material’s yield strength, causing plastic flow of the surface layer. The mechanism:
- The roller contacts a surface peak and compresses it
- Material flows plastically into adjacent valleys
- The surface layer undergoes work hardening
- Compressive residual stress is induced in the surface
Research shows that burnishing increases surface hardness by up to 50% in steel and produces a compressive residual stress layer that improves fatigue life by up to 300%.
Cutting Parameters
Speed
| Operation | Recommended Range | Notes |
|---|---|---|
| Skiving | 150–300 m/min | Optimal surface finish at 250–300 m/min |
| Hot-rolled pipe | Up to 0.3 m/min | Reduced feed speed due to scale and hardness variation |
| Cold-drawn pipe | Up to 1 m/min | Consistent material allows higher speeds |
Feed Rate
| Operation | Range | Notes |
|---|---|---|
| Skiving | 1.2–2.5 mm/rev (typical) | Lower feed = better surface finish |
| Combined SRB | 1–6 mm/rev | Typical operating range |
| Optimized burnishing | 0.5 mm/rev | Achieved Ra 0.130 µm in published research |
Depth of Cut (Stock Allowance)
The radial depth of cut per side depends on tool diameter:
| Tool Diameter | Maximum Radial Depth (AP) |
|---|---|
| 38–56.9 mm | 0.6 mm |
| 57–90.9 mm | 1.0 mm |
| 91–306 mm | 1.6 mm |
Total stock removal in one pass can reach 6 mm on diameter. For most applications, 0.5–1.5 mm on diameter is typical.
Summary of Recommended Parameters
| Parameter | SRB Tool | Scraping Tool Only |
|---|---|---|
| Cutting speed (Vc) | 250–300 m/min | 50–150 m/min |
| Feed rate (fn) | 1.2–2.5 mm/rev | 1–3 mm/rev |
| Depth of cut (ap) | 0.5–2.0 mm | up to 10 mm |
| Motor power requirement | 22–45 kW (diameter dependent) | — |
| Coolant flow rate | 3 × D (L/min) | — |
| Resulting surface finish | Ra 0.05–0.20 µm | Ra 4–10 µm |
Surface Finish Capabilities
Achievable Range
| Condition | Ra (µm) | Rz (µm) |
|---|---|---|
| Before SRB (BTA-drilled or bored) | 1.6–6.3 | 12–50 |
| After SRB (standard production) | 0.2–0.4 | 2–4 |
| After SRB (optimized parameters) | 0.05–0.20 | 0.5–2 |
| With subsequent light honing | 0.05–0.15 | 0.5–1 |
Published research using optimized parameters (speed 505 rpm, feed 0.5 mm/rev, depth 0.04 mm) achieved Ra 0.130 µm with surface hardness of 64 HV increase and circularity error of 0.042 mm.
Bearing Ratio Comparison
The bearing ratio (material ratio curve) is the most functionally significant difference between SRB and honing:
| Property | SRB Surface | Honed Surface |
|---|---|---|
| Bearing ratio curve | “Fat” — high load-bearing material at surface | “Skinny” — low load-bearing material at surface |
| Surface profile | Plateau-like, compressed | Peaked, crosshatched |
| Seal contact area | Higher — reduced seal wear | Lower — point contact on peaks |
| Oil retention | Moderate — grooves retain lubrication | High — crosshatch pattern retains oil |
| Wear resistance | Better — work-hardened surface | Moderate |
The plateau-like finish from SRB provides 50% less seal wear after 100 hours of testing compared to honed surfaces, according to published comparative data. The compressed surface layer eliminates the “stick-slip” phenomenon in slow-displacement hydraulic cylinders.
Dimensional Accuracy
| Parameter | Achievable Value |
|---|---|
| Diameter tolerance | IT7–IT8 |
| Size control | Within 0.05 mm on average |
| Roundness | 0.036 mm per 200 mm diameter |
| Cylindricity | 0.05 mm per 500 mm length |
| Surface finish | Ra 0.2–0.4 µm (standard); Ra 0.05 µm (optimized) |
Comparison with Alternative Processes
| Factor | SRB | Honing | Grinding | Lapping |
|---|---|---|---|---|
| Surface finish (Ra) | 0.05–0.4 µm | 0.05–0.6 µm | 0.2–0.8 µm | 0.025–0.1 µm |
| Production speed | Fastest — 70–90% faster than honing | Slow | Moderate | Slowest |
| Surface hardness increase | Up to 50% | None | Possible thermal damage | None |
| Material removal rate | High (up to 6 mm/pass) | Low (0.01–0.05 mm/pass) | Low | Very low |
| Bearing ratio | High (plateau) | Low (peaked) | Moderate | High |
| Compressive residual stress | Yes | No | Variable | No |
| Equipment cost | High | Low–Moderate | High | Low |
| Abrasive residue | None (cutting only) | Yes (abrasive grit) | Yes | Yes |
| Typical cycle time (1 m tube) | 2–5 minutes | 30–60 minutes | — | — |
SRB vs Honing — Detailed Comparison
SRB and honing are the two most common finishing processes for hydraulic cylinder tubes. Each has distinct advantages.
When SRB is the Better Choice
- High-volume production of cylinders with consistent bore diameters
- Thick-walled tubes where the wall thickness exceeds 10% of the bore diameter (the burnishing effect requires adequate wall support)
- Applications where seal wear is critical — the plateau-like finish reduces seal friction and wear
- When a clean process is required — no abrasive grit remains embedded in the surface
- When surface hardness improvement is beneficial — the cold-working effect increases surface hardness
When Honing is the Better Choice
- Low-volume production or repair work — lower equipment investment
- Thin-walled tubes — the burnishing pressure can distort thin walls
- Applications requiring specific oil retention — the crosshatch pattern of honing provides better lubrication retention
- When precise control of surface finish is required — honing offers more predictable and controllable surface texture
- Correcting bore geometry errors — honing can correct roundness and straightness issues more effectively
Combined Approach
Many manufacturers use SRB for the initial finishing pass (removing the BTA drilling or rough boring marks) followed by a light honing pass to create the crosshatch oil-retention pattern. This combination achieves the production speed of SRB with the surface texture control of honing.
Tool Design
Skiving Blades
- Material: Carbide (ISO K15–K25 for steel)
- Coating: TiAlN or AlTiN recommended
- Number per tool: 2, 3, 4, or 6 blades depending on diameter
- Blade expansion: Hydraulic (0.5–1 MPa) or pneumatic actuation
- Blade retraction: Automatic on return stroke to prevent damage
Burnishing Rollers
- Material: Hardened tool steel or carbide
- Number per tool: 8 rollers (38–50 mm dia.) to 20 rollers (200–300 mm dia.)
- Roller loading: Spring or hydraulic pressure
- Surface condition: Polished to mirror finish (transfers to bore surface)
Tool Diameter Range
| Tool Diameter | Number of Rollers | Max AP (radial) |
|---|---|---|
| 38–50 mm | 8 | 0.6 mm |
| 50–65 mm | 8 | 0.6 mm |
| 65–140 mm | 12 | 1.0 mm |
| 140–170 mm | 12 | 1.0 mm |
| 170–200 mm | 16 | 1.6 mm |
| 200–300 mm | 20 | 1.6 mm |
Machine Requirements
Key Machine Features
A dedicated SRB machine differs from a BTA drilling machine in several important ways:
| Feature | SRB Machine Requirement | Reason |
|---|---|---|
| Workholding | Cone clamps (not 3-jaw chucks) | Prevents tube deformation during burnishing |
| Spindle power | 75–110 kW (large machines) | High torque for burnishing cold-working |
| Spindle speed | 60–1,000 rpm (4-speed gearbox) | Wide speed range for different tube diameters |
| Feed system | Stepless, 5–5,000 mm/min | Smooth feed critical for surface finish |
| Coolant system | High flow (1,000+ L/min), 25 bar, <20 µm filtration | Heat removal and chip evacuation |
| Rotary union | Hydraulic/pneumatic through-tool | Blade expansion and retraction |
| Guideways | Induction hardened, precision ground | Maintains straightness over long strokes |
Commercial Machine Specifications
Profimach SRB-Series (Dedicated SRB machines):
| Parameter | Range |
|---|---|
| Boring diameter | 30–600 mm (up to 850 mm special) |
| Processing length | Up to 18 m |
| Tolerance | IT7–IT8 |
| Surface finish | Ra 0.05–0.20 µm |
| Roundness | 0.036 mm per 200 mm |
| Cylindricity | 0.05 mm per 500 mm |
Premach TGK50/TGK63 Series:
| Parameter | TGK50 | TGK63 |
|---|---|---|
| Boring diameter | 120–500 mm | 120–630 mm |
| Processing depth | 1–12 m | 1–12 m |
| Spindle speed | 60–1,000 rpm (4 gears) | 60–1,000 rpm |
| Main motor power | 75–110 kW | 75–110 kW |
| Feed speed | 5–5,000 mm/min | 5–5,000 mm/min |
| Coolant pressure | 2.5 MPa | 2.5 MPa |
| Coolant flow | 2,000 L/min (4 groups) | 2,000 L/min |
| CNC system | Siemens 808 | Siemens 808 |
| Surface finish | Ra 0.2–0.4 µm | Ra 0.2–0.4 µm |
| Filtration | 20 µm (magnetic + paper) | 20 µm |
Applications
Primary Application: Hydraulic Cylinder Tubes
SRB is the standard finishing process for hydraulic cylinder tubes. The process produces:
- Seal-compatible surface finish — Ra 0.1–0.3 µm is the standard for hydraulic cylinder seals
- Maximum seal life — the plateau finish reduces seal lip wear
- Consistent bore geometry — critical for piston seal performance at high pressures
- Elimination of stick-slip — the compressed surface prevents the adhesion-friction cycle that causes jerky motion at low speeds
Other Applications
- Pneumatic cylinder tubes — where low friction and seal life are required
- Injection molding machine barrels — where a smooth, wear-resistant bore surface is needed
- Lift cylinder tubes — for mobile hydraulic equipment
- Shock absorber tubes — where surface finish affects damping characteristics
- Pressure vessel bores — requiring surface integrity and compressive residual stress
Material Compatibility
SRB is effective on most ferrous materials:
| Material | SRB Suitability | Notes |
|---|---|---|
| Carbon steel (1045, 1026) | Excellent | Most common, good hardness increase |
| Alloy steel (4140) | Excellent | Higher strength, good burnishing response |
| Stainless steel (304, 316) | Good | Requires higher roller pressure |
| Stainless steel (17-4 PH) | Moderate | High hardness limits deformation |
| Cast iron | Good | Graphite provides natural lubrication |
| Aluminum | Good | Lower roller pressure required |
| Copper alloys | Good | Soft, responds well to burnishing |
| Heat-treated steel (> 40 HRC) | Limited | Hardness restricts plastic deformation |
For effective burnishing, the workpiece material should have sufficient ductility to undergo plastic deformation. Hardness above approximately 40 HRC limits the burnishing effect. The wall thickness should generally be at least 10% of the bore diameter to prevent tube distortion during burnishing.
Advantages Summary
| Advantage | Impact |
|---|---|
| One-pass finishing | Eliminates multiple secondary operations |
| 70–90% faster than honing | Reduces cycle time from hours to minutes |
| No abrasive residue | Clean process — no embedded grit |
| Surface hardness increase | Up to 50% improvement |
| Compressive residual stress | Fatigue life improvement up to 300% |
| Superior bearing ratio | Reduced seal wear by 50% |
| Consistent results | CNC-controlled, operator-independent |
| Versatile diameter range | 30–850 mm |
Limitations
| Limitation | Cause |
|---|---|
| Not suitable for thin-walled tubes | Burnishing pressure distorts thin walls |
| Limited geometry correction | SRB follows existing bore; does not correct straightness errors as effectively as honing |
| Higher equipment investment | Dedicated SRB machine costs $200,000–$800,000 |
| Tool cost per tube | SRB tools are more expensive than honing stones |
| Limited to bores > 30 mm | Below 30 mm, tool clearance becomes insufficient |
| Material hardness limit | Above ~40 HRC, burnishing effect diminishes |
For related reading, see the Deep Hole Drilling for Hydraulic Cylinders, the Surface Finish for Deep Hole Drilling Guide, and the Deep Hole Drilling Equipment Guide.