Appearance
A hydraulic cylinder manufacturer producing 80 mm ID × 2,000 mm long bore tubes in 1026 steel (cold-drawn seamless, 200–240 HB) was using a two-step finishing process: BTA drilling to create the bore (leaving 0.5–1.0 mm stock for finishing), followed by roller burnishing in a separate machine to achieve the final surface finish of Ra 0.50 µm. The burnishing-only process had two fundamental limitations: it could not correct straightness errors from the drilling step (the bore remained bowed by 0.12–0.20 mm), and the burnishing interference — the amount the rollers compressed the bore surface — was inconsistent because the drilled bore diameter varied by ±0.10 mm. The result was a 15% scrap rate from surface finish non-conformance and a 20% rework rate for bores whose straightness exceeded the 0.10 mm assembly tolerance. By replacing the two-step drilling-then-burnishing process with a single-pass combined skiving and burnishing process — using a combined tool head with a carbide skiving blade (0.15 mm depth of cut) followed by six hardened burnishing rollers (0.04 mm interference per roller, 48 µm total diametral compression) — the manufacturer achieved: consistent surface finish of Ra 0.12–0.20 µm (well within the Ra 0.50 µm requirement), bore straightness corrected to within 0.05 mm over the full 2,000 mm length (the skiving blade cut the bore to a true axis, removing the geometric errors from the drilling step), diameter tolerance of H8 (0–0.046 mm), and a 25% reduction in cycle time because the two finishing steps were combined into one. The skiving blade required changing after 500–600 meters of cutting (approximately 250–300 tubes), and the burnishing rollers lasted for 8,000–12,000 meters (4,000–6,000 tubes) before requiring re-grinding. The total tooling cost per tube was $1.80, compared to $2.40 for the previous process — a 25% reduction driven by the elimination of the separate burnishing operation and its associated setup and handling costs.
Process Principles
How Skiving Works
Skiving is a cutting process that removes a thin layer of material from the bore wall using a single-point cutting tool (the skiving blade) mounted in a tool head that is pulled or pushed through the bore. The skiving blade is positioned at a specific offset from the tool head axis, and as the tool head rotates and advances, the blade cuts a helical swath of material from the bore wall. The cutting geometry of a skiving blade is similar to a boring tool: the blade has a defined rake angle (typically 8–15° positive), relief angle (6–10°), and nose radius (0.4–1.2 mm).
Skiving differs from boring in two critical aspects: the depth of cut is very small (0.05–0.30 mm per pass, compared to 0.5–5.0 mm for boring), and the tool head is supported by guide pads (similar to BTA drill heads) that contact the existing bore surface and guide the tool head along the bore axis. The guide pads ensure that the skiving blade follows the existing bore axis rather than creating a new axis, which means that skiving can correct straightness errors only to the extent that the guide pads can stabilize the tool head against the bore wall.
The material removal mechanism in skiving produces a characteristic chip form: thin, ribbon-like chips with a width equal to the skiving depth of cut and a thickness equal to the feed per revolution. These chips are easily evacuated through the tool head chip mouth, and the low cutting forces (typically 200–800 N for 40–100 mm diameter bores) minimize tool deflection and heat generation.
How Burnishing Works
Burnishing is a cold-forming process that plastically deforms the bore surface rather than cutting it. The burnishing rollers — hardened steel or carbide rollers (58–65 HRC) with a smooth, polished surface — are pressed against the bore wall under controlled interference (typically 0.02–0.08 mm per roller). As the tool head rotates and advances, the rollers compress the surface peaks into the surface valleys, reducing the surface roughness from Ra 1.0–3.0 µm (as-skived) to Ra 0.05–0.40 µm (burnished).
The burnishing process produces three simultaneous effects on the bore surface: geometric smoothing (the rollers compress the surface profile, reducing Ra by 80–95%), work hardening (the plastic deformation increases the surface hardness by 20–40%, improving wear resistance), and residual compressive stress (the burnishing action creates a compressive stress layer of 200–600 MPa extending 0.05–0.20 mm beneath the surface, improving fatigue life).
The burnishing interference — the amount of radial compression applied by each roller — is the most critical process parameter. If the interference is too low (below 0.01 mm per roller), the rollers do not make consistent contact with the surface and the finish is irregular. If the interference is too high (above 0.10 mm per roller), the rollers may overload, causing surface tearing, flaking, or roller damage. The optimal interference depends on the workpiece material yield strength and hardness — harder materials require less interference because they plastically deform at lower compression.
Combined Skiving and Burnishing
In the combined process, the skiving blade and burnishing rollers are integrated into a single tool head. The skiving blade is positioned ahead of the rollers (in the direction of tool travel) and removes a controlled layer of material from the bore wall. The burnishing rollers follow immediately behind the skiving blade and compress the freshly cut surface.
The critical design feature of the combined tool head is the diametral relationship between the skiving blade setting, the roller setting, and the guide pad position. The skiving blade must be set to cut to a diameter that is slightly smaller than the burnished final diameter — the difference between the skived diameter and the burned diameter is the total burnishing interference. For a typical hydraulic cylinder tube: the skiving blade is set to cut to 79.90 mm (for a final bore diameter of 80.00 mm), and the burnishing rollers are set to compress the surface by 0.10 mm total (0.05 mm per roller for a two-roller head, or 0.025 mm per roller for a four-roller head), producing a final bore diameter of 80.00 mm.
Tool Design and Configuration
Combined Tool Head Types
Skiving and burnishing tool heads are available in several configurations depending on the bore diameter, production volume, and quality requirements.
Multi-roller design (4–8 rollers) — The most common configuration for hydraulic cylinder production (40–300 mm bore diameter). Multiple rollers (typically 4–8) are arranged circumferentially around the tool head, each mounted on a tapered roller bearing or needle bearing. The rollers are spring-loaded or hydraulically loaded to maintain consistent contact with the bore wall. The skiving blade is mounted in a pocket on the tool head periphery, positioned approximately 30–60° ahead of the first roller in the rotation direction. Multi-roller heads provide the best surface finish (Ra 0.05–0.20 µm achievable) and the highest production rates.
Single-roller design — Used for small-diameter bores (15–40 mm) where multi-roller heads cannot fit. A single roller is mounted on one side of the tool head, and a support pad is mounted opposite. The roller applies burnishing pressure on one side while the support pad reacts the force on the opposite side. Single-roller heads produce finish of Ra 0.15–0.40 µm and are slower than multi-roller heads but can operate in smaller bores.
Combined tool with adjustable rollers — For high-precision applications, the rollers are mounted on eccentric bushings or hydraulic pistons that allow the burnishing interference to be adjusted without removing the tool from the bore. Adjustable heads are used when the bore diameter or material hardness varies between parts, requiring different interference settings.
Roller Material and Geometry
Burnishing rollers are manufactured from through-hardened tool steel (AISI D2, AISI 52100, or powder metal grades) hardened to 58–65 HRC, or from tungsten carbide (for high-wear applications). The roller surface is ground and polished to Ra < 0.05 µm to ensure that the roller surface finish is replicated on the bore surface.
The roller profile — the shape of the roller in cross-section — is designed to control the pressure distribution at the roller-bore contact point. The most common profiles are: cylindrical (constant diameter across the roller width — produces uniform pressure distribution), crowned (slightly larger diameter at roller center — concentrates pressure at center and reduces edge loading), and tapered (diameter decreases toward roller edges — produces higher pressure at center). The roller width is typically 5–15 mm for multi-roller heads and 10–25 mm for single-roller heads.
Process Parameters and Quality Outcomes
Process Parameter Guidelines
The table below provides recommended starting parameters for skiving and burnishing of common bore materials.
| Workpiece Material | Skiving Depth (mm) | Burnishing Interference per Roller (mm) | Feed Rate (mm/rev) | Cutting Speed (m/min) | Skiving Insert Grade | Expected Ra (µm) | Expected IT Grade |
|---|---|---|---|---|---|---|---|
| Carbon steel (150–250 HB) | 0.10–0.20 | 0.03–0.06 | 0.08–0.20 | 60–120 | P20–P30 (TiN/TiAlN) | 0.08–0.25 | IT7–IT8 |
| Carbon steel (250–350 HB) | 0.08–0.15 | 0.02–0.04 | 0.06–0.15 | 50–90 | P25–P35 (TiAlN) | 0.10–0.30 | IT7–IT8 |
| Alloy steel (300–400 HB) | 0.08–0.12 | 0.02–0.04 | 0.06–0.12 | 40–70 | P30–P40 (AlTiN) | 0.12–0.35 | IT7–IT8 |
| Stainless steel (austenitic) | 0.12–0.20 | 0.04–0.08 | 0.06–0.15 | 40–80 | M10–M20 (AlCrN) | 0.10–0.30 | IT7–IT8 |
| Cast iron (200–300 HB) | 0.10–0.20 | 0.02–0.05 | 0.10–0.25 | 60–120 | K10–K20 | 0.05–0.20 | IT6–IT7 |
| Aluminum alloys | 0.15–0.30 | 0.05–0.10 | 0.10–0.30 | 150–300 | K10–K20 (polished) | 0.05–0.15 | IT6–IT7 |
| Brass / bronze | 0.10–0.20 | 0.03–0.06 | 0.08–0.20 | 80–150 | K10–K20 | 0.05–0.15 | IT6–IT7 |
Quality Outcomes and Capability
Skiving and burnishing achieves the following quality outcomes in deep bores under properly controlled conditions:
Surface finish — Ra 0.05–0.40 µm, with typical production values of Ra 0.10–0.25 µm for carbon steel. The surface finish after burnishing is 5–20× better than after BTA drilling (Ra 0.8–2.0 µm) and 3–10× better than after honing (Ra 0.2–0.8 µm). The burnished surface has a characteristic mirror-like appearance and reflects light evenly under visual inspection.
Diameter tolerance — IT6–IT8 (e.g., 0–0.030 mm for 50 mm diameter, 0–0.046 mm for 80 mm diameter). The skiving blade establishes the bore diameter, and the burnishing interference expands the bore by a consistent amount determined by the material yield strength. The process capability (Cpk) for bore diameter is typically 1.33–1.67 when the skiving blade condition is maintained and the workpiece material hardness variation is within ±15 HB.
Straightness correction — Skiving can correct straightness errors from the drilling step by 0.03–0.10 mm, depending on the bore diameter and the severity of the initial deviation. The correction is achieved because the skiving blade removes a layer of material from the bore wall, and the blade follows the tool head axis (established by the guide pads) rather than the existing bore axis. The limitation is that the guide pads must have sufficient contact with the bore wall to stabilize the tool head — if the bore is so crooked that the guide pads lose contact on one side, the tool head will follow the crooked bore rather than correcting it.
Roundness — Roundness error after skiving and burnishing is typically 0.005–0.020 mm, limited primarily by the roundness of the tool head and the condition of the spindle bearings on the machine.
Process Limitations
Skiving and burnishing has several limitations that should be considered when selecting the process: the minimum bore diameter is approximately 15 mm (limited by the combined tool head size); the process cannot correct straightness errors exceeding 0.30–0.50 mm (the skiving blade's depth of cut is limited by the guide pad engagement requirements); the burnishing process work-hardens the surface, which may be undesirable for applications requiring subsequent machining of the bore (e.g., threading, grooving); and the burnishing rollers require periodic re-grinding (typically after 8,000–15,000 meters of burnishing, depending on the workpiece material and roller material).
FAQ
What is the difference between skiving/burnishing and honing?
Skiving and burnishing combines a cutting process (skiving) with a cold-forming process (burnishing), while honing uses bonded abrasive stones to cut the surface through abrasive action. Key differences: skiving/burnishing achieves Ra 0.05–0.40 µm in a single pass at feed rates of 0.08–0.30 mm/rev, while honing requires multiple reciprocating passes at slower material removal rates; skiving/burnishing produces a work-hardened surface with compressive residual stress, while honing may produce a surface with tensile residual stress or neutral stress; skiving/burnishing corrects straightness errors more effectively because the skiving blade cuts the bore to a new axis, while honing follows the existing bore axis; and skiving/burnishing uses a simpler machine tool (lathe or boring machine) while honing requires a dedicated honing machine with reciprocating spindle. Honing is preferred for: very tight roundness tolerances (below 0.005 mm), complex bore geometries (keyways, blind holes, interrupted bores), and materials that are difficult to burnish (highly abrasive materials, materials with very low ductility).
What burnishing interference should be used for hydraulic cylinder tubes?
For hydraulic cylinder tubes in carbon steel (200–280 HB, cold-drawn seamless or normalized), the recommended burnishing interference is 0.03–0.06 mm per roller for a multi-roller tool head (4–8 rollers), with the total diametral interference (sum of interference from all rollers) kept below 0.20 mm. The interference should be set at the lower end (0.03 mm per roller) for harder materials (250–280 HB) and the higher end (0.05–0.06 mm per roller) for softer materials (200–240 HB). The bore diameter after burnishing should be 0.02–0.06 mm larger than the skived diameter, and this expansion should be accounted for in the skiving blade setting. The proper interference is verified by measuring the bore diameter before and after burnishing: if the diameter increase is less than 0.02 mm, the interference is too low; if the increase exceeds 0.08 mm for a multi-roller head, the interference is too high and risks surface tearing.
Can skiving and burnishing correct bore straightness errors?
Yes, skiving and burnishing can correct bore straightness errors by 0.03–0.10 mm, depending on the bore diameter, the initial straightness deviation, and the tool head design. The correction mechanism is the skiving blade: as the blade removes a thin layer from the bore wall, it cuts to a new axis established by the tool head guide pads and the machine tool axis. The maximum correctable straightness error is limited by the depth of cut — if the straightness error exceeds the skiving depth of cut, the blade will cut only on one side of the bore at the point of maximum deviation, producing an incomplete cut. For typical hydraulic cylinder tubes with a skiving depth of 0.15 mm, straightness errors up to 0.15 mm can be fully corrected in one pass; errors of 0.15–0.30 mm may require two passes. Straightness errors exceeding 0.30 mm typically require a rough skiving pass followed by a finish skiving pass, or the BTA drilling process must be improved before skiving.
How long do burnishing rollers last before needing replacement?
Burnishing roller life depends on the roller material, workpiece material, burnishing interference, cutting speed, and coolant condition. For tool steel rollers (AISI D2 or 52100, 58–65 HRC) processing carbon steel tubes (200–280 HB) at 60–120 m/min with 0.03–0.06 mm interference per roller and oil-based coolant with 20 µm filtration: the rollers typically last 8,000–15,000 meters of burnishing before requiring re-grinding. Rollers can be re-ground 2–4 times before the roller diameter is reduced below the minimum usable dimension, giving a total life of 24,000–60,000 meters. Tungsten carbide rollers last 3–5× longer than tool steel rollers but cost 4–6× more and are more susceptible to chipping if the burnishing interference is set too high or if the workpiece surface has hard inclusions. Rollers should be inspected every 500–1,000 meters for surface wear (flaking, pitting, or dimensional change) and replaced or re-ground when the surface finish of the burnished bore exceeds the specified Ra value.
What surface finish can be achieved with skiving and burnishing?
Skiving and burnishing achieves surface finishes of Ra 0.05–0.40 µm under production conditions, with typical values of Ra 0.10–0.25 µm for carbon steel hydraulic cylinder tubes. The achievable finish depends on: the burnishing interference (optimal interference produces the best finish), the roller surface condition (rollers must be polished to Ra < 0.05 µm to replicate their finish on the bore), the workpiece material (soft, ductile materials burnish more readily and achieve finer finishes), the cutting speed (higher speeds at the lower end of the recommended range produce finer finishes because the material has more time to flow plastically), and the coolant type and filtration (oil-based coolants with fine filtration produce the best finishes). Under laboratory conditions, finishes as fine as Ra 0.02 µm have been achieved on carbon steel, but production finishes of Ra 0.05–0.15 µm are more realistic for well-controlled processes.
Disclaimer: The process parameters, quality outcomes, and tool life data presented in this article are based on published technical literature, tooling manufacturer specifications, and industry-reported experience with skiving and burnishing processes. Actual results depend on machine tool condition, workpiece material consistency, coolant system capability, tooling quality and condition, and process control. The cutting parameters provided should be used as starting recommendations and verified through process development trials for each specific application. Tooling selection and process design should be reviewed with the tooling manufacturer or a qualified finishing process engineer. No guarantee of specific surface finish, dimensional tolerance, or tool life is expressed or implied. All data is provided for informational purposes and reflects industry practices as of 2026.