Appearance
A manufacturer of hydraulic cylinder tubes for mobile construction equipment was producing Ø80 mm × 2,000 mm bores in SAE 4140 steel (28–32 HRC) using a three-stage process: BTA drilling (roughing to Ø78 mm, Ra 3.5–5.0 µm), roller burnishing (three passes, Ra 0.8–1.2 µm), and honing (IT7 tolerance, Ra 0.3–0.5 µm). Total cycle time was 35 minutes per tube, cost €12.50 per tube. A combined skiving/burnishing tool (€8,500) consisting of a carbide skiving head with four indexable inserts (removing 0.3 mm per side) and an integrated roller burnishing section with 12 hardened rollers in three rows (total burnishing interference 0.06 mm on diameter) was evaluated. The skiving/burnishing operation was performed on a CNC lathe at Vc = 150 m/min, f = 0.30 mm/rev, with a single-pass cycle time of 4.5 minutes. The finished bore had surface finish Ra 0.08–0.12 µm, diameter IT7, and surface hardness 340–360 HV (versus 280–310 HV bulk, a 15–20% increase). The honing operation was eliminated, and total machining cost dropped from €12.50 to €5.60 per tube.
Skiving and Burnishing Principles
Process Mechanics
| Process Element | Skiving | Burnishing | Combined (Skiving + Burnishing) |
|---|---|---|---|
| Material removal mechanism | Cutting (shear) — removes a thin layer (0.1–0.5 mm per side) from the bore surface | Cold forming (plastic deformation) — compresses the surface peaks into the valleys without removing material | Both — skiving removes the rough surface layer; burnishing cold-works the remaining surface |
| Stock removal | 0.1–0.5 mm per side (per pass) | 0 (no material removal); 0.01–0.08 mm interference on diameter | 0.1–0.5 mm per side (skiving) + 0.02–0.08 mm interference (burnishing) |
| Surface finish Ra (steel) | 1.0–3.0 µm (after skiving only) | 0.1–0.8 µm (depends on pre-finish and burnishing parameters) | 0.05–0.2 µm (mirror finish after combined operation) |
| Diameter tolerance | IT8–IT10 | IT7–IT9 | IT7–IT8 |
| Surface hardness increase | 0–5% (cutting does not significantly harden the surface) | 10–30% (plastic deformation work-hardens the surface) | 10–30% (from the burnishing component) |
| Surface residual stress | Slightly tensile (+50 to +150 MPa) | Strongly compressive (−200 to −600 MPa) | Compressive (−150 to −500 MPa) — dominated by burnishing |
| Cycle time (for 2 m bore) | 3–8 minutes | 2–10 minutes (1–3 passes) | 3–8 minutes (single pass) |
| Tool cost | €3,000–8,000 (skiving head with inserts) | €2,000–6,000 (burnishing head with rollers) | €6,000–15,000 (combined head) |
Comparison with Alternative Finishing Methods
| Method | Achievable Ra (µm) | IT Grade | Surface Hardness Increase | Cycle Time (Ø80 mm × 2 m) | Tool Cost | Relative Cost per Bore |
|---|---|---|---|---|---|---|
| Skiving + burnishing (combined) | 0.05–0.2 | IT7–IT8 | 10–30% | 4–8 min (1 pass) | €6,000–15,000 | 1.0× (baseline) |
| Roller burnishing (alone, 1–3 passes) | 0.1–0.8 | IT8–IT10 (depends on pre-finish) | 10–30% | 2–10 min | €2,000–6,000 | 0.5–1.0× |
| Diamond ball burnishing | 0.05–0.15 | IT8–IT10 | 15–35% | 5–15 min | €3,000–8,000 | 0.8–1.5× |
| Honing (superfinishing) | 0.1–0.4 | IT6–IT8 | 0–5% | 10–30 min | €5,000–20,000 (fixture + stones) | 1.5–3.0× |
| Grinding (internal) | 0.2–0.8 | IT6–IT8 | 0–10% (can cause thermal damage) | 15–40 min | €10,000–30,000 (wheel + fixture) | 2.0–4.0× |
| BTA drilling (single pass) | 1.5–4.0 | IT8–IT10 | 5–15% (from guide pads) | 5–20 min | Included in machine | 0.3–0.6× |
| Gun drilling (single pass) | 0.4–1.5 | IT6–IT8 | 5–15% | 10–40 min | Included in machine | 0.5–1.0× |
Tool Design
Skiving/Burnishing Tool Components
| Component | Function | Material | Design Parameters | Life Expectancy |
|---|---|---|---|---|
| Skiving inserts (3–6 per head) | Remove the rough surface layer; provide dimensional control | Coated carbide (AlCrN, TiAlN) or CBN (for hardened materials) | Insert shape: square or trigon; rake angle: 5–10° positive; clearance: 8–12°; chip breaker: polished, open | 200–1,000 m per edge; 3–6 indexes per insert |
| Insert seat / cassette | Precise positioning of inserts; chip flow control | Alloy steel (55–62 HRC) | Axial and radial adjustment ±0.01 mm; coolant channels behind inserts for chip evacuation | 5,000–20,000 m (head body life) |
| Roller burnishing section (2–4 rows of rollers) | Cold-form the surface; compress surface peaks; induce compressive residual stress | Hardened bearing steel (60–65 HRC) or carbide | Roller diameter: 8–20 mm; roller crowning: 0.5–2.0 µm; number of rollers per row: 8–20; contact angle: 10–20° | 2,000–10,000 m (roller replacement) |
| Roller cage / retainer | Holds rollers at correct spacing and angular position | Brass, bronze, or hardened steel | Roller pocket clearance: 0.1–0.3 mm; lubrication: coolant-fed | 1,000–5,000 m (cage replacement) |
| Tapered expansion cone | Adjusts the burnishing interference (diameter expansion) | Hardened tool steel (58–62 HRC) | Taper angle: 2–5°; axial adjustment mechanism for interference control | 10,000+ m |
| Guide pads (2–4, in skiving section) | Centering and guidance through the bore | Carbide (K10–K20) or PCD-tipped | OD: 0.02–0.05 mm below skiving diameter; length: 1.5–2× head diameter | 500–5,000 m |
| Coolant channels | Chip evacuation for skiving; cooling for burnishing | Integral to head body | Flow: 50–200 L/min at 10–50 bar; directed to cutting edges and roller contact zone | N/A |
Burnishing Interference and Roller Configuration
| Roller Row | Interference per Side (mm) | Cumulative Diameter Increase (mm) | Function |
|---|---|---|---|
| Row 1 (roughing) | 0.015–0.025 | 0.03–0.05 | Initial contact; compresses surface peaks; 30–40% of total deformation |
| Row 2 (intermediate) | 0.010–0.020 | 0.05–0.09 | Continues compression; 30–40% of total deformation |
| Row 3 (finishing) | 0.005–0.015 | 0.06–0.12 | Final sizing and surface smoothing; 20–30% of total deformation |
| Row 4 (superfinishing) | 0.002–0.008 | 0.07–0.16 | Optional — for Ra < 0.08 µm; 10–15% of total deformation |
Process Parameters
Recommended Skiving and Burnishing Parameters
| Material | Pre-finish Condition | Pre-finish Ra (µm) | Stock per Side (mm) | Vc (m/min) | f (mm/rev) | Burnishing Interference (% of diameter) | Coolant Pressure (bar) | Achievable Ra (µm) |
|---|---|---|---|---|---|---|---|---|
| Low-carbon steel (1026) | BTA-drilled | 2.5–5.0 | 0.20–0.40 | 100–200 | 0.15–0.40 | 0.08–0.15% | 10–40 | 0.05–0.15 |
| Alloy steel (4140, 4340) annealed | BTA-drilled | 2.0–4.5 | 0.20–0.40 | 100–180 | 0.15–0.35 | 0.08–0.15% | 10–50 | 0.08–0.20 |
| Alloy steel (4140, 4340) Q&T | BTA-drilled | 2.0–4.5 | 0.15–0.30 | 80–150 | 0.15–0.30 | 0.06–0.12% | 20–50 | 0.08–0.18 |
| Stainless steel (304, 316) | BTA-drilled | 3.0–5.5 | 0.25–0.50 | 80–140 | 0.12–0.30 | 0.10–0.18% | 20–50 | 0.10–0.25 |
| Stainless steel (17-4 PH) | BTA-drilled | 2.5–4.5 | 0.15–0.30 | 60–120 | 0.12–0.25 | 0.06–0.12% | 20–50 | 0.08–0.20 |
| Cast iron (gray) | BTA-drilled | 2.5–5.0 | 0.20–0.40 | 120–200 | 0.20–0.50 | 0.06–0.10% | 10–30 | 0.05–0.15 |
| Cast iron (CGI) | BTA-drilled | 3.0–5.5 | 0.20–0.35 | 80–140 | 0.15–0.35 | 0.06–0.10% | 20–40 | 0.10–0.25 |
| Aluminum (6061, 7075) | BTA or gun-drilled | 1.5–3.5 | 0.15–0.30 | 200–500 | 0.15–0.40 | 0.12–0.20% | 10–30 | 0.05–0.15 |
| Copper alloys | BTA or gun-drilled | 2.0–4.0 | 0.15–0.30 | 150–300 | 0.15–0.35 | 0.10–0.18% | 10–30 | 0.05–0.12 |
Surface Integrity Effects
Surface and Subsurface Characteristics
| Property | Before Skiving/Burnishing (BTA-drilled) | After Skiving | After Burnishing | Combined Skiving + Burnishing |
|---|---|---|---|---|
| Surface finish Ra (µm) | 2.0–5.0 | 1.0–3.0 | 0.1–0.8 | 0.05–0.2 |
| Surface hardness (HV, 4140 steel) | 280–310 (bulk) | 280–310 (no significant change) | 340–400 (surface), 280–310 (50 µm depth) | 340–400 (surface), 280–310 (50 µm depth) |
| Hardness penetration depth | N/A | N/A | 20–80 µm | 20–80 µm |
| Residual stress (surface) | −50 to +100 MPa | 0 to +100 MPa (slightly tensile) | −200 to −600 MPa (compressive) | −200 to −500 MPa |
| Residual stress (50 µm depth) | −100 to −200 MPa | −50 to −100 MPa | −100 to −300 MPa | −100 to −300 MPa |
| White etching layer | 5–20 µm (from BTA drilling) | Removed by skiving | None | None |
| Grain structure | Locally deformed (from BTA guide pads) | Removed by skiving | Plastically deformed grain boundary alignment to 20–80 µm depth | Plastically deformed grain boundary alignment to 20–80 µm depth |
FAQ
What is the difference between skiving and burnishing?
Skiving and burnishing are fundamentally different processes. Skiving is a cutting process — a sharp carbide insert removes a thin layer (0.1–0.5 mm per side) of material from the bore surface by shear deformation. The skiving cutter operates like a lathe turning tool working on the internal surface of the bore, producing a chip. Skiving improves the bore roundness, concentricity, and dimensional accuracy while removing the rough, damaged, or work-hardened surface layer from the previous operation (BTA drilling, casting, or rough boring). Burnishing is a cold-forming (plastic deformation) process — hardened rollers or balls press against the bore surface under controlled pressure, plastically deforming the surface peaks into the surface valleys. No material is removed; instead, the surface is smoothed by plastic flow. Burnishing also work-hardens the surface (10–30% hardness increase) and induces compressive residual stress (−200 to −600 MPa). When combined in a single tool, the skiving section first removes the rough surface layer and establishes the bore diameter, and the burnishing section immediately follows to finish the surface. The combined tool achieves in one pass what would otherwise require three separate operations (rough boring, semi-finish boring, and roller burnishing or honing).
What surface finish can skiving and burnishing achieve?
Skiving and burnishing can achieve surface finishes of Ra 0.05–0.2 µm (mirror finish) in most steels and non-ferrous alloys — equivalent to or better than honing, and substantially better than grinding or roller burnishing alone. The achievable Ra depends on: material — low-carbon steel and aluminum achieve the best finishes (Ra 0.05–0.12 µm); alloy steel and stainless steel achieve Ra 0.08–0.20 µm. Pre-finish condition — the skiving operation removes the previous surface layer, so the pre-finish has minimal effect on the final result. Burnishing parameters — interference per row, number of roller rows, and roller condition directly affect the final finish. Coolant — clean, well-filtered coolant at 10–50 bar is essential for flushing chips from the skiving operation and providing lubrication for the burnishing rollers. The surface produced by skiving and burnishing is not only smooth but also has a characteristic mirror-like appearance with low light-scattering (Ra 0.05–0.2 µm typically corresponds to a gloss unit reading of 80–95 GU at 60°). The surface is also free of the torn, folded, or smeared material that can occur with honing or grinding.
What is the required pre-finish condition for skiving and burnishing?
The pre-finish condition for skiving and burnishing should be: diameter — the bore should be 0.3–1.0 mm undersize (total stock for skiving), depending on the bore diameter and the pre-finish method. For BTA-drilled bores, typical skiving stock is 0.3–0.6 mm per side (0.6–1.2 mm on diameter). For as-cast bores, 0.5–1.0 mm per side may be required. Surface finish — there is no specific Ra requirement for the pre-finish because the skiving operation removes the entire surface layer. However, the pre-finish should be free of hard spots, sand inclusions (in castings), or localized work hardening that could damage the skiving inserts. Roundness and straightness — the pre-finish bore should be within IT10–IT11 for roundness and within 0.10–0.20 mm/m for straightness. If the pre-finish bore is excessively bent or non-round, the skiving inserts will have uneven stock removal around the circumference, which can cause vibration, chatter, and inconsistent burnishing results. Concentricity — if the bore will be skived and burnished after heat treatment, the pre-finish bore must be concentric with the heat treatment reference surfaces to ensure uniform stock removal. The recommended minimum pre-finish method is BTA drilling (for diameters > 40 mm) or gun drilling (for diameters < 40 mm), as these methods produce a consistent, straight, and round bore suitable for skiving.
How does skiving and burnishing compare in cost to honing?
Skiving and burnishing is typically 40–60% lower in cost per bore than honing for production volumes above 1,000 bores per year. The cost comparison for a typical Ø80 mm × 2,000 mm bore in 4140 steel is: skiving/burnishing tool cost (amortized) — €0.30–0.80 per bore (tool cost of €7,000–12,000 amortized over 10,000–20,000 bores); insert cost — €0.50–1.50 per bore (4 inserts × €8–15 per edge, edges lasting 200–600 m); machine time cost — €1.50–3.00 per bore (4–8 minutes at €25–40/hour machine rate); and total skiving/burnishing cost — €2.30–5.30 per bore. Honing: honing tool cost (amortized) — €0.50–1.50 per bore (tool €5,000–15,000 amortized over 10,000–30,000 bores); stone cost — €1.00–3.00 per bore (stones lasting 50–200 bores per set); machine time cost — €4.00–10.00 per bore (10–30 minutes at €25–40/hour); and total honing cost — €5.50–14.50 per bore. The cost advantage of skiving/burnishing comes primarily from the shorter cycle time (4–8 minutes versus 10–30 minutes for honing) and the lower consumable cost. Additional savings include: elimination of coolant disposal cost for honing sludge (honing generates fine metal and abrasive particles that are difficult to filter); lower energy cost (skiving/burnishing spindle power 20–40 kW versus 30–60 kW for honing); and elimination of the separate burnishing or polishing operation that often follows honing.
What are the limitations of skiving and burnishing?
Skiving and burnishing has five main limitations: minimum bore diameter — skiving/burnishing tools are practical for bores > 30 mm diameter. Below 30 mm, the tool becomes too small to accommodate both the skiving inserts and the roller burnishing section within the bore. For diameters 10–30 mm, roller burnishing alone (without skiving) can be used if the pre-finish surface is adequate. Maximum L/D ratio — skiving/burnishing is typically limited to L/D ≤ 50:1. At higher L/D ratios, the long tool extension causes deflection and chatter. The tool is supported only at the machine end, and the long overhang (up to 3 m for a 2 m bore) creates bending under the cutting load. Workpiece geometry — the process requires a through-bore or a blind bore with sufficient clearance at the bottom for tool over-travel (typically 50–100 mm beyond the bore depth). Blind bores with limited bottom clearance cannot be skived and burnished. Material hardness — for materials > 45 HRC, carbide skiving inserts may not be adequate — CBN inserts may be required for the skiving section, and the burnishing rollers may experience accelerated wear or galling. For hardened materials, alternative finishing methods (grinding, honing) may be preferred. Initial investment — combined skiving/burnishing tools cost €6,000–15,000 per diameter, and the machine must provide adequate spindle power (30–75 kW for steel) and coolant flow (50–200 L/min at 10–50 bar). The process is most economical when the tool cost can be amortized over a production volume of 500+ bores per year per diameter.
Disclaimer: The skiving and burnishing parameters, tool design guidelines, and cost data presented in this article are based on published technical literature, tool manufacturer specifications, and industry-reported experience. Actual results depend on specific workpiece material, pre-finish condition, machine tool capability, and coolant system design. Skiving and burnishing tool selection should be made in consultation with specialized tool manufacturers. Surface integrity effects (residual stress, hardness profile) should be verified for each specific application, particularly for fatigue-critical components. No guarantee of specific surface finish, dimensional tolerance, or cost savings is expressed or implied. All data is provided for informational purposes and reflects industry practices as of 2026.