Appearance
A hydraulic cylinder manufacturer produces thousands of tube bores that must meet two requirements: accurate diameter (IT8) and mirror-like surface finish (Ra 0.2 µm). The traditional process chain is BTA drilling → rough boring → fine boring → roller burnishing or honing — four separate operations. Each bore takes 45 minutes of total processing time. A UNISIG S-series machine with a combination skiving and roller burnishing tool eliminates the boring and separate burnishing steps. The combination tool mounts on a BTA-style machine: carbide skiving blades cut the bore to final diameter on the forward stroke, and carbide rollers immediately cold-work the surface. The resulting bore is IT8 with Ra 0.08 µm finish — exceeding specification — and cycle time drops to 12 minutes per bore. Tool life exceeds 10,000 metres.
What Is Combination Tooling?
Combination tooling for deep hole drilling integrates two or more material removal or surface finishing operations into a single tool that performs all operations in one pass through the bore. The most common combination for deep holes is skiving (precision cutting) plus roller burnishing (cold working).
Benefits of One-Pass Combination Tooling
| Benefit | Traditional Multi-Operation | One-Pass Combination |
|---|---|---|
| Number of operations | 3–5 (drill, bore, finish bore, burnish/hone) | 1 (drill + combination tool) |
| Setups | Multiple machines, multiple fixtures | Single machine, single setup |
| Cycle time | 45–90 minutes per bore | 10–20 minutes per bore |
| Work-in-progress | High — parts wait between operations | Minimal — complete in one pass |
| Surface finish | Ra 0.2–0.4 µm (burnishing) | Ra 0.05–0.2 µm |
| Diameter tolerance | IT7–IT9 | IT8–IT9 |
| Capital equipment | Multiple machines | One machine with combination tool |
Integration Approaches
| Approach | Description | Typical Configuration |
|---|---|---|
| Guide pad burnishing | Standard BTA guide pads provide partial burnishing | BTA drilling head with optimised pads |
| Drill + burnish | BTA drill head with integrated burnishing rollers behind the cutting zone | Single tube, single pass |
| Skive + burnish (SRB) | Dedicated skiving head with burnishing rollers | Separate tool, post-drilling |
| Bore + burnish | Precision boring head with burnishing rollers | Finish boring operation |
| Triple combination | Drill + ream + burnish | Large diameter custom tooling |
Guide Pad Burnishing Effect
Standard BTA drilling already produces a burnishing effect through the guide pads. As the carbide pads contact the bore wall under cutting forces, they compress and smooth the surface.
Effect of Guide Pad Burnishing
| Parameter | Without Guide Pad Optimisation | With Guide Pad Optimisation |
|---|---|---|
| Surface finish after BTA drilling | Ra 3.2–6.3 µm | Ra 0.8–1.6 µm |
| Surface hardness increase | 10–20% | 30–56% |
| Residual stress | Tensile or neutral | Compressive |
| Cycle time impact | None (standard process) | None (same drilling parameters) |
Research on BTA drilling of cast iron has shown that optimised guide pad geometry and parameters can produce a subsurface hardness increase of 56% and compressive residual stress — effects that are characteristic of burnishing.
Tip: Before investing in dedicated combination tooling, evaluate whether optimised guide pads on your existing BTA drill head can achieve the required surface finish. For many applications requiring Ra 1.6 µm or higher, guide pad optimisation alone may be sufficient.
Skiving and Roller Burnishing (SRB) Tools
Skiving and roller burnishing (SRB) is the most common combination tooling technology for deep holes. It is typically used after BTA drilling to finish the bore to final size and surface quality.
How SRB Works
The SRB tool combines two functional sections on a common body:
Skiving section — 2 to 6 carbide cutting blades arranged around the circumference. These blades remove a thin layer (0.2–1.0 mm radial) to establish the final bore diameter and correct any geometry errors from the preceding drilling operation.
Burnishing section — 6 to 12 hardened carbide or steel rollers that cold-work the surface. The rollers apply high contact pressure (1,000–3,000 MPa) that plastically deforms the surface peaks into valleys, creating a smooth, work-hardened surface.
The tool is typically advanced through the bore with simultaneous rotation and feed. On some tool designs, the skiving and burnishing functions operate simultaneously on the same pass. On others, skiving occurs on the forward stroke and burnishing on the return stroke.
Hydraulic Expansion
Most SRB tools use hydraulic expansion to control the cutting and burnishing elements:
- The tool is inserted through the bore with blades and rollers retracted
- Hydraulic pressure is applied through a rotary union, expanding the blades and rollers to the cutting position
- The tool rotates and feeds through the bore
- At the end of the stroke, hydraulic pressure is released and the elements retract
- The tool is withdrawn without contacting the finished surface
This retraction feature is critical — dragging expanded rollers back through a finished bore would destroy the surface finish.
Warning: Always verify hydraulic pressure before starting an SRB pass. Insufficient pressure causes incomplete blade expansion, resulting in undersize diameter. Excessive pressure can over-expand the blades, causing oversize diameter or blade damage. Follow the tool manufacturer's pressure specification exactly.
SRB Tool Specifications
| Parameter | Typical Range |
|---|---|
| Diameter range | 30–600 mm |
| Skiving blades | 2–6 (carbide, indexable or brazed) |
| Burnishing rollers | 6–12 (carbide or hardened steel) |
| Blade expansion | 0.2–1.0 mm radial |
| Hydraulic pressure | 30–100 bar |
| Coolant pressure | 10–50 bar |
| Coolant flow | 250–1,000 L/min |
Cutting Parameters
Skiving Parameters
| Material | Cutting Speed (m/min) | Feed (mm/rev) | Depth of Cut (mm radial) |
|---|---|---|---|
| Low-carbon steel | 150–250 | 1.0–4.0 | 0.2–0.5 |
| Alloy steel (4140) | 120–200 | 1.0–3.0 | 0.2–0.4 |
| Stainless steel | 100–180 | 0.8–2.0 | 0.2–0.3 |
| Cast iron | 150–300 | 1.5–6.0 | 0.3–1.0 |
| Aluminum | 200–400 | 2.0–6.0 | 0.5–1.0 |
Note: Skiving feed rates are much higher than BTA drilling feed rates because the multi-blade arrangement multiplies the effective material removal rate. A 4-blade skiving head at 3 mm/rev feed removes 12 mm of material per revolution.
Burnishing Parameters
| Parameter | Typical Value |
|---|---|
| Burnishing speed | 80–200 m/min (often same as skiving) |
| Burnishing feed | 0.5–2.0 mm/rev |
| Roller interference | 0.05–0.15 mm (diametral) |
| Number of roller passes | 1 (single pass is sufficient) |
| Coolant | Oil or high-viscosity emulsion |
Achievable Quality
| Quality Metric | Skiving Only | Skiving + Burnishing |
|---|---|---|
| Surface finish (Ra) | 1.5–10 µm | 0.05–0.20 µm |
| Surface finish (Rz) | 5–20 µm | <1 µm |
| Diameter tolerance | IT8–IT9 | IT8–IT9 |
| Roundness | 0.02–0.05 mm | 0.02–0.05 mm |
| Surface hardness increase | — | Up to 50% |
| Residual stress | — | Compressive |
Dedicated Combination Drill-Burnish Tools
Beyond SRB tools for post-drilling finishing, true combination tools integrate drilling and burnishing in a single tool body for one-pass operation from the solid.
Drill + Guide Pad Burnishing
The simplest form of combination tooling: a standard BTA drill head with guide pads optimised for maximum burnishing effect. This is not a separate tool but rather an optimisation of the existing drilling process. It is suitable for applications where Ra 1.6 µm is acceptable.
Drill + Roller Burnishing
These tools integrate BTA drilling inserts at the front with roller burnishing elements behind the guide pads. The tool:
- Drills the bore from solid using standard BTA cutting geometry
- Guides the tool with carbide pads
- Burnishes the bore surface with rollers immediately behind the pads
This configuration requires careful design to avoid interference between the drilling and burnishing functions. The burnishing rollers must be sized to contact the bore at the drilled diameter plus the burnishing interference.
Skive + Roller Burnishing (Post-Drill)
The most common production configuration: the bore is first pre-drilled using BTA drilling, then finished in a second pass using an SRB combination tool. This two-pass approach is more flexible and allows independent optimisation of the drilling and finishing operations.
Comparison with Alternative Processes
| Process | Surface Finish (Ra) | Tolerance | Cycle Time (relative) | Equipment Cost |
|---|---|---|---|---|
| BTA drilling only | 3.2–6.3 µm | IT9–IT10 | 1× | Moderate |
| BTA drilling + SRB | 0.05–0.20 µm | IT8–IT9 | 1.3–1.5× | Higher |
| BTA drilling + honing | 0.1–0.4 µm | IT7–IT8 | 3–5× | High |
| BTA drilling + grinding | 0.2–0.8 µm | IT6–IT7 | 4–6× | Very high |
| BTA drilling + boring + burnishing (separate) | 0.2–0.4 µm | IT7–IT9 | 3–4× | High |
| Combination drill-burnish (one tool) | 0.2–0.8 µm | IT8–IT9 | 1.1–1.2× | Moderate |
SRB is 8–20× faster than honing for equivalent surface finish, making it the most productive finishing method for deep holes where Ra <0.2 µm is required.
Applications
| Industry | Application | Typical Specification |
|---|---|---|
| Hydraulic cylinders | Cylinder bore finishing | IT8, Ra 0.1–0.2 µm, Ø30–300 mm |
| Oil and gas | Drill pipe bores | IT9, Ra 0.4 µm, up to 12 m length |
| Automotive | Shock absorber tubes | IT9, Ra 0.2–0.4 µm, high volume |
| Aerospace | Landing gear struts | IT8, Ra 0.2 µm, high-strength materials |
| Mold and die | Cooling channel finishing | IT9, Ra 0.4 µm, complex materials |
Machine Requirements
Combination skiving and burnishing imposes higher demands on the machine than standard BTA drilling:
| Requirement | Reason |
|---|---|
| Higher spindle power (2–3× BTA drilling) | Burnishing rollers consume significant power for plastic deformation |
| Hydraulic rotary union | For blade and roller expansion/retraction |
| Through-coolant system | 10–50 bar minimum, high flow rate |
| Rigid machine frame | Burnishing generates higher radial forces than cutting |
| Clamping cones (not chucks) | Thin-walled tubes distort under chuck pressure |
| V-shaped steady rests | Support tubes without deformation |
| Chip conveyor | High chip volume at high removal rates |
Machine manufacturers: UNISIG (S-Series), Profimach (SRB Series), Sunnen (SHD Series), Mollart/Ecoroll (Omega System).
Troubleshooting
| Problem | Likely Cause | Correction |
|---|---|---|
| Surface finish above Ra 0.2 µm | Insufficient roller interference | Increase hydraulic expansion pressure |
| Streaking or tearing on bore surface | Worn or damaged burnishing rollers | Replace rollers, check for embedded debris |
| Bore diameter undersize | Blade expansion too low | Check hydraulic pressure, verify blade projection |
| Bore diameter oversize | Blade expansion too high | Reduce hydraulic pressure |
| Chatter marks | Speed too high, insufficient rigidity | Reduce speed, check workpiece support |
| Roller marks on bore surface | Coolant starvation | Increase coolant flow, check nozzle alignment |
| Blades chipping | Depth of cut too high or interrupted cut | Reduce depth of cut, check for hard spots |
| Uneven wear across rollers | Roller alignment or pressure distribution | Check roller diameters, verify hydraulic system balance |
FAQ
What is the difference between skiving and BTA drilling?
Skiving is a finish cutting operation that removes a thin layer (0.2–1.0 mm) from a pre-existing bore. BTA drilling cuts the bore from solid material. Skiving uses multiple cutting edges at high feed rates and produces a better surface finish and geometry than drilling.
Can I combine BTA drilling and burnishing in one tool?
Yes — combination drill-burnish tools exist, but the more common configuration is to drill first, then finish with a separate skiving and burnishing tool in a second pass on the same machine.
What surface finish can combination tooling achieve?
Ra 0.05–0.20 µm is achievable with skiving and roller burnishing. This is equivalent to honing or fine grinding but achieved in a fraction of the cycle time.
Is SRB faster than honing?
Yes — SRB is 8–20× faster than honing for deep holes. A 3 metre bore that takes 30 minutes to hone can be skive-burnished in 2–3 minutes.
What is the typical tool life of an SRB tool?
Skiving blade life is typically 500–2,000 metres of cut per edge. Burnishing roller life is 5,000–20,000+ metres. Rollers can be reground and reused.
Do I need a special machine for combination tooling?
Yes — combination skiving and burnishing requires higher spindle power than BTA drilling, plus a hydraulic rotary union for blade expansion. Standard BTA machines may not have sufficient power or the required hydraulic system.
What materials can be skive-burnished?
All common engineering materials: steel, stainless steel, cast iron, aluminum, brass, and bronze. Very hard materials (above 45 HRC) are difficult to burnish effectively.
How much stock should be left for skiving?
0.2–1.0 mm radial, depending on diameter and material. For hydraulic cylinder tubes, 0.3–0.5 mm radial is typical. Too little stock causes the blades to rub rather than cut.
Can combination tooling correct straightness errors from drilling?
No — combination tooling follows the existing bore path. Straightness must be established in the drilling operation. SRB improves roundness and surface finish but not straightness.
What is the cost premium for combination tooling?
An SRB combination tool costs 3–10× more than a standard BTA drill head of the same diameter. However, the elimination of separate finishing operations typically provides payback within 6–12 months in production environments.
Summary
Combination tooling for deep hole drilling — particularly skiving and roller burnishing (SRB) — transforms the economics of producing high-quality deep bores:
- One-pass finishing — skiving and burnishing in a single pass eliminates separate boring, honing, or grinding operations
- Surface finish Ra 0.05–0.20 µm — equivalent to honing but at 8–20× the speed
- Diameter tolerance IT8–IT9 — consistent, repeatable bore geometry
- Cycle time reduction — typical 60–75% reduction compared to multi-operation processing
- Hydraulic expansion — blades and rollers retract for tool withdrawal, protecting the finished surface
- Machine requirements — higher spindle power and hydraulic rotary union are essential
- The hydraulic cylinder manufacturer in the opening scenario achieved Ra 0.08 µm finish at IT8 tolerance with cycle time reduced from 45 minutes to 12 minutes, justifying the combination tooling investment within nine months