Deep Hole Drilling Finishing: Honing, Lapping, Burnishing

Deep hole drilling produces bores with surface finishes of Ra 1.6–12.5 μm depending on the method (BTA vs gun drilling). When hydraulic cylinders, engine components, or precision aerospace parts demand sub-micron finishes, straightness within microns, or engineered surface textures, post-processing operations transform the as-drilled bore into a finished surface meeting exacting specifications.

Overview ​

Gun drilling typically yields Ra 3.2–12.5 μm with IT8–IT10 diameter accuracy, while BTA drilling achieves Ra 1.6–3.2 μm with IT7–IT10 accuracy. Many applications accept these as-delivered finishes — hydraulic cylinder tubes, fluid passages, and structural bores often go straight to service. However, when functional requirements demand:

• Surface finish below Ra 0.8 μm for seal surfaces, bearing journals, or fatigue-critical areas
• Diameter tolerance tighter than IT7 for valve bores or precision fitments
• Geometric correction of out-of-roundness, taper, or bell-mouth conditions
• Surface engineering such as cross-hatch oil-retention patterns or compressive residual stress

…post-processing becomes necessary. Five principal methods are available, each with distinct capabilities, limitations, and economic profiles.

Honing ​

Honing is an abrasive finishing process in which a rotating tool (mandrel) fitted with bonded abrasive stones expands radially against the bore wall while reciprocating axially. The combination of rotation and reciprocation generates a characteristic cross-hatch surface pattern that aids lubrication and oil retention.

Process Fundamentals ​

The cross-hatch angle is controlled by the ratio of rotational speed to stroke speed:

$$\theta = 2 \cdot \arctan\left(\frac{V_s}{V_r}\right)$$

where:

• θ = cross-hatch angle (typically 40–45° for hydraulic/engine bores)
• V_s = stroke speed (m/min)
• V_r = rotational speed (m/min)

Typical honing parameters for deep hole applications:

Achievable Quality ​

Abrasive Selection ​

Abrasive materials include aluminium oxide (general steel), silicon carbide (cast iron), CBN (hardened steel, production high-volume), and diamond (carbide, ceramics). CBN and diamond stones cost more but maintain geometry longer in high-production environments.

Applications in Deep Hole Components ​

Honing is widely used for:

• Hydraulic cylinder tubes — final sizing and surface finish after BTA drilling or skiving
• Engine cylinder bores — plateau honing for ring-seal and oil control
• Gun barrels — bore smoothing prior to rifling (less common today, replaced by button rifling or lapping)
• Fuel injection components — precision bore geometry for plunger barrels
• Aerospace landing gear — fatigue-critical bores requiring controlled surface texture

Tip: Honing corrects form errors (roundness, cylindricity) but cannot change the bore centerline position. If axis location is critical, boring or skiving must precede honing.

Roller Burnishing ​

Roller burnishing is a chipless cold-working process in which hardened rollers or balls are pressed against the bore surface under high force. The contact pressure plastically deforms surface micro-peaks into valleys, producing a mirror-like finish while simultaneously work-hardening the surface layer and inducing compressive residual stress.

Process Principles ​

Unlike honing or lapping — which remove material — roller burnishing displaces it. The tool rollers (typically 3–12 rollers arranged around a tapered cone) expand radially as the tool advances through the bore. The interference between the roller diameter and the bore wall determines the working pressure.

Key parameter relationships:

Achievable Quality ​

Tooling Types ​

• Mechanical multi-roller tools — rollers expanded by axial movement of a tapered cone; diameter-specific, high feed capacity (3–6 mm/rev). Common for medium-to-large bores (Ø20–400 mm).
• Hydrostatic ball burnishing tools — single carbide ball pressed by hydraulic pressure (up to 600 bar); self-centering, handles oval/trumpet-shaped bores. Suitable for smaller diameters and deep rolling applications.
• Combined skive-burnish tools — discussed in detail below.

Applications ​

Roller burnishing is applied after deep hole drilling when:

• Hydraulic cylinder bores require mirror finish for rod seal longevity
• Connecting rod small ends need surface densification to eliminate bronze bushings
• Piston pin bores demand low friction and high wear resistance
• Valve bodies require leak-free sealing surfaces

Roller burnishing can reduce manufacturing costs by 50–90% compared to honing or grinding in suitable applications, primarily through shorter cycle times and elimination of abrasive consumables.

Skive-Burnishing (Combined Process) ​

Skive-burnishing integrates skiving (a cutting operation using carbide blades) and roller burnishing into a single pass. The tool carries skiving knives at the front, which remove a thin layer (typically 0.2–0.5 mm radial) to correct geometry, followed immediately by burnishing rollers that cold-work the surface to a mirror finish.

Why Combine? ​

Deep hole drilled bores often exhibit:

• Taper along the bore length (larger at entry, smaller at exit)
• Out-of-roundness from tool deflection or workpiece movement
• Surface variability due to coolant pressure fluctuations or chip re-cutting

Skiving corrects these geometry errors before burnishing, so the final bore is both dimensionally accurate and mirror-finished in one operation.

Process Capabilities ​

Machine Requirements ​

Skive-burnishing is typically performed on:

• Dedicated skive-burnishing machines (e.g., UNISIG S-series) with high-torque spindles and rigid workpiece support
• BTA deep hole drilling machines configured with skive-burnish tooling (same machine, different head)
• CNC lathes with live tooling for shorter bores and smaller diameters

The key requirement is axial feed force capacity — skiving generates substantial cutting forces (1000–5000 N depending on diameter and depth of cut) that require a rigid feed drive system.

Applications ​

Skive-burnishing is the preferred finishing method for:

• Hydraulic cylinder tubes (most common application worldwide)
• Pneumatic actuator bores
• Injection moulding barrel bores
• Large-bore landing gear struts

Warning: Skive-burnishing cannot be used on blind holes, interrupted bores (cross-holes, keyways), or materials below 180 HB (the burnishing action requires sufficient material strength to generate compressive stress without tearing).

Lapping ​

Lapping is an abrasive finishing process that uses a loose abrasive compound (slurry) carried by a lapping tool (lap) to remove microscopic amounts of material. For deep hole applications, cylindrical lapping employs a helically-grooved lap (helilap) that rotates and reciprocates within the bore while abrasive slurry is continuously supplied.

Process Characteristics ​

Lap Design and Abrasive Selection ​

The helical lap is typically made of cast iron or soft steel with helical grooves that serve both as abrasive slurry channels and chip clearance paths. The lap is slightly tapered (approximately 0.01 mm per 25 mm of length) and can be expanded by a tapered arbour to control fit.

Abrasive compounds are classified by:

• Type: Aluminium oxide (steel), silicon carbide (cast iron), diamond (carbide/ceramic)
• Grit size: 5–50 μm for rough lapping; 0.5–3 μm for fine lapping
• Vehicle: Oil-based paste or water-based slurry

Wire lapping — using diamond-coated wire pulled back and forth through small-diameter deep holes — is effective for high-aspect-ratio bores up to 30:1 L/D, particularly in hard materials such as ceramics or carbide.

When to Choose Lapping ​

Lapping is appropriate when:

• Extreme precision (< IT5) is required — no other mechanical process matches lapping's tolerance capability
• Very fine surface finishes (Ra < 0.05 μm) are specified
• Geometric errors (out-of-round, bell-mouth, barrel shape) must be corrected in the final pass
• Small batches or one-off tooling where dedicated honing stones or burnishing tools are uneconomical

Lapping is slower and more labour-intensive than honing or burnishing, so it is typically reserved for high-value components where absolute precision justifies the cycle time.

Ballizing ​

Ballizing (also called ball sizing) forces an oversized precision-ground ball through a pre-machined bore. The ball plastically displaces material, simultaneously sizing and surface-finishing the hole in a single, rapid pass.

Process Parameters ​

Achievable Quality ​

Limitations ​

Ballizing cannot correct bore straightness or alignment — it follows the existing hole axis. The process also requires continuous, uninterrupted bores without cross-holes or undercuts. Blind holes are difficult to ballize unless a shaft-mounted ball and push-through arrangement is designed.

Applications ​

Ballizing is most economical for:

• High-volume small bores (Ø2–25 mm) in automotive components (fuel injector bodies, valve guides)
• Die-cast or powder-metal parts where as-sintered bores need sizing
• Bearing housing bores requiring consistent diameter across multiple parts
• Zinc and aluminium alloy components where material softness makes ballizing highly effective

Tip: Ballizing is the fastest post-processing method — cycle time is measured in seconds, not minutes. For high-volume production of small-diameter deep holes, it often provides the lowest cost per part.

Process Selection Guide ​

The table below compares all five methods across the criteria most relevant to deep hole drilling applications:

Summary ​

The post-processing method for deep hole drilled components should be selected based on the functional requirements of the finished bore:

In practice, the most common pairing in deep hole drilling shops is BTA drilling followed by skive-burnishing for hydraulic cylinders, and gun drilling followed by honing for precision mechanical components. Roller burnishing alone is specified when the as-drilled bore is already within roundness tolerance and only surface finish improvement is needed.

FAQ ​

What is the typical surface finish after gun drilling, and when is post-processing needed? ​

Gun drilling typically achieves Ra 3.2–12.5 μm. Post-processing is needed when the application requires Ra below 0.8 μm, tighter diameter tolerances (IT7 or better), or specific surface textures such as cross-hatch patterns.

Can honing correct a bent or misaligned bore? ​

No. Honing follows the existing bore axis and cannot correct centreline deviation. Misalignment must be addressed at the drilling or boring stage.

What is the difference between honing and roller burnishing? ​

Honing removes material using bonded abrasive stones and produces a cross-hatch surface pattern. Roller burnishing displaces material plastically (no material removal) and produces a mirror-like finish while also work-hardening the surface. Ballizing is similar to burnishing but uses a single precision ball instead of multiple rollers.

Does skive-burnishing require a dedicated machine? ​

Not necessarily. While dedicated skive-burnishing machines exist (e.g., UNISIG S-series), the process can also be performed on BTA deep hole drilling machines with appropriate tooling adapters, or on CNC lathes with sufficient feed force capacity for short-to-medium bore lengths.

What materials are suitable for roller burnishing? ​

Most steels, stainless steels, aluminium alloys, and cast irons with hardness up to approximately 40 HRC are suitable. Materials below 180 HB may not respond well, as insufficient material strength prevents the development of compressive residual stress.

Can ballizing be used on blind holes? ​

Ballizing blind holes is challenging but possible with shaft-mounted balls and push-through arrangements. Conventional push-through ballizing is limited to through holes.

Which method provides the best diameter tolerance? ​

Lapping provides the tightest diameter tolerance (IT4–IT5, ±0.0001 mm), followed by honing (IT5–IT7). Skive-burnishing typically achieves IT8–IT9, which is sufficient for most hydraulic applications.

Is post-processing always necessary for deep hole drilled parts? ​

No. Many deep hole drilled parts — especially in structural applications, fluid passages, and non-critical bores — are used as-drilled. Post-processing is applied only when functional requirements (surface finish, tolerance, surface texture) exceed what the drilling process can deliver economically.

How much stock should be left for honing after deep hole drilling? ​

Typically 0.1–0.3 mm on diameter for finishing operations, depending on bore size and length. Rough honing can remove up to 0.2 mm per side, while finish honing removes 0.005–0.020 mm.

What is the economic advantage of skive-burnishing over honing? ​

Skive-burnishing combines geometry correction and surface finishing in a single pass at feed rates of 1–6 mm/rev. Honing requires multiple passes at lower feed rates and uses consumable abrasive stones. For hydraulic cylinder production, skive-burnishing can reduce cycle time by 50–70% compared to honing.