Appearance
Honing is the precision finishing process that corrects the geometric errors left by deep hole drilling and produces the cross-hatch surface texture essential for sealing, lubrication, and fatigue life in deep bores.
Overview
Deep hole drilling processes — gun drilling, BTA/STS drilling, and trepanning — generate the initial bore. These processes are optimised for material removal rate and depth capability, not for final surface finish or geometry. They leave a characteristic surface layer of ruptured and smeared metal, with measurable taper, ovality, and straightness deviations that grow with depth.
Honing addresses each of these deficiencies in a single process. It simultaneously corrects geometric errors, removes the damaged surface layer, and generates a controlled surface texture. A honing tool of bonded abrasive stones rotates and reciprocates inside the bore, and the combined motion creates the cross-hatch pattern — intersecting helical grooves that retain lubricant and provide bearing surfaces.
Unlike boring or internal grinding, honing uses a low-speed, low-temperature process that avoids thermal damage to the workpiece. The abrasive stones are expanded against the bore wall under controlled pressure, and the averaging effect of the complex tool path corrects errors that would be impossible to remove with a single-point boring tool.
Why Honing Follows Deep Hole Drilling
Deep hole drilling introduces specific bore imperfections that honing is uniquely suited to correct. The following table summarises the typical bore defects after deep hole drilling and how honing addresses each:
| Bore Defect | Typical Cause in Deep Hole Drilling | Correction by Honing |
|---|---|---|
| Taper — diameter variation along bore length | Drill wear, coolant pressure variation, chip accumulation | Stones follow the bore, removing more material where diameter is smaller |
| Ovality / Out-of-roundness | Non-symmetric cutting forces, machine spindle runout | Rotating abrasive action averages out non-circularity |
| Drift / Axis deviation | Gun drill deflection from pilot bushing misalignment or material variation | Limited correction — honing follows the existing hole axis |
| Waviness — long-wave surface irregularities | Vibration during drilling, periodic chip breakage | Averaging effect of reciprocating stones smooths long-wave errors |
| Surface damage — ruptured, smeared, or work-hardened layer | Cutting edge rubbing, excessive feed, built-up edge | Removes the damaged layer completely, exposing sound base metal |
The critical insight is that honing removes the damaged layer left by drilling. Gun drilling, for example, creates a surface layer approximately 0.005–0.025 mm deep that contains microcracks, smeared material, and residual tensile stress. Honing removes this layer and leaves a surface with compressive residual stress and controlled texture — both essential for fatigue life in components such as hydraulic cylinders, fuel injection systems, and aerospace actuators.
Stock Removal in Deep Hole Honing
Stock removal in honing must balance two requirements: enough material to clean up geometry errors and surface damage, but not so much that the process becomes uneconomical or risks exceeding the available stock allowance.
Typical Stock Removal Values
| Application | Stock Removal (Diameter) | Purpose |
|---|---|---|
| Finish honing after gun drilling | 0.02–0.08 mm | Surface finish, remove damaged layer, minor geometry touch-up |
| Correction of taper or ovality | 0.05–0.15 mm | Geometry correction + surface finish |
| Heavy stock removal | 0.15–6.35 mm | Resizing, reclamation of worn parts, removing deep damage |
Estimating Required Stock
A practical rule for estimating the minimum stock required for geometry correction: leave twice the measured error in the bore. If a bore has 0.05 mm of ovality, the minimum stock for honing should be 0.10 mm on diameter. This ensures that the largest diameter after honing is below the minimum acceptable finished size.
For very deep holes, the stock allowance must account for cumulative runout. American Hollow Boring recommends allowing 0.001 inch (0.025 mm) per inch of bore length for worst-case cleanup. A 180-inch-long part should therefore allow at least 0.180 inches (4.6 mm) of stock in the bore — though this can often be reduced by specifying critical zones where tight tolerances are required.
Stock Removal Rate
Stock removal rate depends on abrasive type, grit size, stone pressure, and workpiece material. Typical ranges:
- Conventional abrasives (Al₂O₃, SiC): 5–50 cm³/hr per stone
- Superabrasives (diamond, CBN): 10–150 cm³/hr per stone
- Heavy-duty tube hones (Sunnen HTD): Up to 2,500 cm³/hr total (30–40 hp spindle)
Cross-Hatch Pattern and Surface Texture
The cross-hatch pattern is the defining feature of a honed surface and the primary reason honing is specified for functional bores.
How the Cross-Hatch Is Formed
The honing tool simultaneously rotates and reciprocates. Each abrasive grain follows a helical path on the bore surface. The rotation direction reverses at each stroke end, creating overlapping helical paths that cross, forming the characteristic diamond-shaped pattern.
Cross-Hatch Angle
The cross-hatch angle is the included angle between the two sets of helical scratches. It is controlled by the ratio of rotational speed (RPM) to reciprocating stroke speed:
- Higher RPM relative to stroke speed → shallower angle
- Higher stroke speed relative to RPM → steeper angle
Typical cross-hatch angle specifications by application:
| Application | Cross-Hatch Angle | Purpose |
|---|---|---|
| Automotive engine cylinders | 40°–60° | Oil retention, ring seating |
| Hydraulic cylinder tubes | 30°–60° | Seal lubrication, low friction |
| High-performance engines | 35°–45° | Reduced oil consumption, ring seal |
| Pneumatic cylinders | 20°–30° | Minimal lubrication requirement |
| Fuel injection components | 45°–60° | Precise oil film control |
The angle specification is a compromise: steeper angles (60°) retain more oil but produce higher seal friction; shallower angles (30°) reduce friction but retain less oil. Most hydraulic applications converge on 40°–50° as the optimal balance.
Surface Finish Parameters
Honing surface finish is typically specified using multiple parameters:
| Parameter | Typical Range | What It Measures |
|---|---|---|
| Ra | 0.05–0.8 µm | Average roughness |
| Rz | 0.5–5.0 µm | Average peak-to-valley height |
| Rpk | 0.1–0.5 µm | Reduced peak height (bearing surface) |
| Rvk | 0.3–2.0 µm | Reduced valley depth (oil retention) |
| Mr1 / Mr2 | Varies | Bearing area ratios |
For hydraulic cylinder tubes, a typical specification might be Ra 0.2–0.4 µm with Rpk ≤ 0.3 µm and Rvk ≥ 0.8 µm. The plateau honing process (a final light honing pass with fine grit) is used to reduce Rpk while maintaining Rvk, optimising the surface for seal performance.
Geometry Correction Capabilities
Honing is the most effective process for correcting bore geometry errors because of the averaging effect: the abrasive stones are in constant contact with the bore surface, and they remove material preferentially where the bore is tight (smaller diameter). This self-correcting action means honing accuracy often exceeds the machine tool's own positioning precision.
Achievable Geometric Tolerances
| Geometric Parameter | Typical Before Honing | Achievable After Honing |
|---|---|---|
| Diameter tolerance | IT9–IT11 (±0.05–0.20 mm) | IT5–IT7 (±0.002–0.012 mm) |
| Roundness | 0.02–0.10 mm | 0.002–0.008 mm |
| Cylindricity | 0.05–0.20 mm | 0.005–0.020 mm |
| Straightness | 0.01–0.05 mm / 100 mm | 0.003–0.010 mm / 100 mm |
| Surface finish (Ra) | 1.0–3.2 µm | 0.05–0.8 µm |
What Honing Cannot Correct
Honing corrects form errors — roundness, taper, cylindricity, straightness — because the tool follows the existing bore axis. It cannot correct positional errors — bore location relative to external datums, angular misalignment, or axis offset from the true position. Those errors must be addressed by the preceding drilling or boring operation or by using a fixture that references external surfaces.
For deep holes, specifying critical zones rather than requiring tight tolerances along the entire bore length significantly reduces honing cost. A common specification approach is to define three zones: the seal zone (tightest tolerance, typically at one or both ends), the mid-stroke zone (moderate tolerance), and the non-critical zone (relaxed tolerance).
Honing Process Parameters
Successful deep hole honing depends on correct parameter selection for abrasive grit, pressure, speed, and stroke configuration.
Grit Size Selection
| Grit Size | Use Case | Typical Ra Achieved |
|---|---|---|
| 100–150 | Roughing — heavy stock removal, geometry correction | 0.8–1.6 µm |
| 180–240 | Semi-finishing — moderate stock removal | 0.4–0.8 µm |
| 320–400 | Finishing — final surface quality | 0.2–0.4 µm |
| 600–1000 | Plateau honing — surface conditioning | 0.05–0.2 µm |
| 1200–1500 | Superfinishing — mirror finish | 0.025–0.1 µm |
A typical two-stage cycle uses a coarse grit (120–180) for geometry correction and stock removal, followed by a fine grit (320–400) for surface finish. Plateau honing adds a third stage with very fine grit (600–1000) and minimal pressure to remove surface peaks without enlarging the oil-retaining valleys.
Honing Pressure
| Parameter | Typical Range | Effect |
|---|---|---|
| Rough honing pressure | 150–250 PSI (1000–1700 kPa) | Maximum stock removal, but rougher finish |
| Finish honing pressure | 50–150 PSI (350–1000 kPa) | Better surface finish, slower removal |
| Plateau honing pressure | 20–80 PSI (140–550 kPa) | Minimal removal, surface conditioning |
Higher stone pressure increases material removal rate but also increases stone wear and produces a rougher surface. Modern CNC honing machines program multi-stage pressure cycles: high pressure for initial geometry correction, stepped reductions as the bore approaches finished size, and low pressure for the final finishing pass.
Speed Parameters
| Parameter | Typical Range | Effect on Surface |
|---|---|---|
| Rotational speed | 16–60 m/min surface speed (500–2000 RPM for typical bores) | Higher speed = shallower cross-hatch, smoother finish |
| Reciprocating speed | 8–20 m/min (30–100 strokes/min) | Higher speed = steeper cross-hatch |
| Stroke length | Bore length minus 1/3 stone length | Stone should overhang each end by 1/3 of its length |
The cross-hatch angle is determined by the ratio:
tan(θ/2) = v_s / v_r
where θ is the included cross-hatch angle, v_s is the reciprocating speed, and v_r is the rotational speed. This relationship allows precise angle control by adjusting the machine parameters rather than changing the tool.
Coolant and Filtration
Honing coolant serves three functions: flushing chips from the cutting zone, cooling the workpiece and tool, and lubricating the abrasive/workpiece interface. Typical specifications:
- Honing oil: 8:1 kerosene-to-oil mixture or proprietary honing fluids
- Flow rate: 50–200 litres/min per spindle for deep holes
- Filtration: Below 20 µm for superabrasive honing; below 50 µm for conventional abrasives
Abrasive Selection and Grit Sequencing
Abrasive Types
| Abrasive | Best For | Characteristics |
|---|---|---|
| Aluminium oxide (Al₂O₃) | Carbon steel, ductile iron | Low cost, predictable wear, easy dressing |
| Silicon carbide (SiC) | Cast iron, aluminium, stainless steel | Higher hardness than Al₂O₃, sharper grains |
| Cubic boron nitride (CBN) | Hardened steels (> 45 HRC), tool steels | Excellent wear resistance, high thermal stability |
| Diamond | Carbide, ceramics, glass, hardened cast iron | Highest hardness and wear resistance |
Bond Types
| Bond | Characteristics | Applications |
|---|---|---|
| Vitrified | Porous, free-cutting, consistent wear | Conventional abrasives, general-purpose honing |
| Resinoid | Smooth cutting, good finish | Fine finishing, plateau honing |
| Metal bond | Long life, high stock removal | Diamond and CBN superabrasives |
Multi-Stage Grit Sequencing
For deep hole honing requiring both geometry correction and surface finish, a multi-stage sequence is standard:
- Stage 1 — Roughing: 120–180 grit, high pressure (200 PSI), removes 60–70% of stock
- Stage 2 — Semi-finishing: 220–280 grit, medium pressure (120 PSI), removes 20–25% of stock
- Stage 3 — Finishing: 320–400 grit, low pressure (80 PSI), removes remaining 5–10%
- Stage 4 — Plateau (optional): 600–1000 grit, very low pressure (40 PSI), surface conditioning
Deep Hole Honing Equipment
Horizontal Tube Hones
For long bores (hydraulic cylinders, driveshafts, gun barrels), horizontal tube hones are the standard equipment. These machines support the workpiece along its length and use a long reciprocating spindle with the honing head at the end.
| Machine Feature | Typical Specification |
|---|---|
| Bore diameter range | 50–500 mm (2–20 inches) |
| Stroke length | Up to 14,000 mm (52 ft) |
| Spindle power | 22–30 kW (30–40 hp) |
| Maximum stock removal | Up to 2,500 cm³/hr |
Vertical Honing Machines
Vertical honing machines support the workpiece vertically, which is advantageous for shorter bores (L/D < 10:1) where gravity assists chip evacuation and setup is simpler.
CNC-Controlled Honing
Modern CNC honing systems provide real-time control of:
- Stone feed and pressure — closed-loop control based on in-process bore measurement
- Stroke position and reversal — precise over-travel control at each end
- Spindle speed and reciprocation — programmable profiles for variable cross-hatch angles
- Adaptive correction — the machine measures bore geometry and adjusts stone feed to correct specific form errors
CNC control enables multi-stage cycles with automated parameter changes, in-process gauging, and tool wear compensation — essential for high-volume production honing.
Skiving and Roller Burnishing
Skiving and roller burnishing is an alternative to conventional honing for hydraulic cylinder tubes. It combines two operations in a single pass:
- Skiving: A cutting head removes 0.3–0.5 mm of stock, correcting ovality and straightness with a single-point cutting tool
- Roller burnishing: Rollers cold-work the surface, compressing peaks into valleys to achieve Ra 0.2–0.4 µm finish
Comparison with Conventional Honing
| Parameter | Conventional Honing | Skiving + Roller Burnishing |
|---|---|---|
| Stock removal | 0.02–0.15 mm (typical) | 0.3–0.5 mm per pass |
| Cycle time | Multiple strokes (minutes) | Single pass (seconds) |
| Surface finish (Ra) | 0.05–0.8 µm | 0.2–0.4 µm |
| Tolerance | IT5–IT7 | H8–H9 |
| Geometry correction | Excellent — taper, ovality, straightness | Good — ovality and straightness |
| Cross-hatch pattern | Yes — functional texture | No — burnished surface |
| Equipment cost | High | Moderate |
| Cycle time reduction | Baseline | 30–40 % faster |
Skiving and roller burnishing is preferred for high-volume production of hydraulic cylinder tubes where the cross-hatch pattern is not required and H8–H9 tolerance is acceptable. For applications requiring IT7 or better, or where surface texture specification demands a cross-hatch, conventional honing remains the standard.
FAQ
How much stock should I leave for honing after gun drilling?
For gun-drilled bores requiring only surface finish improvement, 0.02–0.08 mm on diameter is sufficient. If geometry correction for taper or ovality is needed, leave 0.05–0.15 mm. A practical rule is to leave twice the measured bore error as stock allowance.
What controls the cross-hatch angle in honing?
The cross-hatch angle is controlled by the ratio of rotational speed (RPM) to reciprocating stroke speed. Increasing rotational speed relative to stroke speed produces a shallower angle; increasing stroke speed produces a steeper angle. The relationship is given by tan(θ/2) = vₛ/vᵣ.
What is the typical cross-hatch angle for hydraulic cylinder tubes?
Hydraulic cylinder tubes typically specify a cross-hatch angle of 30°–60°, with 40°–50° being the most common range. The specific angle is chosen based on seal type, operating pressure, and lubrication requirements. Steeper angles provide better oil retention; shallower angles reduce friction.
Can honing correct bore straightness?
Yes, honing corrects straightness errors within limits. The averaging effect of the reciprocating abrasive stones removes material preferentially where the bore is tighter, gradually straightening the bore axis. However, honing cannot correct large axis deviations or positional errors relative to external datums.
What grit sequence should I use for deep hole honing in steel?
A three-stage sequence is standard: 120–180 grit (roughing, geometry correction), 220–280 grit (semi-finishing), and 320–400 grit (finishing). For plateau honing, add a fourth stage with 600–1000 grit at very low pressure.
Is honing better than internal grinding for deep holes?
For most deep hole applications, honing is preferred over internal grinding. Honing operates at lower speeds and temperatures, avoiding thermal damage. The averaging effect of honing stones corrects geometry errors more effectively. Internal grinding is necessary only for very hard materials (> 65 HRC) where abrasive stone wear would be excessive.
What is the difference between honing and skiving/roller burnishing?
Honing uses bonded abrasive stones that rotate and reciprocate to generate a cross-hatch pattern. Skiving and roller burnishing uses a cutting tool for stock removal followed by rollers that cold-work the surface. Honing produces a functional surface texture; skiving/burnishing produces a smooth, burnished surface without cross-hatch.
What surface finish can honing achieve in deep holes?
Honing reliably achieves Ra 0.2–0.8 µm in production. With fine grits and plateau honing, Ra 0.05–0.2 µm is achievable. The surface finish depends on grit size, pressure, and the number of finishing passes.
Summary
Honing is an essential finishing process for deep hole drilling applications where bore geometry, surface finish, and surface texture are critical for function. It simultaneously corrects taper, ovality, and straightness errors while generating the characteristic cross-hatch pattern that enables seal performance and lubrication retention.
The key parameters for successful deep hole honing are: stock allowance (typically 0.02–0.15 mm), abrasive selection matched to workpiece material, multi-stage grit sequencing, correct cross-hatch angle selection (30°–60° for most applications), and appropriate pressure and speed settings.
For hydraulic cylinder tubes and other long-bore applications, horizontal tube honing machines with CNC control provide the stock removal capacity and geometry correction capability needed for production finishing. The alternative skiving and roller burnishing process offers faster cycle times for applications where the cross-hatch pattern is not specified and tolerance requirements are H8–H9 or looser.
The decision to specify honing after deep hole drilling should be based on three requirements: the need for geometry correction (taper, ovality, straightness), the need for surface texture (cross-hatch), and the required tolerance grade (IT5–IT7 for honed bores).