Appearance
The bore finishing method you choose determines not just surface finish and tolerance, but whether the part seals, slides, rotates, or fails — and choosing wrong means finding out at final inspection.
Overview
Five processes dominate bore finishing: drilling, boring, reaming, skiving, and honing. Each occupies a distinct position in the capability space defined by surface finish, dimensional tolerance, stock removal rate, and cycle time.
| Process | Primary Function | Material Removal | Typical Applications |
|---|---|---|---|
| Drilling | Create initial hole | Full hole volume | Starting point for all holemaking |
| Boring | Enlarge and position | 0.1 – 5.0 mm per side | Precision diameters, corrected location |
| Reaming | Finish to size | 0.05 – 0.5 mm per side | Fit holes, dowel pin holes |
| Skiving | Correct geometry, heavy stock removal | 0.5 – 10 mm per side | Hydraulic cylinders, long bores |
| Skiving + burnishing | Finish in one pass | 0.2 – 1.0 mm per side | Production cylinder tubes |
| Honing | Ultra-precision finish | 0.01 – 0.1 mm | Engine cylinders, valve bores |
Drilling
Drilling creates the initial hole and sets the foundation for all subsequent operations. As a finishing process, it is limited.
| Parameter | Typical Range |
|---|---|
| Surface finish (Ra) | 1.6 – 6.3 µm |
| Dimensional tolerance | IT8 – IT11 |
| Positional accuracy | ±0.1 – 0.3 mm (standard drill) |
| Roundness | 0.02 – 0.10 mm |
Drilling alone is rarely specified as a finished bore for precision applications. It serves as the starting point for reaming, boring, or skiving. The quality of the drilled hole directly affects the performance and tool life of subsequent finishing operations.
Boring
Boring uses a single-point cutting tool to enlarge an existing hole. It is the most versatile bore finishing method, spanning from rough stock removal to precision finishing.
| Boring Type | Surface Finish (Ra) | Tolerance (IT) | Stock Removal per Side |
|---|---|---|---|
| Rough boring | 6.3 – 12.5 µm | IT12 – IT13 | 2.0 – 5.0 mm |
| Semi-finish boring | 3.2 – 6.3 µm | IT9 – IT10 | 0.5 – 2.0 mm |
| Fine boring | 0.8 – 1.6 µm | IT7 – IT8 | 0.2 – 0.5 mm |
| Diamond boring | 0.05 – 0.4 µm | IT6 – IT7 | 0.1 – 0.3 mm |
Boring corrects position errors
Unlike reaming and skiving, boring uses a single-point tool on an adjustable diameter. This means boring can correct hole location errors from previous operations. Reaming and skiving tools follow the existing hole axis — they improve size and finish but cannot move the hole center.
Reaming
Reaming uses a multi-fluted cutting tool to remove a small amount of material from an existing hole, producing a precise diameter with good surface finish.
| Parameter | Standard Reaming | Precision Reaming |
|---|---|---|
| Surface finish (Ra) | 0.8 – 3.2 µm | 0.2 – 0.8 µm |
| Dimensional tolerance | IT7 – IT9 | IT6 – IT7 |
| Stock removal per side | 0.1 – 0.3 mm | 0.05 – 0.15 mm |
| Typical diameter range | 3 – 80 mm | 3 – 50 mm |
Reaming is the most cost-effective method for achieving IT7 tolerance in the 3–50 mm diameter range. It is widely used for fastener fit holes, dowel pin holes, and bearing seats.
Reaming Limitations
- Cannot correct position — the reamer follows the existing hole axis
- Fixed diameter — each reamer produces one specific hole size
- Limited stock removal — excessive material causes chatter and poor finish
- Chip evacuation — reaming in blind holes requires careful chip management
Skiving
Skiving uses multiple carbide cutting blades arranged around a floating tool body to remove significant material from a bore. It was developed for hydraulic cylinder tube finishing where long bores (1–18 m) need geometry correction and surface preparation.
| Parameter | Skiving Alone | Skiving + Roller Burnishing |
|---|---|---|
| Surface finish (Ra) | 4 – 10 µm | 0.05 – 0.20 µm |
| Dimensional tolerance | IT8 | IT7 – IT8 |
| Stock removal per side | 0.5 – 10 mm | 0.2 – 1.0 mm |
| Roundness | 0.03 – 0.05 mm/Ø200 mm | 0.020 – 0.036 mm/Ø200 mm |
| Surface hardness change | None | +30 – 50% |
Skiving vs Reaming
Skiving resembles heavy-duty reaming but differs in key ways:
| Factor | Skiving | Reaming |
|---|---|---|
| Stock removal | 0.5 – 10 mm per side | 0.05 – 0.5 mm per side |
| Tool design | Multiple floating carbide blades | Fixed or floating cutting edges |
| Feed rate | 1 – 6 mm/rev | 0.1 – 0.5 mm/rev |
| Surface after process | Ideal for burnishing | Final finish |
| Bore length capability | Up to 18 m | Typically < 1 m |
Skiving + Roller Burnishing (SRB)
When skiving is combined with roller burnishing in a single pass, the process achieves mirror-like surface finishes (Ra 0.05–0.20 µm) at 8–20 times the speed of honing. The skiving blades remove material and correct geometry; the burnishing rollers immediately follow, cold-compressing the surface to its final finish.
Honing
Honing uses bonded abrasive stones to remove微量 material and achieve the highest geometric accuracy of any bore finishing process.
| Parameter | Standard Honing | Precision Honing |
|---|---|---|
| Surface finish (Ra) | 0.1 – 0.8 µm | 0.025 – 0.4 µm |
| Dimensional tolerance | IT6 – IT7 | IT4 – IT5 |
| Stock removal per side | 0.01 – 0.10 mm | 0.005 – 0.05 mm |
| Roundness | 0.002 – 0.010 mm | 0.001 – 0.005 mm |
| Cylindricity | 0.005 – 0.020 mm | 0.002 – 0.010 mm |
Honing produces a characteristic crosshatch surface pattern that retains oil — critical for engine cylinder and hydraulic applications where lubrication is required between sliding surfaces.
Surface finish alone does not tell the full story
A process that achieves Ra 0.05 µm is not necessarily better for every application. Honing's crosshatch pattern retains oil for lubrication; skived and burnished surfaces are mirror-smooth but do not hold oil. For dynamic seals running on dry or minimally lubricated surfaces, skiving+roller burnishing is superior. For engine cylinders requiring oil retention, honing is essential. Evaluate the full application requirements — not just the Ra number.
Process Selection Guide
| Application Requirement | Recommended Process | Why |
|---|---|---|
| Maximum stock removal | Skiving or rough boring | Removes 0.5–10 mm per side efficiently |
| Tightest tolerance (IT4–IT6) | Honing | Only process achieving below IT6 consistently |
| Best surface finish (Ra < 0.1 µm) | Honing or SRB | Both achieve Ra down to 0.05 µm |
| Fastest cycle time | Drilling or SRB | Single-pass processes |
| Lowest cost per hole | Drilling | No secondary operation needed |
| Position correction | Boring | Single-point tool adjusts to correct location |
| Fixed diameter finishing | Reaming | Most economical for IT7 at small diameters |
| Long bore finishing (1–18 m) | Skiving + burnishing | Only practical process for long tubes |
| Oil retention surface | Honing | Crosshatch pattern retains lubricant |
| Seal surface for PTFE/polyurethane | Skiving + burnishing | Mirror finish improves seal life |
| Surface hardness increase | Skiving + burnishing | Cold-working increases hardness 30–50% |
| Fatigue life improvement | Skiving + burnishing | Compressive residual stress improves fatigue |
Summary
| Metric | Drilling | Boring | Reaming | Skiving | SRB | Honing |
|---|---|---|---|---|---|---|
| Ra range (µm) | 1.6 – 6.3 | 0.05 – 12.5 | 0.2 – 3.2 | 4 – 10 | 0.05 – 0.20 | 0.025 – 0.8 |
| IT grade | 8 – 11 | 6 – 13 | 6 – 9 | 8 | 7 – 8 | 4 – 7 |
| Stock removal (mm/side) | Full hole | 0.1 – 5.0 | 0.05 – 0.5 | 0.5 – 10 | 0.2 – 1.0 | 0.005 – 0.1 |
| Corrects position? | N/A | Yes | No | No | No | No |
| Cycle time | Fastest | Moderate | Fast | Fast | Fast | Slow |
| Tool cost | Low | Moderate | Moderate | High | High | Moderate |
| Surface hardening | No | No | No | No | Yes (+30–50%) | No |
| Oil retention | Poor | Poor | Poor | Poor | Low | Good (crosshatch) |
FAQ
What is the difference between reaming and skiving?
Reaming removes 0.05–0.5 mm per side to produce a precise diameter with Ra 0.2–3.2 µm finish. Skiving removes 0.5–10 mm per side using multiple carbide blades, correcting bore geometry and producing a Ra 4–10 µm surface ideal for subsequent roller burnishing. Skiving is essentially heavy-duty reaming designed for long bores and high stock removal.
Which process produces the best surface finish for bores?
Skiving + roller burnishing (SRB) and precision honing both achieve Ra 0.05 µm or better. SRB produces a mirror-like, work-hardened surface with compressive residual stress. Honing produces a crosshatch pattern that retains oil. The "best" finish depends on the application — sealing surfaces benefit from SRB, while lubricated sliding surfaces need honing's oil retention.
Can reaming correct a misaligned hole?
No. Reaming follows the existing hole axis — if the drilled hole is misaligned, the reamed hole will also be misaligned. Boring is the only process that can correct hole location because it uses a single-point tool with adjustable diameter. For critical position requirements, bore to correct the location, then ream or hone for finish.
What tolerance can drilling achieve?
Standard drilling achieves IT8–IT11 depending on drill condition, material, and setup. For most production work, expect IT9–IT10. Drilling alone is not specified for precision fit applications — reaming, boring, or honing is required for tighter tolerances.
When should I choose skiving over boring?
Choose skiving over boring when: the bore is longer than 1 meter (skiving handles up to 18 m), the bore diameter is consistent and requires geometry correction, the application allows a floating tool (skiving follows the existing bore), or skiving will be combined with roller burnishing for a single-pass finish. Choose boring when: hole position must be corrected, the part is short (< 1 m), or single-point tool flexibility is needed for varying diameters.
Is honing always better than skiving and roller burnishing?
No. Honing achieves tighter tolerances (IT4–IT6 vs IT7–IT8) and provides oil-retaining crosshatch, but it is 8–20 times slower than SRB, does not increase surface hardness, and does not introduce compressive residual stress. For high-volume hydraulic cylinder production where the sealing surface does not require oil retention, SRB is superior in both cycle time and part quality.
Process capabilities depend on machine condition, tooling quality, workpiece material, and operator skill. The values in this article represent typical production ranges. Consult process engineers and tool suppliers for application-specific recommendations. This article reflects industry knowledge as of 2026.