Deep hole drilling rarely delivers the final bore. Reaming, honing, roller burnishing, and skive & burnish bring the as-drilled hole to specification — tighter tolerance, better finish, and a stronger surface layer. This guide covers process selection, achievable results, stock removal, and setup practice.
As-drilled finishes typically land in the Ra 1.6–6.3 µm band depending on method, material, and parameters; gundrilling on a good day holds Ra 0.4–1.8 µm. Functional bores demand far better, and many callouts sit below Ra 0.8 µm — a threshold that virtually always requires a dedicated finishing pass.
Finishing does two jobs at once. It corrects geometry the drill left behind — taper, ovality, bell-mouth, and straightness drift — and it changes the surface layer. Cutting processes leave tensile residual stress and feed marks that act as stress concentrators; cold-working processes (burnishing, skive & burnish) leave a compressive layer that blocks crack initiation and measurably extends fatigue life.
The most common secondary operation for deep holes. A multi-flute reamer removes a thin layer (0.1–0.5 mm on diameter) with a shear cut to improve size and finish while preserving the existing hole axis. Deep hole reamers carry carbide or PCD edges and push coolant through the tool body for chip control at depth.
| Attribute | Value |
|---|---|
| Achievable finish | Ra 0.8–3.2 µm; fine reaming to ~Ra 0.2–0.4 µm demonstrated |
| Tolerance | IT6–IT8; aerospace bores to ±0.0005 in on diameter |
| Stock removal | 0.1–0.5 mm on diameter (rough allowance 0.15–0.35 mm, fine 0.05–0.15 mm) |
| Cutting data | Vc 20–60 m/min, feed 0.05–0.3 mm/rev; HSS reamers run slower (<8 m/min on steel and cast iron) |
| Surface integrity | As-cut, no compressive layer; feed marks can concentrate stress |
BTA reaming (self-piloting reaming) sizes pre-drilled, cast, or rolled holes with light stock removal at high speeds and feeds — accuracy IT6–IT9, finish Ra 0.4–1.6 µm. Heads are available chip-forward (through holes) and chip-back (blind holes), with 45° or 20° lead angles. Per-side depths of cut run ~1 mm on small heads up to ~3 mm on large ones when sizing as-drilled blanks (American Heller RBTU data).
Bonded abrasive stones on an expanding mandrel rotate and reciprocate simultaneously, tracing a figure-eight path that leaves the characteristic cross-hatch. That pattern is functional: intersecting grooves hold oil for lubrication while plateaus carry the load. Honing corrects out-of-roundness, taper, bell-mouth, and barrel shape with very small stock removal.
Also called skiving and roller burnishing. The tool carries carbide skiving knives that cut a thin layer off the bore, immediately followed by rollers that cold-work the freshly cut surface to a mirror finish — geometry from the cutting edge, finish from the rollers, in a single pass.
| Attribute | Value |
|---|---|
| Achievable finish | Ra 0.05–0.4 µm; mirror finishes of 0.13–0.20 µm reported |
| Tolerance | IT6–IT8; bore diameter and roundness to 0.0005 in (0.013 mm) |
| Stock removal | 0.05–0.2 mm on diameter |
| Surface layer | Compressive residual stress; surface hardness up to ~10 HRC higher |
| Productivity | One pass; ~10–20× the speed of honing — roughly 1 m finished in 1–3 minutes |
| Cost | 60–75% cheaper per hole than honing at volume |
Configurations: the tool skives on the feed stroke and burnishes on the return (push-pull), or a one-direction head does both in a single forward pass. Tooling is specific to bore diameter and length. VDI 3209 Blatt 2 gives the reference data for skiving and roller burnishing of bores, and is used alongside VDI 3210 and VDI 3209 Part 1.
Pull boring draws the cutting head back through the workpiece on a tensioned bar instead of pushing it. The bar rotates one way while the workpiece counter-rotates, and coolant around the bar washes chips out through the hollow center. Because the tool works under tension, pull boring is the most effective finishing pass for straightness in long bores.
Chipless and chip-free: hardened rollers press the bore surface so the peaks flow plastically into the valleys. No material is removed — the process rearranges what is there, so the stock allowance is zero. The trade-off is that burnishing only works on a surface that is already close to shape and finish.
| Attribute | Value |
|---|---|
| Achievable finish | Ra 0.05–0.4 µm (tool makers quote Ra ≤0.1 µm with suitable tools); a copper comparison measured Ra 0.09 µm — best of all methods tested |
| Hardness gain | +5–10% from work hardening (up to ~10 HRC reported) |
| Fatigue | Life improved 20–50% by the compressive residual stress layer |
| Size control | Within 0.0005 in or better on diameter |
| Range | Ø3.7–200 mm and up, on through and blind holes |
| Materials | Plastically formable metals to ~HRC 45 / 1400 N/mm² (deep rolling to 65 HRC) |
Lapping uses loose abrasive between the bore and a lap tool to remove sub-micron layers by three-body abrasion. It is the slowest and most expensive finishing option, reserved for the bores where nothing else is good enough.
| Process | Achievable Ra | Tolerance (IT) | Stock Removal | Capability | Relative Cost |
|---|---|---|---|---|---|
| Reaming | 0.8–3.2 µm | IT6–IT8 | 0.1–0.5 mm | Sizes the hole; follows the axis | Low |
| BTA reaming | 0.4–1.6 µm | IT6–IT9 | 0.2–0.8 mm (to ~3 mm/side on large heads) | Sizes as-drilled blanks at high feed | Moderate |
| Honing | 0.1–0.8 µm | IT5–IT7 | 0.01–0.1 mm | Corrects form; cross-hatch for oil retention | Moderate–High |
| Roller burnishing | 0.05–0.4 µm | IT7–IT9 | None (cold deformation) | Mirror + compressive layer; needs a good prior bore | Low–Moderate |
| Skive & burnish | 0.05–0.4 µm | IT6–IT8 | 0.05–0.2 mm | One-pass finish + geometry + compressive layer | Highest tooling, lowest per-part at volume |
| Pull boring | 0.2–1.6 µm | IT7–IT9 | 0.2–1.0 mm+ | Best straightness correction; sizes and rounds | Moderate–High |
| Lapping | 0.025–0.2 µm | IT5+ | Microns | Sub-micron texture; slow | Highest |
For hydraulic and pneumatic cylinder tubes — the classic skive & burnish application — the economics are the whole story. Where conventional honing needs 2–5 passes to size and finish a bore, skive & burnish does it in one: the carbide knives true the round and cut to size, and the rollers behind them cold-work the surface in the same stroke. Published comparisons put it at 60–90% faster than honing and 60–75% cheaper per hole, with a harder surface (up to ~10 HRC) and better seal life.
Rollers iron the surface and leave deep compressive residual stress that seals micro-cracks and pushes crack initiation deeper into the part. For fatigue-critical components — landing gear, turbine shafts, suspension hardware — specify burnishing or skive & burnish: the compressive layer extends component life by roughly 2–5× versus as-drilled or reamed surfaces. It is the single most valuable finishing trick in precision deep bores.
Skive & burnish works best on straight, consistent starting stock — cold-drawn and seamless cylinder tubes are ideal. It is more sensitive to the initial curvature of the raw tube than a boring-plus-burnish line; if raw tubes are heat-treated, tempered, or variable, expect higher rejection unless the process is tuned for it.
| Application | Recommended Process | Why |
|---|---|---|
| Hydraulic cylinder tubes | Skive & burnish | Mirror finish + tight tolerance in one pass; compressive layer improves fatigue life |
| Bearing surfaces (connecting rods, shafts) | Honing | Cross-hatch for oil retention; tightest tolerances (IT5–IT6) |
| Precision valve bores | Reaming or honing | Ream for moderate finish at low cost; hone for the highest precision |
| Static seal surfaces | Roller burnishing | Mirror finish eliminates leaks; cold working raises surface hardness |
| Fuel injector bores | Reaming (PCD) | Sharp edge holds; consistent size for metering accuracy |
| Mold cooling channels | Roller burnishing | Smooth surface improves heat transfer; no cutting edges to break in small bores |
| Gun barrels / rifled bores | Pull boring then honing | Precision geometry required before the rifling operation |
| Large BTA-drilled bores (>50 mm) | BTA reaming or skive & burnish | High stock removal capacity; single-pass finishing to tight tolerance |
Stock removal figures below are per side and assume the bore is within straightness and roundness before finishing. Leave too little and the tool cannot clean up the drilling marks; leave too much and finish quality, tool life, and cycle time all suffer.
| Bore Diameter | Reaming (per side) | Honing (per side) | Skive & Burnish (per side) |
|---|---|---|---|
| < 10 mm | 0.05–0.15 mm | 0.005–0.025 mm | 0.025–0.05 mm |
| 10–50 mm | 0.10–0.25 mm | 0.01–0.05 mm | 0.05–0.10 mm |
| 50–200 mm | 0.15–0.40 mm | 0.02–0.08 mm | 0.075–0.15 mm |
| > 200 mm | 0.25–0.50 mm | 0.03–0.10 mm | 0.10–0.20 mm |
Straightness and size are set at the drill. Finishing cleans up only the last fraction of a millimetre.
Check taper, roundness, and bell-mouth against the finishing allowance before committing a costly tool.
Required before roller burnishing — burnishing cannot fix a rough or out-of-round bore.
Every reamer, hone, and skive head follows what the start bore offers — misalignment shows up as a bell mouth.
Chips recirculated through the bore scratch a finish that cost you money to make. Filter to 10–30 µm for finishing passes.