Beyond gundrilling, BTA and the ejector system, a family of specialized processes handles the jobs the big three cannot do well: trepanning preserves an expensive core, skive & roller burnishing replaces honing in a single pass, pull boring reaches extreme straightness by cutting in tension, and counterboring, step drilling and multi-diameter boring enlarge, true, or shape existing holes.
The three core methods — gundrilling, BTA (single tube system, STS) and the ejector system (double tube system, DTS) — create primary holes from solid material. The specialized methods in this guide do not replace them; they extend them. Some refine a rough bore to a finished surface (skive & roller burnishing), some push straightness beyond what push-cutting can manage (pull boring), some preserve expensive material instead of converting it to chips (trepanning), and some enlarge, step, or shape an existing hole (counterboring, step drilling, multi-diameter and bottle boring).
Every specialized method plugs into one of the three core systems for coolant delivery and chip evacuation.
| Method | Primary role | Works on | Signature result |
|---|---|---|---|
| Trepanning | Large-diameter, material-preserving cutting | Solid, through-hole | Bore + reusable solid core |
| Skive & Roller Burnishing | Finish and surface-treat in one pass | Rough-bored tube | Ra < 0.2 μm, IT7–IT8 |
| Pull Boring | Maximum-straightness finishing | Drilled / cast / rough hole | ≤ 0.001 in/ft straightness |
| Counterboring | Enlarge an existing hole true | Drilled / cored hole | True, larger diameter |
| Step Drilling | Progressive enlargement in stages | Pilot hole | Large or stepped bores, low torque |
| Multi-Diameter / Bottle Boring | Internal profiles and cavities | Existing bore | Stepped / bottle / contour shapes |
Trepanning removes only the outer circumference of the bore, preserving the center slug as a solid cylinder. An annular cutting head — essentially a hollow coring tool with inserts mounted around its periphery — cuts a concentric ring of metal instead of the full cross-section. This cuts chip volume by roughly 60% and, per practical guidance, needs only about 10 hp per 1 inch of cutting diameter — a fraction of the spindle power required for solid drilling at the same size.
Why the core matters: The retained slug can be re-used for other parts, recycled at a far better price than chips, or archived for testing. In expensive alloys such as titanium, Inconel or Hastelloy, the material saving alone can justify the more complex tooling — reclamation yields as high as ~82% material utilization are reported. The retained core typically runs about 1.0–1.5 inch smaller than the trepanned bore.
Chip evacuation: Trepanning uses the BTA (STS) principle — coolant is delivered through the annulus and chips are evacuated through the center of the tube, around the retained slug. Coolant runs at lower pressure but higher flow than solid deep-hole drilling. The slug must be held by a retention device and periodically retrieved; in horizontal setups a long, heavy core may need a support or plug so it does not fall on breakout and chip the inner insert.
| Parameter | Specification |
|---|---|
| Typical diameter range | 90–600 mm (indexable tools); 140–350 mm (industrial systems); large systems to ~500 mm |
| Starting size | ~51 mm (2 in) and up; below ~90 mm solid BTA is usually preferred |
| Depth capability | Up to 11 m; L/D typically < 50:1 |
| Core preservation | ~60% reduction in chip volume; 30–50% less spindle power |
| Straightness | 0.001–0.002 in/ft |
| Surface finish | 250–500 μin Ra (rough) — honed or burnished after |
| Working arrangements | Stationary tool / rotating work; rotating tool / stationary work; counter-rotation of both |
| Typical applications | Large valve bodies, thick-walled cylinders, heat exchanger tube sheets, offshore oil & gas components |
This combined process completes rough cutting and surface finishing in a single pass through the bore. Skiving uses carbide blades — mechanically a twin/multi-edge floating reamer — to remove stock to final dimensional accuracy, while roller burnishing follows immediately: hardened rollers cold-work the surface, plastically deforming peaks into valleys rather than cutting. The result is a mirror-like “plateau” profile with compressive residual stress that improves fatigue life and seal performance.
The process is standardized under VDI 3209 Blatt 2 (2019), which provides official approximate values for skiving and roller burnishing of bores on deep-hole boring machines, primarily for hydraulic cylinder tube production. Combined tools are available in push or pull operating directions, are hydraulically (or pneumatically) activated and retracted, and are increasingly engineered for standard CNC lathes using high-pressure internal coolant (70–120 bar).
Combined vs. two-step: The one-pass skive & burnish route is far more efficient but sensitive to the initial curvature of the raw tube, so it prefers higher-quality cold-drawn material. The two-step route (deep-hole boring, then roller burnishing) delivers more stable straightness and corrects initial tube curvature — at lower efficiency. In a comparison on Ø400 mm cylinders, push-boring + burnishing showed ~30% lower dimensional dispersion and ~35% lower cylindricity dispersion than skive + burnish.
| Parameter | Specification |
|---|---|
| Diameter range | 38–402 mm (typical 50–306 mm); combined tools from ~Ø20 to ~Ø400 mm |
| Stock removal per side (skiving) | 0.05–0.50 mm (typical 0.30 mm); up to 3 mm on diameter |
| Surface roughness (as-processed) | Ra < 0.2 μm, as low as Ra 0.05–0.1 μm; Rz < 1 μm |
| Tolerance grade | IT7–IT8 (can reach IT7) |
| Cutting speed | 100–300 m/min |
| Feed rate (skiving) | 1–3 mm/rev typical (1–5 mm/rev self-feeding) |
| Burnishing feed force | 4,000–15,000 N (approx. 6,000 N at 80 mm diameter) |
| Tool life | Skive inserts ≥ 50 m/edge at 200 m/min; burnishing rolls > 200 m; mandrel > 400 m |
| Machine power required | 40–100 kW (net skiving power 6 kW at 40 mm to 30 kW at 200 mm) |
| Typical parts | Hydraulic, pneumatic and oil cylinders, telescopic booms |
Pull boring is a finishing process for existing holes — drilled, cast or rough-bored. The critical difference from conventional push boring: the tool is pulled through the bore rather than pushed. This places the boring bar in tension rather than compression. A tensioned bar self-straightens along the hole axis; a pushed bar buckles under compression, letting the cutting edge wander off centerline. That single change makes pull boring the straightness champion among boring processes.
Process sequence:
Straightness in practice: Reported hole drift can be less than 0.001 in per foot (0.08 mm/m) of workpiece length under favorable material, tooling and process conditions (several passes may be required). For concentricity, a multi-point head with 2–6 inserts and wear pads positioned in front of the inserts rides on the existing bore. Pull boring is well established in aerospace: dedicated suppliers quote 0.001 in/ft straightness on bores from 0.078 to 24 in diameter at depths up to 480 in, working Inconel and titanium.
Case study — jet engine turbine shaft: Inconel 718, 46 in (1168 mm) long, existing bore. Pull boring achieved hole concentricity of 0.0006 in (0.002 in TIR) with a 32–60 microinch rms finish — a level of precision essentially unreachable with conventional push boring.
| Parameter | Specification |
|---|---|
| Straightness | Up to 0.0005–0.001 in/ft (0.04–0.08 mm/m) |
| Typical tolerance | Tool drift < 0.001 in TIR per foot |
| Surface finish | 32–60 microinch rms (0.8–1.6 μm Ra) typically |
| Push vs pull deviation | Pull boring 0.05 mm/m; push boring 0.15 mm/m |
| Tooling options | Single-point (general straightness) or multi-point, 2–6 inserts (better concentricity) |
| Guide pad placement | Single-point: behind the insert; multi-point: in front of inserts (rides on existing bore) |
| Tool material | Carbide (standard); PCD ~200× longer tool life, ideal for high-silicon aluminum |
| Capability references | Bores 0.078–24 in diameter, depths to 480 in, straightness 0.001 in/ft |
Counterboring enlarges an existing hole — cast, cored or previously drilled — to a larger diameter. The BTA counterboring head pilots on the existing bore, which keeps the new centerline true and removes only the annular stock between the pilot and the final size. It is the standard way to reach large final diameters without the torque and chip load of solid drilling, and it cleans up the misalignment left by a cast or cored hole.
Where it appears: Counterboring is used heavily in hydraulic cylinder production — hot-rolled tubes, whose wall thickness fluctuates, typically get an upstream counterboring pass before skiving and burnishing. In the BTA world it covers a very broad envelope: TU Dortmund’s research program cites d = 10–1500 mm across BTA variants, commercial machines counterbore effectively up to ~630 mm, and indexable counterboring heads are standard from ~Ø18–40 mm upward (the Sandvik CoroDrill 818 covers 40–301.75 mm). Because it is a BTA/STS operation, coolant and chips travel through the drill tube and process tolerances sit around IT9.
| Parameter | Specification |
|---|---|
| Diameter range | ~Ø18–630 mm typical; 10–1500 mm across all BTA variants |
| Typical size bands | 40–301.75 mm indexable standard; to ~630 mm on industrial machines |
| Start point | Existing pilot / cored / drilled hole |
| Chip evacuation | BTA/STS — through the drill tube |
| Hole tolerance | ~IT9 |
| Applications | Hot-rolled hydraulic tubes, valve bodies, large bores reached in stages |
Step drilling builds a final bore from a series of progressively larger diameters instead of one full-cross-section pass. A small pilot is drilled first — by gundrill, twist drill or a first BTA pass — then each subsequent pass enlarges the hole by a modest step. Each pass carries only the annular stock of that step, so torque and chip load stay low even for very large or very deep final bores.
Why step: Step drilling is standard practice for long aerospace turbine shafts, which are often step-drilled with multiple tool diameters in superalloys such as Inconel 718 and Waspaloy. It also lets a machine reach a final diameter beyond its single-pass solid-drilling limit (through counterboring steps), and it lets the operator correct deviation between steps — each pass re-guides the tool on a truer bore than the last.
| Parameter | Specification |
|---|---|
| Principle | Pilot hole, then progressive enlargement |
| Typical step size | 2–8 mm on diameter per pass (material and machine dependent) |
| Chip evacuation | Gundrill or BTA/STS, depending on head type |
| Typical use | Superalloy turbine shafts, large hydraulic bores, stepped ports |
| Depth capability | Inherits the parent system (gundrill L/D to ~100–300:1; BTA to ~200:1) |
| Tolerance build | Each pass improves on the pilot’s position error |
Multi-diameter boring produces internal profiles rather than a single straight bore — stepped sections, tapered walls, or a cavity wider than the access hole. Bottle boring is the signature case: an actuator expands the cutting head once it is inside the part, machining an enlarged internal chamber through a smaller-diameter mouth — exactly like widening the inside of a bottle. Contour boring machines continuously varying internal profiles.
These processes are driven by actuator-extended cutting heads fed through an existing bore, and they are a mainstay of aerospace landing gear production, where internal cavities and profiles carry structural loads and oil galleries. Suppliers such as Hunting Energy Services list bottle bores and contour bores among the high-complexity, mission-critical parts they machine for aerospace.
| Parameter | Specification |
|---|---|
| Principle | Actuator-expanded cutting head machines an internal profile |
| Signature shape | Bottle bore — internal chamber larger than the mouth |
| Profile types | Stepped, tapered, bottle, continuous contour |
| Coolant / chips | Via the BTA/STS drill tube |
| Typical depths | Landing gear bores to ~2 m and beyond |
| Primary use | Aerospace landing gear, structural cylinders, oil galleries |
Roles, envelopes and results side by side.
| Method | Role | Diameter | Depth | Finish Ra | Tolerance | Pre-condition |
|---|---|---|---|---|---|---|
| Trepanning | Material-preserving large bore | 90–600 mm | to 11 m; L/D < 50:1 | 250–500 μin (rough) | Rough — IT10–IT12 | Through-hole; core handled |
| Skive & Roller Burnishing | Finish + surface treatment | Ø20–402 mm | Tube length | 0.05–0.3 μm | IT7–IT8 | Rough-bored tube |
| Pull Boring | Maximum-straightness finish | ~2–600 mm+ | to ~12 m (480 in) | 0.8–1.6 μm | IT6–IT8 | Existing hole |
| Counterboring | Enlarge existing hole true | Ø18–630 mm | L/D to ~50:1 | 3.2+ μm | ~IT9 | Pilot / cored hole |
| Step Drilling | Progressive enlargement | pilot → final | System L/D | Per final pass | Per final pass | Pilot hole |
| Bottle / Contour Boring | Internal profiles & cavities | chamber > mouth | to ~2 m+ | Per application | Per application | Access bore |
Match the scenario to the method, then verify with the master table above.
| Situation | Recommended method | Why |
|---|---|---|
| Large diameter, expensive alloy, material must be recovered | Trepanning | Preserves the core; 30–50% less spindle power |
| Hydraulic / pneumatic cylinder tube needing a seal-grade bore | Skive & Roller Burnishing | One pass to Ra < 0.2 μm with compressive residual stress |
| Long shaft where straightness is the critical callout | Pull Boring | Bar in tension holds ~0.001 in/ft |
| Cast or cored hole to be brought true and larger | Counterboring | Pilots on the existing bore |
| Deep superalloy shaft needing staged enlargement | Step Drilling | Low torque; re-guides on each pass |
| Internal chamber wider than the mouth | Bottle / Contour Boring | Actuator-expanded cutting head |
Specialized methods do not stand alone — they run on the same coolant and chip-evacuation architectures as the three core methods. Trepanning and counterboring are BTA/STS derivatives (chips through the drill tube). Skive & burnish and pull boring are finishing passes that follow a primary hole. Step drilling is a strategy usable inside any of the three systems, and bottle boring extends a BTA/STS bore into an internal profile.
| Core system | Chip path | Machine | Envelope | What it feeds |
|---|---|---|---|---|
| Gundrilling | External V-flute; high-pressure coolant through the tool | Gundrill machines / spindles | Ø0.5–50 mm; L/D to ~100–300:1; tolerance ±0.01–0.025 mm | Small precision bores, pilots, fuel injectors, medical, mold cooling |
| BTA (STS) | Through the drill tube — chips never touch the bore | Dedicated, with pressure head and guide bushing | Ø14.5–1500 mm; IT9; higher coolant pressure and flow | Trepanning, counterboring, bottle boring, pull-boring parent |
| Ejector (DTS) | Through the inner tube via Venturi suction | Conventional lathes / machining centers; no pressure head | Ø18.4–250 mm; ~1000 mm depth (to ~2000 mm with special connector); IT9–IT10 | Retrofit primary holes |