Appearance
There are three ways to make a deep hole: cut it from solid, enlarge a hole that already exists, or cut an annulus and extract the center as a solid core. Each method changes everything — the tool design, the starting requirements, the chip volume, and the quality expectations. Using a BTA solid drill in a counterboring application will produce an oversize, misaligned hole. Using a counterboring tool from solid will break the tool.
Overview
BTA (Single Tube System) and ejector (Double Tube System) drilling tools are not limited to drilling from solid. The same machine and coolant system can be configured for three distinct operations:
| Operation | Starting Condition | Material Removal | Chip Form | Typical Applications |
|---|---|---|---|---|
| Solid drilling | No pre-existing hole | Full cross-section removed as chips | High chip volume per depth | New holes in solid material |
| Counterboring / peeling | Pre-drilled or pre-cored hole | Annular ring removed | Lower chip volume per depth | Enlarging existing bores |
| Trepanning | Centering hole for pilot | Annular groove cut; core extracted intact | Minimal chip volume | Expensive materials, core retention |
Each operation uses a different tool head design, and the workpiece preparation requirements differ significantly.
Solid Drilling: Starting Bush Requirements
When a BTA or ejector drill starts from solid material, the tool enters through a starting bush (guide bushing) mounted in the pressure head. The bush provides initial guidance until the guide pads engage the bore wall.
Starting Bush Specifications
| Parameter | Requirement | Rationale |
|---|---|---|
| Bore tolerance | G6 grade | Provides consistent clearance for the drill head |
| Alignment to spindle | Within 0.02 mm (0.0008 in) | Prevents entry misalignment that causes bell mouth |
| Bush-to-workpiece gap | < 1 mm | Minimizes unsupported tool length at entry |
| Bush material | Hardened steel or tungsten carbide | Carbide for high-production runs |
| Bush length | 1–2× drill diameter | Sufficient guidance length |
G6 Tolerance Values
The guide bushing bore must be manufactured to G6 tolerance relative to the drill head diameter:
| Drill Diameter Range | G6 Tolerance (Lower) | G6 Tolerance (Upper) |
|---|---|---|
| 18 – 30 mm | +0.007 mm | +0.020 mm |
| 30 – 50 mm | +0.009 mm | +0.025 mm |
| 50 – 80 mm | +0.010 mm | +0.029 mm |
| 80 – 120 mm | +0.012 mm | +0.034 mm |
A bush that is too tight will seize on the drill head; a bush that is too loose will not provide adequate guidance and will produce bell mouth.
Pressure Head Function
The pressure head in BTA drilling serves three functions simultaneously:
- Seal — contains the high-pressure coolant (20–120 bar) between the drill tube and bore wall
- Guide — holds the starting bush that aligns the drill head to the workpiece
- Coolant delivery — introduces cutting fluid into the annular space
Counterboring and Peeling
Counterboring (also called peeling or enlarging) uses a BTA tool to enlarge an existing hole. The existing hole may be cast, forged, pre-drilled, or produced by another process.
How Counterboring Differs from Solid Drilling
| Aspect | Solid Drilling | Counterboring |
|---|---|---|
| Cutting edges | Edges cover full diameter | Edges cover annular ring only |
| Guide pads | Pads engage freshly cut bore | Pads engage existing bore or cut zone |
| Chip volume | Maximum (full cross-section) | Reduced (annular area only) |
| Cutting forces | Higher | Lower |
| Starting requirement | Starting bush required | Pilot hole required |
| Tool head design | Solid drill head | Counterboring head |
Pilot Hole Requirements
The pilot hole for counterboring must meet specific dimensional and quality requirements:
| Parameter | Requirement | Consequence of Non-Compliance |
|---|---|---|
| Pilot hole diameter tolerance | +0.020 / −0.000 mm relative to nominal | Oversize: pads lose guidance; undersize: tool tries to drill from solid |
| Pilot hole depth | Minimum 2× drill diameter (enough to engage all guide pads) | Inadequate guidance, tool deflection |
| Pilot hole surface | Clean, no burrs at entry | Burrs deflect tool at start |
| Pilot hole straightness | Within 0.1 mm per 100 mm | Tool will follow existing deviation |
| Pilot hole concentricity | Within 0.05 mm of finished bore center | Uneven stock removal, tool deflection |
| Entry chamfer | 15–20° chamfer at pilot hole mouth | Eases tool entry |
Pilot Hole Production Methods
| Method | Tolerance Achievable | Best For |
|---|---|---|
| Boring on lathe or machining center | ±0.01 mm | High-precision, single parts |
| U-drill (indexable insert drill) | ±0.05 mm | General purpose, moderate precision |
| Gun drilling | ±0.02 mm | Deep pilot holes, high straightness |
| Cast or cored hole | ±0.5 mm or worse | Requires sufficient stock for clean-up |
| Laser or waterjet cut | ±0.1 mm | Thin walls, difficult materials |
Stock Allowance for Counterboring
| Pilot Hole Condition | Minimum Stock per Side | Recommended Stock per Side | Maximum Stock per Side |
|---|---|---|---|
| Bored pilot hole | 0.5 mm | 1.0 – 2.0 mm | 3.0 mm |
| Cast or cored | 1.0 mm | 1.5 – 3.0 mm | 5.0 mm |
| Rough pre-drilled | 0.8 mm | 1.5 – 2.5 mm | 4.0 mm |
| Existing deep hole (re-work) | 0.3 mm | 0.5 – 1.5 mm | 2.5 mm |
Insufficient stock causes the guide pads to run on the existing bore surface without proper cutting edge engagement, producing poor surface finish and oversize holes. Excessive stock overloads the cutting edges and may cause chatter.
Pull Boring
Pull boring is a specialized form of counterboring where the tool is pulled backward through the existing hole on a tensioned boring bar:
| Advantage | Trade-off |
|---|---|
| Exceptional straightness (tool is under tension, not compression) | Requires access to both ends of the hole |
| No buckling risk even at high L/D ratios | Longer setup time |
| Can correct minor straightness deviations in the pilot hole | Limited to through-holes |
Pull boring is the preferred method when maximum straightness is required — typically for hydraulic cylinders, gun barrels, and long structural components.
Trepanning
Trepanning cuts only the outer circumference of the hole, leaving the center material intact as a solid core. The core is extracted when the cut is complete.
Trepanning vs. Solid Drilling
| Aspect | Solid Drilling | Trepanning |
|---|---|---|
| Material removed | Entire cross-section | Annular ring only |
| Core produced | No | Yes — usable for other purposes |
| Power requirement | Higher | Lower (30–50% of solid drilling) |
| Chip volume | High | Low |
| Tool cost | Lower | Higher (more complex head design) |
| Material utilization | Poor (all material becomes chips) | Excellent (core is usable) |
When to Choose Trepanning
| Condition | Choose Trepanning | Choose Solid Drilling |
|---|---|---|
| Material cost | High-value material (titanium, superalloy, precious metals) | Low-cost material |
| Core value | Core has economic value | Core is scrap |
| Power available | Limited machine power | Adequate power |
| Hole diameter | Large (> 50 mm) | Small to medium |
| Production volume | Low to medium | High |
Pilot Hole for Trepanning
Trepanning typically requires a centering or pilot hole for the core guide:
| Requirement | Specification | Purpose |
|---|---|---|
| Pilot hole depth | 3–5× diameter of core | Centers the core within the trepanning head |
| Pilot hole diameter | Matches core diameter within ±0.1 mm | Prevents core from binding or wandering |
| Pilot hole concentricity | Within 0.05 mm of trepanning axis | Ensures uniform wall thickness |
| Pilot hole location | At center of trepanning axis | Core alignment |
The pilot hole is usually produced by conventional drilling or gun drilling before the trepanning operation.
Ejector Drilling (Double Tube System)
The ejector system is a variation of BTA designed for conventional machine tools and machining centers, using a double-tube configuration.
How the Ejector System Differs
| Feature | BTA (STS) | Ejector (DTS) |
|---|---|---|
| Tube configuration | Single tube (drill tube) | Double tube (outer boring bar + inner tube) |
| Coolant path | Through annular space between tube and bore wall | Through annular space between outer and inner tubes |
| Chip return | Through drill tube interior | Through inner tube |
| Workpiece seal | Required (pressure head seals against workpiece face) | Not required (pressure difference is internal) |
| Starting method | Guide bushing in pressure head | Guide bushing (pressure head not required) |
| Typical diameter | 20 – 630 mm | 18 – 200 mm |
Starting Requirements for Ejector Drilling
The ejector system has more flexible starting requirements than BTA:
| Method | Description | When to Use |
|---|---|---|
| Guide bushing only | Guide bushing mounted in a fixture before the workpiece | Diameter < 30 mm, or when maximum straightness is required |
| Pilot hole only | Pre-drilled pilot hole engages guide pads directly | Diameter > 30 mm, or when guide bushing cannot be mounted |
| Guide bushing + pilot hole | Both used together | Extreme L/D ratio (> 50:1), or very tight straightness tolerance |
| No bushing, no pilot hole | Tool starts directly on workpiece surface | L/D < 16:1, flat and square workpiece face, rigid setup |
Pilot Hole for Ejector Drilling
| Parameter | Recommendation | Source |
|---|---|---|
| Pilot hole tolerance | +0.020 / −0.000 mm | Sandvik Coromant |
| Pilot hole depth | Minimum 2× diameter (enough to fully engage guide pads) | Walter USA |
| Pilot hole diameter | At nominal drill diameter (not undersized) | Industry practice |
| Entry chamfer | 20° chamfer recommended | Eases tool entry |
The key difference from BTA: ejector drills can start on a pilot hole without a pressure head seal because the coolant pressure differential is contained within the double tube system. This makes ejector drilling attractive for retrofitting deep hole capability onto standard CNC lathes and machining centers.
Guide Bushing Design and Maintenance
Wear Criteria
| Bush Condition | Symptom | Action |
|---|---|---|
| Wear within G6 tolerance | Normal hole quality | Continue use |
| Wear beyond G6 tolerance (+0.03 mm over upper limit) | Bell mouth at hole entry | Replace bushing |
| Oval wear (0.02 mm + out-of-round) | Hole oversize in one axis | Replace bushing |
| Scoring or galling | Rough tool entry, possible tool damage | Replace bushing immediately |
| Edge breakdown at entry side | Chip packing at bush face | Replace or re-profile |
Maintenance Schedule
| Component | Frequency | Action |
|---|---|---|
| Starting bush bore measurement | Every 500 holes | Check for wear beyond G6 tolerance |
| Bush face condition | Daily visual | Inspect for scoring or edge damage |
| Pressure head seal | Weekly | Check for coolant leakage |
| Bush alignment to spindle | Monthly | Verify within 0.02 mm |
Process Selection Summary
Decision Flowchart
text
Is there an existing hole?
├── No → Is the material expensive or the core valuable?
│ ├── Yes → Trepanning (requires pilot hole for core)
│ └── No → Solid drilling (requires starting bush)
└── Yes → Is the existing hole straight and concentric?
├── Yes → Counterboring / peeling (requires pilot hole preparation)
└── No → Pull boring (requires through-hole access)Parameter Comparison
| Parameter | Solid Drilling | Counterboring | Trepanning | Pull Boring |
|---|---|---|---|---|
| Starting bush | Required | Not required (pilot hole) | Not required (pilot hole) | Not required |
| Pilot hole | Not needed | Required | Required (for core) | Required (through) |
| Tool tension/compression | Compression | Compression | Compression | Tension |
| Typical stock removal | Full diameter | 0.5–3 mm per side | Annular ring | 0.3–2 mm per side |
| Achievable tolerance | IT8–IT9 | IT7–IT8 | IT9–IT11 | IT6–IT7 |
| Surface finish (Ra) | 1.6–3.2 µm | 0.8–1.6 µm | 3.2–6.3 µm | 0.4–0.8 µm |
| Relative power requirement | 100% (baseline) | 40–70% | 30–50% | 30–60% |
FAQ
What is the difference between peeling and counterboring in deep hole drilling?
In BTA practice, peeling and counterboring are often used interchangeably to describe enlarging an existing hole. Some manufacturers distinguish by application: counterboring refers to enlarging a hole to a specific diameter and depth (often with a shoulder), while peeling refers to enlarging a hole over its full length. Both use the same tool design principle — cutting edges arranged around an annular ring with guide pads for self-piloting.
Can a BTA solid drill be used for counterboring?
No. A BTA solid drill head has cutting edges that cover the full diameter, with guide pads positioned to engage a freshly cut bore at the solid-drilled diameter. Using it in a pre-existing hole will cause the guide pads to lose contact with the bore wall, resulting in oversize, misaligned holes and likely tool damage. A counterboring head has a different cutting edge layout and pad geometry designed for the annular cutting condition.
What happens if the pilot hole is too large?
An oversized pilot hole causes the guide pads to lose contact with the bore wall at entry. The tool may deflect, producing an eccentric or oversize finished hole. The critical limit is typically +0.1 mm above nominal — beyond this, the pads cannot provide adequate guidance. The pilot hole must be within +0.020 / −0.000 mm of the nominal diameter for reliable counterboring.
What happens if the pilot hole is too small?
An undersized pilot hole forces the counterboring tool to cut from solid rather than enlarging an existing hole. This overloads the cutting edges, increases cutting forces, and may cause the tool to jam or break. The tool must never encounter solid material at the pilot hole diameter.
How is the pilot hole produced for BTA counterboring?
The pilot hole is typically produced by boring on a lathe or machining center (for highest precision), by gun drilling (for deep pilot holes), or by U-drilling (for moderate precision). For cast or cored holes, sufficient stock must be left for clean-up (minimum 1.0 mm per side). The pilot hole diameter is measured and verified before the counterboring operation.
What is pull boring and when is it used?
Pull boring is a counterboring method where the tool is pulled backward through the existing hole on a tensioned boring bar. Because the bar is in tension rather than compression, there is no buckling risk, enabling exceptional straightness — typically IT6–IT7. Pull boring is used for hydraulic cylinders, gun barrels, and other applications where straightness is the primary quality requirement. The trade-off is that both ends of the hole must be accessible.
Does ejector drilling always need a pilot hole?
No. Ejector drilling can start with a guide bushing (similar to BTA), with a pilot hole, or in some cases directly on a flat workpiece surface for shallow holes (L/D < 16:1). The flexibility comes from the double-tube design, which does not require a pressure head seal against the workpiece. For deep holes (L/D > 16:1), a pilot hole or guide bushing is recommended.
What guide bushing tolerance is required for BTA solid drilling?
G6 grade is the industry standard. The G6 tolerance provides enough clearance for the drill head to rotate freely while maintaining the alignment required for straight-hole entry. For a 40 mm drill, the G6 tolerance is +0.009 / +0.025 mm. A bushing worn beyond this range should be replaced.
Can trepanning be used for any material?
Trepanning is most advantageous for expensive materials (titanium, superalloys, copper alloys) where the solid core has economic value, for materials that are difficult to chip (the core eliminates the need to cut the center), and for large diameters where power is limited. Trepanning is not typically used for small diameters (< 30 mm) or for high-volume production where solid drilling is faster and tooling is simpler.
How do I select between BTA and ejector for a counterboring application?
Choose BTA (STS) when: a dedicated deep hole drilling machine is available, hole diameter is above 30 mm, maximum coolant pressure is needed, and the workpiece face is accessible for sealing. Choose ejector (DTS) when: retrofitting a standard CNC machine, hole diameter is 18–200 mm, workpiece sealing is difficult (complex geometry), or quick changeover between drilling and other operations is required.
The pilot hole and starting requirements in this article represent standard industry practice. Always verify with the specific tool manufacturer's technical data sheet, as requirements vary with tool design, workpiece material, and quality specifications. This article reflects industry knowledge as of 2026.