Appearance
The BOZA is the only component in a BTA drilling system that simultaneously contains high-pressure coolant, guides the rotating drill tube, centres the workpiece, and holds the starting bushing — all while fighting chip abrasion, coolant temperature, and continuous vibration. When it leaks, the entire machine stops.
Function and Operating Principle
The BTA pressure head (BOZA — Bohröldruck-Zuführungs-Aggregat) delivers coolant to the cutting zone through the annulus between the bore wall and the outside of the drill tube. Unlike gun drilling where coolant flows through the drill's internal hole, BTA coolant flows around the outside of the drill tube and returns with chips through the tube's interior.
Four Core Functions
| Function | Description | Consequence of Failure |
|---|---|---|
| Coolant supply | Delivers high-pressure oil to the annulus at 10–50 bar, 300–1,500 L/min | Chip evacuation stops; tool failure within seconds |
| Drill tube seal | Seals the rotating drill tube against coolant leakage | Coolant loss, pressure drop, environmental hazard |
| Guide bushing retention | Holds and centres the drill guide bushing | Bore start misalignment, diameter error |
| Workpiece seal | Seals against the stationary or rotating workpiece face | Coolant spray, pressure loss, chip evacuation failure |
Coolant Flow Path
Coolant pump → BOZA inlet → Annulus between bore wall and drill tube → BTA cutting head
→ Coolant + chips enter drill tube interior → Through drill tube → BOZA return port → Chip separatorBOZA Types
BTA-Tiefbohrsysteme GmbH defines five standard BOZA types, classified by whether the workpiece rotates and how the seal is achieved.
Non-Rotating Workpiece Types
| Type | Seal Mechanism | Typical Application |
|---|---|---|
| 2210 | Non-rotating clamping cone — cone centres the workpiece end and seals through conical metal-to-metal contact | Standard BTA drilling, low-to-medium production |
| 2220 | Non-rotating face seal — a flat seal surface pressed against the workpiece end face | Workpieces with pre-machined end faces, higher pressure requirements |
Type 2210 is the most common for general BTA work. The clamping cone centres the workpiece and seals by wedging into the workpiece centre hole or a pre-drilled starter hole. The cone angle is typically 60° or 90°.
Type 2220 is preferred when the workpiece already has a machined end face. The face seal provides better sealing at higher coolant pressures but requires the workpiece end face to be perpendicular to the bore axis within 0.05 mm.
Rotating Workpiece Types
| Type | Seal Mechanism | Typical Application |
|---|---|---|
| 2250 | Rotating clamping cone — cone rotates with workpiece; bearing-supported | Counter-rotation drilling, rotating workpiece machines |
| 2270 | Rotating face seal pressed by coolant pressure — bearing-supported face seal engages dynamically | High-speed rotating workpieces, variable pressure applications |
Type 2270 is a particularly elegant design — the coolant pressure itself engages the face seal. As pump pressure increases, the seal force increases proportionally. This provides:
- Self-regulating seal pressure
- Reduced wear at low pressure (idle, setup)
- Maximum seal force exactly when needed (full drilling pressure)
Selection Guide
| Application | Type 2210 | Type 2220 | Type 2250 | Type 2270 |
|---|---|---|---|---|
| Non-rotating workpiece, standard | ✓ Best | ✓ Good | — | — |
| Non-rotating, high pressure | ✓ Good | ✓ Best | — | — |
| Rotating workpiece | — | — | ✓ Good | ✓ Best |
| Counter-rotation | — | — | ✓ Good | ✓ Best |
| Face-machined workpieces | ✓ Good | ✓ Best | — | ✓ Good |
| Rough cast/forged ends | ✓ Best | — | ✓ Best | — |
Guide Bushing
The drill guide bushing (also called the boring bush or starting bushing) is a precision component mounted in the BOZA that guides the BTA tool during the first few millimeters of cut.
Function
| Role | Requirement |
|---|---|
| Tool guidance | Maintains tool position during initial entry before full bore guidance |
| Coolant seal | Seals against the rotating drill tube |
| Start accuracy | Determines bore position within ±0.05 mm |
| Diameter control | Bushing ID matches tool diameter within 0.01–0.02 mm clearance |
Bushing Materials
| Material | Hardness | Wear Life | Best For |
|---|---|---|---|
| Hardened tool steel (60–62 HRC) | High | 1,000–5,000 holes | General steel drilling |
| Carbide (WC-Co) | Very high | 5,000–20,000 holes | High-production, abrasive materials |
| Bronze | Low | 200–1,000 holes | Soft materials, non-marring requirement |
Bushing Wear
A worn guide bushing is the most common cause of BOZA coolant leaks and bore start inaccuracy.
| Clearance (bushing ID minus tool OD) | Condition | Action Required |
|---|---|---|
| 0.01–0.03 mm | New/normal | OK |
| 0.03–0.05 mm | Acceptable wear | Monitor |
| 0.05–0.08 mm | Worn | Schedule replacement |
| > 0.08 mm | Excessive | Replace immediately |
Tip: Measure bushing ID weekly on production BTA machines. A 0.05 mm increase in clearance reduces seal effectiveness and allows abrasive chips to bypass the bushing, accelerating wear on the downstream drill tube seal.
Seal Types and Materials
Seal Locations
| Location | Seal Type | Typical Material | Operating Condition |
|---|---|---|---|
| Drill tube (dynamic) | Lip seal, U-cup, or gland packing | Polyurethane (92–95 Shore A), PTFE-filled | 10–50 bar, rotating at 500–3,000 RPM |
| Workpiece interface (static/dynamic) | Face seal or clamping cone | Hardened steel-on-steel, or PTFE-faced | Static or rotating, 10–50 bar |
| BOZA housing joints (static) | O-ring | Nitrile (NBR) or Fluorocarbon (FKM) | Static, up to 50 bar |
| Guide bushing-to-tool | Close-clearance annular gap | Metal-to-metal (lubricated by coolant) | 0.01–0.05 mm clearance |
| Rotary coolant union (rotating types) | Mechanical seal | Silicon carbide / carbon face | Rotating, 10–50 bar |
Drill Tube Seal Details
The drill tube seal is the most maintenance-critical seal in the BOZA. It must seal a rotating shaft (the drill tube) while coping with:
- Surface speed up to 5 m/s (for 50 mm tube at 2,000 RPM)
- Coolant pressure up to 50 bar
- Coolant temperature 30–60°C
- Occasional chip debris in the coolant
- Tube surface condition (joint welds, wear marks, scratches)
Common configurations:
| Configuration | Advantages | Disadvantages |
|---|---|---|
| Single lip seal | Simple, low cost | Limited pressure capability (< 20 bar) |
| Tandem lip seals | Higher pressure capability | More friction, requires lubrication |
| Lip seal + gland packing | Excellent sealing, handles worn tubes | Higher friction, periodic gland adjustment |
| Floating ring seal | Low friction, long life | Higher leakage rate, requires external drain |
Warning: Never replace a drill tube lip seal without inspecting the tube surface. A scratch, weld splatter, or corrosion pit on the tube will destroy a new seal within minutes. Use 600-grit emery cloth to polish minor defects. Replace the tube if surface roughness exceeds Ra 0.8 μm in the seal contact zone.
Seal Material Selection
| Material | Max Temperature | Pressure Limit | Abrasion Resistance | Coolant Compatibility |
|---|---|---|---|---|
| Polyurethane (AU) | 80°C | 50 bar | Excellent | Good (oil-based) |
| Nitrile (NBR) | 100°C | 30 bar | Good | Excellent (oil-based) |
| Fluorocarbon (FKM) | 200°C | 40 bar | Good | Excellent (all) |
| PTFE-filled | 250°C | 20 bar | Fair | Excellent (all) |
| HNBR | 150°C | 40 bar | Good | Excellent (oil + additives) |
Preventive Maintenance
Daily Checks
- Visual inspection for external coolant leaks at all seal locations
- Check coolant pressure reading at BOZA gauge — a 10–15% drop from nominal indicates seal or bushing wear
- Verify chip evacuation flow at return port — reduced flow indicates blockage or pressure loss
Weekly Checks
| Item | Inspection | Action |
|---|---|---|
| Guide bushing ID | Measure with inside micrometer or plug gauge | Replace if > 0.05 mm wear |
| Drill tube surface | Visual + fingernail test in seal contact zone | Polish minor scratches; replace if grooved |
| Cone seal face (Type 2210/2250) | Visual for galling or deformation | Dress lightly with fine stone; replace if damaged |
| Coolant filter differential pressure | Read gauge | Replace filter element if > 1.5 bar ΔP |
Monthly Checks
| Item | Inspection | Action |
|---|---|---|
| Lip seal condition | Remove and inspect for cracking, hardening, lip wear | Replace at first sign of hardening or cracking |
| BOZA bearings (rotating types) | Rotate by hand — check for roughness or play | Replace if not smooth and silent |
| Coolant oil condition | Sample — check for water content, particulate, viscosity | Change oil if contaminated |
Annual / 2,000-Hour Overhaul
| Component | Action |
|---|---|
| All seals | Replace |
| Guide bushing | Replace (regardless of measured wear) |
| Bearings (rotating types) | Replace |
| Coolant hoses to BOZA | Inspect for swelling, cracking; replace if >5 years old |
| Pressure gauge | Calibrate or replace |
| Clamping cone (Type 2210/2250) | Inspect for wear; replace if >0.1 mm wear on cone surface |
Troubleshooting Common Problems
| Symptom | Likely Cause | Solution |
|---|---|---|
| Coolant leaking at workpiece face | Worn or damaged face seal (Type 2220/2270); workpiece end face not perpendicular | Replace seal; re-machine workpiece face |
| Coolant leaking at drill tube entry | Worn lip seal; scored drill tube | Replace lip seal; polish or replace tube |
| Low coolant pressure | Worn guide bushing; leaking seal; pump issue | Check bushing clearance; check seals; verify pump |
| Chips in coolant return (poor evacuation) | Insufficient pressure/flow; leaking pressure head | Check all seals; verify pump output |
| Bore start location drifting | Worn guide bushing | Replace bushing |
| Excessive noise at BOZA | Bearing wear (rotating types); seal starvation | Replace bearings; check lubrication |
| Coolant temperature rising | Internal leakage recirculating hot oil | Check seal leakage; verify chiller operation |
| Sporadic pressure fluctuation | Partially clogged filter; air entrainment | Change filter; check return line for vortex |
Cost of Neglect
| Component | Replacement Cost | Downtime | Consequential Damage If Ignored |
|---|---|---|---|
| Lip seal | $50–200 | 1–2 hours | Scored drill tube ($500–3,000) |
| Guide bushing (steel) | $100–400 | 30 min | Scrap bore, tool breakage |
| Guide bushing (carbide) | $300–1,000 | 30 min | Scrap bore, tool breakage |
| Face seal (Type 2270) | $500–2,000 | 2–4 hours | Coolant pump damage from debris |
| BOZA bearings | $500–2,000 | 4–8 hours | BOZA housing damage ($5,000–15,000) |
FAQ
What does BOZA stand for?
BOZA is a German acronym for Bohröldruck-Zuführungs-Aggregat — oil pressure head or drilling oil feed device.
What are the four functions of a BTA pressure head?
Coolant supply to the cutting zone, sealing of the drill tube and workpiece, centering and holding the drill guide bushing, and determining the bore starting point.
What is the difference between Type 2210 and Type 2270 BOZA?
Type 2210 is a non-rotating clamping cone design for stationary workpieces. Type 2270 is a rotating face seal design for rotating workpieces, where the seal is pressed by coolant pressure.
How often should BTA pressure head seals be replaced?
Lip seals and dynamic seals: inspect monthly, replace at 1,000–2,000 hours or first sign of leakage. Static O-rings: replace annually or during any major service.
What causes coolant leakage at the pressure head?
Worn guide bushing, damaged workpiece seal face, degraded drill tube lip seal, or scored drill tube surface. The guide bushing is the most common root cause.
How is the guide bushing maintained?
Measure ID weekly. Replace when clearance exceeds 0.05 mm beyond nominal. Carbide bushings last 5,000–20,000 holes; steel bushings last 1,000–5,000 holes.
Can the drilling oil be damaged by seal failures?
Seal leakage allows air ingress and chip debris to enter the coolant system. This accelerates oil degradation, reduces filter life, and can damage the coolant pump.
What seal materials are used for BTA pressure heads?
Polyurethane (95 Shore A) for drill tube lip seals, NBR/FKM for static O-rings, silicon carbide/carbon for mechanical face seals in rotary coolant unions.
What pressure does a BTA coolant system operate at?
10–50 bar depending on machine size and drilling diameter. The pressure head must maintain a reliable seal across the full operating range.
How do you select the correct BOZA type?
Based on whether the workpiece rotates (non-rotating: Type 2210/2220; rotating: Type 2250/2270), workpiece end condition (rough cast: cone seal; machined: face seal), and coolant pressure requirements.
Conclusion
The BTA pressure head (BOZA) is a precision component that integrates coolant delivery, dynamic sealing, workpiece centering, and tool guidance into a single unit. Its design must balance conflicting requirements — high-pressure sealing against rotating motion, chip abrasion against seal life, and guide bushing clearance against coolant retention. Four standard types (2210, 2220, 2250, 2270) cover the range from simple non-rotating stationary drilling to complex counter-rotation applications. Preventive maintenance — particularly guide bushing inspection, lip seal condition monitoring, and coolant filter management — is essential for reliable BTA drilling operation. A well-maintained BOZA operates for thousands of hours with minimal leakage; a neglected one is the most common cause of BTA machine downtime.