Appearance
A BTA drilling operation loses coolant pressure during a production run on 316 stainless steel bars. The operator notices coolant spraying from the workpiece entry point, chip evacuation becomes intermittent, and surface finish deteriorates. The supervisor inspects the BOZA pressure head: the face seal has a 0.5 mm groove worn by the rotating drill tube, and the guide bush is loose in its holder. Replacement takes 90 minutes: remove the BOZA assembly, replace the face seal and guide bush seal, check the drill tube for wear (0.05 mm, within tolerance), align the guide bush to the spindle axis within 0.02 mm, and reassemble. Coolant pressure returns to specification. The seals lasted 14 months — within the expected 12–18 month service life.
What Is a BOZA Pressure Head?
The BOZA (German: Bohröl-Zuführapparat — drilling oil feed device) is the pressure head assembly in a BTA deep hole drilling machine. It is the interface between the machine's coolant system and the rotating drill tube, mounted between the workpiece and the machine spindle.
Four Functions
- Coolant supply — delivers high-pressure coolant into the annular gap between the drill tube and the bore wall
- Workpiece sealing — seals against the workpiece face to contain coolant pressure (up to 120 bar)
- Guide bush holder — centres and holds the guide bush that supports the drill at entry
- Drill tube sealing — seals around the rotating drill tube to prevent coolant escape at the machine end
All four functions must operate correctly for the BTA process to work. A failure in any one function causes coolant loss, pressure drop, and chip evacuation failure.
BOZA Types
BOZA pressure heads are classified by whether the workpiece rotates and by the sealing method:
| Type | Workpiece Rotation | Sealing Method | Typical Application |
|---|---|---|---|
| 2210 | Non-rotating | Clamping cone | Standard BTA drilling on dedicated machines |
| 2220 | Non-rotating | Face seal | BTA drilling with flat workpiece faces |
| 2250 | Rotating | Clamping cone | Lathe-based BTA, workpiece rotates |
| 2270 | Rotating | Face seal (coolant-actuated) | High-pressure BTA, automated loading |
Type 2210 — Non-Rotating Clamping Cone
The most common BOZA type for dedicated BTA drilling machines. The workpiece is stationary and the drill tube rotates. The clamping cone centres the workpiece, seals the coolant at the entry face, and mechanically grips the workpiece to resist torque.
Typical specifications:
- Diameter range: 7.76–700 mm
- Maximum pressure: 80–120 bar
- Clamping force: pneumatic or hydraulic actuation
Type 2220 — Non-Rotating Face Seal
Used when the workpiece has a flat face rather than a centre hole. A flat seal ring presses against the workpiece face to seal the coolant. The guide bush is housed in the pressure head body.
Type 2250 — Rotating Clamping Cone
Used on lathe-based BTA setups where the workpiece rotates. The clamping cone rotates with the workpiece. A separate rotary seal on the drill tube handles the coolant seal at the rotating interface.
Type 2270 — Rotating Face Seal
Similar to Type 2250 but uses a face seal that is pressed against the workpiece by coolant pressure. The seal pressure increases automatically with coolant pressure, providing better sealing at higher pressures.
Tip: BOZA Type 2270 is preferred for automated production lines because the face seal pressure is self-regulating — higher coolant pressure automatically increases the sealing force. This eliminates the need for manual seal adjustment when changing coolant pressure for different materials.
BOZA Components
| Component | Function | Typical Material |
|---|---|---|
| Housing | Supports all components, mounts to machine | Steel or cast iron |
| Clamping cone | Centres and seals the workpiece | Hardened steel, nitrided |
| Face seal ring | Flat seal for workpiece interface | Bronze, PTFE, or polyurethane |
| Guide bush holder | Precision bore for guide bush | Hardened steel |
| Guide bush | Supports drill at entry | Carbide or bronze |
| Drill tube seal | Seals around rotating drill tube | PTFE, polyurethane, or carbon |
| Coolant inlet | Connection from coolant pump | Steel with threaded flange |
| Seal retainer | Holds seals in position | Steel |
Guide Bush Holder
The guide bush holder is a precision-bored sleeve within the BOZA that accepts the guide bush. The holder must:
- Maintain concentricity with the spindle axis within 0.01–0.02 mm
- Provide a secure press fit or clamp fit for the guide bush
- Withstand the radial forces from the drill at entry
- Allow for guide bush replacement without removing the BOZA
Drill Tube Seal
The drill tube seal (also called the rotary seal) seals the annular gap between the rotating drill tube and the stationary BOZA housing. It is the most wear-sensitive component in the BOZA assembly.
| Seal Type | Maximum Pressure | Speed Limit | Wear Life | Cost |
|---|---|---|---|---|
| PTFE lip seal | 80 bar | 3,000 RPM | 6–12 months | Low |
| Polyurethane lip seal | 100 bar | 2,500 RPM | 12–18 months | Moderate |
| Carbon ring seal | 120 bar | 4,000 RPM | 18–36 months | High |
| Mechanical face seal | 150 bar | 5,000 RPM | 24–48 months | Very high |
Seal Materials and Coolant Compatibility
| Seal Material | Oil-Based Coolant | Water-Based Emulsion | Max Temperature |
|---|---|---|---|
| PTFE (polytetrafluoroethylene) | Excellent | Excellent | 250 °C |
| Polyurethane | Excellent | Good | 100 °C |
| NBR (nitrile rubber) | Good | Excellent | 100 °C |
| FKM (Viton) | Excellent | Good | 200 °C |
| Carbon (mechanical seal) | Excellent | Excellent | 300 °C |
| Bronze (wear ring) | Good | Good | 400 °C |
Warning: Using the wrong seal material for the coolant type causes rapid seal failure. Polyurethane seals swell and degrade in water-based emulsions — use PTFE or FKM for emulsion systems. NBR seals degrade in oil-based coolants — use FKM or PTFE for oil systems.
Wear and Replacement
Expected Service Life
| Component | Typical Life (Continuous Production) | Failure Mode |
|---|---|---|
| Drill tube lip seal | 6–12 months | Wear, leakage |
| Clamping cone | 24–48 months | Wear, loss of grip |
| Face seal ring | 12–24 months | Grooving, leakage |
| Guide bush | 3–12 months (bronze), 12–24 months (carbide) | Wear, oversize bore |
| Guide bush holder | 5+ years | Damage from mishandling |
Signs of Seal Wear
| Symptom | Likely Failed Component |
|---|---|
| Coolant spraying at workpiece entry | Face seal or clamping cone seal |
| Coolant leaking at machine end | Drill tube seal |
| Coolant pressure drops during cut | Any seal — inspect all |
| Intermittent chip evacuation | Coolant pressure loss from seal leak |
| Guide bush loose in holder | Guide bush holder bore worn |
| Drill tube visible scoring | Seal failure allowed debris into seal area |
Replacement Procedure
- Stop machine and lock out — isolate coolant pump, spindle drive, and hydraulic systems
- Depressurise coolant system — open bleed valve, verify zero pressure
- Remove drill tube — retract tube from BOZA
- Remove BOZA assembly — unbolt from machine frame
- Disassemble BOZA — remove seal retainer, old seals, and guide bush
- Inspect all components — measure guide bush holder bore, check clamping cone for wear, inspect housing for cracks
- Replace worn seals — install new seals per manufacturer specification
- Replace guide bush — press-fit new bush to correct interference
- Reassemble BOZA — torque all fasteners to specification
- Align to spindle — indicate the guide bush bore to within 0.02 mm TIR of the spindle axis
- Pressure test — run coolant at operating pressure, verify no leaks
- Test run — drill a test hole, check pressure, chip evacuation, and surface finish
Alignment Procedure
Proper alignment of the BOZA to the machine spindle is critical for seal life and hole quality:
- Mount a test bar or indicator in the guide bush bore
- Rotate the spindle and measure runout at the guide bush position
- Adjust the BOZA position using the mounting screws
- Target: 0.01–0.02 mm TIR (total indicator reading)
- Re-tighten all mounting bolts and re-check
Misalignment of 0.05 mm or more will cause:
- Accelerated drill tube seal wear (50% reduction in life per 0.03 mm misalignment)
- Guide bush wear on one side
- Bell-mouth at bore entry
- Reduced hole straightness
Troubleshooting
| Problem | Likely Cause | Correction |
|---|---|---|
| Coolant leaks at workpiece entry | Worn clamping cone or face seal | Replace seal, check cone condition |
| Coolant leaks at drill tube entry | Worn drill tube seal | Replace seal, check tube OD for wear |
| Coolant pressure drops during cut | Seal failure or pump issue | Inspect all seals, check pump |
| No coolant flow to cutting zone | Blocked coolant passage in BOZA | Disassemble and clean coolant passages |
| Guide bush spins in holder | Worn holder bore or incorrect fit | Replace holder or bush with correct interference |
| Drill tube seizes in seal area | Seal swollen from incompatible coolant | Replace seal with correct material |
| Chip evacuation stops | Coolant pressure too low at tool | Check all seals, verify pump pressure |
| Bell-mouth at bore entry | Guide bush worn or misaligned | Replace bush, check alignment |
| Spray mist at workpiece entry | Face seal gap too large | Adjust or replace face seal |
| Hydraulic oil in coolant system | Internal seal failure in BOZA | Replace internal seals immediately |
FAQ
What does BOZA stand for?
BOZA is a German abbreviation for Bohröl-Zuführapparat — drilling oil feed device. It is the standard term for the pressure head assembly in BTA deep hole drilling machines.
How often should BOZA seals be replaced?
In continuous production, drill tube seals typically last 6–18 months depending on type. Guide bushes last 3–24 months depending on material. Annual inspection and replacement of all wear components is recommended.
What causes BOZA seal failure?
The most common causes are: normal abrasive wear from the rotating drill tube, incompatible seal material for the coolant type, misalignment between the BOZA and spindle, and contaminated coolant with abrasive particles.
Can I replace BOZA seals without removing the assembly?
Drill tube seals can sometimes be replaced in situ if the seal retainer is accessible. Face seals, clamping cone seals, and guide bush replacement typically require removing the BOZA from the machine.
What alignment accuracy does the BOZA need?
The guide bush bore must be aligned to the spindle axis within 0.01–0.02 mm TIR. Misalignment beyond 0.05 mm causes accelerated seal wear and reduced hole quality.
How do I choose between clamping cone and face seal BOZA types?
Use clamping cone (Type 2210/2250) when the workpiece has a centre hole or chamfer that the cone can grip. Use face seal (Type 2220/2270) when the workpiece face is flat and a cone cannot be used.
What is the maximum pressure for a BOZA pressure head?
Standard BOZA assemblies are rated for 80–120 bar. High-pressure versions with mechanical face seals can handle up to 150 bar. Always verify the pressure rating before operating.
Can a worn BOZA cause poor hole quality?
Yes — a worn BOZA causes coolant pressure loss, which leads to poor chip evacuation. Chips that are not evacuated can score the bore surface, cause oversize holes, or jam the tool.
What is the difference between rotating and non-rotating BOZA types?
Non-rotating types (2210, 2220) are used on dedicated BTA machines where the workpiece is stationary and the drill tube rotates. Rotating types (2250, 2270) are used on lathe-based setups where the workpiece rotates.
How do I measure guide bush holder bore wear?
Use a bore gauge to measure the holder bore at three positions: entry, middle, and exit. Replace the holder if the bore exceeds nominal by 0.02 mm. A worn holder will not grip the guide bush securely.
Summary
The BOZA pressure head is a critical component in BTA deep hole drilling that performs four functions: coolant supply, workpiece sealing, guide bush holding, and drill tube sealing:
- Four standard types — 2210 (non-rotating, clamping cone), 2220 (non-rotating, face seal), 2250 (rotating, clamping cone), 2270 (rotating, face seal)
- Seal materials — PTFE, polyurethane, carbon, and mechanical face seals, each with different pressure, speed, and coolant compatibility characteristics
- Seal life — 6–48 months depending on type and operating conditions
- Alignment requirement — 0.01–0.02 mm TIR to spindle axis
- Replacement interval — annual inspection and replacement of wear components is recommended for continuous production
- The manufacturer in the opening scenario restored coolant pressure and chip evacuation by replacing the worn face seal and guide bush, returning the machine to production within 90 minutes