The pressure head — in German the Bohröl-Zuführ-Apparat, abbreviated BOZA — is the component that makes the BTA single-tube system possible. It seals high-pressure cutting oil against the workpiece, feeds coolant into the annular gap around the drill tube, and houses the drill bushing that starts the bore. Every dedicated BTA machine needs one; the ejector (DTS) system was invented partly to avoid it.
The BTA system (also called the Single Tube System, STS) is defined by its coolant routing: cutting oil is supplied externally, down the annular gap between the drill tube and the machined hole wall, while chips return internally through the center of the drill tube. Because the coolant is pressurized and travels along the outside of the tool, the drilling zone must be enclosed — the pressure head is the seal between the machine, the drill tube, and the workpiece that makes that pressurization possible.
| Function | Why It Matters |
|---|---|
| Coolant supply | Feeds pressurized oil into the annulus so it reaches the cutting edges and flushes chips |
| Sealing | Contains up to 100 bar cutting oil at the workpiece — a leak means loss of pressure and chip evacuation |
| Bushing reception | Holds the drill bushing that locates the bore start and produces the correct hole diameter |
| Workpiece centering | Conus or chuck centers and clamps the part so the bore starts on axis |
The pressure head sits between the high-pressure pump and the cutting zone. Oil from the pump enters the head, is sealed against the workpiece face by the drill bushing and seal element, and is forced into the annular gap between the drill tube outer wall and the bore wall. It travels the length of the hole to the cutting edges, cools and lubricates the carbide inserts and guide pads, and sweeps chips into the central opening of the drill head. Chips and spent oil then flow back through the center bore of the drill tube and out through the machine spindle — no pecking required.
| Stage | What Happens |
|---|---|
| 1 — Pump | High-pressure cutting oil delivered to the pressure head (typical 15–100 bar, 50–500+ L/min depending on diameter) |
| 2 — Pressure head | Seals against the workpiece; bushing contacts the face to contain coolant |
| 3 — Annular gap | Oil flows forward between the drill tube outer wall and the bore wall |
| 4 — Cutting zone | Coolant cools edges and guide pads; flushes chips into the head’s central opening |
| 5 — Internal return | Chips + oil flow back through the center of the drill tube and exit through the spindle |
BOZA pressure heads are classified by whether the workpiece rotates and by how the seal is made. For a non-rotating workpiece, the head uses a non-rotating clamping cone or a non-rotating face seal. For a rotating workpiece, it uses a rotating clamping cone or a rotating face seal that is pressed closed by coolant pressure itself.
| Type | Workpiece | Seal / Clamping |
|---|---|---|
| 2210 | Non-rotating | Non-rotating clamping cone — holds, centers, and seals the workpiece |
| 2220 | Non-rotating | Non-rotating face seal |
| 2250 | Rotating | Rotating clamping cone — holds, centers, and seals the workpiece |
| 2270 | Rotating | Rotating face seal, pressed against the face by coolant pressure |
On a typical BTA deep hole drilling machine the oil pressure head is mounted in the middle of the machine bed and moves axially under hydraulic control to tighten and loosen the workpiece. It carries a tool bar support sleeve on one end (supporting the drill tube) and, on the workpiece-facing end, a guide sleeve and conical disc — the cone positions the part and the guide sleeve directs the tool into the bushing.
There are two broad sealing geometries on a pressure head: cone seals (conus), which simultaneously center the part and seal, and face seals, which press a flat sealing element against the flat end face of the workpiece. In rotating heads, the seal and its bearing package rotate with the part, and the rotating face seal on Type 2270 is energized by coolant pressure — the higher the pressure, the tighter it seats.
The drill bushing contacts the workpiece face to seal in the coolant, so the part face must be flat and form a good seal with the guide bushing — this is a stated equipment requirement of the BTA process. Spot-facing or facing the bore entry before deep drilling is standard practice. In the mechanical seal industry, lapped face seals are finished to sub-micron flatness (less than ~1.8 μm across the sealing periphery) and 5 micro-inch surface finishes to seal reliably.
Seal life is governed by the PV factor — pressure × velocity of relative rotation. Rotating face seals and cone seals see constant rubbing wear, and abrasive fines in poorly filtered coolant accelerate seal and bushing wear. That is why BTA coolant filtration to 50 μm is the minimum, with 20 μm recommended for consistent guide pad and seal life.
The boring bush (drill guide bushing) is received in the pressure head, determines the starting point of the bore, guides the tool as drilling begins, produces the correct hole diameter, and seals against the workpiece face to contain coolant. Bushing geometry is therefore a quality-critical item, and its fit tolerances follow conventional jig-bushing practice.
| Parameter | Typical Value | Notes |
|---|---|---|
| Drill-to-bushing clearance | 0.013–0.026 mm | Tight enough to guide, loose enough to avoid drag |
| Bushing length (deep drilling) | 2–3 × inside diameter | Longer than the 1.5× rule for shallow jig work |
| Headless / liner press-fit interference | 0.013–0.020 mm | For permanent press-fit installations |
| Headed press-fit interference | 0.008–0.013 mm | Headed bushings resist heavy axial loads |
| Bushing material | Tool steel, HRC 60–64 | Concentricity typically 0.01 mm |
Spot-face or face the bore entry flat and square. The bushing seals against this face — an off-square face is the #1 cause of leaks and entry drift.
Carrier lifts the part into place; oil sealing pans on the tailstock and pressure head tighten both ends; the center frame clamps the workpiece; the carrier lowers.
Advance the pressure head axially under hydraulic control so the clamping cone or chuck seats the part on center.
Bring the drill bushing into firm contact with the workpiece face — on some machines the hydraulic pressure head automatically advances the guide bush.
Verify drill tube / spindle concentricity with the bushing bore and the tool bar support sleeve. Check runout before cutting.
Bring coolant pressure up slowly and watch for leaks at the face seal, cone, and fittings before engaging the feed.
Start the cut and watch coolant pressure and flow — a steady gauge reading means the seal is holding and chips are evacuating.
BTA coolant pressure is set by bore diameter, material, and chip load. Small diameters and difficult materials need higher pressure to push oil down a long annulus; large bores lean on flow more than pressure. Reference values below follow the BTA cutting-data standard (VDI 3209) and machine ratings.
| Material | Coolant Pressure | Typical Flow |
|---|---|---|
| Low-carbon steel | 15–40 bar | 50–500+ L/min by diameter |
| Alloy steel (annealed) | 20–60 bar | 50–500+ L/min by diameter |
| Alloy steel (hardened) | 30–80 bar | 50–500+ L/min by diameter |
| Stainless steel (austenitic) | 30–80 bar | 50–500+ L/min by diameter |
| Aluminum | 15–40 bar | High flow, low pressure |
| Titanium (Ti-6Al-4V) | 60–100 bar | High pressure required |
| Cast iron (gray) | 15–40 bar | Moderate flow |
| Problem | Probable Cause | Solution |
|---|---|---|
| Coolant leaking at workpiece face | Damaged face seal, off-square part face, or worn bushing seat | Dress the seal, re-face the part entry, replace bushing / seal |
| Pressure loss / weak chip evacuation | Worn seals, leaking fittings, clogged filtration, pump drop-off | Inspect seals and hose fittings; check filter; verify pump flow |
| Pressure spikes | Blockage in the chip evacuation channel | Retract immediately, clear the chips, check chip shape |
| Entry wander / hole drift | Worn or oversized drill bushing, misaligned pressure head | Check bushing clearance (<0.02 mm); align head to spindle |
| Bushing binding or drag | Clearance too tight; swarf ingress between tool and bushing | Measure fit; clean; open clearance within 0.013–0.026 mm |
| Chatter / vibration marks | Worn guide pads on the drill head; insufficient tool support | Inspect drill head pads; check tool bar support sleeve |
| Rotating seal overheating | PV factor exceeded; poor coolant flow at the seal | Reduce speed or pressure; verify seal cooling flow |