BTA Trepanning Head Design: Inserts Guide Pads and Operation

An oil and gas equipment manufacturer needs large-diameter deep holes in 316 stainless steel bars — 180 mm bore, 3,000 mm deep, Ra 3.2 µm, IT10. Each bar costs €8,000. Standard BTA drilling would convert the entire bore volume into chips, wasting over 60% of the material. The solution is trepanning: cutting an annular groove that removes only the outer ring while preserving a solid core for reuse. The trepanning head must be designed with the correct insert arrangement, guide pad positioning, and coolant flow paths. The manufacturer selects a head with four indexable carbide inserts across the annular face, two carbide guide pads at 90° and 180°, cutting speed 20 m/min at 0.12 mm/rev feed. The core is recovered intact, improving material utilisation from 35% to over 80%.

Trepanning vs. Solid Drilling ​

Trepanning is a deep hole machining method that cuts an annular groove instead of drilling a full bore. The centre core is preserved as a solid cylinder.

Comparison ​

When to Choose Trepanning ​

Trepanning is the preferred method when:

• Material cost is high — stainless steel, titanium, Inconel, or superalloys where the core value justifies the more complex tooling
• The core has value — the extracted core can be used for smaller-diameter components (shafts, pins, smaller tubes)
• Bore diameter is large — typically above 60 mm where the annular cut saves significant power and material
• Machine power is limited — trepanning requires 40–60% less power than solid drilling at the same diameter

Trepanning Head Design ​

Head Types ​

The indexable and cartridge-type heads are more common for production trepanning because the inserts can be replaced without removing the head from the drill tube.

Insert Arrangement ​

The annular face of a trepanning head is divided into cutting zones, each served by one or more inserts. The number of inserts depends on the annular width:

Annular width = (Bore diameter - Core diameter) / 2

A typical rule of thumb: annular width = 0.05 × bore diameter + (3–6 mm).

For a four-insert arrangement across the annular face:

The inserts are arranged with overlapping cutting paths to ensure complete annular groove cutting. Each insert removes a portion of the total annular width, reducing chip load per insert and improving chip control.

Insert Geometry ​

Guide Pad Configuration ​

Guide pads in trepanning heads serve the same function as in solid BTA heads — they balance cutting forces, guide the tool, and burnish the bore wall.

The primary guide pad (90°) carries the highest load and typically wears faster. The pad lag — the setback from the cutting edge corner — is a critical parameter that determines when the pads first contact the bore wall.

Tip: In trepanning, the guide pad on the bore side (outer pad) carries higher load than the pad on the core side because the cutting forces are asymmetrically distributed. Monitor the outer pad wear more frequently.

Cutting Parameters ​

Speed and Feed ​

Parameter Influences ​

Chip Formation ​

Chip control is more critical in trepanning than in solid drilling because the annular space is narrower and chip jamming can damage both the bore surface and the core.

Ideal chip form: Spiral or fragmented chips that evacuate freely through the annular gap.

Problematic chip forms:

Coolant Parameters ​

Core Recovery ​

Core recovery is the primary economic advantage of trepanning.

Core Handling ​

The core must be supported and removed after the trepanning operation is complete:

1. As the trepanning head advances, the core enters the hollow drill tube
2. A core catcher or gripper inside the tube prevents the core from dropping when the tool retracts
3. After retraction, the core is extracted from the drill tube
4. The core is inspected for surface damage and dimensional accuracy

Core Diameter ​

The core diameter is determined by the inner insert position on the trepanning head. Typical core diameters range from 40–80% of the bore diameter.

Material Utilisation ​

For a 180 mm bore with a 120 mm core (annular width = 30 mm):

Annular area = π × (R_bore² - R_core²) = π × (90² - 60²) = 14,137 mm²
Full bore area = π × 90² = 25,447 mm²
Material removed = 14,137 / 25,447 = 55.5%
Core recovered = 100% - 55.5% = 44.5%

Total material utilisation (core + chips recycled) >80%.

Troubleshooting ​

Surface Finish Problems ​

Core Quality Issues ​

Chip Evacuation Problems ​

Warning: A chip jam in trepanning is more dangerous than in solid drilling because the chip is wedged between the tool and the core. Clearing a jammed trepanning head requires retracting the tool with the core still attached — a procedure that should only be performed following the machine manufacturer's specific instructions.

FAQ ​

What is the minimum diameter for BTA trepanning? ​

The practical minimum is approximately 50–60 mm. Below this, the annular width is too small for effective chip evacuation and the core is too slender to survive the cutting forces.

What are the advantages of trepanning over solid drilling? ​

Lower power requirement (40–60% less), higher material utilisation (70–85% vs. 30–50%), reduced thrust force, and the preserved core can be reused for secondary products.

How many inserts does a trepanning head need? ​

Two to four inserts are typical, depending on the annular width. Two inserts for narrow annulus (<15 mm), three to four for wider annulus (>15 mm). Each insert removes a portion of the annular width.

What causes core breakage in trepanning? ​

Core breakage is most common at breakthrough when the remaining material cannot support the core weight. Reduce feed by 50% for the last 10–20 mm of the hole and use a core catcher to support the core during retraction.

Can I convert a BTA drill head to trepanning? ​

No — the head designs are fundamentally different. BTA drill heads have a centre cutting edge, while trepanning heads cut only the annular periphery. Conversion requires a complete head replacement.

What coolant pressure is needed for trepanning? ​

10–30 bar is typical, which is lower than gun drilling (50–120 bar) but at much higher flow rates (200–2,000 L/min depending on diameter).

How is the core removed after trepanning? ​

The core enters the hollow drill tube as the tool advances. A core catcher inside the drill tube grips the core during retraction. The core is then extracted from the drill tube manually or with a hydraulic pusher.

What accuracy can trepanning achieve? ​

Standard trepanning achieves IT10 (±0.05 mm for a 100 mm bore) with surface finish of Ra 3.2–6.3 µm. With optimised parameters, IT8–IT9 and Ra 1.6–3.2 µm are possible.

How do guide pads differ between trepanning and solid BTA? ​

In trepanning, the outer guide pad (contacting the bore wall) carries significantly higher load because the cutting force is asymmetric across the annular face. The inner guide pad (contacting the core) carries less load.

When should I choose indexable over brazed trepanning heads? ​

Choose indexable for diameters above 80 mm and production volumes above 100 parts per year. The higher initial cost is offset by lower per-edge cost and faster changeover.

Summary ​

BTA trepanning is the preferred method for large-diameter deep holes where material cost is high and core recovery adds value:

• Insert arrangement — two to four inserts distributed across the annular face, each removing a portion of the total annular width
• Guide pad configuration — two pads at 90° and 180° from the cutting edge, with the outer pad carrying higher load
• Cutting parameters — speeds of 15–40 m/min and feeds of 0.08–0.20 mm/rev depending on material
• Chip control — spiral or fragmented chips are essential; stringy chips will jam the annular gap
• Core recovery — the economic driver for trepanning, improving material utilisation from <50% to >80%

The oil and gas manufacturer in the opening scenario achieved the required 180 mm bore with a recovered core valued at €3,200 per part, making the operation significantly more profitable than solid drilling.