BTA and Gun Drilling Power and Torque Requirements

Selecting a deep hole drilling machine without understanding the power and torque requirements is like specifying a pump without knowing the head pressure. The spindle power, torque capacity, feed force capability, and coolant system must each be matched to the largest diameter and most demanding material the machine will ever cut — and the relationship between diameter and power is not linear. It is approximately cubic.

Power and Torque Fundamentals ​

Key Formulas ​

The fundamental relationships for deep hole drilling power and torque are the same as for conventional drilling, but the magnitudes differ due to the single-sided cutting action of gun drills and the multi-insert configuration of BTA heads.

Spindle speed:

n = (Vc × 1,000) / (π × D)

Where n = RPM, Vc = cutting speed (m/min), D = drill diameter (mm).

Feed rate (penetration):

vf = fn × n

Where vf = feed rate (mm/min), fn = feed per revolution (mm/rev).

Material removal rate:

Q = (vf × π × D²) / 4,000

Where Q = MRR in cm³/min.

Net spindle power:

Pc = (Q × kc) / (60,000 × η)

Where Pc = net power (kW), kc = specific cutting force (N/mm²), η = machine efficiency (typically 0.7–0.85).

Torque:

Mc = (kc × fn × D² × sin κ) / 8,000

Where Mc = torque (kNm), κ = cutting edge angle.

Feed (thrust) force:

Ff ≈ 0.63 × (D/2) × fn × kc × sin κ

Where Ff = feed force (N).

Tip: The relationships that matter for machine sizing are: power ∝ D² × fn × Vc, torque ∝ D³ (approximately), and feed force ∝ D. This means that doubling the drill diameter increases power requirement by 4× and torque by approximately 8×.

Scaling Relationships ​

Power Requirements by Diameter ​

BTA STS (Single Tube System) — Net Power ​

Data from the ISCAR Drilling Handbook for BTA STS drilling of steel (specific cutting force kc ≈ 2,200 N/mm²):

Practical interpretation: A machine specified for 50 mm diameter BTA drilling in steel at productive parameters needs at least 45 kW of spindle power. If the same machine must handle occasional 60 mm work, 60 kW spindle power is required. These are net cutting power values — the machine's rated motor power should be 20–30% higher to account for transmission losses and auxiliary loads.

Warning: The power values above assume stable cutting conditions. Power consumption can spike by 30–50% during transient events such as entry, exit through cross-holes, or when the tool encounters hard spots in the material. The machine spindle motor must have sufficient overload capacity (typically 150% for 30 seconds) to handle these events without stalling.

Power by Material ​

Torque Requirements ​

BTA STS Torque by Diameter ​

The torque values determine the spindle drive sizing and the drill tube torsional capacity. The drill tube must be capable of transmitting the peak torque without exceeding its torsional yield strength — particularly for long tubes where wind-up becomes significant.

Drill Tube Torsional Capacity ​

The safe limit is approximately 70% of the theoretical maximum torque to provide a margin against transient overloads and tube fatigue over extended use.

Feed Force (Thrust) Requirements ​

Feed Force by Diameter ​

Tip: Feed force is the limiting factor for long, small-diameter BTA drilling. A machine with 50 kN thrust capacity can drill 60 mm holes without concern, but the feed force requirement for a 20 mm drill at 15:1 L/D is limited by column buckling of the drill tube, not by machine capacity. For L/D above 30:1 in diameters under 30 mm, feed force must be reduced to prevent tube buckling.

Machine Thrust Capacity by Class ​

Specific Cutting Force Values ​

The specific cutting force (kc) is the fundamental input for all power and torque calculations. It varies by material and chip thickness:

kc1 Values by Material ​

The actual specific cutting force at a given feed rate is:

kc = kc1 × h^(-mc)

Where h is the chip thickness (mm).

Example Calculation ​

For a 40 mm BTA drill cutting alloy steel (kc1 = 2,200, mc = 0.28) at f = 0.20 mm/rev:

1. Chip thickness: h = fn × sin κ ≈ 0.20 × sin 75° = 0.193 mm
2. kc = 2,200 × 0.193^(-0.28) = 2,200 × 1.56 = 3,432 N/mm²
3. Torque: Mc = (3,432 × 0.20 × 40² × sin 75°) / 8,000 = 0.265 kNm
4. Power at Vc = 80 m/min: Pc = (Mc × 2π × n) / 60, where n = (80 × 1,000) / (π × 40) = 637 rpm → Pc = (0.265 × 2π × 637) / 60 = 17.7 kW

Machine Spindle Specifications ​

Commercial Machine Data ​

Speed-Torque Curve Considerations ​

Electric spindle motors deliver constant torque up to a base speed, then constant power above it. For BTA drilling:

• Large diameters (100+ mm) require high torque at low speed — operation is below the motor's base speed in the constant torque region
• Small diameters (under 30 mm) require lower torque at higher speed — operation is above the base speed in the constant power region

A common mistake is specifying a machine based on power alone without verifying that the motor delivers sufficient torque at the required operating speed. A 50 kW motor with a base speed of 1,500 rpm delivers only 318 Nm — insufficient for a 100 mm BTA drill requiring 400+ Nm at 300 rpm.

Counter-Rotation Effects ​

Many BTA machines incorporate counter-rotation where the workpiece rotates opposite to the tool:

Power Distribution ​

The total cutting power is the same regardless of how the speed is distributed between tool and workpiece. Counter-rotation improves:

• Straightness — symmetric cutting force distribution
• Surface finish — reduced feed mark height
• MRR — ability to increase feed without straightness penalty

Gun Drilling Power and Torque ​

Gun drilling differs from BTA in power requirements because:

• Single cutting edge (vs. multiple inserts in BTA)
• Lower feed rates (typically 0.01–0.05 mm/rev vs. 0.10–0.30 mm/rev for BTA)
• Higher cutting speeds possible in small diameters

Typical Gun Drilling Power ​

These values are substantially lower than BTA at equivalent diameters — but gun drilling coolant pressure requirements are significantly higher (80–150 bar vs 3–10 MPa for BTA), which drives the coolant pump power specification.

Coolant System Power ​

The coolant pump is often the largest single power consumer on a BTA machine, sometimes exceeding the spindle drive.

Pump Power Calculation ​

Pp = (Q × P) / (600 × ηp)

Where Pp = pump power (kW), Q = flow (L/min), P = pressure (bar), ηp = pump efficiency (typically 0.7–0.85).

For gun drilling:

Warning: The coolant pump power for BTA drilling at diameters above 60 mm can exceed the spindle power. A 60 mm BTA operation requiring 60 kW of cutting power and 63 kW of coolant pump power needs a total installed capacity of approximately 150 kW after accounting for auxiliary systems and efficiency losses. Do not size the electrical installation on spindle power alone.

Machine Sizing Procedure ​

Step-by-Step Selection ​

1. Determine maximum diameter — All subsequent calculations depend on this
2. Select cutting parameters — Vc and fn for the hardest material to be drilled
3. Calculate spindle power — Pc = (kc × fn × D² × Vc) / (8,000 × η)
4. Calculate torque — Mc = (kc × fn × D² × sin κ) / 8,000
5. Calculate feed force — Ff from the diameter/feed table
6. Calculate coolant flow — Q = 4.5 × D (L/min) for BTA
7. Calculate coolant pressure — Based on diameter and tube length
8. Calculate pump power — Pp = (Q × P) / (600 × ηp)
9. Add safety margin — Total installed power = 1.3 × (Pc + Pp + auxiliary)

Example: Sizing for 50 mm BTA in Alloy Steel ​

FAQ ​

How do I calculate spindle power for BTA drilling? ​

Use the formula Pc = (kc × fn × D² × Vc) / (8,000 × η) where kc is the specific cutting force (N/mm²), fn is feed per revolution (mm/rev), D is drill diameter (mm), Vc is cutting speed (m/min), and η is machine efficiency (typically 0.7–0.85). For a quick estimate, use the power-by-diameter tables in this article.

What is the relationship between drill diameter and power requirement? ​

Power scales approximately with the square of the diameter (P ∝ D²). A 60 mm drill requires approximately 9× the power of a 20 mm drill at the same cutting speed and feed. Torque scales even more steeply — approximately with D³.

How much spindle power is needed for 40 mm BTA drilling? ​

For productive BTA drilling of steel at 40 mm diameter, expect 19–32 kW net cutting power depending on cutting speed and feed rate. Specify a machine with at least 37 kW rated spindle power for a 40 mm production requirement.

What coolant pump power is required for deep hole drilling? ​

Coolant pump power for BTA drilling is calculated as Pp = (Q × P) / (600 × ηp). For a 40 mm BTA operation (225 L/min at 50 bar), pump power is approximately 23 kW. At 60 mm (380 L/min at 80 bar), pump power reaches approximately 63 kW — potentially exceeding the spindle drive.

How does counter-rotation affect power requirements? ​

Counter-rotation does not change the total cutting power — it redistributes it between the tool spindle and work spindle. The total power at the cutting zone is the same whether the tool rotates alone or both tool and workpiece counter-rotate.

What is specific cutting force and how do I use it? ​

Specific cutting force (kc) is the force required to remove a chip of unit cross-sectional area, measured in N/mm². It varies by material and chip thickness. Multiply kc by the chip cross-sectional area to get cutting force, then use force × speed to get power. Typical kc1 values range from 700 (aluminium) to 3,000 (superalloys).

How do I size a machine for both small and large diameter drilling? ​

Size the machine for the largest diameter, then verify that the spindle drive provides adequate characteristics for small-diameter operation. Large diameters need high torque at low speed; small diameters need lower torque at high speed. A spindle drive with a wide constant-power speed range (at least 4:1) is essential.

What feed force (thrust) capacity is needed? ​

Feed force ranges from 6 kN (20 mm drill) to 18 kN (60 mm drill) at productive feed rates. Machine thrust capacity should be 1.5–2× the calculated maximum feed force. For reference: medium-duty BTA machines typically provide 50–80 kN thrust, which covers most drilling up to 80 mm diameter.

What machine power is needed for gun drilling? ​

Gun drilling requires much less spindle power than BTA — typically 5.5–7.5 kW for diameters up to 25 mm. However, coolant pump power is significant due to high pressure requirements (up to 150 bar). For a 20 mm gun drill, total installed power is typically 15–20 kW split between spindle and coolant pump.

How do I determine total installed power for a BTA machine? ​

Total installed power = spindle motor + coolant pump motor + auxiliary systems (chip conveyor, hydraulics, controls), plus a 30% safety margin. For a 50 mm BTA machine: approximately 35 kW spindle + 25 kW coolant pump + 10 kW auxiliary = 70 kW × 1.3 = 91 kW total. For a 100 mm machine: approximately 120 kW spindle + 100 kW coolant + 20 kW auxiliary = 240 kW × 1.3 = 312 kW total installed.

Conclusion ​

Spindle power, torque, and coolant system capacity are the defining specifications of a deep hole drilling machine — and they scale dramatically with diameter. The relationships are predictable: power ∝ D² × fn × Vc, torque ∝ D² × fn, and coolant pump power can equal or exceed spindle power at diameters above 60 mm. Machine selection must consider not only the maximum diameter to be drilled but the specific cutting force of the most demanding material and the feed rate required for productive operation. The total installed power — spindle drive, coolant pump, and auxiliaries with safety margin — is the single most important specification for evaluating whether a machine can handle a given range of deep hole drilling work.