Spindle power and torque are related but different specifications. Power determines how fast material can be removed. Torque determines how much cutting force the spindle can deliver at low RPM. For deep hole drilling, misunderstanding these two specs leads to undersized spindles that stall in the cut. I have seen a 35 kW spindle stall out on a 60 mm BTA drill because the machine peaked at high RPM and delivered only 80 Nm at the feed rate needed for BTA parameters.
The Difference Between Power and Torque
Power and torque are linked by a simple formula:
Power (kW) = Torque (Nm) x RPM / 9549
A spindle rated at 30 kW delivers 191 Nm at 1500 RPM but only 95 Nm at 750 RPM. This is the trap. A spec sheet that lists only peak power tells you nothing about low-speed capability. I always ask for the full torque curve from 0 RPM to maximum speed.
| Specification | What It Determines | Deep Hole Relevance |
|---|---|---|
| Peak power (kW) | Maximum material removal rate | High |
| Torque at low RPM (< 1000) | Ability to drive large drills | Critical |
| Torque at mid RPM (1000 - 3000) | Typical BTA and gundrill range | High |
| Power at the tool (kW) | Actual cutting capability after losses | Very important |
| Speed range (RPM) | Min and max spindle speed | Medium |
Torque Requirements by Drilling Method
A typical gun drilling application (10 mm drill at 3000 RPM with 0.04 mm/rev feed) requires about 3 to 5 kW of power and 10 to 15 Nm of torque. A BTA drilling application (50 mm drill at 800 RPM with 0.15 mm/rev feed) requires about 20 to 30 kW of power and 250 to 400 Nm of torque. The difference is driven by the chip load per revolution and the tool diameter.
Here is a comparison table I compiled from machines running in production:
| Drilling Method | Tool Diameter (mm) | RPM | Feed (mm/rev) | Power at Tool (kW) | Torque at Spindle (Nm) |
|---|---|---|---|---|---|
| Gundrill | 6 | 6000 | 0.025 | 2.5 | 4 |
| Gundrill | 15 | 3500 | 0.040 | 7.0 | 19 |
| Gundrill | 30 | 2000 | 0.060 | 15.0 | 72 |
| BTA single-tube | 40 | 1200 | 0.120 | 25.0 | 199 |
| BTA single-tube | 60 | 800 | 0.150 | 35.0 | 418 |
| BTA double-tube | 80 | 600 | 0.180 | 45.0 | 716 |
| STS / ejector | 100 | 400 | 0.200 | 55.0 | 1313 |
The torque column tells the real story. A gundrill under 20 mm uses less than 20 Nm. A 40 mm BTA drill needs 10 times that. A 100 mm STS drill needs more than 1300 Nm. If the spindle cannot deliver torque at the RPM you intend to run, the machine will not cut efficiently.
Power Losses Between Motor and Tool
The spindle power specification is usually listed at the motor, not at the tool. Belt and gear losses reduce the power at the tool by 10% to 20%. I check the power at the tool specification when evaluating a machine. On a geared spindle with a belt drive, the losses break down roughly as follows:
| Power Loss Source | Typical Loss (%) | Notes |
|---|---|---|
| Belt drive | 3 - 5 | Increases with belt wear |
| Gear train | 5 - 10 | Higher for multi-speed gearboxes |
| Bearing friction | 2 - 4 | New bearings are tighter |
| Coolant seal drag | 1 - 3 | High-pressure seals cause more drag |
| Total loss | 11 - 22 | Apply to motor power for tool power |
A 30 kW motor with 15% system loss delivers only 25.5 kW at the tool. If the machine is pushing the limit at 25 kW, that 4.5 kW difference decides whether the job runs or stalls half way through a 500 mm deep bore.
Reading a Torque Curve
Machine spec sheets rarely list torque curves. I ask the manufacturer for the torque vs RPM curve when evaluating a machine for deep hole drilling. The curve shows whether the spindle can deliver the needed torque at the required speed.
A continuous-duty torque curve has three regions worth noting:
- Constant torque region. From 0 RPM to the base speed, the spindle delivers rated torque. This is where BTA drilling operates. A high base speed pushes constant torque further up the RPM range.
- Constant power region. Above base speed, torque drops off as RPM increases. This is where gundrills typically run. The torque available is inversely proportional to speed.
- Overspeed region. If the spindle exceeds the rated speed, torque drops steeply and the spindle cannot sustain operation indefinitely.
In my experience, a spindle with a base speed above 1000 RPM is not ideal for large-diameter deep hole drilling. I look for a base speed of 600 RPM or lower to ensure sufficient torque in the BTA operating range.
Why This Matters for Machine Selection
I have evaluated more than a dozen used machine tools where the spindle appeared powerful on paper but failed in deep hole applications. The most common problem is a high-speed machining center spindle repurposed for deep hole work. These spindles deliver 200+ Nm at 3000 RPM but only 60 Nm at 500 RPM. For a 50 mm BTA drill that needs 300+ Nm, the spindle stalls instantly.
When I spec a machine now, I work backwards from the tool. I calculate the required torque and power from the largest drill the machine will ever run, then add 25% margin. That margin accounts for material hardness variation and tool wear over the life of the machine.
Key Takeaways
- Torque at low RPM is the limiting factor for deep hole drilling, not peak power. Always check the full torque curve.
- Power loss between motor and tool is 10% to 20%. Use the at-tool figure for sizing, not the motor rating.
- Gun drilling is power-limited. BTA drilling is torque-limited. Different machines suit each method.
- Machine spec sheets intentionally obscure the torque curve. I do not buy a deep hole machine without seeing the torque vs. RPM graph for the specific spindle configuration.