I estimate cutting forces to check whether the machine can handle the job before I start every deep hole drilling operation. Cutting forces in gun drilling have two components that I track separately: thrust force pushing the drill forward and torque resisting the rotation. Getting these wrong means either stalling the machine feed axis or snapping the drill shaft at depth. I have seen both outcomes on the shop floor, and neither is cheap to fix.
The stakes are higher in gun drilling than in conventional drilling because the cutting forces act at the end of a long, slender shaft. Any force spike that exceeds the drill’s buckling limit snaps the tool instantly. That is why I always calculate cutting forces before I touch a machine.
Thrust Force Calculations
Thrust force in gun drilling depends primarily on the drill diameter, feed rate, and the specific cutting force of the workpiece material. The general formula I use is:
Thrust (N) = Specific Cutting Force x Feed (mm/rev) x Drill Diameter (mm) x Geometry Factor
The geometry factor accounts for the single-lip design of a gun drill. For most gun drills, this factor falls between 0.5 and 0.7 depending on the tip geometry and the chip breaker configuration. I use 0.6 as my default and adjust once I have measured data from the first production run.
For a 10mm gun drill in mild steel at 0.04 mm/rev, the thrust comes out to about 1500-2000 N. I check the machine feed motor rating to make sure it can deliver that continuously without stalling on the feed axis. I also check the thrust bearing capacity on the drill guide bushing — that is a failure point that newer machinists often overlook.
Here is a reference table I have built from production data across different materials and diameters:
| Drill Dia (mm) | Material | Feed (mm/rev) | Thrust (N) | Machine Feed Required |
|---|---|---|---|---|
| 6 | Mild steel | 0.03 | 600-800 | 5 kN rated |
| 10 | Mild steel | 0.04 | 1500-2000 | 10 kN rated |
| 10 | Stainless 304 | 0.03 | 2500-3000 | 15 kN rated |
| 12 | Alloy steel 4140 | 0.04 | 3200-3800 | 18 kN rated |
| 15 | Alloy steel | 0.05 | 4000-5000 | 20 kN rated |
| 18 | Stainless 316 | 0.025 | 4500-5500 | 25 kN rated |
| 20 | Titanium Ti-6Al-4V | 0.02 | 3500-4500 | 20 kN rated |
| 25 | Inconel 718 | 0.015 | 6000-7500 | 30 kN rated |
I add a 50% safety margin when selecting a machine. If the calculated thrust is 2000 N, I want a machine with at least 3000 N of feed force available. The margin accounts for tool wear and material variation between heats. On one job running 4140 steel, I saw thrust climb from 3200 N on a fresh tool to 4800 N just before the tool failed — that 50% headroom saved the drill.
Sandvik Force Formula for Gun Drilling
The Sandvik deep hole machining guide provides a more precise formula that I use for critical applications:
Feed (Thrust) Force:
Ff + Ffμ = 0.65 x ap x fn x kcfz x sin(KAPR)
Where ap is the depth of cut, fn is the feed per revolution, kcfz is the specific cutting force for the material, and KAPR is the cutting edge angle. The Sandvik formula adds a friction component (Ffμ) that accounts for the guide pad contact forces — this extra term matters because gun drill guide pads carry significant load.
Material Factor K Values
For empirical thrust and torque calculations, I use the work material factor K. These values come from published deep hole drilling research and my own shop-floor validation:
| Material | K Factor (N/mm²) |
|---|---|
| Aluminum 7075-T6 | 7,000 |
| Mild Steel 1018 | 18,000 |
| Alloy Steel 4140 (RC30) | 24,000 |
| PH Stainless (RC45) | 28,000 |
| Alloy Steel (RC55) | 31,000 |
| Titanium Ti-6Al-4V | 22,500 |
| Inconel 718 | 35,000 |
| Cobalt Chrome | 32,000 |
The K factor changes with hardness. I multiply torque and thrust by 1.5 for dull tools and 1.25 when chip ejection is poor. These adjustment factors come directly from published empirical models for deep hole drilling cutting forces.
Torque and Power Requirements
Torque in gun drilling depends on drill diameter, feed, and material hardness. For a 10mm gun drill in mild steel at 0.04 mm/rev, the torque at the cutting tip is about 8-12 Nm. The spindle drive must provide that torque at the operating RPM without tripping the drive overload protection.
I calculate the required spindle power from the torque and RPM using the standard formula:
Power (kW) = Torque (Nm) x RPM / 9550
For 10 Nm at 3000 RPM, the required power is about 3.1 kW at the cutting zone. I multiply by 1.2 to account for drive losses through the gearbox and belt drive, giving a spindle rating requirement of 3.7 kW minimum. If the machine has a 5 kW spindle, I have margin for tool wear and depth effects.
The Sandvik power formula for gun drilling adds a friction component:
Power (kW) = (fn x vc x DC x kcfz / 240,000) x 1.34
Where vc is cutting speed in m/min and DC is drill diameter in mm. I have compared both formulas against actual spindle load readings and found the Sandvik formula predicts within 10% of measured power for most materials.
Torque Distribution Between Cutting Edges
The torque splits unevenly between the two cutting edges in a gun drill. The inner cutting edge (near the center) carries about 40% of the torque, and the outer cutting edge carries 60%. This uneven distribution is why gun drills use a single cutting edge design with guide pads on the opposite side — the outer edge generates more radial force, and the guide pads balance that force against the bore wall.
I have measured this split directly using a split-tool dynamometer setup. The inner edge generates higher specific force because of the lower cutting speed near the center, but the outer edge has a larger chip cross-section. The net effect is the 40/60 split I use in my calculations.
Torque Table by Diameter and Material
| Drill Dia (mm) | Material | Feed (mm/rev) | RPM | Torque (Nm) | Power (kW) |
|---|---|---|---|---|---|
| 6 | Mild steel | 0.03 | 5000 | 3-5 | 1.6 |
| 10 | Mild steel | 0.04 | 3000 | 8-12 | 2.5-3.8 |
| 10 | Stainless 304 | 0.03 | 2500 | 14-18 | 3.7-4.7 |
| 15 | Alloy steel | 0.05 | 2000 | 25-35 | 5.2-7.3 |
| 20 | Titanium | 0.02 | 1500 | 30-40 | 4.7-6.3 |
Measuring Cutting Forces in Production
I measure cutting forces indirectly through spindle load monitoring on every production run. Most CNC controls display the spindle load as a percentage of the rated maximum. I record the baseline load for a sharp tool on the first part and set an alarm at 120% of that baseline.
Cutting forces in gun drilling increase with tool wear in a predictable pattern. If the spindle load increases by 20% over the baseline, the tool needs replacement. For a 10mm gun drill in steel, the baseline might be 25% spindle load. When it reaches 30%, I change the tool. This approach catches worn tools before they break — I have saved dozens of workpieces this way.
I have also used a piezoelectric dynamometer on a research setup to measure gun drilling cutting forces directly. The dynamometer sits under the workpiece and measures thrust and torque in real time at 1000 Hz sampling. The data confirmed that my calculated estimates were within 15% of actual forces for most materials. The dynamometer traces also revealed a force spike of 30-50% at drill entry that lasts about 2mm of depth penetration. I now account for that entry spike when setting feed acceleration profiles.
Factors That Change Cutting Forces
Several factors shift cutting forces during a gun drilling cycle:
Tool wear: Forces increase gradually as the cutting edge wears. The thrust force typically rises faster than torque because edge rounding increases the ploughing component. I expect thrust to climb 30-50% over the life of a gun drill tip.
Coolant pressure: High coolant pressure (80-120 bar) reduces cutting forces by improving lubrication at the cutting zone. I have measured a 5-10% force reduction when coolant pressure increases from 60 bar to 100 bar.
Chip jamming: When chips jam in the flute, torque spikes instantly. The spindle load jumps 50-100% in a fraction of a second. This is the leading cause of gun drill breakage in production. For more on chip control, see the chip management article.
Material hardness variation: I have seen thrust vary by 20% between different heats of the same alloy steel grade. I always test a new heat before running production quantities.
For more on how tool wear interacts with chip formation and cutting forces, see the chip breaking article — worn tools produce different chip shapes that directly affect force distribution on the cutting edges. The coolant temperature article also covers how coolant conditions affect the cutting zone and the resulting forces.
Key Takeaways
- Calculate thrust as specific cutting force x feed x diameter x geometry factor — I use 0.6 as the default geometry factor for gun drills.
- Select machine feed capacity with 50% margin over calculated thrust to handle tool wear and material variation.
- Use the Sandvik power formula for precise spindle sizing: P = (fn x vc x DC x kcfz / 240,000) x 1.34.
- Monitor spindle load as a proxy for tool wear in gun drilling — change tools at 120% of baseline load.
- Force spikes at drill entry are normal at 30-50% above steady-state and last about 2mm of depth — account for this in feed acceleration.
- Inner cutting edge carries 40% of torque, outer edge carries 60% — the single-lip design balances forces through guide pads.
- High coolant pressure (80-120 bar) reduces cutting forces by 5-10% through improved zone lubrication.
- Material hardness variation between heats can shift thrust by 20% — always test new stock before production.