Thrust force in BTA drilling is not a theoretical number — it determines whether the machine's feed axis can drill the hole, whether the workpiece will deflect, and whether the fixture can hold the part. Every BTA drilling setup should include a thrust force calculation to verify machine and fixture adequacy.
Thrust Force Components
Force Breakdown
| Component | Description | Typical % of Total Thrust | Variable Factors |
|---|
| Cutting edge thrust | Force required to shear material at each cutting edge | 50–60% | Feed rate, material hardness, edge sharpness |
| Guide pad friction | Friction between guide pads and hole wall | 20–30% | Coolant lubrication, pad material, hole diameter |
| Chip breaking force | Force to break and deform chips | 10–15% | Chip breaker geometry, material ductility |
| Coolant pressure force | Force from coolant pressure acting on drill head cross-section | 5–10% | Coolant pressure, drill head diameter |
| Bushing friction | Friction at the drill bushing (if drill passes through) | 0–5% | Bushing condition, lubrication |
| Formula | Description |
|---|
| F_total = F_cutting + F_pads + F_chip + F_coolant + F_bushing | Sum of all thrust components (N) |
| F_cutting = kc × A × C_edge | Specific cutting force × chip area × edge factor |
| F_pads = µ × F_normal_pad × N_pads | Coefficient of friction × normal force per pad × number of pads |
| F_coolant = P × A_head | Coolant pressure × drill head projected area |
Thrust Calculation Method
Data Required
| Parameter | Symbol | Unit | Source | Example Value |
|---|
| Hole diameter | D | mm | Drawing | 50 mm |
| Feed rate | f | mm/rev | Process sheet | 0.040 mm/rev |
| Specific cutting force | kc | N/mm² | Material data (see table) | 2,500 N/mm² (steel 4140 Q&T) |
| Material hardness | — | HRC or HB | Material specification | 30 HRC |
| Number of cutting edges | z | — | Drill head design | 3 |
| Coolant pressure | P | bar | Process sheet | 80 bar |
| Drill head diameter | D_head | mm | Drill head spec | 49.8 mm |
Specific Cutting Force Table (kc1 for f = 1 mm/rev)
| Material | Hardness | kc1 (N/mm²) | mc (slope) | kc at 0.04 mm/rev (N/mm²) |
|---|
| Low carbon steel (1018) | 150 HB | 1,800 | 0.25 | 3,550 |
| Medium carbon steel (1045) | 200 HB | 2,100 | 0.26 | 4,200 |
| Alloy steel (4140 Q&T) | 300 HB | 2,500 | 0.27 | 5,100 |
| Tool steel (H13) | 350 HB | 2,800 | 0.28 | 5,700 |
| Stainless steel (304) | 180 HB | 2,400 | 0.30 | 5,200 |
| Cast iron (G25) | 200 HB | 1,100 | 0.20 | 1,950 |
| Aluminum (6061-T6) | 100 HB | 700 | 0.22 | 1,300 |
Calculation Steps
| Step | Calculation | Example (50 mm hole, 4140 steel, 0.040 mm/rev) |
|---|
| 1 | Calculate chip area per edge: A = (D/2) × f / z | A = (50/2) × 0.040 / 3 = 0.333 mm² |
| 2 | Calculate specific cutting force: kc = kc1 × f^(-mc) | kc = 2,500 × 0.040^(-0.27) = 2,500 × 2.15 = 5,375 N/mm² |
| 3 | Calculate cutting force: F_cutting = kc × A × z | F_cutting = 5,375 × 0.333 × 3 = 5,370 N |
| 4 | Estimate pad friction: F_pads ≈ 0.25 × F_cutting | F_pads ≈ 0.25 × 5,370 = 1,340 N |
| 5 | Calculate coolant force: F_coolant = P × π × D²/4 | F_coolant = 80 × 10^5 × π × (0.0498)²/4 = 15,600 N |
| 6 | Total thrust: F_total = F_cutting + F_pads + F_coolant | F_total = 5,370 + 1,340 + 15,600 = 22,310 N ≈ 2,270 kgf |
Important: Coolant pressure thrust is often the largest component. It acts against the drill head and must be overcome by the feed axis. Do not overlook this component.
Typical Thrust Values
Estimated Thrust by Diameter
| Hole Diameter (mm) | Feed (mm/rev) | Coolant Pressure (bar) | Steel (4140) Thrust (kN) | Cast Iron Thrust (kN) | Aluminum Thrust (kN) |
|---|
| 20 | 0.025 | 100 | 6.5 | 3.0 | 2.5 |
| 30 | 0.030 | 90 | 11.0 | 5.0 | 4.0 |
| 40 | 0.035 | 85 | 17.0 | 7.5 | 6.0 |
| 50 | 0.040 | 80 | 22.5 | 10.0 | 8.0 |
| 60 | 0.045 | 75 | 29.0 | 13.0 | 10.5 |
| 80 | 0.050 | 70 | 45.0 | 20.0 | 16.0 |
| 100 | 0.055 | 65 | 62.0 | 28.0 | 22.0 |
Machine Feed Axis Capacity Check
| Check | Requirement | Verification |
|---|
| Machine rated thrust | > 1.5 × calculated total thrust | Machine specification sheet |
| Servo motor torque at feed rate | Available torque > required thrust × lead / (2π × efficiency) | Torque-speed curve |
| Ball screw capacity | Dynamic load rating > 3 × calculated thrust | Ball screw specification |
| Fixture clamp force | Total clamp force > 2 × calculated thrust | Fixture design |
| Steady rest capacity | Each rest load rating > thrust / number of rests | Steady rest specification |
Factors That Increase Thrust
Thrust Increasing Factors
| Factor | Effect on Thrust | How to Mitigate |
|---|
| Dull drill | Increases 30–100% | Change drill at planned interval |
| High feed rate | Proportional increase | Optimize feed for material and diameter |
| Hard material | Higher specific cutting force | Reduce feed, use coated tools |
| Worn guide pads | Increases pad friction | Replace or recondition pads |
| Low coolant pressure | No direct thrust increase — but reduces chip evacuation | Maintain operating pressure |
| Coolant with poor lubricity | Higher pad friction | Maintain coolant concentration and quality |
| Chip packing | Can double thrust suddenly | Improve chip breaking, increase coolant pressure |
| Misaligned bushing | Increases bushing friction | Align bushing with spindle axis |
Thrust Monitoring
| Monitoring Method | What It Detects | How to Implement |
|---|
| Feed axis load display | Gradual thrust increase = tool wear | Record feed load at consistent depth |
| Spindle power monitoring | Indirect — correlates with thrust | More sensitive to tool condition than thrust |
| Feed axis torque monitoring | Direct thrust indication | Most common — built into servo drive |
| Dynamometer (research) | Exact thrust measurement | Test lab only — not production |
Machine Implications
Machine Requirements
| Machine Parameter | Why It Matters | Minimum Requirement |
|---|
| Feed axis thrust capacity | Must exceed total drilling thrust | 1.5× calculated maximum thrust |
| Guideway rigidity | Thrust forces must not deflect the axis | Linear guide size and preload |
| Ball screw diameter | Thrust load affects screw life | Dynamic rating > 3× thrust |
| Fixture and clamp design | Must hold workpiece against thrust | Clamp force > 2× thrust |
| Steady rest capacity | Must support thrust without deflection | Rest rating > thrust |
| Bushing holder rigidity | Must not deflect under drill entry forces | Rigid mounting, bracket thickness |
Fixture Design Considerations
| Requirement | Thrust Force (kN) | Minimum Clamp Force (kN) | Recommended Clamp Type |
|---|
| 20 mm hole in steel | 6.5 kN | > 13 kN | Hydraulic or mechanical |
| 50 mm hole in steel | 22.5 kN | > 45 kN | Hydraulic — multiple clamps |
| 100 mm hole in steel | 62 kN | > 124 kN | Hydraulic — heavy-duty |
FAQ
What is thrust force in BTA drilling?
Thrust force is the total axial force required to push the BTA drill head through the workpiece. It includes: the cutting force at the cutting edges, friction from the guide pads rubbing against the hole wall, force to break and deform chips, the force of coolant pressure acting on the drill head cross-section, and friction at the bushing. The coolant pressure component is often the largest.
How do I calculate thrust force for BTA drilling?
Use the formula: F_total = F_cutting + F_pads + F_coolant. Calculate F_cutting = kc × A × z (specific cutting force × chip area per edge × number of edges). Calculate F_coolant = P × A_head (coolant pressure × drill head area). Estimate F_pads ≈ 0.25 × F_cutting. Sum all components. For a 50 mm hole in 4140 steel at 0.040 mm/rev feed and 80 bar coolant pressure, total thrust is approximately 22 kN (2,200 kgf).
How does coolant pressure affect thrust force?
Coolant pressure acts on the projected area of the drill head (π × D²/4). At 80 bar on a 50 mm diameter drill head, the coolant force alone is about 15.6 kN — often the largest single component of total thrust. This force pushes the drill head backward against the feed axis. The machine feed axis must overcome this force before any cutting occurs. Higher coolant pressure increases thrust proportionally.
What happens if thrust force exceeds machine capacity?
If thrust exceeds the machine feed axis capacity: the axis may stall (following error alarm, machine stops), the ball screw life is dramatically reduced (premature fatigue failure), the workpiece may be pushed out of the fixture (safety hazard), the drill tube may buckle (especially in deep holes with small diameters and chip packing), or the guideways may be overloaded (accelerated wear). Always verify machine thrust capacity before selecting parameters.
How do I reduce thrust force in BTA drilling?
To reduce thrust: reduce feed rate (direct proportional reduction in cutting force), improve drill sharpness (sharper edges reduce cutting force by 30–50%), use coolant with better lubricity (reduces guide pad friction), ensure proper coolant concentration (affects lubricity), verify coolant pressure is at target (not higher than needed), and check guide pad condition (worn pads increase friction).
Thrust force calculation is an essential engineering check before setting up any BTA drilling operation. The coolant pressure thrust component is often the largest and most commonly overlooked. Verify that the machine feed axis, ball screw, fixture, and steady rests are all rated for the calculated thrust before drilling the first hole. This article reflects industry practice as of 2026.
