Keyway Broaching Force & Tonnage Calculator: Pitch, RPT & Gullet
Keyway broaching force equals the number of simultaneously engaged teeth ze = L / P multiplied by keyway width w, rise per tooth RPT, and specific cutting force kc: F = (L/P) × w × RPT × kc. Cutting an 8.0 mm wide keyway across a 40 mm SAE 1045 steel hub requires 4.34 kN (0.49 US tons).
Keyway Broaching Tonnage, Tooth Pitch & Chip Gullet Calculator
Compute optimum broach tooth pitch P = 1.75√L, simultaneously engaged teeth ze = L/P, peak arbor press tonnage F = ze·w·RPT·kc, and coiled chip gullet fill ratio.
How Do Tooth Pitch, Engaged Teeth, and Gullet Capacity Control Broaching Tonnage?
Unlike milling or turning—where sheared chips escape freely into open space—an internal keyway broach must curl and carry the entire ribbon of metal removed across the full hub length L inside each closed tooth gullet until that tooth exits the bottom of the bore. Therefore, broach design under DIN 1415 and ASME B17.1 (Keys and Keyseats) balances two strict physical boundaries. First, the optimum tooth pitch P = 1.75√L (in mm) keeps between 2.0 and 6.0 teeth engaged simultaneously so the tool never dives or chatters in thin hubs while avoiding excessive press tonnage in long hubs. Second, the cross-sectional gullet area Agullet ≈ 0.35 × P² must be at least 4 to 5 times larger than the solid chip cross-section Achip = L × RPT to accommodate the coiling bulk of steel chips.
As engineering charts from The duMont Corporation (Minute Man Broaches) and Hassay Savage demonstrate, letting the gullet ratio Agullet / Achip drop below 4.0 causes chips to pack solid inside the root radius and shear off consecutive broach teeth inside the workpiece. Force conversions between kilonewtons, metric tonnes-force, and US short tons (2,000 lbf = 8.8964 kN) follow NIST Special Publication 811, while material cutting resistance scales with tensile strength tested per ISO 6892-1 (Metallic Tensile Testing) and tabulated in Machinery's Handbook (31st Ed., Industrial Press).
| Broach Style & Width w | Hub Length L | Optimum Pitch P & Teeth | Rise / Tooth (RPT) | 1045 Steel Force (k_c = 2500) | 304 Stainless Force (k_c = 3300) |
|---|---|---|---|---|---|
| A (2 mm / 1/16") | 20 mm | 7.83 mm (ze=2.5) | 0.04 mm/tooth | 0.51 kN (0.06 US tons) | 0.67 kN (0.08 US tons) |
| B (4 mm / 3/16") | 25 mm | 8.75 mm (ze=2.9) | 0.05 mm/tooth | 1.43 kN (0.16 US tons) | 1.89 kN (0.21 US tons) |
| B (6 mm / 1/4") | 32 mm | 9.90 mm (ze=3.2) | 0.06 mm/tooth | 2.91 kN (0.33 US tons) | 3.84 kN (0.43 US tons) |
| C (8 mm / 5/16") | 40 mm | 11.07 mm (ze=3.6) | 0.06 mm/tooth | 4.34 kN (0.49 US tons) | 5.72 kN (0.64 US tons) |
| C (10 mm / 3/8") | 50 mm | 12.37 mm (ze=4.0) | 0.07 mm/tooth | 7.07 kN (0.80 US tons) | 9.34 kN (1.05 US tons) |
| D (12 mm / 1/2") | 60 mm | 13.56 mm (ze=4.4) | 0.08 mm/tooth | 10.62 kN (1.19 US tons) | 14.02 kN (1.58 US tons) |
| D (14 mm / 9/16") | 75 mm | 15.16 mm (ze=5.0) | 0.08 mm/tooth | 13.85 kN (1.56 US tons) | 18.28 kN (2.06 US tons) |
| E (18 mm / 3/4") | 90 mm | 16.60 mm (ze=5.4) | 0.09 mm/tooth | 21.96 kN (2.47 US tons) | 28.98 kN (3.26 US tons) |
What Rise Per Tooth (RPT) and Specific Cutting Force Should You Use?
Standard push-type keyway broaches divide total slot depth across 2 to 4 shimmed passes so the slender broach column does not fail by Euler buckling under compressive arbor press loads. For medium-carbon and alloy steels (SAE 1045 at kc = 2,500 MPa and 4140 at 3,000 MPa), roughing teeth are ground with a rise per tooth of 0.050 to 0.075 mm/tooth, followed by 4 full-depth sizing teeth with zero RPT that calibrate final slot depth.
| Workpiece Material | Specific Force kc | Typical Roughing RPT | Face Hook / Clearance Angle | Force on 8×40 mm Keyway |
|---|---|---|---|---|
| SAE 1018 mild steel | 2100 N/mm² | 0.04 – 0.08 mm | 12° – 18° hook / 2° backoff | 3.64 kN (0.41 US tons) |
| SAE 1045 medium-carbon steel | 2500 N/mm² | 0.04 – 0.08 mm | 12° – 18° hook / 2° backoff | 4.34 kN (0.49 US tons) |
| SAE 4140 alloy steel (28 HRC) | 3000 N/mm² | 0.04 – 0.08 mm | 12° – 18° hook / 2° backoff | 5.20 kN (0.58 US tons) |
| 304 stainless steel | 3300 N/mm² | 0.04 – 0.08 mm | 12° – 18° hook / 2° backoff | 5.72 kN (0.64 US tons) |
| Gray cast iron (GG25) | 1400 N/mm² | 0.04 – 0.08 mm | 12° – 18° hook / 2° backoff | 2.43 kN (0.27 US tons) |
| 6061-T6 aluminum | 850 N/mm² | 0.04 – 0.08 mm | 12° – 18° hook / 2° backoff | 1.47 kN (0.17 US tons) |
| C360 free-cutting brass | 950 N/mm² | 0.04 – 0.08 mm | 12° – 18° hook / 2° backoff | 1.65 kN (0.19 US tons) |
Worked Examples: Arbor Press Sizing for 8 mm and 3/8" Keyways
Worked Example 1 (8.0 mm Keyway in a 40 mm 1045 Steel Hub): Using auto pitch P = 1.75√40 = 11.07 mm and RPT = 0.060 mm, exactly ze = 3.61 teeth cut simultaneously. Solid chip area is 2.40 mm² inside a 42.89 mm² gullet (a spacious 17.9× gullet ratio), and peak force is 4.34 kN (0.49 US tons / 0.44 t)—just 5.0% of a 10-ton press.
Worked Example 2 (3/8" / 9.525 mm Broach in a 75 mm 4140 Gear Hub): Pushing a 9.525 mm broach (P = 12.7 mm, RPT = 0.065 mm) through 75.0 mm of 4140 alloy steel (kc = 3,000 MPa) engages ze = 5.91 teeth, requiring 10.97 kN (1.23 US tons) with a gullet ratio of 11.6×. Standard keyway dimensions are listed on our ANSI B17.1 keyway calculator and Woodruff key chart; for higher torque, compare our DIN 5480 involute spline calculator and verify shaft stress on the shaft torsional shear stress calculator.
Cite this page
ToolSpecVault Engineering Data Team. (2026, October 29). Keyway Broaching Force & Tonnage Calculator: Pitch, RPT & Gullet. ToolSpecVault. https://toolspecvault.com/keyway-broaching-force-tonnage-pitch-calculator/