Buttress Thread Dimensions Calculator: 7°/45° ASME B1.9 & 3-Wire MOW Chart

Why Engineers Specify 7°/45° Buttress Threads Over Acme or Square Threads

When a power screw, hydraulic press tie-rod, artillery breech block, or injection molding clamp column must withstand massive unidirectional axial thrust without radial bursting forces, neither a standard 60° Unified thread nor a 29° Acme thread is optimal. A Square thread offers zero radial bursting force (α = 0°), but it is expensive to single-point turn, impossible to grind with a wheel, and has half the root shear thickness of a V-thread. The Buttress Thread governed by ASME Geometric & Mechanical Standards (ASME B1.9) and ANSI Engineering Standards Webstore combines the high axial efficiency and low radial bursting component of a Square thread with the root shear strength and machinability of an Acme thread.

In the ASME B1.9 profile, the load-bearing pressure flank is inclined at 7° from perpendicular to the thread axis (just enough clearance angle to prevent wheel rubbing during thread grinding and allow clean insert relief), while the trailing clearance flank is inclined at 45°. This asymmetric 7° / 45° geometry yields an included groove angle of 52° and a wide, shear-resistant tooth base with a mandatory tangency root radius of r = 0.07141 × P to eliminate fatigue stress risers.

Exact 3-Wire Pitch Diameter Inspection Formula for Asymmetric 7°/45° Threads

Because the flank angles of a Buttress thread are unequal (α_1 = 7° and α_2 = 45°), standard symmetric 60° or 29° 3-wire equations cannot be used. As derived in NIST (National Institute of Standards and Technology) Handbook H28 (Screw-Thread Standards for Federal Services) and Machinery's Handbook (Industrial Press), a cylindrical measuring wire of diameter w seated in an asymmetric groove of half-angles α_1 and α_2 contacts the two flanks at different radial heights! For an ASME B1.9 7° / 45° Buttress thread, the wire size that touches the 7° load flank at the exact pitch line is the Best Wire Diameter:

For an ASME B1.9 7°/45° Buttress screw of axial pitch P and basic pitch diameter d_2 = D - 0.60000 × P, the Best Wire Size is w_best = 0.54147 × P (with an allowable commercial wire diameter range from 0.452 × P to 0.656 × P), and the exact micrometer Measurement Over 3 Wires is M_w = d_2 - 0.89064 × P + 3.15689 × w. When inspecting a European DIN 513 3°/30° saw-tooth Buttress thread, the basic pitch diameter is d_2 = D - 0.75000 × P, the best wire diameter is w_best = 0.45835 × P, and the 3-wire micrometer reading follows M_w = d_2 - 0.96590 × P + 3.69460 × w.

When single-point turning deep Buttress threads on a CNC turning center, always feed the carbide threading tool along the 45° (or 30°) trailing flank using modified flank infeed so the wide chip curls freely off a single cutting edge, then finish the 7° load-bearing flank with two light spring passes of 0.025 mm (0.001") radial depth to achieve a mirror-smooth bearing surface free of chatter marks.

Comparing American ASME B1.9 (7°/45°) vs European DIN 513 (3°/30°) Buttress Threads

Before cutting or ordering threading inserts for a Buttress screw, always verify whether the blueprint originates from an American ASME standard or a European ISO International Metrology Standard / DIN 513 (Saw-Tooth "S" Thread, e.g., S 48×8) standard. DIN 513 uses a steeper 3° load flank and a 30° trailing flank (33° included angle) with a deeper engagement height of H_1 = 0.750 × P. The two standards are completely non-interchangeable! For symmetric power screws and CNC threading cycles, cross-check your dimensions on our 29° Acme & Stub Acme Thread Calculator, 60° Three-Wire Thread Measurement Calculator, Fanuc G76 Threading Cycle Generator, and ASME B1.1 Thread Class 2A/3A Tolerance Calculator.