press-selection

name	press-selection
description	Mechanical press selection — press tonnage calculation (blanking, piercing, bending, drawing), press types (mechanical, hydraulic, servo), flywheel energy and motor sizing, bed size and shut height, stroke and speed, die set compatibility, tonnage at position (eccentric vs. hydraulic), progressive die stamping, deep drawing (draw ratio, blank holder force), blanking clearance (% of thickness), OSHA 1910.217 safety, and SME/STAMPING JOURNAL selection guides.
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Mechanical Press Selection — Complete Skill

Press Types and Characteristics

Mechanical Crank/Eccentric Press

Drive mechanism: Crankshaft or eccentric converts rotary motor energy (stored in flywheel) to linear ram motion Speed: 30–600 SPM (strokes per minute); higher speed for lighter blanking work Flywheel releases energy at bottom of stroke; motor recharges flywheel between strokes

Tonnage at position (eccentric press): F_available(θ) = F_rated × sin(α_max + θ) / sin(α_max) [approximate; α_max = maximum crank angle at rated tonnage] Full tonnage only available within 10–30° of BDC (bottom dead center); earlier in stroke → less force available Typical rated tonnage at 3/16 in (4.8 mm) above BDC (ANSI B11.1)

Advantages: high speed; low operating cost; repeatable stroke Limitations: fixed stroke; tonnage curve (less force high in stroke); die must be designed for tonnage-at-position

Hydraulic Press

Drive: hydraulic cylinder; constant force throughout full stroke F_available = p × A_piston [constant over full stroke; rated at any position] Speed: 2–50 SPM; slower than mechanical; adjustable stroke length and force

Advantages: full tonnage anywhere in stroke; programmable force profile; flexible stroke; overload protection Limitations: lower production rate; oil maintenance; higher initial cost

Servo Press (Servo-Mechanical)

Servo motor directly (or via linkage) drives ram: Programmable motion profile: stop at any position, variable speed, reverse mid-stroke Energy: servo motor + small flywheel or direct servo (no flywheel for full servo) Speed: up to 400 SPM for full-servo; up to 600 SPM for servo-mechanical

Advantages: programmable dwell, variable speed at critical points, reduced noise, longer tool life Applications: precision forming, material-sensitive applications (AHSS, warm forming), quality-critical stamping

Press Tonnage Calculation

Blanking and Piercing

Blanking/piercing force: F = k × L_perimeter × t × τ_ult [N; L = perimeter of cut [m]; t = material thickness [m]; τ_ult = shear strength] τ_ult = 0.65 × S_ult [shear strength; 0.65 × UTS typical; or 0.6–0.8 per material] k = 1.0–1.3 (shock factor for die condition; use 1.0 for new dies with proper clearance)

Blanking clearance: c = a × t [per side; total clearance = 2c] a = 5–10% (soft materials: aluminum, copper); 10–15% (medium: mild steel); 15–20% (hard: HSLA, stainless) Die size = blank size; punch undersized by 2c for blanking Punch size = hole size; die oversized by 2c for piercing

Example (round blank, D = 100 mm, t = 3 mm, SPCC mild steel S_ult = 310 MPa): τ = 0.65 × 310 = 201.5 MPa; L = π × 0.100 = 0.314 m F = 1.0 × 0.314 × 0.003 × 201,500 = 190 kN = 19 tonnes

Tonnage multiplier for compound operations: F_total = F_blank + F_pierce1 + F_pierce2 + ... [concurrent operations in progressive or compound die] Progressive die: not all cuts happen simultaneously → calculate worst station

Bending Force

Air bending (V-die): F_bend = 1.33 × C_b × S_ult × b × t² / W_die [N; b = bend width; t = thickness; W_die = V-die opening; C_b = 1.3 for air bending] F_bend = 1.33 × S_ult × b × t² / W_die (with C_b = 1.0; multiply by 1.33 for safety) Die opening W_die: typically W = 6t (for t < 3 mm) to 8t (for t > 3 mm); minimum W = 6t

Coining (full die contact): F_coin ≈ 3 × S_ult × A_contact [high-pressure coining; A_contact = area being coined; 3-5× S_ult]

Deep Drawing Force

Maximum drawing force: F_draw = π × D_p × t × S_ult × [D_b/D_p - 0.7] [simplified; D_b = blank diameter; D_p = punch diameter] Or: F_draw = π × D_p × t × k_d × S_ult [k_d = 0.5–0.8; depends on draw ratio]

Draw ratio (LDR): DR = D_b / D_p [blank-to-punch diameter ratio] Max DR without intermediate anneal: DR ≤ 2.0 (mild steel); 1.8 (stainless); 2.2 (aluminum) Redraw operations: DR per redraw ≤ 1.3–1.4

Blank holder force: F_BH = p_BH × A_flange [p_BH = blank holder pressure; A_flange = flange annular area] p_BH = 0.005 × (10 + S_ult/1,000) × S_ult [MPa; empirical for steel; or 0.7–1.5% of S_ult × A_flange] Wrinkling: F_BH too low → wrinkles; Splitting: F_BH too high (or excessive draw ratio)

Total press tonnage for draw: F_total = F_draw + F_BH [N]

Flywheel and Motor Sizing

Energy Analysis

Work per stroke: E_stroke = F_avg × stroke_working [J; F_avg = average force during forming; stroke_working = working stroke depth] E_stroke = (F_max + F_min) / 2 × d_work [trapezoidal approximation]

Flywheel energy available: E_flywheel = (1/2) × I_fw × (ω₁² - ω₂²) = (1/2) × I_fw × ω_avg² × (1 - (ω₂/ω₁)²) Coefficient of fluctuation: C_f = (ω₁ - ω₂) / ω_avg = 0.005–0.02 (1–2% speed drop for presses) E_flywheel = I_fw × ω² × C_f [I_fw = flywheel moment of inertia; ω = angular speed]

Required flywheel moment of inertia: I_fw = E_stroke / (ω² × C_f) [kg·m²]

Motor power: P_motor = E_stroke × SPM / 60 / η_mech [W; η_mech = mechanical efficiency 0.85–0.92] Motor must also overcome idle losses: add 10–20% for friction, air clutch, etc.

Press Size Parameters

Bed Size and Shut Height

Bed area: Must accommodate largest die in length × width direction; typically die ≤ 70% of bed dimension Standard beds: 24×36 in, 36×48 in, 48×72 in, 60×96 in (US Imperial standard sizes)

Shut height (closed height): H_shut = distance from bed to ram at BDC (maximum closed position) Die height ≤ H_shut (total assembled die height including die set, stock, bolster plate) Adjustment: most presses allow ±25–50 mm adjustment with ram adjustment screw

Stroke: Mechanical press: stroke = 2 × eccentricity (fixed per press model) Must be sufficient for part ejection and material feed: stroke ≥ 2 × part depth + 25 mm clearance

Daylight (open height): H_open = H_shut + stroke [= space available when ram is fully up for die access]

Tonnage Rating Verification

Eccentric press tonnage-at-position check: Available tonnage at position S above BDC (in inches): F(S) ≈ F_rated × (30/16)^½ × √(S × (C - S)) / C [simplified; C = stroke length; varies by press design] Rule: do not use full-rated tonnage above 1/3 of stroke from BDC for heavy work (use piston press instead)

Snap-through warning: Blanking: spring-back energy release → impact when punch breaks through Energy dissipated: ΔE_snap = F_max × t × (1 - breakthrough_fraction) ≈ 30% of forming energy Excess energy → frame vibration, die fatigue; mitigate with snub plates, staged punches, pre-loaded dies

Safety

OSHA 1910.217 Requirements

Mechanical power presses: Point-of-operation guarding: barrier guard or two-hand control or light curtain Two-hand control: press will not fire unless both buttons pressed simultaneously; hands kept away from point of operation Light curtain: stop signal if beam interrupted; Category 4 SIL (SAE/OSHA requirements) Anti-tie-down: control must be released and re-engaged between strokes (no "tied down" operation) Brake monitoring: monitor brake stopping time; maximum stop time before guard deactivation

Die protection: Press controller interlocked with die sensors: part present/absent; part stuck detection; hydraulic overload protection Tonnage monitor: strain-gauge on frame; alarm at 110% rated; shutdown at 125%

Standards and References

Standard	Scope
OSHA 29 CFR 1910.217	Mechanical power presses safety
ANSI B11.1	Mechanical power press safety standard
ANSI B11.2	Hydraulic power press safety
ANSI B11.28	Safety requirements for servo presses
SME Die Design Handbook	Die geometry, clearances, tonnage
AIAG/OHSAS	Automotive stamping quality

Output

Provide: part description (material; S_ult [MPa]; thickness t [mm]; blank dimensions; operation type), tonnage calculation (F_blank [kN]; F_pierce [kN]; F_bend [kN]; F_draw [kN]; F_BH [kN]; total F [tonnes = kN/9.81 × 0.1018]), press type recommendation (mechanical/hydraulic/servo; justification based on speed, stroke, force profile), tonnage at position check (F_available at working stroke position vs. F_required for mechanical press), bed size (L × W [mm]; die footprint ≤ 70% bed), shut height check (die height [mm] ≤ H_shut [mm]), stroke requirement (part depth + feed clearance [mm] vs. press stroke [mm]), flywheel/motor sizing (E_stroke [J]; I_fw [kg·m²]; P_motor [kW]), SPM (strokes per minute); annual production capacity [parts/year], safety requirements (guarding type; light curtain; OSHA 1910.217; tonnage monitor), and applicable standard (OSHA 1910.217, ANSI B11.1, SME Die Design Handbook).