How to Calculate Servo Press Tonnage: The Complete Engineering Guide

Published February 18, 2026 · Updated March 22, 2026 · By ServoPress Club Engineering Team · 14 min read

1. Tonnage Calculation Fundamentals

A shop I consulted for last year bought a 200-ton servo press for a job that actually needed 260 tons. They didn't find out until the first production run ? the press hit overload on every stroke, the die cracked within 500 hits, and they were down for three weeks waiting for a replacement insert. Total cost of that miscalculation: $47,000 in die repair plus lost production. The tonnage formula takes five minutes to run. Skipping it can cost you a quarter.

Press tonnage (or press force) is the maximum force a press can exert on the workpiece, measured in tons (short tons in the US, metric tonnes elsewhere) or kilonewtons (kN). One metric ton = 9.81 kN. Selecting the correct tonnage is critical: too little force causes incomplete forming and part defects; too much force damages dies and wastes energy.

The fundamental tonnage formula for any cutting operation is:

Where: F = required force (kN), L = cut perimeter (mm), t = material thickness (mm), S = material shear strength (MPa). This formula applies to blanking, piercing, notching, and trimming. For forming operations (drawing, bending, coining), different formulas apply as described below.

Unit Conversions

2. Blanking & Piercing Tonnage

Blanking (cutting the outer profile) and piercing (cutting internal holes) use the same formula. The key variable is the total cut perimeter.

Where k is the shear factor (1.0 for flat cutting, 0.5-0.7 for shear-angle tooling). Shear-angle punches reduce peak force by spreading the cut over a longer stroke, which is especially useful when press capacity is limited.

Stripping Force

After the punch cuts through the material, force is needed to strip the material from the punch. Add 5-10% of blanking force for stripping:

Progressive Die Considerations

In progressive dies with multiple stations, calculate the force for each station and sum them. The total force peaks when the maximum number of stations are cutting simultaneously. Typical progressive die force buildup:

3. Deep Drawing Tonnage

Deep drawing requires two forces: the drawing force (to pull material into the die cavity) and the blank holder force (to prevent wrinkling).

Where: d = punch diameter (mm), D = blank diameter (mm), t = thickness (mm), S = tensile strength (MPa), C = correction factor (0.6-0.7 for first draw).

Blank Holder Force

Where A_BH = blank holder area (mm²) and p = specific pressure (1.5-3.0 MPa for steel, 0.8-1.5 MPa for aluminum). The blank holder force is typically 25-35% of the drawing force.

Drawing Ratio Limits

4. Bending Tonnage

Bending force depends on the bend type (V-bend, U-bend, wipe bend), material properties, and tooling geometry.

V-Bending (Air Bending)

Where: K = die opening factor (1.33 for V-die), S = tensile strength (MPa), L = bend length (mm), t = thickness (mm), W = V-die opening width (mm). Typical W = 8t for material up to 3mm, 10t for 3-6mm, 12t for >6mm.

U-Bending

U-bending requires approximately 2x the force of V-bending for the same material and dimensions, plus additional force for the bottom of the U-shape.

Bottoming (Coining)

Bottoming or coining requires 3-5x the force of air bending because the material is compressed between punch and die to achieve a precise angle. This is where servo presses excel ? the programmable dwell at BDC ensures complete coining without excessive force.

5. Forming & Coining Tonnage

General forming operations (embossing, flanging, ironing) require force calculations specific to each process:

6. Material Shear Strength Reference

Shear strength is approximately 60-80% of tensile strength for most metals. Use these values for tonnage calculations:

Always verify material properties from your supplier's mill certificates. Actual values can vary ?10% from published data.

7. Safety Margins & Derating

Never size a press to exactly the calculated tonnage. Apply safety margins:

• Standard operations: 20% safety margin (multiply calculated force by 1.2)
• Progressive dies: 25-30% margin (force buildup uncertainty)
• UHSS materials: 30% margin (material property variation)
• Prototype/new tooling: 30-40% margin

Derating Factors

Press capacity is rated at a specific distance above BDC (typically 6mm or 12mm). If your working point is higher above BDC, the available tonnage decreases. Check your press's tonnage curve ? a 200-ton press may only deliver 150 tons at 25mm above BDC.

Servo presses have a significant advantage here: they can deliver full rated tonnage through a much larger portion of the stroke compared to mechanical presses, which lose capacity rapidly away from BDC.

8. Servo Press Tonnage Advantages

Servo presses change the tonnage equation in several important ways:

• Full tonnage through stroke: Unlike mechanical presses where tonnage drops off away from BDC, servo presses maintain rated force through a wider working range.
• Reduced peak force: Programmable approach speed means lower impact force at material contact, reducing the peak tonnage spike by 20-40%.
• Dwell capability: Holding at BDC allows material to flow, reducing the instantaneous force needed for deep drawing by 15-25%.
• Force monitoring: Real-time load cell feedback detects overload before damage occurs, protecting both press and die.
• Pendulum mode: For blanking, partial-stroke operation means the press never reaches the high-force zone unnecessarily.

In practice, a 200-ton servo press can often replace a 250-ton mechanical press for the same job, because the servo's controlled force application is more efficient. This means potential capital savings of 15-20% on press purchase. Read more in our Servo vs Mechanical Press comparison.

9. Worked Examples

Example 1: Blanking a Circular Part

Given: Circular blank, diameter 80mm, material AISI 1010 steel, thickness 2mm.

Perimeter: L = π × 80 = 251.3 mm
Shear strength: S = 275 MPa
Force: F = 251.3 × 2 × 275 = 138,215 N = 138.2 kN
Add 8% stripping: 138.2 × 1.08 = 149.3 kN
Add 20% safety: 149.3 × 1.20 = 179.1 kN (18.3 metric tons)

Select a press rated at least 20 tons. Use our Tonnage Calculator to run this calculation instantly.

Example 2: Progressive Die with Multiple Stations

Given: 5-station progressive die, stainless steel 304, 1.5mm thick.

Station 1 ? Pilot holes (4 × ∅5mm): F = 4 × π × 5 × 1.5 × 390 = 36.8 kN
Station 2 ? Notch (2 × 25mm): F = 2 × 25 × 1.5 × 390 = 29.3 kN
Station 3 ? Pierce (6 × ∅3mm): F = 6 × π × 3 × 1.5 × 390 = 33.1 kN
Station 4 ? V-bend (40mm length): F = (1.33 × 515 × 40 × 1.5²) / 12 = 5.1 kN
Station 5 ? Blank (perimeter 180mm): F = 180 × 1.5 × 390 = 105.3 kN
Total: 209.6 kN
Add 25% safety: 262 kN (26.7 tons)

Example 3: Deep Drawing a Cup

Given: Cup diameter 50mm, blank diameter 90mm, low carbon steel 1mm thick.

Drawing force: F = π × 50 × 1 × 365 × (90/50 - 0.65) = 57,400 × 1.15 = 66.0 kN
Blank holder: A = π/4 × (90² - 50²) = 4,398 mm², p = 2.0 MPa, F_BH = 8.8 kN
Total: 66.0 + 8.8 = 74.8 kN
Add 20% safety: 89.8 kN (9.2 tons)

10. Common Mistakes

1. Using tensile strength instead of shear strength ? Overestimates blanking force by 30-40%. Shear strength ≈ 0.6-0.8 × tensile strength.
2. Forgetting stripping force ? Adds 5-10% to total force requirement.
3. Ignoring material thickness tolerance ? If spec says 2.0mm ±0.1mm, calculate at 2.1mm.
4. Not accounting for work hardening ? Stainless steel and UHSS work-harden during forming, increasing force in subsequent operations.
5. Sizing at BDC only ? Check the tonnage curve at your actual working height above BDC.
6. Ignoring temperature effects ? Hot stamping requires different force calculations than cold stamping.
7. No safety margin ? Always add 20-30%. Material properties vary batch to batch.

Real Questions from the Shop Floor

Our tonnage monitor reading thread has practical tips on interpreting force signatures that go beyond textbook calculations. If you are working with UHSS, the discussion on UHSS forming challenges covers the real-world tonnage surprises engineers encounter with DP980 and DP1180.

Related Resources

• Tonnage Calculator ? Calculate instantly
• Energy Calculator ? Estimate energy costs
• SPM Calculator ? Optimize cycle times
• Servo vs Mechanical Press Guide
• Force Curve Optimization
• Maintenance Best Practices

References

• Society of Manufacturing Engineers (SME), "Die Design Handbook," 3rd Edition
• Schuler Group, "Metal Forming Handbook," Springer
• AISI Steel Products Manual ? Mechanical Properties
• ISO 16092-1:2017 ? Machine tools safety ? Presses
• OSHA 29 CFR 1910.217 ? Mechanical Power Presses