Press Tools, Punches, Dies and Sheet-Metal Forming: Double-Action Presses

Double-Action Presses: Greater Reductions

In double-action presses, where the inside of the cup is supported by a bushing during drawing, reductions can be significantly increased:

Note: These double-action press figures can also be used for brass in single-action presses — brass is naturally more ductile and tolerates greater reductions.

Ironing (Wall Thinning)

Sometimes you want the walls to get thinner during drawing — this is called ironing. The range of wall thinning per drawing operation is:

• Standard: 0.002 to 0.004 inch per side
• Final draw (for good finish): Should not exceed 0.001 inch per side

When significant ironing is planned, the diameter reduction per draw must be smaller, because ironing increases the force the punch exerts on the bottom of the shell.

Necking Operations

Necking — drawing out a short portion of the lower part of a cup into a long neck — is a special case. Necking can be done without as frequent annealing as standard reductions, because only a small portion of the shell is being deformed at any time.

Piercing at the Bottom of a Cup

When a hole must be pierced at the bottom of a cup before further drawing:

Always pierce from the OPPOSITE direction to the subsequent draw.

This is critical. Piercing creates a sheared edge with a burr on one side. If the burr faces the direction of subsequent drawing, it acts as a stress concentrator and initiates cracks. Piercing in the opposite direction places the smooth side against the drawing forces.

After piercing, it may be necessary to machine the metal around the hole to remove any features that could initiate cracks during subsequent drawing operations.

Bending Sheet Metal — The Art of Predicting the Unpredictable

Why Bending Calculations Matter

Every bent part starts as a straight piece of stock. The question that haunts every sheet metal worker: How long should that straight piece be?

Cut it too short and the finished part is undersized. Cut it too long and you waste material or — worse — the part doesn't fit.

The difficulty is that metal doesn't behave like paper when you fold it. The outer surface stretches and the inner surface compresses. Somewhere between them is a neutral axis that neither stretches nor compresses. The length of that neutral axis is the true developed length of the bent section.

The Bending Formulas

Three formulas cover virtually all bending situations for 90-degree bends. Each applies to a different range of materials:

Formula 1: Soft Brass and Soft Copper

$$L = (0.55 \times T) + (1.57 \times R)$$

Formula 2: Half-Hard Copper and Brass, Soft Steel, and Aluminum

$$L = (0.64 \times T) + (1.57 \times R)$$

Formula 3: Bronze, Hard Copper, Cold-Rolled Steel, and Spring Steel

$$L = (0.71 \times T) + (1.57 \times R)$$

Where:

• L = Length of straight stock required for the bend (inches)
• T = Thickness of the material (inches)
• R = Inside radius of the bend (inches)

These formulas are based on extensive experiments by the an electrical manufacturer and apply to parts bent with simple tools or on the bench, where limits of ±1/64 inch are specified.

For Angles Other Than 90 Degrees

For bends at any angle, use the formula:

$$L_{actual} = L_{90} \times \frac{A}{90}$$

Where A is the angle through which the material has actually been bent (not necessarily the angle shown on the drawing).

This is a critical distinction. Study these examples carefully:

    ┌─────────┐
    │         │
    │    60°  │  ← Angle on drawing = 60°
    │   ╱     │     Angle of BEND = 180° - 60° = 120°
    │  ╱      │
    │ ╱       │
    └─────────┘

    Figure 1: The drawing shows 60°, but the metal
    actually bends through 120°.

    ┌─────────┐
    │         │
    │         │  ← Angle on drawing = 60°
    │     ╲   │     Angle of BEND = 60°
    │      ╲  │     (They happen to match here)
    │       ╲ │
    └─────────┘

    Figure 2: Drawing angle and bend angle
    are the same.

    ┌─────────┐
    │    30°  │  ← Angle on drawing = 30°
    │   ╱     │     Angle of BEND = 90° - 30° = 60°
    │  ╱      │
    └─────────┘

    Figure 3: Don't be fooled — the actual
    bend is 60°, not 30°.

the practitioner's Rule: "Always ask yourself: how many degrees did the metal actually move? That's your bend angle. Never trust the number on the print without thinking about it."

Worked Example: Two Bends at Different Angles

A part made from soft steel has:

• A 180-degree bend at the left end (R = 0.125", T = 0.125")
• A 60-degree bend at the right end (R = 0.625", T = 0.125")
• A straight section of 3.5 inches between them

For the 180-degree bend (using Formula 2):

$$L = [(0.64 \times 0.125) + (1.57 \times 0.375)] \times \frac{180}{90} = 1.338 \text{ inches}$$

(Note: The values 0.125 and 0.375 represent the specific T and R for this bend)

For the 60-degree bend (using Formula 2):

$$L = [(0.64 \times 0.125) + (1.57 \times 0.625)] \times \frac{60}{90} = 0.707 \text{ inches}$$

Total length before bending:

$$3.5 + 1.338 + 0.707 = 5.545 \text{ inches}$$

Multiple Bends of the Same Radius

If a part has two or more bends of the same radius, calculate the stock length for one bend and multiply by the number of bends.

Lengths of Straight Stock for 90-Degree Bends

The following tables provide pre-calculated values for the most common materials and dimensions. These tables are the sheet metal worker's daily companions.

Table 1: 90-Degree Bends in Soft Copper and Soft Brass

(Length L in inches, using Formula 1: L = 0.55T + 1.57R)

Other Bending Allowance Formulas

The Quick Rule for Shop Use

When bending sheet steel or brass with ordinary precision:

Add 1/3 to 1/2 of the stock thickness, for each bend, to the sum of the inside dimensions of the finished piece.

• 1/3 of the thickness for soft stock
• 1/2 of the thickness for hard material

This rule is adequate for general fabrication work — counters, bank fittings, office fixtures — where precision of ±1/64 inch is not essential.

The V-Die Deduction Formula

For semi-square corners formed in a V-die:

$$X = 1.67 \times B \times G$$

Where:

• X = Amount to deduct from the sum of outside bend dimensions
• B = Number of bends
• G = Decimal equivalent of the gage (stock thickness)

The Drawbench Deduction Formula

For square bends drawn through a block or through rollers in a drawbench:

$$X = 1.33 \times B \times G$$

The drawbench process elongates material more than the V-die process and produces a sharper corner, so less material is deducted.

Allowances for Bends in Sheet Metal — Complete Reference Table

Formed in a Press by V-Die (X = 1.67 × B × G)

Rolled or Drawn in a Drawbench (X = 1.33 × B × G)

Worked Example Using the V-Die Table

A strip with two bends is to have outside dimensions of 2, 1-1/2, and 2 inches. Stock is 0.125 inches thick (11 gage).

1. Sum of outside dimensions: 2 + 1.5 + 2 = 5.5 inches
2. From table (11 gage, 2 bends): deduction = 0.416 inches
3. Blank length = 5.5 − 0.416 = 5.084 inches

Fine Blanking — Precision Beyond Conventional Stamping

When "Good Enough" Isn't

There's a category of stamped parts where conventional blanking simply can't deliver. Gear teeth. Lock components. Safety-critical automotive parts. Surgical instruments. Parts where the edge quality, flatness, and dimensional accuracy must be extraordinary.

That's where fine blanking enters the picture.

What Makes Fine Blanking Different

Fine blanking is a specialized process that uses special presses and tooling to produce flat components from sheet metal or plate with high dimensional accuracy. Developed and refined by firms like Hydrel A.G. of Romanshorn, Switzerland, it differs from conventional stamping in three critical ways:

1. Three Separate Movements

A fine-blanking press must have three distinct, independently controlled movements:

2. Much Higher Forces

Fine blanking requires 1.5 to 2.5 times the force used in conventional stamping. Machines and tools must be designed and constructed accordingly.

3. V-Projections (The Secret Weapon)

The clamping elements include sharp V-shaped projections (90-degree cross-section) that follow the outline of the workpiece. Before the blanking operation begins, these projections are pressed into the material surface. This does two things:

• Prevents lateral movement of the blank during cutting
• Squeezes material toward the cutting edges, reducing the rounding effect that plagues conventional blanking

Press Types for Fine Blanking

Hydraulic presses are also well-suited to embossing, coining, and impact extrusion work.

Tooling for Fine Blanking

Cutting elements are typically made from:

Critical dimension — cutting clearance: Between the intermediate punch and die, clearances are held between 0.0001 and 0.0003 inches. Compare that to conventional blanking clearances of 0.003 to 0.010 inches. Fine blanking operates at tolerances an order of magnitude tighter.

V-Projection Design

The V-projections are incorporated into the stripper plate for thin material, and into both the stripper plate and die plate when material is thicker than 0.15 inches.

When small details like gear teeth must be produced, V-projections on both sides of the work enhance material flow even with thin materials.

V-Projection Dimensions — Stripper Plate Only

V-Projection Dimensions — Both Stripper and Die Plate

    V-Projection Cross Section

         ╱╲
        ╱  ╲        ← 90° angle
       ╱    ╲
      ╱  h   ╲     ← h = height
     ╱        ╲
    ╱──── A ────╲   ← A = width
    ══════════════  ← Material surface

    r = tip radius (rounded for tool life)

V-Projection Placement: The Goldilocks Zone

The positioning of V-projections relative to the cutting line involves a delicate balance:

• Too close to the cut: The projection may move out of the material at the start of cutting, reducing its effectiveness
• Too far from the cut: Both material consumption and blanking force increase
• Just right: Adequate clamping force with reasonable material use and good tool life

The Principle: A small V-projection close to the cut has approximately the same effect as a large V-projection spaced away from the cut. But position also affects tool life — this is where experience and testing come in.

Cutting Force Calculation for Fine Blanking

$$F_{cutting} = 0.9 \times L_c \times T \times UTS$$

Where:

• F = Cutting force (lb)
• L_c = Length of the cut (inches)
• T = Material thickness (inches)
• UTS = Ultimate tensile strength (lb/in²)

V-Projection Clamping Force

$$F_{clamp} = L_v \times h \times UTS \times f$$

Where:

• L_v = Length of V-projection (inches)
• h = V-projection height (inches)
• UTS = Ultimate tensile strength (lb/in²)
• f = Empirical factor (2.4 to 4.4 for UTS of 28,000–113,000 lb/in²)

Rule of thumb: Clamping pressure is approximately 30% of the cutting force.

Results Achievable with Fine Blanking

Strip Width Requirements

Fine blanking requires slightly wider strips than conventional press work:

Strip width ≥ (2 to 3 × material thickness) + width of part (measured transverse to feed direction)

Additional factors: part shape, material quality, V-projection size and location, and spacing between adjacent blanked parts.

Steel Rule Dies — The Elegant Alternative

A 19th-Century Invention Still Going Strong

In 1879, an inventor named the practitioner patented something remarkably simple: a die made from strips of hardened steel — essentially rules, like the steel strips used in graduated measuring rules — mounted on edge in a block.

It sounds almost primitive compared to precision-machined punch-and-die sets. But steel rule dies are one of the most versatile, economical, and enduring tools in manufacturing.

What Steel Rule Dies Are

A steel rule die uses thin strips of hardened steel as cutting edges, held on edge in slots cut into a die block. The sharp edge of the rule faces toward the workpiece. When the press closes, the rule edge pushes through the material, cutting the desired shape.

Originally used in printing and shoemaking for cutting paper, cardboard, leather, rubber, cork, and felt, steel rule dies were later adopted for cutting everything from automotive upholstery to sheet metal parts in copper, brass, and aluminum.

The Economic Advantage

Steel rule dies generally cost 25 to 35 percent of the cost of conventional blanking dies, and can be produced in much less time. For prototyping, short-run production, and complex shapes in softer materials, they're often the obvious choice.

Anatomy of a Steel Rule Die

    ┌─────────────────────────────┐
    │        Upper Die Shoe        │
    │                              │
    │  ┌──────────────────────┐   │
    │  │   Fool-proofing pins  │   │
    │  └──────────────────────┘   │
    │                              │
    │  ╔══════════════════════╗   │  ← Steel rule (held on edge)
    │  ║  Steel rule with     ║   │    with land for shearing
    │  ║  land for shearing   ║   │
    │  ╚══════════════════════╝   │
    │                              │
    │  ┌──────────────────────┐   │
    │  │  Lignostone die block │   │  ← Holds rule in slots
    │  │  (densified wood)     │   │
    │  └──────────────────────┘   │
    │                              │
    ├──────────────────────────────┤
    │      Lower Die Shoe          │
    │                              │
    │  ┌──────────────────────┐   │
    │  │  Male punch / pad     │   │  ← Counter-punch
    │  └──────────────────────┘   │
    │                              │
    │  ┌──────────────────────┐   │
    │  │   Piercing punches    │   │  ← For holes, slots
    │  └──────────────────────┘   │
    │                              │
    │  ┌──────────────────────┐   │
    │  │  Die strippers        │   │  ← Neoprene, spring, or
    │  │  (ejection system)    │   │    positive knockout
    │  └──────────────────────┘   │
    │                              │
    │  ┌──────────────────────┐   │
    │  │   Parallels for       │   │  ← Slug clearance
    │  │   slug clearance      │   │
    │  └──────────────────────┘   │
    │                              │
    │  ┌──────────────────────┐   │
    │  │  Lower die plate      │   │
    │  └──────────────────────┘   │
    │                              │
    └──────────────────────────────┘

Die Block Materials

Lignostone deserves special mention. In the 3/4-inch thickness most commonly used, it's made from approximately 35 plies of highly compressed lignite wood, bonded with phenolformaldehyde resin. The result is a material of great density and strength — far superior to standard plywood for holding steel rules under press forces.

How the Die Cuts

The steel rule is held in a slot in the die block, typically against the flat platen of a die set on the moving slide. The sharp edge faces the workpiece, which is supported by the other die half (which may be flat or may carry a punch).

When the press closes:

1. The cutting edge penetrates the material
2. For blanking, the edge pushes the workpiece past a close-fitting counter-punch (pad)
3. The pad may have a slight height reduction to create a shearing action

After cutting, the material often clings to the sides of the knife. Ejector inserts — made from rubber, cork/rubber combinations, or specially compounded plastics — are installed alongside the rules and punch. These are compressed as the dies close and expand when the dies open, pushing the material clear.

Cutting Edge Profiles

The cutting edges of steel rules can be shaped in several profiles:

The standard workflow:

1. Start with a sharp edge (Profile A) to match the rule to the counter-punch during setup
2. After fitting, grind the edge flat to create a land of about 1/64 inch wide (Profile B)
3. This flat land becomes the working cutting edge

Capabilities Beyond Simple Cutting

Steel rule dies can incorporate:

• Piercing punches of various shapes (holes, slots)
• Forming projections (strengthening ribs and other shapes)
• Multiple nested dies operating together in a single press stroke

This means a single steel rule die can cut the outline, punch holes, and form features — all simultaneously.

Making Steel Rule Dies — The Craft

Slot Cutting in the Die Block

Slots are cut into the die block material (wood or Lignostone) to hold the steel rules. The process requires precision and care:

Trial fitting: Before cutting into the actual die block, trial slots are always cut in scrap pieces to verify that the slot width produces a tight fit for the steel rule thickness being used.

Maintaining verticality: During slot cutting, the saw blade must always be maintained vertical to the board. Magnifying lenses are often used to keep the blade close to the layout line.

Blade selection: Carbide or carbide-tipped saw blades are recommended for both clean cuts and long blade life.

The "Bridge" Technique

When cutting closed shapes (like the center of a circle), the continuous slot would separate the "island" from the surrounding block. The solution: bridges.

At various places in the sawn line, the slot is cut to less than full depth for lengths of 1/4 to 1/2 inch, at heights of 5/8 to 3/4 inch. These partial cuts create wood bridges that hold the island in position. Corresponding slots are cut in the steel rules (on the non-cutting side) to accommodate these bridges.

Bending the Steel Rules

Steel rules are bent to shape using small, purpose-built bending machines with specialized tooling. For bends of small radius, the tooling performs a peening or hammering action to force the rule into close contact with the forming surface, producing accurate small radii — even the intricate curves needed for jigsaw puzzles.

Complex shapes may be made in two or more pieces, then joined by welding or brazing:

1. Edges are mitered for a perfect fit
2. Pieces are clamped securely
3. Electrical resistance or gas torch heating is used for the joint
4. Wet rags are applied to the steel on each side of the joint to keep the material cool and maintain the pre-set hardness level

Heat Treatment of Steel Rules

The hardness of the steel rule is matched to the application:

For intricate shapes:

1. Begin with dead-soft steel (Rb 95)
2. Form the intricate bends
3. Carburize in a liquid compound at 1500°F
4. Quench in oil
5. Temper at 550°F ("tough" tempering)
6. Cool in the furnace

This sequence — form soft, then harden — allows the creation of complex shapes that would be impossible to bend from already-hardened material.

Piercing Punch Materials

Piercing punches incorporated into steel rule dies are preferably made from high-carbon, high-vanadium alloy steel:

The differential hardness is critical: the cutting end must be extremely hard for edge retention, while the head end must be softer to absorb impact without fracturing.

Final Fitting and Clearance Adjustment

After the hardened rule is reinstalled in the die block:

1. The tool is loaded into the press
2. The sharp die edge is carefully used to shear the sides of the pad to match the die contours exactly
3. This creates a close fit with clearances about half those used in conventional blanking dies
4. Fine adjustments are made by grinding the die steel or the punch
5. The sharp rule edge is then ground flat to create the 1/64-inch working land

Clearances for piercing punches in steel rule dies should be similar to those used in conventional piercing dies.

the practitioner's Morning, Revisited

It's 11 AM. The presses have been running for five hours. the practitioner walks the floor and checks the production boards:

Press 3 is stamping stainless steel brackets. The operator correctly switched to a chlorinated high-pressure lubricant when they moved from mild steel to stainless. Zero galling incidents.

Press 7 is drawing cylindrical shells — 3-inch diameter, 4-inch deep, from 1/16-inch stock. The blanks were cut to 6.25 inches (verified against the table: a 3-inch shell at 4-inch height needs a 6.25-inch blank, minus corner radius adjustments). They're on the second draw now, having annealed after the first, and are reducing the diameter by 20% — right on the specification for 1/16-inch stock.

The steel rule die station is cutting aluminum automotive trim pieces — complex shapes with four pierced holes, produced in one stroke. The die cost 30% of what a conventional tool would have cost and was delivered in days rather than weeks.

Everything is running because everyone understands the fundamentals. The clearances are right. The lubricants are matched. The blank sizes are calculated. The speeds are appropriate. The forces are within machine ratings.

That's the transformation this knowledge creates. You go from guessing to knowing. From reactive troubleshooting to proactive planning. From wasted material and broken tools to clean parts and long die life.

The Complete Formula Reference

For quick reference, here is every formula presented in this guide:

Punching Force

Blank Diameter

Bending

Fine Blanking

Troubleshooting Guide — When Things Go Wrong

Common Problems and Their Root Causes