Press Tools, Punches, Dies and Sheet-Metal Forming: The Invisible Craft That Builds Everything

Context and scope

A 40,000-word journey through the craft that shapes the modern world — from the fundamentals of clearance to the art of steel rule dies.

The Invisible Craft That Builds Everything

The Press Room at 5 AM

The fluorescent lights buzz to life at Kovac & Sons Precision Stamping. the practitioner Kovac, a third-generation toolmaker, walks past rows of silent presses — each one a sleeping giant capable of exerting hundreds of tons of force on a piece of sheet metal thinner than a playing card.

In six hours, these machines will stamp out 40,000 brackets, 12,000 enclosures, and 8,000 drawn shells. Every single one must be dimensionally perfect. Every single one depends on the invisible geometry hiding inside a punch and die.

This is the world of press work.

It's a world where a clearance error of two-thousandths of an inch means the difference between a clean, sheared edge and a ragged, unusable blank. Where choosing the wrong lubricant destroys a die in a single shift. Where a miscalculated blank diameter wastes thousands of units of material value in scrap.

If you design parts, build tools, operate presses, or manage a shop floor — this guide is your complete reference. Every formula. Every table. Every principle. Written so you can use it today, and so the apprentice reading it a hundred years from now can still rely on it.

Let's step into the press room.

Punches and Dies — The Fundamentals

What a Punch and Die Actually Do

At its core, every punch-and-die operation is the same: a shaped tool (the punch) forces material through or against a matching opening (the die) to cut, bend, or form a shape.

Think of it like a cookie cutter pressing through dough — except the "dough" is steel, the "cookie cutter" exerts tens of tons of force, and the tolerances are measured in thousandths of an inch.

Every operation in press work falls into one of these categories:

• Blanking — Cutting a flat shape out of sheet material (the blank is the desired part)
• Piercing (Punching) — Cutting holes in material (the slug is discarded)
• Drawing — Forming flat sheet into three-dimensional shapes like cups, shells, and enclosures
• Bending — Forming angular shapes without cutting
• Forming — Creating complex contours, ribs, and features

The magic — and the difficulty — is in the details.

Clearance — The Most Critical Dimension You Can't See

Why Clearance Matters More Than Almost Anything

When the practitioner's grandfather, Josef Kovac, started the shop in the 1960s, he had a saying: "Get the clearance right and the die takes care of itself. Get it wrong and God Himself can't save your parts."

Clearance is the gap between the punch and the die opening. It seems trivial. It is anything but.

Defining Clearance: The Industry Divide

Here's something that still trips up experienced toolmakers: the industry doesn't even agree on how to define clearance.

There are two schools of thought:

The dominant convention — and the one this guide uses — is clearance per side.

Why? Because it's the only definition that makes sense for irregular shapes. If you're cutting a symmetrical circle, "total clearance" works fine — just divide by two. But try applying "total clearance" to an L-shaped blank, a star, or any asymmetric form. It breaks down immediately.

Rule of Thumb: Always specify what you mean by "clearance" in your documentation. Never assume the next person reading your specs uses the same convention you do.

The industry also distinguishes between two related but different terms:

• Cutting clearance — The space between punch and die on each side (the gap that determines cut quality)
• Die clearance — The angular relief provided below the cutting edge so blanks or slugs fall freely through the die

How Much Clearance Do You Need?

The amount of clearance depends on two variables: the kind of material and its thickness.

For thin material: The punch should be a close sliding fit in the die. Too much clearance on thin stock creates ragged, burred edges that require secondary operations to clean up.

For heavy stock: You need proportionally more clearance. Proper clearance on thick material actually reduces the punching force required and dramatically reduces the risk of punch breakage.

Here's the data from decades of production experience:

In punching holes in fairly hard steel, production data shows that a clean fracture can be obtained with a clearance of only 0.03 times the stock thickness. That's an astonishingly tight tolerance — for 0.250-inch plate, that's just 0.0075 inches per side.

Angular Clearance: The Relief Below the Cutting Edge

Once material is sheared, the blank or slug needs to fall freely through the die. That's what angular clearance (also called die relief) provides — a slight taper below the cutting edge.

Two methods exist for providing this clearance:

Method 1 — Full taper from the top face: The clearance angle extends from the cutting edge all the way to the top face of the die. This is used for very soft, thin metals like soft brass. It's fast to make but wears quickly.

Method 2 — Straight land with relief below: A small section (about 1/8 inch) below the cutting edge is left nearly straight, with the angular clearance beginning below that land.

Method 2 is the professional's choice for hard brass, steel, and any production die. Here's why: when you re-sharpen a die made with a straight land, you grind the top face. The hole size barely changes. You can cut thousands upon thousands of blanks with virtually no dimensional drift.

With Method 1, every time you grind the face, you enlarge the opening. Your blanks get progressively bigger.

the practitioner's Lesson: "My grandfather would say: 'A die with a land is a die with a life.' He was right. We have dies in this shop that have been reground fifty times and still hold tolerance."

              ┌──────────────┐
              │   DIE FACE   │ ← Grinding surface
              ├──────────────┤
              │ ▓▓ LAND ▓▓  │ ← ~1/8" straight (Method 2)
              │  (straight)  │
              │    ╱    ╲    │
              │   ╱      ╲   │ ← Angular clearance (1-2°)
              │  ╱        ╲  │
              │ ╱          ╲ │
              └──────────────┘

   Method 2: Straight land with angular relief below
   (Preferred for production dies on hard materials)

              ┌──────────────┐
              │   DIE FACE   │ ← Grinding surface
              │    ╱    ╲    │
              │   ╱      ╲   │ ← Clearance extends to top
              │  ╱        ╲  │    (Method 1)
              │ ╱          ╲ │
              └──────────────┘

   Method 1: Full taper from top face
   (Only for very soft, thin metals)

Lubricants for Press Work — The Unsung Heroes

The Story of the Ruined Die

the practitioner, the practitioner's lead press operator, once ran a batch of 304 stainless blanks with nothing but residual mill oil on the strip. By the 200th stroke, the punch was galling. By the 500th, the die had material buildup that scored every subsequent blank. The die needed to be pulled, disassembled, polished, and reground — a four-hour job that killed half a shift's production.

The fix would have cost pennies: a proper lubricant.

Lubricant Selection in the supplied reference right lubricant depends on what you're doing, what material you're working, and how hard you're working it.

Blanking and Piercing

Application matters as much as selection. The target lubricant film thickness is approximately 0.0001 inches — astonishingly thin. That translates to roughly 1 pint of fluid to cover 500 square feet of material.

During successive strokes, metal debris adheres to the punch and accelerates wear. Applying lubricant to the sheet or strip via rollers or spray reduces this damage. For high-speed blanking, heavier applications or continuous airless spraying of oil may be required.

Drawing and Forming

Drawing operations create a much more demanding tribological environment — the metal is being plastically deformed while sliding against the tool surface under enormous pressure.

The key principle: Lubricant additives are selected for their ability to maintain a physical barrier between tool surfaces and the metal being shaped. When that barrier breaks down, you get galling, scoring, and die failure.

Dry soaps and polymer films are frequently used for the most demanding drawing operations.

Annealing Drawn Shells — Restoring What Drawing Destroys

Why Metal Gets Harder When You Draw It

Every time you draw a shell — forcing flat sheet metal into a cup shape — you're permanently rearranging the crystal structure of the metal. Grains deform, dislocations multiply, and the material work hardens. After two or three draws, the metal becomes so hard and brittle that further drawing will crack it.

Annealing — heating the metal to a specific temperature and allowing it to cool — restores ductility by allowing the crystal structure to recrystallize.

Annealing Rules by Material

Steel and Brass:

• Anneal between alternate reductions, at minimum
• For steel: standard annealing temperatures apply (material-dependent)
• For brass: heat to a temperature just below what would show a dull red in a dark room
• This controlled heating of brass enables drawing of difficult shapes and square corners that are otherwise almost impossible

Aluminum:

• Can be drawn deeper and with less annealing than other commercial metals (provided the proper grade is used)
• If annealing is necessary, heat in a muffle furnace
• Critical: Temperature must never exceed 700°F (371°C)
• Exceeding this temperature damages the grain structure permanently

Tin Plate / Pre-finished Stock:

• Cannot be annealed without spoiling the finish
• Must ordinarily be drawn to final size in one or two operations
• This constraint drives the need for extra-careful blank sizing and die design

The Brass Trick: the practitioner learned this from her father: "When you're drawing brass and the corners just won't form, heat it until you can barely see color in a dark room. Not a full red — just a hint. That's the sweet spot. The metal flows like it's alive."

Drawing Rectangular Shapes — Where the Corners Kill You

The Corner Radius Problem

Drawing a cylindrical shell is relatively forgiving — the material flows uniformly from all directions. But rectangular shapes introduce corners, and corners are where everything goes wrong.

At each corner, the material must compress from a larger radius to a smaller one. The tighter the corner radius, the more severe the compression, and the more likely you are to see:

• Wrinkling (excess material with nowhere to go)
• Tearing (material stretched beyond its limit)
• Thinning (localized reduction in wall thickness)
• Cracking (catastrophic failure)

Maximum Draw Depths in the supplied reference figures come from actual production experience — not theory, not simulation, but decades of stamped parts

The governing principle: Make the corner radius as large as possible. Every increase in corner radius buys you more drawable depth.

If the box needs to be very deep and the radius must be small, two or more drawing operations will be necessary. Each successive draw takes the shell deeper, with annealing between draws as required.

Speeds and Pressures for Presses

How Fast Should You Run?

Press speed is a balancing act between productivity and quality. Run too fast, and you introduce dynamic forces that damage tools and produce poor parts. Run too slow, and you waste money.

Speed depends primarily on material type and thickness:

Pressure Required for Punching

This is one of the most fundamental calculations in press work. Get it wrong and you'll either break your punch, overload your press, or fail to complete the cut.

The General Principle

The cutting pressure equals:

Cutting Pressure = Perimeter of Cut × Stock Thickness × Shearing Strength

For round holes, the formula is:

$$P = \frac{\pi \times D \times S \times T}{2000}$$

Where:

• P = Force in tons
• D = Diameter of hole (inches)
• S = Shearing strength of material (lb/in²)
• T = Thickness of stock (inches)
• 2000 = Conversion factor (pounds to tons)

The Practical Shortcut

For everyday shop calculations, an approximate formula works well:

$$P = D \times T \times F$$

Where:

• P = Force in tons
• D = Diameter of hole (inches)
• T = Thickness (inches)
• F = Material factor

Material factors for the approximate formula:

Tensile Strength Values for Pressure Calculations

To allow for excess pressure, the tensile strength may be substituted for the shearing strength. Approximate values:

Worked Examples

Example 1: Round hole in steel

Find the pressure required to punch a 2-inch diameter hole through 1/4-inch thick steel.

Using the approximate formula:

$$P = 2 \times 0.25 \times 80 = 40 \text{ tons}$$

Example 2: Non-circular hole — Square hole in steel

Find the pressure required to punch a 1-inch square hole in 1/4-inch thick steel.

For non-circular holes, replace the hole diameter with one-third of the perimeter:

• Perimeter = 4 × 1 = 4 inches
• One-third of perimeter = 4/3 = 1.333 inches

$$P = 1.333 \times 0.25 \times 80 = 26.67 \text{ tons}$$

Example 3: Rectangular hole in brass

Find the pressure to punch a 1 × 2 inch hole in 1/4-inch thick brass sheet.

• Perimeter = 2(1 + 2) = 6 inches
• One-third of perimeter = 6/3 = 2 inches

$$P = 2 \times 0.25 \times 65 = 32.5 \text{ tons}$$

the practitioner's Note: "Always round up. Always add a safety margin. A press that's rated at exactly the force you need is a press that's about to surprise you."

Shut Height of Press — The Dimension That Locks Everything Together

What "Shut Height" Means

The shut height of a press is the die space when:

1. The slide is at the bottom of its stroke, AND
2. The slide connection has been adjusted upward as far as possible

It represents the minimum distance between the mounting surfaces — the tightest the press can close.

The Definition Problem

Just like clearance, the industry has two ways of measuring shut height:

The safer convention (and the recommended one): Measure from the top of the bolster to the bottom of the slide, with stroke down and adjustment up.

Why? Because most dies are mounted on bolster plates of standard thickness. If there's a misunderstanding, having too much die space is a recoverable problem (you can shim). Having too little die space means the die physically cannot fit in the press — a much more serious and expensive mistake.

Shut Height of a Die

The term also applies to the die itself:

Die shut height = Distance from the bottom of the lower die section to the top of the upper section (excluding the shank), measured when the punch is at its lowest working position.

Always match your die shut height to your press shut height. And always confirm which definition your press manufacturer uses before you design a die.

Diameters of Shell Blanks — The Geometry of Transformation

The Core Problem

Imagine you need to draw a cylindrical cup — 2 inches in diameter, 3 inches deep. You start with a flat, circular disc of sheet metal. How big does that disc need to be?

Get it wrong and you waste material (blank too big) or produce a short shell that has to be scrapped (blank too small).

The Fundamental Formula

For plain cylindrical shells with sharp corners (thin stock), the blank diameter is:

$$D = \sqrt{d^2 + 4dh}$$

Where:

• D = Diameter of flat blank
• d = Diameter of finished shell
• h = Height of finished shell

This formula is based on a beautifully simple principle: the area of the flat blank equals the area of the drawn shell. It assumes the wall thickness stays constant during drawing — which is a good approximation for thin stock.

Worked Example

Shell diameter: 1.5 inches. Shell height: 2 inches. Find the blank diameter.

$$D = \sqrt{1.5^2 + 4 \times 1.5 \times 2} = \sqrt{2.25 + 12} = \sqrt{14.25} = 3.78 \text{ inches}$$

For Round-Cornered Cups

Most real cups have a radius at the bottom corner. For these, a modified formula accounts for the corner:

$$D = \sqrt{d^2 + 4dh - r}$$

Where r = radius of the corner.

This is accurate provided the radius doesn't exceed about one-quarter of the shell height.

The Complete Blank Diameter Reference Table

This table gives blank diameters for sharp-cornered drawn cylindrical shells across a wide range of dimensions. This is one of the most referenced tables in all of press work.

Diameters of Blanks for Drawn Cylindrical Shells (inches)

How to use this table: Find the shell diameter in the left column, then read across to the column matching your shell height. The intersection gives you the blank diameter. For round-cornered cups, subtract the corner radius from the table value.

When Wall Thickness Changes During Drawing

The formulas above assume constant wall thickness — fine for thin stock with minimal draws. But heavy stock often thins significantly during drawing. When the thickness of the finished shell is much less than the original stock, the standard formula produces a blank that's too large.

Why? Because thinning the walls effectively increases the surface area of the shell beyond what's predicted by constant-thickness geometry.

The Mean Height Method

To account for thickness reduction, first calculate the mean height of the shell:

$$M = \frac{h \times t}{T}$$

Where:

• M = Approximate mean height of the drawn shell
• h = Actual height of the drawn shell
• t = Thickness of the finished shell
• T = Thickness of the original stock

Then use this mean height (instead of the actual height) with the standard blank diameter table or formula.

Worked Example

A shell 2 inches in diameter and 3-3/4 inches high is to be drawn. Original stock thickness: 0.050 inch. Finished shell thickness: 0.040 inch.

Step 1 — Calculate mean height:

$$M = \frac{3.75 \times 0.040}{0.050} = 3.0 \text{ inches}$$

Step 2 — Look up blank diameter for a 2-inch shell at 3-inch height:

From the table: D = 5.29 inches

Step 3 — Adjust for corner radius (if applicable):

If the shell has a 1/4-inch corner radius:

$$D = 5.29 - 0.25 = 5.04 \text{ inches}$$

Important: This formula is accurate unless the thickness reduction exceeds about one-fifth of the original thickness. Beyond that, the calculated blank will produce a shell that's too long — but this error is on the safe side, since drawn shell edges are ordinarily trimmed.

The Volume-Based Formula (For Heavy Reductions)

When thickness reduction is significant, a more rigorous formula based on the volume of metal in the shell can be used:

$$D = \sqrt{a^2 + \frac{(a^2 - b^2) \times h}{t}}$$

Where:

• D = Blank diameter
• a = Outside diameter of the shell
• b = Inside diameter of the shell
• t = Thickness of shell at bottom
• h = Depth of shell

The Weight Method (For Irregular Cross-Sections)

When a shell has an irregular cross-section and its weight is known:

$$D = 1.1284\sqrt{\frac{W}{w \times t}}$$

Where:

• D = Blank diameter (inches)
• W = Weight of the shell
• w = Weight of the metal per cubic unit volume
• t = Thickness of the shell

The Trial-and-Error Method (The Toolmaker's Secret)

For irregular forms, the most reliable method is often the most practical:

1. Make the drawing tool first
2. Cut a trial blank as near to estimated size as possible
3. Scribe the blank outline on a flat sheet
4. Draw the trial blank
5. Evaluate: if the shell is short, the blank was too small; if there's excess material, it was too large
6. Adjust and repeat

If you have a model shell available:

1. Cut a blank somewhat larger than estimated, from the same material
2. Reduce the blank size until its weight equals the weight of the model

This weight-matching method is elegant and accurate — the physics guarantees it.

Drawn Cylindrical Shells — Depth and Diameter Reductions

The Rules of Multi-Draw Operations

No operation in press work tests the limits of material science quite like deep drawing. You're asking a flat disc of metal to transform into a three-dimensional vessel — and the metal has definite opinions about how far it's willing to go in one step.

The First Draw: Setting the Foundation

General Rule: The depth or length of the first draw should never be greater than the diameter of the shell.

Alternative Rule: The depth given to the shell on the first draw should equal one-third the diameter of the blank.

Practical Guideline: Ordinarily, sheet steel of any thickness up to 1/4 inch can be drawn so that the diameter of the first shell equals about six-tenths (60%) of the blank diameter.

Successive Reductions

After the first draw, each subsequent draw reduces the shell diameter further. But the percentage of reduction must decrease with each successive draw, because the metal work-hardens progressively.

Here are the maximum reductions for single-action presses, assuming annealing after the first draw and between every two subsequent operations:

Reading the table: If a shell made of 1/16-inch stock is 3 inches in diameter after the first draw, it can be reduced 20% on the next draw (to 2.4 inches), then 15% (to 2.04 inches), and so on.