Sheet metal punching shears holes, slots, and shaped features through a flat sheet by driving a hardened punch through the material into a matched die below. On a CNC punching machine the sheet is repositioned and struck hundreds of times a minute, so punching is the fastest way to put a high count of holes into a part. The numbers that decide whether a punched feature comes out clean are knowable up front: minimum hole diameter relative to thickness, how far a hole can sit from an edge or another hole, the punch-to-die clearance set by the alloy, and the tonnage the feature demands. Get those on the print and the part nests, runs, and ships. Get them wrong and the shop finds out at the machine, after the blank is loaded.
This guide covers the working figures a designer needs to specify a punched part: minimum hole size and proximity rules by thickness, punch-to-die clearance as a percentage of thickness by material, the tonnage formula and how to read it, the burr and rollover characteristics of a sheared edge, nibbling and forming stations (louvers, extrusions, countersinks), achievable tolerances, and when punching beats laser cutting. Every figure below is tied to a published source.
How Punching Works: Punch, Die, and the Sheared Edge
A punch press forces a hardened steel punch through the sheet into a die cavity, shearing out a slug that drops through the die opening. The cut does not happen all at once. As the punch enters, it first rolls the top surface down (the rollover), then cuts a clean vertical band (the burnish zone), then the material fractures the rest of the way through, leaving an angled fracture zone and a small burr on the die side. That four-part signature, rollover, burnish, fracture, and burr, is the fingerprint of a sheared edge, and the proportions of each band are controlled by the punch-to-die clearance, tool sharpness, and alignment. (The Fabricator, Holemaking in Sheet Metal)
Two machine families do the work. Mechanical and hydraulic punch presses drive a single tool or a fixed tool set and are most economical on dedicated, high-volume runs of the same part. CNC punching machines hold many tools and switch between them automatically off a CAD file, which suits mixed production, complex hole patterns, and prototyping without tooling changes. Tool storage comes in two styles: the traditional rotating turret, and the linear tool changer, which carries tools in a rail and loads them at the head for faster, more flexible tool swaps. Atlas’s punching machines are the linear-tool-changer type, rated to 25 tons of punching force. Modern CNC punching machines run fast: TRUMPF rates the TruPunch 5000 at up to 1,000 strokes per minute in punching mode and higher in marking modes, and published holemaking comparisons put CNC punching in the 400 to 1,600 strokes-per-minute range. (TRUMPF TruPunch 5000; The Fabricator)
| Factor | Punch press (mechanical/hydraulic) | CNC punching machine |
|---|---|---|
| Best for | High volume, simple repeated patterns | Complex patterns, mixed and medium volume |
| Setup time | Longer (manual die changes) | Minimal (automatic tool changes) |
| Speed | Up to 1,000+ strokes/min | 400 to 1,600 strokes/min |
| Flexibility | Limited to installed dies | Many tools loaded in the tool changer |
| Typical thickness | Up to 0.250 in. | Up to 0.250 in. (some to 0.500 in.) |
Turret vs. Linear Tool Changer
How a CNC punching machine gets the next tool under the ram, shown as a 3-tool job (A, B, C)
Tool A indexed under the ram.
Tools ride in stations on the turret ring. To change tools, the whole turret rotates until the next station indexes under the fixed ram.
Tool A loaded at the head.
Tools sit in cartridges on a straight rail. A carriage travels directly to the tool and loads it at the head, so tool swaps are faster and more flexible and changeover is staged without manual setup.
Animation is off because your system prefers reduced motion. The labeled schematics above compare the two mechanisms in their tool-loaded state.
Schematic comparison of the two CNC punching tool-storage styles: a rotating turret that indexes tool stations under the ram versus the linear tool changer used on Atlas’s 25-ton punching machines.
Minimum Hole Size and Proximity Rules
The most common punching drawing error is a hole that is too small or too close to something. A punch is a slender column under compression: when the hole diameter drops below the material thickness, the punch is more likely to deflect, gall, or break, and the slug tends to pull back up through the die. The proximity rules come from the same physics that govern the sheared edge. Material near a hole or an edge is in the shear deformation zone, so holes crowded together or pushed to an edge tear, distort, or break out.
| Feature | Design rule | Why |
|---|---|---|
| Minimum hole diameter | ≥ material thickness (T) | Smaller holes risk punch breakage and slug pull-back; below T needs specialized tooling and slows the run |
| Hole-to-edge distance | ≥ 1.5 × T (hole edge to part edge) | Closer than this weakens the edge and can break out |
| Hole-to-hole spacing | ≥ 2 × T (edge to edge) | Crowded holes let the web between them distort or the slug pull back |
| Hole-to-bend distance | ≥ 2 × T + bend radius (hole edge to bend tangent) | Holes inside the bend zone pull oval when the flange forms |
| Internal corners | Add a radius; avoid sharp inside corners | Sharp corners concentrate stress and accelerate tool wear |
Standard round tooling typically runs from about 0.040 in. up to roughly 4 in. in diameter, with oblong, square, and rectangular tools available off the shelf. Anything smaller than the standard range, or a proprietary shape, moves into custom tooling, which is worth the upfront cost only when it will be amortized across many parts.
Punch-to-Die Clearance by Material
Clearance is the gap between the punch and the die wall, and it is the single most important variable for edge quality and tool life. Too little clearance forces the fracture lines to miss each other, so the material is cut twice, which produces a secondary shear band, more burr, and rapid tool wear. Too much clearance lets the material roll and tear instead of shearing cleanly, producing a large rollover and a heavy burr. The correct clearance is expressed as a percentage of material thickness and scales with the alloy: softer metals take less clearance, harder and work-hardening metals take more.
The figures below are total clearance (both sides combined), the way most shop calculators and tool-builder charts state it. A long-standing die-maker rule of thumb is roughly 10 percent of stock thickness per side for mild steel, which is the 20 percent total shown here. (Dayton Lamina, Engineered Clearance; MetalForming Magazine)
| Material | Total die clearance (% of thickness) | Example at 0.125 in. thick |
|---|---|---|
| Aluminum | ~15% (T × 0.15) | 0.019 in. |
| Mild / cold-rolled steel | ~20% (T × 0.20, ~10% per side) | 0.025 in. |
| Stainless steel (304) | ~25% (T × 0.25) | 0.031 in. |
| Brass / copper (soft) | ~15–20% | 0.019–0.025 in. |
Tonnage: Sizing the Force a Feature Demands
Punching force scales with how much edge has to be sheared and how strong the material is. The governing relationship is:
Force = perimeter × thickness × shear strength
To put the answer in tons, divide pounds by 2,000: Tonnage = (perimeter × thickness × shear strength in psi) / 2,000. For a round hole the perimeter is π × diameter. The widely used shop shortcut for mild steel folds the constants together: diameter × thickness × 80 = tons for a round hole, and one-third of the perimeter × thickness × 80 for a shaped hole. The 80 comes from mild steel’s shear strength expressed in the calculator’s units. (Cleveland Steel Tool, Tonnage Calculator; UniPunch Tonnage Calculator)
The shear strength term is what makes material choice show up at the press. Mild steel runs roughly 50,000 psi, stainless steel is higher and work-hardens during the cut, and aluminum is much lower. Worked example: a 1.0 in. round hole in 0.125 in. mild steel needs about 1.0 × 0.125 × 80 ≈ 10 tons. The same hole in 304 stainless can need on the order of 1.5 times that force, and in aluminum roughly half.
| Material | Approx. shear strength | Relative punching force vs. mild steel |
|---|---|---|
| Aluminum (e.g., 5052) | ~25,000–40,000 psi (170–275 MPa) | ~0.5× |
| Mild / cold-rolled steel | ~50,000 psi (~200–250 MPa) | 1.0× (baseline) |
| Stainless steel (304) | ~80,000+ psi (300–400 MPa) | ~1.5× |
| Copper / brass | ~25,000–35,000 psi | ~0.5–0.7× |
Nibbling and Forming Stations Beyond Simple Holes
A CNC punching machine does more than drop round and shaped holes. Two capabilities extend it well beyond a hole maker.
Nibbling creates long contours and large openings by placing many small, overlapping hits with a single tool instead of a dedicated cutting profile. Each hit removes a sliver; stepping the tool along a path nibbles out a slot, a radius, or an arbitrary contour. It trades edge smoothness (a nibbled edge shows a scalloped step pattern that often needs deburring) for the ability to cut any shape without a custom punch. (TRUMPF, Punching and Nibbling)
Forming stations use special tools that deform the sheet in place rather than cutting through it, producing 3D features in the same setup as the holes:
- Louvers lance and form ventilation openings in one hit, common in electronic enclosures and cooling panels.
- Extrusions (extruded / flow-drilled holes) push a collar of material up around a hole, adding thread engagement length for tapping in thin stock.
- Countersinks form a tapered seat around a hole for flush flat-head fasteners, done as a forming hit rather than a machined chamfer.
- Embosses, electrical knockouts, and card-guide / stiffening forms add rigidity or function without a secondary operation.
The catalog of standard forming tools is wide; UniPunch documents louvers, extruded holes, countersinking, embossing, and knockouts as routine punch-tool operations. (UniPunch, Typical Work Done)
Tolerances You Can Hold
CNC punching is repeatable because the machine’s coordinate system, not a human, positions every hit. Features on the same flat surface hold tighter than features that span a formed bend, because forming introduces its own variation. Atlas’s punching machines are rated at ±0.004 in. (±0.10 mm) positioning accuracy (Pa) and ±0.001 in. (±0.03 mm) repeatability (Ps). The values below are typical industry figures for well-maintained tooling.
| Feature | Typical tolerance |
|---|---|
| Hole diameter (individual) | ±0.003 in. to ±0.005 in. |
| Hole-to-hole (same surface) | ±0.005 in. |
| Hole-to-edge | ±0.010 in. |
| Hole-to-bend | ±0.015 in. |
| Atlas machine rating | ±0.004 in. positioning accuracy (Pa); ±0.001 in. repeatability (Ps) |
Tool condition is part of holding tolerance. Punches dull with use and need periodic regrinding, typically after about 0.004 in. to 0.006 in. of edge wear; a punch can be reground 15 to 20 times over its life before replacement. A dull punch produces a heavier burr and a larger rollover, which is why scheduled regrinding is a quality control measure, not just a maintenance one.
Punching vs. Laser Cutting vs. Stamping
Punching is not always the right call. The choice turns on hole density, contour complexity, and volume. Punching wins on hole-heavy parts and energy cost; laser cutting wins on complex contours and low volumes; stamping wins only at very high volumes where a dedicated die can be amortized.
| Factor | Punching | Laser cutting | Stamping |
|---|---|---|---|
| Hole speed | 400–1,600 / min | Linear contour cutting (slower per hole) | 1,000+ strokes/min |
| Setup cost | Low to medium | Very low | High (custom dies) |
| Ideal volume | ~5,000 to 100,000+ | 1 to ~10,000 | 50,000+ |
| Complex contours | Limited (nibbled, stepped) | Excellent | Requires custom die |
| Energy / operating cost | Lower | Higher (laser + assist gas) | Lowest at volume |
| Best application | Hole-heavy parts, forms | Contours, prototypes | Mass production |
The practical rule: a panel with many identical holes (a ventilated server chassis, a perforated guard, a louvered enclosure) is punching work, and a cluster tool can drop several holes per stroke. A part dominated by a few intricate outside contours and run in small quantity is laser work. Many real parts use both: punch the holes and forms, laser the contour, on the same blank.
Material Thickness Ranges for Punching
| Material | Min thickness | Max thickness (typical CNC punching) | Notes |
|---|---|---|---|
| Mild steel | 0.020 in. | 0.250 in. | Most common; punches cleanly |
| Stainless steel | 0.020 in. | 0.187 in. | More force, work-hardens; lubricate |
| Aluminum | 0.020 in. | 0.250 in. | Adjust clearance to prevent galling |
| Copper / brass | 0.020 in. | 0.125 in. | Soft; excellent hole quality |
For alloy property and specification data, consult mill data sheets and the relevant ASTM specifications, for example ASTM A1008 for cold-rolled carbon steel sheet, ASTM A240 for stainless steel sheet and plate, and ASTM B209 for aluminum sheet and plate, alongside the relevant ASM Handbook volume on forming and stamping.
Applications and File Formats
Punching is the backbone of high-hole-count fabrication: brackets, chassis, enclosures, faceplates, ventilation and louvered panels, server cabinets, cable-management panels, and structural elements with repeated hole patterns. Electronics and data-center work in particular lives on punching, where ventilated panels with hundreds of holes are far cheaper to punch than to laser cut.
Modern CAD systems export directly to CNC punch programming. Atlas accepts DXF, DWG, STEP, IGES, and PDF, along with native CAD files; a 3D model with PMI (Product Manufacturing Information) annotations carries design intent to the floor most reliably.
Punching at Atlas Manufacturing
Atlas Manufacturing runs precision sheet-metal punching out of facilities in Minneapolis, Minnesota and Eau Claire, Wisconsin, serving engineers and procurement teams across telecom, medical device, electronics, industrial, and OEM markets. The Minneapolis facility is ISO 9001:2015 certified. The shop pairs 25-ton linear-tool-changer CNC punching with 4 kW and 6 kW Bystronic fiber laser cutting, forming, welding, hardware insertion, powder coating, and assembly under one roof, so a hole-heavy panel and its formed features and finish move through one supplier.
The value to a design team is catching the punchability problems above before the blank is loaded: confirming hole diameters clear the thickness minimum, holding the edge, spacing, and bend proximity rules, and choosing punching versus laser for the actual hole count and volume. Send a print and the punching group will flag the small-hole, edge-distance, and tonnage issues up front rather than at the machine.
Frequently Asked Questions
What is sheet metal punching?
Sheet metal punching uses a punch and die set to shear holes, slots, and shapes through sheet metal. The punch forces material through the die opening, removing a slug and leaving a sheared edge with a rollover, burnish band, fracture zone, and small burr. CNC punching machines produce hundreds of holes per minute with high repeatability.
What is the minimum hole diameter for punching?
As a baseline, keep the punched hole diameter at or above the material thickness. Smaller holes risk punch breakage and slug pull-back and require specialized tooling that slows the run. Also hold holes at least 1.5x thickness from an edge and space them at least 2x thickness apart.
How much punch-to-die clearance is needed?
Clearance is set as a percentage of material thickness and scales with the alloy. Total clearance runs about 15 percent for aluminum, about 20 percent for mild steel (roughly 10 percent per side), and about 25 percent for 304 stainless. Too little clearance causes a double cut and heavy burr; too much causes rollover and tearing.
How do you calculate punching tonnage?
Force equals perimeter times thickness times shear strength. In tons, tonnage equals (perimeter x thickness x shear strength in psi) divided by 2,000. For a round hole the perimeter is pi times diameter. The mild-steel shortcut is diameter x thickness x 80 = tons for a round hole.
Is punching or laser cutting more cost effective?
Punching is typically cheaper for parts with many holes, especially above about 5,000 units, because it produces 400 to 1,600 holes per minute and uses under half the energy of laser cutting. Laser cutting is more economical for complex contours, prototypes, and very low volumes. Many parts use both.
What is nibbling in CNC punching?
Nibbling creates long contours and large openings by placing many small, overlapping punch hits with a single tool instead of a dedicated cutting profile. It can cut any shape without a custom punch, at the cost of a scalloped edge that usually needs deburring.
