Metal Stamping Design Standards and Considerations - Xometry

In metal stamping, holes and slots get formed via piercing techniques that use steel tools called punches. During the process, the punch compresses a sheet or strip of metal against the opening of a die. As the material begins to yield to the forces, the punch cuts through and shears the material, eventually punching all the way through as the material fully yields and breaks away at the line between the punch and die edges. The result is a hole with a burnished wall on the top face that tapers out towards the bottom, leaving a burr where the material has broken away. By the nature of this process, holes and slots will not be perfectly straight. The walls can be made uniform by using secondary machining operations; however, these can add high cost.

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Minimum Diameters

The design standards for minimum diameter will depend on the chosen material. For ductile materials, such as aluminum, the minimum diameter of holes should be at least 1.2x the thickness of the material. For materials such as stainless steel alloys with higher tensile strengths, our team recommends a minimum diameter of 2x the material thickness. Slot widths should be at least 1.5x the material thickness. It is possible to achieve smaller diameters; however, they require expensive specialized processes or tooling, increasing your part cost and the risk of tool failure.

Distance From Edges

Place holes and slots near edges at a distance of at least twice the material thickness. Failure to do so may result in an outward bulging of the material web between the hole and edge. Holes closer to an edge than the recommended minimum distance may bulge or deform during stamping. These features require secondary machining or other operations that add cost.

Distance From Bends

Design holes or slots less than 0.100” in diameter or width at a distance of at least twice the material thickness (2x MT) plus the radius of the form. For holes or slots larger than this, the minimum distance should be 2.5x the material thickness plus the form radius. Holes and slots can suffer from distortion, bulging, or stretching when located closer than these recommended standards.

Bends and other formed features often come towards the end of progressive die stamping processes. Material grain direction is a crucial consideration to make when it comes to bent features. When the material’s grain is in the same direction as a bend, it is prone to cracking, especially on high-strength materials such as stainless steel alloys or tempered materials. Design bends against the material’s grain for the best results, and note grain direction on your drawing.

Bend Height

It is essential to ensure there is enough material to form bends properly. One way to provide enough material to execute a bend properly is to follow a minimum bend height standard. The recommended height of a bent feature is 2.5x the material thickness plus the bend radius. Shorter bend heights are possible but at the cost of additional operations.

Bends Close to Edges

Bent features near edges, such as bent tabs, should have an offset of material added or relief cuts in the bend. Failure to do so may result in the material tearing on either side of the bent section. When adding material offsets, you should add at least as much as the radius of the bend. Alternatively, designers can put relief notches immediately adjacent to the bend area. Relief notches should be at least twice as wide as the material thickness and as long as the bend radius, plus the material thickness.

Preventing Distortion and Bulges

Relief notches are also helpful in preventing distortion or bulging that can occur when thicker materials are bent. Bulges become especially likely with more minor bends on thicker material. Designing a relief notch on either side of the bend will help mitigate bulging. Using flag notes on your drawings is also recommended, calling attention to areas where bulging is not permissible.

Notches and Tabs

A width of 1.5x the material thickness should be designed to prevent excessive force on punches and tabs. When made smaller, the risk of tool breakage is much greater.

Corner Radii

All corners of the blank design should include a radius of at least half the material thickness. Corners can be left relatively sharp if the material is less than 0.060” thick.

Burrs

Burrs are a typical and expected occurrence on cutout features due to how the stamping process works. The general expectation is that burrs 10% of the size of the material thickness will be present on the bottom side of cutouts. You can mitigate burrs by avoiding sharp corners and intricate cutouts. Drawing notes specifying burr direction can also help the manufacturer account for this during stamping. If your part requires burr removal, Xometry offers this as a selectable option during the quoting process.

Bending is a fundamental process in sheet metal working that involves deforming a metal workpiece into a desired shape by applying force between two tools by a press brake: an upper tool (known as a punch) and a bottom tool (known as a V-die). Bending can improve a part’s structural integrity by increasing part stiffness, redistributing stress within a part, and help achieve specific shapes that are required for certain applications. For instance, shaping a curved profile can improve a part’s ability to withstand certain types of loads.

To fully utilize the capabilities of this process, it is important that your CAD is designed according to a number of recommendations. In this article, we offer a comprehensive guide to the best design practices for Sheet Metal Bending, tolerance guide and cost reduction tips.

Sheet metal bending: designing guidelines

Rules for Designing Bends

The basic bending design guidelines that a designer needs to consider when modelling a sheet metal component include wall thickness, bend radii, and bend allowance.

1. Wall thickness

Sheet metal parts are usually fabricated from a single sheet of metal, so they should have a uniform wall thickness. Generally capabilities of of 0.9mm – 20mm in thickness are able to be manufactured from sheet (<3mm) or plate (>3mm) but this tolerance depends mainly on the part.

2. Bend radii

At a minimum, the smallest bend radius should be at least equal to the sheet thickness to avoid fractures or distortions in the metal part. Keeping bends in the same plane in the same direction helps to save time and money by preventing part reorientation. Keeping the bend radius consistent will also make parts more cost-effective.

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3. Bend allowance

When you bend sheet metal, the neutral axis shifts toward the inside surface of the bend. The ‘K-factor’ is the ratio of the neutral axis location (t) to the material thickness (T), which can be used to to calculate the bend allowance. View the K-factor chart below to calculate the amount of material needed to account for your bend.

K-factor chart

RadiusAluminium (Soft)Aluminium (Medium)Stainless Steel (Hard)Air bending0 – t0.330.380.40t. – 3*t0.400.430.453*t. – >3*t.0.500.500.50Bottom bending0 – t.0.420.440.46t. – 3*t.0.460.470.483*t. – >3*t.0.500.500.50Coin bending0 – t.0.380.410.44t. – 3*t.0.440.460.473*t. – >3*t.0.500.500.50

Rules for Designing Bend Relief

Bend reliefs are two small cuts made in a piece of sheet metal to free up the metal between them. Although they are small features, leaving them out can cause stress to concentrate at the bend line, resulting in deformed holes and slots.

1. Bends close to an edge

If bend reliefs are left out for bends made close to an edge, it can cause unwanted tearing. In some cases, it can make your part un-manufacturable. To ensure successful bending, the width of the relief cuts should be at least equal to the material thickness, and the length should be longer than the radius of the bend.

2. Bends where the flanges aren’t adjoining

Flange in sheet metal parts, is a feature that consists of a face and bend connected to an existing face along a straight edge. For bends where the flanges aren’t adjoining, there are a number of different relief types available for utilisation by designers. Two of the most common types include:

• Oblong Relief: They have rounded ends, which help in distributing the stress more evenly compared to sharp corners. Oblong reliefs are particularly useful useful when the bend is close to holes or slots, as they minimise the distortion of these features by allowing more controlled movement of the material.
• Rectangular Relief: Rectangular reliefs are straightforward to cut and require less complex and costly tooling, suitable for designs where the bend radius is not too tight, and the material thickness is within a manageable range.

Rules for Designing Edge Features

Some components benefit from having special features formed from the remaining edges, two of these main features are curls and hems.

1. Curl edge guidelines

Curls are hollow circular rolls formed at the edge of the sheet via sheet metal bending. Curl features are commonly used to provide strength to a part and to remove sharp edges from the workpiece so that it is safe to handle.

For best results, it is recommended that the outer radius of a curl be at least twice the material thickness, although this will vary depending on the manufacturer and their tooling for curling. The bend should be at least the radius of the curl plus 6 times the material thickness from the curl feature

2. Hem edge guidelines

Hems are similar to curls — they are folds made back onto the metal itself — formed into a U shape. Hem features are commonly used to provide strength to the part and connect parts together. The three main types of hem features industrial and designers should be familiar with include: open hem, closed hem, and teardrop hem.

• Open Hem: This type of hem has a slight gap or space, leaving the fold partially open. The minimum recommended inside diameter equals the material thickness and a return length of 4 times the thickness is recommended.
• Closed Hem: This type of hem is tightly closed with no gap. It is recommended that the minimum inside diameter equals the material thickness, and the hem return length is 6 times the material thickness.
• Teardrop Hem: This type of hem forms a teardrop shape, providing a compromise between strength and material flexibility. The minimum inside diameter should be at least equal the material thickness, and a return length of 4 times the thickness is recommended.

Example of how open hems can be used to connect two parts

Rules for Designing Hole Features

1. Holes and slots positioned too close to bends

Holes and slots which are located close to bends are susceptible to deforming following bending. To ensure successful bending, it is recommended to place holes away from bends at a distance of at least 2.5 times the material’s thickness (T) plus the bend radius (R). For slots, it is recommended to position it at least 4 times the material’s thickness plus the bend radius away from the bend.

• Minimum recommended hole edge from bend face = 2.5T + R
• Minimum recommended slot edge from bend face = 4T + R

2. Holes and slots positioned too close to edge

Holes and slots located too near a part edge can result in a ‘bulging’ effect. Therefore, a good rule of thumb is to leave a minimum space of at least 2 times the thickness of the sheet between the extruded holes and the part edge.

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