Sheet Metal Product Design

What is sheet metal? In simple layman terms, its a sheet made of metal, and that's exactly what it is. On the same time, in terms of engineering, I would say a sheet made of metal with a maximum thickness of 1/4th on an inch (6.35mm). If the thickness is more than that, it is termed as a plate and not a sheet. In my personal experience, I have never seen/used a sheet with a thickness of more than 4mm and understand that it is always advisable to support a design by adding ribs rather than adding to the thickness of the sheet being used.

To design a sheet metal product it is very important for a design engineer to have good knowledge of the following factors:

• Sheet metal Materials
• Design Thumb Rules
• Fabrication Processes
• Welding/Fasteners
• Finish Processes 

Materials:

Before getting into the detail of the materials being used in the sheet metal fabrication industry, it is very important to know how the raw material is being formed into sheets to be used. The materials are being formed into thin sheets by metal rolling process.

Steel is the material being mostly used, followed by other materials like aluminium, brass, copper, etc. These materials are divided into various grades by various Engineering Standards defining organizations like ISO, AISI, DIN, etc in order to easily identify the right material to be used for an application. 

For instance, AISI uses the following nomenclature for easy identification of steel:

 

Though, as one can observe in the above picture, the quantity of the alloys added in the steel under various grades is very less yet, their addition makes significant changes in the Mechanical/Physical/Chemical properties of steel.

The strength of steel can be increased by the addition of alloys such as manganese, niobium and vanadium. However, these alloy additions can also adversely affect other properties, such as ductility, toughness and weldability. And so is the case with other materials.

Apart from that, there are some heat treatments that are usually performed to improve the desired properties of a material.

Design Thumb Rules:

As per my knowledge there are five basic Design thumb rules for sheet metal product design. These rules are made with an idea to keep the product stress free or to keep the stress to the minimum:

• Flange Width: The width of the flange should always be atleast equal to the sum of 4 times the sheet thickness and the inside bend radius.
• Distance b/w a bend and a hole: The distance should be atleast equal to the sum of 3 times the sheet thickness and the inside bend radius.
• Bend Relief: While bending  a sheet, the minimum bend relief to be given should be equal to sum of the sheet thickness and the inside bend radius.
• Minimum Hole Diameter: The minimum hole diameter in a sheet should be atleast equal to the thickness of the sheet and no less than that.
• Minimum Bending Radius: The minimum bend radius depends ductility of the sheet material. The more ductile the sheet metal, the smaller the inner bend radius is possible. Generally, the minimum bend radius should be atleast equal to the thickness of the sheet.

PS: It is advisable but not mandatory to go as per these rules. These are generic rules and as a Design Engineer, one must know them. The final decision is made as per the application or requirement of the design.

Fabrication Processes: