Manufacturing Processes - Sheet Metal Forming Process
Sheet Metal Forming Process
Written by an engineer with decades of practical manufacturing experience, this book is a complete modern guide to sheet metal forming processes and die design – still the most commonly used methodology for the mass-production manufacture of aircraft, automobiles, and complex high-precision parts.
Sheet Metal Forming
The sheet metal forming process comes under the category of forming as a manufacturing process. There are a number of sheet metal forming methods available which all consist of some form of deformation of the material. The main sheet metal forming methods are:
- Blanking, where a shaped piece of metal is sheared from a continuous strip.
- Stretching, where a sheet is gripped at its edges and a form block is pushed in to the sheet to deform it. The material must pass beyond its material yield point to take a permanent deformation but it should be understood that there will always be a certain amount of spring back once the load is removed.
- Bending, this is where a sheet is bent over a shaped tool usually using a folding machine or press brake. The most common bend form is the 'L' shape and other options are the 'V' and 'U' shape bending tools. Again care needs to be taken to ensure the amount of spring back of the material is taken into account when bending to achieve the required angle of bend and steps should be taken to ensure there is no thinning of material through the bend.
- Drawing, this is where the sheet metal is formed between a tool and a die. The edge of the material is suitably clamped and a punch is pushed into the material forcing it into the die. Normal practice is to support the underside of a sheet with a pressure pad which helps alleviate any potential tearing problems. The criteria for deep drawing is defined as a depth to diameter ratio of equal to or greater than 1:1
- Pressing, this process can be considered to be a combination of bending, stretching and drawing. There are 2 side of a mould pushed together with the sheet metal in between to form the required shape. Sometime the bottom mould can be a rubber compound that simply forces the sheet metal around the form of the upper mould, this is useful for small batch production quantities.
Machines and dies can be quite costly but the quality of the product depends on the die design and lubrication used. A better finished die produces a higher quality component.
The main problems associated with sheet metal forming are the aforementioned springback issues, which can be resolved through calculation or, for small batch with general tooling, trial and correction.
Sheet metal forming processes are often accompanied with heat treatment stages, this is particularly true of aluminium alloys which are often annealed then solution treated for final forming and precipitation hardened after final processing. Placing an aluminium alloy component in a freezer at around -19C will delay the period of hardening after solution treatment to keep the material in a soft state for longer.
Sheet Metal Shearing and Bending
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