Sheet metal

As the name suggests, the metal flats are converted in to thin sheets of different thickness with the mechanical property called "malleability". It is the property by virtue of which flat can be transferred in to thin sheets either by pressing / hitting or by industrial process of rolling.

Types of rolling

Cold rolling

The thick flats are passed through the series of rollers, reducing the thickness of the flat at each step and convert the thick cross section in to elongated long thin sheets.

As the name suggest, it is a cold working process that means the metals are processed at room temperature.

The concern is the more number of stages, as the ratio of change in thickness at each step is low, when performed at room temperature.

The pros are good surface quality and low hardness on the outer skin, suitable for more accurate precise components, lower thickness and close tolerance.

Hot rolling

In hot rolling the material is rolled in hot condition at very high temperature around 1700°F, which is above re-crystallization temperature for steel. The steel blocks so called billets are easier to roll at higher temperatures and are flattened to form sheet metal coils.

The main disadvantages are less control over shape, scaling and rounded edges, suitable for higher thickness and low precision parts.

The benefits are low processing cost, thus are cheaper than the cold rolled steels.

Sheet metal process:

Shearing

This process is used to make cut to length pieces from a slit coil, the slit coil is straightened by passing through numbers of rollers prior to shearing process. There is a large cutter blade on the both die and punch edges, normally high speed steel blade inserts are used. The cutting clearance is usually made to suit with wide range of thickness, to produce the right blend of fracture and shear edge with minimum burr.

Blanking

The metal cut out form the sheet or strip is the output in blanking process, generating web of symmetrical scrap called strip layout. The clearance is given to the punch and the die is the master, that means the die size is the same as the output. The output blank produced have the following main edges :

Edge rollover : The edge produced by punch when pushing the material against the walls of the die, mainly material is rollover due to clearance.

Shear edge : The shearing process is the actual cutting happens due to punch pushes the material against the narrow region (clearance), eventually cuts or shear the some portion of material identified by smooth shining band.

Fracture : When the material is forced to push against the walls of punch and die through narrow entrance so called clearance, the major portion of material around 70% is being fractured from the parent sheet or strip. This portion identified by coarse surface having draft angle, the amount of draft is directly proportional to the clearance.

Piercing or Punching

The process is same except the output, the outer part is the output in this case and the inner cut out is the scrap. The clearance is given to the die and the punch size is master, that means the punch size is same as of the output.

Why clearance given to punch in blanking process and to die in piercing process

Blanking process

The clearance is given to the punch and the die is the master, that means the die size is the same as the output.

Piercing process

The clearance is given to the die and the punch size is master, that means the punch size is same as of the output.

Drawing process

The process of making thin walled deep, hollow boxes etc. is called drawing. It can be further classified as shallow drawing or deep drawing. Mainly where there is more depth than the width is called deep drawing and where there width is more than the depth is called shallow drawing.

There is detailed article on deep drawing , refer link How to choose number of Draw process for deep drawn geometry

Bending

The bending the process in which, the material is folded against the small area at an angle of certain degree. The material flow is along with the axis of bending, where the inner area of the formed radius is subjected to compression and the outer area is subjected to tension or stretching. To read more about type of bending, you can refer the article Sheet Metal force calculation, mechanical engineering

Bending is a cost-effective near net shape process when used for low to medium quantities. Parts usually are lightweight with good mechanical properties. A disadvantage is that some process variants are sensitive to variations in material properties. For instance, differences in spring-back have a direct influence on the resulting bend angle. To mitigate this, various methods for in-process control have been developed.

Types of bending

V-Bending or Air bending

The metal piece called blank, is pressed between punch and die to form the shape of the part. The long parts are bend on the mechanical press known as Brake press, where length of the bed is quite long compared to width. The major problem faced in V bending is ‘’springback’’, which occurs due to elastic recovery of the material when subjected to plastic deformation beyond their yield limits. The springback is directly proportional to the tensile strength of the material, depending on material properties, the sheet may be overbent to compensate for springback.

U bending or Channel bending or Wiping

The material is pressed by the punch against the walls of the die, clearance plays the major role. Two main defects are the scratch and springback. The scratches are prevented by certain surface coatings such as chrome plating or PVD coating. The springback is non-controllable, either is allowed or punch size is reduced, generate slightly tapered face and outer face is the output size.

Double bending or offset bending

It is the condition where more than two bends are there, sheet is directly placed on the die and crash formed against the punch.

Edge bending

In this case, the sheet is first pressed with some cushioned part and bend by punch against the die face. The main problem is material slippage and springback, the material slippage is due to friction between punch and material causing material to drag, the solution is to increase the holding force. The springback is killed by relieving the die edge by imparting the negative draft.