Sheet Metalworking

Stretcher-levelling of sheet

Sheet is normally supplied flat, that is to say in a stretched condition. However, it can become dented or bent in transit or in storage. These flaws can be eliminated by stretching the sheet again. Dents are removed by stretching the material at the edge of the dent. This is done with hammer-blows, working in a spiral pattern round the dent from the inside to the outside. It a sheet has several dents, they can be reduced to a single dent by stretching the material, which is then straightened. Dents can also be removed by flame straightening.

Warped sheets always rest on the shorter diagonal. By stretching along this line (hammer-blows), the material is elongated and the sheet will lie flat again (Figure 9.15). Wavy sheet edges can be straightened again by stretching the material from the edge towards the middle, if the change of shape is not too great. The blows should be light, and the hammer itself should not be too heavy in order to avoid renewed stretching. Stretching sheet metal, particularly large areas, calls for considerable experience. On soft materials (brass, light metal), only wooden mallets or rubber faced hammers should be used. Distorted sheet metal can often be straightened easily with the aid of stretching hammers. The hammer head is ground to a diamond pattern. The resulting slight points penetrate a small distance into the metal, relieve stresses and flatten the material. Workpieces or materials can also be flattened in the press, using rough levelling tools.

Removing dents from vehicle body panels

Frequently damaged body panels are usually designed for easy replacement. This applies particularly to mud guards (front and rear sidepanels). Slight damage can be rectified, although this needs much experience. The precautions applicable to the stretching of sheet metal basically apply here too. Steel bodies are produced from thin deep drawn sheet, which acquires additional strength in the deep drawing process. Dents caused by impact can be pressed out without difficulty if the sheet has stretched only slightly. It the change in shape is too great, however, each blow applied to the dent will only magnify it. The area surrounding the dent should therefore be stretched if possible. The dent is often shrunk by flame straightening. ln many cases the dented area is cutout, a replacement section welded in and the surface reworked until it is again acceptable. Deforrned areas can also be cut, straightened and welded together again. By using dozers (Figure 9.17), body realignment jigs and frames and hydraulic compression and tension rams, bodyshells can be straightened again quickly, easily and extremely accurately.