To the uninitiated the levelling of plates or sheets presents one of the most awkward jobs it is possible to have. Yet with the exercise of thought and some practice the difficulties soon disappear. Before commencing to hammer a plate the position of the buckle or looseness should be carefully noted, and the blows placed accordingly. There are only two ways in which a plate may be buckled. It may either be sagged in the middle, as shown on plate "A" in Fig. 337, or it may be tight in the centre and slack along the edges, as shown by plate "B." On one plate there may be a combination of these two ways of buckling; one half may be slack in the middle and the other half slack on the edges.
The cause of buckling is due to unequal contraction of the sheet or plate in cooling, so that one part becomes longer or shorter than the other. To bring the plate level, all strain must be removed, so that no one part of the surface shall be pulling against another. To do this all the short or tightened parts of the plate will require hammering. Thus in plate "A," where the buckle is in the centre, the hammer blows will need to be thickest at the outside of the plate, running away to nothing at the middle. In plate "B," where the edges are buckled or loose, the process will have to be reversed: the blows being placed about the middle and gradually dying away towards the edges.
The strength of the blows will, of course, depend upon the
thickness of the plate. The inexperienced should always err on the side of light blows, as one heavy blow inadvertently given may require a hundred more to remove its ill-effects.
When particular work is required a flatter should be used to avoid the blows coming directly on to the surface of the plate. When the hammer alone is used great care must be taken so that its face edge shall not cut into the plate,
To obviate this, hammer faces for planishing purposes usually have a slightly outward curve.
Straightening rolls and other machines are now used for flattening plates and sheets, but where these are not available it is a good plan to run the sheet through the ordinary bending rolls a few times, reversing the sheet at each operation, as this tends to run the small buckles altogether and generally assists in determining how the sheet shall be hammered.