AT Lockheed (Ref. 18) the double‐acting press has been used toad vantage in this work, as it permits the part to be held at the edges while the punchforms it, thereby building up the desired tensile stress. This is illustrated by Fig. 32. The same general effect may be obtained in the single‐acting press with rubber punch by the use of special concave blocks in which the sheet is clamped at the edges by means of beaded plates or other devices. Mechanical stretching machines are also available for this type of work. These methods are generally not as satisfactory as the double‐acting press, however, as the latter permits greater control over the edge restraint conditions.
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