The term punching is generally understood to include all operations of cutting out blanks from sheet metal The shape cut away is known as the blank, whilst the unavoidable amount of metal sheet left afterwards is only fit for scrap. Some ingenuity is required to so arrange the sequence of successive punchings that the amount of scrap shall be a minimum. _
Shearing is the operation of cutting up sheet metal, flat bars, etc, into lengths, strips, etc. The action of a punch is that of shearing, and a punch may be regarded as an endless shearing tool.
The load that comes upon a punch is wholly compressive, and very heavy. The limiting thickness of metal that can be punched without heating the punch, being dependent upon the relative resistances of the die to compression, and of the plate to shearing.
Taking the case of best Yorkshire iron plates, then fs is, according to Professor Goodman, about 19 tons as the upper limit, and for tempered cast steel fc may be taken at about 85 tons, the exact figure, however, depends upon the amount of hardening and the quality of the steel, then using these values the ratio
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