The shear method of cutting out is usually adopted in the case of large blanking tools, to allow of a large blank being cut with comparatively little power required for the operation, and to reduce the stress on both the tools and machine.

The shear method is carried out successfully when the tool which comes in contact with the usuable material is kept quite flat, and the shear put upon the tool that will distort the scrap only. For instance (1), suppose ventilator grid blanks are being cut out, these blanks would be required perfectly flat. In this case the punch face should be kept perfectly flat, so as to keep the part of the metal flat which is required to be used up for the blank. The shear in this case will be arranged in the cutting edge of the lower tool bed or die, by making it concave or with curved cutting edge, and this would distort the scrap only, leaving the metal ventilator grid blank perfectly flat.

'2. Suppose ventilator plates are being perforated. This is an instance where the order of things must be reversed. The lower tool face, or face of cutting bed, should be kept quite flat, whilst the shear can be arranged upon the cutting edge of the punch, either by making the face of the punch concave or convex; or, if a number of punches are working together, they may be made of variable length, so as to extend the time during which the actual cutting or piercing operations take place. The pieces that are cut out in perforating the plate will be scrap, whilst the ventilator plate itself will be quite flat, owing to the l>ottom tool or die being perfectly flat It is of no consequence if the scrap blanks, scrap piercings, or scrap metal surrounding any blank becomes bent or distorted, as they fall from the cutting tools; whilst the ease and comparative silence in which the work is done is quite a pleasure, as contrasted with the constant repetition of reports incidental to cutting when both the punch and bed are flat and without shear or curved edges. Refering to figs. 172, 173, and 174, it will be noticed that the pair of cutting-out tools fixed in the press are for cutting out round blanks. The bottom tool or die being arranged to shear the blank from these tools will therefore be quite flat, whilst the scrap metal will be distorted or curved as it falls from the tools of the press.

Fig. 183.

Figs. 183 and 184 illustrate a pair of tools for cutting blanks of irregular shape and a pair of round blanking tools respectively. In each case the shear is arranged on the cutting edge of the bed or bottom tool, so that the blanks when cut shall be flat.

Fig. 184.