Page:The Kinematics of Machinery.djvu/512

 430 KINEMATICS OF MACHINERY.

130. The Tool.

In commencing with the part of the machine which directly executes the work the tool we shall first try to find what portion of a few well-known machines answers to this description. In the lathe, planing-machine, band-saw, etc., this is very easy. The chisel or other cutting tool, the saw-blade and so on, are obviously the pieces directly employed in the work. In the screwing-machine there are generally several pieces, the dies, acting together, and these, together with the frame or stock in which they are placed, may fairly be called the tool, as in practice they actually are. In rolling mills of any kind the two rolls serve as the tool, they share equally in shaping and moving the metal passing beween them. In flour mills the stones act as tools, grinding the corn and passing it from them as meal. In the manufacture of wrought iron nails one tool, a compound one, is used to hold the wire, another, also compound, to cut it, a third to form the head, other parts bring forward a new portion of wire, others remove the nail already made.

In the card-making machine, also, several tools act in succession, some to pierce the leather band, others to cut the wire, to bend it, and to insert it in its place in the band, while others act on band and wire together in order to press home the latter. We have, that is to say, a series of tools working in different ways and for differ- ent purposes, and so connected that it is very difficult to say where one ends and the next one begins.

We notice here that the unity of the tool, or indeed of the body to be worked upon, does not appear to be a condition of the ma- chine. This fact has to be kept in mind if it be wished to form accurate definitions, and many writers who have endeavoured to carry scientific exactness through all their work, Eedtenbacher among them, have been compelled for this reason to use extended descriptions instead of definitions. Before we consider this ques- tion let us look into some further examples.

In hoisting machines the hook from which the body hangs as it is lifted has been called the tool. This is perfectly right, according to the definition, for it is clearly the part of the machine which