THE WORK OF THE MACHINE-SHOP 179 cutting lathe if other machines designed specially for cutting short screws are available. Certain jobs are allocated to certain machines because they have proved most suitable in practice. 2. DIMENSIONS.—The dimensions of articles to be machined naturally determine the size of the machines used. The size of the machine is specified in different ways, depending on the kind of machine. The lengths of planer and other beds, the dimensions of tables, the swing, and the centre-distance of lathes, the sizes of chucks, mandrels, and so on, determine the " size " of the machines. The machine may take either a single large piece or several smaller pieces; thus a series of articles may be put in tandem on a machine-table, or be disposed around a chuck, or two or more articles may be. placed on a mandrel. 3. ACCURACY.—The degree of accuracy desired is the factor upon which the interchangeable system of manufacture depends. Certain " tolerances " are allowed, and if parts which are to fit together comply with these tolerances, any part A will fit any part B: for example, suppose a |-in. spindle is to fit a hole approximately J in. in diameter. If the hole is drilled so that its diameter is less than 0-505 in., and more than 0-495 in-> and the spindle I is turned so that its diameter is less than 0-495 in. and greater than 0-49 in., then any spindle turned to these tolerances will fit any hole bored to the | tolerances stated for it. The tolerance allowed in the hole is 0-505 — °"495 = o-oi in. The tolerance allowed for the spindle is (0-495 — 0-49) = 0-005 i*1- Now it is clear that the finer the tolerances the more difficult and costly is manufacture. What then is the advantage of fine tolerances? The advantages are that a noiseless smoothly-running machine can be built I which will have great freedom from wear, because the moving parts have no room in which to knock themselves to pieces. The contrast between a 1 Rolls-Royce and a Ford car engine is largely one of contrast between toler- ances. In one case we have an expensive, smoothly-running car which is cheap to maintain, in the other a cheap car, but one with higher maintenance costs. The Machines.—In a study of the machine-shop, some knowledge of the standard machine-tools must be assumed—we are here chiefly concerned with the later developments which have followed the changing practice of the present day. It does not harmonize with that practice to deal with the machines, as of old, in watertight compartments. The work of allied groups frequently overlaps. What is of moment now is the modern way of regarding the vast subject of machining, the reaction of this view on machine design •I and selection, and on shop practice.