THE WORK OF THE MACHINES 211 \ t I % 4 CHAPTER IV The Work of the Machines DIVISION I The Lathes. —These in- clude some forty to fifty groups, each having well-defined spheres of operation. They range in size from very small to mammoth dimensions, while extreme ma- chines have little in common except the fact that the work revolves between centres or in chucks. The prototype of most of these is the standard, " self- acting, sliding, surfacing, and screw-cutting lathe "—the all- round machine-tool, the econ- omic value of which gets less and less as specialized manufac- ture increases. Short screws, and those of which large quantities are re- quired, are now manufactured on turret lathes, screwing machines, and brass - finishers' lathes. The longer screws and stays are still made in screw- cutting lathes. The later lathes, fig. 47 being an example, nearly all differ from the earlier in the provisions made for speed and feed changes. Stepped belt cones are now almost entirely super- seded by all-geared heads. When cones are retained, with back gears, speeds are arranged in carefully chosen ratios instead of in a haphazard way. All-geared heads, being driven from a single s 64 7 8 S d.