Metal working: Production-oriented machinery including drilling machines, reaming machines, and combined drillers and reamers (introduced by 1923); tapping machines, combined drilling and tapping machines, and pipe-threading machines (introduced 1924); multiple-spindle lathes (introduced 1931); milling machines and combined milling and center-drilling machines (introduced 1941); boring machines (introduced 1944); combined milling-drilling-tapping machines and milling-spinning machines (introduced 1946); multiple operation machines (introduced 1948).