226 



HANDBOOK OF MECHANICAL DESIGN 



Siop /im/fswi^hi 



S-f-arf Siop 



Speed- = 



"'-Reversing limH' swiiches'' 

 Fig. 451. — When milling work having a gap between 

 machined surfaces, production is increased by rapid 

 traverse between machining positions. Jump feed 

 control is accomplished by means of adjustable limit 

 switches, multispeed motors, and suitable magnetic 

 controls. 



Reverse limif swiich' 

 Fig. 452. — Accurate positioning of reversing and stop 

 limits is necessary on tapping machines especially when 

 tapping blind holes. Special alternating-current revers- 

 ing motors for tapping service permit as many as 60 

 reversals per min. The use of two- or four-speed motors 

 reduces the number of gear changes required. Accurate 

 limit switches, quick-acting contactors, and high torque 

 motors are used. A plug stop is used for braking at the 

 "out" position. 



Final posiHon swiich 



.5 



T T 



PosiHon .J 



Slow- down swifch 

 Rapid traverse 



-Slow speed 



Fig. 453. — Accurate location of boring tools for indexing 

 requires extremely slow speed of work table to prevent over- 

 travel when stop limit is reached. A direct-current motor and 

 control is used; heavy armature series resistance and armature 

 parallel resistance provide for creep speeds for final positioning. 



