Figure 23. — Details of a work spindle with work, showing the use of a master lead screw 

 to control the pitch of a precision worm thread being ground. From the 1933 U.S. patent 

 1899654. of F. A. Ward's worm-grinding machine. 



In the manufacture of hobs it is common practice 

 to employ the same machine for grinding hobs of varied 

 diameters, and in order to employ such a machine in this 

 manner the pitch of the lead screw, thereof, which 

 actuates the work carrier, must conform to the axial 

 pitch of the hob to be ground. This will be readily 

 apparent when it is understood that the helix angles of 

 hobs vary in accordance with their diameters and, 

 consequently, the difference between the normal pilch 

 and the axial pitch correspondingly varies. While the 

 requirement for the normal pitch may be the same for 

 hobs of different diameters, it is necessary to change the 

 axial pitch in accordance with a change in the hob 

 diameter, and this axial pitch of the hob is equal to the 

 pitch of the lead screw which actuates the work carrier 

 in grinding machines heretofore used. Hence, in order 

 to adapt such machines to cover a wide range of leads, 

 it is necessary to provide a large number of interchange- 

 able lead screws and obviously this represents a large 

 investment, and the interchanging of these screws requires 

 the expenditure of considerable time in setting up the 

 iii.ii hine for each job. 



Thread-grinding machines wen- being designed 

 concurrent with the development of hob-grinding 

 machines. Many were entirely concerned with 

 features peculiar to the problems of wheel-dressing 



and to automatic characteristics. An invention to 

 embody the use of a master screw and concerned 

 with the precision grinding of worm threads, for 

 use in gearing, was patented by Frederick A. Ward 

 in this era. 8 That part of the invention pertaining 

 to the use of a master screw, "a rotary work holder 

 mounted on said carriage and provided with a driving 

 spindle, an exchangeable master screw and stationary 

 nut detachably secured to said spindle and head, 

 . . ." is shown in figure 23. 



Machines embodying the principle of the master 

 lead screw are found in constant use by industry at 

 the present time for specialized application. When- 

 ever technological changes again reopen the topic of 

 thread-cutting to a new degree of accuracy or call 

 for a reevaluation of popular methods for any 

 other reason, we may expect to see another resurgence 

 of the master-screw method, for no other design 

 eliminates so many variables or rests on such firm 

 and fundamental natural principles as the machine 

 of Das mittelalterliche Hausbuch of 1483, the earliest 

 such machine now known. 



8 U.S. patent 1899654, Bled August (1, 1931, issued to V. \. 

 Ward ol Detroit, Michigan, February 28, 1933, and assigned to 

 the Gear Grinding Company of Detroit, Michigan. 



Foi :ale b) the Superintendent of Documents, U.S. Government Printing Office 

 Washington, D.C. 20402 - Price 20 .cms 



