MACHINERY ' S REFERENCE SERIES EACH NUMBER IS ONE UNIT I N A COMPLETE LIBRARY OF MACHINE DESIGN AND S HOP PRACTICE REV I SE D AND REPUBLISHED FROM MACHINERY NUMBER 133 HOBS AND GEAR HOB G B y J OH N E DGAR CON TE N TS Intro duction :P rinciples of the H obbing P roce ss Bobs f or S pur and S piral Gears S pecial H ob - tooth S hapes The Centering of the H ob in Ge ar I;I obbing e ar H obbing v s . M illing of G s b h M A C H I N E R Y p r h t 1 91 4 T h e n d s t r a P re ss , P s e rs , C , o y ig , I u i l u li of - L e t S t re e t N e w Y r% t 1 4 0 1 4 8 a fay t e , o Ci y Othe r boo%s in this series dealing with the subje ct o f Ge aring are as follows: —WORM GE R N G N o . l A I — E R N G , N o . 1 5 SPUR G A I — P1 RA E R NG N o . 20 S L G A I —B EV EL GE R N G N o . 37 A I M ACHI NERY The L e adin g Mech an ic al Jo u rnal M ACH IN E DE SIGN CON STR UCTION S HOP PR ACTICE TH E IN DUSTRIAL PRESS - 1 4 0 1 4 8 L afayett e St . N e w Y o r% Cit y - e Lo ndo n 51 52 Chance ry L an , “ I N TR ODUCTHHJ PR IN C I P LE O F TH E H O B B IN G PR O C E S S The hobbing proc e ss for cutting the teeth in spur and spiral gears e is beginning to be very widely used . The principl of this method is shown diagrammatically in the accompanying illustration . In the lower part Of the illustration is shown an imaginary rack ( in dott e d h e o f are lines ) ; this rack is in mesh with the ge ar , t e te th which e as Of a to be formed , and if the blank could be imagin d made plastic e material , the rack , i f moved along as indicated by the arrow , whil e ee the ge ar rotate d to correspond , would form theor e tically corr ct t th th e in the gear blank . The teeth of this rack coincide with outlines o f the worm shown in full lin e s , this latter having been set at such an angle as to make the teeth o n its front side parallel with the axis Of h the gear . In other words , it has been set at the angle of its elix , e measured at the pitch line . This worm , when properly flut d , forms e o f th e the hob for cutting the gear t e e th . It will be s en that the teeth e e . h ob , when set in this position , corr spond with the t eth of the rack th e If, now , the hob and blank be rotated at the ratio required by number o f thr e ads in the hob and the number o f teeth in the gear , this movement will cause the teeth of the hob to travel lengthwis e in exactly the same way as the t e eth o f the imaginary rack would trav el , s if in mesh with the ge ar , th e t e eth of which are to be c ut . It will thu be seen that th e h ob fulfills the r e quir e m e nts nec e ssary for molding h the t e eth o f the ge ar to the proper form . In practice t e hob is f rotate d in th e r e quir e d ratio with the work , and e d gradually through - it from one side Of the face to th e other . When it has passed through ‘ e . once , the work is complet d Of the great number o f machines built during the past few years e S e involving this principle , many are arrang d for cutting piral g ars r Of as well as spur ge ars . Of cou se , all of the machines capable T cutting spiral ge ars are capabl e of cutting spur gears also . h e spiral gear- hobbing machine bears ab out th e s ame r e lat ion to the plain spur g ear- hobbing machin e that th e universal does to the plain mill me e ing mac hine . The added adjust nts and mechanism r quired in e ach case tend to somewhat limit the capacity of th e machine in tak ing heavy cuts although th e y add to its usefuln e ss by e xte nding the n range o f work I t is capabl e o f p e rformi g. R e q uire m e n t s o f G e a r H o b b in g Ma c h in e s Th e requirem e nts o f the succe ssful gear hobbing machin e are e First . A frame and m e chanism of gr at rigidity . e e m S e cond . D urabl e and pow rful driving m chanis . e . Third . Accurate indexing m chanism o n The first requirem e nt is e of gr e at importanc e , not only in its influ e nc e o n th e h e avin e ss o f the cut to b e take n and th e cons e qu e nt 3 4 7 5 9 7 NSJEM OB S SAN D GE AR HO B B I N G o o f o n cc utput work , but the matter Of a uracy as well . The co n n ecti on between the h ob and the work , through the shafts and gea ring, is liable to be so complicated that the irregular cutting action Of th e c hob produ es torsional deflections in the connecting parts , leading to serious displacement from the desired relation between the hob and the teeth being cut . This displacement fro m the desired position f results in teeth o inaccurate shape , weak and noisy at high speeds . In its effect o n the output, rigidity is even more imp orta nt in the D i a gram illu strat in g th e P rin ciple of th e H obbin g Proc ess of Formin g S pur Ge ars A hobbing machine than in the orthodox automatic ge ar cutter . On heavier cut is taken , since a greater numb e r of teeth are cutting the work at once . The numb e r o f joints between the cutter and the “ - - w o rk supporting table and spindle must , therefo re , be reduced , to a minimum , and the matter o f overhang both for the work and the cutter must be car e fully looked o ut for . The r e duction Of overhang is hampered at the cutter head by the necessity fo r a strong drive - and an angular adjustment . In the case o f th e work supporting parts , it is difli cult to bring the cutting point close to the b e aring on a o c ount o f the necessity for plenty o f clearance below the work for the hob and its driving gear . S PUR AN D S PI R AL GE AR HOB S 5 Th e matte r o f design Of the driving me chanism for th e hob and th e o t o work is a difficult one . N nly must it be rigid for th e sak e o f ac - curacy , as pr e viously explained , but car e ful attention must b e given to durability as well . It requires gr e at skill to design a durable — mechanism for the purpose within the limitations imposed i n the e cutter h ad by the necessity for reducing the ov e rhang, and in the e e e work tabl by the high sp ed r quired for cutting small ge ars . S ince th e ind e xing wheel works constantly and und e r considerable load , both the wheel and worm must be built o f such materials as e e c will pr serve th ir ac uracy after long continued use . Particular attention should be giv e n to th e homogen e ity o f the mate rial o f the - index worm wheel , to make sure that it does no t wear faster on o n e side than o n the other . e The fi ld of the hobbing process for cutting spur gears has , perhaps , n o t yet been definite ly determined . In some work it appears to have great advantage s over the usual type Of automatic gear—cutting machine , while in other cases it seems to fall behind . It will doubt a f less require continued use , with variety o work , and fo r a con si de rable length of time , to determine just what cases are best suited fo r the hobbing machine , and for the machine with the rotating disk t cutter .