4 2 



Laboratory Arts 



FIG. 32. Outside and inside callipers, 

 showing various patterns of jaw. 



Inside Callipers are closed until rather too narrow to fit 

 the hole they are to measure, and then opened while in position 



until they touch the sides of 

 the hole. While outside cal- 

 lipers have broadened jaws, 

 inside callipers would be 

 faulty if so provided, see 

 Fig. 33 A, as they would not 

 measure the diameter accu- 

 rately. The jaws are, there- 

 fore, not infrequently rounded, 

 as shown in Fig. 33 C, or 

 ground to a knife edge, Figs. 

 32 and 33 B. 



In copying or in com- 

 paring two objects by means 

 of either tool, the process is 

 obviously simple, but it is 

 not quite so simple to mea- 

 sure a hole and turn a pin 



to fit it. Measurements are transferred from one tool to the 

 other in the same way as the measurements were originally 



taken, only with more care, 

 both tools being liable to 

 alter the distance transferred 

 if carelessly handled. 



If a hole is measured 

 with the inside callipers, 

 transferred accurately and 

 closely to the outside callipers, and a pin obtained to fit 

 this gauge, the pin will enter the hole only with difficulty, 

 even when the edges of pin and hole are chamfered. A 

 " driving fit " would necessitate the pin being of such size 

 that it fits the outside callipers comfortably and easily. Then 

 hammering will drive it home. A " working fit " will be some- 

 what smaller than this, and should be allowed for in the first 

 setting of the callipers. The difference between a " driving " 

 and " working " fit may be arranged when taking the original 



FIG. 33 . 



