396 BELL SYSTEAI TECHNICAL JOURNAL 



end of each cut. With this machine a set of die blocks, which would 

 require approximately 300 hours to machine by the hand finishing 

 method, can be completed in 80 hours. This machine was used for 

 working out the openings of the moulding dies for the new hand set. 



Precision Measuring 



One of the essential factors in high grade tool work is precision 

 measuring instruments of sufficient accuracy to check the dimensions 

 to the limits required. The most common and practical method of 

 making precise measurements is by comparison with standard known 

 dimensions and most of the instruments used on a commercial basis 

 for measuring to limits of .0001 in. or less employ this principle. 



The standards used for comparison are "Hoke" or "Johanssen" 

 gage blocks made in 81 sizes as shown in Figs. 23 and 25. The blocks 

 are arranged in four sets, the first consisting of four blocks 1 in., 2 in., 

 3 in. and 4 in. in length. The second set of 19 varies from .050 in. to 

 .950 in. in .050 in. steps. The third set of 49 varies from .101 in. to 

 .149 in. in .001 in. steps and the fourth set of 9 varies from .1001 in. to 

 .1009 in. in .0001 in. increments. In combination any dimension may 

 be obtained within the limit of the set in .0001 in. steps. 



The surfaces of these gages are so flat and smooth that if two or 

 more are wrung together so as to expel the air, they will adhere to 

 each other and resist separation at right angles to the contacting 

 surfaces with a force of over 20 pounds per sq. in. The precision 

 of these gage blocks at 68° F. is within .00001 in. per inch of length 

 of the dimensions stamped on the blocks for the larger sizes and 

 .000005 in. for the smaller sizes under one inch. Although the gages 

 are standard at 68° F., it is of course not necessary to use them at 

 this temperature, or make corrections when measuring metal of the 

 same coefficient of expansion. However, as previously mentioned, 

 it is essential that the work to be measured be at the same tempera- 

 ture as the gages. In addition to being used as standards for checking 

 parts in the different measuring instruments, a variety of other uses 

 are made of the gage block in laying out and measuring the work 

 directly. By the use of accessories and attachments, which are 

 furnished for holding the blocks, they may be made into inside and 

 outside calipers, shape and height gages, etc. One of the sets which 

 has been checked and certified by the Bureau of Standards is main- 

 tained as a standard for checking the other sets and also other master 

 gages. 



One of the most frequently used measuring instruments is the upright 

 dial indicator gage. The part to be measured is placed on the accu- 



