PRECISION TOOL MAKING 



393 



An idea of what this class of workmanship means can be appreciated 

 when it is considered that 10 degrees Fahrenheit difference in tempera- 

 ture will change the length of an inch block more than this .00005 in. 

 limit. Since the change per inch in steel is about seven-millionths 

 of an inch per degree Fahrenheit, the heat of the hands or machines 

 may, in extremely accurate work, make sufficient difference so that 



Fig. 19 — Grinding die block openings after heat treatment. 



parts have to be laid on steel blocks to attain room temperature be- 

 fore the dimensions are checked, in order that they will be of the same 

 temperature as the master gages used. The temperature is also an 

 important factor in producing accurate plane surfaces with a surface 

 lap. Unless the temperature of the lap and the work is the same, a 

 convex surface will usually be produced even though the lap itself is 

 an accurate plane. 



In making the blanking punch for the solderless cord tip tool, 

 it is first rough milled to within 1/64 in. of the nominal dimensions, 



