MACHINE-SHOP PRACTICE 



439 



FIG. 204. de-sinking. 



times work is required to be exact to the ten-thousandth 

 part of an inch. When such accuracy is expected the work 

 is ground to size on a universal grinder, 

 which grinds either cylindrical or plane 

 surfaces. 



Drop forgings, in which a form is 

 made to conform to the desired shape, 

 are made by means of dies or blocks 

 of steel. 



The work of making the dies is 

 called die-sinking (Fig. 204). As it is 

 necessary to work on the metal by 

 various means, making allowance for 

 shrinkage and the flow of metal, a 

 considerable amount of skill is required to work out these 

 forms and finish them so that the product will be properly 

 made. 



Questions 



1. Explain the different steps in finishing and machining 

 castings. 



2. How are the castings made to exact size? 



3. What is high-speed steel? Why is it very useful in the 

 machinists' trade? 



4. What is chipping and filing? 



5. What is the meaning of " bench work " in the machine-shop? 



6. Is it necessary to use mathematical instruments in doing 

 machine-work? Why? 



7. Describe a micrometer. How is it used? Does the heat of 

 the hand affect it? 



8. Name some machine tools built on the principle of rotary 

 motion; reciprocating motion. 



9. Which is the most economical tool, so far as the energy put 

 into it and the useful work it will do is concerned? 



