MACHINE-SHOP PRACTICE 419 



use of grinding and cutting tools, and to assemble, repair, 

 and erect machines. The metal in its original form may be 

 a casting, a forging, or a piece of stock of indefinite form 

 from which the required object is to be shaped. Designing 

 the part often tests the ingenuity of the machinist, as he 

 must devise ways and means to perform the various opera- 

 tions so that the dimensions when finished will be accurate 

 and will correspond to those given on the blue-print. Before 

 a machinist begins to grind or cut a casting he marks out the 

 outlines, holes, and machine cuts required. This is called 

 laying-off work; the measurements are carried out by means 

 of a center punch and dividers. The casting is then at- 

 tached to the plate or table of the machine tool by means 

 of special bolts and plates. When a great many castings of 

 the same kind are to be machined special devices, called 

 jigs, are employed to hold them. A jig is a great aid to 

 quick work. 



The casting is shaped by cutting or grinding the excess 

 metal by means of various power tools. Chips may be re- 

 moved also by means of a hammer and chisel. The surface 

 is smoothed by filing. Benchwork or visework consists 

 in fitting and finishing the machine, and floorwork in as- 

 sembling the parts. Considerable skill is often required to 

 file and scrape the parts true, so that they fit perfectly when 

 the machine is finally assembled. 



480. Measurement of Work. To do good work, a ma- 

 chine must have perfectly plane surfaces, must afford a rigid 

 support to the tool, and must give an accurate motion to the 

 part to be machine'd. There must be no slackness or back- 

 lash between the parts which move over one another. All 

 edges must be truly rectangular, and there must be equal 



