436 APPLIED SCIENCE 



finish soft pieces also by this method. This machine is 

 similar to a lathe, with the exception that instead of a tool 

 for removing the stock, a revolving wheel made of abrasive 

 material is employed. 



A surface grinder is a modification of the planer, but uses 

 a revolving, abrasive wheel instead of a tool for removing 

 stock from flat surfaces. 



504. Power Hammer. Power hammers may be divided 

 into three classes: (1) trip hammers, (2) friction board or 

 drop hammers, and (3) steam hammers. In all classes, 

 dies are used to shape or form the hot metal. Trip hammers 

 are used for light work, drop hammers for work of medium 

 weight, and steam hammers for heavy work. 



505. The Vernier and its Use. An important measur- 

 ing instrument used in machine operation is the vernier (Fig. 

 201), so called from the inventor's name. 



It consists of a bar of metal divided into inches, each inch being 

 again divided into ten parts, and each tenth into four parts, making 

 forty parts to the inch. On the sliding jaw is a line of division 

 called the vernier which consists of 25 parts, numbered 0, 5, 10, 15, 

 25. The 25 parts on the vernier correspond, in extreme length, 

 with 24 parts or -|-J of an inch on the bar. Consequently each 

 division on the vernier is smaller than each division on the bar by 

 one-thousandth of an inch. 



If the sliding jaw of the caliper is pushed along the other, so that 

 the line marked on the vernier corresponds with that marked 

 on the bar, then the two next lines to the right will differ from each 

 other by one-thousandth of an inch. The difference continues to 

 increase one-thousandth of an inch for each division, till they again 

 correspond at the line marked 25 on the vernier. To read the dis- 

 tance when the caliper is open, we begin by noticing how many 

 inches, tenths, and parts of tenths the zero point on the vernier 



