306 



APPLIED SCIENCE 



specifications in thickness. They are then placed on a bench 

 and laid out, " squared up" on the edges, and the location of 

 every rivet hole, nozzle, etc., is marked off with a soapstone 

 pencil. The rivet holes are punched J4 m - I GSS than their 

 finished size, then reamed full size, after which the plates arc 

 brought to a planing machine and planed to the exact size 

 on the edges. Edges that are to be calked (pressed to- 

 gether by a compressed air hammer) are beveled (inclined 

 to an angle other than 90), while the others are planed at 

 right angles to the surface of the plate. 



The cylindrical shell of a boiler retains its shape without 

 the need of a brace or support for the very simple reason that 

 the internal pessure tends to keep it cylindrical. On the 

 other hand, this internal pressure has a constant tendency 

 to force or " bulge" out the flat surface of the heads of the 

 boiler which in consequence are reinforced by means of the 

 diagonal brace and stay-rods. The brace is used for low pres- 

 sure, and the stay-rods for high pressure. Stay bolts extend 

 from head to head. These bolts are often broken by the 



; unequal expansion of the 



fire-box and outer shell. 



7 



(Copyrighted by Millers Fall Cos.) 



FIG. 160. Lap Joint. 



347. Joints of a Boil- 

 er. It is very important 

 that a boiler should safely 

 withstand the pressure of 

 steam for which it has 

 been constructed. Though 

 the tensile strength of the 



boiler plate is marked on it, it is necessa'ry to .test it when 

 the boiler is completed. When a rivet hole is punched, the 

 plate is weakened proportionally because a quantity of 



