PATTERNS, CORES, FLASKS, AND MOLDS 445 



514. Difficulties in Pattern-Making. For the same 

 reason that it is necessary to have a constant draw, making 

 it possible to transfer prints from one pattern to another, 

 it is necessary that the dowels by which the prints are attached 

 to the patterns should be of constant diameter, usually i 7 c 

 or J^ in. It is often possible to make one pattern serve 

 for many castings by this method. For example, certain 

 diameters may be desired with different diameters of holes 

 for the shaft. While the diameter and width of the face 

 are constant, the different diameters of the holes in the hub 

 for the shaft may be obtained by simply changing the core- 

 prints to those of the required diameter. 



For convenience in molding, as well as from necessity, 

 many patterns are made of two or more parts doweled to- 

 gether, so that the parts may be retained in their positions 

 during the process of molding. Very often these joints 

 are made where different parts of the mold separate to with- 

 draw the pattern, and are called the " parting lines." 



Owing to the difficulty of drawing some patterns from the 

 sand, it is sometimes necessary to draw them in sections. 

 To do this, one or more parts are attached to the pattern by 

 wire pins from the outside, so that the pin may be withdrawn 

 after the parts are supported in place by the sand. Such 

 parts are termed " loose pieces." 



Questions 



1. What is pattern-making? 



2. Is it a wood-working or metal-working trade? 



3. Explain the steps in making a pattern. 



4. Why is a pattern varnished with gum shellac cut with alcohol? 



5. What are core-boxes? 



6. Of what wood are patterns usually made? 



