64. 



PATTERN MAKING 



with core prints attached, and is external only, with nothing to 

 show the internal openings, chambers, and winding passages that 

 must be provided for by "coring." The core boxes, in which the 

 cores are to be formed, are not shown in the drawings furnished to 

 the pattern maker, but must be provided by him in correct shapes 

 and sixes, in addition to the pattern itself with its added core prints. 

 In building a pattern the workman, as before stated, must 

 allow for shrinkage. He must also allow for draft and for finish. 

 Shrinkage. The shrinkage of cast iron when cooling in tin; 

 moulds is, as has before been stated, about inch to each foot, 

 and the manner of obtaining the exact sizes for different parts of 

 the pattern has been explained under the head of "Rules" 

 (page 38). For brass or bronze castings a greater allowance must 

 be made, averaging & inch to each 12 inches. Shrinkage rules for 

 brass (-f\ inch to the foot) can be obtained, and must be used for 

 all patterns made from brass. 



Draft. After shrinkage, the second point of importance in a 

 well-made pattern is draft. By this term is meant the bevel or 



taper made on all vertical parts of the 

 pattern so that it can easily be lifted 

 from the sand without injury to the 

 mould. This is best illustrated as in 

 Fig. 134, in which it will be seen that 

 'f the diameter of a pattern at a were to 

 be the same as that at &, the latter point 

 would drag over the whole length of the 

 sand until it reached the former point. 

 As the sand is held together only very 

 lightly, this dragging would be likely to dislodge some of the 

 particles and make it necessary to mend the mould. In order to 

 avoid this, the diameter at a is made slightly greater than at J, so 

 that the body of the gland is tapering, and the moment it is started 

 out the whole surface from a to b is clear of the sand and can be 

 removed without injury thereto. This difference in the diameters 

 at a and /; is called the draft of a pattern. The amount of draft 

 depends /upon the length of the part that is to be drawn out of the 

 sand. 



Fiji. 1-'1A. 



