126 



PATTERN MAKING 



This method of making gear patterns, however, is expensive, 

 and is used only when many wheels are to be cast of the same 

 si/e and number of teeth from the same pattern, and, as in tin- 

 case of pulleys, the wooden pattern is still used for all special 

 sizes of gears. 



For these wooden patterns we shall now give a few hints as 

 to the best methods of construction. As the form of the tooth 

 used by the draftsman will play no part in the construction of 

 the pattern, we think it would be out of place here to enter into 

 a discussion of the relative merits of the single curve, double curve, 

 or other form of tooth. 



The single curve or involute tooth, however, has the great 

 advantage of being the only form of gear which can be run at 



FiK. 236. 



varying distances of axes, and transmit an unvarying velocity and 

 amount of power. The common contention that two gears will 

 crowd harder on their bearings when the single curve, or involute 

 form is used, has not been proven in actual practice. The practical 

 methods for obtaining the ciirves for either the involute or for the 

 epicycloidal tooth, the two forms in most common use. are taken 

 up in Mechanical Drawing. 



In the illustrations here given the single curve form of tooth 

 is used. 



In the construction of gear- wheel patterns, the methods 

 employed in making and fastening the tooth, or the blocks out of 

 which the teeth are to be formed, to the rim of the ; wheel, varies 

 greatly. It was formerly the custom to dovetail the tooth into the 

 rim of the wheel as shown in Fig. 236. This was the case especially 

 when the teeth were large, as in 2 pitch or larger. 



