148 HORSESHOEING. 



3. Shoeing with Screw Heel-Calks. — Any ordinary flat 

 shoe not too tliin and narrow at the ends of the branches can 

 be changed to a shoe with screw heel-calks by punching holes 

 in the ends of the branches and cutting a thread in them. 



The screw heel-calk holes are made either by punching or 

 boring. The punching is done by means of an almost cylindrical 

 hammer-punch, afterwards finishing the holes by driving 

 through them a round punch which tapers from the middle 

 towards both ends. On the ground-surface of 

 ^^' ■ the shoe the hole is moderately counter-sunk 

 (Fig. 162, a,), so that after the thread has 

 been cut and the calk screwed into place the 

 shoulder of the latter will rest on the counter- 

 sinking. 



At present nearly all screw-calks are made 

 by machinery, either of iron or toe-steel. The 

 former is too soft and therefore not sufficiently 

 durable; the latter, however, is quite durable 

 when the calk is properly hardened (tempered) 

 by heating to a cherry-red, sticking the head of 

 the calk as far as the tap into a bed of moist 

 sand, and allowing it to slowly cool. 

 Ground-surface of The chief requirements of a good screw- 



the end of a branch n /. i i 



of shoe, showing calk are, further, a clean, deep, but not too 

 ter-sinidng'^ for^'a coarsc thread, and but 07i€ size of thread and 

 hlirrturai S: i'^P f^"- «^^ ^^^^^^ «o that every calk will fit in 

 every shoe. A calk whose tap measures one- 

 half inch (12.7 millimetres) (Whitworth) in diameter is sirfS- 

 cient for the heaviest shoes. The tap which is used to cut the 

 thread in the holes for the screw-calks must be about y|-g- of an 

 inch thicker than the head of the calk. In the German army 

 tbe calks have a tap fifteen thirty-seconds of an inch in diameter. 

 The coachman should be given four calks (sharp and blunt) 

 for each shoe, and a small screw-calk key for placing and remov- 

 ing them. Screw toe-calks are also used, yet they require 



