Left fore-hoof sharp-shod: a, toe-calk bevelled from in front; b, outer heel-calk directed lengthwise with the branch; c, inner heel-calk, half sharp and directed transversely to direction of the branch.
Hoofs are easily damaged or even ruined by frequently repeated sharpening of the shoes, because every time this is done the shoes must be removed and replaced.
3. Shoeing with Screw Heel-Calks.—Any ordinary flat shoe not too thin 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.
Fig. 162.
Ground-surface of the end of
a branch of shoe, showing (a)
hole and counter-sinking
for a screw-calk
(one-half natural size).
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.
The chief requirements of a good screw-calk are, further, a clean, deep, but not too coarse thread, and but one size of thread and tap for all calks, so that every calk will fit in every shoe. A calk whose tap measures one half-inch (12.7 millimetres) (Whitworth) in diameter is sufficient for the heaviest shoes. The tap which is used to cut the thread in the holes for the screw-calks must be about ¹/₁₂₅ of an inch thicker than the head of the calk. In the German army the 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 removing them. Screw toe-calks are also used, yet they require special security to prevent their becoming loose. Experimentation with the screw toe-calks, though not yet entirely satisfactory, cannot be said to have ended.