Fig. 23.

European Lathes With Reciprocal Motion From Poles  40018

Fig. 24.

European Lathes With Reciprocal Motion From Poles  40019

The straight stemmed hook tool is made both to deepen and widen the grooves, that it may be perfectly free within them.

Fig. 25.

European Lathes With Reciprocal Motion From Poles  40020

Fig. 26.

European Lathes With Reciprocal Motion From Poles  40021

Fig. 27.

European Lathes With Reciprocal Motion From Poles  40022

It is then exchanged for others, slightly curved in their stems, with which, cutting upon their ends and sides, the grooves are carried a little further and deeper, at curves corresponding to the curvatures of the different bowls; these tools in like manner are then exchanged for others with shanks still more curved, until by such continued succession, the curved grooves are carried down to the required depths and shapes.

During these operations, the tools are embraced on all sides within the grooves, their curved stems and the increasing depth, completely concealing their cutting portions. With continuous motion in the work, as in the ordinary lathe, they would soon become fixed or clogged by the accumulation of the chips, full of sap, collecting between the sides of the grooves and the tools, which would render them unmanageable. It is in this respect that the intermittent motion of the pole lathe is valuable, the tool cuts only at the down stroke, while the up stroke, which is against the tool, each time clears it from the chips, which come tumbling out of the groove above it. The grooves completed, the block fig. 25, is removed from the lathe and the different bowls are detached, by being forcibly split out from each other, the fractures being made at the base of each and in the direction of the grain. The under surfaces are turned flat, and some precautions are taken, that the bowls so far prepared green, shall not dry out of shape; occasionally they undergo subsequent finishing.

The smallest of all turning apparatus is to be found in the hands of the watch and clock makers, with whom the bow lathe has never been altogether superseded, and it is extremely well suited to their purposes. Many of these miniature lathes are used only for specific work, from which they derive their names, such as Balance turn, Pivot turn, Screw head tool, and others; fig. 28, the turn, or turn bench, is the most generally useful. All are made entirely in metal and are used held in a table vice; the bow is a light elastic cane or piece of whalebone with a fine catgut, or for delicate work, a single horsehair, the bow, then being also correspondingly slender. The graver, almost the only tool used, is held obliquely to the axis of the work, its chamfer upwards, the work is turned with the extreme point of the tool and is smoothed with one of its side edges; this being almost the only means used by the watchmaker to turn the most delicate hardened steel pivots, some of which, when turned, polished and burnished measure less than the three-hundredth part of an inch in diameter.

The turn bench fig. 28, which measures about nine inches in length, is suitable both to the watch and clockmaker for the larger and smaller parts of their respective works. The turn used for the delicate axes, above referred to, is less than a third this length and proportionately slighter; the bar and the heads are formed of one solid piece, the only movable parts being the cylindrical brass axes neither of which is screwed, and the rest for the tool.

All modes of turning with alternating motion, are alike subject to the same disadvantage, viz., that the tool can only cut whilst the work is moving towards it; the tool is not only inoperative during one half the time spent upon the work, but it is also necessary to withdraw it just out of contact during the opposite retrograde motion, to avoid the destruction of the cutting edge from friction. The tool receiving a restless backward and forward motion simultaneous with every change of direction. Though necessary for some purposes, among others for works in which some projecting stud or other part stands in the path of the tool, and still exclusively employed in many parts of the world; the reciprocating and the pole lathes are rapidly falling into disuse in Europe; nevertheless independently of their simplicity which has a certain merit, the pole lathe has some peculiar advantages.

Fig. 28.

European Lathes With Reciprocal Motion From Poles  40023

The cord being wrapped around each piece of work as it is turned, the speed required for every diameter is adjusted without any attention on the part of the workman; the surface velocity being always equal to the rapidity of the foot. Less speed being required for turning metal, this may be arrived at by attaching a pulley on the work fig. 28, which at the same time affords the cord greater purchase or leverage ; and if the pulley be chosen four, six, or any number of times the diameter of the work, the relations of velocity and power may be variously adjusted. There is also an advantage in the two ends of the cord being carried away in opposite directions; their action being opposite and equal, they counteract each other and there is little bending pressure exerted on the work. The friction of the pole lathe is also scarcely more than that inseparable from every cutting process; the resistance offered by the shaving, to its removal from the material.

A method of imparting a continuous direction to the rotation of the work, by the single cord of the pole lathe, fig. 29, taken from a model in the Museum of the late East India Company, may be considered as of interest and to some extent as a link between the reciprocating and the rotatory motion. The cord from the pole is first passed under the work, around a pulley overhead, and then hitched around the work before proceeding to the treadle below. On depressing the treadle, the half of the cord hitched around the work acts precisely as in the ordinary pole lathe, but also, overcoming the friction of the other half of the cord, half of which is then travelling in the opposite direction, bends the pole; the pressure being relieved from the treadle, the entire half of the cord attached to the pole, continues the revolution of the work in the same direction, while the opposite half of the cord then slips around the cylinder. Both halves of the cord hitched around the work and pulled alternately by the hands, fig. 30, will in like manner cause a continuous direction in the revolution.

Fig. 29.

European Lathes With Reciprocal Motion From Poles  40024

Fig. 30.

European Lathes With Reciprocal Motion From Poles  40025

Little advantage is derivable from this arrangement; the motion although always the same in direction is intermittent, the work suffering a momentary check at its dead points at the termination of every ascent and descent of the treadle; while the power communicated at the ascent and descent is unequal. The re-duplication of the cord, both halves of which should also work upon similar diameters, is inconvenient, and the arrangement, which is more curious than practical, does not appear to have come into use.