Fig. 227.
When the part shown has been cut this tool must be removed and a similar tool bevelled in the reverse direction, adjusted by reference to the central line of the ball as before.[17] It is recommended to roughly shape the work with the gouge, and partially to cut it off with the parting tool so as to relieve the tool as much as possible, and when the last finishing cut is to be taken a freshly sharpened tool is to be made use of. It is evident that in the above and similar work the rest may be placed across the end of the cylinder if preferred to turn the right hand hemisphere, but it would have to be moved for the second half, which should be avoided, if possible. The advantage which the circular rest has over the above is due to the fact that the tool and rest once in position, neither has to be readjusted until the work is complete. The slide rest and semicircular template forms, however, if judiciously used, a very serviceable substitute and makes very satisfactory work. Whether or no the reader has a complete rest for spherical work, he should decidedly provide templates to use as above. They are not only useful for turning or ornamenting spheres, but any forms whatever that may be desired, and they possess this special advantage, that when a dome or other pattern has been thus turned with a plain tool the same template used with revolving cutters will enable the work to be ornamented with perfect ease, doing away in a great measure with the need of a dome chuck. Suppose, again, that a number of pieces are desired precisely similar, as a set of pawns for a set of chessmen, a sectional drawing made and transferred to a piece of sheet-iron, and the latter cut to form a pattern plate, will enable the most unskilful to work satisfactorily. Nothing more need be said of the uses of templates, and for the present the subject will be dismissed, though it may possibly be referred to again in a future page.
[17] Holtzapffel uses a tool, the plan of which is semicircular, like a small round tool, cutting on front and two side edges; the tool is very narrow and bevelled below.
[Chucks with Slides and Compound Movements.]
The first of the chucks comprised under the above head is the oval or elliptical chuck, and it is introduced first in order because it is not essentially a machine for ornamental turning, as are the eccentric and others of this class. There are many plain works required of elliptical section, as bradawl and other tool handles, for which a very simple arrangement is required.
The principle of the oval chuck is as follows:—There is an arrangement of slide, by which as the piece revolves it is drawn gradually further from the tool during half a revolution, and in a similar manner caused to approach it during the remaining half revolution, each point in the circumference alternately partaking of such movement; the whole of these points together, which, of course, form the circumference, will become an ellipse. Let D, [Fig. 228], be the centre of the mandrel, A, B the direction of the slide moving up and down in a right line, and carrying the work upon a screw in the centre of it. C, E become centres, and may be taken as the extremes, for as the work revolves a succession of centres are formed and instantaneously changed. The figure produced will be the oval shown. To render this, however, clearer, [Fig. 229] may be taken, which represents the chuck in its most simple form with separate details of the parts which compose it. A is the chuck with central slider and chamfered bars, as described in speaking of the slide and rest and previous apparatus; B is the slide detached; D, front view of the same. The short arms a, b, pass through slots in the back plate as seen at C, which shows this plate with slide removed. Through these short arms pass a pair of adjusting screws; or still better a and b are themselves cross arms or pallets extending the width of the plate as seen next drawing, and in the chuck of Muir which follows. They are merely flat plates of steel embracing the guide ring, so that some point in their inner surface may rest against it during every part of the revolution of the chuck.
