When the work is held in the vice during the burnishing, the burnisher is held by the tip between the finger and thumb of the left hand, which as a fulcrum rests quiescent on the vice chop, and its edge is forcibly rubbed backwards and forwards over a small space, by the right hand holding the handle. A rather considerable pressure is given, but as the round edge of the tool, the portion principally used, meets and acts only upon an exceedingly narrow line upon the rounded surface of the work, no more than is necessary should be employed; neither should the burnishing be continued on any spot, for a time longer than the few strokes which suffice to produce the effect, after which the friction is deleterious. The contact is then made to fall on the right and on the left of the first line, in which manner a narrow space not exceeding about half an inch in length may be gradually completed. The hands are then slightly shifted, to burnish similar portions lying on either side and merging into the first, until the width is attained, after which the work is shifted round in the vice, and the operation repeated on fresh portions joining the first. With works burnished in revolution, the left hand is placed on the tee of the hand rest, which is fixed close to the work. The pressure as before is given by the right hand, the contact of the edge of the burnisher being gradually and slowly continued all over the space under operation, commencing about the center of the width, and proceeding to its margins, first in the one direction and then in the other; the entire width being thus completed as a series of narrow circular lines in juxtaposition, merging into each other.

To prevent abrasion, the work and the rounded edge of the burnisher are plentifully moistened with clean oil, which soon becomes blackened by the friction; they are both also frequently wiped with clean rag to remove the used oil, and, as a precaution against the possible intrusion of a particle of the metal or other extraneous substance between the two, which would leave scratches. The burnisher is also rubbed from time to time on the buff stick, to keep it clean and in condition, being always again wiped clean and oiled, before it is returned to the work. Usually, the work is entirely burnished over a second time, but with less pressure and oil; some then prefer to moisten the burnisher with the tongue.

The surfaces of brass and gunmetal work are finished with rotten stone and oil in place of crocus, and then with washed whiting (see Vol. III. page 1101), applied dry with the buff stick; the process is otherwise the same, except that far less pressure is required, and the burnisher is moistened with water with or without the addition of a little vinegar.

The cast iron trumpet mould, fig. 489, is one of the larger examples of curves in metal, that are necessarily turned by hand. The moulds for the bell of the instrument vary from about three to eighteen inches in diameter at the base, and are from about six to eighteen inches in height; they are provided with a square projection at the lower end, to be held in the vice or inserted in a hole in the work bench. The conical brass or silver tube for the bell of the horn is gradually and equally beaten out, with round faced hammers and wooden mallets, upon a succession of moulds gradually increasing in size; the thin edge being finally turned over and strengthened by enclosing a wire ring. The prolongation of the bell into the taper tube of the horn, the two parts being soldered together, is beaten upon taper or curved triblets of appropriate sizes; the angles at the end of the mould and those of the triblet being slightly rounded, that they may not indent the tube. The various curves for the moulds for different instruments, also vary somewhat with the views of different makers, but their exact and regular curvature is essential. The mould is first turned, most conveniently with the heel tool, figs. 415-17 Vol. II., to the shape of a wooden template, which is frequently applied to the work its edge smeared with powdered red chalk mixed with oil, to mark the high points of the curve that require reduction. It is then turned a second time and very exactly, with the heel tool and ordinary round tools, to the gage of a thin sheet metal template accurately filed to the shape of the curve. Subsequently to this, the entire surface is carefully filed smooth with a crossing file, applied without rotation of the work, removing all marks of the turning tool; after which the mould is finally polished.

The tubes of the more exactly constructed silver flutes are drawn upon steel triblets, which may be cited as difficult specimens of hand turning. The sides of some of these taper triblets are formed as very long, flat curves, often varying but a few thousandths of an inch from the straight line ; in some extreme cases, the curve rising to no greater extent than one single thousandth of an inch in the diameter, its rise and return extending over half an inch in length. The exact proportions of so slight a curve, and the exact length it covers in gradually merging again into the straight or other line, require the most minute attention in turning and finishing the triblet, and are stated to materially affect the tone of the instrument.

Fig. 491. Fig. 492. Fig. 493.

Metal Cones Cylindrical And Surface Curves Continu 400312

The turning tool is employed to obtain similarity of shape, in the two curved sides of various forms, having flat and other sections. The key fig. 491, which serves as its own carrier, is mounted between centers, and the graver held very firmly underhand is cautiously advanced to turn a series of nicks at short distances all along its contour; these equally indenting the edges, serve as the guide in subsequently filing the two sides alike, during which process the work is held in the vice. The small round end used as a lever, is turned taper after the edges have been marked out with the graver. The forms of many such objects, in which the sides are required alike, and true with other turned portions, of which the frame of the diestock, fig. 488, the carrier, fig. 492, and the oval heads of thumbscrews are instances, are usually shaped in this manner.

The curved faces and contiguous portions of many hammer heads and other tools, are also turned; while some, such as fig. 499, a steel crushing hammer, an ellipsoid modified in shape from the tooth of the hyaena, and used by geologists, are turned all over the entire surface. When the pane of the hammer does not lie across the axis of the head, the center to turn the face, is afforded by an L shaped piece of metal, after the method fig. 228; the shorter limb is placed through the eye, and the longer, the end of which carries the center, is gripped together with the pane in the carrier. The brass plumb-bob, figs. 496. 497, is of double curvature, and is shaped with flat and round tools, applied radially around its curves ; it presents no peculiarity as a specimen of metal turning, but is noteworthy from being made in two halves which unscrew to contain the point, to enable it to be carried in the pocket.