9. - Large-sized Flat Works. - These are in almost every case, castings in iron, wrought in the planing machine; - a machine that produces its results with so much accuracy and precision, that polishing is not frequently required as the concluding step. When however large planed works are polished, it is with rubbers of various kinds applied with emery and oil. Sometimes a flat lump of lead is cast upon the center of an old file, or of a still longer bar of iron; at other times a bar of wood serves as the handle, and to it is fixed by screws or nails a piece of lead or wood, or wood covered with thick leather; such rubbers are generally held in the two hands much after the manner of the spokeshave or drawknife, or they are worked by one very long handle as in smoothing a large slab of stone or marble. When the rubbers are large they are occasionally loaded with heavy weights, so that the workmen have only to drag them to and fro on the works, the forms of which latter are in general too diversified to offer much inducement to the application of machinery to rectilinear polishing.
10. - Medium-sized Flat Works. - Such of these as are of cast-iron, are also for the most part worked in the planing machine, and if at all polished, it is done with emery rubbers nearly or precisely as above described; most of the flat parts of mechanism that are made in wrought iron and steel, are too irregular in their forms to admit of being worked otherwise than with the file. The black oxidized surfaces of forged works are often removed on the grindstone prior to the application of the file; this application of the grindstone is in general highly economical, it being comparatively, much more rapid in its action, and less costly in respect to wear and tear than the file.
Sometimes, indeed, the flat parts of iron works are reduced on the grindstone to accurate plane surfaces, but this requires the assistance of mechanism, which is by no means common; this subject, and also the application of revolving metallic laps to the production of flat works, will be noticed in the first section of Chap. XXXIII.
The coarser and larger of the filed works are sometimes left from the file, or without being subsequently polished; in which case the coarser marks left from the file when used in the customary manner, or from point to heel, are removed by the method known as draw-filmg, in which the file is drawn sideways along the work; draw-filing is particularly employed in narrow pieces. Large broad surfaces are occasionally finished by giving a circulating motion to the file, thereby producing curly marks. Each of these latter processes are more effectual when the file is moderately supplied with oil, which lessens its disposition to become pinny, or clogged, by particles which stick into it, and scratch the work; but the reader is referred to the previous chapter on the File, vol. ii., page 852, for more detailed particulars of these applications of this useful instrument.
Works requiring a finish superior to that of draw-filing, are rubbed with an emery stick, or with rubbers of the various kinds already noticed, and supplied with emery and oil.
11. - Small-sized Flat Works, after having been draw-filed, are more usually finished with the emery stick, and often followed by emery paper of different degrees of coarseness wrapped on a file or a square stick. The emery is moistened with oil for the more finished works, the dry rubber gives however the brighter surface, and it is sometimes applied with a curling motion, so as to diversify the grain left on the work.
Buff sticks supplied with crocus are often used for the last gloss, but on small flat surfaces they must be cautiously applied for fear of rounding them, a defect that is easily distinguished, and very objectionable.
Still smaller works and those required to be very flat are finished with square slips of stone with oil, or slips of mahogany, brass or tin, any of which are used with fine flour emery or oilstone- powder and then with crocus.
12. - Small Flat Works of Hardened Steel. - As it commonly happens that in the process of hardening steel works they are more or less distorted from their intended figures, and as in many cases it is impossible or inadmissible to restore them to the plane figure by the hack hammer, (see vol. 1, p. 247,) grinding is then resorted to, metal laps generally of lead with a little antimony, and laps of copper or of cast-iron are also employed with emery and water. When it is desired the works should present very true plane surfaces, the laps should be themselves very exact and flat. There is however a constant tendency to depreciate the figure of the lap, because the outer part or exterior diameter gets the more worn, on account of the greater rapidity of its action at that part. After the lap has been used, the mode of finishing described in the last article is also sometimes employed.
13. - Watchworks in Steel. - Steel works of this diminutive kind are generally polished by the watchmakers, 1st with a steel rubber and oilstone powder, 2ndly, with a steel rubber and crocus of two degrees of coarseness, which is frequently called red stuff from its colour, and 3rdly with gun metal or glass rubbers supplied with fine crocus. Some of the work is beautifully finished on tin or pewter revolving laps, into which the red stuff is embedded, occasionally with the burnisher, they are used nearly or quite dry, and when the laps are carefully preserved, they themselves present, under the magnifier, a beautiful smooth surface and which they impart to the work.
Many of the grinders and rubbers for watchwork, are made from one to two inches square, and of steel, glass, gun-metal, tortoiseshell, horn, or ivory, etc, the small pieces are laid down upon the anointed grinders, and rubbed about with the fingers, as if the work were a muller used in grinding paint, this mode also preserves the flatness of the respective objects in a most admirable manner.