To illustrate more clearly some of the typical methods of brass work, let our first example be a thin flat plate with decoration in low relief on one side.
Place the pattern face down, a little below the center of flask. Sift on facing through a No. 16 sieve, then tuck, fill, and pack, as previously described. Roll over and make a joint. Now cut a half section of the main runners and risers, but do not connect them with the mold at this stage. Dust on parting material from a bag, and ram the other half of the flask just hard enough to stand handling.
Fig. 177. A - Mold for Thin Piute, B - Mold for Heavy Plate.
Separate the flask; spray the face of the mold with weak molasses water, and dust on it from a bag some finely powdered pumice stone, or any fine strong sand, and over this a little parting dust. Now replace this half over the pattern, and re-ram to the required firmness, and again separate and this time draw the pattern.
The impression of the runners and risers cut in the first half of the mold show as ridges on the second half packed, and serve as guides for cutting the runners to a full round section. Connect the gates in four places, as shown in A, Fig. 177. Skin-dry the mold and it is ready to close and pour.
Dusting fine sand on the face of the mold, then reprinting, as it is termed, ensures a very smooth, perfect mold face. Where the mold is not skin-dried, flour is dusted over the face, allowed to stand for a short time, and then blown off. This makes a good facing.
Holding Capacity (Liquid Measure)
Capaciti Weight orWater
Cutting the heavy runner over the top of the thin plate ensures a sufficient supply of clean hot metal to the gates under a large enough pressure to force the metal into every detail of the mold before it has time to chill.
In B, Fig. 177, is shown the difference in construction of the gate when a heavier piece is cast with the flask setting horizontally. The gate proper is cut in the drag, but a good feeding head is cut out of the cope side to keep the metal in the riser liquid until the casting has solidified.
For duplicating work, the sand match, oil match, or follow board are used, the same as for iron work. Fig. 178 shows a typical set of castings run from the end and made from gated patterns set in an oil match. Steady pins are placed on the gates to facilitate a clean lift.
All alloy metals, and especially zinc and tin, burn if exposed to the air while melting. To prevent this burning the brass melter endeavors to so control the draft in his furnace that all oxygen entering the gates combines with the fuel, and that the gases which may reach the metal shall contain no free oxygen. For this reason, the ordinary brass furnace is a natural-draft furnace, although a forced draft is often connected below the grates to make combustion independent of atmospheric conditions.
Fig. 178. Duplicated Gated Work.
The metal does not come in direct contact with the fuel, but is contained in fire-clay pots called crucibles, which are bedded in the fire. Hard coal or coke is used for fuel. These crucibles, Fig. 179, A, are manufactured from a very refractory fire-clay mixture, and are strong and tough, even at a high temperature. They are lifted in and out of the furnace by the tongs shown at B, Fig. 179. For the larger sizes a crane is used for hoisting the pot. Crucibles are classed by number, as seen in Table VIII. New crucibles should always be annealed before using, that is, brought very slowly to a low red heat.
Furnaces of this type are usually called brass furnaces, and may be bought on the market made up in single complete units. Fig. 180 illustrates one of a battery of several furnaces connecting with a common flue. The top is on a level with the molding floor. The sketch shows clearly the principles of construction. A cast-iron bottom plate A, with a circular opening, carries a shell of boiler plate lined with fire brick. The diameter inside the lining should be 6 inches larger than the crucible to be used. A top plate, with a similar opening, binds the whole together. On one side, below the top, the opening B, which may be formed by a cast-iron box, connects with the flue or stack. Two heavy ribs cast on the bottom plate rest on a pair of rails as shown, and these rails are supported by suitable piers of brickwork about 2 feet high, so that ashes may be conveniently removed when the furnace is dumped.
In the space mode by the ribs, between the bottom plate and the rails, the grate bars C are set. These bars are loose and may be pulled out when it is desired to dump the fire for the day.
Before starting the fire in preparing to run off a heat, a good plan is to use a half fire brick on which to rest the crucible, or the bottom of a worn-out crucible cut off to the height of 4 inches or 5 inches may be turned upside down and used for this purpose.
Sufficient time and special care should be exercised in placing the metal in a crucible. It is more or less dangerous to jam in the charges, so particular care should be taken to see that they are placed in the crucible loosely. Graphite is the crucible's principal ingredient; the only expansion possible to a crucible comes from its clay body, hence, if the charges are wedged in a crucible and jammed to fit tight, their expansion, which is much greater than the expansion of the crucible, cracks the latter before the melting point is reached. The crucible should be kept covered, especially for brass.
Fig. 180. Natural-Draft Funace.
In melting brass, melt down the copper first, then the scrap. When this is melted, charge the zinc and stir well before lifting the pot. Allow the mixture to come to the proper heat again, then pull the pot, skim off the dross, and stir in the lead if any is called for, just before pouring. In bronze the same method is pursued, but both the tin and zinc are stirred in after the pot is drawn. In mixing in the zinc in brass, care must be taken to plunge it well under the surface of the copper with long handled pick-up tongs, and to hold the piece down with the stirring bar until it has melted.
Where a large casting requires more metal than can be melted in a single crucible, several furnaces must be used and the contents of their various crucibles assembled into one large pouring ladle just before pouring.