This section is from the book "Amateur Work Magazine Vol3". Also available from Amazon: Amateur Work.
An iron casting is always somewhat smaller than the mould in which it is made, and consequently smaller than the pattern. This is true of castings made from any of the common metals. Shrinkage of the metals when cooling, causes this difference in size. The amount of skrinkage will vary with the shape and size of the casting, and also with the kind of metal employed. The following allowances for shrinkage are made by pattern makers for ordinary purposes : Cast Iron, 1/8" to 12". Aluminum, 1/4" to 12".
Yellow Brass, 7/32" to 12". Lead, 1/4" to 12.
If the surface of a casting is to be cut away or finished, an allowance equal to the amount removed is always added to the pattern, and is called an allowance for Finish. The allowance must always be great enough to allow for removing all scale and have the clean metal exposed. For ordinary small patterns 1/16" to 1/8" should be allowed, over and above the finished dimensions. As the size of the casting increases, the surface is liable to become rough and uneven, its irregularities and the total amount of shrinkage increase, and so a greater allowance for finish must be made. To avoid the necessity of making frequent calculations to determine the allowance for shrinkage and finish, pattern makers use a rule called a Shrink Rule. This rule is 24 1/4" long, standard measure, and is divided in the same manner as the standard rule into inches and subdivisions of an inch.
For our exercise we will make a pattern for a ribbed plate as shown in Fig. 8. The right end view of the plate is shown in section and indicates that two blocks are to be used for this pattern. The top surface of the pattern is marked a and the arrow indicates the direction in which the pattern is to be drawn from the mould. It will be noticed that the top surface is marked with an f. This is to be always understood as indicating that the surface upon which this mark is placed is to be finished in the machine shop.
Fig. 8. Ribbed Plate.
The dimensions in Fig. 8 indicate the size of the finished casting. If you have carefully studied the meaning of draft and finish, as applied to patterns, you will have no difficulty in making the necessary allowances. The planer blade can be set over from the centre so that the draft can easily be planed on the sides of the blocks. If you are without a block plane, a paring chisel or a wood rasp may be used on the ends. In any case, make sure that the taper is straight and in the right direction. If the taper is put in the wrong direction the pattern is said to have back draft, and if there is no taper there is said to be absence of draft. The tongue C, Fig. 8, is glued and nailed to the plate with four 1 ±" No. 16 wire brads.
Glue naturally suggests itself as an indispensible means of attachment. There is, however, a decided objection to its use in pattern work, as it does not stand contact with the damp sand. If the joint does hold, the glue will ooze out all round, and will, unless carefully removed, cause a hard line of sand to stick to the pattern, thus spoiling what would otherwise be a good mould. Much depends on this glue in cases when patterns cannot be nailed or screwed, and it becomes absolutely necessary that only first class glue be employed. Among the many qualities of glue in the market are liquid, pulverized, and sheet glue. The liquid glue is good in quality and very handy for small light work, as it is always ready for use. The sheet or flake form dissolved and used hot is, however, preferred for general work.
Animal glue is generally considered the best, it comes in thin sheets and is the most expensive. As a rule, the best quality of glue is amber in color, and the flakes are quite thin. Glue should be soaked in cold water before placing it in the glue pot, but the soaking should never be continued for any great length of time, as this injuries the quality. Glue is strongest when freshly prepared and, if of good quality, can be drawn out into very fine threads. As a rule, the harder the glue, the better it will resist moisture. In gluing two pieces together, the glue must be thin enough to spread evenly; if the surfaces to be glued can be warmed, a much better joint may be obtained. Make sure that the surfaces to be glued are wiped clean of any dust before the glue is applied. This is especially true in the case of surfaces that have been sandpapered, for in this case the dust has probably been rubbed into the pores of the wood, and so closing them to the entrance of the glue.
When the end grain of wood is to be glued, give it a heavy sizing coat first. This is done to fill the openings among the fibres. When this sizing coat has become hard, the surfaces are given a second coat of glue and clamped together. If this is not done, the open end grain will probably absorb the glue so rapidly as to seriously weaken the joint. Plenty of time should be given the glue to set; in most cases, twelve hours in a dry place is sufficient.
It will be noticed in Fig. 8, that at the joint between the blocks a sharp corner is avoided, and a rounded corner made instead. Sharp corners, whether inside or outside of a pattern, should be avoided, and whenever there is nothing to interfere, all corners should be slightly rounded. Sharp corners in a pattern will form sharp corners of sand when moulding, and these corners will give the moulder a great deal of trouble. Sharp corners, generally, not only detract greatly from the appearance of the pattern, but also injure its strength.
To overcome these difficulties, a fillet is generally placed at the sharp angle or corner, as indicated in Fig. 8. To form this fillet on the pattern for the ribbed plate, melt some beewax and run into the corner with a short piece of fairly large wire, say, 1/8" in diameter. When this wax is thoroughly hard, any excess may be scraped off and the surfaces lightly sandpapered. All wooden patterns require covering with some protective coating, to prevent warping and cracking from the influence of the moist sand in the mould, and also to prevent the glued joints from coming apart. This protective coating is not affected by moisture, and gives a smooth surface that draws easily from the sand. This will be taken up in the next chapter.