As already explained, the pattern maker must understand working drawings in order to construct patterns from them directly. These drawings are usually made to a scale much les3 than the actual size of the required work, and always represent the completed or finished machine or one of its parts.
Drawings are made for the machine shop to guide the machinist in cutting, turning, planing, and fitting the parts given, so as to produce in the castings the shapes, sizes, and general requirements of the articles to be constructed. Hence there is less liability for mistakes after the castings reach the machinist, as he has before him not only the drawing with its accurate dimensions to work from, but also the castings for the machine or its parts, from all of which the construction and uses of these several parts can easily be understood.
On the other hand, the pattern maker, with the aid of the same drawing, must imagine the casting before him, and must build something in wood which will produce that casting in metal. This pattern, in some cases, will be a duplicate of the required casting, but more often it has only a general resemblance to it, with core prints attached, and is external only, with nothing to show the internal openings, chambers, and winding passages that must be provided for by coring. The core boxes, in which the cores are to be formed, are not shown in the drawings furnished to the pattern maker, but must be provided by him in correct shapes and sizes, in addition to the pattern itself with its added core prints.
In building a pattern the workman, as before stated, must allow for shrinkage. He must also allow for draft and for finish.
The shrinkage of cast iron when cooling in the molds is, as has before been stated, about 1/8 inch to each foot, and the manner of obtaining the exact sizes for different parts of the pattern has been explained in the section on Measuring Tools. For brass or bronze castings a greater allowance must be made, averaging 3/16 inch to each 12 inches. Shrinkage rules for brass allowing 3/16 inch to the foot can be obtained, and must be used for all patterns made from brass.
After shrinkage, the second point of importance in a well-made pattern is draft. By the term draft is meant the bevel or taper made on all vertical parts of the pattern so that it can easily be lifted from the sand without injury to the mold: This is best illustrated as in Fig. 132, in which it will be seen that if the diameter of a pattern at a were to be the same as that at 6, the latter point would drag over the whole length of the sand until it reached the former point. As the sand is held together only very lightly, this dragging would be likely to dislodge some of the particles and make it necessary to mend the mold. In order to avoid this, the diameter at a is made slightly greater than at b, so that the body of the gland is tapering, and the moment it is started out the whole surface from a to 6 is clear of the sand and can be removed without injury thereto. This difference in the diameters at a and b is called the draft of a pattern.
The amount of draft depends upon the length of the part that is to be drawn out of the sand. The allowance for draft, which varies with the pattern, is often greater or less on different parts of the same pattern. For example, the draft on the outside of the pattern of a pulley rim, 24 inches in diameter and 6 inches face, should be 1/8 inch to the foot, while on the inside of the rim and on the hub of the pulley it should be in the ratio of 3/8 inch to the foot. The reason for this difference is that the face of the rim is often turned and finished straight, and for that reason the least possible amount of draft which will allow of the pattern being removed from the sand should be used, while on the inside of the rim a greater amount of draft is necessary if the inside of the pulley mold is to be coped to the center of the arms. This mass of sand hanging from the cope is lifted or drawn from the pattern with the cope flask, and will require a greater draft to be sure of obtaining a perfect lift.
To obtain any required amount of draft correctly, a draft template, kept with other tools and templates, will be found convenient and useful, saving much time when changing from one ratio of draft or bevel to another. It is made as follows:
Take any straight-grained board 14 inches to 16 inches long and 12 1/4 inches wide, as shown in Fig. 133. Having jointed the edge a perfectly straight, draw the line b perpendicular to the edge and 12 inches long, using a steel square and a sharp-pointed knife - not a scratch awl or a lead pencil. On the edge a carefully measure 1/4 inch on each side of b; and at the upper extremity, with the same care, measure 3/8 inch on each side of b; connect the last two points thus found with the first two on the edge a by a sharp knife line, and the result will be a right and a left slanting line, each having, with reference to the perpendicular, a slant of 1/3 inch to a foot. These lines should each be marked " 1/8 inch", as shown in the drawing.
Now draw a second perpendicular c, at a distance of 1 1/2 inches or 2 inches from the first. On the edge of the board a again carefully mark off \ inch on each side; at the other extreme mark off 7/16 inch on each side of c, and again connect the latter points with the former. The result will be a taper of 3/16 inch to a foot. Again repeat the process, making the taper 1/4 inch, and lastly 3/8 inch, to a foot. Mark the pairs of right- and left-hand tapers, " 1/8 inch", "3/16 inch", "1/4 inch", "3/8 inch", respectively, as shown. These lines having been obtained permanently, the width of the board may be cut down from 12 1/4 inches to 6 inches, as shown by the dotted line A B, and the board then shellacked.
To use this template, place the bevel against the edge a of the board, and carefully adjust the blade to the 1/3-inch, 3/16-inch, or other draft, right or left, as may be required. It will readily be seen that whatever may be the width of the surface to which the bevel is applied, the taper or draft will be the exact proportion of the given amount for each 12 inches.
In pattern making, the term finish refers to the additional thickness besides shrinkage and draft, which must be given the pattern in places where the casting is to be planed, turned, chipped and filed, or fitted, in the machine shop. The amount that is to be so added is, to a certain extent, though not wholly, independent of the size of the piece. For small articles whose longest dimension does not exceed 3 or 4 feet, an addition of 1/8 inch to the surface to be finished is usually sufficient. For larger dimensions it may be necessary to add as much as 1/4 inch, or 3/8 inch, but very rarely more than this. In making this allowance it is also well to bear in mind the tendency of the casting to warp in cooling. Where the thickness of the metal varies to any great extent, there is a greater liability to warp than if a uniform thickness prevails throughout the whole. Hence, in such cases, a greater allowance must be made for the finishing.
Fig. 131. Pattern for Plain Bur.
On small pieces, and where the molding is carefully done, it may be possible to make as small an allowance as 1/16 inch, but as a general rule sufficient metal should be put upon the casting to allow the cutting tool of the finishing machine to cut well below the surface so that it shall not become dulled by the sand and the hard scale on the outside.
A pattern for the plain cast-iron bar illustrated in Fig. 134 will afford a good example of the allowance necessary for finish and for draft.
This bar is to be finished all over, the finished sizes being 36 inches long, 1 inch wide, and 1 inch thick. A slender bar of this length is liable to warp or bend when cooling in the mold, and for this reason the bar should have an allowance of at least J inch all over for finish, thus requiring a pattern 36 1/4 inches long, 1 1/4 inches wide, and 1 1/4 inches thick - actual dimensions. Moreover, to enable the molder to remove the pattern from the sand without injury to the mold, we must add on two of the opposite sides a draft of about 1/4 inch to the foot, making a cross-section through the pattern of the shape and dimensions shown in Fig. 135.
When accuracy is required in testing bars - 1 by 1 by 36 inches, and seldom finished - they are often molded partly in the cope and partly in the drag, as shown in Fig. 136, the parting being on the line ab. In this position the inclination of the sides of the pattern in the mold is so great that no draft is required, the pattern being simply a square bar of wood of dimensions 1 by 1 by 36 inches, measured with the shrinkage rule.