Modern Machine-Shop Practice Part 205

Fig. 2771 represents the half check joint, and it is obvious that the thickness at A must equal that at H, and be half that at B, which will give each half equal strength.

A gland for an engine piston rod forms a simple example of the different ways in which a pattern may be formed. Fig. 2772 represents the drawing for the gland.[46]

[46] From the "Pattern Maker's a.s.sistant."

"Let us suppose the pattern-maker to be uninformed of the purpose the casting is to serve, or how it is to be treated: in such a case he is guided partly by his knowledge of the use of such patterns, and a consideration of being on the safe side. The form shown in Fig. 2773 would suggest itself as being a very ready method of making the pattern; by coring out the hole, it can be made parallel, which the drawing seems to require. The advantage of leaving the hole parallel is that less metal will require to be left for boring in case it should be necessary; because, if the hole is made taper, the largest end of the bore will require to have the proper amount of allowance to leave metal sufficient to allow the hole to be bored out true, and the smaller end would, therefore, have more than the necessary amount; while just the least taper given to the exterior would enable the moulder to withdraw the pattern from the mould. Made in this way, it would be moulded as shown in Fig. 2774, with the f.l.a.n.g.e uppermost, because almost the whole of the pattern would be imbedded in the lower part of the flask, the top core print being all that would be contained in the cope; and even this may be omitted if the hole requires to be bored, since the lower core print will hold the core sufficiently secure in small work, unless the core is required to be very true. The parting of the mould (at C D, Fig. 2774) being level with the top face of the f.l.a.n.g.e, much taper should be given to the top print (as shown in Fig. 2773), so that the cope may be lifted off easily. Were this, however, the only reason, we might make the top print like the bottom one, providing we left it on loose, or made it part from the pattern and adjust to its place on the pattern by a taper pin; but another advantage is gained by well tapering the top print, in that it necessitates the tapering of the core print at that end; so that, when the two parts of the mould are being put together, that is to say, when the cope is being put in place, if the core has not been placed quite upright, its tapered end may still arrive and adjust itself in the conical impression, and thus correct any slight error of position of the core. The size of the core print should be, at the part next the pattern, the size of the core required; for if the extremities are made of the size of the core, and the taper or draft is in excess, there will be left a useless s.p.a.ce around the core print, as shown at A B in Fig.

2774, into which s.p.a.ce the metal will flow, producing on the casting, around the hole and projecting from the end face, a useless web, which is called a fin, which will of course require to be dressed off the casting.

[Ill.u.s.tration: Fig. 2772.]

"We will now suppose that our piece, when cast, is to be turned under the f.l.a.n.g.e and along the outside of the hub or body, and that the hole also is to be bored. In this case the pattern made as above would still be good, but could be much more easily made and moulded if it has to leave its own core, its shape being as shown in Fig. 2775; because the trouble of making a core is obviated, and the core is sure to be in the centre of the casting, which it seldom is when a core is used. We must, however, allow more taper or draft to a hole in a pattern than is necessary on the outside; about one-sixteenth inch on the diameter for every inch of height on work of moderate size is sufficient. The allowance for boring should be one-sixteenth inch at the large end of the hole, provided the diameter of the hole is not more than five or six inches, slightly exceeding this amount as the diameter increases; whereas, if the pattern had been made with core prints, an allowance of one-eighth inch for small, and three-sixteenths inch for larger work would be required. These are the advantages due to making the pattern leave its own core. We have still to bear in mind, however, that, if the casting require a parallel hole, a core must be used; and furthermore, if the hole is a long one, we have the following considerations: The separate dry sand core is stronger, and therefore better adapted to cases where the length of the hole greatly exceeds the diameter. Then again, if the hole require to be bored parallel, it can be more readily done if the hole is cast parallel, because there will be less metal to cut out. The casting also will be lighter, entailing less cost, provided it has to be paid for by the pound, as is usually the case. The moulder is given more work by making the core; but the saving in metal, and in turning, more than compensates for this, provided the length of the hole is greater than the diameter of the bore.

"Let it now be required that the casting is to be finished all over. It would, in that case, be preferred that if the casting should contain any blow or air holes, they should not be on the outside face of the f.l.a.n.g.e, and this will necessitate that the piece be moulded the reverse way to that shown in Fig. 2773: that is to say, it must be moulded as shown in Fig. 2776, with the f.l.a.n.g.e downwards; for it may be here noted that the soundest part of a casting is always that at the bottom of the mould; and furthermore, the metal there is more dense, heavier, and stronger than it is at the top, for the reason that the air or gas, which does not escape from the mould, leaves holes in the top of the casting or as near to the top as they can, by reason of the shape of the casting, rise. The bottom metal also has the weight of the metal above it, compressing it, and making an appreciable difference in its density. It must, therefore, be remembered that faces requiring to be particularly sound should be cast downwards, or at least as near the bottom of the mould as they conveniently can. Following this principle, our gland will require to be moulded as shown in Fig. 2777, P P representing the line of the parting of the mould; so that, when the cope is lifted off, the loose hub A will rise with it, leaving the f.l.a.n.g.e imbedded in the lower half of the mould. It is evident that in this case the pattern must be made, as shown in Fig. 2776, the body and core prints being in one piece and the f.l.a.n.g.e in another, fitting easily on to a parallel part on one end, and adjoining the core print, as shown at A. For glands of moderate size, this method is usually adopted, and it answers very well for short pieces; but in cases where the length of the body approaches, say, three diameters, the horizontal position is the best, and the pattern should be made as shown in Figs. 2778, 2779, or 2780. Even in short pieces, when the internal diameter approaches that of the external, this plan is the best, because it is difficult for the moulder to tell when his core is accurately set in position.

[Ill.u.s.tration: Fig. 2780.]

"For a pattern to be moulded horizontally, Fig. 2780 shows the best style in which it can be made. Its diameters are turned parallel; the required draft is given by making the rim of the f.l.a.n.g.e a little thinner than at the hub, and by making the end faces of the hub and the core prints slightly rounding. If the hub is very small, as, say, a half-inch or less, and the f.l.a.n.g.e does not much exceed it, the pattern may be made solid, as shown in Fig. 2778; but if the hub be small and the f.l.a.n.g.e large, it should be made as shown in Fig. 2776.

[Ill.u.s.tration: _VOL. II._ =EXAMPLE IN PATTERN WORK.= _PLATE XIV._

Fig. 2773.

Fig. 2774.

Fig. 2775.

Fig. 2776.

Fig. 2777.

Fig. 2778.

Fig. 2779.]

"To construct the pattern shown in Fig. 2773, we proceed as follows: From a piece of plank we saw off a piece of wood a little larger and thicker than the required f.l.a.n.g.e, and turn it up between the lathe centres, using a pattern makers' contraction rule, which has its measurements larger than the actual standard ones in the proportion of one-eighth inch per foot: so that a foot on the contraction rule is 12-1/8 standard inches, and an inch is 1-1/96 standard inches. The reason for this is, that when the metal is poured into the mould, it is expanded by heat; and as it cools it contracts, and a casting is, therefore, when cold, always smaller than the size of the mould in which it was made. Bra.s.s castings are generally said to be smaller than the patterns in the proportion of one-eighth inch per foot, and cast-iron castings one-tenth inch per foot; and so, to avoid frequent calculations and possible errors, the contraction rule has the necessary allowance in every division of the foot and of the inch. It is not, however, to be supposed that the possession of such a rule renders it possible for the pattern-maker to discard all further considerations upon the contraction of the casting; because there are others continually stepping in. Such, for example, is the fact that the contraction will not be equal all over, but will be the greatest in those parts where the casting contains the greatest body of metal.

"In the smaller sizes of patterns, such as those of 6 inches and less in diameter, there is another and a more important matter requiring attention, which is, that after a moulder has imbedded the pattern in the sand, and has rammed the sand closely around it, it is held firmly by the sand and must be loosened before it can be extracted from the mould. To loosen it, the moulder drives into the exposed surface of the pattern a pointed piece of steel wire, which he then strikes on all sides, causing the pattern to compress the sand away from the sides of the pattern in all directions; and as a result, the mould is larger than the pattern. In many kinds of work, this fact may be and is disregarded, but where accuracy is concerned, it is of great importance, especially in the matter of our example (bra.s.ses for journals), for they can be chipped and filed to fit their places much more rapidly than they can be planed, and it is necessary to have the castings as nearly of the correct conformation as possible. In cases where it is necessary to have the castings of the correct size without any work done to them, the shake of the pattern in the sand is of the utmost importance. If it is required to cast a piece of iron 3 inches long and 1 inch square, supposing the pattern were made to correct measure by the contraction rule, the moulder, by rapping the pattern (as the loosening it in the mould is termed) would, by increasing the size of the mould above that of the pattern, cause the casting to be larger than the pattern; that is to say, it would be longer and broader, and therefore, in those two directions, considerably above the proper size, since even the pattern was too large to the amount allowed for contraction. The depth, however, would be of correct size, because the loosening process or rapping does not drive the pattern any deeper in the mould. It follows that, to obtain a casting of as nearly the correct size as possible, the pattern must be made less in width and in length than the proper size, to the amount of the rapping; and to insure that the moulder shall always put the pattern in the sand with the same side uppermost, the word "top"

should be painted on the face intended to lie uppermost in the mould.

The amount to be allowed for the rapping depends upon the size of the pattern, and somewhat upon the moulder, since some moulders rap the patterns more than others; hence, where a great number of castings of accurate size are required, it is best to have two or three castings made, and alter the pattern as the average casting indicates. For castings of about 1 inch in size, the patterns may be made 1/32 inch too narrow and the same amount too short; but for sizes above 6 inches, allowance for rapping may be disregarded.

"In patterns for small cast gears, the rapping is of the utmost consequence. Suppose, for instance, we have six rollers of 2 inches diameter requiring to be connected together by pinions, and to have contact one with the other all along the rollers; if we disregard the allowance for rapping, the pinions will be too thick, and we shall require to file them down, entailing a great deal of labor and time, besides the rapid destruction of files.

[Ill.u.s.tration: Fig. 2781.]

[Ill.u.s.tration: Fig. 2782.]

"Let it be required to cast a pillow block to contain a babbitt-metal bearing. In this case there requires to be a cavity to receive and hold the babbitt metal. This is provided by casting ridges of metal around the edges of the bearing, as shown in Fig. 2781, at D E and on each side at F, the pieces D E may be made solid with the pattern, but those for the sides must be removable, having dovetails as at _c_ _c_ to hold them in position while being moulded, or in place of the dovetails, wires as at F F may be employed, in either case the pattern would be extracted from the mould, leaving the side strips to be removed afterwards. If, instead of a pillow block, a bracket or frame, such as in Fig. 2782, were required, it must be moulded in the direction of the arrow, and in that event it would be desirable to core out the journal bearing. This would be accomplished by providing a core print to block up the whole opening B. A suitable core box for the bearing would be as in Fig. 2783.

The core print must project below the casting so as to form in the mould a core print for the core, and it is obvious that the core itself must be made of increased depth to the amount allowed for core print; hence the end piece B, Fig. 2783, is increased in thickness to the amount allowed for core print."

[Ill.u.s.tration: Fig. 2783.]

Patterns for cylindrical bodies, especially those that are hollow and thin, are constructed in pieces by a process termed "building up." The pieces are usually segments of circles, and the manner of marking them is as follows:--

[Ill.u.s.tration: Fig. 2784.]

[Ill.u.s.tration: Fig. 2785.]

[Ill.u.s.tration: Fig. 2786.]

[Ill.u.s.tration: Fig. 2787.]

Let it be required to make a pattern for a f.l.a.n.g.ed pulley, such as shown in section in Fig. 2784. It would be constructed in two halves composed of a number of courses as from 1 to 8, and each course would be composed of segments of the form shown in Fig. 2785. The length of the arc of these segments must be such that it will require a certain number of these to complete the circle of that part of the cylinder which the segment is to form; and the manner of accomplishing this is shown in Fig. 2786, in which the circle C is of the diameter of the outside, while circle D is that of the outside of the pulley proper, circle E is of the diameter of the inside of the pulley rim. These circles are divided into as many equal divisions as there are to be segments in the circ.u.mference; hence the number of divisions determines the length of arc of the segments. Thus A would be a segment for the body of the pulley, and F a segment for the rim. A template is then made of each one of these segments, as at A and F. This template must be made slightly larger in every direction than the respective divisions, to allow for the stuff that will be turned off in truing the pattern in the lathe and in jointing the segments to one another during the building. The templates are employed to mark out on the board which should first be planed to the required thickness. This will be a trifle thicker than the course so as to allow for truing the surface of each finished course in the lathe. The courses are best built up on the chuck of the lathe on which they are to be turned, and a saving in time will be effected if there are two chucks, so that a course on one half of the pattern may be built up while the glue of another course on the other half is drying.

On the lathe chuck, and directly beneath, where the joints of the segments will come, pieces of paper as at _a_, _c_, _e_, _g_, Fig. 2787, and if the segments are long ones, intermediate pieces of paper, as _b_, _d_, _f_, _h_, will be necessary. The radial edges of the segments are trimmed on what is termed a shooting board, which is a device such as shown in Fig. 2788, in which A is a piece of board on which is fastened the piece B. S is a piece projecting above B, and is provided to rest the segment S' against, the flat surface of the latter lying on the board B. It is thus held in a fixed position, ready to have its edges E planed, the whole being laid upon the bench against the bench stop G.

If, however, it is more convenient to rest the shooting board across the bench, a piece C may be fastened beneath A, so as to come against the edge of the bench as in Fig. 2789, in which T is the bench. The plane is laid with its side on A as in Fig. 2790, so that the surface of A acts as a guide, keeping the edge of the plane vertical, and thus planing the edges of the segment square. The plane is operated by hand in the usual manner (save that it lies on its side), taking its cut most off the outside or inside of the edge of the segment S', according as the position of the latter is varied. In some of the shooting boards manufactured by tool makers, the height of B from A is adjustable, so that all parts of the plane blade edge may be used, which saves grinding, since only that part of the edge that is used dulls. Also there is provided means whereby the required lateral position of the segment may be adjusted; such a device is shown at P, Fig. 2788, which is a plate having a slot through it, through which pa.s.ses the thumb screw V, which screws into S. Hence the plate may be adjusted so that when one end of the segment rests against the end of S, and the other against the end of P, its edge E will be in the proper position to be planed to correct angle by the plane, whose line of action is in this case rendered positive by means of a slide on the plane, acting in a groove in the base A.

[Ill.u.s.tration: Fig. 2788.]

[Ill.u.s.tration: Fig. 2789.]

[Ill.u.s.tration: Fig. 2790.]

The first segment is glued to the pieces of paper on the chuck, as shown in Fig. 2787, S' representing the segment. A second segment is then added, being set fair to the pencil circle O, and jointed and glued both to the chuck and to the ends of the first segments. Successive segments are added until the whole circle or course is completed, and when dry the radial face of this course is turned in the lathe so as to be true, flat, and of the required thickness, and the diameter is trued. The second course may then be added, but the joints at the ends of the segments should not come over those of the first course, but in the middle as shown by the dotted line. The ends of the segments should be made to bed properly against each other, and glue should be applied to the joint between the two courses and at the ends. By adding the successive courses the whole may be built up on the chuck ready to receive the arms. As each segment is added it should be clamped or weighted to press it firmly to its seat and press out the excess of glue.

If the pattern consists of two, or say three, courses, the glue will be sufficient to hold it to the chuck while turning, but if there are more courses a screw should be inserted through the chuck and into each segment of the first course. The cylinder must then be turned inside and out ready to receive the spokes. These are made of pieces equal in length to the internal diameter of the rim, or a trifle longer, so that the ends may be let into the rim. A line is then marked along the edge of the rim, dividing its thickness into two divisions, and in the centre of the length a recess should be cut out from the face to the line, the width of the recess equalling the width of the arm, so that one arm will let into the other, forming a cross, of which the flat surfaces lie in the same plane. This cross is let into the rim of the wheel and fixed temporarily with brads. The lathe may then be started and the centre of the arms (and therefore that of the cylinder or pulley) be found by a pencil point moved until it marks a point and not a circle when the lathe revolves. The arms may then be marked to shape and a recess turned at their centre to receive the hub. The arms being marked to their respective places and their outside faces being marked with a pencil so that they may be replaced in the same position in the wheel, they may be removed and shaped to the required dimensions and form, and then replaced and glued to the rim.

If the wheel is to have six arms they may be constructed as follows:--

Instead of taking two pieces of the diameter of the rim, as in the case of four arms, three pieces are necessary, and in this case the thickness of the edge of each piece is divided by two marked lines which will divide the thickness of the edge into three equal divisions, as shown by the dotted lines 1 and 2 in Fig. 2791, which will divide the thickness of the edge into three equal divisions of thickness. From the centre of the lengths of each of the three pieces we mark on the flat face a circle whose diameter will equal the width on the flat face of the pieces themselves.

With an angle square having its adjustable blade set to an angle of 60, and set so that the back is fair with the edge of the piece, and one edge coincident with the perimeter of the circle, lines tangent to the circle and crossing each other are drawn on the pieces A C. On the piece B four of such tangent lines (two on each side) must be drawn. The piece A is recessed between one pair of tangent lines to the depth of the second lines on its edge, or, in other words, to a depth of two-thirds its thickness, and between the other pair to a depth of one-third, as shown, the two-thirds at D, the one-third at E. The piece D must be recessed between its tangents on each side to a depth of one-third its thickness, as denoted at F F, while on C the whole s.p.a.ce between the tangent lines must be recessed to a depth equal to two-thirds its thickness, as shown at G. The pieces may then be put together so that the two diametrically opposite arms will be in one piece. If an odd number of arms is employed this form of construction cannot be followed; hence each spoke will be a separate piece, extending from the rim to the centre and jointed at the latter, as in Fig. 2792, which is for five arms.

For this construction draw a circle _c_, Fig. 2792, and divide it into as many equal points of division as there are to be arms in the wheel.

From these points of division draw lines to the centre, and these lines will show the required bevel at the end of each spoke, as shown in the figure. The ends should be verified for bevel by striking from the common centre a second circle, as D; and measuring if the arms are equidistant, measured at the circle and from the edge of the arm to that of the next, finished along the full length. When fitted, corrected, glued and dry, the spokes may be let into the wheel and a recess turned into the centre to receive the hub.

The rim and all parts that can be got at may then be turned in the lathe, the pattern then being reversed in the lathe to turn the inside of the rim, or the other side of the spokes, when the job will be complete. When, however, the rim is to be a very thin one, it may be necessary to fasten the segments together at the ends by other means as well as glue, hence a saw-cut may be made in each end, and a tongue inserted.

[Ill.u.s.tration: Fig. 2791.]

[Ill.u.s.tration: Fig. 2792.]

It is obvious that each half of the pattern is constructed by similar segments, the line of parting being through the centre of the arms, as at A B, in Fig. 2791. To keep the two halves coincident when in the mould, pins are inserted in the rim and arms of one half, fitting closely into holes provided in the other half.

To construct a pattern for a pipe, the pattern would be made in two halves, and constructed of what are termed staves, that is, pieces of wood running lengthways of the pipe. The number of these staves is optional, save that it must be even, so that each half pattern will contain an equal number.

[Ill.u.s.tration: Fig. 2793.]

Let it be required to make a pattern for a pipe 18 inches in diameter, and to be 1 inch thick. Draw the line A B, Fig. 2793, and from a point on it, as C, draw a semicircle A B, equal in diameter to the diameter of the outside of the pipe. Also the circle D E F, equal to the diameter of the inside of the pipe, and these will represent an end view of the pipe. Divide these semicircles into as many equal divisions as it is decided to have staves in the half pattern--as 1, 2, 3, 4, 5, 6; and from one of these divisions make a template as denoted by the oblique lines at 2, leaving it slightly larger than the division, to allow stuff to work on in fitting the staves, &c.

[Ill.u.s.tration: Fig. 2794.]