Fig. 2772.

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 Assistant.”

VOL. II. EXAMPLE IN PATTERN WORK. PLATE XIV.
Fig. 2773.Fig. 2774.
Fig. 2775.Fig. 2776.
Fig. 2777.Fig. 2778.
Fig. 2779.

“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 flange 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 flange, 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 space around the core print, as shown at a b in [Fig. 2774], into which space 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.

“We will now suppose that our piece, when cast, is to be turned under the flange 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 flange, 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 flange 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 flange 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 flange 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.