Possibilities and Limitations of Die Casting
Fig. 16. Die-casting Constructions to be avoided
At the outset we may say that it is possible to die-cast almost any piece, but it is not by any means practicable to do so. It must be remembered that to die-cast on a practical basis the dies must be constructed in such a manner that the cost of their operation and up-keep will be light, or there will be no profit in die-casting. It is impracticable to produce under-cut work, that is, work having no draft and which is therefore impossible to draw from the die. Such an instance is that illustrated at A, [Fig. 16], and by the internal section of M, [Fig. 21], and the internal groove in O, also shown in [Fig. 21]. If absolutely necessary, work of this kind can be done by the use of collapsible cores; but here, again, we meet resistance in maintaining the dies in proper condition, and, moreover, this method is commercially impracticable, owing to the difficulty of operating these cores rapidly. Hollow work, requiring curved cores, like faucets and bent piping of the character illustrated at C in [Fig. 16], are difficult to produce. If, in designing the piece, it can be planned to have the parts of such a shape that the cores can be readily withdrawn, employing a two-piece core with a slight draft in each direction, the division coming as indicated by the core line of C in [Fig. 16], the problem becomes simpler. Oftentimes this work can best be done by casting in a straight piece, afterward bending the die-casting. It does not pay to cast rough heavy work that can be made just as efficiently by sand casting. Generally speaking, the greatest saving can be effected by die-casting small pieces which have previously required a large amount of machining to produce. On large plain work the amount of metal required for the casting makes the cost excessive on account of the difference in cost of the metals. If, however, the large work must be finely finished by polishing, etc., it is oftentimes found of advantage to die-cast. Corners, especially those joining thick and thin sections, as at B, [Fig. 16], should be heavily filleted as shown on one side of this piece. Regarding the casting of thin sections, it is not practicable to try to cast sections under 3/64 inch in thickness, as the metal runs with difficulty into such narrow places. A casting having walls 1/16 inch, like that shown at X, [Fig. 24], is easily cast. Threaded sections, if the threads are fine, say, under twenty-four to the inch, should not be die-cast, because under moderate pressure they will strip. A good way to treat constructions of this kind is to enclose brass or steel bushings in the die-castings in which the threads are required.
Fig. 17. View of the Casting Room
As to the accuracy with which die-castings may be produced, it is possible to keep dimensions within 0.0005 inch of standard size, but to do so requires considerable expense in keeping the dies in condition. A limit of 0.002 inch, however, is entirely practicable, and can be maintained easily. In specifying the accuracy with which die-castings are to be made, only those parts which are absolutely essential should be held to size, in order to keep the cost of the work nominal. One of the great advantages of the use of die-castings is that no finishing is required after the pieces leave the molds. Finish requirements should be plainly stated in ordering die-castings, as the alloy must be suited to these requirements.
Fig. 18. Methods of attaching Die-cast Gears, etc., to Shafts
Another great saving is effected on lettered work, either raised or sunken. One of these jobs is illustrated at Q, [Fig. 22], which shows an example of die-cast lettering. Sunken lettering is to be preferred to raised lettering, as the latter is more easily injured. Knurled work may be produced easily, if straight knurls are used, and threaded sections over ¼ inch in size are entirely practicable, either internal or external. External die cast threads are illustrated at R and S, [Fig. 22]. The casting of gears and segments is a familiar application of die-casting; this is illustrated by the large gear at N, [Fig. 21], and the segment at W, [Fig. 23], which give an idea of the general character of this class of work. The casting of pulleys, gears, and similar parts on shafts may be easily effected as shown by the gear on the shaft at N, in [Fig. 21]. The views shown in [Fig. 18] are intended to convey an idea of three methods of die-casting around shafts. At D is shown a die-casting cast around a steel shaft. If the surface of the shaft coming within the pulley has been previously knurled, the pulley will grip it much better, but for ordinary purposes the shrinkage of the die-cast metal around the shaft is sufficient. If any heavy strain is to be imposed on the work, it is better to provide anchor holes through the shaft, like those indicated at E. It will be readily seen that the die-cast metal runs through these holes in the shaft, forming rivets which are integral with the casting. For locating levers upon the ends of shafts, etc., a good way is to flatten opposite sides of the shaft and cast around them, as shown at F, [Fig. 18]. The screw seen projecting beneath the piece at Q, [Fig. 22], was die-cast in place. Any of these methods are to be recommended, and a proper knowledge of possibilities of this kind will increase the scope of die-casting.
Fig. 19. A Few Possibilities of Die Casting
Another phase of die-casting which can well be borne in mind is the possibility of inserting steel or other parts in the die-casting. Such an instance is shown at G in [Fig. 19]—a die-casting which was made by the Van Wagner Co. as a part of an electrical apparatus, the steel inserts being contact points. Oftentimes it is found advisable to include brass bearing rings to give additional durability at points where the die-cast metal would not stand up. The die-casting shown at U, [Fig. 23], in which the brass ring at T has been incorporated, is typical of such cases. To die-cast pieces like those shown at H in [Fig. 19], and similarly at V in [Fig. 23], having inverted conical openings, might at first thought seem difficult, but this is entirely practicable. Similarly, split bushings like those shown at I, [Fig. 19], and at W, [Fig. 23], may be cast with projecting lugs for the reception of screws for clamping upon shafts, etc., but this construction should not be used if frequent tightening or loosening will be necessary.
Fig. 20. Castings which illustrate Points of Shrinkage and Draft
Fig. 21. Die-castings showing Impractical Under-cut Sections; also a Large Gear die-cast on Shaft
Fig. 22. Die-castings which show Lettering and Thread Castings
The shrinkage problem manifests itself in die-casting in the same measure that it does in other casting operations. Different metals shrink in different degrees, as will be explained later on. However, one important point can be mentioned at this time: that is, the amount of shrinkage is often dependent upon the shape of the piece. For instance, pieces like those shown at K in [Fig. 20] or at X in [Fig. 24], will shrink very little on account of the fact that the steel mold is of such shape that the central core will prevent the die-casting from shrinking. However, pieces like those shown at L in [Fig. 20], or at V in [Fig. 24], which have nothing to hold them from pulling together as they cool, will shrink to the greatest extent. All of these points must be taken into consideration when designing work for die-casting. Practically no draft is necessary on a die-casting, except on very deep sections, as indicated at J in [Fig. 20], where a draft of 0.001 inch to the inch is desirable. Perfectly straight sections, however, can be cast, as the shrinkage of the metal is usually enough to free it from the die.
Fig. 23. Typical Die-castings illustrating Various Points
Fig. 24. Die-castings illustrating the Extremes of Shrinkage
It is the opinion of the Van Wagner Co. that die-casting costs can be materially reduced if designers will bear this point in mind when bringing out new designs. Even though it is often possible to cast special pieces, incorporating several parts in one, and thereby accomplishing what seems to be a great stunt to the designer, it is sometimes more practicable to make the piece in several sections and later assemble it. Not only is this simpler for the die caster, but it is also more economical for the customer. Such points as avoiding thin sections, including large fillets at corners, as well as taking account of the under-cut problem, are simply matters of common sense, but they can profitably be considered by the designer.