CHAPTER III

VAN WAGNER MFG. CO.’S DIE-CASTING PRACTICE

In 1907, Mr. E. B. Van Wagner, of Syracuse, N. Y., established the E. B. Van Wagner Mfg. Co. for the production of die-castings. The factory comprises the office section, the machine shop where the dies and casting machines are built, the metallurgical laboratory where the metals are alloyed, the casting department shown in Fig. 17 where the die-castings are made, and the trimming department.

Possibilities and Limitations of Die Casting

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 If, in designing the piece, it can be planned to have the parts of such a shape that the cores can be readily withdrawn, employing a two-piece core with a slight draft in each direction, the division coming as indicated by the core line of _C_ in Fig. 16, the problem becomes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.

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.

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.

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.

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.

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.

The Van Wagner Die-casting Machine

The first essential to good die-casting is a good casting machine. Perhaps the best known types of casting machines are of the familiar plunger type, of which there are several varieties, the pneumatic type and the rotary or automatic type. (For descriptions of various types of die-casting machines, see “Die Casting Machines,” MACHINERY’S Reference Book No. 108.) For the economical production of die-castings, however, the hand-operated machines are rather too slow, and automatic machines are applicable only to a class of work which may be made in very large quantities. For these reasons, therefore, the Van Wagner Co. employs the compressed air type of die-casting machine which was patented by Mr. E. B. Van Wagner in 1907. In the casting department of the Van Wagner shop, illustrated in Fig. 17, there are installed about thirty machines. Fig. 27 shows a die-casting machine in the open position. Fig. 26 shows a closer view of the die-operating mechanism and Fig. 25 is presented to give a general idea of the construction of the entire machine.

By referring to the line illustration Fig. 25, which shows the Van Wagner pneumatic die-casting machine in part, and comparing this illustration with Fig. 26, which shows the general appearance of the die-operating and other mechanism of the casting machine, a good idea may be obtained of its construction and working. At _A_ may be seen the base of the machine in which is located the melting pot _B_. This melting pot is heated by means of fuel oil passing through the supply pipe _C_ to the burners _C_`1. A vent pipe _D_ is provided to take away the gases incident to combustion. The pressure for “shooting” the metal into the die cavity is supplied by air through the supply pipe _E_. A valve controls this air supply. The pressure is regulated to suit the particular casting or die, the proper amount being determined by experiment. Similarly, an air exhaust pipe _F_, which may be seen directly above the supply pipe, sub-divides into two tubes which extend to the die cavity to exhaust the air before the metal is admitted. There are two methods of overcoming the presence of air in the die cavity--the exhaust method and the venting method, and it is the former that is here described.

A “goose-neck” _G_, shown in Fig. 25, serves to temporarily contain the metal which is forced into the mold. An amount of metal slightly in excess of that required for one die-casting is placed in this goose-neck with a hand-ladle, previous to each operation of the machine. One end of the goose-neck is connected to the air pipe, _E_, while the other end terminates in the nozzle _G_`1. This nozzle may best be seen by referring to the illustration of the machine shown in Fig. 27, in connection with Fig. 25. One of the advantages in using this goose-neck is that the entire air pressure is expended upon the metal in the goose-neck, and, by reason of its isolated position, the goose-neck and its contents are kept slightly hotter than the contents of the melting pot.

The Die-operating Mechanism

The die-operating mechanism of the machine is contained within a hinged framework, shown in position for the removal of the die-casting in Fig. 27. Referring to Fig. 26, in connection with the line illustration Fig. 25, it will be seen that the die-holding mechanism is all supported upon the lower die-holding plate _H_, which is hinged to the edge of the base of the machine. A lock _J_ serves to hold the dies and operating mechanism in the upright operating position, and by means of a counterbalance, suspended from an overhead rope which connects with the top of the mechanism at _P_, the changing of the position of this mechanism is easily effected, and when thrown into the horizontal position, as indicated in Fig. 27, it rests upon a support while the dies are being opened and the castings ejected.

The lower die is shown at _H_`1 and the upper die _K_`1 is mounted upon the upper die-holding plate _K_. Four rods _L_ act as guiding members for the upper die-holding plate to slide upon. These rods _L_ are mounted in fixed positions at the corners of the lower die-holding plate _H_, and at their upper ends the operating shaft supporting plate _M_ is located in a fixed position, serving to support the upper ends of these rods. The position of this plate _M_ is adjustable upon the rods by means of check-nuts, thus providing for the accommodation of thick as well as thin dies. A shaft _O_ is supported in this top plate, and by means of the operating lever _N_ working through slotted levers _O_`1 and links _O_`2, the upper die-holding plate and die can thus be removed from contact with the lower die at will.

The metal enters the die cavity through the nozzle _G_`1 and after setting, it is necessary to cut the sprue formed by the surplus metal that remains outside the die cavity. For this purpose, a sprue-cutter, operated by means of hand-lever _Q_`1, is employed. This sprue-cutting lever is hinged in the fulcrumed link _Q_`2, and is held in its casting position by means of an adjustable stop on bracket _Q_`3.

In many dies, it is necessary that water be circulated through the die-blocks to keep them cool during the die-casting operation. In Fig. 26, the water pipe may be seen at _R_, and hose pipes run from this supply to each side of the die-blocks, thus providing a cooling circulation. In this illustration, the pipes used for exhausting the air from the die cavity are apt to be confused with the cooling pipes, but by following the two pipes leading vertically down to the machine, the exhaust pipes may be seen and kept distinct from the water pipes.

Making a Die-casting

In order to clearly understand the operation of the die-casting machine, let us follow the sequence of events that takes place in producing a casting. Two men are required to operate the machine. In Fig. 27, the operators may be seen in their working positions. The first step is taken by the operator at the left who, with a hand-ladle, dips enough metal for one casting from the melting pot and pours it through nozzle _G_`1 into the goose-neck. The second operator in the meantime is replacing the cores in the dies, adjusting the position of the sprue-cutter and closing the dies preparatory to making a casting. This being done, he elevates the dies and their operating mechanism, which are hinged and counterbalanced, as previously described, bringing them to an upright position. The die operator now mounts the box, raises the sprue-cutter to its open position to admit the metal; after which the machine operator turns the air valve with his left hand. The operation of this air valve admits the air behind the metal, forcing it into the die, and the same movement opens the exhaust valve slightly in advance. The exhaust valve is located upon the second length of piping just above the air valve, and as a link connects the two valves, the single motion exhausts the air from the die cavity and immediately afterward the air is admitted behind the metal, thereby “shooting” the metal into the die. This being done, the air is shut off and the die operator cuts the sprue by means of lever _Q_`1, withdraws the cores in the die, throws the dies to the open position (which is indicated in Fig. 27), and operates the ejecting mechanism, thus removing the casting from the die. In the meantime, the machine operator is tending to his metal supply and getting a ladle full of metal ready for the next die-casting operation. By referring to the machines shown in Fig. 17, it will be noticed that only a few are provided with exhaust piping for venting the dies. Another venting method will be described later.

The number of die-castings which can be made on one machine per day of ten hours varies with the character of the pieces being die-cast, the number of pieces made at each operation of the machine and the ease with which the dies may be worked, which depends, of course, upon the number of cores and parts to be handled at each die-casting operation. The dies shown in the machine in Fig. 26, produce four bearings at each operation.

Trimming Die-castings

At the end of each run the operators of the machines go over their work, breaking the castings from the sprues and throwing out all that are defective. No matter how carefully the die-casting molds have been made, there is always a certain amount of trimming to be done on the finished die-castings, on account of the crevices left in the die for air vents, or which exist from improper fitting of the parts of the dies. These “fins,” as they are called, are trimmed by hand operators in a special department. A general view of this trimming room is shown in Fig. 28. Usually it is sufficient to scrape these fins off with a scraping knife, but if the casting is especially difficult to produce, so that a large opening is required to admit the metal, it is sometimes necessary to trim unusually thick sprue sections by filing. Fig. 29 illustrates the method of trimming such die-castings on a filing machine.

The Dies Used

Next to the casting machine, the dies or molds are the most important necessary factor. A general view of the Van Wagner Co.’s die-making department is shown in Fig. 30. In order to gain a proper conception of the work required in producing a high-grade die-casting mold, we will follow the different steps which are necessary in making the mold. The first and most important step is the proper planning of the die. Before any work at all can be done, it is necessary to plan the die, _i. e._, to decide just where the parting lines will come; just what method will be used for ejecting the piece; what alloy will be used; where the casting will be gated; and a hundred and one minor points, all of which have a direct bearing upon the performance of the finished dies. All these decisions have to be made by the diemaker, and in Fig. 37 he is shown, micrometer in hand, computing the shrinkage allowances that he will make in the dies. This is a very important factor on accurate work as the shrinkage varies from 0.001 to 0.004 inch, according to the alloy and the general shape of the piece.

Before taking up the actual machining operations of the mold-making as conducted in this factory, it will be well to take a typical die-casting mold and note its general construction. Fig. 31 shows a typical die-casting mold closed, while Fig. 32 shows the same mold disassembled on the bench to show its construction. The piece for which the mold has been made is also shown. Fig. 33 shows a similar die in section. From the three illustrations a good idea of an average die-casting mold can be obtained. Referring to these illustrations, the principal parts of this die are the ejector box _A_, and the ejector plate _B_ which is operated by the racks _C_. For operating the ejector plate, the pinion shaft _D_ having a handle suitable for turning, is furnished. This, of course, fits into a bored hole in the ejector box, bringing the pinion into mesh with the racks for raising the ejector plate. In the ejector plate are three ejector pins _E_ for removing the casting from the mold. The ejector pins operate through holes _F_. Beyond the pinion shaft may be seen the casting for which this mold has been made. It will be noticed that the top side of the casting has three projecting lugs through which are small holes. Provision for forming this side of the die-casting is made in the lower half of the mold _G_, while the upper half of the die-casting is taken care of by the top plate _H_. One of the toggles for operating the core pins through these three lugs is shown at _I_. These parts will be described more fully later. The sprue cutter is shown in position in the die at _J_.

Machining the Die Cavities

As will be noticed from Fig. 30, the machinery in the die-making department is of modern design, for no other class of work demands as good tool equipment and as much skill in the making as die-casting molds. The die-blocks are made of machinery steel. Fig. 34 illustrates the first step in making a die-casting mold after the die-block has been shaped approximately to size. This operation consists in carefully facing off the die surfaces on a vertical-spindle grinding machine. This, of course, is a quick method of surfacing the die-block, and it insures that the top and bottom surfaces of these plates will be parallel, permitting the die-faces to come together properly.

The next step consists of laying out the die, as shown in Fig. 36. This is done in the usual manner, by working on a coppered surface, using dividers, scales, and a center punch. When laying out the die, the necessary allowances are made for shrinkage and finish, these points having been planned before actual work on the die has been started. As in other phases of die-work, the machining operations are performed, as far as possible, before any hand-work is done. In Fig. 38 may be seen a die-maker turning the cavity in a part of the die-casting mold. The highest type of skilled workmanship is called for on this machine work, and as may be surmised from Fig. 38, where the die-maker is shown measuring the die with a vernier caliper, the measurements must be exact, for no grinding operations follow the machine work.

Figs. 35 and 39 show typical milling operations being performed on die-casting molds. In Fig. 39 the diemaker is shown indicating a pin in one corner of the mold cavity, preparatory to doing additional milling. The block is held in the usual manner by being clamped on the bed of the milling machine, and after it has been properly located under the cutter head, tools are substituted for the indicator and the milling of the cavity is completed. Fig. 35 shows one of the sections of the die-casting mold which is to be used in producing the casting shown at the right of the work. In this case the diemaker is milling the recess for the steel arbor which may be seen directly in the foreground. This will be fitted in place to provide for the forming of the hole in the side of the piece.

Fig. 40 illustrates several important points in the making of a die-casting mold. This illustration shows the ejector box with the lower half of the mold on it, the ejector plate being held against the under side of the die-plate by means of the pinion shaft. The operation being done is the drilling of the ejector-pin holes. Referring back to Fig. 32, which by the way shows the die here illustrated disassembled, the holes being drilled are those shown at _F_ for the reception of the pins _E_. The method employed is to drill the holes through the die and into the ejector plate, afterward reaming all holes to size and driving the pins into position in the ejector plate, while they are allowed to slide freely through the die-plate. We will now assume that the ejector box and plate have been completed and fitted, a pinion shaft for operating this plate also fitted, the lower and upper dies completed by the machining operations previously described, and all assembled. The final operation of the fitting of the pins is shown in Fig. 41 in which the die-maker may be seen filing off the ends of these pins so that when dropped to the lower position they will lie flush with the surface. If of uneven lengths, these pins will cause irregular spots in the casting. It now remains to describe the toggles used for operating the cores which form the holes through the three lugs in the casting. One of these toggles, of which there are three, is shown at _I_, in Fig. 31, and also in Fig. 32. These toggles consist of brackets which are attached to the die-plate, and levers which are fulcrumed at the ends of the brackets so that their operation works the core pins. It is necessary to remove these core pins after each casting has been made and position them before another casting can be produced.

The fitting of the parts of a die-casting mold is one of the most important parts of the work. It demands the highest type of workmanship, for a poorly fitted die means a die which works hard in addition to producing poor castings. It is very important that all movable parts should work freely. Fig. 42 shows the assembling operation on a die-casting mold, the casting which is to be duplicated being shown in the immediate foreground. These parts must all be screwed into their respective places, making the joints as nearly air-tight as possible. One cause of poor die-castings arises from the trapping of air in the die, and different methods are employed for overcoming this trouble.

Venting the Dies

There are two methods of preventing air from being trapped in die-casting molds; either by constructing the dies so that the air may be exhausted from the mold cavity before admitting the metal, or by venting the die so that the air may be forced out by the inrushing metal. In the first of these methods it is necessary that the joints in the mold be made as close as possible, otherwise it will be impossible to produce anything like a vacuum in the mold cavity. If, however, it has many parts which must be fitted, it is usually considered advisable to provide the die with vents consisting of milled recesses a few thousandths inch deep. Several vents are provided, from which the air can escape when the metal is admitted to the dies. The hot metal, of course, “shoots” through them in thin ribbons, but not enough escapes to affect the pressure on the metal which goes into the casting.

No matter how carefully a die may have been constructed, or how carefully it has been assembled, there is always a certain amount of “babying” to be done before it will work satisfactorily. The casting may stick a little here, or there may be a rough spot there, and it is the successful elimination of these troubles which constitutes the production of a good die-casting.

Die-casting Metals

One of the purposes of this book is to correct several erroneous impressions which are prevalent in regard to die-casting possibilities. Many people seem to think that nearly all metals can be die-cast, but as a matter of fact, those metals which can be successfully die-cast can be numbered on the fingers of one hand, being alloys of lead, zinc, tin, copper and antimony. The tin base metals shrink very little, while the zinc base metals shrink considerably, and those with a large per cent of aluminum have a very high shrinkage. Without doubt, the most used die-casting metals are the zinc base metals. A typical metal of this class contains about 85 per cent zinc; 8 per cent tin; 4 per cent copper and 3 per cent aluminum. The melting point of this metal is about 850 degrees F. While this alloy is one of the most common, it is not by any means the best, as there is too little tin employed, but it is a comparatively cheap metal, which probably accounts for its large use. This metal is easily affected by heat and cold, and rapidly deteriorates with age. The lead base metals may be typified by an alloy containing 80 per cent lead; 15 per cent antimony; 4 per cent tin; and 1 per cent copper. This composition melts at approximately 550 degrees F. and is used for castings subjected to little wear and where no great strength is required. The weight of this metal is its greatest objection, and it is also quite brittle because of the large percentage of antimony.

For the best class of die-castings, the tin base metals are employed. These range from 60 to 90 per cent tin, and from 2 to 10 per cent copper, together with a little antimony. The melting point of a mixture of this composition is about 675 degrees F. The castings have a good color and they are much better in quality than any of the other alloys. It is absolutely essential that tin base metals be used for carbureter parts or other parts coming in contact with gasoline. Also, the tin base metals must be used for parts which come in contact with food products, as the lead or zinc alloys have a contaminating effect.

Aluminum alloys have been cast in France and Germany in limited quantities, but very seldom in this country on account of their high melting point, as well as their effect upon the die. After aluminum alloys have been run in the dies for a short time, the surfaces of the molds become pitted. Through some unexplained cause, the metal seems to flake out particles of the steel in the molds. When an aluminum alloy is to be used, a good mixture is 80 per cent aluminum, 3 per cent copper and 17 per cent zinc. This alloy has a high shrinkage and it has also the same deteriorating effect upon the dies, but to a much less degree than pure aluminum.

Transcriber’s Notes

Punctuation, hyphenation, and spelling were made consistent when a predominant preference was found in this book; otherwise they were not changed.

Simple typographical errors were corrected; occasional unbalanced quotation marks retained.

Ambiguous hyphens at the ends of lines were retained.

Text uses “die-cavity” and “die cavity”, “die-maker” and “die maker”; none changed here.