Transcriber’s Note: Subscripts are preceded by an acute accent mark `;
italic text is enclosed in _underscores_.
[Illustration]
MACHINERY’S REFERENCE SERIES
EACH NUMBER IS ONE UNIT IN A COMPLETE LIBRARY OF MACHINE DESIGN
AND SHOP PRACTICE REVISED AND REPUBLISHED FROM MACHINERY
NUMBER 109
DIE CASTING
DIES--MACHINES--METHODS
By CHESTER L. LUCAS
CONTENTS
Die Casting 3
Making Dies for Die-Casting Machines 15
Van Wagner Mfg. Co.’s Die-Casting Practice 27
Copyright, 1913, The Industrial Press, Publishers of MACHINERY,
49-55 Lafayette Street, New York City
CHAPTER I
DIE CASTING
Die-casting, a comparatively recent method for producing finished
castings, is rapidly proving itself an important factor in the
economical manufacture of interchangeable parts for adding machines,
typewriters, telephones, automobiles and numerous other products where
it is essential that the parts be nicely finished and accurate in
dimensions. The term “die-casting” is self-explanatory, meaning “to
cast by means of dies”; described briefly, the process consists of
forcing molten metal into steel dies, allowing it to cool in them, and
then opening the dies and removing the finished casting. It is the
purpose of this treatise to give a general outline of the die-casting
process, showing its possibilities and limitations, and also to give
a description of the die-casting machinery and its operation, of
the fundamental principles involved, and of the methods used in the
die-making. Illustrative examples of the best types of dies, based on
results obtained from actual experience, will also be given.
Origin of Die Casting
The origin of the die-casting process is somewhat difficult to
ascertain. We may look into the history of type founding and find
that away back in 1838, the first casting machine for type, invented
by Bruce, was a machine that involved the principles of die-casting
as it is practiced to-day. More recently, in 1885, Otto Mergenthaler
brought out the linotype machine. This machine is a good example of
a die-casting machine. However, as we interpret the word to-day,
die-casting is a broader term than type-casting or linotyping, although
its development without doubt is due to the success of the linotype
machine. It is doubtful if die-casting, properly speaking, was
originated until about fifteen years ago, and it is certain that it is
only during the past few years that the activities in this line have
been very noticeable.
One of the first experiments in the direction of die-casting was
undertaken to get out some rubber mold parts cheaply enough to leave a
profit on a job that was beginning to look dubious from the financial
side. The molds were for making rubber plates about three inches
square and one-eighth inch thick, the top side of which was decorated
with fine raised scroll work; it was this latter feature that gave
the trouble. After wasting much time and money trying to stamp the
mold parts, a metal-tight box was made as shown in Figs. 1 and 2 with
a block screwed in it, the purpose of which was to shape the mold
impression and impart to it the scroll design. As shown, the ends of
the box were removable, being screwed on. This box was placed under
a screw press and a straight plunger that just filled the top of the
box was fitted to the head of the press. After the two were lined up,
molten type metal was poured into the box, and as soon as the metal
had cooled to the “mushy” state, the ram of the press was forced down
as shown in Fig. 2. Next, the ends of the box were removed, the screw
holding the block taken out, and the die-casting pushed from the box.
The object in having the inclined side to the box was to produce a
piece shaped with the proper inclination for its position in the final
mold used for casting the rubber plates. The illustrations give an idea
of the compression that took place. The die-casting was found to be
sharp at the corners and free from flaws, and the scroll work came up
in fine shape. Naturally the rest of the mold parts were made in the
same way and the job turned from failure into success.
[Illustration:
Fig. 1. An Early Experiment in Die Casting--Before Applying Pressure
Fig. 2. An Early Experiment in Die Casting--After Applying Pressure
]
From such simple experiments as these, the die-casting industry has
developed to its present stage. In view of the advances that have been
made in die-casting, it is singular that there are to-day only about
a dozen concerns in the business in this country, but as the subject
becomes better understood, and the possibilities of the process are
realized, the demand for this class of castings will result in many
other firms going into the work, and it is not improbable that a large
number of factories will install die-casting plants of their own to aid
them in producing better work in a more economical way.
Advantages, Possibilities and Limitations of Die Casting
The greatest advantage of die-casting is the fact that the castings
produced are completely and accurately finished when taken from the
dies. When we say completely, we mean that absolutely no machining is
required after the piece has been cast, as it is ready to slip into its
place in the machine or device of which it is to be a part. When we say
accurately, we mean that each piece will come from the die an exact
counterpart of the last one; and if the dies are carefully made, the
castings will be accurate within 0.001 inch on all dimensions, whether
they are outside measurements, diameters of holes or radii. All holes
are cast and come out smoother than they could be reamed; lugs and gear
teeth are cast in place; threads, external and internal, and of any
desired pitch can be cast. Oil grooves can be cast in bearings, and, in
a word, any piece that can be machined can be die-cast.
The saving in machining works both ways; not only is all machine work
eliminated by the one operation of casting, but the machine tools and
the workmen necessary for their operation and up-keep are dispensed
with, the expense of building jigs and fixtures is stopped; and no
cutters, reamers, taps or drills are required for this branch of the
production. In addition, the labor required for operating the casting
machines may be classed as unskilled. No matter how intricate and
exacting the machine work on a piece has been, and how skillful a
workman was required to produce the work when machine-made, the same
result may be brought about by die-casting, and usually the work is
excelled, and, excluding the die-making, unskilled men can make the
parts.
From a metallurgical standpoint a die-casting is superior to a
sand-casting on account of its density, strength and freedom from
blow-holes. Also, when the hot metal comes in contact with the cool
dies, it forms a “skin” similar to the scale on an iron sand-casting.
As the die-casting requires no machining after leaving the dies, this
skin increases the wearing qualities of the casting.
The possibilities of die-casting are numerous. By this method of
manufacturing it is possible and practical to cast pieces that could
not possibly be machined. It is an every-day occurrence to make
castings with inserted parts of another metal, as, for instance, a
zinc wheel with a steel hub. It is also possible to make babbitt
bearings that are harder and better than can be made in any other way.
Often there are two or more parts of a device that have formerly been
made separately, machined and assembled, that can be die-cast as one
piece. In such cases the saving in production is very great. Figures
and letters may be cast sunken or in relief on wheels for counting
or printing, and of course ornamentation may be cast on pieces that
require exterior finish. As to size, there is no definite limit to the
work that can be cast. One job that is being done at the present time
is a disk 16 inches in diameter with a round flange 1 inch in diameter,
around the rim.
[Illustration: Fig. 3. Examples of Die-castings]
“There is no great gain without some small loss,” is just as true of a
process like die-casting as it is of anything else. The limitations of
this work are few, however, and they are here given so as to state the
situation fairly. Generally speaking, a part should not be considered
for die-casting if there are but few pieces required, because the cost
of the dies would usually be prohibitive. Often, however, it happens
that because of the large amount of accurate machine work being done on
a machine part, it is economical to make a die for the comparatively
small number of parts required and die-cast them. A case illustrating
this phase of the matter recently occurred in actual practice. In
getting out an order of two hundred vending machines, it was decided
to try die-casting on a part that was difficult to machine. The dies
were expensive, costing $200, and as there were only 200 pieces to be
cast, the die cost per piece was one dollar; but even with that initial
handicap, it was found that on account of the difficult machining that
had formerly been required, the die-cast parts effected a large saving,
and of course the results were superior.
A rough part that would require little or no machining should not
be die-cast, because pound for pound, the die-casting metals cost
more than cast iron or steel. The casting machine cannot make parts
as rapidly or of as hard metals as the punch press or the automatic
screw machine. For this latter reason a part that necessarily must be
made of brass, iron or steel, cannot be die-cast, although mixtures
approximately equal in strength to iron and brass are readily die-cast.
To give added strength to a die-cast part it is often advisable to
add webs and ribs or to insert brass or iron pins at points that are
weak or subject to hard wear. Roughly speaking, it is the part that
has required a good deal of accurate machine operations that shows the
greatest difference in cost when die-cast, and sometimes the saving is
as great as 80 per cent.
The Metals used in Die Casting
The metals that produce the best die-castings are alloys of lead, tin,
zinc, antimony, aluminum and copper, and the bulk of the die-castings
made at the present time are mixtures of the first four of these
metals. From them, compositions may be made that will meet the
requirements of nearly any part.
For parts that perform little or no actual work, save to “lend
their weight,” such as balance weights, novelties and ornaments for
show windows, etc., a mixture consisting principally of lead, often
stiffened with a little antimony, is used. There is but little strength
to this metal, but it is used because of its weight and low cost.
For parts that are subject to wear, such as phonograph, telephone,
gas-meter and adding machine parts, an alloy composed of zinc, tin and
a small amount of copper is used. This alloy may be plated or japanned,
and is a good metal to use on general work.
Another metal, used chiefly for casting pieces that have delicate
points in their design but are not subjected to hard wear, consists
principally of tin alloyed with lead and zinc to suit the requirements
of the work. This mixture casts freely, and the finished castings come
out exceptionally clean. Still another metal, used chiefly for casting
pieces that have letters and figures for printing, is similar to the
standard type metal--5 parts lead and 1 part antimony; but if there are
teeth cast on the sides of the printing wheel a harder mixture will be
required to give longer life to the gears.
The following mixtures are typical of die-casting or “white brass”
alloys: copper, 10 parts; zinc, 83 parts; aluminum, 2 parts; tin, 5
parts. Another is copper, 6 parts; zinc, 90 parts; aluminum, 3 parts;
tin, 1 part. Another containing antimony is copper, 5 parts; zinc, 85
parts; tin, 5 parts; antimony, 5 parts. Shonberg’s patented alloy is
copper, 3 parts; zinc, 87 parts; tin, 10 parts. Alloys containing 15 to
40 per cent copper and 60 to 85 per cent zinc are brittle, having low
strength and low ductility. An alloy of 8 per cent copper, 92 per cent
zinc has greater resilience and strength but not the ductility of cast
zinc.
Aluminum may be cast, but it is a difficult metal to run into thin
walls and fine details; it plays, however, an important part in some
good mixtures used for die casting. Experiments are now being conducted
for die-casting manganese bronze, and it is said that some very good
castings have already been made. Its wearing qualities are so valuable
that it is particularly desirable for making die-castings.
The Die-casting Machine
The three important requisites for good die-casting are the machine,
the dies and the metal. The casting machine is fully as essential as
either of the other requisites, and although there are a number of
different styles of casting machines in use, each of which has its
advantages over the others, especially in the eyes of their respective
designers, the fundamental principles upon which they all operate are
the same. In each there is the melting pot and the burner, the cylinder
and the piston for forcing the metal into the dies, and the dies with
the opening and closing device. In some machines pressure is applied to
the metal by hand, in others power is used, and in still another class
the metal is forced into the dies with compressed air. The provisions
for opening and closing the dies vary in the different machines; there
are various means employed for cutting the sprue, and the styles of
heaters are numerous.
One or two of the largest firms in the die-casting industry have
automatic casting machines for turning out duplicate work in large
quantities very rapidly. These machines are complicated and are only
profitable on large quantities of work, and for that reason their
use is not extensive. In general, their operating principles are the
same as in the case of the hand machines, but provision is made for
automatically opening and closing the dies, compressing the metal, and
ejecting the castings.
The Soss Die-casting Machine
The Soss die-casting machine, manufactured and sold by the Soss
Manufacturing Co., Brooklyn, N. Y., was the first die-casting machine
to be placed on the open market. This machine is shown in Figs. 4 and
5, and in section in Fig. 6. The Soss Manufacturing Co. originally
manufactured invisible hinges exclusively. At the advent of the
die-casting era, they commenced to make these hinges from die-castings,
and placed orders with a leading die-casting concern amounting to
thousands of dollars each year. After the die-cast hinges had been on
the market for a short time, complaints commenced to come in, some to
the effect that the hinges were breaking and others that the hinges
were corroding. Either of these faults was serious enough to blast the
reputation of the hinge, but the first trouble, breakage, was the more
important. Examination of the broken hinges showed that the castings
were porous and full of flaws, and as the makers of the castings
could not produce castings sufficiently strong for the hinges, Mr.
Soss started to experiment for himself. This experimenting led to the
production of the Soss die-casting machine.
[Illustration: Fig. 4. General View of the Soss Die-casting Machine]
Referring to the illustrations Figs. 5 and 6, _A_ is the base and frame
of the machine, _B_ is the heating chamber located at one end of the
machine, and within this heating chamber is the tank _C_, shown in Fig.
6. This tank contains the metal from which the die-castings are made,
and the metal is heated by the burners _D_. These burners are fed by
air and gas through piping on the side of and beneath the furnace. To
facilitate lighting the burners and inspecting their condition at any
time, there is an opening (not shown) through the firebrick lining of
the furnace and the outer iron wall, on a level with the top of the
burners. There is also another opening through the furnace wall to
allow the gases due to the combustion to escape. Through the bottom of
the tank, well to the inner side of the furnace, runs the cylinder _E_.
Below the bottom of the tank, the cylinder makes a right-angle turn,
extending through the furnace wall and terminating just outside of the
wall. The orifice of this cylinder is controlled by gate _F_. In that
part of the cylinder that extends upward into the tank, there is an
opening _G_ that allows the molten metal to run into the cylinder from
the tank. Working in this cylinder, is the piston _H_, that is used in
forcing the metal into the dies. The compression lever _I_, hinged over
the inner furnace wall, is kept normally raised by spring pressure, and
is connected to the piston by means of the link _J_.
[Illustration: Fig. 5. Working Parts of the Soss Die-casting Machine]
At the opposite end of the machine from the furnace, is the mechanism
for operating the dies. This mechanism consists of a pair of square
rods _K_, upon which are mounted the sleeves _L_. These sleeves have
a long bearing surface and are attached to the die-plate _M_. Lever
_N_ at the end of the operating mechanism controls the movement of
these sleeves by means of links _O_. Upon these sleeves is mounted a
secondary set of sleeves _P_, attached to the other die-plate _Q_,
and whose movement is controlled by lever _R_, through links _S_.
This second set of sleeves is free to travel with the first set, and
in addition has an independent movement of its own on the primary
sleeves. It is the function of lever _R_ to bring die-plate _Q_ up
to die-plate _M_ by means of links _S_ and sleeves _P_; and it is
the function of lever _N_ to bring both of the die-plates up to the
outlet of the cylinder by means of links _O_ and sleeves _L_. This
system of sleeve-mounting is one of the distinctive patented features
of the Soss machine. The orifice of the cylinder _E_ is conical in
shape and exactly fits the cup-shaped opening in die-plate _M_, so
that when the two are brought together, the joint is metal tight. At
the center of this opening, and extending through the die-plate _M_,
is an opening that leads to the dies mounted on the inner faces of the
two die-plates, and a continuation of this opening extends through
die-plate _Q_ in which the sprue-cutter _U_ works. Attached to the
outer side of this die-plate are two slotted brackets. In the slot of
one of these is pivoted the lever _T_, and in the slot in the opposite
bracket are bolted two stops that limit the motion of the lever. This
lever operates the sprue cutter _U_, that works through the opening in
die-plate _Q_. The sprue-cutting mechanism is best shown in Figs. 5 and
6. At the left of Fig. 5 may be seen a rubber hose connected to the air
piping. This hose is used for cleaning out the dies after each casting
operation.
Operation of the Die-casting Machine
The metal for the die-casting machine is mixed in the proper
proportions for the work in hand by means of a separate furnace, before
being poured into the tank of the machine itself. The burners are
lighted and the dies are set up on the two die-plates. As soon as the
machine has “warmed up,” so that the metal is in a thoroughly melted
condition, the sprue-cutting lever _T_ is thrown back, leaving a clear
passageway to the die cavities. Lever _R_ is pulled backward, thus
bringing die-plate _Q_ up to die-plate _M_, which operation closes
the two halves of the die. Then lever _N_ is thrown forward, thereby
bringing the closed die up to the body of the machine, with the nozzle
in close contact with the outlet of the cylinder. Next, the gate _F_
is opened, and the man at the compression lever _I_ gives the lever a
quick, hard pull, forcing the metal in the cylinder downward and into
the dies. The molten metal literally “squirts” into the dies. Gate _F_
is now closed; lever _N_ is pulled back to remove the dies from the
cylinder outlet; and the sprue-cutting lever _T_ is pushed forward,
cutting off the sprue and pushing it out of the nozzle into the kettle
placed beneath it. The lever _R_ is pushed forward, and a finished
casting is ejected from the dies.
An important advantage that this machine has over other die-casting
machines is the fact that the metal for the castings is taken from the
_bottom_ of the melting pot, whereas most other machines use metal from
the top of the tank. At the bottom of the tank the metal is always the
best, as it is free from impurities and dross; hence, there is little
chance for the formation of blow-holes. A handful of rosin thrown
into the melting tank occasionally helps to keep the metal clean,
but the metal nearest the surface always contains more or less foreign
matter.
[Illustration: Fig. 6. Section of Soss Die-casting Machine]
While this description of the operation of the die-casting machine may
convey the idea that the process is a slow one, as a matter of fact,
the time required is, on the average, not over a minute and a half
for turning out a finished casting. With the ejection of the casting
from the dies, the product is completed, in theory; but in practice
there are always a few small thin fins, caused by the air vents or by
improperly fitted portions of the dies. It is, however, but the work
of a few seconds to break off these fins, and unless there are many of
them, or they are excessively thick, they are detrimental neither to
the quality nor the quantity of finished castings.
Points on the Operation of the Die-casting Machine
We have now taken up the description and general operation of the
die-casting machine, but like every other machine, there are numerous
little kinds and practices in its working the observing of which makes
the difference between good and poor die-casting. Some of these points
are here given.
The casting machine is best operated by three men, one of whom attends
to the compression lever and the metal supply in the tank. The other
two men stand on each side of the die-end of the machine, and it is
their duty to operate the sprue-cutter, open the dies and remove the
finished casting, clean the dies with air and close them, throw back
the die-plates to their casting position over the cylinder outlet, and
do any other work incident to the operation of the machine. While it
requires three men to operate a die-casting machine in the best manner,
the man who attends to the compression lever has a good deal of spare
time between strokes, and if two or even three of the machines are
conveniently placed, one man can easily pull levers for all three.
The metal should be kept just above the melting point and at a
uniform temperature. If the metal is worked too cold, the result will
manifest itself in castings that are full of seams and creases, and it
will be difficult to “fill” the thin places in the dies. If, on the
other hand the metal is allowed to get too hot, the die will throw
excessively long fins, the castings will not cool as quickly in the
die, and consequently they cannot be made as rapidly. On account of
the importance of keeping the metal at a uniform heat, the fresh metal
that is added to that in the tank from time to time, is kept heated in
a separate furnace. Therefore, when the metal in the tank gets low, the
new supply does not reduce the temperature of the metal being worked.
Some casters use a thermometer to indicate the heat of the metal.
Casting-dies require lubrication frequently. Just how often they should
be lubricated depends on the shape of the die, the composition of the
casting metal, and the general performance of the dies. Beeswax is
the common lubricant, and the lubrication consists in merely rubbing
the cake over the surfaces of the dies that come in contact with the
casting metal. In die-casting large parts, the dies must be kept cool
by some artificial means, for hot dies are conducive to slow work and
poor castings. To reach this end, large dies are sometimes drilled and
piped so that water may be circulated through them to keep them cool.
In the Soss machine, the burners are so placed that the metal in the
cylinder is kept at a slightly higher heat than that in the tank
proper. This condition is brought about by having the cylinder directly
over the burners. The value of this feature lies in the fact that
gas is not wasted in heating the entire tank full of metal to this
higher heat, but still the metal under compression is at the required
temperature. The gas consumption of the average die-casting machine is
about 100 cubic feet per hour.
The speed at which die-castings may be produced varies with the size of
the castings being made, the composition of the metal being cast, and
the style of dies that must be employed. In many cases, in die-casting,
separate brass or steel pieces are used, that must be placed in the
dies before each operation so that they will be inserted in and become
a part of the finished casting. The dies may be difficult of operation
on account of draft problems or pins and screws that must be inserted
in the dies and removed from each casting before another can be made.
These different types of dies will be more fully described in the next
chapter. Taken as a whole, from ten to sixty pieces per hour are the
maximum limits for speed in die-casting, and with a well-working die,
of simple construction, a speed of forty pieces per hour is considered
good production. It is possible, however, when the castings to be
produced are small in size and simple in shape, to gate a number of
them together, or rather to construct the dies so that six or more
castings may be made at once. By this means it is often possible to
cast five or six thousand pieces per day of ten hours, on a hand
die-casting machine.