MAKING DIES FOR DIE-CASTING MACHINES
The making of casting-dies calls for ingenuity and skill of the highest
order on the part of the die-maker. There is probably no class of
die-making in which the work produced is more faithful to the dies,
both in showing up the little details in the making that reflect credit
on the dies, and in exposing the defects and shortcomings in the
workmanship, if there be any. The castings from casting-dies or molds
as they are sometimes called, may be produced in dimensions down to
ten-thousandths for accuracy if necessary, and once the dies are made
the castings will not vary in the slightest degree, if the working
conditions are kept uniform.
In spite of the close work required in making casting-dies, the work is
very fascinating. Perhaps it is on account of this accuracy; possibly
it is on account of the fact that they are made from machine steel;
but most likely it is because there are no hardening troubles to be
contended with. Another factor that makes the work interesting is the
ingenuity required in the work, for almost every die-maker, if he is
worthy of the name, likes to figure out and plan for the best way of
building a die for a difficult job.
General Principles of Casting-die Making
Casting-dies, or molds, have little in common with sand molds. It
is true that the dies for die-casting are composed of two parts
corresponding to the cope and nowel of the sand mold, but they are
so different in every other way that no benefit would result from a
comparison.
Generally speaking, casting-dies are made of machine steel; the parts
which are exceptions are the heavy bases and frames, which are made of
cast iron, and the dowel pins and small cores, usually made of tool
steel. Except in rare instances, there are no hardened parts about a
casting-die; this is the case because the melting points of some of the
alloys that are die-cast are high enough to draw the temper from any
hardened parts of the dies.
The ideal die is simple in construction, with as few parts as
practicable; the castings should be easily ejected and should come
from the dies as nearly free from fins as possible. To meet these
requirements in the best way is the proposition that confronts the
ingenuity of the die-maker. As the die is primarily in two parts, there
must be a parting line on the casting. This line is always placed at
the point that will permit the casting to be ejected from the dies in
the easiest manner possible, bearing in mind the effect the joint will
have on the appearance of the finished casting; this is a point far
less important than with sand casting, for, if the dies are properly
made this seam will be barely perceptible. When it is practicable to
do so, it is wise to have the parting line come on an edge of the
die-casting. Draft is unnecessary on the straight “up-and-down”
places, but of course it is impossible to draw any parts that are
undercut. Means must be provided for ejecting the casting from the
dies after completion and it is usually done by means of ejector pins,
though frequently it is better to have the bottom of the die or some
other section movable and do the ejecting on the same principle that
is used on drawing dies of the compound type. On close work, shrinkage
plays an important part, and the amount of shrinkage varies from 0.002
to 0.007 of an inch per inch. Aluminum shrinks the greatest amount,
Parsons white brass shrinks considerably, while tin shrinks but little.
Thus, it may be easily seen that to figure the shrinkage allowance for
an alloy that contains three or four metals with different shrinkages,
requires judgment. To prevent the air from “pocketing,” air vents are
necessary at frequent intervals around the die-cavity. These vents are
made by milling a flat shallow cut from the die-cavity across the face
of the die to the outside edges of the block. From ¼ inch to ½ inch
is the usual width and from 0.003 to 0.005 of an inch, the customary
depth, varying with the size and shape of the die in question.
[Illustration: Fig. 7. Disk cast in Simple Casting-die]
The dies or molds for die-casting are of various styles, as are also
punch-press dies, and it would be difficult to lay down specific
rules for their classification. There are the plain dies, without
complications of any kind; slide dies with one or more slides; dies
for bearings, both of the “half-round” and of the “whole-round” types;
dies for gated work; and many other less important classes. Then there
are dies that have features that belong to more than one of these
types, so that it is easily seen that to decide upon the style of die
that would be best for a given piece of work requires a good deal of
experience. Some of the most important of these types can best be shown
by illustrating dies made in the various styles, showing, step by step,
how the dies are made and assembled. To begin with, consider the making
of a casting-die of the very simplest form.
In Fig. 7 is shown a plain flat disk made by die-casting. In actual
practice, a die would not be made for such a simple piece, unless
there were some features about it that would prevent it being made
on a screw machine or with press tools. It might have a cam groove
cut in one of its flat sides, the sides might be covered with scroll
work, there might be gear teeth around its circumference, or a hundred
and one other conditions to make die-casting a desirable method of
manufacturing. All these complications are omitted for the sake of
simplifying this initial description of a casting-die.
[Illustration: Fig. 8. Simple Casting-die for Casting Block shown in
Fig. 7]
Fig. 8 shows the die for this piece in plan and sectional elevation.
_A_ is a square cast-iron frame, made from a single casting. This frame
or box, as it is generally called, is planed on the top and bottom
only. Next, the two die-halves _B_ and _C_ are shaped up from machine
steel. In this casting-die, and in the majority of others, these
die blocks are square. The lower half of the die _B_ is held to the
cast-iron frame by fillister head screws, set in counterbored holes,
thus sinking the screw-heads under the surface of the block. The upper
half of the die _C_ is located upon _B_ by dowel pins driven into _B_
which have a sliding fit in the reamed holes in _C_. This being done,
the die-half _B_ is fixed to the faceplate of the lathe and the recess
bored for the die-cavity. This operation is a simple one in this case,
for it is merely a straight hole one-half inch deep and three inches in
diameter. Of course this recess must be carefully finished with a tool
that has been stoned up to a sharp edge, using lard oil. Emery cloth
should be used as little as possible. It is unnecessary to give this
hole draft, but it must be free from ridges or marks that would prevent
the casting from being pushed out. If the faces of the dies are spotted
with a small piece of box wood or rawhide held in the drill press and
kept charged with flour emery, the die-casting will reproduce this
“bird’s-eye” finish and the appearance will well repay the few minutes
additional time that it will take. The spotting should be done with dry
emery (without oil) to get the brightest finish. The upper die-half _C_
is simply ground on its working face. The outside corners and edges of
the faces of both die-halves should be well rounded off so as to insure
the absence of slight dents or rough places that might prevent the dies
from fitting perfectly.
The ejecting mechanism must next be considered. Lever _D_, pivoted
from bracket _E_, has a steel pin _F_ that engages in the elongated
hole in bracket _G_, so that an upward pull of the lever _D_ raises
bracket _G_, which is attached to ejector-pin plate _H_. This plate is
a loose fit over the guide screws _I_ that are attached to the lower
die-half _B_. The ejector pins _J_, four in number, in this die, are
riveted into the ejector-pin plate, and they work through holes drilled
and reamed through the lower die-half. The ends of these pins must be
finished off so as to lie perfectly flush with the inside of the die
when ready for operation and, of course, they must be a sliding fit in
the holes in the die.
An important feature of a casting-die is the sprue cutter, shown in
this die at _K_. If the disk for which this die was made, had had a
hole or central opening of any kind, the sprue cutter would best be
operated at that point; but, as this disk is plain, the sprue cutter
must be placed at the edge. At the outside of the die-cavity, as shown
in Fig. 8, the opening for the sprue cutter is laid out, drilled
and filed to shape. It is obvious that the side of the sprue cutter
adjacent to the die must fit the outline of the die perfectly, so that
there will be no break in the appearance of the casting. The opening
for it is extended through the upper die-half, and from a point ¼ of an
inch from the inside face of the die this hole is flared out nearly as
large as the opening through the die-plate of the machine. Of course
the apperture in the upper die-half must be no larger than the opening
through the die-plate; otherwise the sprue could not be pushed out. The
sprue cutter itself is a long rod, whose section is of the same shape
and size as the openings just made, and it is connected to the sprue
cutting mechanism of the machine. Of course it is unnecessary to shape
the entire length of the sprue cutter to size; after the working end is
milled to shape for a distance of six or eight inches, the rest of the
rod may be left round. The sprue cutter is finished first, after which
both the openings in the die are fitted to it; and while the fit should
be metal tight, it must be perfectly free to slide.
The dies are mounted on the die-plates of the casting machine by means
of straps, much the same as bolsters are held on punch press beds. The
position of the die on the die-plate must be such that the opening for
the sprue cutter will line up with the nozzle at the outlet of the
cylinder. At the time of casting, the position of the sprue cutter is
as shown in the illustration of this die, Fig. 8. In this position
there is room for the metal to enter the die-cavity, and yet there is
but a small amount of metal to be cut off and pushed back after the die
has been filled with metal.
With slight modifications, the above style of die may be used for
die-casting any piece that will draw or pull out of a two-part die. If
holes must be cast through the piece, it is only necessary to add core
pins to the lower die _B_, a point that will be more fully described
later. It is unnecessary to add that both halves of the die may be
utilized in making the cavity for the die, should they be needed. Also,
it is often easier to machine out the recess larger than is needed, and
set in pieces in which parts of the outline of the die-casting have
been formed. Gear teeth are put in the die in this way; a broach is cut
similar to the gear desired, then hardened and driven through a piece
of steel plate which is afterward fitted to its place in the die.
Slide Dies
The die illustrated in Fig. 9 is one of the most successful of the
various types of casting-dies, and if properly made is an interesting
piece of die work. The principal use of this particular style of die,
called a slide die, is to cast parts like the one shown in Fig. 10,
which is a disk similar to the one which the last die described was
to cast, except that it has raised letters at the edge and a hole in
the center. It is obvious that the die last described, (Fig. 8), would
not do for disks or other pieces having projections or depressions
around their edges, as, for instance, printing or counting wheels with
raised or sunken characters, or grooved pulleys. Briefly, this style
of die is similar to the simple casting-die, except that slides are
provided, to the required number, which form the edge of the casting.
A die for a plain grooved pulley would require but two slides, while
a die for a printing wheel with forty letters around its edge would
necessitate forty slides, one for each of the letters. The die about to
be described, shown in Fig. 9, was made to cast a wheel with six raised
letters.
Referring to Fig. 9, _D_ is the cast-iron box or frame, _E_, the lower
die, and _F_ the upper die. In making the lower die-half, the stock is
first shaped to size and doweled to the blank for the upper die-half,
and the holes for attaching to the frame are drilled. For the sake of
clearness, these holes and screws are omitted from the illustration as
are also the vents, since they have been fully explained. The lower
die is next strapped to a faceplate, trued up, and bored out nearly
to the diameter of the body of the piece to be cast, exclusive of the
raised letters. The depth of this recess is equal to the thickness of
the printing wheel plus 3/16 inch to allow for the cam ring _G_ that is
used to reciprocate the slides of the die. The cam ring is made large
enough to cover the die-cavity as well as the slides that surround
it, with an allowance of an inch or two for the cam slots _H_. The six
slides _I_ are made long enough to have good bearing surfaces. With the
size of the cam ring determined, the die is next bored out to receive
this cam ring and the last inch of the recess is carried down to the
depth of the die cavity so as to make an ending space for the slots
that the slides are to work in. The die is now taken from the faceplate
and the slots for the slides laid out.
[Illustration: Fig. 9. Slide Die for Casting the Printing Wheel shown
in Fig. 10]
These slots may be milled or shaped, but milling is to be preferred.
The next step is the making and fitting of the slides, which are of
machine steel, having a good sliding fit in the slots. The six slides
are fitted in position and left with the ends projecting into the die
proper. The slots _H_ are next profiled in the cam ring _G_, and the
pins _J_ that work in them are made and driven into the holes in the
slides. With the slides and cam ring in place, the cam ring is rotated
to bring all the slides to their inner position where they are held
temporarily by means of the cam ring and temporary screws. The die-half
with the slides thus clamped in the inner or closed position, is set up
on the lathe faceplate and the die-cavity indicated up and bored out to
the finish size, which operation also finishes the ends of the slides
to the proper radius. The die may now be taken down and the slides
removed to engrave the letters upon their concave ends. The engraving
can be done in the best manner on a Gorton engraving machine, but if
such a machine is not available they may be cut in by hand. Stamping
should never be resorted to for putting in the letters, because the
stock displacement would be so great that it would be impossible to
refinish the surface to its original condition. Before fitting the
cam ring, an opening must be milled in the die to allow the handle to
be rotated the short distance necessary. After the cam ring has been
fitted, it is held in by the four small straps _K_, attached by screws
to the lower die-half at the corners.
[Illustration: Fig. 10. Printing Wheel cast in a Slide Die]
The sprue cutter, which is not shown, is operated through the hole
in the center of the piece and is, of course, round in this die.
Its action is the same as was the one previously described, and the
ejecting device is similar, with the exception that the brackets _L_
that are attached to the ejector-pin plate M, are widely separated so
as to make room for the sprue cutter that works through a hole in the
plate _M_.
Die for Casting with Inserted Pieces
For making die-castings that are to have pieces of another metal
inserted, it is necessary to have a die with provisions for receiving
the metal blank and holding it firmly in position while the metal is
being cast around it, and of course the piece must be held in such a
manner that it can be easily withdrawn from the die with the finished
casting.
The die illustrated in Fig. 11 is for a part that is used as a swinging
weight, shown in Fig. 12. The upper part of the piece is made from a
sheet steel punching, so as to lighten this part of the piece as well
as to give increased strength, especially at the hole at the pivoted
end of the work. The cast portion of the piece is slotted lengthwise,
as the illustration shows; and three holes pass through the casting,
piercing the sides of the slot. In addition to showing the method of
making dies for inserted pieces, this die shows the principles of
simple coring.
[Illustration: Fig. 11. Casting-die for Making Castings with Inserted
Pieces like that shown in Fig. 12]
In making this die, two machine-steel blanks are planed up for the
upper and lower halves of the die, _A_ and _B_, the lower die being
made nearly twice as thick as the upper die because it is in this part
that the most of the die-cavity will be made. In this lower half of
the die the stock is milled out to the same shape as the outline of
the plan view of the casting, being carried down to the exact depth
of the thickness of the casting. From the wide end of this recess the
stock is milled or shaped out in a parallel slot to the outside of the
die-block. At the bottom of the side of this wide slot are T-slots
to guide the slide _E_ that is to work in this opening. The side is
milled and fitted to the T-slots and opening in the die, but is left
considerably longer than the finish size. Next, the slide is mounted
on the faceplate of a lathe and turned out on the end with the proper
radius and a tongue to form the slot that is to be in the curved end
of the casting. At the outer end of the slide is left a lug that is
drilled and tapped for the operating lever _F_ that reciprocates the
slide, using the stud in bracket _K_ as a fulcrum.
Two pieces of machine steel are next shaped and finished up to form the
chamfered part of the casting and to locate the inserted steel punching
in the die. The combined thickness of these pieces _C_ and _D_ is equal
to the thickness of the casting, less the thickness of the inserted
piece. It is now an easy matter to seat section _D_ in the bottom of
the milled part of the lower die-half, and to locate section _C_ in its
proper position on the upper half. A pilot pin _M_ is fitted in _D_ to
hold the steel punching in position by means of the hole that is in the
extreme upper end of the punching. The pilot pin extends through this
hole into a corresponding hole in section _C_. At the lower end of the
steel part that is inserted, there are two holes the object of which
is to secure the punching to the die-casting, for the molten metal
runs through these holes, practically riveting the die-casting to the
inserted piece.
[Illustration: Fig. 12. Die-cast Weight with Inserted Sheet-steel
Punching]
Provision has now been made for holding the sheet-metal part that is
to be inserted, and the cavity has been completed for the casting,
including the tongue at the end; it now remains to describe the manner
of forming the holes that pierce the casting through the slotted
portion. In the lower die-half the positions of the three holes _H_ are
laid out, drilled and reamed. Then, with the two die-halves together
and the slide clamped at its inner position, the holes are transferred
through the slide and the upper die. This being done, it is an easy
matter to make core pins and drive them into the upper die at the two
end holes, the center hole being taken care of by the sprue cutter _L_
that will be described later. The core pins should be a nice sliding
fit through the slide and in the holes in the lower die, into which
they should extend from a quarter to half an inch. In addition to
coring the holes, these pins act as a lock to hold the slide _E_ in its
proper position at the time of casting.
The sprue cutter _L_ is most conveniently operated in the center hole,
thus doing away with the core pin that would otherwise be required. The
sprue cutter needs little description in this die, for as in the slide
die, it is merely a plain round rod that fits closely in the holes
through the dies and slide. The ejector mechanism is the same in this
die as in the dies already described; therefore further description is
unnecessary.
[Illustration: Fig. 13. Casting-die for the Half-round Bearing shown in
Fig. 14]
The operation of this die is very simple. The sheet-steel piece is laid
in the recess in the open die, being located by the pin _M_. Slide
_E_ is thrown in by means of lever _F_, and the dies are closed. At
the time of casting, the sprue cutter in is the position shown in
the sketch, being nearly through the die-cavity. As before explained,
this position admits the molten metal to pass into the die-cavity, but
still leaves very little sprue to be cut off after the die-casting is
completed. It should be stated that the steel piece that is inserted
must be perfectly flat and free from burrs that would prevent the
die-halves from coming together properly.
Bearing Dies
Bearing dies are one of the most important of the various classes of
casting-dies. The bearings produced by die-casting are so far superior
to those made by other casting methods and machining that their use is
now very extensive. Dies are made for “half-round” and “whole-round”
bearings. There is little out of the ordinary about a whole-round die,
but the half-round die involves many interesting methods of die-making,
and for that reason is here described.
Fig. 13 shows a casting-die for half-round bearings. Half-round bearing
dies are usually made to cast two bearings at a time, for the reason
that it is just as easy to cast two pieces of such a shape as it is to
cast one, and, in addition, the die is balanced in a better manner. As
with other dies, the first step is to machine up the frame _A_ and the
two die-halves _B_ and _C_. The pieces _D_ and _E_ that are to form
the insides of the bearings are then turned up and one side of each
shaped and keyed to fit the slots that have previously been milled in
die-half _C_. These parts are held in place by dowels and screws. One
of the bearings produced by this die is shown in Fig. 14, and it will
be noticed that there is an oil groove within that covers the length
of the bearing. To produce this groove in the die-castings, a shell
must be turned up and bored out whose inside diameter is that of the
inside of the bearing, and whose thickness equals the depth of the oil
groove. This being done, the oil grooves are laid out upon the shell
and cut out by drilling and filing. After rounding the outside corners,
these little strips are pinned to the cores _D_ and _E_ in their proper
places.
[Illustration: Fig. 14. Die-cast Half-round Bearing, Showing the Cast
Oil Grooves]
Another little kink in this connection is worthy of noting. So many
different styles and sizes of bearings are made by a concern doing much
die-casting that it is essential that the die-cast bearings should
bear some distinguishing number to identify them. As this number is
of no consequence to the user it is well to have the number in an
inconspicuous place, but it must be where it will not be effaced by
scraping, etc. Bearing in mind that it is much easier to produce raised
lettering by die-casting than to produce sunken lettering, it will be
readily seen that the oil groove affords a good place in which to
put the bearing number. This is easily done by stamping the figures
upon the narrow strip that forms the oil groove. In this place on the
bearing it may be easily found if needed, and of course there is no
danger of its being taken out by machining.
The lower die consists of two blocks _F_ and _G_, each of which
contains an impression of a bearing. The best way to make these parts
is to lay out the ends of each of the blocks with the proper radius,
taking care to have the center come a little below the surface of the
face of the block. Then the blocks should be shaped out to get the bulk
of the stock out, before setting up in the lathe. After the lathe work
is done on each piece, which of course is usually done separately, the
faces of the two blocks are faced down just to the exact center of the
impression. It will be noticed that two blocks are used for the lower
part of the die. The reason is to facilitate the locating of the female
parts of the die in proper relation to the male parts. After properly
locating, they may be doweled and screwed to baseplate _B_.
[Illustration: Fig. 15. Interesting Examples of Die-castings]
The sprue cutter _H_, better shown in the plan view, is square in shape
and connects with the die-cavities in a thin narrow opening on either
side of the sprue cutter. The ejector pins, _I_, two to each die, are
at the ends of the bearings. The ejector-pin plate _J_ is necessarily
large, and is operated by lever _K_.
Fig. 15 shows a number of interesting examples of die-castings.