IN 1943 the Sperry Gyroscope Co.
used approximately 4 million die cast
ings as component parts o f precision in
struments built for the military forces and government departments. In 1944 it used approximately 7% million, and as this is written, plans are going for
ward for additional conversion to die casting.
The reason for this rapid increase in die casting applications is that early in 1944 Sperry set up its own die casting department, equipped with four high- pressure die casting machines such as those shown in Fig. 1. As a result, a good percentage of die castings previ
ously purchased outside are now made in the Sperry plant, and design engineers who have this process available in the plant are now intensely interested in its possibilities and by their new designs are causing the total use of die castings to expand tremendously.
By S. U. SIEN A
Die C astin g S u p erin ten d en t Sperry Gyroscope Co. Inc.
G reat Neck, I. I., N. Y.
A number of distinct advantages have been realized from the installation of our own die casting department. We have been able to speed up delivery of die castings from design to finished part, which has served as a stimulus to de
sign engineers. F ar more important is die fact that we are able to produce, day after day, first rate die cast parts which must pass the most severe inspec
tion requirements, and we are able to
make them at high speed with minimum rejection rates. W e are thus able to avoid the losses resulting from machining castings which are found to be defective.
Design engineers, once skeptical of die casting as a practical process for parts where high degree of density, strength, stability and unformity are involved, can now design critical parts for die casting with confidence that the finished parts will meet Sperry specifications, which are among the strictest employed in man
ufacture for military use.
The millions of die castings made at Sperry are all produced by die "slow- squeeze" method of cold chamber in- j'ection, most of them on machines equipped with pre-fill injection systems, which are capable of the extremely high injection pressures necessary for die uniform production of high-density aluminum alloy die castings.
These machines, designed by the Lester Engineering Co. for
Lester-F ig. 4— T his unusual d ie casting job is a “squirrel ca g e” fo r a sm all induction m otor with 36 annular lam inations cast in place. F in ished rotor is show n at 4A, p ie c e as cast at 4B and test p ie c e tvitli lam inations etch ed out with
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Phoenix Inc., Cleveland, are built to give greater concentration of pressure at the instant the injection plunger comes home to squeeze the cooling metal into the die cavities.
There has been considerable emphasis in the past upon high injection speeds for cold chamber die casting and hy
draulic accumulator bottles have been widely used to step up the speed of injection. Actually such high speeds have been the source of considerable difficulty in the production of sound aluminum die castings.
Porosity, the chief defect of inferior castings, has two causes: (1) voids caused by shrinkage; (2) voids caused by trapped air and gases. Molten metal, shot into a die at high speed, rapidly spreads over the die walls sealing the air vents and trapping air and gases within the casting. Without high pressure to squeeze the resulting voids out of the metal and the ability to maintain such high pressure on the casting as it cools, inferior castings may result.
The designers of the Lester-Phoenix machines and of the pre-fill injection sys
tem, were familiar with the success of the permanent mold process, in which metal is slowly poured by gravity into heated metal dies, flowing from the bot
tom upward, allowing air and gases to escape ahead of metal. Castings so pro
duced were virtually free of porosity.
The slow squeeze injection method and the pre-fill injection system were
developed to simulate permanent mold
ing and also to provide high final in
jection pressure on the casting as. it chills. The pre-fill system (see Fig. 2) eliminates the necessity for the nitrogen filled hydraulic accumulators usually used for high injection speeds and steps up injection pressure as high as 33,000 p.s.i. in the production of 3-pound alum
inum castings. Accumulators shoot the metal into the die at high velocity but have little energy left for the applica
tion of pressure on the casting at the end of the injection stroke, while pre
fill injection applies and sustains its highest pressure at that point. In the pre-fill system injection speed and pres
sure can be varied independently of one another, affording a variety of speed- and-pressure combinations for various types of dies and castings.
The pre-fill injection system consists (see Fig. 3) of a hydraulic cylinder fitted with a large actuating piston, the
hollow piston rod of which contains a passageway to a smaller inner fixed pis
ton. Oil under 1000 p.s.i. hydraulic pressure is introduced through the hol
low piston rod, displacing at high ve
locity the small piston opposing it, which piston also carries with it in its forward movement the piston rod and also the attached main or large piston.
As the latter moves forward at high speed, oil flows by gravity from a ver
tical storage tank through the pre-fill check valve to occupy the space back of the large piston. W hen the die cavi
ties have been filled and the injection plunger meets resistance, 2 0 0 0 p.s.i. oil pressure from a motor driven booster pump is applied directly to both pistons, resulting in the high sustained final in
jection pressure which packs the metal into the die. When the injection pistons reverse, the oil is returned to the gravity tank, ready for the next cycle.
As may be seen in Fig. 2, the ma-Fig. 2— C lose-up o f operator pouring m etal fo r a “shot." N ote large injection cylinder w ith oil reservoir rising at right and m assive fram e an d d ie locking
m echanism at le ft o f operator
Fig. 3— D rawing o f th e pre-fill injection unit show s large an d sm all pistons Fig. 1— Section o f the Sperry G yroscope Co.'s d ie
casting departm en t show ing th ree Lester-Phoenix H H P-3-C C m achines
May 14, 1945 109
piece was reduced to 5 minutes per piece, when the parts were produced by die casting.
All Sperry die castings are subjected to fluoroscope, X-ray, or air pressure in
spection; some of them are required to meet all three. The percentage of cast
ings subjected to X-ray inspection ranges from 1 0 per cent on the average run of parts to 1 0 0 per cent on a few high
ly critical parts.
The castings shown in Figs. 4 to 13 inclusive are parts for gun sights, bomb sights, flight instruments, gyro-pilots, stabilized turrets, firing cut-off controls and similar instruments. Fig. 4A shows an extremely difficult part which we are now producing very satisfactorily.
It is a “squirrel cage’' for a small in
duction motor, and is made by stacking 36 annular laminations of electrical iron, each 0.008-inch thick, in a stacking fix
ture in such a manner that the 17 slots around the inside circumference of each ring are aligned to form oblique chan
nels. The laminations then are pressed tightly together and placed on an arbor to be used as an insert in tbe die.
In casting aluminum around this in
sert, the injection plunger must force metal through the 17 channels, in such a way that no gas or air is trapped. The gentle action of “slow-squeeze” injection permits the escape of tire gas and air and packs the metal in solidly. If ex
cessive porosity were present in these castings, it would be easily detected after machining by inspection and electrical test and would result in the automatic re
jection of the part, which would be doubly wasteful because of loss of the laminations. A criterion of the success in the production of this part is the fact that it is being made at the rate of 100 to 125 per hour in a single cavity die, with very few rejections.
Fig. 4B shows tire casting as it comes from the die, with a heavy section of the laminated insert protruding. This protuberance is machined off and then the part is ground inside and out, tak
ing 0.040-inch off each diameter and chines in this department are equipped
with accumulator bottles. These are not regularly used in the die casting of aluminum alloys but are kept available for use, along with the pre-fill system, for the die casting of magnesium, which chills very rapidly and may require ex
tremely high injection speed, particularly if thin wall sections of large area are to be filled.
These pre-fill equipped machines have enabled us to produce a great number of very difficult die castings, many of which had never been made by the die casting process. Conversion from other processes to die casting has saved con
siderable money on long runs and has proved practical even on short runs. For instance, a $ 2 2 0 0 die was used to make 2 0 0 pieces, yet the final cost of tire finished part, including amortization of the die, was 1 0 per cent less than that of sand cast parts. Time required for production of the sand cast part (in
cluding cleaning, sand blasting, machin
ing and heat treating) of 2 hours per Fig. 5— L ittle m achin e w ork is required on stabilizer chassis
(5A ) and b racket ( 5 B ) Fig. 6 — L in ks em p loy ed in the
m echanism o f a gyro d ev ice
Fig. 7 — At 7A is show n fou r d ie castings com prising com p lete housing K- 12 gun sight at 7B
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- ' O ’ ~ ~ ‘ y V I W U V n / k v i u i l l / l / o U i t < I l M U \ y n K U O i / V / l / l t / U I U U l / U U l U l U l l U g o u i
blan k fo r a K-13 V ector sight, bein g 0.001-inch in case o f slot at left o f hub.
Stainless steel insert in center serves as bearing
Fig. 9— Sperry produ ces 40 o f these gun sight pedestals p er hour. L ittle m a
chining is necessary
Fig. 10— Pendulum b od y fo r an A and N instrument. It must b e extrem ely solid and sound b ecau se o f m achining requirem ents
Fig. 11-13— Sperry m akes m ost d ie castings, such as th ese three exam ples, in single cavity d ies
Fig. 14— T h e infinite variety o f d ie castings requ ired by Sperry for instru
m ents is w ell illustrated h ere producing die finished part (Fig. 4B).
Fig. 4C shows the aluminum die cast portion with all the iron laminations dis
solved out by acid.
The parts shown in Fig. 5 (Fig. 5A is a chassis and 5B a bracket for stabilizing unit) must also be sound structurally.
In order to obtain early production, these parts were designed for both die cast
ing and sand casting, and the latter proc
ess got into production first. The parts now are all die cast. The chassis re
quires no machining of its inside and outside surfaces as in the case of sand castings. On the bracket, tolerances tor the adjustable parts were so close that several surfaces had to be created by machining, notably the lugs, which could not be sand cast to the correct toler
ances but had to be milled individually.
A bearing screws into the large threaded hole which can be seen inside the chassis, which hole must be aligned with a pivot to within 0.0003-inch. Any porosity in the casting would make this alignment impossible because holes and pits in the structure of the metal would throw off the chasing of the thread, causing inac
curacy of alignment.
Figs. 7A and 7B show a complete housing for a K- 1 2 gun sight, which is now made of four die castings. Convert
ing them to die casting reduced machin
ing 80 per cent, reduced the weight 30 per cent (which is always a desirable factor in an airborne instrument) and resulted (because of the accuracy of die cast parts) in a much more compact unit, occupying less space and requiring less clearance in places where moving parts must function adjacent to each other.
Without die casting machines of ade
quate holding capacity and high injec
tion pressure, it would be impossible to make castings of this size, weight and complexity, holding the required dimen
sional tolerances and obtaining the mi- ( P lease turn to P age 146)
May 14, 1945 111
I L E C JS O O t slower speeds give greater penetration.
However, facts show that faster speeds give greater penetration, while the slow
er speeds tend to build up more of the metal on the surface. A fillet weld with greater penetration, resulting from faster travel speed appears smaller but penetration resulting from faster arc speeds to obtain the required weld further demonstrated by tire comparison of water flowing from a hose. The ac