Method of electrical discharge machining for manufacture of Belleville springs

A circular steel blank (10) having a central opening (12) is first press formed into a frusto-conical shape and then subjected to a heat treating process to provide the unfinished spring with the requisite shape and hardness. The spring is then mounted in the electrical discharge machining (EDM) apparatus to remove material on one side using a first electrode. The spring is then electrically discharge machined on a second side with a second electrode. In this manner a Belleville spring with an exact machining tolerance can be manufactured.

BACKGROUND OF THE INVENTION 
1. Field of Invention 
The present invention relates to a method for machining an electrode 
workpiece and the electrode workpiece so manufactured. In particular, the 
present invention relates to a method of machine finishing a 
frusto-conical disc spring by an electro-erosion process, wherein exact 
tolerances for the spring are attainable. 
2. Prior Art 
One of the many uses for frusto-conical disc springs, and more particularly 
for Belleville springs, is as a control mechanism for a gas pressure 
regulator. The Belleville spring provides a reference force in the gas 
pressure regulator for precise control of flow rate and airflow pressure 
such as is needed in a face piece for a breathing apparatus. Breathing 
apparatus are particularly useful in hostile environments including those 
typically encountered by firefighters, airplane crews and the like and 
precise regulation of the breathing air pressure at the face piece 
necessitates that the Belleville spring be manufactured to exacting 
tolerances. In order to achieve such preciseness, as well as fast 
production time, the Belleville spring of the present invention is 
produced by electro-erosion using an electro-discharge machining (EDM) 
process. 
The electro-discharge machining process involves providing a high frequency 
pulsed current across a formed conductive electrode and a grounded 
workpiece submerged in a dielectric fluid. The electrode is rotated while 
being moved axially along a work centerline, through the dielectric fluid 
and towards the workpiece, maintaining a constant gap therebetween. A 
dampening rod is used to depress resonant vibration in the workpiece 
during the EDM process while the discharge of an electric spark from the 
electrode at a chosen frequency and current erodes a portion of material 
from the workpiece. Distortion in the surface finish of the workpiece due 
to heat generated during the EDM process is reduced by circulating the 
dielectric fluid through the gap between the electrode and the workpiece, 
thereby carrying away the generated heat as well as the eroded material. 
The power supply frequency, current and electrode movement is numerically 
controlled to regulate the metal removal and thereby provide the proper 
surface finish. 
Prior to subjecting the workpiece to the EDM process to thereby manufacture 
the Belleville spring of the present invention, the general shape of the 
spring is provided by subjecting a piece of steel stock to a press-forming 
process. Such a process is described in U.S. Pat. No. 3,668,917 to Komatsu 
et al., wherein the distortion problems frequently encountered in using 
successive and separate steps of press forming and quenching a piece of 
steel stock to produce the desired shape of a Belleville spring are 
overcome. This prior art method provides for simultaneously press-forming 
and quenching the steel stock placed between a pair of cooperating die 
members upon heating the steel stock to its austenitizing temperature. The 
die members apply opposing pressure to the steel stock while they are 
maintained at a working temperature to rapidly conduct heat, thereby 
quenching the steel stock under forming pressure. The Komatsu et al 
process does not completely eliminate distortion in the final product and 
the resulting frusto-conical disc spring is described as being useful as 
an automotive clutch. This magnitude of tolerance control is unacceptable 
for use as a control mechanism, such as a gas pressure regulator where 
extremely close machining tolerances are required. 
U.S. Pat. No. 4,039,354 to Schober describes a Belleville spring formed 
from a steel blank having a deliberately produced carbon gradient through 
the blank thickness. The level of carbon determines the transformation 
temperature at which austenite transforms into martensite, the desired 
final product. Providing a carbon level at the surface of the steel blank 
that is higher than the internal carbon level is designed to balance the 
temperature gradient set up through the blank thickness during quenching 
wherein the transformation temperature at the surface is lower than the 
transformation temperature range at the core. This enables the spring core 
to martensite prior to the spring surface which reduces the formation of 
internal tensile stresses that frequently lead to cracks and similarly 
undesirable imperfections in the surface finish. Tailoring the carbon 
gradient through the thickness of the steel blank to the correct 
formulation is difficult to regulate. 
U.S. Pat. No. 4,135,283 to Kohlhage describes a method of hardening and 
roughening the surface of a Belleville spring by causing a stream of 
shot-peen to impinge on the spring surface. The surface is then at least 
partially smoothed by grinding and polishing in a drum, or by an 
electro-chemical process. This process does not provide a Belleville 
spring having a surface finish that is acceptable for use as a control 
mechanism for use as a gas pressure regulator, and the like. 
OBJECTS 
It is therefore an object of the present invention to provide an improved 
frusto-conical disc spring having a uniform cross sectional thickness and 
a smooth surface finish. 
It is another object of the present invention to provide a method for 
manufacturing a frusto-conical disc spring by an electro-discharge 
machining process wherein the spring has a uniform cross-sectional 
thickness and a smooth surface finish. 
Still another object of the present invention is to provide a method for 
manufacturing a frusto-conical disc spring that is acceptable for use as a 
control for a gas pressure regulator. 
Finally, another object of the present invention is to provide a method 
that can be rapidly carried out to produce a frusto-conical disc spring 
having a high degree of dimension tolerance such that the unit cost of the 
spring is relatively low. 
These and other objects will become increasingly apparent to those of 
ordinary skill in the art by reference to the following description and to 
the drawings.

DETAILED DESCRIPTION 
Referring now to the drawings, FIGS. 1 to 6 show the process for 
manufacturing a steel blank 10 into a frusto-conical disc. The 
frusto-conical disc preferably comprises a spring and most preferably a 
Belleville spring having a frusto-conical shape provided by an inner 
annular circumference which is spaced axially from the plane of the outer 
spring periphery or outer annular circumference and approaches that plane 
as the spring distorts under a compression force. The amount and shape of 
the distortion is directly related to the machining tolerances of the 
Belleville spring, which are extremely critical if the spring is intended 
for use as a gas pressure regulator, as is well known to those of ordinary 
skill in the art. 
As shown in FIG. 1, steel blank 10 is a generally circular member having a 
central opening or aperture 12. Prior to being deformed into the 
frusto-conical shape, a first machining tool 14 shown schematically in 
FIG. 1, and having a cutting member 16 positioned normal to the plane of 
blank 10, is used to provide a machined finish to the peripheral edge 18 
formed by the outer annular circumference by rotating the blank 10 against 
the cutting member 16. A second machine tool 20, also shown schematically 
in FIG. 1, is provided having a cutting member 22 with blank 10 rotated 
against cutter 10 to thereby provide a machined finish to the inner 
annular circumference 24 of opening 12. 
As shown in FIG. 2, the steel blank 10 having the machine finished inner 
and outer annular circumferences 18 and 24, respectively, is then formed 
into the frusto-conical shape by a press-forming process, as is well known 
to those of ordinary skill in the art. The press forming process comprises 
a hydraulic press (not shown) having a press fixture 30 that includes an 
upper die member 32 providing an upper working surface 34 having a 
generally concave shape for registry with the corresponding first side 36 
of the steel blank 10 and a lower die member 38 having a lower working 
surface 40 provided with a generally convex shape for registry with the 
corresponding second side 42. The steel blank 10 is initially positioned 
on the lower die member 38 with aperture 12 being in registry with a guide 
pin 44 and with the second side 42 of blank 10 facing the lower working 
surface 40 of die member 38. The upper die member 32 is positioned on the 
guide pin 44 so that the upper working surface 34 faces the first side 36 
of blank 10. The assembled press fixture 30 is then mounted in the 
hydraulic press (not shown) that provides for moving the die members 32 
and 38 towards each other with upper die member 32 traveling along pin 44 
and towards blank 10 supported on lower die number 38, as indicated by 
arrow 46 in FIG. 2. Thus, the hydraulic press applies mechanical pressure 
to deform the steel blank 10 into a frusto-conical shape forming the 
frusto-conical disc spring 48 when the blank 10 is squeezed between the 
upper and lower die members 32 and 38 moved towards relative engagement 
with respect to each other, as is well known to those of ordinary skill in 
the art. 
The hydraulic press is then reciprocated to a release position so that the 
press fixture 30 can be removed from the press. The die members 32 and 38 
are separated and the spring 48 is removed from the press fixture 30 and 
positioned in a heat treatment fixture 50, as shown in FIG. 3, for the 
purpose of heat treating the spring 48. Heat treatment fixture 50 an upper 
member 52 having a generally concave working surface 54 for registry with 
the corresponding convex first side 56 of spring 48 and a lower member 58 
having a generally convex working surface 60 for registry with the concave 
second side 62 of spring 48. The upper and lower members 52 and 58 are 
provided with respective axial guide channels 64 and 66 that receive a 
bolt 68 having a threaded end for engagement with nut 70. Upper member 52 
further has an enlarged recess 72 that receives the head 74 of the bolt 
68. This provides for securing the members 52 and 58 in registry with the 
respective sides 56 and 62 of the intermediate spring 48 tensioned between 
the die members 52 and 58 when the bolt 68 is positioned through the axial 
guide channels 64 and 66 and through the aperture 12 in spring 48 with nut 
70 threadingly engaged with bolt 68. 
The assembled heat treatment fixture 50 is then placed in a suitable 
autoclave (not shown) and heated to a temperature of about 900.degree. F. 
for about one hour. The heated fixture 50 and spring 48 are then removed 
from the autoclave and allowed to air cool to complete the heat treatment 
process. The purpose of the heat treatment fixture 50 is to eliminate as 
much as possible any distortion in the material comprising the spring 48 
to thereby obtain the desired hardness and generally to set the spring 48 
to its final shape. 
The preferred material for spring 48 is commercially available 17-7 ph, 
condition C stainless steel. After being subjected to the heat treatment 
process, the stainless steel material has a condition CH 900, as is well 
known to those of ordinary shell in the art. 
As previously discussed, springs that are intended for use as gas pressure 
regulators and the like require exact tolerances that are unattainable 
through conventional stamping and forming processes. Thus, the method of 
the present invention comprises a finishing process wherein the formed and 
hardened frusto-conical disc spring 48 is further machined by 
electro-erosion. FIG. 6 shows a typical electrical discharge machining 
apparatus 80, hereinafter referred to as an EDM apparatus, with portions 
of the machine 80 broken away to show the internal construction comprising 
a base 82 supporting a tank or enclosure 84, partly filled with a 
dielectric fluid 86 of the type particularly useful for electro-erosion 
machining, such as, for example, kerosene, paraffin, light oil and the 
like. Base 82 supports a vertically extending post or column 88 having a 
cantilever portion 90 projecting over tank 84. Cantilever 90 is adapted to 
support a feed and guide mechanism for an electrode tool 92. The feed and 
guide mechanism comprises a stationary cylinder 94 affixed to the end of 
cantilever 90 and a reciprocable piston member 96 disposed for 
reciprocating movement along an internal bore 98 formed within cylinder 
94. Piston 96 is coaxially formed integrally with or, alternatively, is 
affixed to a cylindrical member 100 at a position intermediate the ends of 
cylinder 100. Cylinder 100 is provided with an interior bore 102 (shown in 
dashed lines in FIG. 6) that supports a rotatable shaft 104 with a rotary 
motor 106 mounted on a plate 108 provided at a distal, upper end of 
cylinder 100 and connected to shaft 104 for rotation thereof by coupling 
110. 
The opposite ends of bore 98 in cylinder 94 are closed by end plates, 
identified respectively at 112 and 114. Each end plate is provided with a 
cylindrically shaped plate bore, shown at 116 and 118, respectively, 
forming hydrostatic bearings that serve to support and linearly guide the 
cylindrical member 100 proximate the end portions thereof projecting from 
the cylinder bore 98 and through the end closure plates 112 and 114. The 
lower end of cylindrical member 100 threadingly mates with an insulator 
120 having a recess that carries an extension member 122 provided with a 
rotating union 124 mounted coaxially around shaft 104 (FIGS. 4 and 5) and 
in a rotatable and fluid flow communication with an internal shaft bore 
126 (FIGS. 4 and 5), as is well known to those of ordinary skill in the 
art. Union 124 supplies dielectric fluid from an external pump (not shown) 
through conduit 128 to the rotating union 124, as indicated by arrow 130, 
and then to the electrode tool holder 92, and the interchangeable 
electrode tool means, shown as electrode 194 and 200 in FIGS. 4 and 5, 
respectively, for flushing against the spring 48. The electrode tool means 
is adapted to machine, by an electro-erosion process, an appropriate 
surface of the diaphragm spring 48 to be electrically discharge machined 
into a finished diaphragm spring, preferably of the Belleville type, that 
is suitable for use as a gas pressure regulator and the like. A bellows 
sleeve member 132 is preferably disposed around the end portion of 
cylindrical number 100 to provide protection for the surface of the member 
100 from splattering dielectric fluid and from fumes that my cause a 
superficial attack of the rod surface. 
The advance and retraction of electrode tool holder 92 supporting the 
interchangeable electrode tool means along the work centerline defined by 
the longitudinal axis of cylinder 100 is controlled by a hydraulic 
servomechanism comprising a reservoir 134 filled with an appropriate 
hydraulic fluid 136 circulated by a pressurizing and circulating pump 138 
provided with a pressure regulator 140. Pressurized hydraulic fluid 136 is 
supplied by pump 138 to an electrical solenoid actuated four-way valve 
142, via conduit 144, for the purpose to be hereinafter described. The 
machining electrical discharges between the electrode tool means and the 
spring 48 are supplied by a pulse generator 146 that is connected to the 
cylinder 100 supporting shaft 104 and electrode tool holder 92 and a 
support pedestal means 148 provided in tank 84 to support the spring 48, 
by respective power lines 150 and 152. A control mechanism, such as a 
computer numerical controller 154, is electrically connected across this 
machining gap between pedestal means 148 and electrode tool holder 92 by 
transmission cables 156 and 158, respectively, and is adapted to supply an 
electrical signal to amplifier 160 via cable 162. The output of amplifier 
160 is electrically connected to the solenoid coil 164 of valve 142 via 
power lines 166 and 168, and is arranged to controllably cause valve 142 
to direct pressurized hydraulic fluid to either end of cylinder bore 98 
via upper and lower hydraulic lines 170 and 172 so as to cause 
displacement of piston 96, and consequently cylinder 100, shaft 104 and 
electrode tool holder 92 mounted on the end thereof, in an appropriate 
direction along the work centerline. 
Numerical controller 154 is adapted to sense the voltage across the 
machinery gap provided between the pedestal means 148 and electrode tool 
holder 92 via transmission cables 156 and 158 as previously discussed, and 
to compare this gap voltage to an adjustable reference voltage 174. A 
higher than normal voltage across the machining gap indicates to the 
numerical controller 154 that the machining gap is too wide and presents a 
resistance preventing the machining current from reaching the 
predetermined valve for which current generator 146 and the voltage 
reference 174 have been set. Numerical controller 154 through amplifier 
160 then controls valve 142 to introduce hydraulic fluid 136 into the 
upper portion of cylinder bore 98, or chamber 176, while hydraulic fluid 
136 is exhausted from the lower portion of cylinder bore 98 or chamber 178 
through hydraulic line 172 and through valve 142 in fluid communication 
with return line 180 returning hydraulic fluid to reservoir 134. 
Therefore, piston 96 is urged downwardly, as seen in FIG. 6, thereby 
downwardly displacing cylinder 100, and consequently advancing the active 
or working face of the electrode tool means associated with electrode tool 
holder 92 along the work centerline towards the diaphragm spring 48, 
thereby reducing the width of the machining gap. 
This decrease of the machining gap width causes an increase in the current 
and consequently a decrease of the voltage across the gap towards the 
predetermined valve corresponding to the current setting of the pulse 
generator 146 and the voltage of voltage reference 174. In the event that 
the machining gap is decreased to the extent that there is caused an 
increase in the current flowing across the gap with an accompanied drop in 
the voltage across the gap, which is, to a greater extent, the case in the 
event of a short circuit between the electrode tool means and the spring 
48, the decreased gap voltage below the predetermined voltage reference 
level is sensed by numerical controller 154 which, through amplifier 160, 
controls four-way valve 142 to introduce hydraulic fluid into the lower 
chamber 178 in cylinder 94, resulting in an upward motion of piston 96, 
cylinder number 100 and electrode tool holder 92 to thereby reestablish 
the predetermined voltage level across the machining gap. The advance and 
retraction of the electrode tool holder 92 towards and away from the 
spring 48 along the work centerline, as controlled by valve 142 is thereby 
monitored by the numerical controller 154 to maintain a predetermined 
constant voltage across the gap which is continuously compared to the 
voltage of voltage reference 174. As the width of the machining gap is 
regulated to maintain the predetermined constant gap voltage, the motor 
106 mounted on plate 108 provides for rotating the shaft 104 inside the 
cylinder 100. This rotational movement is indicated by arrows 182 in FIG. 
6, and will hereinafter be explained in detail. 
In a typical electro-erosion process using EDM apparatus 80 just described, 
the electrode tool means and the frusto-conical disc spring 48 are 
separated by a small gap, typically 100 .mu.m, filled with the dielectric 
fluid 86. An applied voltage, usually about 80 V, is applied across the 
machining gap. Current flow is preferably in excess of 5 amperes and 
results in the formation of a dielectric vapor bubble due to Joule heating 
that plays a significant part in the sparking action in the EDM process. 
After an "ignition delay," typically of about 0.1 to 5 .mu.s, material 
breakdown of the spring 48 surface occurs. Sparking then takes place 
across the inter-electrode machining gap. In order to prevent arcing, the 
voltage is removed after a short interval. A further short interval is 
then allowed to elapse before the next voltage pulse so that the 
dielectric fluid in the inter-electrode machining gap can deionize. The 
consequence of a series of voltage pulses applied across the gap is the 
production of a set of random discrete discharges. The discharges affect 
both the electrode 110 and spring 48, causing the local temperature to 
rise to about 4,000.degree. to 10,000.degree. K. This intense heat at the 
electrodes results in metal removal by vaporization. 
FIGS. 4 and 5 show in detail the process of finish electro-erosion of 
material from the respective concave and convex sides 62 and 56 of the 
spring 48 by means of the EDM apparatus 80 previously described in detail. 
The spring 48 is initially mounted on a first pedestal 184 that is 
supported in tank 84 immersed in the dielectric fluid 86, pedestal 184 
having a recessed concave support surface 186 for registry with the convex 
side 56 of the spring 48. 
A threaded opening 188 is provided along the axis of pedestal 184 and mates 
with a dampening means, such as screw 190 having an enlarged head 
contacting the inner annular circumference of aperture 12 to hold spring 
48 on pedestal 184 in the desired position and to quell resonance 
vibrations that may be set up in spring 48 as material is removed by means 
of electro-erosion between the convex shaped working surface 192 of 
electrode tool 194 attached to tool holder 92 threaded onto shaft 98 and 
the concave side 62 of spring 48. As shown in FIG. 4, tool holder 92 is 
provided with a manifold 196 that is in communication with a plurality of 
parallel through channels 198 for directing dielectric fluid 86 against 
the concave side 62 during the short interval when sparking is not 
occurring between tool 194 and spring 48. This serves to flush any 
material that has been removed during the previous electro-erosion event. 
Manifold 82 is supplied with dielectric fluid 86 from tank 84 by means of 
a suitable pump (not shown) that feeds through conduit 128, as shown by 
arrow 130 in FIG. 6, to the rotating union 124, in communication with 
manifold 196. The dielectric fluid 86 washes against the concave side 62 
of spring 48 to cleanse any removed material and to provide a clean 
surface with which to reestablish another electro-erosion event between 
electrode tool 194 and spring 48, as previously discussed in detail, to 
further remove material from the concave side 62 of spring 48. This 
process is repeated until side 62 is similar in shape to that of the 
working surface 192 of tool 194. The rotational movement of tool 194 
prevents the formation of built-up material on spring 48 at positions 
corresponding to the fluid channels 198. Machining tolerances that are 
obtainable with this type of process enable material removal to be held to 
a tolerance of .+-.0.0002 inches. 
Upon completion of the electro-erosion of the concave side 62 of spring 48, 
electrode tool 194 is retracted from its gapped relationship with spring 
48 by appropriate numerical control of solenoid 164 which actuates valve 
142 to introduce hydraulic fluid 136 into cylinder chamber 178, as 
previously described in detail, thereby reciprocating piston 96 and 
cylinder 100 and shaft 104 along cylinder bore 98 so that tool 198 can be 
removed from holder 92 and replaced with a second electrode tool 200 
having a concave working surface 202 adapted to machine material from the 
convex side 56 of spring 48. Pedestal 184 is then removed from tank 84 and 
replaced with a second pedestal 204 having a recessed support surface 206 
for registry with the concave side 62 of spring 48 held in position on 
pedestal 204 by screw 190 threaded into opening 188. As previously 
described with respect to electrode tool 194, electric tool 200 is 
provided with a plurality of parallel through channels 208 that are 
supplied with dielectric fluid 86 from manifold 196 in tool holder 92 to 
direct the dielectric fluid against the convex surface 56 of spring 48 
during the short interval when sparking is not occurring between tool 200 
and spring 48, to thereby flush removed material from side 56. The 
electro-erosion process previously described in detail is thus carried out 
to remove material from the convex side 56 of spring 48 until this side 
has a shape similar to that of the working surface 206 of electrode tool 
200 with machining tolerance of .+-.0.0002 inches obtainable. 
Due to this high degree of surface finish tolerance, the spring produced by 
the process of the present invention is particularly suitable for use as a 
gas pressure regulator and the like. This requires a high degree of 
machining tolerance that heretofore was not possible with previous methods 
for making frusto-conical disc springs, and more particularly, Belleville 
springs. 
It is appreciated that various modifications to the inventive concepts may 
be apparent to those of ordinary skill in the art without departing from 
the spirit and scope of the invention and the invention is therefore to be 
limited only by the hereinafter appended claims.