Method of producing magnetron anode

In order to provide a method of producing magnetron anode at a good mass-produceability, according to the invention, so-called straight type steel cylinder blank the inner peripheral surface of which is not tapered is used and, at the same time, the step of quench-hardening of the steel cylinder blank is omitted. After fitting a copper block to the inside of the straight type steel cylinder blank which has not been quenched, both members are compacted to each other. Then, a press working is effected on the copper blank such that the steel cylinder blank and a back-up punch are forced to move relatively to each other, when a press-working shaping punch is forced into the copper block, thereby to form a copper anode. In order to avoid the bursting of the steel cylinder blank, the press work on the copper block is effected at a temperature of 200.degree. to 650.degree. C.

BACKGROUND OF THE INVENTION 
The present invention relates to a method of producing magnetron anode for 
use in household microwave oven. 
One of the conventional magnetron anodes for use in household microwave 
oven has, as shown in FIG. 1, a circular steel cylinder 1 to the inner 
peripheral surface of which secured is a copper anode, i.e. a circular 
copper cylinder 2 which in turn is provided at its inside with a plurality 
of radial copper vanes 3. Also, a process as shown in FIG. 2 has been 
proposed, for example, in the specification of U.S. Ser. No. 819,056 filed 
July 26, 1977, now U.S. Pat. No. 4,200,217, as a method of producing the 
magnetron anode of the type described above. 
In order to clarify the drawback of the prior art, a description will be 
made hereinafter as to the conventional method with reference to FIGS. 2a 
to 2f. 
As shown in FIG. 2a, a steel cylinder blank material 21 having a wholly or 
partially tapered inner peripheral surface and a disc-shaped copper member 
22 are used as the blank materials for the circular steel cylinder and the 
copper anode, respectively. After cleaning the inner peripheral surface A 
of the steel cylinder blank material 21 and the outer peripheral surface B 
of the disc-shaped copper member 22 as shown in FIG. 2a, the steel 
cylinder blank material 21 is placed in a die 25 and the disc-shaped 
copper member 22 is fitted to the inside of the steel cylinder blank 
material 21, as shown in FIG. 2b. Then, a plastic working is effected on 
the disc-shaped copper member 22 in the steel cylinder blank material 21, 
by means of a shaping punch 24 and a back-up punch 26, so that the outer 
peripheral surface of the disc-shaped copper member 22 may be closely and 
tightly fitted to the inner peripheral surface of the steel cylinder blank 
material 21, as shown in FIG. 2c. This step for obtaining the close and 
tight fit will be referred to as "compaction", hereinafter. The composite 
member as shown in FIG. 2e is then subjected to a press work by a shaping 
punch 27 having the illustrated shape, after a preheating, so that a 
plurality of vanes 23 as shown in FIG. 2e are shaped. This step will be 
referred to as vane-shaping, hereinafter. During this vane-shaping, the 
steel cylinder blank material 21 is naturally raised, due to the presence 
of the tapered interface. Finally, the half-finished article as shown in 
FIG. 2e is subjected to an after processing and then to a diffusion 
annealing, so as to become a magnetron anode having a construction as 
shown in FIG. 2f. The copper anode consisting of a circular copper 
cylinder 2 and a plurality of copper vanes 3 integral therewith is thus 
attached to the inner peripheral surface of the circular steel cylinder 1 
at a high reliability. 
As has been stated, the conventional method requires the use of a material 
having tapered inner surface, as the blank for the circular steel 
cylinder. Usually, this tapered inner surface is processed by means of a 
lathe or the like, at a cost of considerably long working time. This 
inconveniently hinders the mass production of the magnetron anode. 
It is therefore expected that the mass-produceability of the magnetron 
anode can be remarkably improved if the taper of the inner peripheral 
surface of the steel cylinder blank material is dispensed with. 
It is impossible, however, to shape the copper anode vanes having a 
complicated shape as shown in FIG. 1 with a required precision and a good 
compaction to the inside of the circular steel cylinder, by the described 
conventional method, if the steel cylinder blank material lacks the 
tapered inner surface, i.e. if a straight steel cylinder blank material is 
used as the blank material for the circular steel cylinder. In other 
words, it has been essential in the conventional method to wholly or 
partially taper the inner peripheral surface of the steel cylinder blank 
material. 
The described conventional method involves another problem. In this 
conventional method, the copper blank is subjected to a plastic working 
while it is embraced by the steel cylinder blank material. Therefore, 
during the pressing under room temperature or a secondary plastic working 
which is effected at a high temperature, the steel cylinder blank material 
1' is likely to be deformed plastically by the shaping force exerted on 
the copper blank 2', as shown in FIG. 3, or even made to burst by such a 
force. 
To avoid this, it has been necessary to previously quench-harden the steel 
cylinder blank material. The adoption of this quench-hardening requires a 
considerably long time, not only for the heating and quenching, but also 
for the elimination of oxidation film and distortion in shape which is 
caused as a result of the quenching, which also hinders the mass 
production of the magnetron anode. 
SUMMARY OF THE INVENTION 
Under these circumstances, the present invention aims as its major object 
at providing a method of producing magnetron anode having a good 
mass-produceability. 
Another object of the invention is to provide a method of producing 
magnetron anode, which is suitable, due to the use of a steel cylinder 
blank material the inner peripheral surface of which is not tapered, for a 
mass production of the magnetron anode. 
Still another object of the invention is to provide a method of producing 
magnetron anode, which is suitable, thanks to the elimination of 
quench-hardening of steel cylinder blank material which has posed a 
problem in the conventional method, for a mass production of the magnetron 
anode. 
To these ends, according to the invention, there is provided a method of 
producing magnetron anode, in which a straight steel cylinder blank 
material is used, and the quench-hardening of the same blank material is 
eliminated. 
More specifically, according to the method of the invention, a copper block 
is fitted to the inside of a straight steel cylinder blank material which 
has not been quenched, and both members are compacted with each other. 
Then, a press working is effected such that the steel cylinder blank 
material and the back-up punch are forcibly moved relatively to each 
other, when a shaping punch for press working is inserted into the copper 
block, thereby to shape the copper anode. In order to avoid the 
aforementioned bursting of the steel cylinder blank material during the 
press work, the pressing work is conducted at an elevated temperature of 
the copper block of between 200.degree. C. to 300.degree. C.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Hereinafter, the invention will be described through a specific embodiment. 
According to the invention, in order to achieve the aforementioned objects 
of the invention, the copper anode is press-formed by making use of a 
straight steel cylinder blank material and both members are pressfitted or 
welded to each other. This is made by forcibly raising the straight steel 
cylinder blank material 31, when the vane-shaping punch 37 is forced into 
the copper blank 32, in the direction reverse to the penetration of the 
shaping punch 37, as illustrated in FIG. 4. 
In order to obtain a good welding, it is important to maintain a constant 
length l.sub.1 between the upper end of the steel cylinder 31 and the 
copper blank, i.e. not to allow a sliding of the portion of length 
l.sub.1. It is therefore necessary that the amount of rise l.sub.F of the 
steel cylinder blank material 31 is equal to or smaller than a length 
(R-1) l, where l and R represent, respectively, the amount of penetration 
of the vane-shaping punch 37 and the extrusion ratio (cross-sectional area 
of copper disc as shown in FIG. 7 (c)cross-sectional area of copper anode 
as shown in FIG. 7 (f). 
The raising of the steel cylinder blank material can be effected by various 
ways. For instance, it is possible to raise the die 35, which is loaded 
with the steel cylinder blank material 31 as shown in FIG. 4, by means of 
a hydraulic servo mechanism. When this raising measure is adopted, the 
time at which the raising of the die 35 is commenced is preferably 
selected to fall within a time region between the instant at which 
vance-shaping punch 37 comes to contact with the copper blank and the 
instant at which the vane shaping is actually started. More preferably, 
the raising of the die 35 is started simultaneously with the start of the 
actual shaping of vanes. 
FIG. 5 shows the relation between the pressing force to be exerted by the 
vane-shaping punch and the timing of raising of the die. In FIG. 5, 
symbols a and b denote, respectively, the press-working force and the 
amount of rise of the die. The instant at which the press working is 
commenced and the instant at which the shaping of vanes is actually 
started are denoted, repectively, by symbols S and P. The constant plastic 
flow of metal is commenced at an instant Q. 
As stated before, it is preferred to start the raising of the die at an 
instant between the instants S and Q. 
More preferably, the raising of the die is commenced at the instant P, at 
which the shaping of the vanes is actually started. 
At the same time, for the reason stated before, the rate of die travel is 
(R-1) times that of the rate of penetration of the vane-shaping punch or 
smaller. 
FIG. 6 shows the relation between the inside diameter Di of the steel 
cylinder blank material and the extrusion ratio R. In FIG. 6, the level of 
the extrusion ratio R and the level of (R-1) corresponding the level of R 
are shown by linear curves a and b, respectively. Thus, the linear curve b 
shows the upper limit of the ratio of the amount of rise of the steel 
cylinder blank material to the amount of the shaping-punch penetration, or 
the ratio of rate of die travel to the rate of shaping-punch penetration. 
More specifically, the (R-1) takes values of about 1.8, 1.6, 1.4 and 1.25, 
for respective diameters Di of 42, 43, 44, and 45 mm. The linear curve c 
represents the optimum values of the above-mentioned ratio for respective 
inside diameters Di. Namely, the optimum ratios are about 1.2, 1.1, 1.0 
and 0.9, for these inside diameters. These optimum values amount, 
respectively, to about 70% of the upper limit values, and further denote 
the lower limit values for the respective inside diameters Di. 
Thus, provided that a steel cylinder blank material having an inside 
diameter Di of 43 mm is used, the amount of rise l.sub.F of the steel 
cylinder blank material has to be 1.6 times as large that of the 
shaping-punch penetration l or smaller, and is preferably 1.1 times as 
large that of the penetration l. In other words, the rate of rise of the 
steel cylinder blank material, i.e. the rate of die travel has to be 1.6 
times as large that of the rate of penetration of the shaping punch or 
smaller, and is preferably 1.1 times as large that of the rate of 
penetration. 
It is possible to use, instead of the aforementioned hydraulic mechanism 
for raising the steel cylinder blank material, a specific press machine 
having an additional function to rise the die. It is also possible to 
retract the back-up punch at a rate corresponding to the penetration of 
the vane-shaping punch into the copper blank, instead of raising the die. 
Other advantages offered by the invention reside in that the quenching of 
the steel cylinder blank material is eliminated, without being accompanied 
by the unfavourable bursting of the same, and the welding strength between 
the circular steel cylinder and the copper anode is enhanced. 
These advantages are brought about by the following measures: (1) to reduce 
the level of force required for the compaction, by using a pre-formed 
copper blank, (2) to use a steel having a flow stress at least twice as 
large that of the copper, as the material of the steel cylinder blank 
material, and (3) to adopt an appropriate temperature under which the 
shaping of vanes is performed. 
These measures (1) to (3) will be described in detail hereinunder. 
Referring first to the use of the pre-formed blank, the purpose of the 
compacting is to prevent the region of welding required in the product, 
i.e. the region represented by lv in FIG. 2f from being oxidized during 
heating at high temperature. Therefore, the length ls of the region over 
which the steel and the copper intimately contact each other as shown in 
FIG. 2c must be larger than the length lv. And then, lv must be equal to 
or larger than the length l's as shown in FIG. 2c. 
In some cases, the adopted length ls is smaller than the length lv. In such 
a case, as shown in the later-mentioned FIG. 7b, the copper blank is 
provided with a cylindrical portion C of a length l4, and is compacted to 
the inner peripheral surface of the circular steel cylinder at the 
periphery of the cylindrical portion C. This conveniently contributes to 
the economization of the copper material. 
In this case, the steel cylindrical blank material is moved simultaneously 
with the penetration of the vane-shaping punch into the copper blank, so 
that the vanes are formed down to the portion of the copper blank where 
the cylinder C is provided. It is necessary, however, to make the size of 
the back-up punch at each step of the process to allow a plastic spreading 
of the cylindrical portion of the copper, in order that the copper blank 
may not be separated from the inner peripheral surface of the steel 
cylinder blank material. 
In the conventional process in which the copper is shaped in the steel 
cylinder blank material as shown in FIG. 2, the force required for the 
shaping is so large that the steel cylinder blank material may be bursted 
as shown in FIG. 3, unless the steel cylinder blank material is suitably 
quenched. 
In order to avoid this bursting of the steel cylinder blank material, the 
disc-shaped copper blank 32 having a shape as shown in FIG. 7a is 
pre-formed into a form as shown in FIG. 7b. Then, the surfaces indicated 
by arrows A' and B' of the steel cylinder blank material 31 and the copper 
blank 32 are cleaned. Subsequently, both members are fitted to each other 
and a compaction effected under the room temperature, as shown in FIG. 7c. 
Then, after preheating both members as shown in FIG. 7d, a press work is 
effected on the copper blank 32 in the steel cylinder blank material 31, 
so as to shape the vanes. In the above-stated process, it is possible to 
reduce the compacting force down to such a low level at which the plastic 
deformation of the copper is started. At the same time, the oxidation of 
the region of length Ls is prevented during the heating at a high 
temperature. 
Referring now to the aforementioned measures (2) and (3), i.e. the flow 
stress of the steel cylinder blank material and the temperature at which 
the vanes are shaped, in order to avoid excessively large distortion or 
the bursting of the steel cylinder blank material, it is necessary that 
the flow stress of the steel cylinder blank material has to be at least 
two times as large that of the copper, during the compaction which is 
conducted under the room temperature. At the same time, in order to 
prevent the excessively large distortion or the bursting of the steel 
cylinder blank material during the vane-shaping which is conducted at a 
high preheating temperature, the flow stress of the steel cylinder blank 
material has to be at least 4.5 times as large that of the copper. 
In order to obtain above specified ratios of the flow stress, it is 
necessary to suitably select the material of the steel cylinder blank 
material and the temperature at which the vanes are shaped. 
Particularly, such a steel as would exhibit a blue-brittleness phenomenon 
and, accordingly, an increased flow stress at the vane-shaping temperature 
is recommended as the material for the steel cylinder blank material. 
It is thus possible, by adopting the above-stated specific material and the 
vane-shaping temperature, to eliminate the quench-hardening of the steel 
cylinder blank material. 
FIG. 7 shows the process in accordance with a practical embodiment of the 
invention. In this process, a work-hardened carbon steel tube for machine 
structural purpose (JIS STKM-17C, 0.45 to 0.55%C, 43 mm inner dia., 48 mm 
outer dia. and 35 mm high) was used as the straight steel cylinder blank 
material, while a disc-shaped blank material (43 mm diameter, about 11 mm 
thick) of oxygen free copper was used as the blank material of the copper 
anode. 
At first, the copper blank 32 having a form as shown in FIG. 7a was 
pre-formed into a form as shown in FIG. 7b (ho=8 to 8 mm, l4=5 mm, total 
thicknesss 14 to 15 mm). Then, both of the copper blank 32 and the steel 
cylinder blank material 31 were subjected to a degreasing. Then, the inner 
peripheral surface of the steel cylinder blank material 31 and the outer 
peripheral surface of the copper blank 32 were cleaned by machining and by 
means of a wire brush, respectively. Subsequently, both members were 
fitted to each other as shown in FIG. 7c, while paying attention not to 
contaminate the cleaned surfaces, and a cold compaction was conducted 
under the room temperature. For the compaction, the force by which the 
copper blank 32 can be plastically deformed is required. 
The steps down to the step of compaction may be made in different ways. For 
instance, the process may be modified such that, after cleaning the outer 
peripheral surface of the copper blank 32 as shown in FIG. 7a and the 
inner peripheral surface of the steel cylinder blank material 31 as shown 
in FIG. 7b, the two members are fitted to each other in a manner as to 
preclude the contamination of the cleaned surfaces, as shown in FIG. 7c, 
and then the cold compaction is effected. 
The compacted body including the copper and steel materials was then 
preheated up to the vane-shaping temperature which ranges between 
300.degree. C. and 650.degree. C. Then, a press working was effected on 
the copper blank 32 embraced by the steel cylinder blank material 31, so 
as to form and shape a plurality of vanes 33 as shown in FIG. 7e. 
In this process, as stated before, it is necessary to forcibly raise the 
steel cylinder blank material 31, during the penetration of the 
vane-shaping punch 37 into the copper blank 32. 
In the described embodiment, a hydraulic servo mechanism was used for 
lifting the die 35 by which the steel cylinder blank material 31 is held, 
thereby to raise the latter. In this case, the back-up punch 36 was kept 
stationary. The instant of raising of the die 35 was made to coincide with 
the instant at which the shaping of the vane is actually started by the 
vane-shaping punch 37 as the latter is lowered. At the same time, the 
amount or rate of die travel was selected to be 1.1 times as large that of 
the penetration of the vane-shaping punch 37. 
After the shaping of the vanes, a post-treatment was conducted to obtain a 
product as shown in FIG. 7f is which lv is 12 mm. Further, in order to 
enhance the weld strength, a diffusion annealing was effected at an 
annealing temperature of 800.degree. C. for 10 to 60 minutes. 
Consequently, a copper anode having a plurality of vanes was formed within 
a straight circular steel cylinder having high precision and good welding 
strength, without taking the step of quench-hardening of the steel 
cylinder blank material, while avoiding the bursting of the latter. 
FIG. 8 shows the hardnesses at elevated temperature measured for the 
materials as used in the described embodiment. Since in usual case the 
hardness is in direct proportion to the flow stress, it is possible to 
know the ratio of the flow stress of the steel cylinder blank material to 
that of the copper blank from the data as shown in FIG. 8. 
Namely, the ratio of the flow stress of STKM-17C to that of oxygen free 
copper is about 2.5 under the room temperature and about 5 in an 
atmosphere of 350.degree. C. or higher temperature. 
The above-mentioned ratio is somewhat lowered when a material obtained by 
normalizing the STKM-17C, i.e. a material corresponding to STKM-17A, is 
used as the material for the steel cylinder blank material. In this case, 
however, the bursting of the steel cylinder blank material as shown in 
FIG. 3 was not observed also, when the shaping of vanes is effected at a 
temperature between 420.degree. C. and 600.degree. C. In consideration of 
the above, it is derived that, in the process of the invention, the 
above-mentioned ratio of flow stress has to be at least 2 under the room 
temperature, and at least 4.5 at the vane-shaping temperature. 
Although STKM-17C steel or STKM-17A steel were used in the described 
embodiment, the material of the steel cylinder blank is not limited to 
these steel containing more than 0.45% of carbon. For instance, low 
alloyed steels such as those containing Ni, Cr or the like, which are 
liable to exhibit a blue brittleness, can be used as the material for the 
steel cylinder blank. In such a case, the vane-shaping can be conducted 
without incurring the breakage of the steel cylinder blank, even at a low 
temperature of around 200.degree. C. 
Similarly, the oxygen free copper as mentioned in the description of the 
embodiment is not exclusive, and the copper blank may be made of other 
copper materials such as commercially available pure alloy of 99.9% 
purity. When, different materials are used, the processing conditions such 
as vane-shaping temperature, annealing temperature, annealing time and so 
forth are suitably selected to optimize the processing. 
As a result of examination of the region of welding of the steel cylinder 
blank material and the copper blank, it has proved that, in order that a 
sufficiently high welding strength is obtained at the cylindrical portion 
C provided on the copper blank, it is necessary that the portion C keeps 
an intimate and close contact with the steel cylinder blank material, 
throughout all steps after the compaction. To this end, as stated before, 
it is necessary to suitably select the sizes of the back-up punches used 
in respective steps. 
More specifically, referring to FIG. 9, the outer diameter d.sub.p of the 
tip of the back-up punch, as well as the angle .theta..sub.p of the taper 
of the same, has to be at least equal to that of the preceding step. 
The sizes and taper angles of the back-up punches used in the described 
embodiment are shown below, by way of example. 
(1) back-up punch for pre-forming of copper blank: 
d.sub.p =38.4 mm, .theta..sub.p =5.degree.20' 
(2) back-up punch for compaction: 
d.sub.p =38.6 mm, .theta..sub.p =6.degree.30' 
(3) back-up punch for vane-shaping: 
d.sub.p =39 mm, .theta..sub.p =6.degree.30' 
Consequently, the anti-oxidation effect provided by the cylindrical portion 
C of the copper blank is ensured throughout all steps of process, and it 
has become possible to increase the length of region of welding up to 
ho+l4. 
The sizes of the back-up punches as shown above are not of limiting sense. 
Rather, the sizes of the back-up punches are suitably selected, so as to 
afford the plastic spreading of the cylindrical portion C of the copper 
blank as the process proceeds. 
Further, various numerical data as used in the description of the 
embodiment are not exclusive, and can be changed over wide ranges in 
accordance with the shape and size of the magnetron anode to be produced. 
As has been described in detail, according to the invention, it is possible 
to produce a magnetron anode in which a copper anode having a plurality of 
vanes is formed precisely within a straight circular steel cylinder with a 
large welding strength to the latter, at a good mass-produceability, 
without making use of steel cylinder blank material having tapered inner 
peripheral surface which is indispensable in the conventional process, and 
without necessitating the quench-hardening of the steel cylinder blank 
material which is also essential in the conventional process. 
In addition, thanks to the pre-forming of the disc-shaped copper blank, it 
becomes possible to obtain a larger area of welding of copper to the steel 
cylinder, with a smaller volume of the copper blank. 
Consequently, the method of the invention contributes to the reduction of 
production cost of the magnetron anode, and offers various other 
advantages in the production of the magnetron anode.