Cut wire for electrical discharge machining
BACKGROUND OF THE INVENTION 
     The present invention relates to a cut wire used as an electrode in electrical discharge machining which is superior in high-temperature strength. 
     A fine brass wire of 65% Cu - 35% Zn in material and 0.1-0.3 mm in diameter has hitherto been used as a cut wire for electrical discharge machining. Since a brass wire is low in tensile strength at normal and high temperatures, it is difficult to expect a high degree of accuracy in machining using a finer wire. By using the finer wire with higher tension applied, the higher accuracy in electrical discharge machining is attainable. But, in the case of brass wire, the possibility of the wire breaking increases because of its low high-temperature strength, and it cannot meet the above requirement. 
     Also, because of its low high-temperature strength, wear progresses rapidly and the wire feed speed must be increased accordingly. Further, it is difficult to use a finer wire and to apply a higher tension to the wire, and thus the machining speed is low. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a cut wire which can improve the accuracy in electrical discharge machining and increase the machining speed. 
     In order to solve the above problems, the present invention provides a cut wire 1 which is a composite wire having a covering layer 3 of copper or its alloy around a core material 2 of stainless steel as the cut wire 1 as shown in the attached drawing. The diameter of the cut wire should be 0.05-0.4 mm and the ratio of the sectional area of the core material 2 to the sectional area of the cut wire, that is, SI/SI+SII, wherein SI designates the sectional area of the core material 2, while SII does that of the covering layer 3, should be 0.3-0.9 (30-90%), and the tensile strength of the cut wire should be 40-200 Kg/mm 2 . 
     The tensile strength may be either the strength after annealing (solution treatment) or the strength after wire drawing. 
     High tensile strength at normal and high temperatures is assured owing to the characteristics of the core material 2, and the electrical conductivity necessary for electrical discharge machining is assured by the covering layer 3. 
     Other objects and advantages of the present invention will become apparent from the following description taken with reference to the accompanying single drawing, which is a sectional view of the cut wire according to the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The FIGURE shows a sectional view of the electrode wire according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
     In this invention, the diameter of the cut wire 1 is limited to the abovesaid values. This is because, if the diameter is smaller than 0.05 mm, there is the possibility of breaking of wire under harsh conditions owing to insufficient strength at normal and high temperatures in spite of the existence of the core material for reinforcement. If the diameter of the wire is more than 0.4 mm, it becomes difficult to expect high accuracy in electrical discharge machining. The ratio of the sectional area of the core material 2 to the whole sectional area is limited to 30-90%. If it is less than 30%, the rate of increase in strength at normal and high temperatures is too small to exhibit sufficient effect of the present invention. If it is over 90%, the percentage conductivity will be less than 10%. The specific properties as a cut wire for electrical discharge machining will not be sufficient. 
     Furthermore, if the tensile strength is less than 40 Kg/mm 2 , the cut wire cannot exhibit its advantage as high-strength material. If it is over 200 Kg/mm 2 , curling tendency necessary for use as a cut wire will not be obtained. 
     In order to confirm the effect of the invention, the specimens of the cut wire shown in Table I were prepared. 
     The specimens No.1-No. 8 in this table are the cut wires of the present invention. Among them, No. 1-4, 6, and 7 were prepared to the final wire diameters by cladding a copper pipe of 5.0 mm in outer diameter and 0.8 mm in thickness on the core material so that the ratio of the sectional area of the core material to that of the entire cut wire will be the values as shown in the table, and repeating wire drawing and annealing. Also, the specimens No. 5 and No. 8 were prepared to the final diameter by plating a stainless wire of 1.25 mm and 0.5 mm, respectively, in diameter and repeating wire drawing and annealing. 
     The SUS 304 stainless steel described in Table I is a stainless steel composition specified by the Japanese Industrial Standard (JIS) to have the following composition: 
     
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C              ≦   0.08 wt %                                       
Si             ≦   1.00                                            
Mn             ≦   2.00                                            
P              ≦   0.045                                           
S              ≦   0.030                                           
Ni                        8.00-10.50                                      
Cr                        18.00-20.00                                     
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     The conductivity and tensile strength at normal and high temperatures for each cut wire shown in Table 1 were evaluated. The results are shown in Table 2. 
     As seen from Table 2, although the conductivity of No. 8 cut wire was lower than that of the conventional wires, those of the other wires were higher compared with the conventional wires. The tensile strength of all the specimens increased substantially not only at normal temperature but at high temperature, compared with the conventional wires. 
     Further, the specimens were tested for machining speed when used for actual electrical discharge machining. The aim of this test was to evaluate their machining capability when machining SKD-11 steel 5 mm thick, feeding the cut wire at a speed of 30 m/sec. and applying a current of 6A. The maximum machining speed is the maximum value within the limit where the wire can be used stably without wire breaking down. The results are shown in Table 3. 
     As seen from the Table, the machining speed when employing the wires according to the present invention is faster by 20% or more than the conventional wires if the diameters of the wires are equal. Even if the diameter of the wire is smaller than those of the conventional ones, the machining speed is sometimes as fast as or even faster than the conventional wires. 
     As described above, this invention increases the tensile strength of the cut wire considerably at normal and high temperatures with the help of the core material of stainless steel. The surrounding cover layer 3 ensures high conductivity. Thanks to the above characteristics of the wire, when the cut wire is used for electrical discharge machining, the machining speed increases and the possibility of wire breakdown decreases. If the machining speed is identical, it is possible to improve the machining accuracy by using a finer wire. 
     Further, by selecting properly the material and the ratio of the sectional areas of the core material to the covering layer, it is sometimes possible to speed up the machining even if the wire diameter is smaller. 
     
                                           TABLE 1                                 
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Specimen No.                                                              
          Wire dia. (mm)                                                  
                   ##STR1##       Covering method                         
                                             Final process                
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Cut wires of                                                              
present invention                                                         
1         0.2     Pure copper on SuS 304 (35%)                            
                                  Copper pipe cladding                    
                                             Wire drawing                 
2         0.2     Pure copper on SuS 304 (50%)                            
                                  Copper pipe cladding                    
                                             Wire drawing                 
3         0.2     Pure copper on SuS 304 (75%)                            
                                  Copper pipe cladding                    
                                             Wire drawing                 
4         0.2     Pure copper on SuS 304 (50%)                            
                                  Copper pipe cladding                    
                                             Annealing                    
5         0.2     Pure copper on SuS 304 (75%)                            
                                  Copper plating                          
                                             Wire drawing                 
6         0.1     Pure copper on SuS 304 (50%)                            
                                  Copper pipe cladding                    
                                             Wire drawing                 
7         0.3     Pure copper on SuS 304 (50%)                            
                                  Copper pipe cladding                    
                                             Wire drawing                 
8         0.3     7-3 brass on SuS 304 (50%)                              
                                  Brass plating                           
                                             Wire drawing                 
Conventional wires                                                        
9         0.2     35% Zn--64% Cu single layer                             
                                  --         Annealing                    
10        0.2     35% Zn--64% Cu single layer                             
                                  --         Wire drawing                 
11         0.25   35% Zn--64% Cu single layer                             
                                             Wire drawing                 
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                       TABLE 2                                                     
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                Strength (kg/mm.sup.2)                                    
Specimen No.                                                              
         Conductivity (%)                                                 
                      Normal temp.                                        
                                 200° C.                           
                                       400° C.                     
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Cut     1    73            89       82   64                               
wires of                                                                  
        2    49           131      120   91                               
present                                                                   
        3    34           182      171   138                              
invention                                                                 
        4    48            56       54   49                               
        5    33           179      169   132                              
        6    48           133      119   90                               
        7    50           129      118   91                               
        8    15           139      123   92                               
Conven-                                                                   
        9    28            44       35   13                               
tional 10    28           104       75   14                               
wires  11    27            99       73   15                               
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 *In the table, the hightemperature strength at 200° C. and        
 400° C. are the values after heating for 10 minutes and conducting
 a tensile test.                                                          
 
    
     
                       TABLE 3                                                     
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           Max. machining speed                                           
Specimen No.                                                              
           (mm.sup.2 /min.)                                               
                          Machining accuracy                              
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Cut     1      60             o (Good)                                    
wires of                                                                  
        2      64             o                                           
present 3      61             o                                           
invention                                                                 
        4      62             o                                           
        5      58             o                                           
        6      49             ⊚ (Very good)                
        7      74             o                                           
        8      71             o                                           
Conven- 9      48             Δ (Not very good)                     
tional  10     51             o                                           
wires   11     57             Δ                                     
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