Abstract:
An apparatus and method of electrochemical polishing a workpiece with ring-form electrode is provided. A mechanism with a tool electrode, a DC power supply and electrolysis-supply tank of the present invention can be installed on the traditional production equipment. The tool electrode is connected with the negative pole of the DC power supply, while the workpiece is connected with the positive pole of the DC power supply and kept a fixed distance from the tool electrode. The electrode or the workpiece advances at a predetermined feeding speed while the workpiece is electrochemically polished. The present invention uses the centrifugal force of rotational tool electrode to discharge electrolytic byproducts, making electrochemical polishing more effective.

Description:
FIELD OF THE INVENTION 
     The invention relates to an apparatus and method of electrochemical polishing utilizing a ring-form electrode, and in particular to an apparatus and method capable of electrochemical polishing a workpiece continuously processed by a shaping machine. 
     DESCRIPTION OF THE RELATED ART 
     In the conventional techniques, after a workpiece has been through a shaping process, such as rolling, extrusion and drawing, etc., manual polishing or mechanical burnishing is performed to complete the surface treatment. However, the effectiveness of manual polishing is limited by the experience of the operator, and the man-hours and cost are relatively high. In addition, the contact pressure between the tools used and the workpiece is not so easily controlled during manual polishing or mechanical burnishing, causing the local generation of non-uniform residual stress on the surface of the workpiece. The residual stress is usually higher than the maximum strength of the workpiece; therefore, the surface of the workpiece may collapse and cause the formation of small cavities on the surface of the workpiece. Thus, the life of the workpiece is reduced. In addition, it is difficult to find operators with the technique and experience needed for manual polishing in today&#39;s society. Mechanical burnishing is limited by the shape and characteristics of the machine; as a result, its application is very limited and inconvenient. 
     Electrochemical processing uses a combination of electric energy and chemical energy. In the electrochemical processing, electrolyte is supplied to the space between the workpiece, connected with the positive pole of the DC power supply, and the tool electrode, connected with the negative pole. The circulation of the electrolyte serves the secondary purpose of removing electrolytic byproducts generated during the electrochemical processing. This method is suitable for materials with high hardness, heat-resistance or corrosion resistance. 
     Electrochemical polishing is a technique using electrochemical processing to reduce the roughness of the workpiece. It can be applied in research or industry as a highly efficient surface treatment method to obtain a high-quality workpiece without residual stress or burrs. 
     However, electrochemical polishing is presently limited in application to stainless steel which has been mechanical processed in order to smooth cavities on the surface of the workpiece and prevent the residue from remaining on the surface of the workpiece. The workpiece has better effect about corrosion resisting after electrochemical polishing. However, after such workpiece has been processed by the traditional electrochemical polishing, it must be put in an additional electrolytic tank; hence, the polishing time is much longer and the amount of material removed from the workpiece is extremely little. 
     Nevertheless, the labor savings and accuracy of electrochemical polishing have lead to continued investigation into its application. Electrochemical techniques such as electrochemical drilling, electrochemical grinding and electrochemical deburring, etc, have been developed. A Japanese company has developed an apparatus for the electrochemical polishing of materials other than stainless steel. However, because the cost of such an apparatus is very expensive and the design of its electrode is very difficult, its practical application is still limited. 
     SUMMARY OF THE INVENTION 
     In view of the disadvantages of the conventional electrochemical polishing technique, an object of the invention is to provide an electrochemical polishing method and its apparatus using a rotatable ring-form electrode. It offers advantages of economical equipment, a minimum-polluting and low-cost process, and easy assembly and automation. Bars and tubes, produced by traditional machining techniques, for example, turning, drawing, rolling, and extrusion, can be continuously processed by the apparatus of the present invention. A mechanism with a tool electrode, a DC power supply and an electrolysis-supply tank of the present invention can be installed on the traditional production equipment. The tool electrode is connected with the negative pole of the DC power supply, while the workpiece is connected with the positive pole of the DC power supply and kept a fixed distance from the tool electrode. The electrode or the workpiece advances at a predetermined feeding speed while the workpiece is electrochemically polished. The present invention uses the centrifugal force of rotational tool electrode to discharge electrolytic byproducts, making electrochemical polishing more effective. The present invention is also designed to obtain fast improvement of the surface roughness of the workpiece, and to effectively reduce residual stress. 
     Another purpose of the present invention is to provide an electrode-supporting mechanism with a low-cost tool electrode, easy assembly and rotational power. For the workpiece with circular shape, such as circular tubes or circular rods, the electrode-supporting mechanism of the present invention can be rotated and use the centrifugal force of the rotational tool electrode to discharge the electrolytic byproducts, which makes electrochemical polishing more effective. 
     Furthermore, another purpose of the present invention is to provide an electrochemical polishing method. The DC power supply, the electrolyte-supplying tank, a pump, a filter and a tube of the present invention can be installed on traditional equipment for drawing, rolling, or extrusion and so on. During electrochemical polishing, the tool electrode is connected with the negative pole of the DC supply power, while the workpiece is connected with the positive pole of the DC supply power. The size of the inner diameter of the ring-from electrode is 0.2˜1.0 mm bigger than the outer diameter of the workpiece. The electrolyte is a solution comprising 20%˜40% of NaCl or NaNO 3 . The feeding speed of the electrode is about 1.5˜2.5 mm/min, the rating current is about 5˜10 mm/min when the average diameter of the workpiece is 10 mm, the voltage is about 10˜15V, and the width of the pulse is about several to several tenths of a sec. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is hereinafter described in detail by reference to the accompanying drawings in which: 
     FIG. 1 is a schematic view showing the structure of the present invention assembled on an apparatus used to extrude a circular rod; 
     FIG. 2 is a schematic view showing the relative position between the tool electrode and the workpiece during the electrochemical polishing; 
     FIG. 3A, FIG.  3 B and FIG. 3C are schematic diagrams showing various types of the tool electrodes; and 
     FIG. 4 is a graph showing the experimental results of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, the structure of an embodiment of the present invention consists of a DC power supply  1 , a first electrolyte-supplying tank  2 , a second electrolyte-supplying tank  3 , a tool electrode (a ring-form electrode)  10 , a supporting mechanism  11  and a feeding mechanism  13 . 
     The current, voltage and pulse values of the DC power supply  1  are adjustable. A positive pole of the DC power supply  1  is connected with a shaping machine  14  connected electrically with the workpiece  12 . A negative pole of the DC power supply  1  is connected with a base  11 - 3  of the supporting mechanism  11 . 
     Electrolyte with proper concentration is loaded inside the first electrolyte-supplying tank  2 . The electrolyte is a solution preferably comprising 20%˜40% of NaCl or NaNO 3 . The electrolyte is pumped by a pump  5 , is filtered by a filter  6 , flows through a tube  7 , a flow meter  8 , is sprayed to a gap between the tool electrode  10  and the workpiece  12  by a nozzle  9 , and flows into the second electrolyte-supplying tank  3 . The flow rate of the flow meter  8  is preferably above 4 l/min, and the gap is preferably 0.3 mm. After the height of the electrolyte inside the second electrolyte-supplying tank  3  is higher than the height of the workpiece  12 , the electrolyte will flow back into the first electrolyte-supplying tank  2  through a drain valve  4 . An electrolyte-supplying device of the present invention consists of the first electrolyte supplying tank  2 , the electrolyte supplying tank  3 , the drain valve  4 , the pump  5 , the filter  6 , the tube  7 , and the nozzle  9 . 
     The supporting mechanism  11  comprises a sleeve  11 - 1 , provided with an annular groove and disposed inside a bearing  11 - 2 , for the tool electrode  10  disposed therein. The bearing  11 - 2  is fixed on a base  11 - 3 . A belt  11 - 4  is put around the annular groove of the sleeve  11 - 1 ; therefore, a belt pulley  11 - 5 , connected with a second motor  11 - 6 , rotates when the second motor  11 - 6  rotates. Meanwhile, the belt  11 - 4  is activated to force the sleeve  11 - 1  and the tool electrode  10  to rotate in order to polish the workpiece, wherein the effect of removing the electrolytic byproducts is obtained as a secondary benefit. The rotational speed of the second motor  11 - 6  is about several hundreds rpm. In summary, the supporting mechanism  11  of the present invention consists of the sleeve  11 - 1 , the bearing  11 - 2 , the base  11 - 3 , the belt  11 - 4 , the belt pulley  11 - 5 , and the second motor  11 - 6 . 
     The feeding mechanism  13  consists of a feed roller  13 - 1 , a first motor  13 - 2  and a support  13 - 3 . After the shaping machine  14  has shaped the workpiece  12 , it is supported on the feed roller  13 - 1  of the feeding mechanism  13 . The rotational speed of the feed roller  13 - 1  depends on the first motor  13 - 2 . The workpiece  12  is fed into an entrance  3 - 1 , of the second electrolyte-supplying tank  3 , and the tool electrode  10  by means of the first motor  13 - 2 . 
     After the traditional shaping machine has shaped the workpiece, the surface of the workpiece needs to be polished. The steps of the processing method are described in detail as follows: 
     Step 1: The positive pole of the DC power supply  1  is connected with the shaping machine  14 , electrically connected with the workpiece  12 . The negative pole of the DC power supply  1  is connected with the metal base  11 - 3  of the supporting mechanism  11 . 
     Step 2: The voltage, rating current, pulse values of the DC power supply  1  are selected as follow: the voltage is about 10˜15V, the rating current is about 5˜15A when the average diameter of the workpiece is 10 mm, and the width of the pulse value is several to several tenths of a second. 
     Step 3: The shape and size of the required tool electrode  10  is predetermined. The inner diameter of the tool electrode  10  is 0.3 mm bigger than the outer diameter of the workpiece, as shown in FIG.  2 . 
     Step 4: The predetermined tool electrode  10  is mounted inside the sleeve  11 - 1  of the supporting mechanism  11 . If the workpiece  12  is a circular rod or a circular tube, the rotational speed of the second motor  11 - 6  can be adjusted to, for example, at least 200 rpm. When the second motor  11 - 6  rotates, the belt pulley  11 - 5 , connected with the second motor  11 - 6 , rotates. The belt  11 - 4  forces the sleeve  11 - 1  and the tool electrode  10 , disposed inside the sleeve  11 - 1 , to rotate to attain polish the workpiece, wherein the effect of removing the electrolytic byproducts during the electrochemical polishing is a secondary benefit. 
     Step 5: The electrolyte, for example, NaCl or NaNO 3 , with proper concentration, for example, 20˜40%, is put into the first electrolyte-supplying tank  2 . The electrolyte is blended uniformly, and its height inside the second electrolyte-supplying tank  3  is higher than the height of the workpiece  12 . The electrolyte from the nozzle  9  is aimed at the gap between the workpiece  12  and the tool electrode  10  to remove the electrolytic byproducts during the electrochemical polishing. 
     Step 6: The flow rate of the electrolyte through the drain valve  4  of the second electrolyte-supplying tank  3  is above 4 l/min preferably to maintain the height of the electrolyte inside the second electrolyte-supplying tank  3  during the electrochemical polishing. The electrolyte flows through the drain valve  4  into the first electrolyte-supplying tank  2 . By means of the pump  5 , it continuously flows back to the second electrolyte-supplying tank  3  through the filter  6 , the tube  7 , the flow meter  8  and the nozzle  9 . 
     Step 7: The rotational speed of the first motor  13 - 2  of the feeding mechanism  13  is adjusted to provide a proper feeding speed of the workpiece  12 , for example, several millimeters per minute. 
     Step 8: The DC power supply  1  and the pump  5  is activated to supply the electrolyte into the second electrolyte-supplying tank  3  and keep the height of the electrolyte inside the second electrolyte supplying tank  3  higher than the height of the workpiece. Meanwhile, the first motor  13 - 2  of the feeding mechanism  13  is activated. 
     Step 9: The shaping machine  14  is activated to shape the workpiece  12  into a predetermined shape. Then, the workpiece  12  is supported by the feed roller  13 - 1  of the feeding mechanism  13 , fed into the tool electrode  10  of the second electrolyte-supplying tank  3  to be electrochemically polished. 
     FIG. 3A, FIG.  3 B and FIG. 3C are schematic diagrams showing various types of the tool electrode of the present invention. Among them, the tool electrode shown in FIG. 3A is a basic type, the shape of the inner portion of the tool electrode shown in FIG. 3B is tapered, and the inner portion of the tool electrode shown in FIG. 3C is provided with several convex pins. 
     The experimental results with four different mold materials using the electrochemically polished with the method of the present invention are shown in FIG.  4 . From the graph in FIG. 4, it can be seen that the roughness of the surface of the workpiece undergoing the method of the present invention is improved. Table 1 provides the difference between the roughness of the workpiece processed by the method with the electrode rotating and the roughness of the workpiece processed by the method without the electrode rotating in order to prove that the method with the electrode rotating has the advantage of enhancing the polishing effect. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 SKD61 
                 SKD11 
                 NAK80 
                 SNCM8 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Relative improvement ratio (%) 
                 20 
                 20 
                 19 
                 18 
               
               
                   
               
             
          
         
       
     
     While the present invention has been particularly shown and described with reference to a preferred embodiment, it will be readily appreciated by those of ordinary skill in the art that various changes and modifications may be made without departing from the spirit and scope of the invention. It is intended that the claims be interpreted to cover the disclosed embodiment, those alternatives which have been discussed above and all equivalents thereto.