- Email: [email protected]
Elastic and electrolytic ultraprecision polishing
Elastic and Electrolytic
Ultraprecision
Polishing
by Wang Zhenlong, Luan Yingyan, Pang Tao, and Liu Weidong. Harbin Institute of Technology, Harbin, China -
H
ow to machine surfaces with high efficiency and high accuracy is a hot research topic in the field of mechanical machining, which will produce a direct affect on the development of the die industry. Statistical data show that more than 60% of the machine parts for automo. biles, tractors, and electrical machinery and equipment are made by dies. In light industry more than 855% of the machine parts are made by dies. According to the development of science and technology, dies will play a more important role in industry. It has been forecasted by CIRP that 70%: of the roueh machining and 50% of the fine machining oi‘ parts will he made by dies in the 21st century. It i> common knowledge that tho final surface roughness and the damage layer of the die cavity xurface have a great influence on the properties and the useful life of the die. Common circumstances show that when the die cavity surface roughness improves one‘ step, the die life will be raiacd about 50%~: moreover, the workload of the final finish machining is more than ‘~1 that of the total workload of the die machining: however. the main means at present to polish die cavity surfaces and to gain low surface roughness with little or no damage is manual polishing. Obviously, the machining efficiency and accuracy are both low using these methods. In order to explore a new ultraprecision polishing method that can polish with high efficiency and high accuracy, numerically controlled (NC) machining. ultrasonic vibration, electrolylic machining, and mechanical polishing were combined. This method was named elastic and electrolytic ultraprecision polishing (EEUP1. Small polishing equipment that can bring EEUP into use was also developed. THE PRINCIPLE
polishing locus becomes very complicated and activates the abrasive powder. The flattening effect can be improved greatly. The structure and shape of the tool head are very simple so it is very easy to make. These structures keep the electrolytic gap between electrode and machining surface constant. In the m;lchining process the tool head moves according to a defined program so the EEUP process is very similar to NC milling. THE POLISHING
EQUIPMENT
In order to bring EEUP into use, a small polishing apparatus was developed. Its principle drawing is shown in Figure 2. In this figure the pressure-adjust system and the rotation axes are fixed IOthe swing plate, which can move along the % direction. Durmg machining, the polishing pressure can be adjusted automatically by the pressure-adjustment system, and the swing plate can swing slowly to adjust the polishing head into the best state suitable for freesurface machining. The gymtion center of the swing plate matches together with that of the polishing ball. These make it easy to generate the machining program and avoid the change of the position of the polishing head when the swing plate rotates. The worktable and swing plate are driven by step motors and the NC program is developed based on a Winrk,~~.s 9.7 operating system. ‘l’hls KC system can realize two- or three-dimensional
OF EEUP
During traditional mechanical polishing the abrasive powder scratches the machining surface lightly because the polishing cloth is far softer than the 22
machining materials. This method can polish the surface very smoothly but its machining efficiency is too low. Electrolytic polishing can machine the surlace with high efficiency but its machining accuracy is nol ideal. Moreover. the motion of the polishing tool head in the process of the abovementioned polishing ih usually a \implc rotation combined with a slow straight movement. In this way the machining locus of abrasive powder on the machining surface is a straight line or arc. thus confining the improvement to both machining efficiency and machining accuracy. To ~lve this problem. EEUP combines electrolytic polishing, mechanical polishing. and ultrasonic vibration together. Figure I ib a schematic drawing of the polishing tool head. There are three small holes on the polyurethane ball surface. ‘These holes can be ~rscd IO fit the negative electrode and to allow the electrolyte and polishing liquid\ to flow through them. In the machining process the polishing tool head rotates with high
Figure 1. Schematicdrawing of EEUP. 0 Copyright
Elsevier
Science
Inc.
Figure 2. Drawing of the polishing equip merit. METAL
FINISHING . JULY 1998
linkage movement and can display the machining simulate figure on the screen according to the NC code selected by the user. The swing plate moves along the Z axis slowly according to the polishing pressure. When the polishing pressure is at a suitable range the pressure is adjusted automatically by the pressure-adjust system: in the meantime the plate does not move along the Z axis (pressure micro-adjust). The plate will move along the Z axis when the pressure goes beyond the normal range. The movements of this equipment are enough to polish the common free surface.
STUDY AND EXPERIMENT TO EEUP Test Conditions The workpiece material is 1CrlX Ni9Ti stainless steel and its surface roughness before machining is about 1.6 urn. The electrolytic and polishing liquid has as chief chemical components sodium nitrate and silicon carbide micro-abrasive powder. The concentration of sodium nitrate in this solution is about 20% (weight) and that of silicon carbide powder is about 5% (weight). The size range of silicon carbide powder is between 1.O to 2.5 urn. The pH value of this solution is 7. The ultrasonic vibration frequency is 20 KHz, and its amplitude is about IO pm.
Technical
Test
Analyses show that during EEUP the chief action of electrolysis is to form a thin-layer passive membrane that is far softer than the base materials. Mechanical polishing erases this soft film to form the final machining surface, and ultrasonic vibration enhances the action of mechanical polishing. The experiments have supported the above-mentioned analyses, and it was noted that there is a best match among the above three actions. In the machining process the passive membrane on the convex part of the surface must be removed first by the abrasive powder. The new surface will be electrolyzed again to form a new membrane, but the passive membrane on the concave part of the surface must remain because the abrasive powder can’t touch it. This remaining film prevents the concave surface from being electrolyzed continually. The machining surface roughness 24
Figure 3. The relationship between current density and machining efficiency.
can be dropped down rapidly by repeating this process again and again. During the above process the electrolytic electric current density can’t exceed 0.6 A/cm’, otherwise, the machining surface will be damaged because such electric current density make the action of electrolysis too strong, damaging the passive membrane to form a pit. But the machining efficiency will drop down obviously if the electric current density is less than 0.3 A/cm’. So in order to give attention to the machining efficiency and accuracy, the electric current density is selected between 0.3 to 0.6 A/cm’. The relationship between the electric current density and machining efficiency or surface roughness can be shown in Figure 3. Obviously, the polishing pressure has a great affect on the machining quality and efticiency. It is the reason why there are two pressure-adjust systems in the equipment. In ordinary circumstances the higher the polishing pressure is the higher machining ehiciency can be but the rougher the machining surface becomes. Although the smooth action of ultrasonic vibration allows the polishing pressure to be a little bigger than that of ordinary polishing there still is a limit of pressure. Experiment shows that the polishing cloth can be crumpled under too much pressure and in this case the polishing pressure is almost concentrated onto the cloth. The pressure on the cloth is so strong that it can press the abrasive powder into the machining surlace and then leave mechanical scratches on the machined surface. Moreover, high polishing pressure can bring the tool head into vibration and then damage the contact state between the tool head. and the machining efficiency or surface roughness is very similar to Figure 3. Because the EEUP is a cornpound polishing action of pulse electrolytic
Figure 4. The relationship between tool head rotation speed and machining process efficiency.
polishing and ultrasonic polishing, the rotation speed of the tool head has a great affect on the machining effectiveness. The rotation speed can’t be selected too high to form a layer of soft passive membrane completely on the machining surface. On this occasion the machining efficiency is low and there will be distinct scratches on the machining surface. Figure 4 is the relationship between the rotation speed of‘ the tool head and the machining efficiency or the machining surface roughness. The experinients show that the rotation speed of the tool head in EEUP can be selected to be higher than that of ordinary polishing, and the best value of the rotate speed is about 2,000 rpm. Beside the above-mentioned fattors there are a number of other factors that affect EELJP such as tho polishing liquid. the kind and size of the abrasive powder, the direction of ultrasonic vibration, and the polishing track.
CONCLUSION The EEUP, which combines electrolytic polishing. ultrasonic vibration, mechanical polishing. and NC machining, is an eftective method to polish a free surface into a mirror finish with high machining efficiency and high accuracy. To realize the above-mentioned technique a small polishing system ha\ been developed. Using this equipment to polish ICrl XNiOTi stainless steel, ;I surface whose roughness is KmaXc0.025 pm. can be obtained with a removal rate 3 times the traditional polishing method by selecting the parameters of electrolytic and ultrltsonic and mechanical polishing properly.
Acknowledement This project (59375234) was supported by National Natural Science Foundation of China. MF
METAL
FINISHING
l
JULY 1998
Ultraprecision
Polishing
by Wang Zhenlong, Luan Yingyan, Pang Tao, and Liu Weidong. Harbin Institute of Technology, Harbin, China -
H
ow to machine surfaces with high efficiency and high accuracy is a hot research topic in the field of mechanical machining, which will produce a direct affect on the development of the die industry. Statistical data show that more than 60% of the machine parts for automo. biles, tractors, and electrical machinery and equipment are made by dies. In light industry more than 855% of the machine parts are made by dies. According to the development of science and technology, dies will play a more important role in industry. It has been forecasted by CIRP that 70%: of the roueh machining and 50% of the fine machining oi‘ parts will he made by dies in the 21st century. It i> common knowledge that tho final surface roughness and the damage layer of the die cavity xurface have a great influence on the properties and the useful life of the die. Common circumstances show that when the die cavity surface roughness improves one‘ step, the die life will be raiacd about 50%~: moreover, the workload of the final finish machining is more than ‘~1 that of the total workload of the die machining: however. the main means at present to polish die cavity surfaces and to gain low surface roughness with little or no damage is manual polishing. Obviously, the machining efficiency and accuracy are both low using these methods. In order to explore a new ultraprecision polishing method that can polish with high efficiency and high accuracy, numerically controlled (NC) machining. ultrasonic vibration, electrolylic machining, and mechanical polishing were combined. This method was named elastic and electrolytic ultraprecision polishing (EEUP1. Small polishing equipment that can bring EEUP into use was also developed. THE PRINCIPLE
polishing locus becomes very complicated and activates the abrasive powder. The flattening effect can be improved greatly. The structure and shape of the tool head are very simple so it is very easy to make. These structures keep the electrolytic gap between electrode and machining surface constant. In the m;lchining process the tool head moves according to a defined program so the EEUP process is very similar to NC milling. THE POLISHING
EQUIPMENT
In order to bring EEUP into use, a small polishing apparatus was developed. Its principle drawing is shown in Figure 2. In this figure the pressure-adjust system and the rotation axes are fixed IOthe swing plate, which can move along the % direction. Durmg machining, the polishing pressure can be adjusted automatically by the pressure-adjustment system, and the swing plate can swing slowly to adjust the polishing head into the best state suitable for freesurface machining. The gymtion center of the swing plate matches together with that of the polishing ball. These make it easy to generate the machining program and avoid the change of the position of the polishing head when the swing plate rotates. The worktable and swing plate are driven by step motors and the NC program is developed based on a Winrk,~~.s 9.7 operating system. ‘l’hls KC system can realize two- or three-dimensional
OF EEUP
During traditional mechanical polishing the abrasive powder scratches the machining surface lightly because the polishing cloth is far softer than the 22
machining materials. This method can polish the surface very smoothly but its machining efficiency is too low. Electrolytic polishing can machine the surlace with high efficiency but its machining accuracy is nol ideal. Moreover. the motion of the polishing tool head in the process of the abovementioned polishing ih usually a \implc rotation combined with a slow straight movement. In this way the machining locus of abrasive powder on the machining surface is a straight line or arc. thus confining the improvement to both machining efficiency and machining accuracy. To ~lve this problem. EEUP combines electrolytic polishing, mechanical polishing. and ultrasonic vibration together. Figure I ib a schematic drawing of the polishing tool head. There are three small holes on the polyurethane ball surface. ‘These holes can be ~rscd IO fit the negative electrode and to allow the electrolyte and polishing liquid\ to flow through them. In the machining process the polishing tool head rotates with high
Figure 1. Schematicdrawing of EEUP. 0 Copyright
Elsevier
Science
Inc.
Figure 2. Drawing of the polishing equip merit. METAL
FINISHING . JULY 1998
linkage movement and can display the machining simulate figure on the screen according to the NC code selected by the user. The swing plate moves along the Z axis slowly according to the polishing pressure. When the polishing pressure is at a suitable range the pressure is adjusted automatically by the pressure-adjust system: in the meantime the plate does not move along the Z axis (pressure micro-adjust). The plate will move along the Z axis when the pressure goes beyond the normal range. The movements of this equipment are enough to polish the common free surface.
STUDY AND EXPERIMENT TO EEUP Test Conditions The workpiece material is 1CrlX Ni9Ti stainless steel and its surface roughness before machining is about 1.6 urn. The electrolytic and polishing liquid has as chief chemical components sodium nitrate and silicon carbide micro-abrasive powder. The concentration of sodium nitrate in this solution is about 20% (weight) and that of silicon carbide powder is about 5% (weight). The size range of silicon carbide powder is between 1.O to 2.5 urn. The pH value of this solution is 7. The ultrasonic vibration frequency is 20 KHz, and its amplitude is about IO pm.
Technical
Test
Analyses show that during EEUP the chief action of electrolysis is to form a thin-layer passive membrane that is far softer than the base materials. Mechanical polishing erases this soft film to form the final machining surface, and ultrasonic vibration enhances the action of mechanical polishing. The experiments have supported the above-mentioned analyses, and it was noted that there is a best match among the above three actions. In the machining process the passive membrane on the convex part of the surface must be removed first by the abrasive powder. The new surface will be electrolyzed again to form a new membrane, but the passive membrane on the concave part of the surface must remain because the abrasive powder can’t touch it. This remaining film prevents the concave surface from being electrolyzed continually. The machining surface roughness 24
Figure 3. The relationship between current density and machining efficiency.
can be dropped down rapidly by repeating this process again and again. During the above process the electrolytic electric current density can’t exceed 0.6 A/cm’, otherwise, the machining surface will be damaged because such electric current density make the action of electrolysis too strong, damaging the passive membrane to form a pit. But the machining efficiency will drop down obviously if the electric current density is less than 0.3 A/cm’. So in order to give attention to the machining efficiency and accuracy, the electric current density is selected between 0.3 to 0.6 A/cm’. The relationship between the electric current density and machining efficiency or surface roughness can be shown in Figure 3. Obviously, the polishing pressure has a great affect on the machining quality and efticiency. It is the reason why there are two pressure-adjust systems in the equipment. In ordinary circumstances the higher the polishing pressure is the higher machining ehiciency can be but the rougher the machining surface becomes. Although the smooth action of ultrasonic vibration allows the polishing pressure to be a little bigger than that of ordinary polishing there still is a limit of pressure. Experiment shows that the polishing cloth can be crumpled under too much pressure and in this case the polishing pressure is almost concentrated onto the cloth. The pressure on the cloth is so strong that it can press the abrasive powder into the machining surlace and then leave mechanical scratches on the machined surface. Moreover, high polishing pressure can bring the tool head into vibration and then damage the contact state between the tool head. and the machining efficiency or surface roughness is very similar to Figure 3. Because the EEUP is a cornpound polishing action of pulse electrolytic
Figure 4. The relationship between tool head rotation speed and machining process efficiency.
polishing and ultrasonic polishing, the rotation speed of the tool head has a great affect on the machining effectiveness. The rotation speed can’t be selected too high to form a layer of soft passive membrane completely on the machining surface. On this occasion the machining efficiency is low and there will be distinct scratches on the machining surface. Figure 4 is the relationship between the rotation speed of‘ the tool head and the machining efficiency or the machining surface roughness. The experinients show that the rotation speed of the tool head in EEUP can be selected to be higher than that of ordinary polishing, and the best value of the rotate speed is about 2,000 rpm. Beside the above-mentioned fattors there are a number of other factors that affect EELJP such as tho polishing liquid. the kind and size of the abrasive powder, the direction of ultrasonic vibration, and the polishing track.
CONCLUSION The EEUP, which combines electrolytic polishing. ultrasonic vibration, mechanical polishing. and NC machining, is an eftective method to polish a free surface into a mirror finish with high machining efficiency and high accuracy. To realize the above-mentioned technique a small polishing system ha\ been developed. Using this equipment to polish ICrl XNiOTi stainless steel, ;I surface whose roughness is KmaXc0.025 pm. can be obtained with a removal rate 3 times the traditional polishing method by selecting the parameters of electrolytic and ultrltsonic and mechanical polishing properly.
Acknowledement This project (59375234) was supported by National Natural Science Foundation of China. MF
METAL
FINISHING
l
JULY 1998