MICRO-BURNISHING APPARATUS USING ULTRASONIC VIBRATION

Abstract: A micro burnishing apparatus using ultrasonic vibration wherein a structure is contacted at a surface thereof by a ball of a burnishing head while the burnishing head is inserted into a horn, the horn being coupled to an ultrasonic oscillator for generating ultrasonic by oscillating high frequency received from a high frequency generator and the horn being pressurized by a spring, where the ball touches the structure and is vibrated to burnish the surface of the structure, such that surface hardness is increased with improved degree of surface roughness and sharp reduction of processing time.

Description: MICRO-BURNISHING APPARATUS USING ULTRASONIC VIBRATION BACKGROUND ^ THE INVENTION FIELD OF THE INVENTION The present invention relates to a micro-burnishing apparatus using ultrasonic vibration. DESCRIPTION OF THE PRIOR ART Inner surface of hydraulic or pneumatic cylinder, or inner surface of mold and the like require a surface roughness of less than 1.6S or 0.8S through super precision micro-finishing process, where S is a surface roughness according to maximum height calculation method and is indicated as "Rmax." Such being the case, the inner surface is first finished by a precision boring machine or grinding machine and in order to obtain such a high degree of surface roughness, the surface is finished via such finishing method as honing, micro-grinding, lapping, superfinishing, ultrasonic machining or burnishing methods. The honing process is a finishing method by way of a honing machine where an angular rubstone called hone softly touches the surface of a workpiece to grind the surface. The lapping process is a finishing method where lap material of fine powdery state and lubricating material are inserted between a workpiece and a lap tool and surface of the workpiece is neatly smoothed by relative motion therebetween. The lapping process includes two methods, one being a wet lapping method and the other being a dry lapping method. The wet lapping method calls for mixture of lap material and fluid to be infused into a workpiece for machining. On the other hand, the dry lapping method needs the lap tool to be buried in the lap material, which is then wiped off with cloth. Then, lapping is carried out in the dry state. The super finishing is performed by pressing small particles of soft whetstone with light force to the surface of workpiece to feed and vibrating the whetstone to polish the surface of the workpiece. The ultrasonic machining is a sort of ultrasonic lapping finishing method in which ultrasonic vibration is applied to tools to collide the lap material to the workpiece. As shown in' Fig. 1, ultrasonic vibration is generated out of the ultrasonic oscillator 14 through high frequency oscillator 12 from power source 10. The ultrasonic vibration amplifies its amplitude with an effect from a conical part of fixed means 16 to be conveyed to a tool 18, which is then to collide lap material 20 to finish a workpiece 22. At this time, the workpiece 22, tool 18, lap material 20 are put in a container 24, and feeding means 26 moves the tool 18 up and down to finish holes of the workpiece. The burnishing process, as shown in Fig. 2, is performed by firstly finishing a hole 32 in a workpiece 30, forcibly pressing a ball 34 having a diameter a little larger than the inner side of the hole 32 as a presser 36 for smoothly polishing like mirror surface the machining scratches of the surface made during a cutting or grinding process. The workpiece 30 is fixed on a machine tool 38 by fixing means (not shown). Along with recent technological improvements, there has been a strong demand on a variety of machinery with smooth surfaces through quiet finishing operations resulting in the surface roughness of under 0.8S, much finer surface than 1.5S, for better energy consumption efficiency. In other words, a high precision of a surface machining process is required for finishing metallic surfaces such as the inner surface of hydraulic or pneumatic pressure cylinder used for automobiles, industrial heavy equipment, various automated machines and other general machinery, the outer surface of various metal shafts, the plane or slant surfaces of machines, plane and the free curved surfaces of press mold, forging mold, injection mold and die casting mold. In the surface finishing processes as such, a high precision grinding or honing machine, more effective than a common precision machining device, having a longer life time is required to achieve a high degree of surface smoothness, under 0.8S, so that there is a problem in increasing manufacturing cost. SUMMARY OF THE INVENTION The present invention is disclosed to solve the aforementioned problems and it is an object of the present invention to provide a micro-burnishing apparatus using ultrasonic vibration for performing a high precision machining process by using ultrasonic energy and contact pressure, but reducing the time to be taken for the machining process. In order to accomplish the object of the present invention, there is provided a micro-burnishing apparatus using ultrasonic vibration comprising: a burnishing head contacting the surface of a workpiece; holding means for holding the burnishing head; an ultrasonic oscillator for generating ultrasonic vibration to be applied to the holding means; high frequency generating means for generating high frequency to be supplied to the ultrasonic oscillator; and pressing means for forcibly pressing the holding means to tightly place the burnishing head at the surface of the workpiece. The ultrasonic oscillator is fixed at a case, and the pressing means is made with pressure controlling means for controlling the pressing force to be applied to the case. Furthermore, the case is made to fluctuate inside guiding means according to the pressure of the pressing means. The guiding means is to be moved by driving (transfer) means and control means. The driving (transfer) means is constructed with a variety of means including a ball screw motor. BRIEF DESCRIPTION OF THE DRAWINGS For fuller understanding of the nature and object of the invention, reference should be made to the following detailed description taken in conjunction with the accompanying drawings in which: FIG. 1 illustrates the structure of a general ultrasonic machining apparatus; FIG. 2 is a view for illustrating a principle of a general burnishing process; FIG. 3 illustrates the structure of a micro-burnishing apparatus using ultrasonic vibration in accordance with the present invention; FIG. 4 illustrates a state of finishing an inner surface of a cylinder with a micro-burnishing apparatus of the present invention; FIG. 5 illustrates a state of finishing an outer surface of a rod with an apparatus of the present invention; FIG. 6 is a test used for examining an apparatus of the present invention; FIG. 7 is a graph for illustrating the relation between transmission of pressing force and surface hardening depth when a finishing process is completed by a micro-burnishing apparatus in accordance with the present invention; FIG. 8 illustrates the structure of a micro-burnishing apparatus in accordance with another embodiment of the present invention; and FIG. 9 illustrates the structure of a burnishing head shown in FIG. 8. DETAILED DESCRIPTION OF THE INVENTION Objects and aspects of the invention will become apparent from the following detailed description of a preferred embodiment with reference to the accompanying drawings. FIG. 3 illustrates the structure of a micro-burnishing apparatus using ultrasonic vibration in accordance with the present invention. The micro- burnishing apparatus comprises: a burnishing head 40 contacting the surface of a workpiece; holding means 42 for holding the burnishing head 40; an ultrasonic oscillator 44 for generating ultrasonic vibration to be applied to the holding means 42; high frequency generating means 46 for generating a high frequency to be supplied to the ultrasonic oscillator 44; and pressing means 48 for forcibly pressing the holding means 42 to tightly put the burnishing head 40 at the surface of the workpiece. The ultrasonic oscillator 44 is fixed at a case 50, and the pressing means 48 is made with pressure controlling means 52 for controlling the pressing force to be applied to the case 50. At this time, the case 50 is made to fluctuate inside guiding means 54 according to the force of the pressing means 48, and the ultrasonic oscillator 44 contained in the case 50 is to be cooled hydraulically or pneumatically by cooling means (not shown here). The guiding means 54 is to be moved by driving (transfer) means and control means. The driving (transfer) means is constructed with a variety of means including a ball screw motor. The burnishing head 40 is constructed with a ball 55 to be in a direct contact with the workpiece and a fitting part 56 for getting the ball 55 inserted. The ball 55 is fixed at the fitting part 56. The ball 55 is usually made of steel for finishing soft material, but super-light alloy for hard material. The burnishing head 40 can be made of only chrome-molybdenum steel (for instance, SCM4), high alloy steel (for instance, SKD11). The ball 55 can be made of tool steel (SKD, SKH, SKS, etc.), super light alloy steel, ceramic tool, CBN, artificial diamond, etc. The ball 55 can be selectively made of various material according to finishing material and conditions. The fitting part 56 of the burnishing head 40 is inserted into the holding means 42. The holding means 42 can be called horn, and screwed at the bottom of the ultrasonic oscillator 44. The holding means 42 can be integrated with the ultrasonic oscillator 44. The burnishing head 40 is fixed at the holding means 42 with a bolt (not shown). The ultrasonic oscillator 44 is constructed in the generally known structure that the coil connected to the high frequency generating means 46 is wound at the external surface thereof. The ultrasonic oscillator is made in the structure using piezo-electric ceramics. The ultrasonic oscillator 44 receives a high frequency from the high frequency generating means 46 to generate ultrasonic vibrations of 16,000 ~ 100,000,000 cycles/second. The high frequency generating means 46 is made into a magnetron or other well-known high frequency generating apparatus, being connected with power source (not shown here). The pressing means 48 has a compression spring, being accommodated in the cylindrical guiding means 54 and pressurizing the case 50 with elasticity. The pressure of the compression spring is conveyed to the ball 55 of the burnishing head 40. The pressing means is included for keeping pressure constant between material and burnishing tool (ball), and also used as other means like a spring, pneumatic pressure, a compliance value of a tool, or a means to forcibly insert a burnishing tool (ball) into the working surface. The pressing means 48 is mounted on the bottom of the guiding means 54, has a screw adapted to control elasticity of the pressing means 48. The pressure controlling means 52 is made to control elasticity of the pressing means (48: spring) by changing its length with fluctuations of a screw mounted in a base plate 58 where the spring is to be mounted. In the micro-burnishing apparatus using ultrasonic vibration of the present invention thus constructed, the guiding means 54 is moved by the driving or transfer means (not shown) to contact the ball of the burnishing head 40 with the surface of a workpiece to be finished, and the pressure controlling means 52 is manually or automatically controlled to adjust the force of the spring 48 to push the case 50, that is, the pressure of the ball 54 of the burnishing head 40 to be applied to the surface of the workpiece. When the high frequency generating means 46 is switched on, the ultrasonic oscillator 44 generates ultrasonic vibration to vibrate the burnishing head 40 to polish surface of the workpiece smooth. With the micro-burnishing apparatus of the present invention thus constructed, the surface roughness of 0.05S can be achieved within a short period of time along with an additional surface hardening effect. FIG. 4 illustrates a state of finishing an inner surface of a cylinder with an apparatus of the present invention, and FIG. 5 illustrates a state of finishing an outer surface of a rod. As shown in the drawings, the burnishing apparatus is machining the surface of a workpiece, pressing at the direction of an arrow mark P and transporting to the direction of an arrow mark s. The micro-burnishing apparatus of the present invention can be used for machining various surfaces like free curved surfaces, plane surfaces and both surfaces of a brake disc. The micro-burnishing apparatus can also be added to a machining center or robot for other uses. In the burnishing process, the basic factors to determine the level of a surface finishing process are the contact pressure of the ball 55 to be applied to the surface of a workpiece, contacting area, transmission of pressure from the machining tool to the surface of the workpiece, and the dimension, transfer speed and rotations of the workpiece. The frequency of ultrasonic vibration, the shape of a machining tool and the like can also make an influence on the level of a surface finishing process. The contacting pressure determines the hardness and hardening efficiency of a finished surface. FIG. 7 is a graph for illustrating the relation between transmission of pressing force (F) and hardening depth (h). In the graph, the amplitude of curve 1 is 10 μm, and that of curve 2 is 20 μm. Among affecting factors, the transfer speed determines a transformation rate of the finishing surface. The quality of the finished surface basically depends on static weight applied to the workpiece and amplitude of ultrasonic vibration. As a result, the selection of adequate transfer speed and rotations significantly affect the control of the surface hardening process. As shown in FIG. 6, several positions (A, B, C, D) are selected on a plurality of levels of a shaft. The machining process is performed by the micro- burnishing apparatus of the present invention for three minutes under the following conditions: frequency of 20 kHz, transfer speed of 0.16 mm/rev, rotations of 450 rev/min, transmission of pressing force of 12 kg, and the result of the surface finishing process is included in Table 1. Table 1 FIGS. 8 and 9 illustrate the structure of a micro-burnishing apparatus in accordance with another embodiment of the present invention. At this time, a burnishing head 40' is shaped in a rod with its circular tip 55'. The tip 55' is forcibly inserted into a fitting part 56', being screwed into a holding means 42'. Preferably, the fitting part 56' is made of chrome-molybdenum steel (SCM4), and the tip 55' is made of carbon tungsten WC. The basic structure of the micro-burnishing apparatus shown in the drawings is the same as that shown in FIG. 3, so as to omit all the detailed descriptions. However, the scope of the present invention is not limited in the preferred embodiments, but can be practiced with various modifications. Therefore, there are advantages in the micro-burnishing apparatus using ultrasonic vibrations of the present invention in that the ball vibrates in contact with a workpiece and smoothes its surface, thereby improving the surface hardness at a lower degree of surface roughness with a reduction in the surface finishing time, and, also, the smoothness of the surface of molds when the micro-burnishing apparatus is attached to a vertical or horizontal machining center or a robot for other uses.

Claims: What is claimed is: 1. A micro-burnishing apparatus using ultrasonic vibration comprising: a burnishing head contacting the surface of a workpiece; holding means for holding the burnishing head; an ultrasonic oscillator for generating ultrasonic vibration to be applied to the holding means; high frequency generating means for generating a high frequency to be supplied to the ultrasonic oscillator; and pressing means for forcibly pressing the holding means to tightly put the burnishing head at the surface of the workpiece. 2. The apparatus, as defined in claim 1, wherein the ultrasonic oscillator is fixed at the case, and the pressing means is made with pressure controlling means for controlling the pressing force to be applied to the case.

Inventors: PYOUN YOUNG SHIK, PYOUN, YOUNG, SHIK
Applicants: DESIGNMECHA CO LTD, PYOUN YOUNG SHIK, SUN MOON EDUCATIONAL FOUNDATIO, DESIGNMECHA CO., LTD., PYOUN, YOUNG, SHIK, SUN MOON EDUCATIONAL FOUNDATION
Application year: 2000
Main classification: B24B39
IPC codes: B24B1, B24B35, B24B39