Abstract:
Apparatus for ultrasonic vibration-assisted machining, the apparatus comprising an ultrasonic transducer for generating ultrasonic waves in a vibration horn; first clamp on the vibration horn at a static node of the ultrasonic waves; and a second clamp between the first clamp and a lowermost end of the vibration horn. The second clamp comprises a linear bearing for reducing vibration of the vibration horn in a direction laterally of the vibration horn, and allowing vibration of the vibration horn in the direction of a longitudinal axial of the vibration horn.

Description:
This is a national stage of PCT/SG04/000260 filed Aug. 26, 2004 and published in English. 
   FIELD OF THE INVENTION 
   This invention relates to apparatus for ultrasonic vibrations-assisted machining and refers particularly, though not exclusively to apparatus for ultrasonic vibration-assisted machining of metals such as steel and stainless steel. 
   BACKGROUND OF THE INVENTION 
   Ultrasonic vibration-assisted machining has several important advantages. One important advantage flows from the friction reducing action, which arises when the cutting edge or point of the tool separates from the work during each vibration cycle and which introduces an ultrasonic pumping action. This pumping action allows transmission of cooling fluids to all areas of the workpieace area. The cooling fluid may be air. 
   Normal machining has an accuracy of greater than 5 μm and uses ordinary machine tools. Ultrasonic vibration-assisted machining was introduced in the 1960s to provide advantages over normal machining. The expected advantages have not been realized, however, as it did not increase tool life nor improve surface finish. Also, cutting efficiency is low as the cutting speed must be less than the vibration speed. 
   Since 1980, ultra-precision machining was developed to meet the demands of fabricating complicated optics. A diamond tool bit is the only tool bit that can be used to generate an optical mirror surface finish. However, a diamond tool bit cannot be used on steel due to the strong chemical reaction between the diamond and the steel. The chemical reaction causes graphitization of the diamond. 
   With prior art systems, the vibration horn is clamped at two static node points of the vibration wave as shown in  FIG. 1 . The diamond tool bit is mounted on the free end of the vibration horn. To achieve a mirror surface finish, the induced lateral vibration of the horn in the radial direction must be significantly reduced. 
   For the prior art method of the vibration horn with two static clamping points, there is some distance from the lower static node point to the free end where the tool bit is mounted. In such a case, the point at the free end has reduced stiffness in the lateral/radial direction and lateral vibration in the lateral/radial direction can easily be induced. 
   To increase the lateral stiffness, the diameter of the horn is increased and, in turn, the power of the ultrasonic vibration generator is also increased. Therefore, the overall size of the apparatus is increased. Furthermore, the operational temperature of the device increases due to the larger power, and is easily damaged due to the higher temperature. Also, lateral (or radial) vibration damages the tool bit. It also causes deeper cuts and therefore lower quality surface finish. Such lateral radial vibrations are normally of the order of 4 μm. 
   To prevent excessive tool wear, a two stage process has been proposed using electroless nickel plating or coating on a workpiece before machining when fabricating optical mold inserts for the injection molding of plastic lenses. However such methods have many disadvantages. Also, they are not capable of manufacturing moulds with high durability as the nickel tends to lift off the steel workpiece. 
   SUMMARY OF THE INVENTION 
   According to a first aspect there is provided an apparatus for ultrasonic vibration-assisted machining, the apparatus comprising: an ultrasonic transducer for generating ultrasonic waves in a vibration horn; a first clamp on the vibration horn at a static node of the ultrasonic waves; and a second clamp between the first clamp and a lowermost end of the vibration horn. The second clamp comprises a linear bearing for reducing vibration of the vibration horn in a direction laterally of the vibration horn, and allowing vibration of the vibration horn in the direction of a longitudinal axial of the vibration horn. 
   The linear bearing may be axially spaced from the first clamp by less than half a wavelength of the ultrasonic waves. 
   According to a second aspect there is provided apparatus for ultrasonic vibration-assisted machining, the apparatus comprising: an ultrasonic transducer for generating ultrasonic waves in a vibration horn; a first clamp on the vibration horn at a static node of the ultrasonic waves; and a second clamp between the first clamp and a lowermost end of the vibration horn. The second clamp is axially spaced from the first clamp by less than half a wavelength of the ultrasonic waves. 
   The second clamp may comprise a linear bearing for reducing vibrations of the vibration horn in a direction laterally of the vibration horn and allowing vibration of the vibration horn in the direction -of a longitudinal axis of the vibration horn. 
   For both aspects, the second clamp may be generally U-shaped and may comprise an upper clamp, a lower clamp, and an intermediate portion between the upper clamp and the lower clamp. The upper clamp and the lower clamp may be radially adjustable relative to the linear bearing. 
   The second clamp may be removably and adjustably mounted in a mounting block. The first clamp may be removably attached to the mounting block. The mounting block may be removably and adjustably mounted on a tool post. The tool post may comprise: an upper portion, a lower portion, a gap between the upper portion and the lower portion, and an adjusting mechanism for adjusting the gap. 
   The apparatus may further comprise a tool bit releasably secured to the vibration horn at the lowermost end thereof. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order that the present invention may be fully understood and readily put into practical effect, there shall now be described by way of non-limitative example only preferred embodiments of the present invention, the description being with reference to the accompanying illustrative drawings in which: 
       FIG. 1  is a schematic side view of a prior art apparatus; 
       FIG. 2  is a schematic side view of the preferred embodiment; 
       FIG. 3  is a perspective view of the preferred embodiment; and 
       FIG. 4  is a horizontal cross sectional view of a preferred form of linear bearing. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   To refer to  FIG. 1 , there is shown a prior art apparatus  8 . Here, there is an ultrasound transducer  10  with a vibration horn  12 . These are mounted to a mounting block  14  by upper clamp  16  and lower clamp  18 . The tool bit  20  is at the lowermost end of the vibration horn  12 . The ultrasound transducer  10  produces sound waves  22  at, for example, 40 KHz. The waves have maximum amplitude at ultrasound transducer  10 , and at tool bit  20  to maximize the movement of tool bit  20 . To hold the vibration horn  12  in place, and to prevent unwanted radial (or lateral) movement, the vibration horn  12  is secured to mounting block  14  by upper clamp  16  and lower clamp  18 . Clamps  16 ,  18  are located at static node points  24  to allow them to function without interfering with the operation of tool bit  20 . However, that means there is inherently a gap between lower clamp  18  and tool bit  20 . This allows unwanted motion to occur at tool bit  20 , the unwanted vibration being induced by vibration horn  12 . 
   To now refer to  FIGS. 2 and 3 , there is shown apparatus  28  according to a preferred embodiment. Here, there is a tool post  30  having an upper post  30  and a lower post  34  with a gap  36  therebetween. An adjusting knob  38  on a threaded shaft (not shown) is used to adjust gap  36  to enable the apparatus  28  to be correctly aligned and positioned. 
   An ultrasound transducer  40  is provided with a vibration horn  68 . The ultrasound transducer  40  produces vibration waves  44  in the axial direction of vibration horn  68 . The waves  44  are at their maximum amplitude  98  at ultrasound transducer  40  and at tool bit  50  mounted at the lowermost end of vibration horn  68 . The waves  44  have a single static node  46 . At static node  46  is provided a clamping ring  42 . In this way clamping ring  42  holds the vibration horn  68 . Between static node  46  and maximum amplitude  48  at tool bit  50  is a linear bearing  52 . Linear bearing  52  allows vibration in the direction of the longitudinal axis of vibration horn  68 , but minimizes vibration in the radial (or lateral) direction of vibration horn  68 . 
   Linear bearing  52  is attached to mounting block  62  by a U-shaped bearing mounting  54  that has an upper clamp  56  and a lower clamp  58  joined by an intermediate portion  60 . Intermediate portion  60  is removably and adjustably mounted to mounting block  62 . Mounting block  62  is removably and adjustably mounted to tool post  30 . 
   Upper clamp  56  and lower clamp  60  each is split, and each has a gap  64 . A tightening screw  66  is used to control the clamping force by clamps  56 ,  58  on linear bearing  52 . In this way the radial/lateral vibration of vibration horn  68  can be adjusted, and controlled. 
   The linear bearing  52  may be of any known construction such as, for example, the linear bearing known as “KUGELBUCHSE” available from Hiwin Technologies Corp of Glenview, Ill., USA. Such a linear bearing  52  is schematically shown in  FIG. 4 . It has an outer casing  70  with a gap  72  therein. The casing  70  is normally of a metal such as, for example, steel, and is preferably relatively thin. Within casing is a cylindrical body  74  in which are rotatably mounted a plurality of balls  76 . The cylindrical body  74  is preferably of plastics material. The balls  76  are preferably of a metal such as, for example, steel and are held in body  74  in the manner of a snap fit. Balls  76  project beyond inner  78  and outer  80  surfaces of body  74 . The gap  72  allows clamps  56 ,  58  to lighten on casing  71  and thus increase the clamping by body  74  on vibration horn  68 . 
   In this way the vibration horn  68  may be of reduced axial length as the distance between static node points  24  is removed. Also, the axial distance between the clamping by clamp  42  and linear bearing  52  is preferably less than half the wavelength of waves  44 . The lowermost clamping location (by lower clamp  58 ) is preferably adjacent the tool bit  50 . However, it may be at any location on horn  68 . Therefore, vibration horn  68  may be made with a reduced diameter as it does not require the relatively high structural strength of prior art vibration horn  12 . For example, the lower clamp  58  may be in the range 3 to 15 mm from tool bit  50 , preferably 10 mm. By having the vibration horn  68  of shorter axial length, and of smaller radius, the power required for ultrasound transducer  40  can be reduced. This reduces heat generation at tool bit  50 . Also, it enables the apparatus to be used with high precision machines. 
   By way of example, ultrasound transducer  40  may be at 40 KHz and the vibration amplitude may vary within the range 0 to 24 μm, preferably 2 to 4 μm, depending on the cutting parameters. The cutting speed may be less than 10 meters per minute, with a depth of cut and feedrate being less than 10 micrometers and 10 micrometer per revolution, respectively. The lateral/radial, and random, vibration may be in the range 0.1 to 0.2 μm. Steel may therefore be machined to a mirror finish with an Ra&lt;8 nm. 
   Furthermore, the productive life of tool bit  50  may be lengthened due to reduced graphite formation as a result of the reduced temperature. Increases in tool bit life of up to 600 times have been experienced. 
   The workpiece may be of any size, but for sizes greater than 40 mm in diameter a higher frequency and/or a lengthy machining time may result. Workpiece sizes down to 10 μm in diameter have been able to be machined. Workpieces may be of any suitable material such as, for example: glass, glass for lenses, steel, stainless steel, magnetizable stainless steel, moulding/tooling steel, and so forth. 
   Whilst there has been described in the foregoing description preferred embodiments of the present invention, it will be understood by those skilled in the technology concerned that many variations or modifications in details of design or construction may be made without departing from the present invention. 
   LIST OF REFERENCE NUMERALS 
   
       
         8 —prior art apparatus 
         10 —ultrasound transducer 
         12 —vibration horn 
         14 —mounting block 
         16 —clipper clamp 
         18 —lower clamp 
         20 —tool bit 
         22 —waves 
         24 —static node points 
         26 —maximum amplitude locations 
         28 —apparatus of embodiment 
         30 —tool post 
         32 —upper part of  30   
         34 —lower part of  30   
         36 —gap between  32  &amp;  34   
         38 —adjusting knob 
         40 —ultrasound transducer 
         42 —clamping at static node 
         44 —waves 
         46 —static node 
         48 —maximum amplitude 
         50 —tool bit 
         52 —linear bearing 
         54 —bearing mounting (U-shaped) 
         56 —upper clamp 
         58 —lower clamp 
         60 —intermediate portion 
         62 —mounting block 
         64 —gap in  56 ,  58   
         66 —tightening screw 
         68 —vibration horn 
         70 —casing 
         72 —gap in  70   
         74 —body 
         76 —balls 
         78 —inner surface of  74   
         80 —outer surface of  74