Abstract:
Pressed shape steel, made of a low cost general steel material that is not hardened, forms a frame with long grooves cut therein. Guide rails, made of a specialized steel material that can be subjected to a hardening treatment, are hardened. Thereafter, outer surfaces of the hardened guide rails are ground and the guide rails are integrally joined in the long grooves. Ball-rolling grooves are formed in the guide rails, thereby completing a guide-equipped frame for an actuator. Because the general steel material and the specialized steel material are principally ferrous materials, both exhibiting a Young&#39;s modulus at or above 170 GPa with substantially the same coefficient of linear thermal expansion, the frame needn&#39;t be increased in size or have a complex internal structure to reinforce the strength of the frame, and the guide rails remain securely retained in the grooves even if the actuator experiences changes in temperature.

Description:
This application is a continuation in part of U.S. patent application Ser. No. 10/852,756, filed on May 25, 2004, now abandoned, which is a divisional of U.S. patent application Ser. No. 10/108,901, filed on Mar. 29, 2002, now abandoned. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an actuator comprising a guide-equipped frame having guide grooves formed on the frame. The present invention also relates to a method for producing such a guide-equipped frame. 
   2. Description of the Related Art 
   Various actuators are conventionally used to transport or position a workpiece. Japanese Laid-Open Utility Model Publication No. 2-12554, for example, discloses an actuator having a guide-integrated frame which has guide grooves integrally formed on inner wall surfaces. 
   The actuator comprises the guide-integrated frame having ball-rolling grooves (guide grooves) axially extending on the inner wall surfaces on both opposed sides. The guide-integrated frame has a ball screw shaft which extends substantially in parallel to the ball-rolling grooves. Further, the guide-integrated frame has a slider. The slider reciprocates along the ball-rolling grooves under a screwing action by means of the ball screw shaft. 
   A method for producing the conventional guide-intergrated frame will be briefly explained. A pillar-shaped member is drawn to form a drawn product. Warpage of the drawn product is straightened. Next, cutting machining is performed on outer surfaces of the drawn product that cannot be straightened. Then straightening is performed again. 
   Next, a hardening process, such as vacuum hardening or high frequency hardening, is performed. Thereafter, straightening and polishing are performed on the outer surface. Groove-polishing is also performed to form ball-rolling grooves along the inner wall surfaces, using a disk-shaped grinding wheel or the like. Thus, the guide-integrated frame is completed. 
   However, a large number of treatment steps are required in the method for producing the conventional guide-integrated frame. Therefore, production costs are high. Further, it is impossible to improve production efficiency, because an extremely long period of time is required to polish the outer surface. 
   U.S. Pat. No. 5,755,515 discloses an actuator body having a base portion fabricated from a light metal or a light metal alloy and supporting a slider, made of the same light metal or light metal alloy as the base, for reciprocal movement along the base portion. The slider and base portions of the actuator include respective guide rails. More specifically, the base portion includes a pair of base rails, made of hardened steel, and the slider includes a pair of slider rails, also made of hardened steel, which are fitted into side grooves provided in the base portion and the slider respectively. 
   In the above structure, the base rails are made according to the following process. First, the base rails are formed by grinding or by a plasticity rolling process, and rolling body tracks are formed in the rails by a heat treatment and hardening process. Under this condition, the base rails are fitted into the base side grooves, and the rolling body tracks are ground to have a transverse cross-sectional shape of a Gothic style arch. The slider rails, which are formed by the same process, are fitted into the slider side grooves. To reduce adverse effects caused by a difference in the coefficients of linear thermal expansion of aluminum and steel, critical dimensional features of the actuator are established, such that the thickness D 2  of the base rails (as well as the thickness of the slider rails) is set to be smaller than the diameter D 3  of the balls, and equal to or less than 10% of a center distance L 2  between the balls on respective sides of the slider, and further, the width B of the base rails is set to be equal to or less than twice the diameter D 3  of the balls, thereby satisfying the relationships, D 2 &lt;D 3 , D 2 ≦0.1×L 2 , and B≦2×D 3 . 
   Noted advantages associated with the above structure are, (1) since most of the base and slider are made of aluminum and only the base rails and slider rails are made of steel, the overall structure is light in weight, (2) thermal expansion related problems are reduced because the portions associated with rolling of the balls are made of the same material on both the base and slider sides, (3) the mechanical strength of the steel base rail and the steel slider rail are high so as to ensure stable performance over time, and (4) owing to the critical dimensional features discussed above, it is possible to reduce the influence of dimensional changes caused by the difference in thermal expansion between aluminum and steel. 
   Nevertheless, in the actuator of U.S. Pat. No. 5,775,515, although attempts are made to minimize the adverse effects brought about by the difference in thermal expansion between aluminum and steel, a complete solution to this problem cannot be obtained. In particular, since the coefficients of linear thermal expansion differ markedly (23.6×10 −6 /° C. for aluminum alloys verses 10.7×10 −6 /° C. for steel), when moved into a hotter environment, the aluminum alloy portions of the base and slider, including the side grooves thereof in which the guide rails are positioned, expand roughly two times greater than the steel guide rails themselves, which leads to gaps occurring between the side grooves and the guide rails, along with potential dislodgement or loosening of the guide rails within the side grooves. 
   Moreover, a further problem results from the disclosed structure owing to the difference in Young&#39;s modulus, which is a measure of rigidity, between aluminum and steel. In particular, because the Young&#39;s modulus of a light aluminum alloy is only about one-third that of steel and other ferrous based metals, the cross-sectional shape and size of the base frame in U.S. Pat. No. 5,755,515 must be made much larger, and a complex hollowed structure must be used, in order to provide sufficient rigidity comparable to that of a steel frame, increasing both size and cost, as well as complexity in fabricating the actuator body. 
   SUMMARY OF THE INVENTION 
   A general object of the present invention is to provide an actuator and method for producing an actuator having a guide-equipped frame, which reduces production costs by simplifying the production steps to conveniently produce the actuator. 
   A principal object of the present invention is to provide an actuator and method for producing an actuator having a guide-equipped frame, which improves production efficiency by simplifying the production steps to conveniently produce the actuator. 
   Another major object of the present invention is to provide an actuator employing a method in which both the frame and guide rails are made from similar ferrous (i.e., iron based) materials, yet wherein only the guide rails are subjected to a hardening treatment, thus allowing the frame to be made of a low cost general steel material, whereas the guide rails are formed of a specialized steel material that can be subjected to hardening. As a result, since the principal elemental base material is iron (Fe) for both the frame and the guide rails, the Young&#39;s modulus is substantially the same for both, and although the frame is not subjected to hardening, the frame and guide rails can still exhibit a uniform solid shape and rigidity. A complex hollowed or internal ribbed structure for the frame is not required. 
   Yet another major object of the present invention is to provide an actuator employing a method in which the general steel material used for the frame and the specialized hardened steel material used for the guide rails exhibit substantially the same coefficient of linear thermal expansion. Therefore, even when used in different temperature environments, gaps or clearances do not develop between the frame and the guide rails due to thermal expansion, and the guide rails always remain firmly secured within accommodating long grooves provided in the frame. 
   The above and other objects, features, and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which a preferred embodiment of the present invention is shown by way of illustrative example. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view illustrating an actuator according to an embodiment of the present invention; 
       FIG. 2  is an exploded perspective view illustrating the actuator shown in  FIG. 1 ; 
       FIG. 3  is a partial exploded perspective view illustrating the actuator shown in  FIG. 1 ; 
       FIGS. 4A to 4G  illustrate steps for producing a guide-equipped frame respectively; 
       FIG. 5  is a perspective view illustrating the amount of flexion when a load is applied with one end of the guide-equipped frame being fixed; 
       FIG. 6  shows characteristics illustrating the relationship between the load and the strain for a heated frame and a non-heated frame; 
       FIG. 7  is a vertical sectional view illustrating a guide-equipped frame according to a first modified embodiment; 
       FIG. 8  is a vertical sectional view illustrating a guide-equipped frame according to a second modified embodiment; 
       FIG. 9  is a vertical sectional view illustrating a guide-equipped frame according to a third modified embodiment; and 
       FIG. 10  is a vertical sectional view illustrating a guide-equipped frame according to a fourth modified embodiment. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In  FIG. 1 , reference numeral  10  indicates an actuator according to an embodiment of the present invention. 
   The actuator  10  comprises a guide-equipped frame  12 , a rotary driving source  14 , a feed screw shaft mechanism  16 , a slider  18  and a guide mechanism  20  (see  FIG. 3 ). 
   The guide-equipped frame  12  has a recess including a plurality of attachment holes  11 . The rotary driving source  14  is connected to one end of the guide-equipped frame  12 . The feed screw shaft mechanism  16  is a unit detachable with respect to the guide-equipped frame  12 . The feed screw shaft mechanism  16  transmits the rotary driving force of the rotary driving source  14  by the aid of an unillustrated coupling member. The slider  18  is reciprocated in the axial direction of the guide-equipped frame  12  by the driving force transmitted by the feed screw shaft mechanism  16 . The guide mechanism  20  (see  FIG. 3 ) guides the slider  18  along the guide-equipped frame  12 . 
   As shown in  FIGS. 2 and 3 , the guide-equipped frame  12  comprises a bottom  12   a  of a flat plate shape and a pair of sides  12   b ,  12   c . The pair of sides  12   b ,  12   c  are substantially perpendicular to the bottom  12   a . The pair of sides  12   b ,  12   c  are integrally formed so that they may face one another. 
   As shown in  FIG. 2 , the feed screw shaft mechanism  16  includes a ball screw shaft (feed screw shaft)  28  coaxially coupled to the drive shaft of the rotary driving source  14  by the unillustrated coupling member, and a ball screw nut (feed screw nut)  30  having a penetrating screw hole for the ball screw shaft  28  to be screwed therein. 
   The ball screw nut  30  includes a cylindrical section  32  and a pair of flanges  36   a ,  36   b . The cylindrical section  32  includes the penetrating screw hole. The pair of flanges  36   a ,  36   b  are integral with one end of the cylindrical section  32  and are fixed to side surfaces of the slider  18  by screws  34 . 
   The feed screw shaft mechanism  16  includes a housing  42 , an unillustrated bearing mechanism and a bearing-holding member  48 . 
   The housing  42  has a support section  38  for supporting the rotary driving source  14  and is connected to one end of the guide-equipped frame  12  by screws  40 . The unillustrated bearing mechanism is connected to one end of the ball screw shaft  28 . The bearing-holding member  48  is connected to the housing  42  by screws. A pair of dampers  49   a ,  49   b  are disposed on the bearing-holding member  48 . The pair of dampers  49   a ,  49   b  are substantially horizontally spaced from each other by a predetermined distance and protrude toward the slider  18  on the bearing-holding member  48 . 
   An end plate  50  is installed by screws  52  to the other axial end of the guide-equipped frame  12 . The end plate  50  rotatably supports one end of the ball screw shaft  28 . 
   As shown in  FIG. 3 , the guide mechanism  20  includes a pair of opposed first ball-rolling grooves  60   a ,  60   b , a pair of second ball-rolling grooves  62   a ,  62   b , and a pair of ball-rolling holes  64   a ,  64   b.    
   The pair of opposed first ball-rolling grooves  60   a ,  60   b  extend in the axial direction of the guide-equipped frame  12  on the inner walls of both sides  12   b ,  12   c  of the guide-equipped frame  12 . Each of the pair of opposed first ball-rolling grooves  60   a ,  60   b  has a vertical cross section having a circular arc shape. The pair of second ball-rolling grooves  62   a ,  62   b  are formed on side surfaces of the slider  18  facing the inner walls of the guide-equipped frame  12 . Each of the pair of second ball-rolling grooves  62   a ,  62   b  has a vertical cross section having a circular arc shape. The pair of ball-rolling holes  64   a ,  64   b  are disposed near the second ball-rolling grooves  62   a ,  62   b  and penetrate axially through the slider  18 . 
   Long grooves  116   a ,  116   b  (see  FIG. 4C ) are formed on the inner walls of both sides  12   b ,  12   c  of the guide-equipped frame  12 . The long grooves  116   a ,  116   b  extend axially. A pair of guide rails  114   a ,  114   b  having the first ball-rolling grooves  60   a ,  60   b  are secured to the long grooves  116   a ,  116   b  (see  FIG. 4G ). 
   The guide mechanism  20  includes plates  68  and covers  70 , and return guides  72 . The plates  68  and the covers  70  are integrally connected to lower portions of the slider  18  by screws  66 . The plates  68  and the covers  70  are substantially parallel to the flanges  36   a ,  36   b  of the ball screw nut  30 . The return guides  72  are installed respectively on side surfaces of the slider  18 . The plate  68 , the cover  70 , and the return guides  72  are preferably formed of a resin material. 
   The plate  68  and the cover  70  are installed on the lower side surface of the slider  18 . In other words, the plate  68  and the cover  70  are not installed on the upper side surface of the slider  18 . Therefore, the upper side surface thereof can be used as an abutment surface for enabling each of the dampers  49   a ,  49   b  to abut against each other. 
   Components of the plate  68 , the cover  70  and the return guides  72  are the same on one and the other axial side surfaces of the slider  18 . 
   Ball return grooves  74  are formed on the cover  70 . Endless circulating tracks are constituted by mutually opposed first and second ball-rolling grooves  60   a ,  60   b ,  62   a ,  62   b , the penetrating ball-rolling holes  64   a ,  64   b  formed through the slider  18 , and the ball return grooves  74 . The endless circulating tracks enable a plurality of balls  76  to roll therein. 
   As shown in  FIGS. 2 and 3 , an opening  78  having a U-shaped cross section is formed at an upper center portion of the slider  18 . The opening  78  extends axially. The opening  78  is of a large recessed shape, which is open upwardly. The cylindrical section  32  of the ball screw nut  30  is installed detachably upwardly. 
   As shown in  FIGS. 2 and 3 , a hole  80  is formed through the slider  18 . The hole  80  penetrates from the opening  78  downwardly through the slider  18 . The hole  80  has a rectangular cross section. Return tubes (not shown) are accommodated in the hole  80 . The return tubes are installed in the ball screw nut  30  and serve as passages for enabling the plurality of balls  76  to roll therein. Therefore, the hole  80 , which accommodates the return tubes therein, reduces the height of the slider  18 . 
   The actuator  10  according to the embodiment of the present invention is basically constructed as described above. Operations, functions, and effects of the actuator  10  shall be explained below. 
   First, the steps for producing the guide-equipped frame  12  of the actuator  10  shall be explained. 
   A flat plate-shaped formable steel member  110 , which is composed of a general low cost steel material, for example, any of materials such as SS400 and S45C in accordance with the Japanese Industrial Standard (JIS), is pressed to form the frame  112  (see  FIGS. 4A and 4B ) comprising the bottom  12   a  and both sides  12   b ,  12   c  which are integrally formed. The pressed frame  112  is straightened. Thereafter, cutting machining is roughly performed. Cutting machining is further performed to form the long grooves  116   a ,  116   b  which are substantially in parallel to the axis of the frame  112  (see  FIG. 4C ). The guide rails  114   a ,  114   b  are inserted into the long grooves  116   a ,  116   b , as described later on. 
   The prism-shaped guide rails  114   a ,  114   b  are hardened by a step, which is different from the step performed for the formable steel member  110 . Each of the prism-shaped guide rails  114   a ,  114   b  is formed of a specialized steel material that is capable of being hardened. Specialized steels that can be subjected to a hardening treatment are exemplified, for example, by materials such as SKH9, SCM420 and SUJ2, in accordance with the Japanese Industrial Standard (JIS). Next, the outer surfaces of the guide rails  114   a ,  114   b  are ground (see  FIGS. 4D and 4E ). 
   The guide rails  114   a ,  114   b  are inserted into and coupled integrally to the long grooves  116   a ,  116   b  of the frame  112  (see  FIG. 4F ). The guide rails  114   a ,  114   b  are polished to form the ball-rolling grooves (guide grooves)  60   a ,  60   b . Thus, the guide-equipped frame  12  is completed (see  FIG. 4G ). 
   Adhesion, forcible insertion fitting, welding fusion, and so on, may be used to connect the guide rails  114   a ,  114   b  into the long grooves  116   a ,  116   b  of the frame  112 . 
   In the method for producing the guide-equipped frame  12 , the main frame body is not hardened. Only the guide rails  114   a ,  114   b , having the ball-rolling grooves  60   a ,  60   b , and which are formed of the specialized steel material, are subjected to hardening. The frame  112  tends to be thermally deformed by hardening. However, since the frame body is not subjected to hardening, it is not necessary to straighten the frame  112  and to polish the outer surface of the frame  112 . Therefore, the production steps can be simplified, thereby reducing production costs. 
   According to the conventional method, the main frame body is hardened by means of a heat treatment, which results in thermal deformation. Therefore, it is necessary to perform straightening and polishing of the outer surface of the frame after hardening has been performed. By contrast, according to the production method of the present invention, only cutting machining is performed on the pressed main frame body. Therefore, it is possible to greatly reduce costs and to improve production efficiency. 
   Further, according to the conventional method, an extremely long period of time is typically required to polish the outer surface of the main frame body. According to the present invention, however, cutting machining may be performed by using a milling cutter or the like. Therefore, the time required for machining can be greatly reduced. 
   The surface and/or the interior of the main frame body has conventionally been subjected to a hardening treatment by heating. However, in this case, if the outer surface of the main frame body is further machined to form the attachment hole and the attachment groove, it is necessary to use a cemented carbide bit or the like, so as to be capable of cutting the hardened material. This increases production costs, since a cemented carbide bit or the like must be purchased. By contrast, in the production method of the present invention, the frame  112  is not subjected to hardening by heating. Therefore, any additional machining can conveniently be performed, for an unillustrated attachment hole or the like, using ordinary cutting machining methods. 
   A specialized metal material, which can be hardened, has conventionally been used for the frame. Therefore, purchase costs for the specialized metal material are expensive. By contrast, in the present invention, the frame  112  does not require an expensive metal material, which can be hardened. Therefore, purchase costs for the material of the frame  112  are low, making it possible to decrease material costs for producing the actuator. 
   In the production method of the present invention, the guide rails  114   a ,  114   b  alone are heated, without heating the frame  112 , for the following reasons. 
   In the guide-equipped frame  12 , comprising the bottom  12   a  and both sides  12   b ,  12   c  which are integrally constructed, it is sufficient only to harden the portions having the first ball-rolling grooves  60   a ,  60   b  for enabling the plurality of balls  76  to roll therein, e.g., such that only the guide rails  114   a ,  114   b  are hardened by heating, for increasing the surface hardness of such portions. 
   For example, it is assumed that a load (P) is applied substantially vertically downwardly with respect to the guide-equipped frame  12 , with one end of the guide-equipped frame  12  being fixed, as shown in  FIG. 5 . The load (P) generates flexion (δ) of the guide-equipped frame  12 . The amount of flexion (δ) is identical whether the guide-equipped frame  12  comprises a heated and hardened frame, or a non-heated and non-hardened frame. 
   Specifically, the amount of flexion (δ) is calculated by the following expression (1), in which the Young&#39;s modulus (E) is constant. The amount of flexion (δ) generated by the load (P) is identical for the heated frame and the non-heated frame.
 
δ= Pl   3 /3 EI    (1)
 
   wherein P represents the load, l represents the length, E represents the Young&#39;s modulus, I represents the second moment of area, and δ represents the amount of flexion. 
   The hardened frame extends the elastic limit, and is tough, as shown in  FIG. 6 . However, the amount of flexion (δ) generated by the same load (P) is identical with respect to the hardened frame and the non-hardened frame. The slope θ is the same as the Young&#39;s modulus (E). 
   In the actuator  10  of the present invention, therefore, the rigidity of the guide-equipped frame  12 , which is not heated, can be the same as that of the heated frame. 
   As discussed above, in the present invention, a flat plate-shaped formable steel member  110 , which is pressed to form the frame  112  (see  FIGS. 4A and 4B ), is composed of a general low cost steel material, whereas each of the prism-shaped guide rails  114   a ,  114   b  is formed of a specialized steel material, which is capable of being hardened. The general low cost steel material and the specialized steel material are selected taking into account the following considerations. 
   The Young&#39;s modulus of both the general low cost steel material and the specialized steel material should be substantially the same, and well in excess of that of the light aluminum or light aluminum alloy materials utilized for the base in U.S. Pat. No. 5,775,515. For illustrative purposes, the approximate values and/or ranges of Young&#39;s moduli [GPa] for some typical elemental metals, alloys, and steel materials, are listed below. 
   
     
       
             
             
             
           
         
             
                 
                 
             
             
                 
               Metal 
               Young&#39;s Modulus (GPa) 
             
             
                 
                 
             
           
           
             
                 
               Tungsten (W) 
               406 
             
             
                 
               Chromium (Cr) 
               289 
             
             
                 
               Nickel (Ni) 
               214 
             
             
                 
               Iron (Fe) 
               196 
             
             
                 
               Low Alloy Steels 
               200-207 
             
             
                 
               Stainless Steels 
               190-200 
             
             
                 
               Cast Irons 
               170-190 
             
             
                 
               Copper (Cu) 
               124 
             
             
                 
               Titanium (Ti) 
               116 
             
             
                 
               Brasses and Bronzes 
               103-124 
             
             
                 
               Aluminum (Al) 
               63-75 
             
             
                 
                 
             
           
        
       
     
   
   As is immediately apparent from the above listing, aluminum, as well as other light metal aluminum alloys, exhibit a relatively low Young&#39;s modulus, roughly only one third that of elemental iron (Fe) and ferrous based steels. Importantly, concerning the steel materials, the Young&#39;s modulus thereof remains constant and does not change, whether such steels are subject to a hardening treatment or not. 
   In particular, it is preferable for the material making up the frame member  112  and the material making up the guide rails  114   a ,  114   b  both to possess a Young&#39;s modulus which is at or above 170 GPa, and more preferably, at or above 190 GPa. 
   More specifically, in the aforementioned embodiment, the general low cost steel material may be either one of SS400 and S45C, in accordance with the JIS standard, wherein the steel material is pressed to form the frame  112  (see  FIGS. 4A and 4B ). In particular, S45C, which is a carbon steel commonly used in fabricating machine structures, has been determined to exhibit a Young&#39;s modulus of 199.9 GPa, whereas SS400 is a rolled steel used in fabricating general structures, and possesses a Young&#39;s modulus of 206 GPa, wherein both of these steel materials, generally, exhibit a Young&#39;s modulus in excess of 170 GPa. 
   The specialized steel making up the guide rails  114   a ,  114   b  and which is subjected to the hardening treatment may be any of SKH9, SCM420 and SUJ2, in accordance with the JIS standard, wherein SKH9 is a high-speed tool steel, SCM420 is a chrome molybdenum steel, and SUJ2 is a high-carbon chromium bearing steel. According to the document, “Standard Mechanical Design Chart Handbook” published by Kyoritsu Shuppan Co., Ltd., the Young&#39;s modulus for such specialized steels generally ranges from 199.9 GPa to 210 GPa. In particular, the Young&#39;s modulus of SUJ2 is 208 GPa. 
   Accordingly, the rigidity of the frame  112  is significantly greater than would be possible using a light aluminum alloy. As a result, the frame  112  can be formed from a flat and solid flat plate-shaped formable steel member  110 , which is simply pressed to form the frame  112 . A large sized frame is not required, and there is no need for internal structural features, such as hollowed cavities or internal ribs, to impart added strength to the frame. 
   Moreover, since the frame member  112  and the guide rails  114 ,  114   b  are both made principally of iron (Fe) based or ferrous materials, the coefficient of linear thermal expansion of the material making up the frame member  112  and the coefficient of linear thermal expansion of the material making up the guide rails  114   a ,  114   b  are substantially the same. Preferably, the difference ΔC e  between the coefficients of linear thermal expansion of the material of the frame member  112  and the material of the guide rails  114   a ,  114   b  respectively (ΔC e =|coefficient of linear thermal expansion of general steel material−coefficient of linear thermal expansion of specialized steel material|) should not exceed 5×10 −6 /° C., more preferably, should not exceed 3×10 −6 /° C., and even more preferably, should not exceed 2×10 −6 /° C. Namely, as one example, the general low cost steel material S45C has a coefficient of linear thermal expansion of 12.1×10 −6 /° C., whereas a typical known specialized steel material, such as the hardenable steel material indicated for the guide rails in U.S. Pat. No. 5,755,515, has a coefficient of linear thermal expansion of 10.7×10 −6 /° C. Thus, in this example, ΔC e =1.4×10 −6 /° C. As another example, the general low cost steel material SS400 has a coefficient of linear thermal expansion of 12.6×10 −6 /° C., whereas the specialized steel material SUJ2 has a coefficient of linear thermal expansion of 12.5×10 −6 /° C. Thus, in this example, ΔC e =0.1×10 −6 /° C. 
   Since the coefficients of linear thermal expansion of the frame member  112  and of the guide rails  114   a ,  114   b  are substantially the same, when the actuator is subjected to changes in temperature, the frame member  112  and the guide rails  114   a ,  114   b  will expand or contract at the same rate and magnitude, and hence, the secure positioning of the guide rails  114   a ,  114   b  within the long grooves  116   a ,  116   b  is not adversely affected, and gaps or clearances between the grooves  116   a ,  116   b  and the guide rails  114   a ,  114   b  do not develop. 
   First to fourth modified embodiments of the guide-equipped frame  12  produced by the above production method are shown in  FIGS. 7 to 10 . 
   As shown in  FIG. 7 , a guide-equipped frame  120  according to the first modified embodiment has a pair of guide rails  114   a ,  114   b  facing one another on the inner wall surfaces of the sides  12   b ,  12   c  of the guide-equipped frame  120 . Preferably, the upper surfaces of the sides  12   b ,  12   c  are partially cut out, and inclined surfaces  122   a ,  122   b  inclined by a predetermined angle extend axially. 
   As shown in  FIG. 8 , in a guide-equipped frame  124  according to the second modified embodiment, connecting portions between the bottom  12   a  and both sides  12   b ,  12   c  are thicker than the central portion of the bottom  12   a.    
   As shown in  FIG. 9 , in a guide-equipped frame  126  according to the third modified embodiment, two strips of guide rails  114   a ,  114   b  are disposed along the inner wall surface on both sides  12   b  and  12   c . The two strips forming the guide rails  114   a ,  114   b  face the other two strips that form the guide rails  114   a ,  114   b.    
   As shown in  FIG. 10 , a guide-equipped frame  128  according to the fourth modified embodiment has a pair of mutually opposed guide rails  114   a ,  114   b  formed on the inner wall surfaces of both sides  12   b ,  12   c , and a pair of guide rails  114   a ,  114   b  substantially in parallel to one another on the bottom surface  130  of the recess of the guide-equipped frame  12 . 
   A method for assembling the actuator  10  shall be explained. 
   The pairs of plates  68  and covers  70  are installed to both of the end surfaces of the slider  18  by the screws  66 . The slider  18  is assembled into the recess of the guide-equipped frame  12  (see  FIG. 3 ). The plates  68 , the covers  70  and the return guides  72 , which are composed of the same components, are installed on both axial ends of the slider  18 . Therefore, the plates  68 , the cover  70 , and so on, can be installed from any direction to each of the ends of the slider  18  in the actuator  10 . 
   In other words, it is possible to conveniently assemble the same components to one and the other axial ends of the slider  18  without considering the installing direction. Further, the components of the guide mechanism  20  can be standardized to make it possible to reduce the number of components and to decrease production costs. 
   As shown in  FIG. 2 , next, the cylindrical section  32  of the ball screw nut  30  is inserted along the opening  78  upwardly from the slider  18 . The flanges  36   a ,  36   b  are fastened to the side surface of the slider  18  by the screws  34 . The feed screw shaft mechanism  16 , to which the ball screw shaft  28 , the ball screw nut  30 , the end plate  50 , and the housing  42  are integrally assembled, is installed to the guide-equipped frame  12 . 
   The slider  18  does not form an obstacle, because the opening  78  having a U-shaped cross section is formed at an upper surface of the slider  18 . The feed screw shaft mechanism  16 , to which the ball screw shaft  28 , the ball screw nut  30 , the end plate  50 , and the housing  42  are integrally assembled, can conveniently be installed in the guide-equipped frame  12 , upwardly from the slider  18 . Inversely, the feed screw shaft mechanism  16  can be conveniently disengaged from the guide-equipped frame  12  through the opening  78  of the slider  18 . 
   Operations of the actuator  10  shall now be explained. 
   An unillustrated power source is energized and transmits a rotary driving force from the rotary driving source  14  to the ball screw shaft  28 . The rotated ball screw shaft  28  is screwed through the screw hole of the ball screw nut  30 . The slider  18  connected to the ball screw nut  30  is integrally displaced in the axial direction of the guide-equipped frame  12  by the guide of the guide mechanism  20 . When the polarity of the current flowing through the rotary driving source  14  is inverted by an unillustrated controller, the slider  18  can reciprocate in the axial direction of the guide-equipped frame  12 . 
   While the slider  18  reciprocates in the axial direction of the guide-equipped frame  12 , the plurality of balls  76  roll along the first ball-rolling grooves  60   a ,  60   b  and the second ball-rolling grooves  62   a ,  62   b.    
   While the invention has been particularly shown and described with reference to preferred embodiments, it will be understood that variations and modifications can be effected thereto by those skilled in the art without departing from the spirit and scope of the invention as defined by the appended claims.