referring to fig1 - 2 , an apparatus for cylindrically grinding workpieces 40 according to a first preferred embodiment , includes a first holding tool 100 , a second holding tool 200 , and a grinding wheel 300 . the holding tools 100 , 200 are configured for holding workpieces 40 . the first holding tool 100 includes two first holding members 22 , a first back plate 24 , two first resisting members 26 , and two first fixing members 28 . each first holding member 22 is rectangular in shape , and has two opposite side surfaces 222 , and a working surface 224 between the side surfaces 222 . each first holding member 22 defines a through - hole 225 extending through the side surfaces 222 . the working surface 224 defines a first groove 226 for containing pre - grinding workpieces 40 . the pre - grinding workpieces 40 can be optical elements having a non - cylindrical shape . in this preferred embodiment , the workpieces 40 are substantially square - shaped . therefore , the first groove 226 is configured to have a v - shaped cross section along a traverse direction , for compliantly receiving the pre - grinding workpieces 40 therein . the pre - grinding workpieces 40 are partially ground on the first groove 226 and centers of the workpieces 40 are higher than the working surface 224 , thus the grinding wheel 300 cannot touch the working surface 224 when the grinding wheel 300 grinds the workpieces 40 . understandably , the cross section along a traverse direction of the first groove 226 may be of other shape depending on the shape of the pre - grinding workpieces 40 . the first back plate 24 , which is substantially rectangular shaped , defines two spaced bores 242 . the bores 242 correspond to the through - holes 225 of the first holding members 22 . the first back plate 24 has a top surface 243 paralleling the axes of the bores 242 . two protruding columns 244 are formed on the top surface 243 . a cross - section along a traverse direction of the protruding column 244 is semicircular shape , and a radius of the protruding column is same as that of the finished product , corresponding to the shape of the first groove 226 . each protruding column 244 has an end surface 246 which is used to resist the workpieces 40 . the end surface 246 is flat enough to avoid harming the workpieces 40 . the first resisting members 26 include a base plate 262 , and a column 264 extending from one surface of the base plate 262 . a diameter and a length of the column 264 are similar to those of the through - hole 225 of the first holding member 22 . a screw hole 269 is defined at the end of the column 264 . the screw hole 269 is coaxial to the column 264 . the cross section shape along a traverse direction of protruding column 266 is the same as the protruding column 244 of the first back plate 24 . the extension direction of the protruding column 266 is the same as that of the column 264 . each protruding column 266 has a end surface 268 which is used to resist the workpieces 40 . the fixing member 28 , includes a head portion 282 , a screw portion 284 , and a connecting portion 286 . the connecting portion 286 connects the head portion 282 and the screw portion 284 . a diameter of the head portion 282 is larger than that of the connecting portion 286 , thus the diameter of the connecting portion 286 is larger than that of the screw portion 284 . the configuration of the second holding tool 200 is almost the same as that of the first holding tool 100 . the second holding tool 200 includes two second holding members 32 , a second back plate 34 , two second resisting members 36 , and two second fixing members 38 . the back plate 34 includes two protruding columns 342 having an end surface 344 . each resisting member 36 includes a protruding column 362 having an end surface 364 . compared with the first holding tool 100 , the second holding tool 200 has some differences , which are as follows : each holding member 32 defines a second groove 322 having a semicircular shaped cross section along a traverse direction ; a radius of the second groove 322 is same as the finished product . in assembling the first holding tool 100 , the first holding members 22 are joined together with the first grooves 226 facing upward . the protruding columns 266 face upward , and each column 264 is inserted through the through - holes 225 . the first back plate 24 abuts against the side surfaces 222 of the first holding members 22 . finally , each first fixing member 28 is inserted through the bores 242 and the screw portion 284 is screwed into the screw holes 269 of the columns 264 . the process of assembling the second holding tool 200 is the same as that of the first holding tool 100 . referring to fig3 - 5 , an exemplary process for cylindrically grinding workpieces 40 includes the steps of : ( 1 ) a stack of workpieces 40 ( i . e . pre - grinding workpieces ) are placed in the first groove 226 of the first holding tool 100 . the first fixing members 28 are screwed down and the pre - grinding workpieces 40 are clamped by the protruding columns 244 , 266 . a first portion 42 of the pre - grinding workpieces 40 projects out of the first grooves 226 . ( 2 ) the first portion 42 of the pre - grinding workpieces 40 is partially ground to a semicircular shape . ( 3 ) the first fixing members 28 of the first holding tool 100 are released so that the first back plate 24 and the first resisting members 26 are detached from the first holding tool 100 . the second holding tool 200 is placed on the partially ground workpieces 40 with the first portion 41 in the second groove 322 . the second fixing members 38 are screwed down and the partially ground workpieces 40 are clamped . ( 4 ) the holding tool 200 is reversed and a second portion 44 of the partially ground workpieces 40 projects out of the second grooves 322 . ( 5 ) the second portion 42 of the workpieces 40 is ground to a semicircular shape . thus a plurality of cylindrical workpieces 40 are obtained . it is believed that the cross section along a traverse direction of the first grooves 226 can be of other shape , for example , a square groove can be defined under the original first groove 226 . the first grooves 226 and the second grooves 322 may extend to the end surfaces of the holding members 22 , 32 . it is believed that the number of the holding members 22 , 32 can be changed and the number of the corresponding resisting members 26 , 36 can be changed accordingly ; the first resisting members 26 of the first holding tool 100 can be integrally formed and the second resisting members 36 of the second holding tool 200 can also be integrally formed . the process can easily and compactly transfer workpieces 40 from the first grooves 226 to the second groove 322 using the resisting members 26 , 36 and the back plate 24 , 34 which can promote the working efficiency of the cylindrical process . also the process does not use adhesive , and thus does not require water or other solvents to dissolve the adhesive , so the workpieces 40 can avoid becoming disarrayed . referring to fig6 , an apparatus for cylindrically grinding workpieces according to a second preferred embodiment includes a first holding tool 100 and a second holding tool 200 . the first holding tool 100 includes one holding member 62 , two back plates 64 , two resisting members 66 , and two fixing members 68 . the holding member 62 defines two through - holes 622 and two grooves 624 with a v - shaped cross section along a traverse direction . each back plate 64 defines a bore . the second holding tool is same as the first holding tool 100 except for the cross section shape along a traverse direction of the grooves 624 being semicircular . it is believed that the present embodiments and their advantages will be understood from the foregoing description , and it will be apparent that various changes may be made thereto without departing from the spirit and scope of the invention or sacrificing all of its material advantages , the examples here before described merely being preferred or exemplary embodiments of the invention .