fixing grooves 140 the fixing balls 139 function to accurately set the fixing pins 112 in the fixing grooves 140 and to prevent the fixing pins 112 from sliding out of the second fixing grooves 140 even when the vacuum produced in the fixing grooves 140 is relieved referring next to fig4 e and 4f at least two fastening units 144 may be provided on a sidewall of the grinding mount 130 each of the fastening units 144 includes a head portion 144 a a second guide 144 b defining a passage and a fixing member 144 c integral with the head portion 144 a such that the fixing member 144 c can be moved along the second guide 144 b when downward pressure is exerted on the head portion 144 a the second guide 144 b and the fixing member 144 c are provided with snap projections to keep the fixing member 144 c in place the grinding plate 110 has third fixing grooves 116 in a sidewall thereof at positions corresponding to the passages defined by the second guides 144 b the third fixing grooves 116 receive the fixing members 144 c respectively in a press or snap fit manner when the head.