in which they are arranged in the particular example illustrated in fig5 the rectangular shaped wafers 40 are arranged end to end in rows and side by side with adjacent wafers offset with respect to one another so that the join lines 42 of the wafers are offset although this offset may not be used in all applications the rows are arranged parallel so that the adjoining elongated longitudinal edges 44 are parallel to one another as depicted in fig5 alternatively a single row of wafers 40 can be arranged end to end instead of an arrangement of two or more parallel rows the number of rows of wafers is a matter of choice depending on the dimensions of wafer used desired dimensions for the wear surface and other specific design considerations the base plate 34 is provided with a reference or straight edge 46 extending upward from the upper surface 38 which forms an alignment surface for the mulitplicity of wafers as they are placed in position the wafers may have a variety of configurations and as shown they are small and thin in structure thus they can be inexpensively manufactured and quickly efficiently assembled to form the wear surface for the foil 20 it has been found that wafers which are two inches long by 5 8th of an inch wide and 1 8th of an inch thick work effectively for this purpose as the wafers are placed in position they are pushed against previously placed wafers and accordingly are urged against the straight edge 46 and thus are maintained in tight aligned interengagement the vacuum applied through openings 36 and chamber 37 assists in retaining this desired.