second parallel side walls 54 and 56 respectively a laterally extending cam roller shaft 58 fig2 is supported between the first and second side walls 54 and 56 and a cam roller 60 is axially supported for rotation about the cam roller shaft 58 fig1 through 4 show elongate first and second keeper plates 62 and 64 respectively mounted on outer surfaces of the respective first and second side walls 54 and 56 of the slide 14 with fasteners such as screws 66 as illustrated best by fig3 and 4 the first keeper plate 62 extends beyond the cam roller 60 to engage the first positive return member 38 as shown by fig2 and 3 in the partially blocked view of the first keeper plate 62 the inner side of the first keeper plate 62 has a relieved portion that leaves a linear surface 65 the linear surface 65 is substantially parallel to the first return flange 44 of the first positive return member 38 to engage the first return flange 44 when the driver 16 moves upwardly although this feature is not shown the second keeper plate 64 is configured in reverse of the first keeper plate 62 to engage the return flange 45 of the second positive return member 40 as shown by fig2 and 5 the lower portion 50 of the slide 14 has first and second parallel sides 68 and 70 respectively that depend vertically from the central portion 48 the lateral thicknesses of the upper portion 46 and of the lower portion 50 of the slide 14 are less than the width of the.