opening discs 32 thereby rest at a zero penetration setting and depth control assembly 60 can be adjusted for proper registration of handle 108 at the zero penetration setting control assembly 82 is adjusted such that handle 108 is engaged with the appropriate notches 104 106 for a zero penetration setting bolt 136 is then adjusted relative to threaded block 146 to advance further into or to withdraw further from threaded block 146 while at all times remaining threadedly engaged there with in this way the overall length of depth control linkage arm 84 is adjusted so that when wobble bracket 86 is re attached thereto it will properly engage control segments 70 of gauge wheel arms 64 and without unacceptable looseness it should be understood that the appropriate length for control linkage arm 84 can be determined in numerous ways including measurement between the end thereof and a fixed point such as for example depth setting register 100 or another fixed point of reference a convenient way for determining the proper overall length of linkage arm 84 is by the use of a gauge 180 as illustrated in fig9 gauge 180 includes wings 182 184 to be received in wobble bracket slots 186 of control segments 70 bolt 136 is rotated so as to advance further into or withdraw further from threaded block 146 and thereby change the overall length of depth control linkage arm 84 until the outer end of control linkage.