vertical position by the swinging of the boom to an outboard and depressed position disengagement of the catch 50 can be accomplished by use of a cable 70 which is pivotally attached to the catch by pin 72 see fig3 when the cradle is rotated to the vertical position it will bottom out against the swing tower at point 82 see fig3 as a result there will be an open space between top member 56 and the back faces of the cradle side pieces 37 stop block 58 see fig1 will when positioned as shown in fig3 lock the cradle in the working position in the unit reduced to practice stop bar 58 was manually inserted to secure the cradle in the working position it is to be understood that a hydraulically or electromagnetically actuated thrust pin or pins securing the sidewalls of the tower to side pieces 37 of the cradle at a location between pins 42 and 44 see fig3 would accomplish the same result fig7 and 8 show the manner in which boom 26 was implemented box beam construction was utilized thus providing strength at reasonable cross sectional size implementation is arranged such that the boom sylinder 28 and the dipper cylinder 32 operate.