its lowest elevation when one of the depressions 162 engages the ground the height of the axis of rotation relative to the ground rises and falls as the control wheel 150 ground engagement point transitions between the maximum at the apex of the lobes 160 and the trough at the bottom of the depressions 162 on the control wheel surface as the cutting assembly 120 is coupled to the control wheel 150 the height of the arbor rises and falls with the height of the axle of the control wheel 150 the arbor 122 also rises and falls in a sinusoidal pattern the rising and falling causes the arbor to grind a sinusoidal pattern such a sonic noise alert pattern is shown in fig1 it can be appreciated that the depth of the pattern can be varied by varying the relative position of the arbor relative to the control wheel 150 however the depth may be set so that the arbor 122 does not grind when the lobes engage the ground it can also be appreciated that the grinding is at its deepest when the depressions of the control wheel 150 engage the ground as shown in fig3 when one of the lobes is engaging the ground the arbor 122 is raised as the grinder 100 advances and the control wheel 150 rolls along.