PaulRowntree wrote:Sorry Mike, I don't follow. Why does the bit move in X or Y when shifting Z?
It moves in Y when you apply a force in the Z direction. The motor carriage hangs off the side of the cross beam. When you pull up on the motor, the resulting torque on the beam (Plus any deflection in the carriage) causes it to twist in the YZ plane and the the bit tip to move in the Y direction. Plunging into the material causes the same upward force on the carriage.
The amount of movement may be trivial, or it can be quite obvious on machines like the Shark. One's choice of bit can affect the amount of movement, a downcut bit would probably increase the problem.
Just to be sure I knew what I was talking about, I ran a quick test. I cut two holes in hard maple, using a 1/4" diameter upcut spiral bit, .625 deep with the Drilling Toolpath. The results were elongated circles. They measured .251" in the X direction and .259" in the Y direction. Note that my Shark Pro Plus has has Paul's bearing rail modification using the Glacern Machine Tools supported linear ball bearing rails, which makes it considerably stiffer than the stock machine.
Good judgement comes from experience.
Experience comes from bad judgement.