Engraving small font - the bit digs deeper and deeper as the job goes on. 6040z

[lucan07]

I cannot see this being the problem more likely the simple options, ER11 thread damaged and not locking collet properly, material insufficiently secured if clamped at edges acrylic will lift in centre with any sort of up cut helix, wrong collett size used, whatever is causing a 3.75mm shift is gong to be measurable so measure collet protrution, relative position of ballscrew, deflection in frame or material plane with dial indicator etc and identify movement.
Before trying to remedy a problem identify it and save a lot of unecessary speculation.