Z axis drops during a carve

[manfulldesign]

On the carve shown in the pics attached, the machine starts on the interior of the design, carves it for a few hours, then carves a little around the perimeter for a while.  When it returns to the interior and connects up with the previously carved areas, the z-axis is not the same height.  These differences are not shown in the Vcarve previews.  What's going on?

[dadealeus]

Could be a number of things for minor heights like that.

More than likely, it's your material flexing rather than a loss of z coordinates by the machine.

To elaborate, your stock may be bowing upward slightly as material is removed - causing what looks like a loss of calibration in the cutting head, but is actually a slight change in the height of the wood.

I found that this occurred a lot for me and was also affected by the bristles on the dust shoe. For example, as I removed material, the center of the piece would begin to lift slightly off the table. Depending on the cut path, the bristles from the brush would push it back down in some places, but allow it to rise up in others. If it was a straight cut that dragged the bristles, the force of the bristles was not enough to overcome the bow in the stock, so it would rise. However, if the cutter was performing a lot of different moves in a small area, the bristles would be more-or-less straight up and down - overcoming the force of the bow and pushing the stock flat - resulting in inconsistent cut heights.

What's what I would check first.

If that's the case, you can use double-sided tape to hold down the center of the piece (where your clamps can't reach without being in the way of the cutter) - or, a vacuum hold-down system.