Workpiece moved, lifted or was thrown

Most likely causes

1. Upcut bit lifting a taped-down partMost common
   An upcut spiral pulls the work upward by design. On tape that is exactly the failure mode. Use a downcut, or hold it mechanically. See upcut versus downcut.
2. Tape not burnished downMost common
   The tape and CA method fails almost entirely because of this step being skipped. See workholding.
3. Tabs too small or too fewCommon
   They break on the final pass, which is exactly when they are all that is holding the part. See tabs.
4. Cut too heavy for the holding methodCommon
   Cutting force scales with depth and engagement. Lighter passes are also gentler on workholding.
5. Clamps only at the edges of a large sheetCommon
   The middle lifts and chatters. Hold sheets across their whole area.
6. Dusty or contaminated surfacesCommon
   Tape and vacuum both need clean flat surfaces to work.
7. Small parts with little areaOccasional
   Vacuum and tape both scale with area, so very small parts hold poorly. Use tabs as well.
8. Porous material on a vacuum tableOccasional
   Air passes straight through. Needs a gasket or a different method.

Diagnostic steps

1. Jog the tool back to work zero before touching anything. If the tool sits on the original corner, the machine is fine and only the part moved.
2. Find the moment in the cut where the path first goes wrong. Near the end of a profile is tabs. Early in a deep pocket is lift. At a corner is a push.
3. Inspect the holding. Tape that never bonded to the board, tabs that were thinner than the final pass, clamps only at the edges.
4. Check the bit type. An upcut spiral on a taped-down part is the cause often enough to check first.

Fixes

1. Match the holding to the forces. For upcut bits and deep cuts, use mechanical holding rather than adhesive.
2. Burnish tape properly on both surfaces before applying CA glue.
3. Use more and thicker tabs on small or heavy parts.
4. Consider onion skinning instead of tabs for small parts, so nothing can move at all.
5. Reduce depth per pass, which reduces cutting force directly.
6. Hold sheets across the middle, not just at the corners.
7. Clean the surfaces before every setup.

Why it happens

A cutting tool pushes on the work in two directions at once: sideways, in the direction it is feeding, and along its axis, up for an upcut spiral and down for a downcut. Workholding has to resist both, and the two common hobby methods are each weak against one of them. Tape and glue resist sideways push well and lift poorly. Clamps at the edges resist lift at the edges and nothing in the middle. Tabs resist everything until the last pass cuts them thinner than the force they are holding. The failure is nearly always the method being asked to resist the one force it is bad at.

After a failure

• Stop the machine before investigating. Not while it is still running.
• Check the bit for damage before using it again.
• Check whether the machine lost position, and re-home if it has homing.
• Work out which force caused it, lift or push, and choose a holding method that resists that specific force.

Prevention

• Choose the holding method from the bit and the cut, not from habit: downcut or mechanical holding for anything taped down.
• Burnish tape onto both surfaces every time, and give CA glue its full cure before cutting.
• Size tabs to survive the final pass with margin, and add more on small parts than feels necessary.
• Clean the spoilboard between jobs. Dust is a release agent.
• Watch the first pass of every profile and the last pass of every cut-out.