How to hold plywood down on a CNC router
The short answer: hold plywood down with screws or brad nails in the waste area, with a vacuum table, or with double-sided tape for small parts, and leave small tabs on the parts so they do not move when they are cut free. Whatever you use, keep it out of the toolpath.
Ways to hold a sheet
- Screws in the waste: the simplest method. Put them in the offcut areas between and around the parts, never inside a part and well clear of the toolpath. Check the layout before you screw.
- Brad nails: a pneumatic nailer fixes the sheet to the spoilboard fast and leaves tiny holes. Place the nails in waste areas too.
- Vacuum table: clean and quick when the spoilboard is sealed and the pump is strong enough for your sheet. Small parts can lose suction as they are cut free.
- Double-sided tape: good for small parts and for holding the last pieces. It needs a clean, flat surface and a smooth underside.
- Clamps: fine for the edges, but only if they cannot be reached by the bit or the spindle.
Tabs keep parts from moving
A tab (or bridge) is a short section the bit does not cut through, so the part stays attached to the sheet until the job is done. Without tabs, the last bit of the cut can free the part, and it can shift, jam against the bit and ruin the cut. Typical settings are a few tabs per part, about 6 to 12 mm (1/4" to 1/2") wide and 1.5 to 3 mm (0.06" to 0.12") thick. Cut them off with a flush-trim saw or a chisel and sand the stub flat.
Order of cuts
- Cut holes, slots and inside pockets first, while the sheet is still held on all sides.
- Cut the outside of the parts last, starting with the smaller parts if they are not tabbed.
- Do not stand in front of the machine, and watch the first part of the cut: the first minutes show if something is moving.
Check before you start
- Is every screw, nail and clamp outside every toolpath?
- Is the sheet flat on the spoilboard? A bowed sheet changes the real depth of the cut.
- Are the tabs placed where they are easy to cut off and sand?
Planning a layout so there is enough waste for the fixings is easier when the parts are nested with a few millimetres of margin.
Frequently asked questions
Where should I put the screws?
In the waste areas between and around the parts, well clear of every toolpath. Never inside a part you want to keep.
What are tabs on a CNC?
Tabs, or bridges, are short sections the bit does not cut through, so the part stays attached to the sheet until the job is done. You cut them off afterwards and sand the stub flat.
Do I need a vacuum table?
No. Screws or brad nails in the waste hold plywood well, and a vacuum table is a convenience that works best on larger parts.