CNC workholding: clamps, tape, vacuum and what fails
Short answer
Screws into the waste area are the most reliable method and the easiest to get wrong by hitting one. Tape and CA glue is excellent for flat parts. Clamps are fast but stand proud, so they are the main collision risk. Vacuum is the production answer and struggles with small parts.
What workholding has to resist
It is not just weight. The cutter applies real sideways force, and depending on the bit it also lifts or presses:
- Sideways push from the cut, which is what slides a part out of position.
- Lift from an upcut bit, which is what peels a taped part off the spoilboard.
- Rotation. A part held at one point spins around that point.
- Vibration, which loosens anything that was only just tight.
The methods
| Method | Holds | Strengths | Weaknesses |
|---|---|---|---|
| Screws into the spoilboard | Very strongly | Cheap, fast, reliable, holds sheets flat across the middle | You must know exactly where every screw is. Hitting one destroys a bit instantly. |
| Clamps (low profile, toe, cam) | Well at the edges | Fast to set and release, no marks in the work | Stand proud: the main collision risk. Cannot hold the middle of a sheet. |
| Double sided CNC tape | Surprisingly strongly over area | Nothing stands proud. Holds thin parts flat. | Needs clean flat surfaces. Can let go under an upcut bit. Removing it can tear soft wood. |
| Painter's tape plus CA glue | Very strongly, and releases cleanly | The hobby favourite. Cheap, strong, no residue on the work. | Needs curing time. Fails if the tape is not well stuck first. |
| Vacuum table | Over large flat areas | No obstructions anywhere, fast loading, production speed | Expensive, useless on porous material without a gasket, weak on small parts |
| Vacuum pods | Individual parts | Flexible placement, strong per pod | Same porosity and small part limits |
| Pin nails or brads | Small parts, thin stock | Almost invisible, very quick | Same hazard as screws, and less holding force |
| Fixtures and jigs | Repeated identical parts | Perfect repeatability, fast changeover | Time to build. Only worth it for repeat work. |
The tape and CA glue method, done properly
It is the most useful technique most hobby users are not using, so here it is in full:
- Stick strips of good quality painter's tape to the spoilboard where the part will sit.
- Stick matching strips to the underside of the workpiece, in the same positions.
- Burnish both sets of tape down hard. This step is the one people skip and it is the one that decides whether it holds.
- Run a bead of thin or medium CA glue along the tape on the spoilboard, and spray activator on the tape on the workpiece.
- Press the workpiece down and hold it for a few seconds.
The bond is between the two tape layers, so it is strong in shear and it releases by peeling the tape rather than by tearing the wood. When you are done, a putty knife under an edge pops it free.
Why it fails when it fails
Almost always because the tape was not burnished down, or because the surfaces were dusty. Both are free to fix. A part that lets go mid-cut is dangerous, so take the thirty seconds.Collisions
The clamp you forgot about
The most common serious crash in hobby CNC is the gantry or the bit hitting a clamp during a rapid move. Every workholding method that stands above the work surface carries this risk. Before every job: check the toolpath preview for rapid moves crossing your clamps, check that your safe Z is above the tallest thing on the table, and run the job in the air once if you are not certain.- Screws: mark their positions on the design, or photograph the setup before you start.
- Clamps: keep them outside the toolpath extents, not just outside the part.
- Safe Z: set it above the tallest obstruction, not just above the stock.
- Dust shoe: it is an obstruction too, and a brush skirt can catch on a clamp.
Choosing for the job
| Job | Usual answer |
|---|---|
| Full sheet of plywood | Screws in waste areas, spread across the whole sheet |
| A single sign blank | Clamps at the edges, or tape and CA glue |
| Thin material | Tape and CA glue, or a vacuum, so nothing flexes |
| Small parts | Tape and CA glue plus tabs |
| A slab being flattened | Shims and wedges so it cannot rock, plus screws or hot glue in waste |
| Repeat production | A fixture with locating pins |
| Round or irregular blanks | A jig that registers them, or hot glue to a sacrificial base |