Stop the Slip: CNC Vacuum Workholding Tips and Tricks
In CNC routing, preventing parts from slipping is a constant challenge. The problem arises once cuts penetrate the workpiece, since each cut introduces leaks that reduce vacuum pressure and therefore your ability to hold parts firmly in place. To compensate, the system must provide more CFM to maintain vacuum and thus holding power.
Challenges faced by operators surround keeping vacuum system performance in the “sweet spot” by maintaining steady airflow as new leaks develop during machining. Aim for 7.5 CFM per square foot when using a medium-pressure pump, or 5.5 CFM per square foot with a high-pressure pump. Good news is there are proven strategies that help.
Spoilboard Management
- Density matters: Low-density fiberboard allows more airflow and better vacuum performance but can be harder to source. Medium-density fiberboard (MDF) is more available but restricts flow. Skimming 0.04”–0.08” from both faces with a flycutter removes the dense outer layer, improving porosity and airflow
- Surface maintenance: Regularly mill the spoilboard flat to ensure consistent contact with the workpiece
- Secure attachment: Bond or fasten the spoilboard to the phenolic base using double-stick tape, silicone, or non-metallic fasteners
Toolpathing
The sequence of toolpaths directly affects vacuum loss.
- Start with cuts that do not penetrate through material thickness—such as pocketing and engraving
- Perform any drilling or tapping that does not break through the material
- Execute cutouts with long 3D sloping lead-ins
- Save through-cuts for last, leaving a thin “onion skin” layer to hold the part in place until the final cut, thereby decreasing the risk of vacuum loss early in the job file
- Perform final cut so that the end of the toolpath is adjacent to/sits against the largest amount of sheet material remaining. The larger intact piece acts like a dam, holding back air to keep the vacuum strong. Ending the cut next to the “dam wall” helps keep the part from “washing loose,” AKA, part shift
Zoning and Gasketing
Zoning with gasket tape divides the table into controllable areas, focusing vacuum only where needed. This concentrates airflow (CFM) under the material, minimizes leakage from unused zones, and ensures a stronger hold, especially with small or porous parts.
The Bottom Line on Vacuum Hold
Part slippage in CNC machining isn’t caused by a single issue. It is the result of multiple factors working together, from spoilboard density and gasket layout to toolpath strategy, zoning, and airflow management. By understanding how each of these elements influences vacuum performance, operators can take practical steps to stabilize parts, reduce waste, and achieve more consistent machining results.