Next to software and operator training, CNC shops struggle most with properly securing work to the table.
An easy and inexpensive method is to just screw the material to a spoil board. But it’s almost guaranteed that at least one screw will be in the path of the cut, resulting in a damaged tool.
Another option is to use T-track and edge clamps, which doesn’t work so well when machining 4x8 panels. Some edge clamps push into the panel and cause a hump that will rear its head in assembly.
Vacuum clamping is the most efficient and consistent option. It works well for sheet goods and small stock. Components include a pump, distribution ducts or pipes, a permanent plenum table, and spoil board.
Vacuum pumps and blowers are available in many different types and sizes. Irrespective of the type or size, there are two important measures to be aware of: the amount of vacuum (inches of mercury, Hg) and air flow (specific cubic feet per minute). The latter, SCFM, refers to the level of air being drawn by the pump or blower at a specific level of vacuum, usually the optimum level of inches of Hg relative to the type of wood machining that the CNC router will be mostly used for.
Leaks must be mitigated in a closed system that moves air, otherwise the pump will have to work overtime. Use materials and structures that will serve to minimize the chance of leaks or restricted flows. Flexible ducting or piping should be avoided.
After the CNC router has been placed in service, searching for leaks needs to be a part of the preventative maintenance procedure. The loss of vacuum is just as harmful to production as a poorly running spindle or gantry that will not move at its optimum speed. It is a good idea to incorporate an inches of mercury gauge into the system with easy view to alert the operator when there is a leak as the readings will begin to decline.
An under table ducting or piping system allows the vacuum to be evenly spread. You can zone with valves, shutting off the vacuum where it is not needed. Zoning can also be accomplished with strategically placed rubber flooring mats.
The permanent plenum table is often part of the CNC machine’s structure. It may be MDF or plywood, but more often it is some form of plastic or aluminum, machined with plenum channels to evenly distribute the vacuum. The channels are often milled side-to-side and front-to-back, and sized to fit rubber gaskets that can be used to define vacuum zones. Even if the CNC router is going to be used for just 4x8 sheets, a gasket should be placed all the way around to prevent vacuum loss between the plenum table and spoil board.
MDF is a good material choice for the spoil board. The vacuum can penetrate this porous material, holding it tight and flat. But the edges need to be covered with banding, two or three coats of paint or varnish, or duct tape to prevent vacuum loss. Next, fly cut .03” to .05” both sides of the MDF spoil board to remove the less permeable hard polished surfaces created during the manufacturing process.
Many advocate adding a second sheet of MDF (1/2”) to lessen airflow loss and concentrate the vacuum in areas where it is needed. But in my experience, rubber flooring mats are just as effective and easier to handle and store.
The spoil board should be kept clear of all dust and debris for maximum vacuum.
One solution for machining small parts is to cut at a lower inch per minute rate to lessen the lateral forces of the router bit, while maintaining the tool’s RPM. But be careful. If you cut too slow, sawdust can ignite from the heat generated. So make sure the dust removal hood is as low as possible, have air at a high volume blowing on the material to carry the heat away, and keep a water bottle or fire extinguisher handy.
Adding fixtures is a good method for limiting side-to-side movement. Fixturing is more of an art than a science, so a woodworker’s creativity learned from building wood holding devices for cutting small or oddly shaped parts on a table saw or sharper should help.
Another technique is to place rubber flooring mats over areas of the material that have been cut. This can be especially effective when cutting very porous material like foam.
Other techniques for holding small parts include machining tabs between parts to prevent side-to-side movement or leaving a thin piece of material, called an “onion skin”, on the bottom side of the cut. This technique limits loss of vacuum though areas that have be cut away or pocketed. To remove the onion skin, use a smaller router bit to finish the cut. This method takes a bit of thought, so make a plan first and experiment with scrap material before heading into production. A third method is to use a pressure foot that attaches to the spindle.
It is tempting to solve vacuum clamping issues with a larger pump that has more horsepower. But minimizing the loss of vacuum is a better approach than just throwing money at the problem.
This article originally appeared in the June 2018 issue.