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Alain Albert: Ensuring vacuum hold-down strength on CNC Machines

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During my visits with manufacturers who work with nested based CNC machines, I often get asked about hold-down vacuum performance. Surprisingly, apart from the specs of the vacuum pumps, information is pretty scarce.

Manufacturers have told me when they have problems holding down parts on their table they usually refer to the machine distributor who sold them the system. Some are told their pump is not strong enough and that they must purchase a bigger pump. When the problem persists they are told the vacuum is not the problem and they must be doing something wrong.

In order to hold down a piece of wood to a table using vacuum, more than just the performance of the vacuum pump has to be taken into consideration. The air that will be drawn by the pump must first travel through a spoilboard, then through a plenum in the table, then through different valves and plumbing fittings and finally through a pipe or tube to the pump itself.

Let’s take a look at all these elements separately.

The spoilboard is usually made of MDF. MDF is permeable and as such it lets air pass through its entire surface. However, different brands of MDF and even different batches of the same brand will let air pass through differently. Since MDF is made of sawdust and resin, using different proportions or sawdust from different wood species will affect the permeability of the material.

It is important to select a type of spoilboard material that has been tried and tested by other manufacturers, or to experiment with different suppliers. In the process of making MDF, the top and bottom layers are polished by the press’ hotplate and become denser than the middle layer. It is important to fly cut both these surfaces down at least 50-thousandths of an inch to get down to a more permeable layer.

A proper spoilboard will let air through with little pressure loss. However, good airflow will not necessarily translate into good hold down power. In fact, you are looking for a balance between airflow and pressure in order to give the best hold down force.

This can be achieved by using a second sheet of MDF on top of the first one, which we will call the sacrificial sheet. This panel can be machined from a 3/8 inch or 1/2 inch sheet and will bear the brunt of the cutting. The sacrificial sheet will serve two purposes: it will restrict the airflow just enough to concentrate the vacuum onto the pieces of wood that we want to hold down and it will make it less expensive to change after multiple cuttings.

Another very important aspect of the spoilboard is that it comes into contact with the table plenum without incurring leakage on the edges. Usually the table has a gasket along the edge where the spoilboard will lay. But that may not be enough and it can be useful to add to this gasket by taping the edges of the MDF directly to the table base. Air will pass through the path of least resistance and if the material we want to cut covers the entire surface of the table, then air will try to pass through the edge of the spoilboard or through any leaks it can find. That is why we seal the edges of the spoilboard with a good lacquer sealer or edgeband tape (shellac also works very well).

The plenum of the table is also important as it distributes the air evenly from the plumbing to the surface of the table. If you measure the diameter of the intake pipe at the vacuum pump and determine its surface, it is very important to understand that this surface cannot be smaller anywhere down the line as a narrower diameter will restrict the air passage.

Another source of restriction for air can occur in the plumbing underneath the table. Sometimes valves are too small and occasionally sharp-cornered fittings will offer too much airflow resistance.

The length of the pipes also impacts airflow. The longer a pipe and the more it turns, the more resistance it offers. Short and straight pipe will give the best performance.

After studying a number of vacuum systems on nested CNC routers, FPInnovations developed a series of tests that measures the performance of a vacuum system at the table, where it counts. By doing a vacuum audit on your nested CNC machine, FPInnovations can now guide you in improving the vacuum performance.

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