By Sergio Castro

When machining parts, quality fixturing is essential to obtain a superior finish. A fixture is simply a device or system to hold material in place in order for it to be machined. The fixturing must be strong and reliable; otherwise it will lead to a poor finish, create safety problems and reduce tool life. An adequate fixture must resist the side forces created by the cutter while routing, keeping the part in place and stable.

Clamping is one of the strongest methods of hold-down. The clamps can be either mechanical or pneumatic and can apply pressure on top of the part, against the table, or on one side against a guide.

There are several hold-down or fixturing systems available in the market: clamps, vacuum and rollers.

Clamps
This method represents one of the strongest methods of hold-down. Clamps can be either mechanical or pneumatic, and they can apply pressure on top of the part, against the table, or on one side against a guide.

Advantages of the clamping method include its stability. Small parts, where heavy cutting is needed, can be held down using this system.

Disadvantages to this system are that it is time consuming and offers limited access. It may require a complex array of clamps in order to hold the part. Also, the clamps will not give access to all the sides of the part. When this occurs, the part has to be moved and relocated for additional machining.

Vacuum
This is the most common way of holding parts. Under this method, the part is placed on a sealed fixture on top of the router table. Then, using the vacuum pump, the air is removed from inside the seal, forcing the outside air to push the part against the fixture.

	With the roller method, several rollers are located in front and behind the router head. An advantage of this system is that it allows for the flattening of warped material and can hold more than one sheet.

There are several methods in which to use vacuum: a conventional vacuum jig, vacuum pods and the universal spoilboard.

To use a conventional vacuum jig, this method requires a fixture with a rubber gasket insert. The gasket is placed into a machined groove in the fixture rather than directly on top of it. This will allow the gasket to compress only a fraction of its thickness while the part touches the top of the fixture. The machined groove has to be around the closed area that will hold the part down.

An advantage of the vacuum jig method is that it provides a strong vacuum so that any side of the part can be machined, except the bottom face which is typically vacuumed. One disadvantage is that it normally requires a dedicated fixture for every part.

Vacuum pods are a more flexible method than the conventional vacuum jig. This system consists of square or round vacuum pods (or cups) that fit into the router table grooves. In order to machine a different part, the cups have to be moved to support the piece.

An advantage of the vacuum pod system is that it works really well for large panels. This method eliminates the need for dedicated spoilboards. However, a disadvantage is that the size of the part is limited to the size of the cup or pod. Also, the part may not be entirely supported. The hold-down force is limited compared to other systems.

One advantage of the vacuum jig is that it provides a strong vacuum. Any side of the part can be machined, except the bottom face, which is usually the side held down by the vacuum.

A third vacuum method is by utilizing a universal spoilboard. This system requires a high flow vacuum pump which will provide a constant vacuum, even under high air leakage, because the high flow will compensate for the leaks. The CNC table is covered with a low density fiberboard, which allows the air to flow through the board, thereby creating a low-pressure area on top of it. This low-pressure area will hold a flat panel without the need for fixtures.

An advantage of using a spoilboard is that different parts can be machined without changing the fixture or setting up clamps or cups. However, the disadvantages are that the size of the part has to be considerably larger to allow the low-pressure to hold it down. Also, warped material can be difficult to hold down.

Rollers
This method uses several rollers located in front and behind the router head. The router head moves between the rollers, which applies pressure on top of the material against the CNC table.

This system allows for the flattening of warped material, and it can also hold more than one sheet. A disadvantage is that the dimensions of the parts are limited to the distance between the rollers. Also, the rollers can cause scratching on soft materials.

Therefore, the best method for fixturing depends upon the application itself. So while there is not one universal system for holding down material, today’s CNC routers offer flexible tables where you can choose the optimal method to fixture your parts.

Sergio Castro is a CNC sales engineer for SCM Group USA Inc. For more information, he can be reached at (770) 883-8818.