CNC Fixture Part/s Hold Down White Paper
One of the challenges while operating a CNC machining center/router is the fixturing of the parts. The methods of fixturing often determine the final result of the part quality and the machines performance relative to the fixturing. Several methods of hold down fixture systems are available dependent on the industry.
The most common method in the WOOD,PLASTIC, COMPOSITE industries is the use of Vacuum/Blower fixture hold down
The most common method in the WOOD,PLASTIC, COMPOSITE industries is the use of Vacuum/Blower fixture hold down
1. Vacuum Fixture Hold Down
The most common method in the WOOD,PLASTIC, COMPOSITE industries is the use of Vacuum/Blower fixture hold down. Vacuum table defined by Wiki:
"A vacuum table is a system for holding workpieces during machining. The device consists of a perforated table top containing a vacuum chamber, and a vacuum pump to keep the vacuum chamber below ambient enough pressure. The workpiece is placed on the top of the vacuum chamber and thus held down by the pressure differential between the vacuum chamber and the outside air." In simple terms, with vacuum hold down applications, suction is created by using vacuum to move air in the opposite direction of the component being held in place. When air is removed beneath the part or component, the air above pushes down. The sucking of the air does not hold down the part. The atmospheric pressure above the part pushing down is what provides the hold down force and keeps the part or component in the correct place. Machines employing vacuum with full table and Rail/Pod(cup) hold down systems are typically with vacuum pumps. OEM's size the amount of vacuum based on the square inches of work surface engaged. Pumps range from 1-1/2 hp to 60hp. Factors to consider to ensure optimum hold down:
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Below is a chart from OEM Becker Pumps highlighting relative vacuum to surface requirements. Generally speaking, when you want to match the right pump to the table size, you need 5.5 CFM per square foot of surface area for spoil board or nested base tables. Due to the small suction cups utilized by Pod and Rail tables, they require far less flow and typically use one size pump. (This sizing chart is for estimating purposes only, and you should consult a trained Becker Representative to match the correct pump to your specific table. All calculations are based on sea level applications.) Link to Becker Vacuum Pumps Guide to Vacuum
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We will focus on Full Sheet Hold Down using full table NESTING CNC machines. Using a spoil board/slave board (Spoil Board best preparation guidelines), placed directly on top of the CNC work surface, vacuum is drawn through the spoil board and holds the sheet of material. HOW DO YOU KNOW YOU ARE GETTING THE BEST HOLD DOWN?
Create a standard for your process, this starts with evaluating the vacuum gauge installed on the machine. The gauge is the heartbeat of the vacuum's performance and provides real time feedback of the vacuum loss during machining operations. The challenge is most operators do not recognize or understanding what the gauge represents and how to read it during daily operations. Below is a guide to assist CNC operators with setting up a standard for their machines. What does the vacuum gauge indicate? We need to refer to the image on the left and the "baby on board" suction cup. In its simplest terms it is the purest form of a PERFECT vacuum. A smooth surface and no leaks! Now imagine a razor blade cutting one edge and allowing the smallest of leaks, and your ability to use your finger to lift the perfect seal away from the surface. The more cuts with the razor blade the greater the likely hood that the suction cup will no longer stay. With spoil board and sheet production, the same methodology is in effect. The gasket seal between the table and the spoil board and the seal between the material to machine requires the attempt for a perfect seal. Any loss in the standard will result in movement of parts. The loss of air(CFM) has to be replaced by the pump and if not sized correctly will require more attention to the vacuum gauge on how this loss is effecting your parts. Most CNC OEM mfg will typically engineer the machine's vacuum system with the correct pump size, table grid matrix, plenum chambers, multi-zones, and other solutions to maximize performance. Some CNC mfgs employ these methods, others do not. The most important thing for an operator is to create the "Standard" for you machine, your sea level, and the parts you plan to process. Help creating your standard is below.
Create a standard for your process, this starts with evaluating the vacuum gauge installed on the machine. The gauge is the heartbeat of the vacuum's performance and provides real time feedback of the vacuum loss during machining operations. The challenge is most operators do not recognize or understanding what the gauge represents and how to read it during daily operations. Below is a guide to assist CNC operators with setting up a standard for their machines. What does the vacuum gauge indicate? We need to refer to the image on the left and the "baby on board" suction cup. In its simplest terms it is the purest form of a PERFECT vacuum. A smooth surface and no leaks! Now imagine a razor blade cutting one edge and allowing the smallest of leaks, and your ability to use your finger to lift the perfect seal away from the surface. The more cuts with the razor blade the greater the likely hood that the suction cup will no longer stay. With spoil board and sheet production, the same methodology is in effect. The gasket seal between the table and the spoil board and the seal between the material to machine requires the attempt for a perfect seal. Any loss in the standard will result in movement of parts. The loss of air(CFM) has to be replaced by the pump and if not sized correctly will require more attention to the vacuum gauge on how this loss is effecting your parts. Most CNC OEM mfg will typically engineer the machine's vacuum system with the correct pump size, table grid matrix, plenum chambers, multi-zones, and other solutions to maximize performance. Some CNC mfgs employ these methods, others do not. The most important thing for an operator is to create the "Standard" for you machine, your sea level, and the parts you plan to process. Help creating your standard is below.
Creating a Vacuum Hold Down Standard
On the left is a vacuum gauge, installed on most CNC machines with a vacuum hold down system. The gauge is sometimes analog, like the picture and sometimes it's digital. The units of measure are inches of water, or mm of water (defined below) and has a maximum value at sea level of (29.92 in. HG and -735 mm HG). The higher the elevation the less the vacuum. To create a standard, we must first create the optimum scenario, just like the baby on board sticker. The perfect vacuum seal starts with PREPARATION of the vacuum system consisting of the CNC matrix table, the sealing Gasket and the MDF Spoil Board. Please click on the Iink for dowloadable document on the instructons to PREPARE your CNC MDF spoil Board-spoilboard_applications_tips_sb-2023.pdf CNC SPOILBOARD APPLICATION TIPS. Once completed, an operator evaluate his/her optimum machine pump performance, using the vacuum gauge to create a standard.
- Confirm your pump is operating at its maximum performance, ie when pump is new, or new vanes, oil change, for maximum pressure and CFM.
- Measure the vacuum gauge when vacuum valve is closed to create standard "ceiling" for optimum pump performance.
- spoilboard_applications_tips_sb-2023.pdfPrepare your spoil board/slave board with new gasket, seal the edges and install nonporous board on top of table.
- Measure the gauge to create your "ceiling" standard.
- Continue your production and record values in a chart to predict future reference for changes.
- Test against other materials and create alternative charts.
- In wood nested mfg, typically its best to replace the spoil board/slave board once its reaches less than 3/8" or 10mm.
Vacuum Jigs - Dedicated Spoil Boards
On occasion a specialty or dedicated CNC Jig will be required to focus the vacuum to the individual parts and the use of various CNC gaskets can be used to create "on" board, or "in" board channels. Please contact us for further help in this use of specialty fixtures.
Mechanical Hold Down
Parts that are too small or have uneven surfaces, may require the use of mechanical clamping. Some of those methods or vises, pneumatic clamps @ 90~120psi, will grip the parts while allowing little to zero movement while under process. Some table systems that employ the use of mechanical clamp systems are considered to be Rail and Pod machines that have options to install these mechanical devices.
Blower Vacuum
For a lot of Porous applications including FOAM and the ratio of pressure to CFM requires further review and the side pressure of the manufacturing process is less while using a hot knife or reciprocation knife while using the CNC.
Some additional hold down methods that are common to the CNC industry include:
- Vacuum fixture hold down-utilizing vacuum pumps and or blowers.
- Pneumatic fixture hold down-piston cylinders with pneumatic pressure
- Hydraulic fixture hold down-piston cylinders with hydraulic pressure
- Magnetic fixture hold down-Magnets in the metal industry
- Manual Clamp Fixture hold down-bar clamp or mechanical vise with acme screw hold down