In 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.

The first tool for better vacuum hold-down is the vacuum gauge installed on most CNC machines with a vacuum hold-down system. The gauge is sometimes analog, like the one pictured, and sometimes it's digital. The units of measure are inches of water or mm of water, Pascal or 1 bar (defined below) and has a maximum value at sea level of (29.92 inHG=1 Bar). 

The higher the elevation from sea level the less the vacuum. To create a standard, we must first create the optimum scenario. The perfect vacuum seal starts with the preparation of the vacuum system consisting of the CNC matrix table, the correct sealing CNC gasket and the MDF spoilboard.

Once completed, an operator can evaluate his/her optimum machine pump performance, using the vacuum gauge to create a standard.

1. Confirm your pump is operating at its maximum performance, i.e., when the pump is new or with new vanes, or had a recent oil change, for its maximum pressure and CFM. 
2. Measure/record the vacuum gauge in inHG when the vacuum valve is closed to the table to create your "pump ceiling standard."
3. Prepare your spoilboard with the new gasket, sealing the edges, and install a nonporous board on top of the table. Remeasure your gauge in inHG to see the loss from your "pump ceiling standard" and record this value. This is your "production ceiling standard." 
4. Continue with your production and continue to record the values in a chart to predict future changes. 
5. Test against other materials and create alternative charts. 
6. In wood nested manufacturing, typically it's best to replace the spoilboard once it reaches less than 3/8 inch or 10mm as the CFM-to-pressure ratio becomes unbalanced. 

What does the gauge state/read when you perform/create this standard? Typically, the range of vacuum achieved for a "production ceiling" measurement will be at its optimum between the range of 24inHG~27inHG or alternatively (609mmHG~685mmHG) / (0.914Bar- 0.812Bar). This beginning measurement is the ceiling of maximum performance and while the machine works, cutting of the spoilboard allows pressure loss, and this ceiling value will decrease. 

It is the operator’s responsibility to evaluate/observe this loss using the pressure gauge as the reference. The operator can determine if the "production ceiling" is moving closer to your "production floor," where parts have the potential to move.
Typical loss experienced while operating, in our experience, is around 4~6" inHG/ alternatively (101mmHG~152mmHG(0.135Bar~0.203Bar). 

While operating, the razor slicing/cutting process can create grooves in the spoilboard table creating an uneven surface that impacts the perfect seal for your "production ceiling" standard. The process of flycutting or resurfacing the spoilboard is important in order to go back to the original "production ceiling." The operator will need to determine whether the potential operating loss of 4-6 inHG while running will impact the parts machined.

An important fact to remember is in most operations, if your vacuum value is lower than 17" inHG (43mm), your parts will begin to move. At 16" or lower, CNC manufacturing will typically E-stop the machine to due safety concerns. If you continue to have parts that lose more vacuum/CFM than the pump can supply, the only option is to increase CFM to make up for the loss. In certain applications, a blower is required rather than vacuum.

Spoilboard preparation tips
• Spoilboard material: 1/2 inch, 45 pound MDF. 
• Seal the spoilboard edge to prevent vacuum loss. Option 1 is to seal with a paint primer. Option 2 is to seal with a mix of 60% PVA glue, 40% water.
• Gasketing between the spoilboard and table. Types of gaskets include round and square (adhesive).
• Machine the spoilboard to increase flow characteristics. Machine both sides approximately 0.020-0.040 inch (0.5-1mm). Feed speed: Run machine to create a 0.028”~0.032” chip load. The result should create a very light fuzzed surface and the MDF fibers raised which is desirable for part holding. The spoilboard can be machined once per day to once per month, depending on specific applications; once per week seems to be the norm.

Source: CNC-Tool.com. For more information call 636-447-3439 or visit CNC-Tool.com. 
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