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Vacuum Hold-Down in CNC Routers

 

Determining how much vacuum is needed to hold down parts on your CNC router is a very important step when preparing to purchase a vacuum pump.  Several other variables enter the equation, which also need to be considered.CNC router users typically use one or more vacuum pumps to create vacuum to hold down work pieces on the router table. This applies to materials such as wood, plastics, aluminum and other non-magnetic materials. A significant advantage of vacuum chucking is that work pieces can be held on the router table with great force while preventing damage or distortion to the parts and also eliminating the need for fixtures.

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Vacuum chucking can be achieved by using individual rubber pots such as the Carter Pod system, which are the preferred choice for smaller parts, or by installing MDF board on the router table and drawing a vacuum through the board to pull down the parts to be machined on the MDF board. The purpose of this article is to explain the relationship between vacuum level and hold-down force.

For industrial vacuum pumps the degree of vacuum is expressed in inches Hg (mercury). The barometric pressure at sea level is 30 inches Hg or 14.7 lbs/in2  (PSI). By creating a vacuum in the channels of the router table under the MDF board we create a differential pressure by utilizing the barometric pressure to apply force on the work piece on top of the MDF board. The degree of vacuum we can achieve depends on the quality of the seal between the router table and the MDF board, the leakage rate through the MDF board and the size of the work piece in relation to the size of the router table.  The smaller the work piece the more leakage through the MDF board and thus the larger the required vacuum pump capacity to achieve the required vacuum level.   In addition, the leakage rate will increase as more parts are routed and more of the MDF board is exposed.

The hold-down force required depends on the type and quality of the router tools and the force generated by the dept of cut and feeding speed. As a rule of thumb the average hold-down force on a single piece should be around 2000 lbs.  

 

 

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Altitude has an effect on the hold-down force and must be taken in to consideration especially on smaller parts.

It should be understood that a certain size vacuum pump, which achieves 20” Hg vacuum at sea level when operating on a CNC router with a certain size work piece and quantity of air leakage will not achieve the same vacuum level at 5,000 ft altitude under the same conditions. This can be explained as follows: Inlet capacities for all vacuum pumps are rated in ACFM or actual CFM measured at inlet conditions. Air leakage through the MDF board and seals is measured in CFM under atmospheric conditions. The volume pressure ratio is calculated by the formula P1 x V1 = P2 X V2 

The following example shows the relationship: Assume the total air leakage to be 100 SCFM at a vacuum level of 20”Hg at sea level, barometric pressure is 30” Hg. P1 = 30” Hg, V1 = 100 SCFM, P2 =(30 – 20) = 10” Hg absolute.

The calculated vacuum pump inlet capacity is: (30 x 100)/10 = 300 ACFM at 20” Hg. To achieve the same vacuum level at an altitude of 5000 feet where the barometric pressure is 25” Hg the required actual pump capacity would be:

P1 = 25” Hg, V1 = 100 CFM, P2 = (25 – 20) = 5” Hg absolute and V2 = (25 x 100 )/ 5 = 500 ACFM at an equivalent sea level vacuum of 25” Hg.

 

Inlet Filtration: The woodworking industry is probably one of the most difficult applications for vacuum pumps due to the incredible dusty operating conditions and it is therefore critical to use efficient inlet filtration and proper maintenance with regular cleaning. Lack of maintenance will cause increased carry-over of wood dust in to the vacuum pump and possible rupture of the filter element. It will also increase the pressure loss over the filter resulting in a reduction of the pump capacity and vacuum level. As an example: At a vacuum level of 24” Hg the pump capacity loss is 33% at a pressure drop of 2” Hg and 50% at 3” Hg. Because of the heavy dust load the pressure drop can increase rapidly. It is therefore highly recommended to install a two stage inlet filtration system whereby the first stage utilizes a cyclonic separation filter to separate the large particles, followed by an efficient 10 micron filter.    Proper care should be taken when cleaning an inlet filter.  Make sure that the inlet filter doesn’t get ruptured by knocking it against a wall or garbage can.  Care should also be taken if using compressed air to clean the filters because the filter may be ruptured by the force of the air.

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