There are several different types of vacuum pumps available for use with a CNC router. To ensure you’re not paying too much or not getting optimal performance of your machine, it’s important that you make the right selection of vacuum pump to suit your needs.
When looking at the specifications of a pump, there are two main variables to consider: vacuum level and flow. Vacuum level is most commonly measured in inches of mercury (Hg), which is a term that refers to atmospheric pressure. CNC vacuum hold-down works by applying 15 lbs/sq.in. (psi at sea level) in all directions. During operation, when you use the vacuum pump you are removing air from the underside of the work piece and putting pressure on the topside, thus increasing the atmospheric pressure and causing clamping force. To calculate how much force is applied to a work piece is determined by multiplying the reading on the vacuum level gauge by the workpiece surface area (sq.in.) and multiplying that number by 0.5.
For example, let’s look at a gauge that reads 15″ Hg for a material workpiece that is 36″ square (1296 sq.in.).
Gauge Reading (Hg) x Material Surface Size (sq.in.) x 0.5
15 x 1296 x 0.5 = 9720 lbs
Vacuum flow is an important specification to consider when considering a CNC vacuum table. Flow is measured most commonly in terms of cubic feet per minute (cfm), which refers to the volume of air pulled in by the vacuum pump without any restrictions on the air being drawn in (i.e., no workpiece/material applied). Some manufacturers also provide a specific flow rate (scfm), which refers to the amount of air drawn in for optimum performance of the vacuum pump with reference to atmospheric pressure in Hg. Scfm should be considered over cfm, as it is the most relevant specification in regards to optimal performance of the vacuum pump.
When considering a vacuum pump, there are many types of pumps to choose from, such as rotary vanes, positive displacement blowers, and rotary screw pumps. The different systems come in a wide range of prices, so it’s important to weigh the pros and cons of each option against your own needs.
Important factors include:
- Running cost
- Annual maintenance costs
- Hg at scfm
- Noise levels
- Common applications
Rotary Vane Vacuum Pump
Rotary vane vacuum pumps consist of an electric motor coupled with a direct drive to the vacuum pump impeller made up of self-lubricating carbon vanes. The vanes rotate in the pump housing drawing air in the inlet and discharging the air through the exhaust to create a vacuum. Greater vacuum pressure results from the close tolerances allowed by the carbon vanes. CANCAM CNC Machines offers rotary vane pumps that can attain 82 cfm from a 8 hp unit.
The main pro of a rotary vane pump is superior hold-down capability. Also, they operate at a lower and more tolerable decibel range (65-75 dB). Rotary vane pumps are ideally suited for wood, plastic, aluminum and other nonporous sheet-like materials.
The main con of these pumps is that the carbon vanes should be replaced roughly every 6000-8000 hours. To ensure maximum operating life, you need to confirm the filters are clean and the pump is operating at optimal temperatures. High heat conditions significantly decrease the life of the carbon vanes.
Regenerative Blower Vacuum Pumps
Regenerative pumps contain a rotating impeller driven by a belt or direct drive motor, and are ideal for holding less dense materials such as foam and other porous materials.
The main pros for regenerative pumps are their low cost and minimal maintenance. They tend to be the lowest-cost alternative, starting around $5,000. Maintenance is also limited to replacing the air filter as needed.
The main cons for regenerative pumps are lower vacuum pressure and louder operation. Higher design tolerances allow air to escape during operation, which can cause lower vacuum pressure. Regenerative pump vacuums are also generally louder compared to other vacuum designs, ranging 65-105 decibels.
Positive displacement pumps contain either coupled belt or direct drive motors. Two rotors rotate in opposite directions. As the rotors pass the blower inlet, it traps a quantity of air in the blower housing and the discharge results in compressed air out the exhaust. These pumps are ideal for industries working with materials such as wood, plastic, and other nonporous sheet-like materials.
A 10 hp unit, which operates at 15 Hg at 250 scfm, costs $9,000. Standard maintenance consists of oil changes roughly every 5000 hrs.
The main con of positive displacement pumps is they represent one of the loudest alternatives, operating at 95-115 decibels.
Rotary screw vacuum pumps are the most complex alternative. This type of pump contains a belt or direct drive motor that connects to two counter rotating screws. They are most commonly used with wood, plastic, and other nonporous sheet-like materials.
Rotary screw vacuum pumps offer the greatest vacuum pressure, 30 Hg at 150 scfm, which is the main pro.
The main con of rotary screw vacuum pumps is cost. These pumps cost around $20,000 and maintenance can be costly too. Tight tolerances are required to retain optimum vacuum pressure. Cooling systems with complex electrical systems are required to operate the unit. Rotary screw pumps also operate at a high decibel range of 100-110 dB.