Choosing and maintaining a CNC’s vacuum pump
As with most machines in a woodshop, performance can decrease with time and failure is always an option.
As with most machines in a woodshop, performance can decrease with time and failure is always an option. Downtime then occurs as you are forced to consider repair, overhaul or replacement options.
A critical component of a CNC router is the vacuum pump, used to hold material to the spoil board or in a fixture. So, it is important to have and follow a preventative maintenance program to lengthen its life. The single most important action is to keep the filters between the vacuum distribution system and pump clear, dry and intact.
A physical inspection is one method to check the condition of the filters. Another is a gauge or sensor that measures the amount of vacuum and/or air flow. Measurements from a gauge should be recorded daily. A sensor emits an audible sound or can disable the router when the vacuum or air flow drops below a certain range.
Filters eventually have to be replaced based on a manufacturer’s recommendations or a failed inspection. But the cost is minimal, especially when compared to the downtime caused by a disabled vacuum pump.
Some vacuum pumps require regular oil changes, simply because they are in a dusty environment. It doesn’t matter if your pump is well protected or isolated; the fine dust generated from a CNC router will infiltrate even the smallest cavities. The continued buildup of impurities lowers the oil’s viscosity, which will cause a failure of the pump’s bearings and other mechanical parts.
Overhauling a vacuum pump is best left to a specialist. Kits are widely available to accomplish the task, but a specialist has the advantage of having the right tools and vast experience. A specialist is also likely to offer a warranty.
In my experience, a vacuum pump with a regenerative blower is seldom worth the time or expense of overhauling. A regenerative blower generates high air flow with lower vacuum and is ideal for a CNC router that machines sheet goods. But their low cost and tendencies for catastrophic failures make them rather disposable.
Regenerative blowers fail for two common reasons. First, a foreign matter gets stuck between the impeller and its housing, causing the impeller to lock up and eventually disintegrate. There can also be insufficient air flow from the vacuum side, causing the impeller to expand from heat and fuse with the housing. This failure can be avoided by the installing a pressure relief valve.
Conversely, a liquid-ring vacuum pump will almost never have a failure or be down for any unscheduled maintenance. That’s because the pump rotor, mounted offset within the pump body, has voids that provide the vacuum on the input side and the compression on the output side. The rotor is submerged in a liquid, usually water, and can’t contact the housing. The liquid acts like a piston and compresses air into the void between the vanes. Think of this area between the vanes as an irregular cylinder, which forces the compressed air through an outlet. The only parts that need replacement are shaft bearings, seals and gaskets, which are externally mounted.
Liquid-ring pumps can operate for decades with virtually no maintenance and are relatively inexpensive. These pumps provide very high vacuum with low air flow and are better suited for CNC routers that machine small parts, which have less surface area for the vacuum to act on.
Rotary vane vacuum pumps are probably the most widely used. They typically have horsepower ratings from 0.25 to 30 and high vacuum ratings from 20 to 30 “Hg. However, the air flow is lower, 150 to 400 cfm, compared to a single-stage regenerative blower type, in the range of 500 to 700 cfm.
Rotary vane pumps are also well suited to machining small parts and a good choice for vacuum chucks on rotary axes. But again, they are best left to specialist to overhaul. A typical total overhaul starts with cleaning the exterior of the pump and removing the paint. After disassembly, the technician looks for unusual wear patterns. The electrical motor is tested and the pump is reassembled with new shaft seals, O-rings, gaskets, valves and vanes. Finally, the pump is tested for air flow and vacuum.
While choosing a vacuum pump certainly depends on what you need it to do, don’t ignore the maintenance and rebuilding aspects.
This article originally appeared in the August 2018 issue.
