Air Compressors for CNC Machining Centers

CNC machining centers depend on compressed air more than most people expect. Tool clamping, workpiece fixturing, chip blowing, spindle sealing, automatic tool change, all these functions need air source support.

CNC

Machining Center Air Supply

Functions of Compressed Air in CNC Machines

Spindle drawbar mechanism pulls tool holder into taper bore forming tight fit. BT40 tool holder pull force requirement is about 3,400 lbf. BT50 even higher. Air pressure drop directly weakens pull force. During heavy cutting, cutting force might yank the tool out of the spindle.

Pneumatic chucks and pneumatic vises, clamping force is proportional to supply pressure. 0.1 MPa difference between 90 psi and 75 psi, converting to clamping force is a significant number. Workpiece shifts during machining, dimensional tolerance is out the window.

Chip blowing air consumption frequently gets underestimated. Machining aluminum parts, chips are long and sticky. Don't blow them away in time, they'll wrap around the tool. Cast iron dust falls into linear guide ways and becomes abrasive. Multiple air nozzles opening simultaneously, flow demand spikes.

High speed spindle internal bearings need air seal protection. Compressed air flows outward from inside the spindle, forming positive pressure barrier at seal gap. Keeps coolant and dust out. Flow rate is small. Interrupt it and coolant will quickly seep into bearing area.

Automatic tool change mechanism concentrates the most cylinders. Tool magazine indexing, robot arm swinging, tool unclamping and clamping. One tool change involves four or five cylinders actuating in sequence. Instantaneous flow hits peak. Atlas Copco and Kaeser technical documents both mention that tool change instantaneous flow demand can be three to four times the average.

Air Consumption Calculation

Single machine air consumption from equipment manual gives a number. VMC850 class vertical machining center rated value is mostly 14 to 18 CFM. VMC1060 slightly higher. Horizontal machining centers generally use more air than verticals. Models with pallet exchange systems, consumption goes up another notch.

Numbers in manuals need discernment. Some manufacturers note peak value. Some note average value. Test conditions also vary. Mazak and DMG MORI manuals usually distinguish their notations. Fanuc system machine manuals often just give one general number. Using 25% above rated value for design margin is fairly safe.

Ten-machine shop can't just multiply single machine consumption by ten. Tool changes don't happen simultaneously. Chip blowing has timing differences. Simultaneity factor at 0.6 to 0.7. Ten machines configured with 85-125 CFM total supply capacity, enough to handle. Machine count increases to over twenty, factor can come down a bit more.

Instantaneous peaks rely on air receiver to absorb. Compressor response speed can't keep up with those few seconds of flow surge during tool change. Receiver capacity insufficient, pressure will collapse a big chunk. Drops below machine alarm threshold and it's shutdown.

Pressure Configuration

Machine manual will note minimum working pressure. Most at 75 or 80 psi. Below this value, some machines directly alarm. Some don't alarm but clamping force quietly weakens. Latter is more dangerous because the problem isn't easily noticed.

From compressor station outlet pressure to machine inlet, air passes through dryer, filters, tens of feet of piping. Each segment eats pressure drop. Dryer eats 3 to 4.5 psi. Filter when new 3 psi, dirty can rise to 12 psi. Pipe pressure drop depends on diameter and distance. SMC and Festo selection manuals both have pressure drop calculation formulas. Run through the formula once, end pressure 15 to 22 psi lower than station outlet is very common.

Pipe pressure drop is where problems hit easiest. 1-inch pipe serving three machines is fine. Serve eight and it's not enough. Flow velocity goes up, pressure drop grows by the square. Lines with many elbows and valves suffer more. Some shops have main pipe running east to west, 200-250 feet, with several turns in between. End machines barely have enough pressure. Add two more machines and it can't hold.

Air receiver capacity and compressor output have a ratio relationship. CNC shops have drastic load fluctuation. Ratio needs to run high. 35 CFM output pairs with roughly 100 gallons of receiver capacity. Pressure fluctuation can be controlled in reasonable range.

Moisture Treatment

50 hp screw machine running full load one day can precipitate 8-10 gallons of water. All this water comes from water vapor in the air. When compressed air temperature drops to room temp, water vapor condenses as liquid.

Water entering machine air circuits. Cylinder lubricant will emulsify. Solenoid valve seals will accelerate aging. Water seeping through spindle air seal directly threatens bearing life. Northern winters, pipe freezing and blocking air circuits also happens from time to time.

Refrigerated dryer is most common moisture removal equipment. Drops compressed air temp to 35-37°F. Water vapor condenses and drains out. Treated pressure dew point about 37°F. As long as shop temp doesn't go below this, no more condensate. Climate controlled shop, refrigerated dryer is more than enough.

Shops without climate control, winter temperature might approach freezing. Refrigerated dryer dew point margin isn't enough. Desiccant dryer can push dew point to -20 or -30°F. Trade-off is regeneration consumes compressed air. Heatless type eats ten-something percent of output. Equals compressor running 10%+ for nothing. Heated and blower heated types can bring losses down. Equipment price goes up accordingly.

Treatment

Dryer & Filter

Piping

Ring Main Network

Oil Content Treatment

Oil-flooded screw machine exhaust carries atomized lubricating oil. Concentration in several to tens of ppm. Oil mist entering machine air circuits, solenoid valve action gets sluggish, air seal surfaces get contaminated, oil-air lubrication system ratio gets thrown off.

ISO 8573-1 divides oil content into several grades. Class 2 is below 0.1 mg/m³. Class 3 is below 1 mg/m³. Precision equipment gets Class 2. General machining, Class 3 is enough.

To reach Class 2, main line gets an oil removal filter at 1 ppm precision. Machine end gets another precision filter at 0.01 ppm. Elements are consumables. Changed too late they'll clog. Clogged, pressure drop is big. Changed way too late, element punctures. Filtration equals nothing. Parker and Donaldson compressed air filters have good reputation in the industry. Element replacement cycles can reference suggested values in their technical manuals.

Oil-free machines eliminate oil content at source. Equipment expensive, maintenance expensive, energy efficiency also a bit worse. Unless cleanliness requirements are exceptionally high, oil-flooded machine with filtration is the better value proposition.

Particle Treatment

Dust in air, rust flakes in piping, powder ground off desiccant, fibers from damaged filter elements, all count as solid particle contamination. These particles entering proportional valves or precision regulators, just a few microns is enough to jam the spool or cause regulation failure.

ISO 8573-1 Class 2 particle grade requires particles larger than 1 micron concentration not exceeding 100,000 per cubic meter. Main line with a 1-micron precision element can achieve this.

Filter pressure drop rises as element gets dirty. New element pressure drop around 3 psi. After few months might rise to three or four times that. Differential gauge needle enters red zone, time to change. Delay and pressure drop loss is one thing. Element getting blown through by differential is another.

Machine's Built-in Air Treatment

Vast majority of CNC machines have an FRL unit at air inlet: filter, regulator, and sometimes lubricator. This setup's design premise is upstream air quality already meets spec. Filter only handles catching occasional residual contaminants.

Expecting that little machine-end filter element to handle dirty air the main system didn't process, element clogs in weeks. Not changed in time it'll blow through. Contaminants march straight in. Pushing air treatment responsibility to the machine end, the thinking is wrong from the root.

Pipe Network Layout

Multi-machine shops recommend ring main network. Main pipe loops around the space connecting head to tail. Each machine can draw air from two directions. Pressure distribution is even. Local maintenance, close valves at both ends of that section, other machines keep producing. Tree branch network can't do this. Further from compressor station, more pressure the machine loses.

Each machine's branch pipe inlet gets a ball valve. Maintenance on this machine, close its valve, doesn't affect neighbors.

Galvanized steel pipe threaded joints are prone to corrosion and accumulation of thread tape debris. Over years these contaminants get carried by airflow into machines. Aluminum alloy quick-connect piping has smooth inner walls, doesn't rust, installs fast too. Recent years seeing more and more use in CNC shops. Transair and Infinity type aluminum pipe systems have growing installed base.

Piping needs slope, toward drain points. Even with dryer, big temperature swings can still precipitate small amounts of water. Low points without drainage, accumulated water gets swept up by airflow into machines. That's a hazard.

Selection

System Design

VFD

Pressure Stability

System Selection Approach

Compressor capacity selected by total air consumption plus margin. VFD machine provides more stable pressure under fluctuating loads. Air receiver capacity calculated by peak flow and allowable pressure drop, better bigger than smaller. Dryer selected as refrigerated or desiccant based on shop temperature conditions. Filters configured by oil content and particle grade requirements. Piping diameter calculated by flow and allowable pressure drop. End pressure must account for dryer and filter pressure drop margin.