Spindle purge air must be dry and free of particles and oil aerosols, to the ISO 8573-1 class stated in the machine manual. In practice that means a refrigerated dryer and filtration after the compressor, then a fine filter at the machine. Wet or dirty air carries water and dirt straight to the spindle bearings.
What does spindle purge air do?
The spindle runs fast, surrounded by coolant and chips. A small, constant flow of air passes through it and escapes through the seal at the spindle nose. That slight overpressure stops coolant mist from working its way back to the bearings. On some spindles, the air also carries the bearing lubricant (air-oil lubrication) or cleans the taper during tool changes.
Either way, this air ends up touching precision parts. Of all the uses described on our page on the compressor for CNC machining, it is the most demanding in air quality.
What has to come out of the air?
- Water. The compressor draws in ambient air with its moisture. Once compressed and cooled in the pipework, that air drops liquid water. In a spindle, water encourages bearing corrosion and spoils lubrication. A refrigerated dryer lowers the pressure dew point to a few degrees: water condenses in the dryer, not in the machine.
- Particles. Dust drawn in, rust from old pipework, seal debris. They lodge in the fine passages of the spindle.
- Oil. Our compressors are oil-lubricated: a small amount of oil is carried over with the air. Filtration after the compressor, with a coalescing filter, holds it back.
Read the required class in the manual
ISO 8573-1 grades air on three criteria, in this order: particles, water, oil. The class is written as three numbers. The lower the number, the cleaner the air. Find the class required for spindle air in the manual: it is sometimes stricter than for the rest of the machine.
We do not set this class in place of the machine builder. Our technicians work out with you the treatment that reaches it, using our guide to ISO 8573-1 air quality.
A small flow, but a constant one
Spindle air uses little, but it never stops. Take 30 l/min per spindle as an example: the real value is in the manual.
- Four spindles: 4 × 30 = 120 l/min.
- Per hour: 120 × 60 = 7,200 L, or 7.2 m³.
- Over a 16-hour day: 7.2 × 16 = about 115 m³.
Three consequences. This flow is a base load the compressor supplies even when the machines are not cutting: a variable speed compressor follows it without running unloaded. The filters see this flow all the time and clog over the months. And some builders ask for spindle air to stay on as long as coolant can reach the spindle: check this before switching the compressor off at night.
What installation gives clean spindle air?
- A compressor that dries and filters as standard. The K 75 (1.1 m³/min at 8 bar) and the K 110 (1.7 m³/min at 8 bar) have a built-in refrigerated dryer and filtration.
- Or a standalone compressor with treatment sized for it. The S 110 delivers 1.7 m³/min at 8 bar; dryer and filters must be chosen for that flow. See which air dryer to choose.
- A fine filter at each machine, with its regulator, as close to the inlet as possible. See fine filters.
- Take-offs from the top of the main pipe and a drain at the low points, with an automatic condensate drain.
Pitfalls that damage a spindle
- A clogged filter. A saturated element drops the pressure and eventually lets through what it should stop. Change elements on schedule and watch the differential pressure indicator.
- A bypassed dryer. A bypass opened during a service job and then forgotten sends wet air into the whole network.
- Polluted intake air. A compressor that breathes coolant mist loads the air from the start. Install it in a separate room: see compressor room installation.
- Postponed servicing. First service at 500 hours, then every 1,500 hours, with a reminder on the controller. Our screw compressor maintenance guide covers each step.
Send us your machine manuals with a quote request: our technicians will define the air treatment that reaches the class required for the spindle.
For your trade: Compressor for CNC machining and machining centres.