Types of Filtration for CNC Machines

A CNC machine uses cutting fluid or coolant to cool the tool, lubricate the cut and wash away chips. The fluid picks up metal particles, dirt and sludge as it circulates. A filter removes them so the fluid goes back to the machine clean.

Dirty fluid wears tools, spoils the surface finish and damages pumps and slideways. The type of filter decides what is removed and how much work the filter needs.

Paper filters

A paper filter uses a roll or sheets of filter paper. The fluid passes through the paper. Particles larger than the pores stay on the surface. When the paper is loaded, fresh paper is fed in or the sheet is changed.

Strengths

• Low cost to buy.
• Easy to find, in many sizes and grades.

Limits

• The paper is used up quickly on a busy machine, so it is changed often.
• It can block in heavy work, and it can tear and let dirt through.
• Particles smaller than the pores pass through.
• Used paper goes to waste, wet with fluid and metal.

Cartridge filters

A cartridge filter uses a cylinder of filter material. The material can be paper, fabric or metal mesh. The fluid passes through the wall of the cartridge and the dirt stays behind.

Strengths

• Lasts longer than paper, so it is changed less often.
• Catches a wide range of particle sizes.
• Many filter materials are available to suit the job.

Limits

• Costs more to buy than a paper filter.
• Elements must be cleaned or replaced.
• A dense cartridge makes the pump work harder.
• Used cartridges add to waste, and some materials cannot be recycled.

Bag filters

A bag filter is a bag of filter cloth held in a housing. The fluid flows into the bag and out through the cloth. The bag holds a large amount of dirt before it is full.

Strengths

• Holds a lot of dirt, so it suits heavy contamination.
• Low running cost where there is a lot of coarse dirt.

Limits

• Less good at holding fine particles.
• The cloth can tear under hard use.
• Used bags go to waste.

Centrifugal filters

A centrifuge spins the fluid at high speed. The spinning force throws the solid particles outwards, where they collect. The clean fluid is taken from the centre. A centrifuge separates particles by their density, so it can remove very small particles.

Strengths

• No filter media to buy or replace.
• Removes a wide range of particle sizes, both ferrous and non-ferrous.

Limits

• High cost to buy.
• It is a machine with moving parts. Rotors and seals wear, need servicing and can fail.
• Less suited to very large volumes of fluid.
• It can warm the fluid as it runs.

Magnetic filters

A magnetic filter passes the fluid over strong magnetic rods. Iron and steel particles are pulled onto the rods and held. The clean fluid flows on. To clean the filter, lift the rods out, wipe off the iron and put them back. How magnetic filtration works explains this in more detail.

Some non-magnetic dirt also gets caught in the layer of iron on the rods.

Strengths

• Holds very fine ferrous particles.
• The open path past the rods does not block, so the flow stays steady and there is no pressure build-up.
• Nothing to replace. The rods are cleaned and used again.
• Needs no power to run.
• The iron comes off the rods as metal that can be sold as scrap.

Limits

• Removes only ferrous particles. Aluminium, brass, plastic and general dirt pass through.
• Costs more to buy than a simple paper or bag filter.

Summary table

Using more than one filter

Many machines use two filters together. On machines that cut steel or cast iron, most of the dirt is ferrous. Fit the magnetic filter first. It removes the iron, and the paper, cartridge or bag filter after it catches the non-ferrous dirt. The second filter then fills more slowly, lasts longer and can often be a coarser grade.

For a closer look at magnetic and barrier filters side by side, see magnetic filters compared with other filters.

The SMAG range

Santosh Magnetic Works builds the magnetic coolant filter to order in Mumbai, sized to the flow rate and connection of your machine. It can be fitted ahead of an existing paper, cartridge or bag filter. See the filtration range or contact us with the machine, the fluid and the flow rate.