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When and Why to Fine Filter Machine Coolant

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When and Why to Fine Filter Machine Coolant

For most purposes flat bed, cyclone, centrifugal and similar filter systems may be more than adequate.  There are some situations where fine filtering is a definite advantage.  Refer back to our Filtration Index for more information on why to filter or to find other Machine Coolant Cleaning Systems. 

Fine filtering involves filtering particles from 20 microns to 1 micron in size.  This is a range of filtering that is often ignored. This number comes from the aerospace industry and is based on improving optimal performance overall.  Actually the finer you can filter the better.  However, below one micron filtering starts to get much more expensive and the payback starts to drop drastically. 

Unfortunately this level of filtering is well below the level that "makes sense" to a lot of operators.  This is bad enough in grinding operations and even worse in chip machining.  It is a common perception that removing the chips leaves the machine coolant clean.   Large chip operations generate large, visible chips but they also generate a significant amount of particles too small to detect readily.

 Centrifugal systems are generally described as effectively removing particles down to ten microns.  This is pretty well true.  These systems also remove a great deal of material that includes smaller particles.  Centrifugal systems do not remove smaller particles as effectively as they remove larger particles because of the difference in weight.  Think of how dust can float on water while dirt clods will sink.  Centrifugal systems remove a large percentage of material of all sizes. 

Fine filtering systems remove particles by size.  Typically it is some sort of a sieving process.  

When to Fine Filter

As a rule of thumb you should filter to a level where the particle size is 10% of your tolerances.  If you are working to 0.001" (25 microns) then you should filter out all particles down to 0.0001" (2.5 microns).

In grinding operations you want to filter out all the particles that are larger than 10% of the grit size of the wheel.   In other machining you want to filter out all particles down to 10% of the tightest dimension specified. 

Probably the best way to explain this to shop personnel is with the tire and 2x4 analogy.    An automotive tire is about twenty inches in diameter.  You can run over a rough surface such as asphalt and the ride is smooth.  If you run over the two-inch dimension of a 2x4 you may feel a little bump.  If you run over the four-inch dimension you will definitely feel a bump.

Why to Fine Filter

The reasons to fine filter are the same as the reasons to filter in the first place.  Better tolerances, better machine life, better finish, longer tool life, less heat, longer machine coolant life, reduced maintenance and overall reduced costs. 

The problem with fine particles is that they get where they should not.  These are particles that are suspended in the machine coolant.  This means that they get between the grinding wheel or tool and the work as well as in the general atmosphere around the machine.  10% is not a magic number but just a minimum number as a rule of thumb.    

If particles of any size come between the tool and the work it will tend to damage both the tool and the surface of the work the same way that a rock in your shoe will try to make a hole in both your foot and the shoe. 

Machine damage is much harder to quantify.  One way of determining acceptable particle size is to ask what size grit sandpaper you would allow to be used on the hydraulic cylinders of the machine.  As the machine coolant gets sprayed it gets on extended hydraulic cylinders and is then abraded as the cylinder moves in and out.  Even fine particles can get trapped in a wiper or seal and still abrade somewhat.  The same effect also occurs wherever there is a tight space and movement.  If you are working to 0.001" then obviously you need tolerance all the way through of no more than 0.001".  Sloppiness in equipment tends to multiply.  A hydraulic cylinder that is 90% accurate, which is moving a rod that is 90% accurate will yield 81% accuracy.     

Obviously, also, the more abrasion there is in a machine the shorter the service life between rebuilds.  

One scratch can ruin a part where dimensions and surface finish are critical.  What is less obvious is that dirty machine coolant can cause extreme scratching when grinding hard materials such as ceramics.  Each scratch can serve as a stress concentrator and promote breakage at that point.  This can have an effect on the tool, the work or both.  Dirty machine coolant shortens tool life by simple abrasion as well as possibly contributing to premature tool breakage due to the creation of stress concentrating surface gouges.  Remember that at 0.001" tolerances a significant gouge is 0.0001".

Filtering in Carbide Tool Grinding Operations

This example is based on a saw grinding operation.  In these test results you may not need to fine filter if you are using a 60 grit wheel since you are at 100 microns and the centrifugal system will probably remove all particles down to that size.  If you are using a 250-grit wheel you need to fine filter.  



Filter to

If you use

Grit Size  

this level

Wheel Grit 

in Microns 

in Microns



































1.  The particle count in a carbide tool grinding operation was done on particles suspended in the machine coolant only.  Heavier particles tended to settle to the bottom of the sump rapidly. 

2.  This is an assumption.   We believe 98% removal to be a good figure for centrifugal systems.  Some may do better than that.   We also assumed 98% of all sizes whereas these systems tend to lose efficiency as the particles get smaller.