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Particles in Machine Coolant

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Particles in Machine Coolant

Microscopic Particles Damage Tools In Precision Machining

 Many machine shops remove the big chips from the machine coolant and consider it clean.  Very few shops adequately filter their machine coolant to fine enough levels.  Particles too small to see can be more than big enough to deflect the tool, chip the tool or scratch the work.   View our Article to see when it is necessary to Fine filter Machine Coolant or when other Types of Machine Coolant Cleaning Systems may be adequate.

Particles in the machine coolant get between the tool and the work during machining.  The carbide is hard but it chips. As the tool edge rotates into the work it traps a particle.  The hard particle then micro-fractures the edge of the tool. 


This is 5.9 oz. of sand on a 12 inch ruler

Chunks of carbide, aluminum, etc. in the coolant   

Total Suspended Solids




parts per million



under 200

These are results from an analysis of particulates in coolant collected at the nozzle.  This is about 5.9 ounces of sand, silt and clay sized particles in the 40-gallon sump of a $500,000 machining center doing precision aerospace work to tolerances of 0.0001" 

These are chips from the sump


10-x magnification Note big chip in lower left. 


60 x photo. This is a piece of aluminum (the dinosaur) welded to a piece of carbide (the surf board.)    Chip is 0.075" high by 0.122" wide









The line in the upper right in the left photo and in the bottom of the right photo is a paper clip to show scale.

This coolant was collected coming out of the nozzle so it is representative of the coolant being sprayed into the cut.  You can see that these are very big chips.  Nobody would deliberately put a paper clip between the tool and the material being cut but dirty coolant is doing the equivalent.  

Edge chipping of a two flute drill run in the dirty coolant. 




These are pictures of a drill taken at 10-x magnification on the left and at 60-x magnification in the middle and right.  The middle picture shows a chip in the side of the drill and the right picture shows at least three distinct fracture planes where the carbide was sheared off because of force concentration on a particle that got between the drill and the work.  In this case the failure mechanism was chipping rather than dulling from straight wear. 

End mill run in coolant with a high count of microscopic particles.

(End mill at 10x and four corners of the end mill at 60-x magnification.)

This end mill that was cutting with one corner when it was removed.  Corners 2 and especially 3 have been chipped off to the point where they are cutting properly because the edge does not stick out far enough.  Corner one has the edge slightly retarded and what is left presents a flat surface into the cut instead of an edge.  When you chip one edge then it makes it much more likely that you will chip the edge behind it because the trailing edge hits with much more force.  




Corners 1, 2 & 3 show the fracture planes that indicate a fracture.  Tool 4 is more typically wear but it also looks like chemical attack was a factor.  The tool became dull because pieces chipped off rather than through wear.  



Filtering Will Remove

1. Particles - of the rated filter size and larger.

2. Tramp oils and greases because they are much larger globules then the coolant they get even larger as they combine with and absorb particles.  Much as sand or clay on a floor helps pick up spilled oil.

3.  Particles much smaller than the rated filter size because the filters are designed not to pass any particles larger than their rating but they also trap smaller particles.  As the filters get used the holes close up and they trap ever-smaller particles. 

4.  Bacteria and fungal colonies.  It will also keep the coolant oxygenated, which will help prevent formation.  Bacteria and fungi typically grow in the sludge on the bottom and feed on tramp oils and greases which and filtering removes.