The Boring Bar

The M&M's of Coolant. A Guide to Mixing and Maintaining Your Coolant.

The M&M's of Coolant. A Guide to Mixing and Maintaining Your Coolant.

Authored By Arthur Alexiou

In a modern machine shop 5 axis simultaneous machining in the most demanding materials doesn't seem to be a problem anymore.

We have adopted complex CAD systems, machine controls, navigated demanding setups and can achieve reaching high tolerances, so why does coolant remain a mystery to some companies?

Is coolant a necessary evil?

A machine shop that has never experienced the full benefit of coolant will typically only want these three factors:

   •  A coolant that does not rust the machine.

   • A coolant that will not go rancid.

   • Cooling of the cutting tool.

Coolant is more than just a liquid to reduce temperatures of your workpiece and cutting tool. When the optimal cutting fluid is selected and mixed correctly, it will:

   • Reduce cycle times by eliminating the need for secondary tapping compounds and cutting speeds.

  • Increase tool life.

 • Increase surface finish.

Mixing & maintain your coolant – The M&M’s

Just like running your cutting tools at the right feeds and speeds, coolant has parameters. Correct mixing and maintenance of coolant is not hard but there are steps every machine shop must take.


Today we're going to look at a high-level view to ensure that you will get the true benefit from your coolant.


 There are three main factors we need to consider:

1.     Water quality.

2.     Coolant concentration.

3.     Mixing methods.

1: Water quality

Coolant is 90% water. Water is the base of every coolant.

It’s the main constituent, so why don’t we place importance in this factor? Test your water to see if it is within your manufacture’s specifications found on the manufacturers data sheet.

 2: Coolant concentration

Before even mixing your coolant, make sure to read the manufacturer data sheet.

Here you will find the best ratio for your machining operation and refractive Index multiplier.

3: Mixing methods.

For your coolant to do its job, it must be mixed correctly. The process of emulsification (mixing) is the most important part and improper mixing of coolant leads to most issues people experience such as:

•       Foam.

•      Short sump life (Coolant going rancid).

•       Heavy residues.


A simple change in mixing methods can be the biggest single improvement to coolant performance if your shop is experiencing problems.

So how should we mix coolants?

The process of emulsification takes place when you mix coolant with water. The “phase” or simply put, the arrangement of the water and coolant droplets can go 2 ways:

  1. Water over coolant – Correct
  2. Coolant over water – Incorrect (reverse emulsion)

The good old-fashioned method of mixing coolant to water in a bucket is still a reliable method today. The introduction of coolant (dispersed phase) to water (continuous phase) ensures your coolant mixture is in the correct phase (water over coolant).

The only problem with this method is it is labour intensive and slow.

There are mixing units (proportionators) available, but which one is the correct one to choose?

Mechanical vs Venturi Mixers

Mechanical: A mechanical mixer recreates the same process as mixing coolant concentration in a bucket. Mechanical mixers add the dispersed phase of the emulsion (coolant) to the continuous phase (water) a mechanical dosing pump creates a more stable emulsification, which lowers foaming, reduces consumption, and decreases residues.

Venturi: A venturi mixer reverses the process, adding the continuous phase (water) to the dispersed phase (coolant) resulting in a less stable emulsion (reverse emulsion), leading to poor coolant performance, foaming, residue and sump life.


You've done a great job by testing your water, running at the correct ratio, and mixing it correctly.

Now what?

Keeping your coolant at a constant concentration will ensure you avoid problems. By following these steps, you will give your coolant what it needs for a smooth operation.

There are 2 main factors we need to consider:

1.     Topping up

2.     Contaminates

1: Topping up

The first step is to know where you are at. Again, check your manufacturer data sheet. Here you will find the best ratio for your machining operation and refractive Index multiplier. Measure the concentration with a refractometer and take into account the refractive Index multiplier.

If your coolant is over the recommended concentration, you can dilute with a weak mix of 1~3% to bring down the concentration.

Never add straight water to your coolant.


By adding straight water, you are diluting your coolant’s chemical composition to the point which can cause rust and bacterial growth.

By making a mix (1~3%) you are recharging your coolant with the additives it needs be at its peak performance.


What is a contaminate? Anything that is not coolant.

Most coolants machine shops know that tramp oil is a contaminate but there are others that need to be considered.

Typical examples of contaminates:

•       Hydraulic, slideway, grease and spindle oils.

•       Cutting fluids (cutting & tapping paste) that are water insoluble.

•       Non approved machine cleaners and anti-rust sprays.

•       Metal fines and coatings.

•       Organic materials.

•       Mixing coolants.

•       Water, yes water.

Contaminates in coolants cause 2 main problems:

1.     Short sump life.

2.     Poor cutting performance and surface finishes.

Steps to keep out contaminates:


•       Make sure machines are maintained and checked for oil leaks.

•       Clean off excess oils, greases and coatings from bar stocks.

•       Have an oil skimmer fitted to every machine.

•       Use only approved machine cleaners and protective anti-rust sprays that do not affect coolant.

•       Use tapping compounds that are water soluble.

•       Use a vacuum that is fitted with a coolant filtration filter to remove metal fines and excess tramp oils.

•       Keep the machines sump free from organic matter. (Food, coffee, cigarette ends etc)

•       Do not mix coolant with other coolants without manufactures advice.

•       Always make a mix of coolant to top up and never add straight water to bring down concentrations.

•       Explain the importance of the above steps with regular toolbox meetings.


By following these steps, you are getting all the bang for your buck from your coolant and not wasting it on disposal costs and lost time. 


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