Selecting the Right Coolant: A Practical Guide for Machine Shops

Choosing the right coolant is one of those decisions that quietly affects everything. Tool life, surface finish, machine condition, and even downtime. Get it right and the shop runs smoother. Get it wrong and problems creep in fast.

Here’s a clear, practical guide to coolant selection and care, based on what we see every day supporting workshops across Australia.

Why Coolant Choice Matters

Coolant does more than keep things cool. A good system should:

• Control heat at the cut
• Prevent rust and corrosion on machines and parts
• Lubricate to reduce tool wear
• Flush chips away from the cutting zone

When the coolant is wrong, or poorly maintained, you’ll usually see corrosion, foaming, bad odours, inconsistent finishes, or premature tool failure.

Preventing Rust and Corrosion

Rust is usually a sign the coolant isn’t doing its job. Common causes include:

• Incorrect concentration
• Poor water quality
• Old or contaminated coolant
• Long periods of machine downtime

What to check:

• Keep concentration within the recommended range
• Use clean water with low mineral content
• Circulate coolant regularly, even on idle machines

This is where a quality, corrosion-inhibited coolant makes a difference. That’s why we often recommend proven options like Excision’s XDP range for general machining applications.

Which Coolant for Which Machine?

Not all machines have the same needs.

Manual lathes and mills

• General-purpose soluble oils are usually sufficient
• Focus on rust protection and operator comfort

CNC milling machines

• Require stable emulsions that resist tramp oil and bacteria
• Consistent cooling and clean finishes matter

CNC lathes and turning centres

• Need good lubrication to support tool life and surface finish
• Foam control is important due to higher flow rates

If you’re running mixed machines, a high-quality multi-purpose coolant can simplify things. We typically steer customers towards a single, well-managed coolant rather than multiple products.

How to Keep Coolant Healthy

Healthy coolant lasts longer and performs better.

Key habits that matter:

• Skim tramp oil regularly
• Remove chips from the sump
• Top up with correctly mixed coolant, not straight water
• Avoid letting coolant sit stagnant for long periods

Neglect is usually what shortens coolant life, not the product itself.

How to Mix Coolant Properly

Incorrect mixing is one of the most common issues we see.

Best practice:

• Always add coolant concentrate to water, not the other way around
• Use a proper mixing device or venturi where possible
• Match the concentration to the application, not guess

Too lean leads to rust and bacteria. Too rich wastes product and can cause residue or skin irritation.

Managing Concentration Levels

Concentration drifts over time due to evaporation and carry-out.

How to stay on top of it:

• Use a refractometer
• Check weekly as a minimum
• Record readings so trends are easy to spot

Keeping the concentration stable is one of the simplest ways to avoid corrosion and extend tool life.

How to Prevent Foaming

Foaming usually comes from:

• Excessive agitation
• Soft water
• Over-concentrated coolant

What helps:

• Check and correct concentration first
• Reduce unnecessary high-pressure return flow
• Use a low-foam formulation designed for CNC systems

We’ve seen good results in high-flow machines using Excision coolants designed specifically for foam control.

What to Expect When It’s Right

When the coolant is correct and well maintained, you should see:

• Cleaner machines
• Longer tool life
• More consistent surface finishes
• Fewer smells, less downtime, fewer surprises

That’s exactly how it should be.

Need a Hand?

At QMT, we don’t just supply machines. We help you keep them running properly. If you’re unsure about coolant selection, setup, or ongoing maintenance, talk to our team. We’ll point you in the right direction for your machines, materials, and workload.