Application expertise and proactive maintenance – a cool approach to cutting fluids

Industrial Lubrication and Tribology

ISSN: 0036-8792

Article publication date: 27 September 2011



Bird, P. (2011), "Application expertise and proactive maintenance – a cool approach to cutting fluids", Industrial Lubrication and Tribology, Vol. 63 No. 6.



Emerald Group Publishing Limited

Copyright © 2011, Emerald Group Publishing Limited

Application expertise and proactive maintenance – a cool approach to cutting fluids

Article Type: Mobil industrial lubricants From: Industrial Lubrication and Tribology, Volume 63, Issue 6

Tougher metals, more advanced, severe metal working processes and the ongoing drive by machine shops to increase productivity and reduce costs has seen the importance of cutting fluids rise up the agenda of metal working operators in recent years.

Cutting fluids have three key functions to perform:

  1. 1.

    Lubricating the chip/tool and tool/workpiece contacts to reduce friction, helping to extend tool life and improve the surface finish of the workpiece.

  2. 2.

    Cooling the workpiece and tool to dissipate heat in the cutting zone, helping to prevent chip/tool welding and to improve dimensional accuracy.

  3. 3.

    Flushing and removing metal chips (swarf) from the cutting zone.

However, with a multitude of factors including the operating environment, machining application and machined metal type all having an impact on a cutting fluid, no one cutting fluid can provide the lubrication, cooling and protection required in each and every operation. As such, leading lubricant and metal working fluid providers such as Mobil Industrial Lubricants have developed a range of fluids to meet many of the varied machining operations in machine shops today.

There are two cutting fluid types.

Neat (straight)

Neat fluids are not mixed with water and are used in machining operations such as tapping and threading of high alloy steels, which are beyond the typical performance profile of aqueous coolants. Properly formulated products can improve machining in high speed automated machining centres through outstanding cutting performance, reduced tool wear and enhanced surface finish. Maintenance professionals should seek products that:

  • are light coloured to allow clear visibility of the workpiece;

  • have low misting characteristics to help workplace safety and product usage; and

  • are chlorine-free to support environmental concerns, while balancing with a high degree of lubricity and machining performance

Aqueous (soluble/water miscible)

While neat cutting oils are provided in packs for immediate use, aqueous cutting fluids are provided in a concentrate form and must be diluted with water at the machine shop site before use. Optimum performance for these aqueous coolants requires an ongoing partnership approach from both the lubricant supplier and machine shop operator to help maximise productivity and reduce unscheduled downtime.

First, operators need to work with their lubricant supplier to identify the most appropriate type of aqueous cutting fluid for the operating conditions. There are three different types, all with different performance characteristics:

  1. 1.

    Milky or conventional cutting fluid. A cutting fluid with a high oil content (60-75 percent) which forms a milky looking emulsion on dilution with water.

  2. 2.

    Semi-synthetic cutting fluid. A cutting fluid with a low to medium oil content (10-50 percent) that when mixed with water forms a translucent emulsion which you can see through.

  3. 3.

    Synthetic cutting fluid. Cutting fluids which do not contain any oil (chemical solutions) and are generally used for grinding.

Second, different concentrations are required for specific machining operations and metal type so it’s important that the correct concentration level is used. Using the correct concentration is vital to the performance of the fluid and typically varies between 3 and 10 percent. For example, if a recommended concentration is 5 percent, then the fluid has been designed to offer 100 percent protection and machining performance at that level. If the concentrate is just 1 percent outside of this value there will be 20 percent more or less additive within the working fluid. Too high a concentration and there is the potential of issues such as skin irritation, foaming and filtration problems to occur and too low can lead to severe problems such as bacteria growth, corrosion and poor surface finish.

In addition, for machine tool applications, the mixing of different lubricating oils (slideway and hydraulic) and aqueous fluids is unavoidable. Using lubricants which are fully compatible with the aqueous cutting fluid is important to help remove the build up of “tramp oil”. “Tramp oil” can compromise the effectiveness of the coolant by shortening its effective life and adversely altering cutting performance. The use of high quality, compatible metal working fluids should be used in conjunction with a regular programme of removing (skimming) as much “tramp oil” as possible to extend the life of the coolant and avoid other potential performance and health and safety issues.

Ongoing monitoring is key to maximising productivity

Once the correct type of coolant and operating concentration has been selected it is crucial to monitor the condition of the fluid going forward. Four parameters to monitor are fluid concentration, pH value, bacterial and fungi count and dissolved salts concentration and hardness. Fluid concentration is the most important factor to monitor and should be checked and recorded formally.

During the lifetime of a coolant in service, the fluid concentration can change widely due to water evaporation from heat generation during the cutting process and losses due to highly pressured circulation. Close monitoring is therefore required on a daily basis with carefully measured counter actions carried out to help control the fluid concentration. Simply approximating the amount of water and concentrate to balance the fluid can lead to problems such as lowering the pH value of the fluid and increased bacterial activity, which will result in shorter coolant life, lower quality products and increased costs.

The following actions should be carried out on a daily or weekly basis:

  • check the concentration of coolant with a refractometer at the beginning of every day/shift;

  • check pH value with pH meter or pH paper at the beginning of every day/shift;

  • check diluting water and coolant hardness with water hardness strips every week;

  • remove as much tramp oil as possible every week after the coolant has been static for one hour;

  • always top up with diluted coolant, never with just water; and

  • keep good records of coolant progress using monitoring charts. Take timely corrective actions as required.

Getting the most from your cutting fluids

Cutting fluids, especially water soluble coolants are sometimes seen as potential problem products due to the amount of variables involved in formulating and monitoring the product in use. However, by following the suggestions provided above, businesses can help enhance overall efficiency and maximise productivity.

For more information about ExxonMobil’s range of metal working fluids or other Mobil-branded lubricants and services, please contact the ExxonMobil Lubricants Technical Help Desk on: or Tel.:+420 221 456 426, or visit:

Peter BirdIndustrial Marketing, Europe, Africa and Middle East, ExxonMobil Lubricants and Specialties

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