How to use coolant and cutting fluid when turning

The main functions of cutting fluid are chip evacuation, cooling and lubrication between the tool and the workpiece material. If applied correctly, it will maximize production, increase process security and improve tool performance and component quality.

In some cases, machining without coolant (dry machining) can have environmental and cost advantages. If you machine dry, contact your Sandvik Coromant expert to select the best tool, geometry and grade.

Many applications require coolant to meet tolerance, surface and machinability factors. If coolant is required, it should be optimized to maximize its true potential.

Several aspects of coolant are important to the cutting process:

Cooling medium

Coolant outlet

Coolant pressure

Cooling medium

There are many different cooling media used when turning:

Emulsion, a mixture of water and oil (5-10% oil in water) is the most common cooling medium

Oil, used in some machines instead of emulsion

Compressed air, used for chip removal, but does not carry away heat very well

MQL - Minimum Quantity Lubrication - compressed air with a minimum amount of oil for lubrication

Cryogenic coolants, using liquefied gas as coolant to maximize cooling effect

Emulsions, oils and air can be applied through the coolant channels in the turning tool. When referring to coolants in general, we mean cooling with emulsions or oils. MQL and cryogenic coolants require special equipment.

Coolant outlet

Most modern turning tools are equipped with internal coolant, and many of them actually offer a precision combination of top and bottom coolant. The outlets in the tool can be of the following types, bringing different benefits to your machining:

• Precision coolant or precision coolant, nozzles (or similar devices) direct the coolant jet to the cutting zone on the front side of the tool. Reduces temperature and improves chip control. Can be used at high pressure to improve chip breaking

  
  

  Coolant jet below the coolant, located on the side, effectively removes heat from the insert, thereby extending tool life

  
  

  Traditional coolant outlets, such as adjustable nozzles, in most cases have a larger outlet diameter than precision coolant nozzles. Designed to flow coolant over the insert and component during machining (can be called flood coolant). These tools are not suitable for use at high pressure

Conventional Coolant vs. Precision Coolant

Precision Coolant

Modern turning tools are equipped with nozzles that direct precision coolant precisely to the cutting zone on the rake side, thus controlling chip breaking and providing secure machining. To optimize machine performance and further improve tool life and chip formation, coolant delivery and speed can be fine-tuned by varying the nozzle diameter.

The positive effect of precision coolant begins at low coolant pressures, but higher pressures allow more demanding materials to be successfully machined.

With precision coolant, you get better chip control, longer tool life, greater process security and increased productivity.

Without precision coolant, chip jamming can become a problem, leading to machine downtime, service calls, increased tool wear and poor surface finish.

Lower coolant

Most modern turning concepts also feature bottom coolant. Bottom coolant controls the heat in the cutting zone, which results in longer tool life and predictable machining.

At lower coolant pressures, the coolant is already very efficient, but at higher pressures we can see a greater impact on tool life extension. Cutting speeds or feeds can be increased to increase production.

Above or below the coolant? Or both?

If using a tool with both top coolant (precision coolant) and bottom coolant, turning off top coolant may be beneficial in certain operations. This depends greatly on the workpiece material, grade, and cutting data you are machining.

For thin coated grades, such as the preferred PVD grades of ISO S, it is best to use both top and bottom coolant to protect the insert from heat and avoid plastic deformation.

Thick coated grades, such as the preferred CVD grades of ISO P and ISO K, have good thermal protection in the coating. These grades achieve the best tool life with bottom coolant only in roughing to medium machining applications. See the blueprint below and the description for ISO P.

For medium coated grades, such as the preferred CVD grades of ISO M, it is recommended to use both top and bottom coolant. However, if crater wear is present in the application, try using bottom coolant only and compare tool life.

Coolant recommendations for steel turning

• Use bottom coolant for longer tool life

  
  

  Use top coolant (and bottom coolant) where improved chip control is needed, usually within the blue depth of cut ( a p ) and feed ( f n ) zones

  
  

  Outside the blue zone, too much coolant may cause slight edge wear and increased crater wear. Crater wear can be difficult to assess, which means unpredictability and reduced tool life. That’s why bottom coolant is recommended. (If bottom coolant is not available, use a tool with conventional coolant outlets)

Advantages of above and below coolant for different materials

Coolant pressure

High-pressure coolants increase energy consumption, which needs to be considered from a sustainability and cost perspective. But high pressure can also increase productivity in different ways.

High pressure precision coolant

High pressure in the machine, combined with the nozzle, creates a high-velocity coolant jet that forms a hydraulic wedge. The coolant jet has three main effects:

Provides more effective cooling of the insert in the contact zone (A)

Quickly forces chips away from the insert face, reducing insert wear (B)

Helps break chips into smaller pieces and removes chips from the cutting area

Use the correct pressure

7–10 bar (100-150 psi)

Precision coolants improve chip control and process security in steel and other common materials. Thanks to their high accuracy, you can increase cutting data while maintaining process security.

70–80 bar (1000-1200 psi)

With higher pressures, chip breaking can also be achieved. You can achieve even better results by using geometries designed for precision coolants.

150–200 bar (2200–2900 psi)

For more demanding materials, such as duplex stainless steel and HRSA materials, higher pressures are required. Use toolholders with nozzles for precision coolant supply and geometries designed for precision coolants.