How to apply coolant and cutting fluid in turning

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

In some cases it may be an environmental and cost perspective benefit to machine without coolant (dry machining). Contact your Sandvik Coromant specialist to choose the best tool, geometry, and grade if you go for dry machining.

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

There are different aspects of coolant that is important for the cutting process:

• Coolant media
• Coolant outlet
• Coolant pressure

Coolant media

There are a number of different coolant medias used when turning:

• Emulsion, a mix of water and oil (5-10% oil in the water) is the most common coolant media
• Oil, in some machines oil is used instead of emulsion
• Compressed air, used for chip evacuation but does not take away heat in a good way
• MQL - minimum quantity lubrication - compressed air with a minimum quantity of oil for lubrication
• Cryogenic coolant, a liquefied gas is used as coolant to maximize the cooling effect

Emulsion, oil and air can be applied through the coolant channels in the turning tools. When mentioning coolant in general terms, we mean cooling with emulsion or oil. MQL and Cryogenic coolant requires special equipment.

Coolant outlet

Most modern turning tools are equipped with internal coolant through the tool, many of these actually offer the combination of precision over coolant and under coolant. The outlets in the tool can be of the following types giving different benefits to your machining:

• Precision coolant, or precision over coolant, a nozzle (or similar) directs a coolant beam directly towards the cutting zone on the rake side. Reduces temperature and improves chip control. Can be used with high pressure to improve chip breaking
• Under coolant, a coolant beam on the flank side, that effectively take away heat from the insert which gives longer tool life
• Conventional coolant outlet, for example, adjustable nozzles that in most cases have a bigger outlet diameter than the precision coolant nozzles. Meant to flow coolant over the insert and component during machining (may be referred to as flood coolant). These tools are not meant to be used with high pressure

Conventional coolant versus precision coolant

Precision coolant

Modern turning tools feature nozzles that deliver precision coolant directed exactly to the cutting zone on the rake side, which controls the chip breaking and offers secure machining. To optimize the machine capabilities and further improve tool life and chip formation, coolant delivery and velocity can be fine-tuned by changing the nozzle diameter.

The positive effects of precision coolant start at low coolant pressure, but the higher the pressure is, the more demanding material can successfully be machined.

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

Without precision coolant chip jamming may be a problem, causing machine stoppages, service call outs, increased tool wear and poor surface finish.

Under coolant

The most modern turning concepts are also featured with under coolant. The under coolant controls the heat in the cutting zone which leads to improved tool life and predictable machining.

Under coolant is very efficient already at low coolant pressure, but the higher the pressure is, the bigger effect we can see in tool life increase. It’s possible to increase the cutting speed or the feed to improve the output.

Over or under coolant? Or both?

If using a tool that feature over (precision coolant) and under coolant, turning off the over coolant can be beneficial in certain operations. It depends a lot on which workpiece material, which grade and what cutting data you machine in.

For thin coated grades, like first choice PVD-grades for ISO S, it is best to use both over and under coolant to protect the insert from heat and avoid plastic deformation.

Thick coated grades, like first choice CVD-grades for ISO P and ISO K, have a good heat protection in the coating. These grades may in roughing to medium applications get the best tool life with under coolant only. See the blue diagram and the explanation for ISO P below.

For medium coated grades, like first choice CVD-grades for ISO M, it is recommended to use both over and under coolant. However, if crater wear occur in the application, try to use only under coolant and compare the tool life.

Read more about insert wear.

Coolant recommendations for steel turning

• Apply under coolant for longer tool life
  
• Use over coolant (and under coolant) where you need improved chip control, normally needed within the blue cutting depth (a_p) and feed (f_n) area
• Outside the blue area, over coolant might cause minor edge wear and increased crater wear. The crater wear might be difficult to evaluate, which means unpredictable and shortened tool life. That is why under coolant is recommended. (If under coolant is not available, use a tool with conventional coolant outlet)

Benefits with over and under coolant in different materials

Coolant pressure

High pressure coolant increases the energy consumption which need to be considered from a sustainability and cost point of view. But high pressure can also increase the productivity in different ways.

Precision coolant with high pressure

High pressure in the machine together with a nozzle creates a high velocity coolant jet which, in turn, creates a hydraulic wedge. The coolant jet has three main effects:

1. To provide more efficient cooling of the insert in the contact zone (A)
2. To quickly force the chip away from the insert face, reducing wear on the insert (B)
3. To help break the chip into smaller pieces and evacuate the chips from the cutting area

Use the right pressure

7–10 bar (100-150 psi)

Precision coolant give improved chip control and better process security in steel and other common materials. Thanks to the precision, you can increase cutting data with maintained process security.

70–80 bar (1000-1200 psi)

With higher pressure, you can also achieve chip breaking. By using geometries designed for precision coolant, you get even better results.

150–200 bar (2200–2900 psi)

For demanding material, such as duplex stainless steel and HRSA material, higher pressure is needed. Use tool holders with nozzles for precision coolant supply and geometries dedicated for precision coolant.