Cutting Fluids in Machining Operation

Cutting fluid is a type of coolant and lubricant designed specifically for metalworking and machining processes. Most metalworking and machining processes can benefit from the use of cutting fluid, depending on work piece material. There are some exceptions to it like in machining cast iron or brass, which are machined dry. Also, interrupted cuts, such as milling with carbide cutters, are usually recommended to be used dry due to damage to the cutters caused by thermoshock. Major functions of Cutting fluids are:

1. improving tool life;
2. reducing workpiece thermal deformation;
3. Lubricating the cutting process primarily at low cutting speeds
4. Cooling the work piece primarily at high cutting speeds
5. improving surface finish and flushing away chips from the cutting zone.
6. Corrosion protection of the machined surface

The cutting fluids can be categorized as:
• Straight oils
• Soluble oils
• Semi synthetic fluids
• Synthetic fluids

Straight oils are non – emulsifiable and are used in machining operations in an undiluted form. They are composed of a base mineral or petroleum oil and often contain polar lubricants such as fats, vegetable oils and esters as well as extreme pressure additives such as Chlorine, Sulphur and Phosphorus. Straight oils provide the best lubrication and the poorest cooling characteristics among cutting fluids.

Synthetic Fluids contain no petroleum or mineral oil base and instead are formulated from alkaline inorganic and organic compounds along with additives for corrosion inhibition. They are generally used in a diluted form (usual concentration = 3 to 10%). Synthetic fluids often provide the best cooling performance among all cutting fluids.

Soluble Oil fluids form an emulsion when mixed with water. The concentrate consists of a base mineral oil and emulsifiers to help produce a stable emulsion. They are used in a diluted form (usual concentration = 3 to 10%) and provide good lubrication and heat transfer performance. They are widely used in industry and are the least expensive among all cutting fluids.

Semi-synthetic fluids are essentially combination of synthetic and soluble oil fluids and have characteristics common to both types. The cost and heat transfer performance of semi-synthetic fluids lie between those of synthetic and soluble oil fluids.

Various methods of applying cutting fluid are: flooding, spraying, dripping, misting, brushing.


Cutting Fluid Selection Criteria:

The principal criteria for selection of a cutting fluid for a given machining operation are:

• Process performance:
    o Heat transfer performance o Lubrication performance
    o Chip flushing o Fluid mist generation
    o Fluid carry-off in chips o Corrosion inhibition
                   o Fluid stability (for emulsions)
• Cost Performance
• Environmental Performance
• Health Hazard Performance


Cutting fluid maintenance involves checking the concentration of soluble oil emulsions (using refractometers), pH (using a pH meter), the quantity of tramp oil (hydraulic oil leaking into the cutting fluid system) and the quantity of particulates in the fluid. Action taken to maintain the fluid includes adding make-up concentrate or water, skimming of tramp oil, adding biocides to prevent bacterial growth and filtering the particulates by centrifuging.


Note: An Emulsion is a mixture of two or more immiscible liquids. In an emulsion, one liquid is dispersed in the other. The dispersed phase is dispersed in continuous phase.Examples of emulsion : milk and cutting fluid for metal working.