CUTTING FLUIDS IN MACHINING
Introduction to Cutting Fluids
Cutting fluids are used in metal machining for a variety of reasons such as improving tool life, reducing workpiece thermal deformation, improving surface finish and flushing away chips from the cutting zone. Practically all cutt ing fluids presently in use fall into one of four categories: - Straight oils
- Soluble oils
- Semisynthetic 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 contains 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 concent ration = 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 esentially 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 sol uble oil fluids.
Cutting Fluid Application Strategies:
The principal methods of cutting fluid application include: - Flood Application of Fluid:
a flood of cutting fluid is applied on the workpiece
- Jet Application of Fluid:
a jet of cutting fluid is applied on the workpiece directed at the cutting zone
- Mist Application of Fluid:
cutting fluid is atomised by a jet of air and the mist is directed at the cutting zone
a flood of cutting fluid is applied on the workpiece
a jet of cutting fluid is applied on the workpiece directed at the cutting zone
cutting fluid is atomised by a jet of air and the mist is directed at the cutting zone
Cutting Fluid Effects in Machining
The primary functions of cutting fluids in machining are :- Lubricating the cutting process primarily at low cutting speeds
- Cooling the workpiece primarily at high cutting speeds
- Flushing chips away from the cutting zone
- Corossion protection of the machined surface
- enabling part handling by cooling the hot surface
- Longer Tool Life
- Reduced Thermal Deformation of Workpiece
- Better Surface Finish (in some applications)
- Ease of Chip and Swarf handling
Cutting Fluid Selection Criteria:
The principal criteria for selection of a cutting fluid for a given machining operation are:- Process performance :
- Heat transfer performance
- Lubrication performance
- Chip flushing
- Fluid mist generation
- Fluid carry-off in chips
- Corrosion inhibition
- Fluid stability (for emulsions)
- Cost Performance
- Environmental Performance
- Health Hazard Performance
Cutting Fluid Maintenance and Disposal:
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 f luid. 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:
The cutting fluid within a coolant system degrades with time due to bacterial growth and contamination with tramp oil and fine metal swarf from the machining operation. When it becomes uneconomical to maintain the fluid by regular make-up operations it is dumped. Prior to letting the fluid flow into a sewer system, it should be treated to bring the fluid composition to safe disposal levels.
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