A TUTORIAL ON CUTTING FLUIDS IN MACHINING

Introduction to Cutting Fluids
Cutting Fluid Application Strategies
Cutting Fluid Effects in Machining
Cutting Fluid Selection Criteria
Cutting Fluid Maintenance and Disposal


Picture Citation: All images used in this tutorial are from "Manufacturing Engineering" magazine


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 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.


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Cutting Fluid Application Strategies:


The principal methods of cutting fluid application include:
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Cutting Fluid Effects in Machining


The primary functions of cutting fluids in machining are : Secondary functions include: Process effects of using cutting fluids in machining include:
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Cutting Fluid Selection Criteria


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


Click here for more information on fluid waste disposal


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