Cutting fluid is a medium with both cooling and lubrication effects applied to the tool-workpiece contact interface during metal cutting. It has four functions: take away heat, lubricate the cutting edge, wash away chips, and inhibit corrosion. Cutting fluids are mainly divided into four types: straight oils, soluble/emulsion oils, semi-synthetics and synthetics. Each type is suitable for specific materials and processing procedures.
Why Is It So Complicated to Choose Cutting Fluid?
The cost of using the wrong cutting fluid is not only the increase of cost, but also the blade damage, the decrease of surface quality and the decrease of spindle start-up rate. Turning free-cutting brass on a chucker is almost opposite to the requirement of cutting fluid chemistry on a sliding-headstock machine with a gun-drilling titanium alloy implant.
Due to the mutual restriction of heat dissipation, lubricity and chip removal, selecting cutting fluids in metal cutting needs to be considered comprehensively. This article will introduce several variables that directly determine the tool life.
The Role of Cutting Fluid in Metal Cutting
During the cutting process, the plastic deformation of the shear zone and the friction of the tool rake face will convert the cutting energy into a highly concentrated local heat. If not controlled, these heats will soften the tool, accelerate flank wear, and may cause deformation of parts processed according to small tolerances.
According to the summary of ScienceDirect materials science entries, metalworking fluid has three core functions: lubrication, cooling, and conveying chips out of the cutting zone. The fourth function, corrosion control, protects both the finished product and the machine guide rail.
Between cooling and lubrication: water with high specific heat capacity can efficiently take away heat; oil can lubricate sliding chips and inhibit built-up edges, but the heat dissipation rate is much slower than water. Each type of cutting fluid is a different solution to this trade-off.
Comparison of Four Types of Cutting Fluid
The market is mainly divided into four categories, the difference is mainly reflected in the oil content and the way of use:
- Pure oil-based cutting oil (straight/neat oils) does not contain water, and mineral oil or vegetable oil is used as the base oil. Its lubricity and rust resistance are the strongest, but its cooling capacity is the weakest.
- Soluble/emulsion oils are mineral oil-rich concentrates that need to be mixed with water in a sump to form an emulsion. This emulsion is a general choice with balanced performance, economical and practical.
- The semi-synthetics concentrate has a lower oil content and is enhanced by polymers, providing a compromise between cooling and lubrication.
- Synthetics are completely free of mineral oil and are prepared from chemical additives. It has the best cooling capacity and tank life, but the boundary lubrication performance is weak.
| Тип течност | Typical in-use concentration | Охлаждане | Смазване | Най-подходящи приложения |
|---|---|---|---|---|
| Straight (neat) oil | Run neat – undiluted | ниско | Най-висока | Swiss lathes, screw machines, threading, gun-drilling, titanium/medical parts |
| Soluble (emulsion) oil | ~5 – 10% concentrate in water | добър | добър | General turning and milling, heavy roughing, aluminum, tapping |
| Полусинтетичен | ~4–8% concentrate in water | Високо | Умерена | Mixed job-shop work spanning mills and lathes |
| Синтетичен | ~3–8% concentrate in water | Най-висока | Спуснете | High-speed milling, grinding, carbide tooling, long sump life |
In the table, the concentration after typical dilution is used; pure oil cutting fluid is used directly. The specific concentration should be based on each supplier’s data sheet, and the concentration should be controlled using a refractometer.
How to Match the Cutting Fluid According to the Material and Process?
The choice of cutting fluid mainly depends on two factors: what kind of material to cut and what kind of cutting method to use.
Match by material
Steel is suitable for the use of mineral oil added with lubricant. Aluminum parts need strong cooling capacity, and cutting fluid without active sulfur should be selected. Active sulfur will stain the surface of aluminum parts. Stainless steel and titanium alloy are difficult to process. Because of their low thermal conductivity, the heat is easy to concentrate in the cutting area.
Therefore, many workshops use high-pressure water-based coolant with a concentration of 8-10%, and sometimes the heat is quickly taken away by the through-tool cooling method at a pressure of 40-70 bar. In contrast, cast iron and most brass are often machined dry.
Matching according to the tool and process
Cemented carbide is easy to heat up and is susceptible to thermal shock. It is advisable to choose a fully synthetic cutting fluid with strong cooling capacity.
High-speed steel is suitable for using emulsified oil or semi-synthetic type. Cooling is the first step in high speed turning and milling. While tapping and broaching are carried out under low speed and high pressure conditions, extreme-pressure lubrication should be preferred.
Why Swiss-type Lathe Prefers Pure Cutting Oil?
On Swiss type lathes with an enclosed, sliding headstock, the workshop experience is almost the same: neat oil is used instead of water-based coolant in most cases. The reason is related to the machine tool structure. The guide bushing and micron-level tolerances require that the cutting fluid has better lubricity and can penetrate into the tiny gap between the tool and the workpiece.
It is also critical that the water-based cutting fluid, trapped inside the enclosed cavity of a Swiss-type lathe, tends to cause corrosion of internal components and, with prolonged use, may even attack the sensitive subspindle windings.
The heat dissipation capacity of oil is far less than that of water, so the feeds and cutting speeds often need to be reduced; oil can also produce mist and increase the risk of fire. In the closed machine tool of unattended cutting titanium alloy, this is a hidden danger that needs to be paid attention to.
Plant-based or soybean-based cutting oils, with flash points higher than that of mineral oil, are less prone to producing smoke and catching fire, making them a common countermeasure. For the most demanding machining tasks involving stainless steel and titanium alloys, some workshops would rather risk corrosion and switch to high-pressure water-based coolants, purely to exploit their superior heat dissipation.
If you’re considering which model is best for your part, JIANKE’s CNC Swiss Type Lathe vs Conventional CNC Lathe links model selection to materials, tolerances and batch sizes – the same variables that influence cutting fluid decisions.
Oil Mist, Concentration & Tank Maintenance
Choosing cutting fluid is not limited to the moment of opening the barrel. There are two numbers that should be written on the wall of each workshop.
First, the exposure limit. - United States NIOSH standard document (Pub.98-102) recommended that the exposure of metalworking fluid aerosols (aerosol) should be limited to 0.4 mg/m3 (inhalable particulate matter in the chest), according to the time-weighted average (TWA) of 10-hour working days, due to long-term oil mist exposure associated with respiratory diseases. Oil mist collection is not an option on high-load machine tools.
Second, concentration. Water-based cutting fluid is usually used as 3-10% concentrate. Below about 3%, lubrication and anti-rust protection will collapse; higher than about 12%, the concentrated solution is wasted and a heavier residual film is left on the part. Since the water-based liquid will also breed microorganisms, it is a standard practice to maintain the liquid tank from deterioration by using a рефрактометър to detect and cooperate with the biocide scheme every week.
Both of them have low thermal conductivity, so they are dominated by heat dissipation management. Many workshops use 8-10% concentration of water-soluble cutting fluid, often in the form of through-tool coolant to take away the cutting zone heat. If the machine tool design is more suitable for oil, the high flash point plant-based cutting oil is a common alternative to titanium processing.
The water-based cutting fluid is tested with a refractometer at least once a week, and the high-yield machine tool is tested daily. The concentration is maintained in the range of 3-10%: too dilute will lose rust and lubrication protection, too concentrated will waste concentrated liquid and leave residual film. Detection should be combined with a fungicide regimen to control microbial growth.
There may be. The oil mist produced by pure oil can be ignited in closed machine tools, especially when cutting titanium alloys or when unattended. Mitigation measures include oil mist collection, fire extinguishing systems, and high-flash plant-based or soybean-based oils that are less likely to smoke and fire than mineral oils.
Usually. Dry cutting is often used for cast iron and most brass, because its chips are not easy to adhere and heat accumulation is controllable. Dry cutting also completely eliminates tank maintenance and oil mist. Instead, dust extraction has become a top priority.
Заключение
There is no unique optimal cutting fluid, only the right match. Starting from the balance between cooling and lubrication, the material, tool and process are superimposed. Swiss type lathe and thread machine usually point to pure oil, in order to lubricity and anti-rust; high-speed mass production on difficult-to-machine alloys often points to water-based coolants for heat dissipation. No matter what is poured into the machine tool, it is necessary to control the concentration and oil mist.
Choosing a machine tool, not just a cutting fluid. Your cutting fluid strategy and machine tool selection are two aspects of the same problem. If you want to process small, complex, strict tolerance parts, and want to make the model fit their own materials and batch, welcome to consult our engineers on the appropriate CNC lathe and walking machine solutions.
