When using a lubricating cutting fluid, such as cutting oil, it should be applied to the friction surface in order to form an oil film. On the other hand, if a cooling cutting fluid, such as a water-based cutting fluid, is used, it should be directed towards the cutting edge of the tool.
The cutting fluid is typically delivered to the cutting area through the pressure method, helping to remove heat generated by friction and deformation of the tool, workpiece, and chip. Continuous application of cutting fluid is more effective than intermittent application.
Intermittent application of cutting fluid can result in thermal cycling, leading to cracking and chipping of hard and brittle tool materials, such as carbide tools. It also shortens tool life and causes the workpiece surface to become rough and uneven.
Correct use of cutting fluid effectively removes chips, extending tool life. Proper placement of the cutting fluid nozzle prevents blockage or chipping of the milling cutter or drill bit’s chip discharge groove.
For large workpieces or heavy cutting and grinding, multiple rows of coolant nozzles may be used to ensure full cooling, improving processing efficiency and quality.
1.Cutting fluid cooling – Manual refueling
Solid or pasty lubricants can be applied or dripped onto the tool or workpiece using a brush pen or brush, mainly for tapping threads or die sleeve threads.
A portable liquid dispenser has also been developed, which atomizes the lubricant through pressurization and sprays it onto tools and workpieces.
For machines without a cooling system, manual refueling is an effective solution if the number of holes or taps is not extensive.
When two different processes are to be performed on the same machine, manual refueling can be used in combination with the overflow cooling system on the machine.
2.Cutting fluid cooling – Overflow method
The overflow method is the most commonly used method for utilizing cutting fluid. In this method, a low-pressure pump drives the cutting fluid into a pipeline and it flows out of a nozzle near the cutting area through a valve. After passing through the cutting area, the cutting fluid is directed to various parts of the machine tool and then collected in an oil collecting pan. From there, it is recycled back to the cutting fluid tank.
To ensure proper cooling and settling of chips and abrasive particles, the cutting fluid tank must have sufficient volume. The volume of the cutting fluid tank varies depending on the type of processing, with a range of 20-200 liters for normal processing and larger volumes (500-1000 liters or more) for drilling deep holes and strong grinding.
A coarse filter is placed in the oil collecting pan to prevent large cuttings from entering the cutting fluid tank, and a fine filter is installed at the suction port of the pump. For high-quality surface processing, such as grinding, refining, deep hole drilling, and deep hole boring, finer filtering equipment is necessary to remove grinding debris, grinding wheel particles, and cutting particles. For example, in gun drilling deep hole processing, a 10-micron filter paper may be used for filtering.
Filtration equipment helps to keep the cutting fluid clean and extends its lifespan by avoiding excessive contamination or metal particles. Modern automatic machine tools often have cutting fluid filtration, separation, and purification devices.
The overflow method provides a continuous flow of cutting fluid to the cutting area and flushes away chips. The flow of cutting fluid must be sufficient to submerge the tool and workpiece in the cutting fluid and prevent abnormal temperature rises. If the cutting fluid tank is too small in deep hole drilling, the cutting fluid temperature can quickly rise and cutting must stop when the temperature exceeds 60°C. As a result, deep-hole drilling machines often have large cooling oil tanks.
The placement of the nozzle affects the efficiency of the cutting fluid. It should be placed in a position where the cutting fluid will not be thrown away from the tool or workpiece due to centrifugal force. It’s best to use two or more nozzles, one for supplying cutting fluid to the cutting area and the other for assisting in cooling and flushing chips.
For turning and boring, the cutting fluid must be directed directly to the cutting area to provide adequate cooling. A second nozzle may be necessary for heavy-duty turning and boring to supply cutting fluid along the underside of the tool. For drilling horizontally and reaming holes, it’s best to send the cutting fluid to the cutting area through the inner hole of the hollow tool to ensure that the cutting edge has enough cutting fluid and to punch chips out of the hole. Only hollow bits can solve the problem of insufficient cutting fluid in the cutting area, as the spiral groove of a drill bit (for discharging chips) removes cutting fluid from the cutting area, even in vertical drilling.
3.Cutting fluid cooling– High pressure
For some processing, such as deep hole drilling and sleeve drilling, a high-pressure (0.69-13.79 MPa) cutting fluid system is commonly used for oil supply.
In deep hole drilling, a single-edged drill bit is used, similar to boring, with a path for the cutting fluid inside the drill bit. Sleeve drilling is a method of drilling a cylindrical hole in a workpiece while leaving a solid cylinder. As the tool enters the workpiece, the drilled solid cylinder passes through the hollow cylindrical cutter head, and a pressure pump sends cutting fluid around the tool, forcing chips to flow out from the center of the tool.
For sleeve drilling, the cutting fluid must have good extreme pressure and sintering resistance, and have a low viscosity to flow freely around the tool. It should also have good oiliness to reduce the friction coefficient between the tool and workpiece and between the tool and chips.
The main challenge in deep hole drilling is maintaining adequate cutting fluid flow in the cutting area. One solution is to use the cutting flute as a path for the cutting fluid, with a pressure of 0.35-0.69 MPa flowing into the drill bit through the rotating sealing sleeve and directly into the cutting area. The cutting fluid flowing out of the hole helps remove the chips. Compared to the overflow method, the use of oil hole drilling in deep hole drilling has greatly improved the drill bit service life and productivity.
The high-pressure method helps the cutting fluid reach the cutting area, and is sometimes used on other machine tools. For example, in grinding, the high-pressure nozzle facilitates cleaning of the grinding wheel.
4.Cutting fluid cooling– Spraying
Cutting fluid can also be applied to tools and workpieces in the form of an oil mist spray. The cutting fluid is dispersed into small droplets by a small nozzle and compressed air with a pressure of 0.069-0.552 MPa and sprayed into the cutting area. In this method, water-based cutting fluid is preferred over oil-based cutting fluid as the oil mist from oil-based cutting fluid can pollute the environment, be harmful to health, and contain larger oil droplets.
The spray method is best suited for machining with high cutting speeds and low cutting areas, such as end milling. The cutting fluid with good cooling performance is used and the small droplets contact the hot cutting tool, workpiece, or chips, evaporating quickly to remove the heat. Spray cooling does not require splash plates, oil pans, or oil return pipes and uses only a small ball. The workpiece remains dry, even with a little oil, as it is easy to dry.
The spray method has several advantages, including:
- Longer tool life compared to dry cutting;
- Can provide a cooling effect when there is no or an inadequate overflow system;
- The cutting fluid can reach places that are not accessible by other methods;
- The velocity of the cutting fluid between the workpiece and the tool is higher than the overflow method, resulting in a much higher cooling efficiency when calculated based on the same volume of cutting fluid;
- Potential cost reduction under certain conditions;
- Improved visibility of the workpiece being cut.
However, the spray method also has limitations, including limited cooling capacity and the need for ventilation.
There are three types of spraying devices:
- Attractive type:
This type operates on the same principle as a household sprayer, using the thin waist tube principle. Compressed air draws the cutting fluid out of the tank, mixing and atomizing it in the air stream. It has a tube for compressed air and another tube for drawing cutting fluid, connected at a mixing joint. This type is suitable for spraying low-viscosity cutting oil and emulsions.
- Air pressure type (pressurization method):
In this type, the cutting fluid is stored in a sealed liquid cylinder and pressurized with 0.2-0.4 MPa compressed air. When the solenoid valve is opened, the cutting fluid is pressed out, mixed with the compressed air stream, and atomized through the mixing valve. This device is suitable for spraying water-based synthetic liquids and emulsions, but the aqueous solution and emulsion must not contain fatty oils or suspended solids. The atomization mixing ratio can be adjusted by the mixing valve and pressure regulating valve.
- Spray type:
This type uses a gear pump to pressurize the cutting fluid and sprays it directly into the compressed air stream through the mixing valve to atomize it. This device is suitable for atomizing transparent cooling water and low-viscosity cutting oil. Spraying can be applied to end milling, turning, automatic machine processing, and CNC machine processing. The spray device with solenoid valve control is suitable for tapping threads and reaming holes on CNC machine tools.
5.Cutting fluid cooling– Cooling liquid method
There are various methods of cooling liquids that can be used, such as compressing nitrogen, argon, carbon dioxide, and other gases into a liquid and storing them in a cylinder. Freon gas can also be compressed into a liquid by mechanical devices and released for use. The cooling liquid is regulated by a valve and directly injected into the cutting area through a nozzle, cooling the tools, workpieces, and chips through gasification and heat absorption.
This method is highly effective for cooling and is suitable for cutting difficult-to-machine materials, such as stainless steel, heat-resistant steel, and high-strength alloy steel. It can significantly improve tool durability.
6.Cutting fluid cooling– Centralized supply system for cutting fluid
For large and medium-sized machinery processing plants, it is ideal to use a centralized circulation system to supply cutting fluid to multiple machine tools, as long as all the machines are using the same cutting fluid. This system can be linked to a conveyor system to handle abrasive debris from several grinders.
Centralized processing of fine chips and abrasives moistened by cutting fluid can improve working conditions and reduce manual processing. The centralized cutting fluid supply system also makes it easier for the factory to maintain the cutting fluid. The cutting fluid is stored in a large pool, and regular sampling inspections are conducted to replenish the original fluid or water according to the inspection results. This makes it easier to control the concentration of the cutting fluid and reduces the number of sampling inspections, allowing for more items to be checked to ensure the quality of the cutting fluid during use.
Compared to separate multi-cutting fluid supply systems, the maintenance cost of the cutting fluid is reduced, leading to a relatively lower cost. The main advantage of the centralized supply system is its ability to effectively remove the oil slick and metal particles in the cutting fluid through centrifugation.
At the same time, it can reduce the growth of bacteria in the cutting fluid by removing half of them. Regular quality inspections and the systematic use of additives or stock solutions based on these inspections are important factors that extend the life of the cutting fluid in a centralized system. This also reduces the waste disposal of water-soluble cutting fluids.