Grinding Fluids: Types, Applications & Best Practices

I. Requirements for grinding fluids

Grinding fluids are mainly used to reduce grinding temperature, improve machining surface quality, increase grinding efficiency, extend wheel life, and avoid workpiece burning, cracking, and thermal deformation. From the perspective of improving grinding effects, grinding fluids should serve the following functions:

1. Cooling effect

The cooling effect of grinding fluids mainly removes grinding heat from the grinding zone, reducing the grinding temperature.

2. Lubrication effect

Grinding fluids can penetrate between abrasive grains and the workpiece surface, adhering to the metal surface to form a lubricating film, reducing friction between abrasive grains and the workpiece surface, improving the cutting performance of the wheel to obtain a smaller surface roughness value.

3. Cleaning and chip flushing effect

Grinding fluids have high fluidity and can promptly wash away chips and fallen abrasive grains through the chip removal groove.

4. Rust prevention effect

Adding rust inhibitors to grinding fluids can form a protective film on the metal surface, protecting the workpiece and machine from rusting for a certain period.

In addition to the above effects, grinding fluids are also required to be non-toxic, odorless, non-irritating to the skin, non-corrosive, chemically stable, resistant to spoilage and deterioration, not prone to foaming, easy to treat and recycle waste fluids, and avoid environmental pollution.

Additives are commonly used to improve the performance of grinding fluids, including oiliness additives, extreme pressure additives, rust inhibitors, antifoam additives, and emulsifiers.

Some common fluid supply methods include: pouring method, high-pressure cooling method, internal cooling supply method, ultrasonic supply method, and wheel impregnation method.

II. Types and applications of grinding fluids

There are many types of grinding fluids, generally divided into two main categories: water-soluble grinding fluids and oil-soluble grinding fluids. Water-soluble grinding fluids can be further divided into: emulsions, transparent aqueous solutions, and electrolyte aqueous solutions.

The main component of water-soluble grinding fluids is water, with other additives added. They have excellent cooling effects, are easy to prepare, cost-effective, and less likely to cause pollution.

The main components of oil-soluble grinding fluids are mostly mineral oils. Common mineral oils are low or medium viscosity mineral oils with rust inhibitors added. For example, fatty acids are added to machine oil, light diesel oil, or kerosene to enhance lubrication.

Additionally, extreme pressure additives containing elements such as sulfur, chlorine, and phosphorus are added to grinding fluids to form extreme pressure oils, which have better penetration and lubrication abilities, suitable for processes requiring small surface roughness values.

Oil-soluble grinding fluids have good adhesion, can isolate air, and prevent undesirable chemical reactions such as oxidation and hydrolysis in the grinding zone. For example, CBN wheels are prone to react with water at high temperatures, so oil-soluble grinding fluids should be used for high-speed grinding with CBN wheels.

Oil-soluble grinding fluids are used to increase grinding speed and rate, particularly suitable for CBN wheel grinding. Oil-soluble grinding fluids made from residual mineral oils are typically used as coolants and lubricants, which age quickly under hot conditions. With proper wheel quality selection, oil-soluble grinding fluids can adapt to most grinding operations.

Synthetic oil-soluble grinding fluids have good characteristics and aging stability, with higher flash points at low viscosities, suitable for high-productivity grinding processes. Compared to conventional oil-soluble grinding fluids, the cost of synthetic grinding fluids is about doubled.

1. Advantages of oil-soluble grinding fluids

• Due to good lubrication performance, they require less power.
• Double the grinding rate at the same power.
• Less wheel wear, extended dressing intervals.
• High surface quality.
• Can be kept clean regularly with proper machine treatment.
• Essentially maintenance-free.
• Does not contain biocides, not susceptible to bacterial or fungal invasion.

2. Disadvantages of oil-soluble grinding fluids

• Flammable and explosive (if mixed with air in volume fractions between 0.6% and 7%).
• Machines must be completely enclosed.
• Must have oil mist extraction devices and explosion-proof doors.
• Must be equipped with carbon dioxide fire extinguishing systems.
• Due to low heat dissipation, double the amount of coolant-lubricant is required.
• Seals, pipes, and cables must be oil-resistant.

Table 1 provides a performance comparison of various grinding fluids; Table 2 shows the composition, properties, and usage instructions of common grinding fluids for reference when selecting.

Table 1 Performance comparison of various grinding fluids

Table 2 Composition, performance and instructions for common grinding fluids

III. Solid Grinding Agents

In addition to the above grinding fluids, solid grinding agents are sometimes used as supplements for grinding difficult-to-machine materials. Solid grinding agents mainly serve as fillers for grinding wheels. For example, using cryolite as a filler for grinding wheels produces good results when grinding stainless steel. Graphite, tungsten disulfide, and molybdenum disulfide can also be used as solid grinding agents.

For instance, when grinding tool steels with high alloy content, grinding becomes more difficult, especially dry grinding, where the workpiece is prone to burns and cracks. To address this, a mixture of molybdenum disulfide and alcohol can be used to soak the grinding wheel before dry grinding. This can improve the surface quality of the machined part and increase grinding efficiency by 2 times. Generally, the soaking process is simple, low-cost, and easy to promote.

Molybdenum disulfide can also be used as an additive in cutting to enhance the lubricating ability of grinding fluids. It can be added to machine oil or kerosene at a mass fraction of 10% molybdenum disulfide and 4% to 5% neutral soap.

IV. Correct Use of Grinding Fluids

• Grinding fluid should be poured directly onto the contact area between the grinding wheel and the workpiece.
• The flow of grinding fluid should be sufficient and evenly sprayed across the entire grinding width of the wheel to achieve a cooling effect.
• Grinding fluid should be injected into the grinding area with adequate pressure to achieve good cleaning action and prevent grinding debris from clogging the wheel surface in the grinding area.
• Properly configure splash guards to prevent grinding fluid from splashing out of the grinding machine.
• Maintain a certain fluid level in the cutting fluid tank.
• Grinding fluid should be kept clean regularly, minimizing the content of grinding debris and abrasive particles in the cutting fluid. Deteriorated cutting fluid should be replaced promptly. For ultra-precision grinding, specialized filtration devices can be used.
• Ensure smooth flow of grinding fluid and prevent blockages in the fluid passages caused by grinding debris. Remove any clogged debris promptly.
• Do not introduce other foreign objects into the fluid tank.
• In summer, pay special attention to prevent emulsion from corroding workpieces and machine table surfaces. The concentration of emulsion can be higher.
• Prevent grinding fluid from splashing into eyes, and especially avoid ingestion or inhalation of sodium nitrite from the cutting fluid. Take care to protect your health.
• Cultivate environmental awareness.

V. Methods of Supplying Grinding Fluid

During grinding, grinding fluid often has difficulty entering the grinding zone. The main reason is that when the grinding wheel rotates, it also produces a rotating airflow around the wheel. The higher the speed, the greater the airflow influence. Moreover, the obstruction of the wheel guard increases the dynamic pressure of the airflow.

1) The general method of supplying grinding fluid is by pouring. To improve the effectiveness of the pouring method, the following measures are often taken:

• Position the nozzle as close as possible to the grinding zone.
• Install a wheel guard or airflow deflectors on the nozzle.
• Increase the flow rate of grinding fluid. Generally, the flow rate should not be less than 0.5L/min per 1mm width. For high-speed grinding, as the wheel speed increases, the supply should also increase accordingly, and should not be less than 2-3L/min.
• Wheel guards often reduce the effectiveness of fluid supply, so the nozzle position should be as far away from the guard opening as possible, or the guard structure should be improved.

2) During high-speed grinding, the airflow around the wheel also increases. To improve fluid supply effectiveness, the following measures are often taken:

• Increase the grinding fluid supply volume and spray pressure.
• Reduce the contact area of the nozzle outlet while appropriately increasing the outlet width. This can improve spray speed and cooling range, and also reduce fluid splashing caused by high-speed airflow.
• Adjust the spray direction.

VI. Filtering Methods for Grinding Fluids

Grinding fluids need to be kept clean regularly. Impurities such as chips and loose abrasive particles produced during the grinding process cause the grinding fluid to deteriorate, not only affecting workpiece quality and scratching the machined surface but also impacting environmental hygiene.

Especially for grinding that requires high precision and low surface roughness, the requirements for filtering and purifying grinding fluids are becoming increasingly stringent. Common filtering and purification devices include metal mesh sedimentation filters, centrifugal filters, magnetic filters, paper filters, and cyclone separators. Various grinding fluid purification devices are shown in Table 3.

Table 3 Various Grinding Fluid Purification Devices

Filter Type		Cyclone Separator	Magnetic Filter	Paper Filter	Centrifugal Filter	Metal Mesh Sedimentation Filter
Purification Performance	Purification Capacity/(L/min)	12.5～100	25～100	<25	<50
	Purification Rate (%)	95	Agitated 99, Not agitated 87
	Particle Size/μm	3～40	<74	2～10		>100
Economic Effect	Power Consumption/kW	0.6～1.1	0.12	0.12	0.17
	Cutting Fluid Usage Duration	Long	Relatively Long	Long	Short	Shortest
	Filter Material	No Consumption		High Consumption
	Equipment Cost	Relatively Low	Relatively High	Relatively High	Relatively High	Low
Floor Space		No Space Required	No Space Required	Relatively Little	Relatively Little	Relatively Little
Noise		None	Slight	None	Relatively High	None
Possibility of Filtering Non-magnetic Impurities		Possible	Not possible	Possible	Possible
Possibility of filtration when the medium viscosity is extremely high		Not possible	Possible	Possible	Possible
Continuity		Possible	Possible	Not possible	Clean once per shift	Clean once per shift
Structural complexity		Simple	Complex	Relatively complex	Complex
Flow loss		Relatively large	Relatively large	Large	Relatively small

VII. Problems encountered in the use of grinding fluids and their solutions

During the use of grinding fluids, expected results are often not achieved or unexpected problems occur, such as high surface roughness values, short wheel life, low processing efficiency, etc.; there are also management issues with grinding fluids, such as corrosion of grinding fluids, rust on workpieces and machines, etc.; there are also safety and health issues, such as operators developing dermatitis. When these problems occur, refer to Table 4 to analyze the causes and take corresponding measures.

Table 4 Problems encountered in the use of grinding fluids and corresponding measures