Selection and use of cooling lubricants for stone sawing

I. Mechanism of Action of Cooling‑Lubricants

Cooling‑lubricants significantly improve stone‑cutting conditions. They are mainly composed of emulsifiers and base oils, and are also referred to simply as lubricants. Since these substances generally form an emulsion after being dissolved in water, they are sometimes called emulsion flushing fluids. The working mechanism of cooling‑lubricants in stone cutting is rather complex; they act in different ways on different targets and involve both physical and chemical phenomena.

1. Mechanism of Action on Diamond

(1) Reducing the temperature at the diamond‑rock sawing interface

A cooling‑lubricant is a kind of surfactant. It adsorbs onto the surfaces of diamond and rock to form a lubricating film, which improves lubrication at the diamond interface and lowers frictional resistance. This minimizes heat generated during sawing, reduces the interface temperature, and slows the oxidation or graphitization of diamond, helping to preserve diamond hardness and strength. Laboratory tests show that a 0.2 % cooling‑lubricant solution reduces the friction coefficient at the diamond‑rock interface by more than 60 % compared with plain water. In addition, the oriented adsorption of cooling‑lubricant molecules on diamond and rock surfaces creates a lubricating film that further cuts frictional resistance. Adsorption strength and lubricating‑film integrity depend on the molecular structure and adsorption groups of the cooling‑lubricant.

(2) Accelerating heat diffusion at the diamond‑rock sawing interface

Diamond is oleophilic and hydrophobic. The aqueous cooling‑lubricant has a low surface tension of only 32.5 × 10⁻³ N/cm, less than 50 % of that of water. It wets diamond effectively. The low surface tension greatly raises the rate of heat diffusion at the interface, shortens the duration of thermal exposure of diamond, mitigates diamond oxidation and graphitization, improves the working environment of diamond, reduces its wear, and therefore extends diamond service life.

2. Mechanism of Action on the Metal Bond

Diamond segments (tool tips) are manufactured by mixing and sintering diamond with various powdered metal bond materials. After sintering, the powder matrix retains the diamond grains and forms the tool body. During sawing, continuous wear of the metal bond (matrix) exposes fresh diamond grains and sustains the cutting process. The wear rate of the bond is a key factor affecting saw‑blade service life.

(1) Cooling‑lubricant reduces frictional resistance at the bond‑rock interface

The cooling‑lubricant adsorbs not only on diamond but also on the metal bond surface. It improves lubrication at the bond‑rock interface and reduces frictional resistance, slowing bond wear. Laboratory tests indicate that with a 0.2 % cooling‑lubricant solution, rock‑induced bond wear falls to one‑tenth of the value obtained with plain water. This creates favourable conditions for the bond to protect diamond grains.

(2) Cooling‑lubricant accelerates the sedimentation and removal of stone powder in the coolant

Cooling‑lubricants contain a certain amount of polymeric precipitant. It rapidly flocculates stone‑dust particles into larger agglomerates that settle quickly. As a result, the concentration of stone powder in the coolant drops sharply, which reduces matrix wear during coolant circulation, keeps the bond and diamond wearing at a matched rate, prevents premature diamond pull‑out, and prolongs tool‑tip life. Measurements show that for quartz particles of 20 μm diameter, 1 g of particles can adsorb 0.1 mg of cooling‑lubricant molecules, while quartz particles below 5 μm adsorb more than 0.8 mg. The rock dust generated in stone cutting is flushed away by the coolant in good time, decreasing repeated crushing of rock fines.

3. Mechanism for Reducing Saw‑Blade Body Deflection

During sawing, severe deflection of the saw blade may occur, raising equipment current and making cutting impossible. There are two main causes.

The first is deformation of the saw‑blade body, which leads to deflection and an excessively wide kerf; this can be corrected by straightening the blade body.

The second is friction between the saw‑blade body and the rock walls on both sides of the kerf. In this case the kerf width remains roughly normal; the machine runs normally with a normal current during shallow cutting, but the feed depth or traverse speed of the saw blade must be reduced when cutting deeper.

For the second condition, cooling‑lubricants provide an effective solution. The cooling‑lubricant undergoes physicochemical adsorption at the sawing interface (between blade body and rock kerf wall) and forms a continuous lubricating film. Without lubricant, the friction coefficient can exceed 0.38; when a 0.2 % cooling‑lubricant solution is used, the friction coefficient falls immediately to approximately 0.1. That means the frictional resistance between the blade body and the kerf rock wall can be reduced by more than 70 %. Lower sawing resistance improves the loading condition of the saw blade, suppresses deflection and vibration of the blade body, and extends saw‑blade service life.

II. Composition of Cooling‑Lubricants

A complete cooling‑lubricant for stone cutting consists of emulsifier, lubricating oil and precipitant. In some applications only emulsifier and precipitant, or lubricating oil and precipitant, are used. Each of the three components has many varieties with different performance characteristics.

(1) Emulsifier

Emulsifiers are primarily composed of surfactants. A surfactant molecule contains an oleophilic group and a hydrophilic group. Even a small addition can greatly lower the surface tension of the solvent (usually water) or the liquid‑solid interfacial tension and modify the interface state of the system. According to the structure of the hydrophilic group and the ionic behaviour when dissolved in water, cooling‑lubricants are divided into four categories:

‑ Anionic cooling‑lubricants: sodium alkyl carboxylates, sodium alkyl sulfonates, sodium alkyl sulfates, sodium alkyl phosphates.

‑ Cationic cooling‑lubricants: secondary amine salts, tertiary amine salts, quaternary ammonium salts.

‑ Amphoteric cooling‑lubricants: amino‑acid type, betaine type.

‑ Non‑ionic cooling‑lubricants: polyethylene‑glycol type, polyol type.

The emulsifier mainly provides lubrication and cooling.

(2) Lubricating Oil

Lubricating oils fall into three broad groups. The first group comprises animal and vegetable oils. The second is mineral oils, mainly petroleum‑derived products. The third is synthetic oils.

Mineral oils are the most widely used because of abundant supply, low cost, broad applicability and good stability. Animal and vegetable oils contain large amounts of stearic acid, exhibit poor chemical stability and limited availability, so they are used less. Synthetic oils ar