Concept of MQL-machining
A machining method called MQL (Minimum Quantity Lubrication) machining or semi-dry machining can be put to practical uses to improve environmental conditions and reduce manufacturing costs.
By supplying the minimum required quantity of coolant to the cutting edge in the form of a mist, tool life can be increased when machining steels. MQL only offers lubrication and it has little or no effect in terms of cooling the cutting edge. Additionally the MQL method has the same chip disposal properties as air-blow. When MQL machining, it is necessary to select cutting conditions under which heat is not likely to be generated. It is also advisable to reduce the contact area and time between the cutting edge and the workpiece.
Mist coolant generator
In order to carry out MQL machining, a special mist coolant generator is needed.
The generator does not have any mechanical moving parts and the generation and carrying of mist coolant is made possible by the use of compressed air. The ultra fine particle mist coolant is supplied directly to the cutting edge. The amount of coolant consumed per component differs according to the machining method and the workpiece properties, but the quantities are very small and usually between 3ml / h～25ml / h.
Mist coolant is changed into ultra-fine particles by mixing a vegetable oil with high pressure air, the resultant mixture is harmless to humans the environment. The mist particle diameter is roughly between 0.7μm and 1.0μm.
When high-speed machining with drill the particles are not influenced by the rotary motion of the spindle and the cutting tool. Additionally, the uniform, stable flow prevents condensation between the particles.
Ideal mist particles
Due to uneven particles (sometimes a maximum particle diameter of 70μm～80μm is possible) poor dispersion and an unstable spray condition creates a poor wetting property of the tool and the workpiece surface.
Owing to the ability to generate a spray of uniform ultra-fine particles (particle diameter of 0.6μm～1.0μm, optimum of 0.7μm), a high wetting property can be maintained and a create an efficient spray with no waste.
IMist coolant used for drilling
The image above shows the condition of mist coolant particles. Finer mist coolant particles ensure a larger area of lubrication and improved wetting properties. It also makes it easier for the coolant to permeate the cutting edge shearing plane and the workpiece. This resulting film generates a fluid friction phenomenon.
Solid friction occurs between the drill and the workpiece and can lead to increasing welding, cutting resistance and heat generation and can also accelerated tool wear and a deterioration of machining accuracy.
Whereas, fluid friction is a phenomenon in which a film of coolant is generated between the cutting edge and the workpiece. The film prevents welding and reduces cutting resistance. This leads to a reduction of heat and can extend tool life and machining accuracy. The key point to MQL machining is to ensure that the particles are delivered in an ultra fine state to the actual cutting edge.
Drilling using MQL
The image shows a machining example of a mild steel component. A 7×D hole (L=126mm) is being drilled using an φ18mm brazed drill with through coolant holes. A total of 182 holes were machined, total machined length of 23m. At this point, the drill's wear condition was normal and further machining was possible.