In machining operations, heat energy plays a very important role. Because of heat energy, various unwanted effects are observed on tool and work piece. Cryogenic cooling is applied for lubricating as well as for cooling the work piece while performing machining process. The quality of machined products can be observed with respect to how near they are with finishing and its properties for the reliability, performance and life. We also need to take into consideration the tool wear, quality of surface finish and dimensional accuracy.
It is clear that the cutting temperature, especially when it is a little high, is very detrimental for cutting tools and work piece which is going to be machined. It needs to be controlled by reducing it as far as possible without losing the quality of the product and productivity.
While machining, friction is generated because of which large amount of heat is evolved. This needs to be controlled and metal chips need to be removed to provide proper shape, size and good surface finish to the work piece. Coolants fulfill two important functions - lubrication and cooling. The cooling effect removes heat energy from the tool, work piece and chips. It also increases tool life
Purpose of coolants
During machining operation, huge amount of heat is liberated due to the friction in between tool and work piece, where energy is lost. Air can also be used as a coolant but for very light cuts and low duty cycle. But during production, heavy cutting operation is done for over a long period of time. So for doing production in a continuous way, the tool has to be cooled using liquid coolant to remove heat more rapidly and help doing speed cutting and lower the friction and tool wear.
Rather than cooling, cutting fluid has also been used as lubricant in the interface between cheap and cutting edge of tool. Lubrication helps chips to not be welded on to the tool due to the heat generated during machining.
Delivery method of coolants
There are many preferable methods of applying cutting fluids, which majorly depends upon the application and the availability of equipment. In a metal cutting operation, usually the coolant is being pumped by high pressure, with high volume directly to the chip-tool interface, with valves being surrounded by the machine to contain the splatter and some to catch filter and recirculation of fluid. With advancement of technology, flooding paradigm is decreasing day-by-day. And now, the new practice is done by permutation of liquid, aerosol and gas delivery. For example, minimum quantity lubrication through the tool tip cryogenic cooling.
Through tool coolant system or through-spindle coolant system, is used to deliver coolant with the help of passages present in the spindle and through the tool, directly to the cutting interface. High-pressure coolant systems are also used in which operating pressure is being obtained between hundreds to several thousand psi pressure.
Types of coolants
Usually, there are four types of liquid coolants – straight oil, synthetic oil, soluble oil and semi-synthetic oil, which are combined with the best property of water by suspending emulsified oil and water base. The major properties of liquid coolant are rust prevention, ability to work with many metals and thermal resistivity.
Straight oils are those oils which are non emuslifiable and are used in machining operation where they function in an undiluted form. Generally, its composition is base mineral or petroleum oil. It also contains polar lubricants, like vegetable oil, fats and easters. They may also contain additives, like sulphur, chlorine and phosphorous.
It doesn’t have mineral oil base or petroleum. In place of it, they are formulated from alkaline organic and inorganic compounds along with additives to prevent corrosion. It gives best results in diluted form. It believes that synthetic fluid gives best cooling performance out of different cutting fluids.
Soluble oil usually forms an emulsion after being mixed with water. The resulting concentration contains emulsion and a base mineral oil to give a suitable emulsion. It gives good performance while present in diluted form and provides good lubrication in addition to heat transfer performance. Soluble oils are cheap and mostly used fluid in industry.
When soluble oils and synthetic fluids are mixed, semi-synthetic fluid is obtained. Besides, the heat transfer performance and cost of semi-synthetic fluid fall between those of the soluble and synthetic fluid.
In some of the applications, cutting fluid can also be taken in the form of paste or gel, particularly in operations, such as, drilling and tapping. In sawing metal with a band saw, one has to periodically run a stick of paste against the blade. It is usually similar to lipstick or beeswax present.
Aerosols can be defined as a cutting fluid in which air with tiny droplets of liquid is scattered throughout. The delivery of aerosols is done directly through the flutes of the tool. Aerosols delivery is being done with respect to location and timing, which is almost like dry machining from the operator’s point of view. The chips which are generated are very close to dry machine chips, which requires no draining.
Carbon-dioxide is used as a coolant in which application is done through pressurised liquid. CO2 is an alloy to expand, which helps in dropping the temperature, which is enough to change the phase into solid. These crystals are then spread on to the cutting zone with the help of external nozzle or through the spindle delivery to give temperature control cooling of the cutting tool and work piece.
Health and safety
For the past decade, due to increasing concern over safety and health hazards in the workplace, a number of approaches have been put forward in response. These approaches come from government agencies holding a congressional mandate for these activities.
To those in the cutting fluid and lubricant production areas, for more than 40 per cent of all reported occupational diseases, it comes as no surprise that of all occupational injuries and illnesses, skin disease and disorders account. Use of cutting fluid causes illness and injury to workers; it can also cause cardio vascular disease, external skin diseases or breathing problems when it enters the mouth or through the nose.
Prevention can be achieved by the following practices:
Concentration of fluids at use should not be excessively irritating
Fluids should not exceed recommended concentration levels
Hypersensitive operators should be prescreened or transferred
Fluids should be physically cleaned — fines and tramp oil removed
Suitable barrier cream should be provided
Protective gloves should be provided
Mildly abrasive cornmeal soap with low pH should be provided
The work force should be educated about the value of good personal hygiene
Properties of coolant
The main purpose of a coolant is to provide cooling effect on grinding wheel and work piece at the time of machining process and also to lubricate the work piece surface. Cooling of the tool and the work piece is very important to terminate the unnecessary effects of heat on the work piece, while tool also cures the work piece from corrosion.
Most important properties of coolants are as follows:
For the better lubrication and cooling purpose
It prevents from corrosion of work piece surface
It is helpful for chemical, physical and technical stability while in use
No excessive forming
Recycling and disposal of cutting & lubricants
The recycling of metalworking fluids is a topic that has commanded increasing interest in recent years, spurred by the impact of more stringent local, state, and federal environmental regulations and the increasing cost of oil. Today, manufacturers are facing an increasingly complex fluid recycling and disposal problem. The following is a list of definitions associated with the topic of oil disposal and recycling:
Used oil: Oil whose original characteristics have changed during use whether or not, has thereby, been rendered unsuitable for further use for the purpose for which it was intended
Waste oil: Oil which through use, storage, or handling has become unsuitable for its original purpose due to the presence of impurities or loss of original properties and which is not considered recyclable
In-service oil: Used oil that is suitable for further use for the purpose for which the original oil was intended
Oily waste: A mixture of oil with substances, such as, water, dirt and/or other materials
Recycling: The generic term for re-refining, reclaiming or reprocessing used oil for reuse. The product is recycled oil
Getting a 360° view on coolants
While the purpose of coolants is well-understood, keeping in mind the possible health hazards and delivery method is important. Also, knowing which coolant is suitable for the job can assist in enhancing one’s shop’s cost-effectiveness, proficiency, and complete machining performance.
Courtesy: International Journal of Advance Engineering and Research Development