Cooling agents in a cutting process
Keeping cool in tough times is important!!!
Temperature rise is a global crisis. World is in the face of danger of Global Warming. Machining process is also the victim of rising temperature. No need to worry though, coolants are there to keep things under control.
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Cutting is one of the machining process which involves a sharp single point tool making a precision cut on the workpiece. The workpiece can be stationary or in constant rotatory motion. The tool is slowly and gradually brought in contact with the workpiece. Let us consider turning operation, the workpiece is rotating at a high r.p.m. and the tool is approaching the surface of the workpiece. The contact needs to be slow and in tiny increments. If the contact happens rapidly, the tool and workpiece will get damaged along with the machine. Hence, the tool is brought in contact slowly. Now when the tool is in contact with the workpiece and the the cutting happens, there is a lot of heat generated. Where does this heat come from though? The answer is simple. In this machining process, there is frictional heat generation when the workpiece and tool are in contact. Higher the r.p.m. will be, higher will be the rise in temperature.
But does this increase in temperature affect our machining process?
Any change in temperature during the machining process will affect the accuracy and precision of the cut. A rise in temperature will cause temporary deformation in the workpiece i.e. the metal will expand. The geometry of the cut will change when the metal workpiece comes to normal temperature. Along with this, there are some material and chemical changes which happens with the increase in temperature which affect the process.
How can this be prevented?
There are different sources for temperature. The ambient temperature or as we call it the room temperature is a factor, this can be prevented by creating a control environment. Another source is the machine vibrations or heat from motor. This can be prevented by lubricating the machine parts and improve machine stability. And lastly the one having higher impact, frictional heat generation due to tool & workpiece contact. To reduce or minimize the impact of this rise in temperature COOLANT is used. Today we will take a look at what are the different types of coolants one can use in a machining process and how does it work?
What are coolants?
Coolant is a part of cutting fluid which is quite instrumental in the various machining processes such as grinding, milling or turning. A coolant is generally used in combination with lubrication, together they form cutting fluid. They are beneficial in improving the life of the cutting tool as well as provide an improved surface finish for the workpiece. A coolant is composed of many different chemicals substances. These substances have heat absorbing and/or conducting properties.
In the process of machining, the area of machining is flooded with the coolant substance. The added benefit to using coolant is that it washes off the chips and particles away from the area of machining. The coolant in liquid form is pumped and supplied at the work area via nozzles, there is sump collected at the bottom which can be recirculated to the work area.
Composition of coolants
The liquid type of coolants are normally categorized as mineral, semi-synthetic and synthetic. Emulsified oil suspended in water is combined with the best qualities of oil to form synthetic or semi synthetic coolant. The qualities that qualify for the coolant are rust inhibition, tolerance of a wide range of water hardness, able to function with many metals, tolerate thermal breakdown and environmental safety.
Aerosol or mist is uses the same liquid coolant but the application method is different. The coolant is sprayed in for of tiny droplets. The challenge with using this type of coolant is that the mist is sprayed everywhere, not just the tool. The workers handling the machine are exposed to the coolant spray. Breathing the chemical tainted air can be hazard to health.
Liquid form of CO2 is created using pressure and is allowed to expand and this causes the temperature to decline. The crystalized CO2 are applied to the cutting area via nozzles to provide a control over the temperature on the cutting tool and workpiece.
Pastes or Gel:
One of the forms in which the coolants is used is paste or gel based on the applications, particularly in the hand operations such as drilling and tapping. When using a handsaw or hacksaw, it is a common practice to run a stick of paste against the blade. The product is similar to a glue-stick or lipstick in applying.
Using plane water was a common practice, but, the disadvantage to using water is that it causes corrosion.
As we discussed earlier, coolant is a combination of various chemicals. The effectiveness of the coolant is determined by the proportion in which these chemicals are used. But, this proportion or concentration varies based on the type of cutting operation and material involved. One of the common issues is having a low concentration of coolant. The low concentration causes below impact:
Machine and workpiece corrosion
Reduction in tool life
On the other hand, if the concentration of coolant is too high, this results in:
Lesser heat transfer
Formation of residue that shortens tool life
Staining of machine and machined parts
Toxicity (skin irritation)
It is important to keep constant checks on the pH and concentration level of the coolant. The suspended particles in the coolant solution should always be at acceptable level. The concentration of coolant reduces due to various factors such as evaporation, splashing, misting and dragouts.
In the early to mid 1900, machining operations were done excessively and industries were facing the problem if wear out of tools and machines. To manage the heating issue, it was a common practice to use water to get rid of the heat. In case of HSS, carbonated water or 'coke' was often used as coolant and lubrication.
Large amount of frictional heat is generated when the cutting tool and the workpiece are brought into contact with each other.
This heat could cause damage to the machine, tool and workpiece, and as well result inaccuracy in the cutting process.
Coolants, in combination with lubricants are used to get rid of or minimize the impact of this heating.
During a machining operation, the coolant is supplied directly at the area of machining by the means of nozzles and pumps.
Regenerative use of coolants is a fairly common practice. When doing so, it is important to keep track of the chemical concentrations in the coolant. Low or high concentration has significant impact on the final product.
There are continuous developments on the chemical composition of the coolants to make them more efficient and lubricative. This is all for now. Stay cool and follow Mech n Flow, also check out Mech n Fun for more cool stuff.