Among manufacturing industries, metal-cutting plays a major role, as practically, all functioning parts will require some form of finishing in order to work properly during an operation. Developmental work on metal-cutting technologies began from the beginning of the industrial revolution that has contributed to mass production. As technologies in various sectors have been progressing, the same is happening with metal-cutting technologies as well.
Conventional metal-cutting operations
In the machining process, cutting tools remove material from the work piece by means of shear deformation to get the desired geometry. Though the actual material removal is by plastic deformation, friction energy accounts for a large amount (20-30%), which is detrimental to the productivity. Since friction is a major impediment for the productivity, efforts need to be made to reduce and also provide conductive conditions in the machining zone to improve the productivity. Also, most of the plastic deformation energy is converted into heat that needs to be reduced in order to improve the life of the cutting tool, while also improve the machined surface quality & integrity. A number of approaches have been tried in order to achieve better results.
Providing a hard, anti-friction, chemically inert and thermal isolating layer of coating on the cutting tool that will reduce the friction during the cutting processes. It is noticed that with suitable coatings, tool lives have been increased upwards by 200%. The coatings are generally achieved by the CVD and PVD processes.
The cutting tools are provided with micro surface textures that will be able to break the contact surface between the cutting tool and chip so that the friction & adhesion will be reduced
Use of cutting fluids in the Minimum Quantity of Lubricant (MQL) form as micro droplets, so that sustainable operations could be achieved
There is also the use of vegetable oils with nano particles of solid lubricants during the MQL operation, so as to achieve near dry machining
Unconventional metal-cutting operations
To overcome the problems of high hardness during the conventional machining operations, unconventional metalcutting operations, such as Electric Discharge Machining (EDM), Electrochemical Machining (ECM), Laser Assisted Machining (LAM) were developed that utilise different methods to achieve the metal removal process. Some of these processes have now become mainstream, such as EDM, ECM and their offshoots.
In EDM of AISI D2 die steel, the material removal rate can be enhanced by adding silicon powder with the dielectric fluid called powder mixed EDM (PEDM). The MRR in powder mixed EDM is significantly affected by the parameters, peak current and the concentration of silicon powder. It was also noted that surface generated is generally good.
Another variant of EDM is ultrasonic assisted cryogenically cooled copper electrode EDM (UACEDM). It was found that while machining M2 high speed steel, it significantly reduced Tool Wear Rate (TWR) and Surface Roughness (SR) compared to the conventional EDM process.
Wire EDM is one of the processes used to separate the metal parts produced from the Additive Manufacturing (AM) machine from the build plate. GF produced a horizontal axis wire EDM machine for the purpose. In this machine, the build plate will be mounted upside down. Since it is not ergonomically convenient, the user mounts in normally on the table horizontally. After mounting, the machine rotates the table to make it upside down. Because of the horizontal axis, as the cutting is complete, the parts just fall into the bottom of the tank.
Difficult to work materials (e.g. titanium, nickel alloys)
Machining hard materials, and that with lower machinability, is a challenge for the manufacturing personnel. Titanium & its alloys, nickel & its alloys and tool steels & stainless steels form a part of this group. It is, therefore, necessary to establish special procedures depending upon the material characteristics, as generic methods do not work well with these materials.
Titanium alloys should be machined with a rigid high torque spindle with through the spindle coolant to avoid built-up edge. The right processes for titanium machining are using lower cutting speeds and high feeds, removing chips as soon as possible to eliminate the chip recutting, using a generous quantity of the right cutting fluid to prevent the build-up of temperature and keep the flutes clean & clear of debris and reducing the radial engagement of a tool.
The use of cryogenic cooling with liquid nitrogen & liquid carbon dioxide has been found useful in many titanium machining applications
Electric Discharge Machining (EDM) was also tried for machining titanium alloys with significant improvement in machining performance, with better surface finish & with fewer defects under ideal conditions. Low discharge energy is preferred. The copper electrodes are preferred as they provide higher Material Removal Rate (MRR) and lower Electrode Wear Rate (EWR) compared to brass & aluminium electrodes.
For Ni-based super alloys, conventional machining methods are generally not very effective in view of the poor machinability of these alloys similar to titanium alloys. Hence, non-traditional methods, such as EDM and AWJM, are the preferred methods. However, when conventional methods are used, the use of proper coatings on the cutting tools helps. For example, it was shown that AlTiN/Cu coating showed a good functionality in view of its selflubricating behaviour. Cemented carbide tools need to be used at lower cutting speeds in the range of 30 m/min to 70 m/min because of their poor thermochemical stability along with high feeds due to their high toughness.
Cutting tools and tool materials
A majority of the cutting materials these days are based on the cemented carbides. However, with the finishing requirements of the components being higher, a number of new materials are also becoming available with improved performance. For example, Cubic Boron Nitride (CBN) finish-turning inserts have three times the tool life that runs at three times faster compared to cemented carbide, providing a productivity that is nine times compared to cemented carbide in machining Ti6AL-4V, while the cost is only about two to three times. In view of better chip control and with relatively less tool wear, it will be an ideal candidate for finish turning operations.
Cutting tool technology is advancing so well that through-tool coolant is now available for a majority of operations
Self-adjusting cutting tools are now a reality. They use in-process measurement (such as, touch trigger probes) to provide a closed-loop feedback to control the cutting process parameters for certain critical machining operations, such as boring, such that, out of tolerance parts are practically eliminated.
Solid carbide gundrills are available for machining tough materials, such as titanium (Ti6Al-4V) and 440 stainless steel for medical machining. The gundrills have the tip and body in a single piece of carbide that is brazed to the driver/shank with the flute ground down the length of the head and body. It is also provided with the high-pressure coolant delivery capabilities, with coolant delivery holes.
The nano onions infused cutting fluid is being used that provides the performance gains during difficult metal-cutting operations. The nano onions are the 10 nm size carbon particles carried in an oil- or water-based solution that flushes heat from the cutting zone, reduces the friction at the intersection of the cutting-tool & workpiece and polishes surfaces.
AM has been used to manufacture special bodies of the cemented carbide milling cutters with higher strength and lower weight. In addition, these tools are also provided with built-in vibration dampers to allow for chatter-free machining at high cutting speeds, thereby, improving productivity. Sandvik CoroMill 390 utilises AM produced titanium body cutter that has half the weight of a comparable steel body cutter.
Powder metallurgy is also being used to produce cutting tools with complex geometries to near-net shape, which is not possible when conventional manufacturing methods are used.
As one can see that there are large amounts of developments taking place in metal-cutting technologies, practically in all aspects, to make them simplified as well as efficient while also considering the environment. Manufacturing industries, therefore, should embrace them continuously to reap the benefits.