Fabric cutting is the second irreversible process of garment manufacturing where a fabric layer is converted into cut parts. Multiple layers of fabric are cut either by reciprocating knife action, unidirectional knife action or guillotine action. The cutting principle in reciprocating and unidirectional knife mechanism is friction where sharp edge of the blade rubs against the fabric layer in perpendicular direction and the principle in guillotine mechanism is chopping, where the sharp edge of the blade cuts through the fabric layer by pressure in parallel direction. The shearing principle of cutting is followed in a pair of scissors and not used in layer cutting. Although reciprocating knife mechanism results in vibration in knife, thereby fraying of raw edges, unidirectional knife has no vibration, gives clean cut look and is more effective. Chopping action also gives clean cut appeal and is most effective. Cutting can happen either by a moving knife and stationery fabric layer or stationery knife and moving fabric layer. Interestingly, while the basic medium of cutting blade remains metal, laser, water-jet or ceramic are hardly used.
The most universally used machine for multiple layer cutting is a straight knife which works on the principle of a vertically reciprocating knife, while the round knife works on rotating blade principle. Both the machines are manually manoeuvred through a fabric layer laid-out horizontally. Initially the cutting principles for straight knife and round knife machine were based on that of friction; however to increase the cutting efficiency for specific types of fabric, the sharpness of blade was modified to incorporate the principle of chopping. Use of a wavy blade in a straight knife and use of octagonal blade in a round knife are examples of the same. Features like variable speed, blade cooling, variable cutting stroke, blades with different bevel angle were incorporated to increase the cutting efficiency and accuracy of the cutters. However, such features remain in catalogues and machines with basic minimum features which are popularly sold to keep the cost to the minimum and some of these features are not even present in modern day cutters of Chinese-make.
The major differentiator in the round knife cutter is the direction of the knife’s movement and its speed. The latter is dependent on the density of fabric or the thread count, which will determine the effective speed of the knife. Knit fabric can be cut at an average speed of 2,500 RPM, whereas 14 oz. denim fabric may require 7,500 RPM to give good results. In knit fabrics, lower RPM and faster linear thrust is required, which is absolutely opposite in the case of heavier and dense fabrics. The band knife cutting machine also works on the principle of friction but due to the possibility of using narrow blades and absolutely no possibility of vertical tilt in blade, these machines have one of the most accurate cutters for bulk layer cutting. Unlike straight and round knife, fabric layer needs to be pushed in band knife. Therefore to ease the movement of fabric layer, some improvements have happened over the years. Earlier machines used to have air cushion, whereas nowadays, air-suspended bearings are used to reduce energy consumption and sound pollution while improving the operational efficiency. Die cutting works on the principle of chopping, and therefore is one of the most efficient forms of cutting; however low productivity restricted its popularity to leather industry only. Continuous automatic die cutting addressed the issue of productivity but was only feasible for undergarments and T-shirt manufacturers, because of being restricted to small components and smaller marker sizes.
Computer Numerical Controlled (CNC) Cutting or CAM is the most efficient and accurate, multiple layer cutting machine for small and large marker. CAM also uses vertically reciprocating knife, but the linear thrust is machine-driven unlike in case of straight or round knife. The surface where the fabric layer is spread in CAM is made out of bristles.
Computer numerical controlled cutting or CAM is the most efficient and accurate, multiple layer cutting machine for small and large marker. CAM also uses vertically reciprocating knife, but the linear thrust is machine driven unlike in case of straight or round knife. The surface where the fabric layer is spread is different in CAM, as it is made out of bristles.
The surface is actually a conveyor made of bristle blocks which are anywhere from 2”X2” to 4”X4” in area and depending upon the material’s elasticity, the knife plunges through the bristles which sway sideways to move away from the knife. The development that happened over the years in a bristle conveyor is the size of the bristle block, because often the bristles are cut by knife, especially while cutting curved edges and replacement cost needs to be minimized. While some bristle blocks are changed within 6 months, some last for as long as 18 months.
But clever repositioning of bristle blocks can also minimize the cost of replacement as sometimes more used blocks are moved to the edges and less used blocks from edges are moved to the centre of the bed where most cutting takes place. But, in any case, the user ends up changing the entire bed eventually. The size of the bristle will ultimately determine the cost of replacement. Instead of changing the entire block that may have been cut from one end only, the smaller block gives better options to change one-quarter block.
While vacuum is holding the layers, the knife plunges from the top, cuts through the plastic and makes a contour shape of the pieces and that is where the air starts gushing in, the vacuum loss is through these cut lines and the re-sealer generally covers those cut lines. Traditionally, there are two types of vacuum systems: one is Tub Vacuum and the other is Localized Vacuum that is covering the part of the top surface. The conveyor is a continuous surface that moves from right to left at the top, goes below the machine and after a full circle, comes back to the same position on the top surface. If the entire surface is travelling within a Tub, there is no vacuum loss. But in Localized Vacuum, the conveyor leaves the vacuum area, goes under without vacuum and when it reappears from the other side, it comes in contact with the vacuum area, which is very inefficient as the suction is only at the top bristle conveyor. As the surface turns, the air can go into the sealed layer from both the ends, though a rubber sealer is in-between. The bristles try to stop lateral air flow, but get cut by knife and need to be changed at least four times per year. Loss of air will cause problems such as lower capacity of vacuum hold down, high electricity consumption and high noise levels. Gradually all CAM manufacturers are moving towards Tub Vacuum principle.
Apart from the mechanical cutting functions accomplished by CAM, the equipment is also integrated with other software and hardware systems like CAD and computerized spreading. There are numerous features that are added to CAM over the years such as cut path intelligence wherein a sequence of cut path is automatically calculated by the computer to minimize the dry hauling time of blade and knife bend prevention to prevent the knife from bending beyond a pre-determined limit during cutting and opposite thrust is exercised to keep the knife straight. Re-sealer, to minimize the vacuum loss during cutting, post-print for automatic label printing on the top ply of cut stack for easy identification of cut parts, are some of the important interventions that improve efficiency. The new generation CAM already incorporates M2M technology through embedded electronic sensors which allow the support team to monitor and detect machine behaviour remotely in real time. Technology providers like Lectra have gone on to imbibe this technological leap with their Vector range of cutting machines. It has around 150 sensors, which gauge dynamically the machine parameter and any odd operational behaviour, which is sent to the Customer Care Centre through internet. The data is analyzed to act on critical issues that need immediate attention and further improvement of machine features.
Apart from the development of hardware features, cut order management software solutions have been developed and improved upon to link various stages from marker making to roll management to spreading and cutting. Variables such as material width, roll length and colour can be changed instantaneously throughout the system. With optimized information exchange between sites and stages, companies now have the flexibility to switch to production models quickly and efficiently.
Although the cutting medium predominantly has remained knife, other mediums like water jet and laser were prohibitively expensive in comparison to benefits generated, but are recently gaining popularity due to the reduction in cost and application in niche processes. While water jet is suitable for leather, laser is used for intricate and precise scoop cut and largely for technical textiles. Surprisingly the evolution of developments in straight knife, round knife, band knife and die cutting were limited to mechanical kinematics and even today, there is no use of PLCs or microprocessor in any of these machines. Future developments are likely to happen in that direction. Another area of development is safety features like auto-stop mechanism, i.e. when a body part comes in close proximity to moving blade, the blade automatically stops. Such features will have tremendous value in coming times due to the importance of occupational safety in the whole supply chain.
