Tony Atkins

Food and Food-Cutting Devices and Wire Cutting

This chapter discusses the mechanical properties of various foods eaten by humans and other animals and presents the methods and machinery for cutting foodstuffs consumed by humans and other animals. Mechanical properties of foodstuffs are rate, temperature, and environment dependent. The size and shape of cells, volume of intercellular spaces, and thickness of cells walls all affect the mechanical properties of fruit and vegetable tissue. In addition, the turgor pressure controls, to a large extent, the stiffness of the liquid-filled cells by altering the water potential. One of the tools employed to cut the foodstuffs is delicatessen slicer that cuts through thick diameters of salami or substantial sides of bacon where considerable parts of the circular blade are in contact with the foodstuff as it is fed by hand through the cutter. Machines are often angled to one side so that the slices fall under gravity to a pile. Some material/cutting blade combinations display high friction and cutting is often performed with a fine plain wire in order to reduce the contact area between blade and material, and so reduce cutting forces and obtain a better surface finish. Single crystals and semiconductor wafers are cut by a plain wire in slurry of abrasive material or by wire itself coated with abrasive particles.

Simple Orthogonal Cutting of Floppy, Brittle and Ductile Materials

Publisher Summary This chapter focuses on simple orthogonal cutting of floppy, brittle, and ductile materials. In orthogonal cutting, a straight-edged tool overhangs the workpiece and cutting takes place along the surface. The relative motion of blade and workpiece is perpendicular to the edge of blade: the blade may move over the workpiece. In analyses of orthogonal cutting, there is lot of material ahead of the tool so that the far end of the workpiece where the cut ends has no effect. The deformation patterns of offcut formation are investigated by a variety of techniques: grid or circle patterns scribed or etched either on the outer edge of the workpiece or on the inner surface of a split workpiece. When polymers that stress-whiten are cut, the extent and severity of the deformation is revealed. Deformation zones are delineated by microhardness surveys over the area encompassing the ingoing undeformed workpiece through the deformation chip-forming zone to the completed offcut. Video films enable the deformation to be studied slowly; pattern recognition software enables the changing strains through deformation fields to be evaluated. The manner in which the offcut/chip is removed (by bending or shear), and its state after cutting, depends on material properties, cutting blade geometry, and friction.

Chapter 9 – Sharpness and Bluntness: Absolute or Relative?: Tool Materials and Tool Wear

This chapter provides an introduction to the concept of sharpness. It is particularly important when the cutting edge is always in contact with the newly separated material ahead of the tool. When cutting ductile metals with offcut formation by shear, the tool remains in intimate contact with the workpiece at the point of separation between cut surface and chip. During the transient indentation start to a cut, tool sharpness will concentrate the stresses and strains in the tip region as the load is increased, leading to the build-up of localized material damage. Eventually, at the appropriate load, the hydrostatic stress/effective strain criterion for fracture will be satisfied and a bifurcation in deformation mode occurs, from the plasticity and friction of inclined indentation to the plasticity, friction and separation of cutting. Sharpness of tools is important in guillotining and punching of ductile metal plates, not only in determining load levels but also in controlling the direction of the paths along which separation occurs and hence the quality of the cut edge. Sharpness of tools is particularly important for cutting highly extensible and flexible solids that are very difficult to separate by simple tearing. Whereas a stiff sheet of glass will break at a surface scratch, a sheet of some highly extensible material will not, as its deformation characteristics limit the degree of stress and strain concentration that is achievable.

Chapter 1 – Controlled and Uncontrolled Separation of Parts: Cutting, Scraping and Spreading

This chapter provides an introduction to cutting, scraping, and spreading and discusses the tools employed to accomplish these tasks. A characteristic feature of some of the controlled cutting is that the cutting tool or blade is not deformed when cutting, and remains ready for reuse. The only significant deterioration may be wear and blunting; unless corrected, the quality of the cut deteriorates. An issue that needs to be addressed to ensure effective cutting is how hard a tool should be to avoid itself being deformed in cutting. Mutual cutting is possible where both tool and workpiece deform (bullets and the target). When it is difficult to insert a woodscrew, the high torque will distort the blade of a poor-quality screwdriver and also cut slivers from the side of the slot in the head of the screw. Many tools can be resharpened and used again. Sometimes tools are used once only (disposable scalpels) or thrown away when blunt (disposable razors, or indexable tool inserts). Hollow needles that pierce the skin and through which liquids may be inserted into the body (hypodermics) or removed (cannulae) may sometimes be reused depending on conditions. Improvements in tool material qualities, and reductions in cost, mean that it is often uneconomic to resharpen tools.

Chapter 2 – Fracture Mechanics and Friction: Muscles, Impact and New Surfaces

This chapter covers the basics of stress analysis, fracture mechanics, and ideas on friction and discusses how muscles function, since the force and work necessary for cutting are often supplied by hand or foot when animals attack prey and afterwards chew food in the mouth. In order to determine the mechanical (strength) properties of materials, a body may be deformed by applying known loads to it, for example by hanging on weights to the suspended body. Deformation may be pulling, compressing, twisting, and bending, including combinations of the different ways of loading. The resulting deformations, extension in tension, reduction in height in compression, angle of twist, and rotation in bending, can be measured. Load–deformation behavior depends upon the temperature and the rate at which loading takes place; it also depends upon the environment, water particularly affecting the behavior of biological materials. Notch sensitivity concerns by how much the load-bearing capacity of a body containing a crack or notch is reduced compared with the uncracked body. Employment of fracture mechanics requires knowledge of the crack size. When no crack is obvious, in brittle solids the effective starter crack is related to microstructural features.

Burrowing in Soils, Digging and Ploughing

Soils are assemblies of particles that are stuck together to a greater or lesser extent. The particles are ground-down rock and humus—dark organic matter produced by the decomposition of vegetable and animal matter and essential to the fertility of the earth. Soils also contain air and water. The water may be chemically bound to the soil particles or may be free within the network of particles. The amount of water depends on the weather and changes with the climate. The properties of soils vary markedly from loose particulate behavior (dry sand) to hard, stiff paste-like behavior (clays). In the first case, the behavior is that of a free-flowing medium having interparticle cohesion; in the second it is more akin to plasticity. Shear deformations in sand and silt usually produce a local volume increase so that particles move apart and create increased void space in the microstructure. Water flows into these larger voids from the surrounding soil and, when the rate of loading is slow, there is plenty of time for this to occur. At faster rates of loading, the water pressure in the dilating pores has to drop more rapidly to create sufficient pressure gradients to permit water ingress. The resulting suction in the fluid thereby presses the soil particles more firmly together, thus increasing the resistance to deformation.

Unintentional and Accidental Cutting: Supermarket Plastic Bags, Falling Objects, Ships Hitting Rocks and Aeroplanes Hitting Buildings

Publisher Summary Unintended cutting may concern a deliberate cutting operation that goes wrong, or may start out as something not remotely connected with cutting, such as, holes in socks formed by overgrown toenails, when a ship hits a rock and is holed below the waterline. Many accidents, especially with colliding vehicles, can occur at high rates, and cutting during an impact can be different from quasi-static experience. All accidental cutting do not occur at high speed, but very often happens when control is lost and something slips. A device to absorb energy in steering columns of cars by tube inversion/eversion involves a metal tube being completely rolled up inside or outside. The efficiency of the device is impaired should the tubes crack, and prediction of the number of cracks that forms shows how a transition from plastic flow alone, to plasticity, and fracture, can take place even though one energy sink becomes two. An understanding of the mechanics of fault and accident situations where cutting occurs enables the forces, and energy of deformation, and cutting to be estimated. This enables either a priori risk assessments to made or a posteriori forensic investigations to be conducted.

Punching Holes: Piercing and Perforating, Arms and Armour

Publisher Summary This chapter focuses on a variety of processes where holes are made by different processes. Solid punches driven into the surface of a brittle solid produce craters with the displaced material removed as radially broken-up lumps like slices of a cake. The action of flat-ended solid punches when making holes in floppy or ductile materials is by pushing a plug of material from the near (proximal) surface through to the far (distal) side of the workpiece. Separation of the center portion from the rest of the sheet can either be by scissor-like cutting or by ductile shear in a narrow band around the separated edges. A fid is a heavy-duty needle used in, for example, sailmaking with canvas, or with leather, and adapted to industrial sewing machines. A pin used to pop a balloon or blister probably causes fracture not by cutting but by facilitating, through its sharpness, crack initiation in the stretched membrane at the strain induced in the balloon by the particular inflation pressure. It is possible to sit on an inflated balloon without bursting it.

Chapter 10 – Unrestrained and Restrained Workpieces: Dynamic Cutting

Publisher Summary The cutting forces, during all cutting, have to be reacted in some way. It is possible for an initially restrained workpiece to become less restrained as cutting proceeds, for example, the stiff skin of an orange can be scraped or cut to get the zest, and eventually the pith at the interface between skin and fruit is reached, after which the surface of the fruit deflects and gives a bow wave that is difficult to cut with a controlled-depth device. A lot depends on the flexibility of the workpiece. Some materials have large Youngs moduli and others have low. The forces for cutting by hand are sometimes generated by impact with a hammer on the tool. Wave effects may become important, as in the way a bricklayer can divide off part of a brick from another by striking the brick with the edge of a trowel. Comb cutters are devices such as hedge cutters, hair trimmers, electric sheep shears, and combine harvesters that have reciprocating blades in the shape of overlapping zig-zags, the triangular gaps between every pair of inclined blades opening and closing every stroke. As the device is fed into the material to be cut, material is gathered into the open V-shaped recesses, and cut by the sideways movement.

Chapter 5 – Slice–Push Ratio: Oblique Cutting and Curved Blades, Scissors, Guillotining and Drilling

Publisher Summary Metal-cutting tools often have two cutting edges, both of which are angled to the direction of cutting, and in round-nosed tools the inclination continuously varies. In orthogonal cutting, the cutting edge is always at right angles across the workpiece. When a straight blade is angled to the direction of motion of the workpiece, it is called oblique cutting. The slice–push ratio that is given by blade displacement, or velocity parallel to the cutting edge/blade displacement, or velocity perpendicular to the cutting edge is important in making cutting seem easier. A slice–push ratio is obtained when an orthogonal blade is driven sideways as well as down; driven straight down but at an angle, since the cutting feed velocity has components along and across the inclined blade; and when an angled tool fed into the workpiece with feed has its own independent motion parallel to the cutting edge. Reduction of forces by slice–push produces better surfaces whatever the material. In the laboratory, the best sorts of junctions with fiber-optic cables and scintillators are obtained when a slicing cut is made with a warm razor blade. Lower tractions across a cut surface reduce the tendency for components in the microstructure to separate (fat from meat in bacon slicing).