Analysis of the sharpness of blades for food cutting
Introduction
The sharpness of a knife is of fundamental importance to initiate a cut at the surface of the cutting material and to further separate the material. Commonly, sharpness is understood as state of a blade that, under ideal conditions, provides one-dimensional contact between edge and cutting material. In practice, there generally is a two-dimensional contact area that depends on the blade tip radius. From the processing point of view, sharpness also depends on properties of the cutting substrate, and refers to the ability of a blade to initiate a cut at low force and deformation (Atkins, 2009a). Overall, there is neither an unambiguous definition for sharpness, nor a unique method with which it can be determined (McCarthy et al., 2007, McCarthy et al., 2010). In the context of cutting of bioengineering materials, Reilly et al. (2004) suggested that sharpness can be classified either by qualitative techniques using optical methods, or by quantitative techniques using force measurements. Although optical methods may provide additional information on quantitative parameters such as blade tip offset, blade tip radius or wedge angle, Schuldt et al. (2013) showed that, on the basis of repeatability data for blades with a different abrasion state, force measurements are more sensitive for the detection of differences in sharpness.
A fundamental critique to existing sharpness evaluation strategies came in by McCarthy et al. (2007) who pointed on the lack of a standardized test that quantifies the sharpness of a cutting edge. Therefore they developed an objective, dimensionless blade sharpness index BSI that relates the energy WCI (Nm) necessary to initiate a cut to the product of cut initiation depth CI (m), thickness x (m) and fracture toughness J (J/m2) of the testing materialwhere BSI = 0 indicates a blade with ideal sharpness, and an increase in BSI can be interpreted as decreasing sharpness. These tests were performed by fixing thin sheets of polymer materials (polyurethane, 2.25 mm thick; silicone, 1.6 mm thick) between two anti-buckle clamps to mimic surgical applications. The testing material was cut between the clamps, and all parameters needed for BSI calculation could be achieved by choosing an appropriate experimental setup. McCarthy et al. (2007) concluded that the BSI is independent of the cutting material and of the cutting velocity, and that it is only influenced by the geometrical properties of the blade. In a subsequent study, McCarthy et al. (2010) used a finite element model and showed by simulation that an increase of blade tip radius and wedge angle significantly increases the BSI.
McCarthy et al. (2007) dedicated their cutting tests to biological materials such as skin and used very thin testing materials. Especially for blunted blades, this led to experimental problems such as buckling and winkling and thus a three-dimensional deformation of the material where only two-dimensional deformation was supposed to occur. As a consequence, the authors detected qualitatively different force and cutting stiffness courses for virgin and dull blades which could have been a result of inadequate testing material dimensions. Some more limitations in transferring the conclusions of McCarthy et al. (2007) to food cutting come from their use of surgical scalpel blades and razor blades that are very sharp and slender but do not have the geometry of knifes that are used in continuous cutting operations in industrial food processing. Such processing is in need of blades with edge properties that remain stable over longer production time: they have a more robust geometry with a higher blade tip radius; they are less prone to wear (Lau et al., 2000) and, consequently, they are also less sharp (Schuldt et al., 2013). Nevertheless McCarthy et al. (2007) provided a theoretically valid and interesting approach for objective knife sharpness evaluation.
Sharpness is an important but not the only parameter that affects cutting performance as cutting forces come from the superposition of separating or fracture energy but also deformation and friction energy (Schneider et al., 2002, Schuldt et al., 2016). In general, a high cutting quality can be achieved if product deformation and cutting forces are minimized. Depending on the specific mechanical and physicochemical properties of a particular food, other important issues that contribute to cutting quality are the minimisation of product deformation, and/or the reduction of friction forces between blade and product (Atkins, 2009b, Schneider et al., 2010).
The food industry usually processes agricultural materials into more or less complex materials with liquid, semi-solid or hard and tough texture. In practice, foods can assume any condition between pure elastic and pure viscous behavior and, in most cases, a time and deformation dependent combination, denoted as viscoelasticity, is observed (Miri, 2011). The time dependency of viscoelastic materials is responsible for phenomena such as stress relaxation and creep, and is the reason for the dependency of cutting performance on cutting velocity (Schuldt et al., 2016, Vliet et al., 1993).
The aim of this work was to validate the concept of the blade sharpness index in the context of food cutting, and to evaluate the BSI as compared to other optical and mechanical sharpness parameters at two cutting velocities by using differently blunted blades, and blades with different wedge angles. Cutting experiments on different polymers were performed to assess whether the BSI is independent of the testing material. In a second step we demonstrate how blade sharpness affects cutting forces of foods with varying cutting properties in orthogonal cutting, and we discuss the influence of blade sharpness on cut initiation of the foods.
Section snippets
Elastomers for blade sharpness index determination
For the determination of sharpness, synthetic materials which ensure high reproducibility, flexibility in sample geometry and quantity, have already been used in other studies (McCarthy et al., 2007, Kohyama et al., 2004, Schuldt et al., 2013, Shergold and Fleck, 2004). The elastomers used in this study were two ethylene propylene diene monomer rubbers from Karl Treske GmbH (Berlin, Germany) with different hardness (EPDMs, 50 ± 5 Shore A; EPDMh, 65 ± 5 Shore A), both provided in sheets with a
Influence of blade geometry on mechanical cutting parameters
In a previous study, Schuldt et al. (2013) showed that blade tip radius and blade tip offset are highly correlated (r = 0.95, p < 0.01), that both are equivalent measures for the geometrical characterization of blades with the same wedge angle, and that both can be taken as appropriate measure to quantify the abrasion of blades that leads to sharpness reduction. For blades with different wedge angle, however, this relation cannot be taken as constant: a similar tip radius accompanied by a
Conclusions
The blade sharpness index is suitable for characterizing the sharpness of blades for food processing. It is independent of the cutting velocity and, in contrast to McCarthy et al. (2010), independent of the wedge angle. Normalized to geometrical and fracture properties of the specimen substrate, BSI only depends on the blade tip radius itself. CI and FCI provide measures of sharpness equivalent to the BSI once a particular specimen substrate and a single velocity is used. In case a simple and
Acknowledgement
The study was supported by funding of the Excellence Initiative by the German Federal and State Governments. We additionally thank Astor Schneidwerkzeuge GmbH for providing knives and advice.
Symbols and units
- A
- contact area [m2]
- Al2O3
- corundum
- BSI
- blade sharpness index [−]
- CI
- cut initiation depth [m]
- d
- cutting stiffness [N/mm]
- EPDMs
- ethylene propylene diene monomer rubber (soft)
- EPDMh
- ethylene propylene diene monomer rubber (hard)
- F
- force [N]
- FCI
- force at cut initiation, normalized to unit thickness of the specimen [N/cm]
- J
- fracture toughness [J/m2]
- l
- displacement [m]
- NBR
- nitrile butadiene rubber
- P
- free pass [N]
- p
- level of significance
- r
- regression coefficient
- SiO2
- seasand
- SSC
- steady state cutting
- v
- cutting velocity [mm/min]
- WCI
- energy
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