A simple apparat us is constructed in order to fatigues test heated nichrome wire under cycled strain. The purpose of the fatigue testing is to examine how the life span of hot nichrome wire varies with different temperatures, pretension treatments, strains, and clamp geometries. The resulting data from these tests are meant to provide the basis for improving the design of hot nichrome wire cutters used in a wide range of manufacturing processes. The theory and results of the strain cycling process are also briefly discussed.
1 Hot Wire Cutters: Superior Forming Tools
The increasing demand for plastic foams makes it desirable to improve the designer’s ability to process various foamstocks into finished parts. Earlier methods involve band saws and circular saws to cut the stock into an approximate shape. Details are then smoothed out by sanding. However, during the last 30 years, hot nichrome wire has become a more popular tool than conventional saws. Rather than separating material by tearing through it mechanically, the hot wire cutter melts the plastic foam as the two come into contact…
1.1 Disadvantages of Hot Wire Cutters
…Two major features diminish the practicality of the hot wire cutter when compared to that of mechanical cutting.
Tolerance. Because nichrome wire is thin and flexible, fast cutting operations may put a slight bend in the wire. The desired wire shape is linear. Hence, the cut may not follow the intended course by an amount dependent on the curve of the wire. The problem may be lessened by pre-tensioning the wire prior to clamping the ends. However, pre-tensioning lessens the life span of the wire.
Fatigue life. All cutting tools wear out or break after a certain amount of working time. Saw stend to last longer than hot wires. For manufacturers, this limitation of nichrome wires is expensive. Every time a cutting tool breaks, the production process must stop. Replacement of the part may take hours. This disruption idles the manufacturing operation and may be costly.
1.2 Research objectives
This investigation seeks to identify some approaches to prolonging the working life of nichrome wire in plastic foam cutting porcesses. Several viarables known to influence nichrome wire life are discussed theoretically and plotted against the life span of the hot nichrome wire. The variables include (1) temperature, (2) wire deflection, (3) wire pretension, and (4) clamp geometry. An apparatus for testing these influences is described, test results are given. And design suggestions are drawn from the results.
2 Theoretical Conditions of Fatigue
Each of the key variables that influence working life of nichrome wire may be understood in its theoretical design context by examining the forces at work.
2.1 Wire temperature
The temperature of nichrome wire may lead to degradation of the wire temperature for two resons. First, at high temperatures, most metals will evaporate. The typical operating range for nichrome cutting wire is between 250°C and 600°C …
2.2 Intrinsic Strain
The nichrome wire is often placed under the tension before it is clamped in place. This practice is followed because it lessens wire deflection during cutting and thus improves tolerances…
2.3 Clamp Geometry
The clamp geometry influences the strain distribution and magnitude at ends of nichrome wire. In contrast to the extrinsic strain caused by wire pre-tensioning and deflection, clamp geometry gives rise to local strain at the clamp edges…
Results of the test show that the main cause of nichrome wire failure is strain, as the wire cuts the sample and is deflected, D/L mm. The variables studied were found to influence wire life, Q, as follows.
5.1 Clamp Geometry
Clamp geometries affected both the life of the wire and the location of the most common wire failure. For example, in clamp geometry No. 1, the wire consistently failed in the region of maximum deflection, when bending over a rounded edge…
5.2 Wire Temperature
Although temperature variations did not notably influence the strength of the nichrome wire, hotter wires did reduce cutting drag, causing less deflection in any given material. This pattern can be seen…
Pre-tensioning, as seen in Figures 4-8, significantly decreased the Fatigue life (Q) of the nichrome wires. Altough pretension is desirable for maintaining tolerance at higher cutting speeds, it does…
Designers for hot nichrome wire cutters should configure and produce the cutters in such ways as to minimize the strain placed by cutting materials on the wire. In this study, the four major factors of clamp geometry, wire temperature, cutting speed, and wire pretension have been shown to affect wire fatigue life in different ways. On the basis of these factors, the following design guidelines should be followed…
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