Wavelength dependence of machining performance in

Authors: Satoshi Fukuta Masaki Nomura Takeshi Ikeda Masaki Yoshizawa Mariko Yamasaki Yasutoshi Sasaki

Publish Date: 2016/04/07

Volume: 62, Issue: 4, Pages: 316-323

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Abstract

Cutting of wood using shortwavelength 266 355 532 and 1064nm lasers was carried out and wavelength dependence in relation to machining performance and postprocessing appearance was investigated We found that a 355nmwavelength laser achieves the greatest machining performance The variation in machining performance between different wavelengths was due to the different light absorptances of the woods when we measured the spectral reflectance of the woods we found that the greater the machining performance for a wavelength the lower its reflection of light and thus the greater its absorption Assuming that the absorption of ultraviolet wavelength of 355 nm by wood is due to lignin a ligninfree cellulose board was prepared for further investigation and the same machining and measurements were conducted The results showed that machining progresses well via the absorption of laser light by lignin at the ultraviolet wavelength of 355 nm On the other hand when this cellulose board was impregnated with phenolic resin there was high absorption of light between the ultraviolet range and wavelengths near 600 nm machining progressed well at wavelengths of 355 and 532 nmAs organic materials exhibit good absorption of CO2 laser light CO2 laser of wavelength 106 µm has been applied almost exclusively in the machining of wood 1 2 As the laser can provide high output power at a relatively low cost CO2 laser has been used for cutting and welding applications that require highspeed machining with high output power not only of wood but also other materials Meanwhile in the field of precision machining shortwavelength lasers are being used that have for example wavelengths in the vicinity of NdYAG lasers wavelength 1064 nm and other even shorter wavelengths This is because as the theoretical focal point diameter is proportional to wavelength the use of shortwavelength lasers enables precise machining In addition lasers are normally irradiated at a short pulse width as shortening the pulse width allows heat effects to the surrounding area to be controlled while increasing the peak power In recent times shorter and shorter pulse widths are being achieved reaching nano pico and even femtosecondsMeanwhile in our previous research 3 we attempted wood machining using a type of shortwavelength laser ultraviolet UV lasers specifically thirdharmonicgeneration NdYLF laser and NdYVO4 laser wavelengths 349 and 355 nm respectively and collected basic performance data In practical terms when we performed cutting holedrilling and then incising at the drilling site we observed the superiority and effectiveness of these lasers we were able to conduct precise cutting with little heat effect as well as holedrilling with an extremely high aspect ratio in a relatively short machining time While one could use various other wavelengths that are generally classified as “shortwavelength” other than the wavelengths of 349 and 355 nm used in our previous study we have not found any reports of testing for a suitable wavelength in relation to wood machining While there is one case 4 of holedrilling at a depth of several millimeters by exposing wood to a pulseirradiation ruby laser with a wavelength of 694 nm this case was from an emerging era when laser had only just been discovered and it is likely that the types of lasers that could be irradiated were limited Hattori et al 5 compared the critical machining energy of NdYAG laser and CO2 laser but their findings were only on NdYAG laser and no detailed quantitative evaluation has since been conductedThus in this study we tested for suitable wavelength for wood machining by comparing the woodmachining performance of the base wavelength of NdYVO4 laser 1064 nm and of its secondharmonic generation wavelength 532 nm its thirdharmonic generation wavelength 355 nm and its fourthharmonic generation wavelength 266 nm obtained by wavelength conversion under uniform conditions We also investigated the causes behind the resultsThe specimens used were heartwood of Japanese cedar Cryptomeria japonica hereafter cedar heartwood of Japanese larch Larix kaempferi hereafter larch and beech Fagus crenata From these woods straight grain test specimens of 12 mm L × 6 mm T × 70 mm R were prepared with the exception of the beech which was 4 mm T The partial density variation of cedar and larch is large because of the effect of annual rings All machining was conducted on early wood density was measured for each segment of early wood cut out with the airdried density of the cedar and larch being 031 and 035 g/cm3 respectively The machining area was not limited for the beech and the airdried density of the beech was 062 g/cm3 All specimens were collected with end matching in the grain direction and all were airdried specimens having around 10  moisture contentIn addition to the above woods as a ligninfree material a cellulose board specimen was prepared with 077 g/cm3 density using the following procedure Briefly a 6mmthick board was prepared by layering a predetermined number of sheets of cellulose nonwoven fabric BEMCOT M3II Asahi Kasei Fibers Co while spraying with water such that its water content was homogenous compressing the specimen to 6 mm with a 120 °C hot press and then drying it in this state This nonwoven fabric uses cotton linter the fuzz hairs attached to cotton seeds as its raw material and is a longfiber nonwoven fabric considered to be composed of almost 100  celluloseFurthermore we prepared a phenolic resinimpregnated cellulose board hereafter impregnated board by impregnation treatment with watersoluble lowmolecular phenolic resin Its density was 079 g/cm3 and its weight ratio of impregnated resin was 40  This board was formed by diluting a watersoluble lowmolecular phenolic resin BRL120Z Showa Denko Co to a resin density of 337  impregnating to the nonwoven fabric with it layering and then drying it and finally thermosetting the resin in a 140 °C hot press

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