【作者】 郭晓磊；

【导师】 曹平祥；

【作者基本信息】 南京林业大学 ， 木材科学与技术， 2012， 博士

【摘要】 中密度纤维板（Medium Density Fiberboard，缩写为MDF）已经在人造板生产和应用中，占据了令人瞩目的地位。但MDF的二次加工，如切削加工，存在加工表面质量差、粉尘污染严重等诸多问题。本文作为国家自然科学基金资助项目“木材及木质复合材料高速切削过程中切屑流控制机理的研究”的重要部分，主要侧重于MDF切削过程中的切屑形成机理、切削力、铣削粉尘特性及切屑流特性进行分析研究。在MDF切屑形成机理研究方面，采用高速摄像技术和材料微观分析技术，进行了MDF正交切削试验，分析了切屑形成过程的基本特征，建立了MDF切削机理模型；进而分析了切削厚度、刀具前角及切削速度等不同切削参数对切削变形的影响，分析了切屑形态演变过程；建立了切削参数与切削加工表面破碎率的关系。研究结果表明：在MDF切削过程中，随着切削厚度的增大，MDF切屑由以粉尘状切屑为主首先逐步向以颗粒状切屑为主转变，进而向以准连续状切屑为主转变；在较大的切削厚度条件下，降低刀具前角或增大切削速度有助于MDF切屑由以准连续状切屑为主向以颗粒状切屑为主转变；在较小的切削厚度条件下，增大刀具前角或减小切削速度有助于MDF切屑由以粉尘状切屑为主向以颗粒状切屑为主转变。采用图像处理技术计算表面破碎率，不仅可以作为MDF已加工表面质量的评价指标，而且可以作为系统研究MDF切削机理、切削力等的重要手段。在MDF切削力研究方面，分析了不同切屑形成过程中切向力时域波形特征，分析了切削参数对MDF切向力和径向力的影响趋势，并且建立MDF切向力和径向力经验计算公式。研究结果表明：MDF准连续状切屑生成过程中，切削力‐时间曲线上的切向力呈一定周期性波动，切向力变化较大。颗粒状切屑形成时，切削力‐切削时间曲线上开始出现比较有规律性的锯齿状波形，这些高高低低的锯齿波峰值点对应着大大小小的MDF挤压颗粒状切屑的形成过程。粉尘状切屑形成时，平均切削力较小，围绕在42N左右变化，切削力‐时间曲线波动平稳。切削厚度对切削力的影响显著，随着切削厚度增大，切向力急剧增大，而径向力逐渐减小；刀具前角对切削力的影响较显著，随着刀具前角的增大，切向力减小较快，而径向力减小趋势较慢；切削速度对切削力的影响较显著，随着切削速度增大，切向力是逐渐减小，而径向力趋势表现并不明显。在MDF铣削加工产生的粉尘特性研究方面，分析了平均铣削厚度对MDF铣削粉尘粒度分布、典型粒度形貌及MDF已加工表面粗糙度的影响，并且分析了粉尘粒度对甲醛释放量的影响。研究结果表明：MDF铣削粉尘粒度分布主要集中在0.5-450μm区间，铣削MDF粉尘颗粒基本上为狭长型的纤维束、胶束或者短小型纤维束。随着平均铣削厚度增加，MDF铣削粉尘的平均粒度{D（4，3），D（3，2）}及中位径D（50）呈逐渐增大的趋势，而铣削速度影响较进给速度影响显著。随着平均铣削厚度的增加，MDF铣削表面质量降低，而铣削速度影响较进给速度影响显著。因此，可以认为，适当地增加铣刀转速有助于提高MDF铣削表面质量。此外，相同质量的MDF铣削粉尘，随着平均粒径减小，其甲醛释放量趋于增大。在MDF切屑流研究方面，基于高速摄像技术和图像识别与处理技术，采集木质复合材料铣削过程中切屑流的瞬态图像，分析MDF铣削切屑流形成机理、流场速度及扩散角度的变化规律。研究结果表明：切屑流形成过程分成两个阶段，第一个阶段为切削层材料离开工件形成切屑开始，到与齿槽接触；第二个阶段为切屑离开齿槽之后向外飞射，最后扩散在大气中。切屑流流场速度变化也分为两个阶段，第一个阶段为切屑流从形成时，具备一定流场速度后，开始趋于减小，第二个阶段为切屑流流场速度突然达到峰值，然后速度随之趋于减小。木质复合材料高速铣削时，随着刀齿切入工件，铣刀转速会趋于降低；而随着刀齿离开工件瞬间，刀齿线速度迅速恢复到额定速度并保持稳定。在切屑流流场内部的不同位置，切屑的速度存在差异。在同一时刻，切屑流中部的切屑颗粒速度最大，靠近刀齿部分速度次之，远离刀齿部分速度最小。随着铣刀转速的增加，切屑流的扩散角减小，在不同的木质复合材料铣削加工中，MDF铣削过程中的扩散角最大，变化范围为59.1‐66.5°。本文研究成果具有一定理论及应用价值，填补了国内外关于MDF切削机理研究的空白，为MDF切削过程中产生粉尘污染的控制，提供了可靠的理论依据。更多还原

【Abstract】 The production and application of Medium Density Fiberboard (MDF) were developing sofast that it has already taken a significant position in the man-made board industry. But thefurther processing of the MDF boards, cutting for example, causes a series of problems such asbad processing quality, serious dust pollution and so on. This paper was an important part of theproject named “Study on the dust flow control of the wood and wood based materials in a highspeed cutting condition”, which was subsidized by Natural Science Foundation of China(NSFC). And this paper placed an emphasis on the study of chip forming mechanism, cuttingforce, dust characteristics of milling and the dust flow control.The MDF chip forming mechanism was investigated by testing the MDF orthogonalcutting with the technology of high speed camera and micro-analysis of the materials. The basiccharacteristics of the chip formation process and the influence of the cutting parameters such ascutting thickness, rake angle of the tool and cutting speed to the chip formation process havebeen studied. And the relationship between cutting parameters and the ratio of the surfacedamage had been analyzed. The result showed that: with the increasing of the cutting thickness,the dominated style of MDF chips changed from dust chip to granular chip and thenquasi-continuous chip; by increasing the rake angle or the cutting speed under the condition oflarge cutting thickness, the main chip style of the MDF changed from quasi-continuous chip togranular chip. While the cutting thickness was small, the leading chip type of MDF changedfrom granular chip to dust chip with the decreasing of the rake angle or the increasing of thecutting speed. The calculated ratio of surface damage based on the image processing technologycan not only be used as the evaluation index of the quality of processed surface, but also used asthe main approach, which was reasonably used in this paper, to study the cutting mechanismand cutting force of MDF systematicly.The characters of the time domain wave of cutting force during the chip forming processand the influence of the cutting parameters on the variation of MDF tangential force and radialforce were studied in the aspect of MDF cutting force research, which were leaded to anestablishment of experience formula for calculation tangential force and radial force of MDFcutting. It was showed that: approximate periodic fluctuations of Cutting Force-Time curve ofthe quasi-continuous chip forming process are found, and the variation of the tangential forceand radial force were obvious. During the formation of the granular chip formation, the Cutting Force-Time curve showed a regular serrated waveform, which corresponding to the differentsizes of MDF granular chip caused by the extrusion. When the cutting thickness was small, theaverage tangential force was comparatively small, more or less than42N, and the wave of theCutting Force-Time curve is comparatively smooth. The cutting thickness had obviousinfluence on the cutting force, and with the increasing of the cutting thickness the tangentialforce increased sharply, while the radial force decreased slowly. The rake angle of the tool alsohad some influence on the cutting force, with the increasing of the rake angle the tangentialforce decreased quickly while the radial force decreased slowly. The cutting speed also hadsome influence on the cutting force, with the increasing of the cutting speed the tangential forcedecreased gradually while the trend of variation of radial force had no significant impact.By not only testing the particle size distribution of the dust, typical particle sizemicrograph and the processed surface roughness with different average milling thickness butanalyzing the influence of different particle sizes on the formaldehyde emission, thecharacteristics of the dust produced during the MDF milling had been researched. The studyresult showed that: The distribution of the particle size was mainly ranging from0.5to450mand the particles of the dust chip can be classified into long-narrow fiber bundle, micelle orsmall fiber bundle. And with the increasing of the average milling thickness, the averageparticle size {D (4,3), D (3,2)} and the median-particle-size D (50) all tended to increase. Andthe cutting speed had more significant influence than the feed rate. However, with theincreasing of the cutting thickness, the processed surface quality decreased, in this case, thecutting speed also had more significant influence than the feed rate. Based on this result, properincreasing of the cutting speed can be considered as a helpful method to improve the processedsurface quality.Based on the high speed video technology and the image identifying and processingtechnology, the transient images of the chip flow during the wood-based composites’ millingprocess were recorded. The formation mechanism and the variation law of the velocity of flowfield and the diffusion angle were studied as a main part of the research on MDF chip flow. Thestudy result showed that: The forming process of the chip flow can be classified into two stages;the first stage was ranging from the uncut chip leaving away from the work piece to the chipcontacting with the gullet, and the second stage was that the chips left away from the gullet anddiffused into the air in the end. The velocity of the flow field can also be classified into twostages; the first one was the velocity of the flow field, got at the beginning of the chip flowformation, started to decrease, and the second one was the velocity of flow field increasedsuddenly to a peak and then decreased. While milling the wood-based composites with high speed, when the cutter tooth started to cut into the work piece the milling speed tended todecrease and at the moment of the cutter tooth leaving the work piece the linear velocity of thecutter tooth recovered quickly to the rated speed and then kept steady. In the same time, thevelocity of the chip in the middle of the chip flow was the fastest, the second location was theplace near the cutter tooth and the location away from the cutter tooth had the lowest velocity.With the increasing of the milling speed, the diffusion angle of the chip flow decreased. Amongthe wood-based composites, the MDF had the largest diffusion angle during the milling process,ranging from53.3to66.5°.Some significant theoretical and application value were showed in this paper which notonly filled in a gap on the study of the mechanism of MDF cutting at home and abroad, but alsoa theoretical basis for the dust pollution control during the MDF cutting process was provided.更多还原