NAK80模具鋼於不同輔助銑削環境切削性能之探討

Investigation of Cutting Performance for NAK80 Die Steel under Different Assisted Milling Conditions

王智寬(Zhi-Kuan Wang)

指導教授 ： 林盛勇

國立虎尾科技大學/工程學院/機械與電腦輔助工程系碩士班/碩士(2017年)

摘要

NAK80模具鋼是目前常用的塑膠專用模具材料，其放電加工表面細緻、美觀、鏡面研磨性良好，主要用於需鏡面拋光模具，是高性能、高精度的高級鏡面塑膠模具鋼。近年來業者對模具鋼表面品質的要求越來越高，由於銑削完的工件表面粗糙度值較大，業界大多的作法為銑削完後進行磨削，最後進行拋光的動作，期間所花費的磨削及拋光時間成本為一大考量，若能以不同切削輔助的應用來達到一定的表面水準，節省後續磨削、拋光的時間及成本，甚至以銑削的方式達到與磨削相同水準的表面品質，將可以直接省去磨削的步驟，節省多道製程的表面處理，並可提供模具業者作參考以減少後續拋光時間及磨削加工成本，增加生產效率。
本文使用五種不同的輔助銑削NAK80模具鋼，包含無輔助、雷射輔助、切削液輔助、MQL輔助及冷風輔助並驗證輔助效果之差異及優劣。首先進行27組無輔助銑削實驗，接著進行各18組的雷射輔助、切削液輔助、MQL輔助及冷風輔助銑削實驗。探討表面粗糙度、切削力、刀具磨耗及工件表面形貌於各加工製程參數之變化趨勢，藉由不同切削輔助以提高NAK80模具鋼之切削性能。於雷射輔助銑削實驗前，先行測試雷射預熱於工件表面之時間，據以設定雷射-刀具距離。實驗採用極細微粒碳化鎢TiSiN鍍層端銑刀，透過加工製程參數（主軸轉速、進給率、徑向切削深度）組合的規劃，針對NAK80模具鋼進行上述五種輔助之銑削加工。於實驗過程中使用動力計監測銑削力的變化，以工具顯微鏡觀測加工完成面之表面形貌及量測刀具之刀腹磨耗量，並以表面粗糙度儀進行表面粗糙度之量測。結果顯示，在相同條件下，表面粗糙度值與切削力值均隨著主軸轉速的增加而下降，隨著進給率及徑向切深的增加而上升，最大刀腹磨耗量隨著主軸轉速、進給率及徑向切深的增加而上升。在切削輔助方面，冷風輔助不僅優於上述各輔助，且明顯地改善刀具磨耗與表面粗糙度，降低切削力。本實驗較佳製程參數組合為使用冷風輔助搭配主軸轉速15000rpm、進給率300mm/min、徑向切深0.1mm，表面粗糙度為0.216μm、切削力為40.311N。

關鍵字

NAK80模具鋼 ； MQL輔助 ； 冷風輔助 ； 雷射輔助 ； 切削性能

並列摘要

NAK80 mold steel is a high-performance, high-precision, and advanced plastic mold steel with mirror surface and hence is a specific material commonly used for plastic die manufacturing currently. The machined surface may exhibit fine pattern and artistic appearance in electrical discharge machining, and is also featured by good mirror surface quality in grinding. Recently, the demand of surface quality for mold steel is getting more stringent, but the machined surface after milling operation is usually rougher. Therefore, most manufacturers solved these problems by grinding followed by polishing subsequently. However, the time wasted and cost for grinding and polishing procedures are considerable. If the surface quality machined by milling only may be promoted to a certain degree through different assisted ways, the time and cost for these machining operations can thus be reduced. As a result, the same standard of surface quality may be attained in replacement of grinding operation eventually and the grinding procedure may be omitted directly. It is expected that some machining processes for surface quality enhancement can be omitted based on the advantages of different assisted ways, which can serve as a reference for mold manufacturing to save the time wasted in some intermediate machining procedures, and enhance the production efficiency.
In this study, five different assisted ways, including without assistance, laser assisted, cutting fluid assisted, MQL assisted and cold air assisted were employed for NAK80 mold steel milling. The assisted effect on cutting performance could be verified and the difference, merit and drawback among them may be compared. Milling experiments without assistance were conducted, followed by laser assisted, cutting fluid assisted, MQL assisted and cold air assisted milling experiments subsequently. The 18 set experiments were performed in each assisted way but 27 sets were executed in the first situation. Under each assisted milling circumstance, the effects of process parameters on the variations of surface roughness, cutting force, tool wear and surface morphology of workpiece are thus investigated. It is expected that the cutting performance of the NAK80 mold steel could be promoted through these assisted ways. Before the use of laser assistance, the laser preheating time related to the workpiece surface should be tested in advance for a proper spacing distance setting between laser-spot and cutting-tool. In this study, milling experiments of NAK80 mold steel by cutting-tool of extra-fine particle tungsten carbide with TiSiN coating were conducted. And the experiments of process parameter combinations such as spindle speed, feed rate and radial depth of cut were also planned. During the experiments, dynamometer is used to monitor the variation of cutting forces. Flank wear of cutting-tool and machined surface morphology of the NAK80 mold steel were measured by tool-microscope off-line. Surface roughness measurement through a probe contact type instrument is also performed. The results show that, under the same milling conditions, the surface roughness and cutting force are decreased as the spindle speed is increased, but both of them is increased as the feed rate and radial cutting depth are increased. In addition, the maximum flank wear of cutting-tool is increased as the spindle speed, feed rate and radial cutting depth are increased. In terms of cutting assisted, the cutting performance exhibited in the cold air assisted milling prevails over that in milling without assistance as well as with the other each assisted way. As a result, the cutting-tool flank wear, surface roughness and cutting force are all better than the other assisted ways. The better result obtained in this study was achieved by using the cold air assisted in NAK80 mold steel milling under the conditions of the spindle speed of 15000 rpm, feed rate of 300mm/min and radial cutting depth of 0.1 mm. The cutting force of 40.31N and surface roughness could be reduced to 0.216μm accompanied by these process parameter combinations.

並列關鍵字

NAK80 mold steel ； MQL assisted ； cold air assisted ； laser assisted ； cutting performance

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