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標題: | 鈦合金螺紋車削之研究 Studies on Thread Cutting of Titanium Alloy |
作者: | Chin-Nan Chen 陳清南 |
指導教授: | 廖運炫 |
關鍵字: | 螺紋車削,未變形切屑面積,鈦合金,切削深度規劃,不等刀鼻半徑,多峰牙刀,切削速度, Thread cutting,Undeformed chip area,Titanium alloy,Cutting depth strategy,Unequal nose radius (UNR),Multi-point threading tool,Cutting speed, |
出版年 : | 2014 |
學位: | 博士 |
摘要: | 鈦合金被廣泛使用在航太科技和生醫科技上,僅就人工牙根一項就有相當龐大的市場需求,但是,市面上的人工牙根絕大部分還是仰賴進口,雖然台灣已經開始自主開發人工牙根,但牙根上的螺紋加工一直有刀具快速磨損及毛邊不易處理等問題。
螺紋加工時,除根據工件材料性質選擇適合的刀具材料外,最重要的車削參數為車削速度及每次進刀的深度,這兩個參數嚴重影響螺紋車削時的效率及刀具壽命。以現行的螺紋加工參數進行鈦合金螺紋加工時,經常會遭遇到刀具快速崩斷的情形,仔細計算未變形切屑面積後發現,在數道次(pass)的車削過程中,切削力的變化大小超過兩倍,且較大的切削力造成刀具的快速損壞。因此本研究提出每道次(pass)進刀時的未變形切屑面積相等的等面積車削觀念,重新規劃螺紋車削的加工道次與每道次的切削深度,目的是讓每道次車削時,作用在刀具上的切削力維持一定。為了維持切削力相同,每一次道次車削時的未變形切屑面積必須相等,而每道次的切削深度就是從每一道次切削面積為定值的條件下,反過來推算得到。市面上人工牙根所用的螺紋規格,通常節距在0.5 mm到1.6 mm之間,本研究使用刀鼻半徑0.05 mm的60度牙刀,以鈦合金為工件材料,車削節距為0.5 mm之ISO公制規格螺紋,進行實驗驗證,結果發現,等面積車削的規劃方式車削鈦合金螺紋時,相同的切削速度下,粗切削的道次可以從四次減少為三次,相當於比現行廠商建議的規劃方式提升25%的效率,且刀具的磨耗量相同,也就是刀具壽命不變。 重新規劃每一道次的切削深度,雖然可以提升加工的效率,但是,在多次進刀車削的過程中,每一次加工都有可能使刀尖損壞,尤其是鈦合金材料,刀具尖端會因為黏附切屑,而容易非常發生崩斷的現象。本文提出一個新型的不等刀鼻半徑(Unequal nose radius)多峰牙刀概念,將螺紋車削時的多道次加工過程,利用多峰牙刀,以較少的加工次數,甚至於僅加工一次便完成螺紋車削。 由於刀尖的損壞和刀鼻的半徑大小有直接關係,透過改變多峰牙刀各個刀刃的刀鼻半徑大小,除了讓不同的刀刃分別擔任粗車削和精車削的不同任務外,也希望後方刀刃的壽命略大於前方刀刃,如此一來,即使發生刀刃損壞情形,只要後方刀刃維持完好,螺紋的加工依然可以順利完成。為了找出各個刀刃刀鼻半徑的最佳組合,本文利用遞迴演算的方式,以有限元素法求得各刀刃應變量作為刀具壽命的比較依據,先假設第一刀刃的半徑值後,依序算出每一刀刃的應變量與刀鼻半徑,比較最後一刀刃是否滿足螺紋規格的刀鼻半徑,若尚未滿足則調整第一刀刃的半徑值再重新計算所有刀刃的半徑值。如此反覆遞迴演算直到找出最後一刀刃滿足螺紋規格的刀鼻半徑組合為止。以三峰牙刀車削0.5 mm節距的鈦合金螺紋為例,演算後求得的最適當刀鼻半徑分別為0.15 mm、0.08 mm、0.05 mm,經由實驗結果驗證,發現此刀具可以58.2 m/min的切削速度一道次車削即完成螺紋,和一般常用的單峰牙刀必須經過五次車削,且切削速度只能限制在20 m/min以下的情形相比較,大約提升了15倍的效率。 Thread cutting is one of the most important manufacturing processes in production precision threads. Several passes are needed in completing a thread by cutting, and the choice of appropriate cutting speed and depth of cut in each cutting pass is essential. The cutting efficiency and tool life are strongly affected by these two parameters especially in cutting thread of difficult-to-cut material such as titanium alloy. In the paper the concept of equal undeformed chip area for all cutting passes is proposed for the determination of the depth of cut in each pass. The main idea is to maintain the same cutting force throughout cutting process. Based on tool geometry the relationships between the cumulated depth of cut and the undeformed chip area in each cutting pass are derived. The depth of cut of the corresponding cutting pass can be solved once the dimension of the thread and number of cutting pass are specified. Experiments were conducted to cut an ISO metric screw thread of the pitch of 0.5 mm on a 40 mm in diameter bar. It was found that the tool wear was less using the depth of cut in each pass determined by the proposed approach than that suggested by the tool makers for the same total number of cutting passes. The thread could be cut by a higher cutting speed which led to much shorter machining time. In addition, it could also be successfully finished in less cutting passes by the proposed strategy. For the specific thread in the experiment, a 25% increase of the efficiency was obtained. Chips created in threading titanium alloy can adhere to the tool tip, leading to tool breakage and reduced efficiency. A novel design of an unequal nose radius (UNR) multi-point threading tool is presented in this paper. Instead of keeping the nose radii of all teeth the same, only the nose radius of the last tooth conforms to the specifications of the thread. The nose radii of the other teeth are determined based on the principle the tool life of the following tooth should not be shorter than that of the preceding one. The strain of each tooth during threading is computed by the finite element method (FEM), and is taken for tool life assessment. A 12% reduction of the strain for the two consecutive teeth is found appropriate, and a recursive algorithm to search for optimal design is established accordingly. A 60 degrees and 0.5-mm pitch ISO metric screw thread was turned to verify the proposed threading tool design. The most appropriate combination of the nose radii for a triple-point threading tool was determined to be 0.15 mm, 0.08 mm, and 0.05 mm. The experimental results show the proposed and designed triple-point threading tool performs satisfactorily; a very good quality thread of titanium alloy is finished in only one pass at a cutting speed of 58.2 m/min. By comparison, a single-point threading tool, commonly used in practice, requires five passes to complete the same task, and its cutting speed is limited to less than 20 m/min. Hence, approximately 15 times efficiency is achieved when the triple-point tool presented in this study is used. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/56297 |
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顯示於系所單位: | 機械工程學系 |
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