Studies on Thread Cutting of Titanium Alloy
Date Issued
2014
Date
2014
Author(s)
Chen, Chin-Nan
Abstract
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.
Subjects
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
Type
thesis
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