首页 考试资料幻灯片工程技术公务员考试小学教学中学教学大学教学外语资料
毕业设计论文


53

设计内容
流量向同时,两个方向上的推力和速度是相等的。 三.液压缸各部分的结构及主要尺寸的确定 尽可能按推荐的结构形式和设计标准进行设计,尽量做到结构简单、 紧凑、加工、装配和维修方便。正确确定液压缸的安装和固定方式。考虑 液压缸的热变形,它只能一端固定。 缸筒内径 D: 根据负载大小和选定的工作压力,或运动速度和输入流 量计算,再参考 GB2348-80 标准选取缸筒内径 D=78.5mm 活塞杆直径 d: 按工作时受力情况, 参考 GB3458-80 标准确定 d=27mm 钢筒长度 L: 根据最大工作行程确定 L=126mm 四.强度校核 对于液压缸的缸筒壁厚 δ 活塞杆直径 d 缸盖处固定螺钉的直径,在高 压系统中,必须进行强度校核。 筒壁厚 δ,在中、低压液压系统中,缸筒壁厚往往由结构工艺要求决 定,一般不要校核。在高压系统中,须按下列情况进行校核。 当 D/δ>10 时为薄壁,δ 可按下式校核,δ ?
py D

主要结论

2?? ?

= 23mm

式中

D----缸筒的内径 py=1.5pn;

py----试验压力,当缸的额定压力 pn≤16Mpa 时,取 pn>16Mpa 时,取 py=1.25

[ζ]----缸筒材料的许用应力, [ζ]=ζb/n, ζb 为材料抗拉强度,n 为安全 系数,一般取 n=5。 活塞杆直径 d 的校核 d ?

? ?? ?

4F

=26.66mm

式中 F----活塞杆上的作用力。

54

[ζ]----活塞杆上的许用应力,[ζ]= ζb/1.

Temperature Variation in the Cutting Tool in End Milling
55

This paper describes the cyclic temperature variation beneath the rake face of a cutting tool in end milling. A newly developed infrared radiation pyrometer equipped with two optical fibers is used to measure the temperature. A small hole is drilled in the tool insert from the underside to near the rake face, and an optical fiber is inserted in the hole. One of the optical fibers runs through the inside of the machine tool spindle and connects to the other optical fiber at the end of the spindle. Infrared rays radiating from the bottom of the hole in the tool insert during machining are accepted and transmitted to the pyrometer by the two optical fiber. For a theoretical analysis of the temperature in end milling, a cutting tool is modeled as a semi-infinite rectangular corner and a Green's j'unction approach is used. Variation in tool-chip contact length in end milling is considered in the analysis. Experimentally, titanium alloy Ti-6Al-4V is machined in up and down milling with a tungsten carbide tool insert at a cutting speed of 2/4 m/min. In up milling, the temperature beneath the rake .face increases gradually during the cutting period and reaches a maximum just after the cutting. In contrast, in down milling, the temperature increases immediately after cutting starts; it reaches a maximum and then begins to decrease during cutting. This suggests that the thermal impact to the cutting tool during.g heating is larger in down milling than in up milling, whereas that during cooling is larger in up milling than in down milling. Temperature variation is measured at difj'erent depths from the rake face. With increasing depth from the rake face, the temperature decreases and a time lag occurs in the temperature history. At 0.6 mm from the major cutting edge, the temperature gradient toward the inner direction of the tool insert is about 3000 C/0.5 mm. the calculated and experimental results agree we//. End milling is a basic machining operation in the manufacture of mechanical components to generate a flat surface and/or a three-dimensional free-formed surface; it requires high geometric accuracy and high productivity. During the machining, most of the power consumed for plastic deformation is converted into heat, and high temperatures are generated. Excessive temperature is known to cause various types of thermal damage to the cutting tool and workpiece, such as rapid tool wear and thermal expansion- soon of the workpiece. To clarify the effect of temperature on the thermal damage and to determine suitable cutting conditions, ac- curate knowledge of milling temperatures is necessary. Since end milling is an intermittent machining process, the cutting tool is subjected to cyclical heating and cooling, and temperature variation- lion in the cutting tool is more complicated than in turning. Many valuable studies on milling temperatures have been conducted. Chakraverti et al. [1] created an approximate model of cyclic temperature changes at a tool-chip interface during intermittent cutting as a problem of periodic heating of a semi-infinite body. They showed that the temperature gradient in the cutting zone increase with cutting frequency, but it diminishes with higher thermal diffusivity of the tool material. Palmai [2] used an empirical formula to calculate the cutting temperature change during intermittent cutting and showed that the cutting temperature first increases with cutting speed and then starts decreasing because of the short heating periods. Stephenson et al. [3] modeled a tool insert as a semi-infinite rectangular corner and used a Green's function approach to calculate the temperatures in interrupted cutting. They showed that the temperatures are generally lower in interrupted cutting than in continuous cutting under the same conditions. Radulescu et al. [4] developed an analytical model for predicting tool temperature fields, which can be applied to
56

1234567891011121314151617

 


 

  【Top

最新搜索

 


 

热点推荐