word文档 下载后可任意复制编辑
Question 12
Which is used to establish and maintain the position of a part in the fixture by constraining the movement of the part? ( ) A. locators
B. clamping device
C. guiding and tool-setting elements D. fixture body
Text 4
Quality and accuracy are major considerations in making machine parts or structures. Interchangeable parts require a high degree of accuracy to fit together. With increasing accuracy or less variation in the dimension, the labor and machinery required to manufacture a part is more cost intensive.[l] Any manufacturer should have a thorough knowledge of the tolerances to increase the quality and reliability of a manufactured part with the least expense.
An engineering drawing must be properly dimensioned in order to convey the designer’s intent to the end user. Dimensions of parts given on blueprints and manufactured to those dimensions should be exactly alike and fit properly. Unfortunately, it is impossible to make things to an exact dimension. Most dimensions have a varying degree of accuracy and a means of specifying acceptable limitations in dimensional variance so that a manufactured part will be accepted and still function. It is necessary that the dimensions, shapes and mutual position of surfaces of individual parts be kept within a certain accuracy to achieve their correct and reliable functioning. Routine production processes do not allow maintenance (or measurement) of the given geometrical properties with absolute accuracy. [2] Actual surfaces of the produced parts therefore differ from ideal surfaces prescribed in drawings. Deviations of actual surfaces are divided into four groups to enable assessment, prescription and checking of the permitted inaccuracy during production:
1. Dimensional deviations; 2. Shape deviations; 3. Position deviations;
word文档 下载后可任意复制编辑
4. Surface roughness deviations.
As mentioned above, it is principally impossible to produce machine parts with absolute dimensional accuracy. In fact, it is not necessary or useful. It is quite sufficient that the actual dimension of the part is found between two limit dimensions and a permissible deviation is kept with production to ensure correct functioning of engineering products. The required level of accuracy of the given part is then given by the dimensional tolerance which is prescribed in the drawing. The production accuracy is prescribed with regards to the functionality of the product and to the economy of production as well. The principal factor used to set a tolerance for a dimension should be the function of the feature being controlled by the dimension. Unnecessarily tight tolerances lead to high cost of manufacture resulting from more expensive manufacturing methods and higher reject rates.
Question 13
What the phrase “more cost intensive” means in the 3rd sentence of the 1st paragraph? ( ) A. With increasing accuracy or less variation in the dimension, the production cost will increase. B. The labor’s salary will increase. C. The cost of machine will increase. D. The cost of material will increase.
Question 14
What the word “convey” means in the 1st sentence of the 2nd paragraph? ( ) A. 传达 B. 运输 C. 让与 D. 传播
Question 15
Which sentence is not right about jigs and fixtures? ( )
A. It is principally impossible to produce machine parts with absolute dimensional accuracy. B. it is not necessary or useful to produce machine parts with absolute dimensional accuracy.
word文档 下载后可任意复制编辑
C. It is quite sufficient that the actual dimension of the part is found between four limit dimensions.
D. The required level of accuracy of the given part is then given by the dimensional tolerance which is prescribed in the drawing.
Question 16
What means is the Ra6.3 ? ( ) A. Dimensional deviations B. Shape deviations C. Position deviations
D. Surface roughness deviations
Text 5
How mating parts or assemblies fit together with component parts is referred to as fit, which includes clearance fit, interference fit, or transition fit, see Fig. 7.3.
Fit is the general range of tightness resulting from the application of a specific combination of allowance and tolerances in the design of mating parts.
Clearance fit. It is a fit enabling a clearance between the hole and shaft in the coupling. The lower limit size of the hole is greater or at least equal to the upper limit size of the shaft.
Interference fit. It is a fit always ensuring some interference between the hole and shaft in the coupling. The upper limit size of the hole is smaller or at least equal to the lower limit size of the shaft.
Transition fit. It is a fit where (depending on the actual sizes of the hole and shaft) both clearance and interference may occur in the coupling. Tolerance zones of the hole and shaft partly or completely interfere.
Question 17
When is there gap between the hole and shaft in the coupling? ( ) A. clearance fit B. interference fit.
word文档 下载后可任意复制编辑
C. transition fit D. basic fit
Question 18
What the word “shaft” means widely in the last sentence of the 3rd paragraph? ( ) A. axis
B. cylinder C. column
D. outside surface
Question 19
When the upper limit size of the hole is smaller or at least equal to the lower limit size of the shaft? ( ) A. clearance fit B. interference fit. C. transition fit D. basic fit
Question 20
When the tolerance zones of the hole and shaft partly or completely interfere? ( ) A. clearance fit B. interference fit. C. transition fit D. basic fit