机械制造作业考研题目答案分享——加工精度和误差

Homework 13

版权声明

本内容由狂小虎原创整合，请不要售卖，为了防止爬虫以及保持免费性，设置为仅粉丝可见。另外，题目以及解释可能不完全正确，仅供参考，同时也欢迎各位在讨论区指点出内容的错误之处。

文章目录

• 版权声明
• 1 举例说明加工精度、加工误差的概念以及两者的区别与关系。
• 2 说明原始误差、工艺系统静误差、工艺系统动误差的概念以及加工误差与原始误差的关系和误差敏感方向的概念。
• 3 当车床主轴径向跳动频率为主轴回转频率4倍时，画图说明车削外圆时可能得到的工件形状。
• 4在车削前，工人经常在刀架上装上镗刀修整三爪卡盘三个卡爪的工作面或花盘的端面，其目的是什么？能否提高主轴的回转精度（径向跳动和轴向窜动）？
• 5 何谓工艺系统的刚度、柔度？它们有何特点？工艺系统刚度对加工精度有何影响？怎样提高工艺系统的刚度？

1 举例说明加工精度、加工误差的概念以及两者的区别与关系。

Definition：
Machining accuracy
The degree of the practical dimensions after machining conforms to the desired geometrical parameters. The higher the degree conforms, the higher the machining accuracy.
Machining error
The degree of the practical dimensions after machining deviates from the desired geometrical parameters.
Differences:
Machining accuracy is a degree of real dimensions correlated with to the desired geometrical parameters, but machining error is a degree of real dimensions deviated from the desired geometrical parameters.
Relationship:
The size of the machining error denotes the size of machining accuracy. They are negative correlation. The lower machining error is, the higher machining accuracy is.
Example:
When machining screw, the axial moving of spindle causes cycle error of pitch. The more severe the axial moving is, the higher machining error is and the lower machining accuracy is. And machining error caused by axial moving of spindle can be measured, which can denote the degree of machining accuracy.

2 说明原始误差、工艺系统静误差、工艺系统动误差的概念以及加工误差与原始误差的关系和误差敏感方向的概念。

Original error:
Factors that can influence machining error is regarded as original error.
Static error of operation system:
The error existed before machining is considered as static error. For instance, error of machining principle, error of machining tool, error of jigs and fixture, error of cutters, clamping error and error of adjustment.
Dynamic error of operation system:
The error produced during machining is regarded as dynamic error, including deformation caused by force, thermal deformation, tool wears.
Relationship between machining error and original error:
Original error is the factor can cause machining error directly. For example, clamping error is not machining error, but with this error, the cutters are not perfectly located in the desired place, which would cause error during machining.
Sensitive direction of error:
The direction which most considerably influence machining accuracy is sensitive direction, which is vertical to machining surface. By contrast, non-sensitive direction is tangential to machining surface.

3 当车床主轴径向跳动频率为主轴回转频率4倍时，画图说明车削外圆时可能得到的工件形状。

Eclipse. If the direction of radial round bounce of spindle is horizontal, then the oval long edge is vertical.

4在车削前，工人经常在刀架上装上镗刀修整三爪卡盘三个卡爪的工作面或花盘的端面，其目的是什么？能否提高主轴的回转精度（径向跳动和轴向窜动）？

提高三爪自定心卡盘3个卡爪的工作面所确定的圆心与主轴回转中心的同轴度，
提高花盘的端面所确定定位平面与主轴回转中心的垂直度， 总之是为了减少夹具（三爪自定心卡盘和花盘）在机床主轴上的安装误差，提高工件加工精度，但不可能提高主轴的回转精度（径向跳动和轴向窜动）。

5 何谓工艺系统的刚度、柔度？它们有何特点？工艺系统刚度对加工精度有何影响？怎样提高工艺系统的刚度？

System rigidity is vertical component of force $F_y$ divided by vertical deformation $y_s$ystem.

$$K_{s}ystem=F_y/y_{s}ystem$$

And system flexibility is the countdown of rigidity.

$$C=1/K_{s}ystem$$

Characteristics:
整个工艺系统的刚度比其中刚度最小的那个环节的刚度还小；
工艺系统刚度在沿工件轴向的各个位置是不同的。

Influence for machining accuracy:
During cutting processes on different cross-sections, system rigidity varies form workpiece on the axial direction, which change the location of force due to different degree of elastic deformation of the workpiece. Thus, the diameter of each cross-section is different, which cause machined shape error.
During cutting processes on the same cross-section, the magnitude of cutting force is changing due to uneven hardness of materials or changing of machining allowance. As the result of K_system is static on one cross-section, the deformation of each place of the cross-section is different, which causes machining error of workpiece.

Measurements to increase system rigidity:
Proper design for structures. For example, reduce mating surfaces as much as possible, consider mating rigidity and prevent from low rigidity phase.
Increasing contact rigidity of interface.
Using proper clamping and machining method. For instance, change axial tension into axial pressure for workpiece.