主要原因是卧式铣床几何精度误差和机床刚性不足。
1卧式铣床几何精度：
在加工工件的两个移动方向上，机床的坐标轴的轴线运动的直线度误差；
主轴轴线和加工工件的两个移动方向轴线专运动的垂直度误差；
主轴轴向窜动和主轴锥孔的径向跳动误差。
2机床刚度：
机床刚性不足。在用大刀盘加工平面时，由于切削力和主轴扭矩的增大，使得机床变形加大，导致主轴轴线与加工工件的两个移动方向轴线运动的垂直度误差增加；
3其他因素：
卧式铣床工件装夹不可靠，大刀盘加工时工件窜动属、振动；
工件本身刚刚不足，大刀盘加工时工件让刀、振动；
加工刀具刚性不足或者装夹不牢固，大刀盘加工时刀具窜动、让刀；
上述所有因素均能使卧式铣床加工的零件的平面度超差。
The main reason is the geometric accuracy error of horizontal milling machine and the lack of rigidity of machine tool.
1. Geometric accuracy of horizontal milling machine:
The straightness error of the axis movement of the coordinate axis of the machine tool in the two moving directions of the workpiece;
The Perpendicularity Error of the special movement of the axis of the spindle and the two moving directions of the workpiece;
The axial movement of the spindle and the radial runout error of the taper hole of the spindle.
2. Machine tool stiffness:
The rigidity of the machine tool is insufficient. When machining a plane with a large cutter head, the deformation of the machine tool increases due to the increase of cutting force and spindle torque, resulting in an increase in the perpendicularity error between the spindle axis and the axis in the two moving directions of the workpiece;
3 other factors:
The workpiece clamping of horizontal milling machine is not reliable, and the movement and vibration of workpiece during machining with large cutter head are belong to;
The workpiece itself is just insufficient, and the workpiece gives way to the cutter and vibrates when machining the large cutter head;
The rigidity of the cutting tool is insufficient or the clamping is not firm, and the cutter moves and gives way to the cutter during the large cutter head processing;
All the above factors can make the flatness of parts processed by horizontal milling machine out of tolerance.
