Machining error refers to the deviation between the actual geometric parameters (dimensions, shape and mutual position) and the ideal geometric parameters. Machining accuracy is determined by the degree of conformity between actual and ideal geometric parameters.
The smaller the machining error, the higher the degree of compliance and machining accuracy. Both machining error and machining accuracy are used to measure the same problem. Consequently, the size of the machining error indicates the level of machining accuracy.
1. Machine tool manufacturing error
The main manufacturing errors in machine tools include errors in the rotation of the spindle, guide rail and drive chain.
Spindle rotation error refers to the deviation of the spindle's real axis at any time from its mean axis, which directly affects the accuracy of the processed part.
The causes of spindle rotation error include spindle coaxiality errors, bearing errors, coaxiality errors between bearings, and spindle winding.
The guide rail serves as a reference for determining the relative positions of machine tool components and also guides the movement of the machine tool. Guide rail error may be caused by guide rail manufacturing errors, uneven wear and poor installation quality.
Drive chain error refers to the relative motion errors between the drive elements at both ends of the chain, and is caused by manufacturing and assembly errors in each link and wear during use.
2. Tool geometric error
In any cutting process, tool wear will result in changes to the size and shape of the part. The impact of tool geometric errors on machining errors varies depending on the type of tool. With fixed size tools, manufacturing errors will directly affect the machining accuracy of the part. On the other hand, for general tools (such as turning tools), manufacturing errors do not have a direct impact on machining errors.
3. Geometric error of the device
The purpose of a fixture is to ensure that the part is positioned correctly in relation to the cutter and machine tool. As a result, geometric errors in the fixture have a significant impact on machining errors, particularly positional errors.
4. Positioning error
The positioning error mainly consists of inaccuracies in the reference datum and manufacturing inaccuracies in the positioning pair.
When machining the workpiece on the machine tool, certain geometric elements of the workpiece must be selected as a reference point. If the chosen reference point is not aligned with the design reference point (the reference point used to determine the size and position of a specific surface in the part drawing), a reference point mismatch error will occur.
The workpiece positioning surface and the fixture positioning elements form the positioning pair. The maximum positional variation of the workpiece caused by inaccuracies in the manufacturing of the positioning pair and the adjustment gap between the positioning pairs is known as positioning pair manufacturing inaccuracies.
Inaccuracies in the positioning pair can only occur when the adjustment method is used, not the trial cutting method.
5. Error caused by forced deformation of the process system
Part stiffness
In a processing system, if the part has lower rigidity compared to the machine tool, tool and fixture, then under the influence of cutting forces, deformation due to insufficient rigidity of the part will have a significant impact on machining errors .
Tool stiffness
The stiffness of a cylindrical turning tool in the normal (y) direction of the machined surface is very high and its deformation can be disregarded. However, when drilling an inner hole with a smaller diameter, the cutting bar has low rigidity, and the deformation induced by the force of the cutting bar greatly affects the machining accuracy of the hole.
Rigidity of machine tool components
Machine tool parts are made up of many components and there is currently no simple calculation method available to determine their stiffness. The stiffness of machine tool parts is mainly evaluated through experimentation. Factors that affect the stiffness of machine tool parts include deformation of joint surfaces during contact, frictional force, low stiffness components, and backlash.
6. Error caused by thermal deformation of the process system
Thermal deformation in the processing system has a significant impact on machining errors, especially in precision machining and large part machining. In some cases, machining errors caused by thermal deformation can account for up to 50% of the total error in the part.
7. Adjustment error
In every machining process, the processing system must be adjusted in some way. However, as the adjustment cannot be perfectly accurate, there will be adjustment errors. Mutual positional accuracy of the workpiece and cutter in the machine tool is ensured through adjustments to the machine tool, cutter, accessory or workpiece.
When the original accuracy of the machine tool, cutter, fixture and blank meets the process requirements without taking into account dynamic factors, the adjustment error becomes the main factor in determining the machining error.
8. Measurement error
When measuring parts during or after machining, the measurement accuracy is directly affected by the measurement method, the accuracy of the measuring instrument, the workpiece, and subjective and objective factors.
9. Internal stress
The stress that exists within a part without any external force is known as internal stress. Once internal stress is generated in a part, the metal is in a state of high energy potential and instability. To reach a state of lower energy and stability, the metal undergoes deformation, causing the loss of the original machining precision of the part.
