Machining accuracy is mainly used to measure the degree of production. Machining accuracy and machining error are terms used to evaluate the geometric parameters of the machining surface. Machining accuracy is measured by tolerance grade. The smaller the grade value, the higher the accuracy. Machining error is expressed by numerical value. The larger the numerical value, the greater the error. High machining accuracy means small machining error, and vice versa.
There are 20 tolerance grades from IT01, IT0, IT1, IT2, IT3 to IT18. Among them, IT01 indicates that the part has the highest machining accuracy, and IT18 indicates that the part has the lowest machining accuracy. Generally, IT7 and IT8 are medium-level machining accuracy.
The actual parameters obtained by any processing method will not be absolutely accurate. From the function of the part, as long as the processing error is within the tolerance range required by the part drawing, the processing accuracy is considered to be guaranteed.
The quality of the machine depends on the processing quality of the parts and the assembly quality of the machine. The processing quality of the parts includes two parts: processing accuracy and surface quality.
Machining accuracy refers to the degree to which the actual geometric parameters (size, shape and position) of the parts after processing are consistent with the ideal geometric parameters. The difference between them is called processing error. The size of the processing error reflects the level of processing accuracy. The larger the error, the lower the processing accuracy, and the smaller the error, the higher the processing accuracy.
Adjustment Method of Machining Accuracy
Reduce Machine Tool Errors
- Improve the Manufacturing Accuracy of the Spindle Components
The rotation accuracy of the bearings should be improved:
- Select high-precision rolling bearings.
- Use high-precision multi-oil wedge dynamic pressure bearings.
- Use high-precision static pressure bearings.
The accuracy of the accessories with bearings should be improved:
- Improve the machining accuracy of the box support hole and the spindle journal.
- Improve the machining accuracy of the surface matching with the bearing.
- Measure and adjust the radial runout range of the corresponding parts to compensate or offset the errors.
- Appropriate Pre-tightening of Rolling Bearings
Can eliminate clearance;
Increase bearing stiffness;
Equalize rolling element errors.
- Make the SpindleRotation Accuracy Not Reflected on the Workpiece.
Make Adjustments to the Process System
Trial Cutting Method Adjustment
Through trial cutting – measuring the size – adjusting the cutting amount of the tool – cutting – trial cutting again, repeat until the required size is reached. This method has low production efficiency and is mainly used for single-piece small batch production.
Adjustment Method
The required size is obtained by pre-adjusting the relative positions of the machine tool, fixture, workpiece and tool. This method has high productivity and is mainly used for large-scale mass production.
Reduce Tool Wear
The tool must be re-sharpened before dimensional wear reaches the acute wear stage.
Reduce Transmission Chain Transmission Errors
- The number of transmission parts is small, the transmission chain is short, and the transmission accuracy is high.
- The use of speed reduction transmission is an important principle to ensure transmission accuracy, and the closer the transmission pair is to the end, the smaller its transmission ratio should be.
- The accuracy of the end piece should be higher than that of other transmission parts.
Reduce Stress Deformation of Process System
Improve the Rigidity of the System, Especially the Rigidity of the Weak Links in the Process System
Reasonable structural design
- Minimize the number of connection surfaces.
- Prevent local low-rigidity links from appearing.
- The structure and cross-sectional shape of the base and support parts should be reasonably selected.
Increase the contact stiffness of the connection surface
- Improve the quality of the joint surface between parts in the machine tool components.
- Preload the machine tool components.
- Improve the accuracy of the workpiece positioning reference surface and reduce its surface roughness value.
Use reasonable clamping and positioning methods
Reduce Loads and Their Changes
- Reasonably select tool geometry and cutting parameters to reduce cutting force.
- Group the blanks and try to make the blank machining allowance uniform during adjustment.
Reducing Residual Stress
- Add a heat treatment process to eliminate internal stress.
- Arrange the process reasonably.
Reduce Thermal Deformation of Process Systems
- Use reasonable machine tool component structure and assembly datum
- Use thermal symmetric structure – in the gearbox, arrange the shaft, bearing, transmission gear, etc. symmetrically, so that the temperature rise of the box wall is uniform and the deformation of the box body is reduced.
- Reasonably select the assembly datum of machine tool parts.
- Reduce the heat generation of heat sources and isolate heat sources
- Use smaller cutting amount.
- When the precision requirements of parts are high, separate the rough and fine processing processes.
- Separate the heat source from the machine tool as much as possible to reduce the thermal deformation of the machine tool.
- For heat sources that cannot be separated, such as spindle bearings, screw nut pairs, and high-speed guide rail pairs. Improve their friction characteristics from the aspects of structure and lubrication, reduce heat generation or use heat insulation materials.
- Use forced air cooling, water cooling and other heat dissipation measures.
- Balance the temperature field
- Accelerate the achievement of heat transfer equilibrium
- Control the ambient temperature
Causes of Machining Accuracy Errors
Processing Principle Error
The machining principle error refers to the error caused by using an approximate blade profile or an approximate transmission relationship for machining. The machining principle error often occurs in the machining of threads, gears, and complex curved surfaces.
For example, the gear hob used to machine involute gears. In order to facilitate the manufacturing of the hob, the Archimedean basic worm or the normal straight profile basic worm is used instead of the involute basic worm, which causes an error in the involute tooth shape of the gear. For example, when turning a modulus worm, the pitch of the worm is equal to the pitch of the worm wheel (i.e. mπ), where m is the module and π is an irrational number. However, the number of teeth of the replacement gear of the lathe is limited. When selecting the replacement gear, π can only be converted into an approximate fractional value (π =3.1415) for calculation. This will cause the tool to be inaccurate in the forming motion (spiral motion) of the workpiece, resulting in a pitch error.
In machining, approximate machining is generally used to improve productivity and economy under the premise that the theoretical error can meet the machining accuracy requirements (<=10%-15% dimensional tolerance).
Adjustment Error
The adjustment error of a machine tool refers to the error caused by inaccurate adjustment.
Manufacturing Errors and Wear of Fixtures
The errors of the fixture mainly refer to:
- The manufacturing errors of the positioning components, tool guide components, indexing mechanisms, fixture bodies, etc.
- The relative size errors between the working surfaces of the above components after the fixture is assembled.
- The wear of the working surface of the fixture during use.
Machine Tool Error
Machine tool error refers to the manufacturing error, installation error and wear of the machine tool, which mainly includes the guide error of the machine tool guide rail, the rotation error of the machine tool spindle and the transmission error of the machine tool transmission chain.
Machine Tool Guide Rail Guidance Error
Guide rail guidance accuracy – the degree of conformity between the actual movement direction of the guide rail pair moving parts and the ideal movement direction. Mainly includes:
- Guide rail straightness Δy in the horizontal plane and straightness Δz in the vertical plane (bending).
- Parallelism (twist) of the front and rear guide rails.
- Guide rail parallelism error or verticality error in the horizontal and vertical planes to the spindle rotation axis.
The influence of guide rail guidance accuracy on cutting processing. Mainly consider the relative displacement of the tool and the workpiece in the error-sensitive direction caused by the guide rail error. The error-sensitive direction in turning processing is the horizontal direction, and the processing error caused by the guidance error in the vertical direction can be ignored. The error-sensitive direction in boring processing changes with the rotation of the tool. The error-sensitive direction in planing processing is the vertical direction, and the straightness of the bed guide rail in the vertical plane causes the straightness and flatness errors of the machined surface.
Machine Tool Spindle Rotation Error
Machine tool spindle rotation error refers to the drift of the actual rotation axis relative to the ideal rotation axis. It mainly includes spindle end face circular runout, spindle radial circular runout, and spindle geometric axis inclination swing.
- The influence of spindle end face circular runout on processing accuracy:
- There is no effect when processing cylindrical surfaces.
- When turning or boring the end face, the verticality error between the end face and the cylindrical axis or the end face flatness error will be generated.
- When processing threads, pitch cycle errors will be generated.
- The influence of spindle radial circular runout on processing accuracy:
- If the radial rotation error is manifested as its actual axis making simple harmonic linear motion in the y-axis coordinate direction, the hole bored by the boring machine is an elliptical hole, and the roundness error is the radial circular runout amplitude; while the hole turned by the lathe has little effect;
- If the spindle geometric axis moves eccentrically, a circle with a radius equal to the distance from the tool tip to the average axis can be obtained regardless of turning or boring.
- The influence of the inclination swing of the geometric axis of the spindle on the machining accuracy:
- The geometric axis forms a conical trajectory with a certain cone angle in space relative to the average axis. From the perspective of each section, it is equivalent to the geometric axis center moving eccentrically around the average axis center, while the eccentricity values at different locations are different from the axial direction.
- The geometric axis swings in a certain plane, which is equivalent to the actual axis center moving in a simple harmonic linear motion in a plane from the perspective of each section, while the amplitude of the runout at different locations is different from the axial direction.
- In fact, the inclination swing of the geometric axis of the spindle is the superposition of the above two.
Transmission Error of Machine Tool Transmission Chain
The transmission error of machine tool transmission chain refers to the relative motion error between the transmission elements at the first and last ends of the transmission chain.
Process System Stress Deformation
The process system will be deformed under the action of cutting force, clamping force, gravity and inertia force, thus destroying the mutual position relationship of the components of the adjusted process system, resulting in machining errors and affecting the stability of the machining process. The main considerations are the deformation of the machine tool, the deformation of the workpiece and the total deformation of the process system.
The Influence of Cutting Force on Machining Accuracy
Only considering the deformation of the machine tool, for machining shaft parts, the deformation of the machine tool under force makes the machined workpiece appear in a saddle shape with thick ends and thin in the middle, that is, cylindricality error occurs. Only considering the deformation of the workpiece, for machining shaft parts, the deformation of the workpiece under force makes the workpiece appear in a drum shape with thin ends and thick in the middle after machining. For machining hole parts, considering the deformation of the machine tool or the workpiece separately, the shape of the workpiece after machining is opposite to that of the machined shaft parts.
The Influence of Clamping Force on Machining Accuracy
When the workpiece is clamped, due to the low rigidity of the workpiece or the improper clamping force application point, the workpiece will produce corresponding deformation, resulting in machining errors.
Manufacturing Errors and Wear of Cutting Tools
The influence of tool error on machining accuracy varies according to the type of tool.
- The dimensional accuracy of fixed-size tools (such as drills, reamers, keyway milling cutters and circular broaches, etc.) directly affects the dimensional accuracy of the workpiece.
- The shape accuracy of forming tools (such as forming turning tools, forming milling cutters, forming grinding wheels, etc.) will directly affect the shape accuracy of the workpiece.
- The blade shape error of the developing tool (such as gear hobs, spline hobs, gear shaping tools, etc.) will affect the shape accuracy of the machined surface.
- The manufacturing accuracy of general tools (such as turning tools, boring tools, milling cutters) has no direct impact on the machining accuracy, but the tools are prone to wear.
Environmental Impact of the Processing Site
There are often many small metal chips at the processing site. If these metal chips are in contact with the positioning surface or positioning hole of the part, the processing accuracy of the part will be affected. For high-precision processing, some metal chips that are too small to be seen will affect the accuracy. This influencing factor will be identified, but there is no very effective way to eliminate it, and it often relies heavily on the operator’s operating skills.
Thermal Deformation of the Process System
During the processing process, the process system is heated and deformed due to heat generated by internal heat sources (cutting heat, friction heat) or external heat sources (ambient temperature, thermal radiation), thereby affecting the processing accuracy. In large-scale workpiece processing and precision processing, the processing error caused by thermal deformation of the process system accounts for 40%-70% of the total processing error.
The impact of thermal deformation of the workpiece on the processing metal includes two types: uniform heating of the workpiece and uneven heating of the workpiece.
Residual Stress Inside the Workpiece
Generation of residual stress:
- Residual stress generated during blank manufacturing and heat treatment.
- Residual stress caused by cold straightening.
- Residual stress caused by cutting processing.
