Comprehensive knowledge of CNC machining precision
Machining accuracy refers to how closely the three geometric parameters of a machined part’s surface—size, shape, and position—match the ideal geometric parameters outlined in the drawing.
For dimensions, ideal geometric parameters indicate average values. For surface geometry, they correspond to perfect shapes such as circles, cylinders, planes, cones, and straight lines. Regarding the relative positions of surfaces, ideal parameters include absolute parallelism, perpendicularity, coaxiality, and symmetry.
The difference between the actual geometric parameters of a part and these ideal parameters is known as machining error.
1 Introduction
Machining accuracy is a critical measure for assessing the quality of finished products. The terms “machining accuracy” and “machining error” are both used to evaluate the geometric parameters of machined surfaces. Machining accuracy is quantified by tolerance grades, where smaller grades signify higher accuracy. In contrast, machining error is represented numerically, with larger values indicating a greater error. Thus, high machining accuracy corresponds to low machining error, and vice versa.
There are 20 tolerance grades, ranging from IT01 to IT18. IT01 indicates the highest level of machining accuracy, while IT18 indicates the lowest. Generally, IT7 and IT8 represent intermediate levels of machining accuracy. It is important to note that the actual parameters achieved through any machining method can never be absolutely precise. However, from a functional perspective, as long as the machining error stays within the tolerance range specified in the part drawing, the machining accuracy can be considered acceptable.
The quality of a machine relies heavily on the quality of its parts, which is influenced by both machining and assembly processes. The quality of part machining encompasses two key components: machining accuracy and surface quality.
Machining accuracy refers to how closely the actual geometric parameters (size, shape, and position) of a machined part align with the ideal geometric parameters. The difference between these parameters is known as machining error. The size of the machining error reflects the level of machining accuracy—larger errors suggest lower accuracy, while smaller errors indicate higher precision.
2 Related Content
Dimensional accuracy
The term refers to how closely a machined part’s actual dimensions conform to the center of the tolerance zone for those dimensions.
Form accuracy
Refers to the degree of conformity between the actual geometric shape of a machined part’s surface and its ideal geometric shape.
Positional accuracy
Refers to the difference in actual positional accuracy between related surfaces of a machined part.
Interrelationships
When designing machine parts and specifying machining accuracy, it is important to ensure that form errors fall within the positional tolerance and that positional errors are within the dimensional tolerance. In other words, for precision parts or critical surfaces, the requirements for form accuracy should be stricter than those for positional accuracy, and the requirements for positional accuracy should be stricter than those for dimensional accuracy.
3 Adjustment Methods
1. Adjust the Process System
Trial Cutting Method
This involves trial cutting, measuring the dimensions, adjusting the tool engagement, performing a continuous cut, and then re-trial cutting, repeating this process until the desired dimensions are achieved. This method has low production efficiency and is primarily used for small-batch production.
Adjustment Method
The desired dimensions are achieved by pre-adjusting the relative positions of the machine tool, fixture, workpiece, and tool. This method is highly productive and is primarily used for large-scale mass production.
2. Reduce Machine Tool Errors
1) Improve Spindle Component Manufacturing Precision
Bearing rotational accuracy should be improved:
① Select high-precision rolling bearings; ② Use high-precision multi-oil wedge hydrodynamic bearings; ③ Use high-precision hydrostatic bearings.
The accuracy of bearing accessories should be improved:
① Improve the machining accuracy of the housing support holes and spindle journals; ② Improve the machining accuracy of the mating surfaces with the bearings; ③ Measure and adjust the radial runout range of the corresponding components to compensate or offset errors.
2) Properly preload the rolling bearings.
① Eliminate backlash; ② Increase bearing stiffness; ③ Even out rolling element errors.
3) Ensure spindle rotation accuracy is not reflected on the workpiece.
3. Reduce transmission chain errors
1) To achieve higher transmission accuracy, it is important to minimize the number of transmission components and shorten the transmission chains. Additionally, utilizing reduced-speed transmission (where the transmission ratio, i, is less than 1) is essential for ensuring this accuracy. Furthermore, the transmission ratio should be smaller for pairs that are closer to the end of the chain.
3) The end aluminum component‘s accuracy should be higher than that of other transmission components.
4. Reduce tool wear
Tools must be resharpened before dimensional wear reaches the acute wear stage.
5. Reduce stress and deformation in the process system
1) Increase system rigidity, especially the rigidity of weak links in the process system; 2) Reduce loads and their variations.
Improving System Rigidity
① Rational Structural Design: 1) Minimize the number of connecting surfaces; 2) Prevent localized low-rigidity links; 3) Choose the appropriate structure and cross-sectional shape of base and support components.
② Improving the contact stiffness of connecting surfaces: 1) Improve the quality of the mating surfaces between machine tool components; 2) Apply preload to machine tool components; 3) Improve the accuracy of the workpiece positioning reference surface and reduce its surface roughness.
③ Adopting appropriate clamping and positioning methods
Reducing Load and Load Variation
① Rationally select tool geometry and cutting parameters to reduce cutting forces; ② Group the blanks to ensure uniform machining allowance during adjustment.
6. Reduce Thermal Deformation in the Process System
① Reduce and isolate heat sources: 1) Use smaller cutting parameters.
2) Separate roughing and finishing operations when high-precision machining is required.
3) Isolate heat sources from the machine tool as much as possible to minimize thermal deformation.
4) For heat sources that cannot be separated, such as spindle bearings, screw-nut assemblies, and high-speed guideways, enhance their friction characteristics through structural improvements and lubrication methods to reduce heat generation, or utilize thermal insulation materials.
5) Implement heat dissipation measures such as forced air cooling and water cooling.
② Balance the temperature field
③ Adopt a reasonable machine tool component structure and assembly datum: 1) Implement a thermally symmetrical design in gearboxes by symmetrically arranging shafts, bearings, and transmission gears to ensure uniform temperature rise along the casing wall and reduce casing deformation. 2) Choose an appropriate assembly reference for machine tool components.
④ Accelerate the achievement of heat transfer equilibrium
⑤ Control the ambient temperature
7. Reduce Residual Stress
1. Add a heat treatment step to eliminate internal stress; 2. Rationally arrange the process.
4 Influencing Factors
① Machining Principle Error
Machining principle error refers to errors caused by using approximate tool blade profiles or drive relationships during machining. Machining principle error often occurs in the machining of threads, gears, and complex curved surfaces.
For example, gear hobs used to machine involute gears often use an Archimedean basic worm or a normal straight-flank basic worm instead of an involute basic worm to facilitate hob manufacturing, resulting in errors in the gear’s involute tooth profile. Another example is when turning a modular worm, because the worm’s pitch is equal to the worm wheel’s pitch (i.e., mπ), where m is the module and π is an irrational number, the number of teeth on a lathe’s replacement gear is limited. Therefore, when selecting a replacement gear, π can only be converted to an approximate fraction (π = 3.1415). This can lead to inaccurate tool movement (spiral motion) relative to the workpiece, resulting in pitch error. In processing, approximate processing is generally adopted to improve productivity and economy, under the premise that the theoretical error can meet the processing accuracy requirements (≤10%~15% dimensional tolerance).
② Adjustment Error
Machine tool adjustment error refers to errors caused by inaccurate adjustment.
③ Machine Tool Error
Machine tool errors refer to mistakes in manufacturing, installation, and wear and tear. These primarily include errors in guideway alignment, spindle rotation, and transmission within the machine tool drive train.
④ Machine Tool Guideway Guidance Error
1. Guideway Guidance Accuracy refers to how closely the actual movement of the guideway’s components aligns with the intended movement. This accuracy is primarily determined by the following factors:
1. Guideway Straightness: Variations in the horizontal plane (Δy) and vertical plane (Δz) due to bending.
2. Parallelism: The alignment or twist of the front and rear guideways.
3. Perpendicularity and Parallelism Errors: These refer to deviations of the guideway in relation to the spindle axis in both horizontal and vertical planes.
2. The impact of guideway accuracy on cutting performance primarily depends on the relative displacement between the tool and the workpiece in directions sensitive to errors, which are caused by guideway inaccuracies. In turning operations, the error-sensitive direction is horizontal, and machining errors due to vertical guideway errors can be disregarded. In boring operations, the error-sensitive direction changes as the tool rotates. In planing, the error-sensitive direction is vertical, with the straightness of the bed guide rail in the vertical plane contributing to straightness and flatness errors of the machined surface.
⑤ Machine Tool Spindle Rotation Error
Machine tool spindle rotation error refers to the deviation of the actual rotation axis from the ideal rotation axis. It primarily encompasses spindle end face circular runout, spindle radial runout, and spindle geometric axis inclination.
1. Impact of Spindle End Face Circular Runout on Machining Accuracy: Machining cylindrical surfaces has no effect. However, turning and boring end faces can lead to errors in perpendicularity or flatness between the end face and the cylindrical axis. When machining threads, it may result in errors in pitch period.
2. Impact of Spindle Radial Runout on Machining Accuracy: If radial rotation error results in simple harmonic linear motion of the actual spindle axis along the Y-axis, the hole produced by a boring machine will appear elliptical. In this case, the roundness error will correspond to the amplitude of the radial runout. However, when holes are created using a lathe, the radial rotation error has no effect on the shape. If the spindle’s geometric axis shifts eccentrically, both turning and boring operations will create a circular hole, with the radius equal to the distance from the tool tip to the mean axis.
3. The influence of the inclination swing of the spindle geometric axis on the machining accuracy: The geometric axis traces a conical trajectory in space, characterized by a specific cone angle relative to the average axis. From the viewpoint of each cross-section, this is analogous to the geometric axis center moving eccentrically around the average axis center; however, the eccentricity values vary along the axial direction. Additionally, the geometric axis center oscillates within a defined plane. This motion can be understood as the actual axis center performing simple harmonic linear motion within that plane, albeit with different amplitudes of deviation at various locations along the axial direction. In essence, the inclination swing of the spindle geometric axis is the result of the superposition of these two types of motion.
⑥ Transmission Error of the Machine Tool Drive Chain
Transmission error in a machine tool drive chain denotes the relative motion discrepancy between the transmission elements at both ends of the drive chain.
⑦ Fixture Manufacturing Error and Wear
Fixture errors primarily refer to: 1) Errors in the manufacturing of positioning elements, tool guides, indexing mechanisms, and fixture bodies; 2) Dimensional discrepancies between the working surfaces of these components after fixture assembly; and 3) Wear on the fixture’s working surfaces over time.
⑧ Tool Manufacturing Error and Wear
The impact of tool errors on machining accuracy varies based on the type of tool used.
1. The dimensional accuracy of sizing tools (such as drills, reamers, keyway milling cutters, and circular broaches) directly influences the dimensional accuracy of the workpiece.
2. The shape accuracy of forming tools (including forming turning tools, forming milling cutters, and forming grinding wheels) directly affects the shape accuracy of the workpiece.
3. The shape errors of the blades on generating tools (such as gear hobs, spline hobs, and gear shaping tools) influence the shape accuracy of the CNC machined surface.
4. The manufacturing accuracy of general tools (like turning tools, boring tools, and milling cutters) does not directly affect processing accuracy; however, these tools are susceptible to wear.
5. Measurement Methods
Ensuring accurate processing requires different measurement methods based on specific accuracy requirements. Generally, these methods can be categorized as follows:
1. Measurement Type: Measurements can be classified as direct or indirect based on whether the parameter is directly obtained.
- Direct Measurement: This involves measuring the parameter directly to obtain the dimension. For example, using a caliper or comparator to take a measurement.
– Indirect Measurement: This method involves measuring geometric parameters related to the desired dimension, which are then calculated to find the measured value. Although direct measurement is more straightforward, indirect measurement becomes necessary when direct measurement cannot meet the required accuracy.
2. Measurement Representation: Measurements can also be categorized as absolute or relative based on whether the reading of the measuring instrument directly indicates the value of the measured dimension.
- Absolute Measurement: In this case, the reading directly reflects the size of the measured dimension, such as when using a vernier caliper.
– Relative Measurement: Here, the reading indicates the deviation of the measured dimension from a standard. For instance, when measuring the diameter of a shaft with a comparator, the instrument’s zero position must be adjusted using a gauge block before taking the measurement. In this case, the measured value is the difference between the diameter of the shaft and the gauge block size. Relative measurement is generally more accurate, but it is also more complex.
In summary, both direct and absolute measurements are usually simpler and more intuitive, while indirect and relative measurements offer higher accuracy but require more complicated processes.
3. Measurements can be categorized into two types based on whether the measuring surface comes into contact with the measuring instrument’s head: contact measurement and non-contact measurement.
Contact measurement: In this method, the measuring head makes physical contact with the surface, applying a mechanical force. An example of this is measuring a part using a micrometer.
Non-contact measurement: In this approach, the measuring head does not touch the surface of the part being measured. This method helps to eliminate any influence that measuring forces might have on the results. Examples of non-contact measurement techniques include projection measurement and light wave interferometry.
4. Measurement can be categorized into two types based on the number of parameters assessed at a time: single-item measurement and comprehensive measurement.
Single-item measurement involves measuring each parameter of the part separately. In contrast, comprehensive measurement focuses on a single indicator that reflects all relevant parameters of the part. For instance, when using a tool microscope to measure threads, one can evaluate the actual pitch diameter, thread profile half-angle error, and cumulative pitch error individually.
Comprehensive measurement is typically more efficient and reliable for ensuring the interchangeability of parts, and it is commonly used for inspecting finished components. On the other hand, single-item measurements are useful for determining specific errors of individual parameters and are generally applied in process analysis, process inspection, and for measuring specified parameters.
5. Based on the role of measurement in the machining process, it can be divided into active and passive measurement.
Active Measurement: This involves measuring the workpiece during the machining process. The results are directly used to control the machining conditions, helping to prevent the creation of scrap.
Passive Measurement: This type of measurement takes place after machining is completed. It can only assess the quality of the finished workpiece and is limited to identifying and eliminating scrap.
Based on the state of the part being measured, measurements can be categorized as either static or dynamic.
Static Measurement: This measurement occurs while the part is relatively still. An example of this is using a micrometer to measure diameter.
Dynamic Measurement: In this case, the measured surface and the probe move relative to each other as they simulate the actual working conditions. Dynamic measurement methods provide valuable insights into the condition of the part when it is used as intended and reflect advancements in measurement technology.
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