Technical analysis of linear guide parallelism

Technical analysis of linear guide parallelism

First, the definition and geometric nature of parallelism

Linear guide parallelism refers to two (or more) guide rails in the installation, its guiding surface in the horizontal plane (X-axis) and the vertical plane (Y-axis) within the relative position deviation, the essence of the measurement of the guide rail between the "parallelism error" of the geometric accuracy index. Specifically manifested as:

Horizontal parallelism (lateral parallelism): guide rail lateral guiding surface and the reference surface in the horizontal direction of the parallel deviation, the unit is usually μm/1000mm;

Vertical parallelism (height parallelism): the top surface of the guide rail and the reference surface in the vertical direction of the parallel deviation, affecting the slider running when the pitch accuracy.

Second, the parallelism error quantitative standards and testing methods

Accuracy level and tolerance range

HIWIN HG series guideway as an example:

Ultra-precision (UP level): parallelism ≤ 5μm/1000mm;

Precision (P level): ≤ 10μm/1000mm;

Ordinary (N level): ≤ 20μm/1000mm.

Mainstream inspection tools and processes

Laser interferometer inspection:

Take a guide rail as a reference, laser beam scanning along the length of the guide rail;

Measurement of the distance change between another guide rail and the reference guide rail, such as the maximum deviation value within 1000mm travel.

Precision ruler + micrometer detection:

the ruler across the two guide rails, micrometer sliding along the ruler, reading the amount of fluctuation that is the parallelism error (for short stroke detection).

The multi-dimensional impact of parallelism on system performance

Guiding accuracy and motion straightness

Insufficient parallelism will cause the slider to run with a "yaw", for example:

Parallelism error of 10μm/1000mm, the slider sideways deflection of 10μm in 1000mm travel, resulting in CNC lathe machining long axis cylindricity exceeded (e.g. φ 50mm axis diameter deviation＞0.01mm).

Case: A lithium battery pole cutting machine, due to the guide rail parallelism error of 15μm/1000mm, the edge of the 1000mm long pole cutting waviness reaches 0.03mm, far exceeding the process requirements (≤0.01mm), and after the replacement of the UP grade guide rail, the waviness is reduced to 0.005mm.

Load distribution and life loss

Parallelism deviation will lead to uneven force on the balls in the slider:

When the error of parallelism of the vertical plane is 10μm, the load on the front end of the slider increases by 25%, and the back end decreases by 15%, the local contact stress exceeds the rated value by 30%, which accelerates the wear of the raceway.

Experimental data: after 1000km operation of 10μm/1000mm parallelism guideway, the fatigue spalling area of the raceway is 2.3 times that of 5μm/1000mm guideway.

Vibration noise and dynamic characteristics

Parallelism error makes the slider run with periodic lateral force, which triggers vibration:

A high-speed transplanting machine guideway parallelism 20μm/1000mm, at 80m/min speed produces 110Hz resonance noise (82dB), whereas the parallelism 5μm/1000mm noise is only 65dB.

Fourth, the source of parallelism error and installation control strategy

error generated by the three major factors

machining errors: guide rail installation base surface (such as aluminum or cast iron platform) flatness is not enough (such as the base surface flatness> 20μm/1000mm);

installation process defects: bolt pre-tensioning force is not uniform (such as neighboring bolts pre-tensioning force difference of> 10%) resulting in deformation of the guide rail;

environmental impact: temperature changes (such as temperature difference> 5 ℃) so that the guide rail thermal expansion and cold shrinkage inconsistency. Guide rail thermal expansion and contraction inconsistency, 1000mm steel guide rail temperature difference of 10 ℃ when the length changes 11μm.

High-precision installation implementation points

Base surface pretreatment:

with a grinder to install the base surface flatness control at ≤ 5μm/1000mm, roughness Ra ≤ 1.6μm;

step-by-step pre-tightening process:

first 30% of the rated torque pre-tightening of all the bolts;

micrometer to monitor the degree of parallelism, section by section (every 200mm) will be the bolts torque increased to 100%, to ensure that the parallelism deviation of ≤ 3μm/1000mm;

temperature Temperature compensation:

Control the ambient temperature of 20±1℃ during the installation of precision equipment, and install temperature sensors at both ends of the guide rail to correct the thermal deformation error in real time.

Five, a typical industry parallelism requirements and selection of reference

Sixth, parallelism failure diagnosis and repair program

non-destructive means of detection

Strain Gauge Monitoring: Strain Gauge paste in the key position of the slider, the strain fluctuation amplitude of the parallelism is bad>150 με (normal ≤ 50 με);

Oil Film Distribution Detection: Push the slider after the smear of the red Dan oil to observe the gap of the oil film fracture, the gap>10 μm, that is, the tip of the parallelism is too poor.

Repair technology path

local grinding correction: the parallelism of the super poor section (such as a 200mm area error of 15μm) with precision grinding head grinding guideway mounting surface, each time the amount of grinding control in 2-3μm, until parallelism ≤ 5μm/1000mm;

elasticity compensation structure: in the guideway and the substrate between the installation of stainless steel elastic shims 0.1mm thick, through the elastic deformation absorption of ≤ 5μm parallelism error.

Summarize

Linear guide parallelism is the core index to determine the precision of precision machinery movement accuracy, and its error by affecting the guiding straightness, load balance and dynamic stability, which has a direct effect on the equipment processing accuracy, life and noise control. From nano-scale semiconductor equipment to millimeter-scale industrial machinery, parallelism control needs to run through the "selection - installation - maintenance" cycle: selection based on the working conditions to calculate the allowable error (such as load F = 1000N, parallelism error Δ ≤ F × 0.001mm / N = 1mm), the installation of thermal compensation and step-by-step pre-tensioning process. Fault warning is realized through strain monitoring during operation, and finally the optimal balance between precision and cost is realized.

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