What are the requirements for accuracy in the design of automobile chassis checking fixture?

As the core tool to verify the dimensional accuracy and assembly compliance of chassis parts (such as control arm, subframe, transmission shaft, suspension bracket, etc.), the design accuracy directly determines the quality control effect of chassis parts, and then affects the driving stability, handling and safety of the whole vehicle. Combined with the functional characteristics (bearing, transmission and positioning) of chassis parts, the design accuracy requirements should be developed around four core dimensions: benchmark accuracy, dimensional tolerance accuracy, shape and position tolerance accuracy and functional simulation accuracy. The specific requirements are as follows:

1. Benchmark accuracy: Ensure that the checking fixture is unified with "design benchmark-process benchmark-detection benchmark".

The assembly of chassis parts depends on strict datum positioning (such as holes, surfaces, shoulders, etc.), and the datum of inspection tools should be completely consistent with the design datum (3D digital model datum) and process datum (production and processing positioning datum) of the parts to avoid detection errors caused by misaligned datum. Specific accuracy requirements include:

Datum accuracy

The positioning datum plane of the fixture (such as "datum face" of supporting parts) shall meet the following requirements:

Flatness: ≤0.02mm/100mm (small parts, such as suspension bracket); ≤0.03mm/200mm (large parts, such as subframe);

Surface roughness: Ra≤0.8μm (to reduce the contact error between parts and datum plane and avoid positioning deviation caused by surface unevenness).

Accuracy of reference hole/shaft

The positioning reference hole (such as "datum hole" for locating the bolt) or reference shaft (such as "datum shaft" for supporting shaft parts) of the fixture shall meet the following requirements:

Dimensional tolerance: design according to 1/3~1/5 of the tolerance of the reference hole/shaft of the parts (for example, the tolerance of the reference hole of the parts is φ 10 0.02 mm, and the tolerance of the reference hole of the fixture is φ 10 0 0.004 ~ 0.006 mm);

Roundness: ≤0.005mm (to avoid bolt positioning deviation caused by non-circular reference hole);

Coaxiality: If there are multiple coaxial reference holes, the coaxiality tolerance shall be ≤0.01mm (such as locating holes at both ends of the transmission shaft).

Reference repeatability

The repeated positioning error of the fixture benchmark is ≤0.01mm (verified by calibration tools to ensure that the positioning state of the parts is consistent during each inspection).

Second, dimensional tolerance accuracy: matching the requirements of "key characteristic dimensions" of chassis parts.

The dimensions of chassis parts are divided into "general dimension" and "key characteristic dimension (KC)"-KC dimensions (such as the distance between mounting holes of the control arm ball head and the size of connecting holes between the subframe and the car body) directly affect the assembly clearance and function, and the inspection fixture should focus on ensuring the inspection accuracy of such dimensions. Specific requirements are as follows:

Detection accuracy of key dimension (KC dimension)

The detection tolerance of the fixture for KC size should be controlled within 1/3 of the design tolerance of parts (in line with the industry standard of "Measuring System Error MSA≤1/3 Product Tolerance"):

Example 1: If the design tolerance of the "center distance of the connecting hole of the car body" of the subframe is 500±0.1mm (tolerance zone is 0.2mm), the detection error of the fixture for this dimension should be ≤0.067mm (that is, the machining tolerance of the fixture itself should be ≤ 0.03 mm);

Example 2: The design tolerance of the "bushing mounting hole diameter" of the control arm is φ 25 0.03 mm, and the dimensional tolerance of the corresponding "stop gauge" or "measuring pin" of the fixture should be φ25(+0.03/+0.027)mm (general gauge) and φ25(+0.033/+0.03)mm (stop).

General size detection accuracy

The tolerance of non-KC gauge can be relaxed to 1/2 of the design tolerance of parts (for example, the edge contour size of chassis bracket, the design tolerance is ±0.2mm, and the tolerance of gauge can be set to ±0.1mm), but it must meet the requirements of assembly compatibility testing.

3. Accuracy of form and position tolerance: Control the "geometric conformity" of chassis parts.

The form and position errors of chassis parts (such as bending, twisting and jumping) will lead to assembly sticking, stress concentration or functional failure (such as excessive dynamic balance of transmission shaft). Therefore, it is necessary to design the form and position tolerance detection structure for the fixture, and the accuracy requirements are as follows:

Flatness/straightness

The flatness of the "simulated assembly surface" of the fixture (such as the joint surface between the subframe and the car body) is ≤ 0.02 mm/200mm;

The straightness of the "guide shaft/locating pin" of the checking fixture is ≤0.005mm/100mm (to avoid the deviation of parts installation caused by the bending of the guide).

Verticality/parallelism

The perpendicularity between the reference hole and the reference plane is ≤0.01mm/100mm (for example, the axis perpendicularity between the "ball hole" and the "bushing hole" on the control arm, and the fixture needs to be checked by "right angle block" or "coordinate detection" to ensure this accuracy);

Parallelism of parallel datum plane ≤0.015mm/200mm (e.g. upper and lower supporting surfaces of chassis beam).

Roundness/cylindricity

The roundness of the "supporting hole of shaft parts" in the fixture is ≤0.005mm (such as the detection hole of the supporting sleeve of transmission shaft);

The cylindricity of the "measuring pin" of the fixture is ≤0.003mm (to ensure even contact clearance with the hole of the parts).

Runout tolerance

For rotating parts (such as hub bearing seat), the radial runout of the "radial runout detection benchmark" of the fixture (such as the simulated hub mounting surface) is ≤ 0.01 mm;

The end-face runout of the "end-face runout detection standard" is ≤ 0.008 mm.

Fourth, the accuracy of functional simulation: restore the "actual assembly conditions" of chassis parts.

Chassis inspection tools need to simulate the real assembly state of parts in the whole vehicle (such as bolt connection, clearance fit and motion interference), so the accuracy of "functional simulation structure" directly affects the inspection effectiveness. Specific requirements are as follows:

Simulation accuracy of assembly clearance

The fit clearance between the "locating block of simulated car body/frame" and the parts in the fixture shall be consistent with the actual assembly clearance, with an error of ≤0.02mm;

Example: The design and assembly clearance between the sub-frame and the car body is 0.5±0.1mm, and the fit clearance between the "simulated car body positioning block" of the fixture and the sub-frame should be controlled at 0.5±0.02mm, so as to avoid misjudging the qualification of parts due to the clearance error.

Accuracy of motion interference detection

For movable chassis parts (such as control arm and steering tie rod), the fixture shall be designed with "motion trajectory simulation structure" with trajectory deviation ≤0.03mm;

For example, to control the motion trajectory of the arm under the limit steering angle, the "guide groove" or "limit block" of the fixture should be completely matched with the digital-analog trajectory. When the deviation exceeds the limit, the interference can be detected by the "plug gauge", and the accuracy of the plug gauge should be 0.01 mm.

Simulation accuracy of bolt connection

The coaxiality of the "locating pin/clamping mechanism of the simulated bolt" in the checking fixture is ≤0.01mm (with the bolt hole of the parts), and the clamping force should be matched with the pre-tightening force of the actually assembled bolt (controlled by a torque wrench, the torque error is ≤5%), so as to avoid the deformation of the parts due to improper clamping force and affect the testing results.

V. Additional accuracy requirements: ensure the stability and durability of the checking fixture.

In addition to the core precision, the "long-term stability" and "environmental adaptability" of the checking fixture also need to be guaranteed by precision design:

Material stability

The main material of the fixture should be high rigidity and low deformation (such as 45# steel quenched and tempered, aluminum alloy 6061-T6), and the thermal expansion coefficient of the material should be close to that of the chassis parts (if the parts are steel, the fixture should be made of steel first; Parts are made of aluminum alloy, and the checking fixture can be made of aluminum alloy or steel+heat insulation layer), so as to avoid the accuracy drift caused by temperature change (generally, it is required that the size change of the checking fixture is ≤0.001mm/m for every temperature change of 1℃).

Wear compensation accuracy

The easily worn parts (such as locating pins and stop gauges) in the checking fixture need to be designed with "replaceable structure", and the dimensional accuracy deviation after replacement is ≤0.003mm (realized by reserving adjustment amount or standardized spare parts).

Calibration reference accuracy

The fixture shall be provided with "calibration datum points/holes" (such as three coordinate calibration balls). The dimensional accuracy of the calibration datum shall be ≤0.005mm, and it shall be calibrated by coordinate measuring machine (CMM) regularly (generally every six months) to ensure the accuracy of the fixture itself within the allowable range.

Summary: the core principle of chassis checking fixture accuracy

The design of chassis checking fixture should follow the principle of "accuracy classification and key control": KC size, datum positioning and shape and position tolerance that affect the safety and control of the whole vehicle should be designed according to "parts tolerance 1/3~1/5"; For general size and non-critical structures, the design is based on "1/2 tolerance of parts", and the accuracy stability is guaranteed through material selection, structure optimization and regular calibration. Finally, it is necessary to meet the requirement that "the comparison error between the test results and CMM is ≤0.02mm", so as to ensure that the fixture can not only efficiently detect in batches, but also accurately reflect the real size state of the parts.