What are the factors affecting the machining accuracy of auto parts mold?

The machining accuracy of automobile parts mold is influenced by many factors, including equipment, technology, materials, human operation and other dimensions. The following is an analysis of key influencing factors and countermeasures:

A, processing equipment and cutting tool factors

1. The accuracy of the machine tool is insufficient

Influence: The spindle accuracy, straightness of guide rail and clearance of ball screw of machine tool directly determine the machining error. For example, spindle jumping will lead to eccentricity of hole processing, and guide rail wear will lead to flatness deviation.

Coping with:

Regularly calibrate machine tools (such as laser interferometer to detect positioning accuracy) and replace worn parts (such as lead screws and guide rails).

Choose high-rigidity machine tools (such as gantry machining center) to meet the needs of milling and boring high-precision molds.

2. Tool wear and clamping problems

Influence: Tool wear will lead to cutting dimension deviation (such as diameter reduction and surface roughness deterioration); Poor matching between the tool holder and the spindle will cause vibration and affect the machining accuracy.

Coping with:

Adopt coated cemented carbide cutter or diamond cutter to improve wear resistance; Set a reasonable tool life threshold and replace it regularly.

Ensure that the tool shank and the spindle taper hole are clean, and use hydraulic or heat shrinkable tool shank to improve the clamping accuracy (runout ≤0.005mm).

Second, the process parameters and cutting strategy

1. Improper selection of cutting parameters

Impact:

Rotational speed and feed rate: too high rotational speed may lead to heating and deformation of the tool, while too low rotational speed will lead to low efficiency; Excessive feed is easy to cause vibration or tool damage.

Cutting depth: Too large a single cutting depth will cause the machine tool load to surge, resulting in elastic deformation (such as "giving way to the tool" phenomenon).

Coping with:

Parameters are optimized by cutting simulation (such as UG and Mastercam), and "AG gives low cutting depth" is adopted for rough machining and "low feed and high rotation speed" is adopted for finishing.

Use cutting fluid or oil-gas lubrication to reduce cutting temperature and thermal deformation.

2. The processing path planning is unreasonable.

Influence: The chaotic tool path may lead to obvious tool contact marks and large error of surface contour, especially in complex cavity machining, which is prone to over-cutting or under-cutting.

Coping with:

Adopt spiral cutting and arc cutting to reduce the impact; Keep continuous cutting during finishing to avoid frequent start and stop.

For high-precision surfaces (such as automobile panel dies), a five-axis linkage machining center is used to realize smooth cutting through multi-axis interpolation.

Third, materials and heat treatment factors

1. Mold material characteristics

Impact:

Uneven hardness of materials (such as stress concentration in forging blank) will lead to fluctuation of cutting force and dimensional deviation.

High-hardness materials (such as die steel above HRC50) wear the tool seriously, which easily leads to the decrease of machining accuracy.

Coping with:

Blank pretreatment: isothermal annealing is adopted to eliminate internal stress and ensure uniform hardness (deviation ≤HRC2).

Choose cutting tools according to the hardness of materials: cemented carbide below HRC45, ceramic or CBN cutting tools above HRC50.

2. Heat treatment deformation

Influence: During quenching, the die is deformed due to thermal expansion and cold contraction and structural transformation, which leads to the out-of-tolerance size after finishing.

Coping with:

Adopt vacuum quenching, step quenching and other processes to reduce deformation; Aging treatment (such as 180℃×24h) shall be carried out before finishing to eliminate internal stress.

Leave enough machining allowance (such as 0.3~0.5mm grinding after quenching), and correct the deformation error by grinding.

Four, measurement and clamping factors

1. Error of measuring tools and methods

Influence: The calibration of coordinate measuring machine (CMM) is not in place, the probe accuracy is insufficient (such as ±0.002mm) or the measurement datum is selected incorrectly, which will lead to the deviation of size judgment.

Coping with:

Calibrate the measuring equipment regularly (for example, once a year) and verify the accuracy with standard balls; Select the mold design datum as the positioning datum during measurement.

The complex surface is scanned by blue light or detected by CT to realize full-scale three-dimensional deviation analysis.

2. Workpiece clamping deformation

Influence: excessive clamping force leads to die deformation (such as rebound of thin-walled parts after clamping), and improper clamping position leads to cutting vibration.

Coping with:

Use elastic clamp or hydraulic clamp to evenly distribute clamping force; Thin-walled parts are clamped by vacuum adsorption.

Follow the "six-point positioning principle", avoid over-positioning or under-positioning, and ensure the stability of the workpiece during machining.

V. Human operation and environmental factors

1. Skill level of operators

Impact: programming errors (such as G code instruction errors) and tool alignment errors (such as manual tool alignment deviation > ＞0.01mm) will directly lead to processing scrap.

Coping with:

Strengthen the training of programmers, use CAM software to automatically generate programs and carry out simulation verification; Popularize automatic tool alignment instrument (such as Lei Nishao tool alignment instrument) to reduce human error.

2. Workshop environment fluctuation

Impact:

Temperature change: for every 1℃ fluctuation of room temperature, the longitudinal error of steel parts is about 0.0012mm/m, and the temperature of precision molds (such as tolerance of ±0.005mm) should be strictly controlled.

Vibration interference: Vibration of peripheral equipment (such as punching machine and large fan) will lead to cutting chatter and affect surface accuracy.

Coping with:

The precision machining workshop is equipped with a constant temperature system (20 0.5℃) to reduce thermal deformation; Install vibration isolation pads on the machine tool, away from the vibration source.

Six, a typical case: automobile panel die precision control

Taking the automobile door panel die as an example, its profile accuracy is required to be ≤±0.03mm, which needs to be controlled emphatically:

Processing sequence: rough milling → semi-finish milling (with 0.5mm allowance) → quenching → aging → finish milling → grinding and polishing.

Detection means: Use gantry coordinate measuring machine to measure the grid points of key surfaces (such as drawbead and fillet) (with a distance of 5mm) to ensure the surface fit.

Compensation technology: through the pitch error compensation and thermal deformation compensation functions of machine tool numerical control system, the inherent error of equipment is corrected.

Summary: the core logic of precision control

The machining accuracy of automobile parts dies needs to be realized through the closed-loop management of the whole process of "equipment precision guarantee → process parameter optimization → material state stability → clamping measurement accuracy → environmental artificial control". For high-precision molds (such as injection molds with tolerance of ±0.002mm), advanced technologies such as on-line inspection and error compensation algorithm (such as machine learning to predict machining deformation) should be introduced to continuously improve machining consistency.