Blog

How should the tolerances for stamped parts be determined?

With the increasing maturity of laser welding technology and the popularity of welding robots, more and more parts are being processed using stamped parts due to their simple assembly, firm welding, and good strength.

This article is divided into unmarked tolerance analysis, marked tolerance analysis, and marked tolerance analysis, which are further divided into flat stamping parts and formed stamping parts. After continuous production for 5000 times, the punching diameter may increase by 0.02~0.05mm, and the mold needs to be regularly adjusted (such as grinding the blade edge).

The formulation of tolerances for stamped parts requires a combination of stamping process characteristics (such as plastic deformation and rebound), material properties (such as strength and elongation), functional requirements (such as assembly and sealing), and manufacturing feasibility. The core logic is to ensure that the parts meet dimensional accuracy and assembly requirements through "standard specifications+rebound compensation+process control". The following are the specific development process and key points:

1. Core standard basis: Specification tolerance marking and grade

The formulation of tolerances for stamped parts must strictly follow national/industry standards to ensure interchangeability and consistency. The commonly used domestic standards include:

GB/T 15055-2021 "Maximum Deviation of Dimensions without Tolerance Indications for Stamped Parts": specifies the linear and angular dimension tolerance levels (f/m/c/v four levels) and maximum deviation for metal stamped parts without tolerance indications, applicable to non fitting dimensions (such as shell contours and reinforcing ribs). Among them, f level (precision level) is suitable for high-precision requirements (such as electronic device casings), m level (medium level) is a universal standard (such as automotive coverings), c level (roughness level) is used for non critical structural components (such as bases), and v level (coarsest level) is used for free size (such as decorative parts).

GB/T 13914-2013 "Dimensional tolerances for stamped parts": For stamped parts with specified tolerances, they are divided into two categories: flat stamped parts (ST1~ST11) and formed stamped parts (FT1~FT10), with precision decreasing from ST1/FT1 (highest) to ST11/FT10 (lowest). For example, ST1 level is suitable for precision electronic components (such as connector pins), while FT1 level is suitable for high-precision bent parts (such as car door sills).

2. Tolerance Level Selection: Balancing Accuracy and Cost

The selection of tolerance levels should take into account both functional requirements and manufacturing economy, avoiding excessive design:

High precision level (ST1~ST3/FT1~FT3): used for precision fitting parts (such as mobile phone charging interfaces, car sensor mounting holes), with a tolerance range of ± 0.05~± 0.2mm (flat stamping parts) and ± 0.1~± 0.3mm (formed stamping parts). This type of part requires precision stamping technology (such as ring gear pressing plate stamping), which is relatively expensive (about 2-3 times that of ordinary stamping).

Medium precision level (ST4~ST7/FT4~FT7): used for general fittings (such as home appliance casings and car door inner panels), with a tolerance range of ± 0.2~± 0.5mm (flat stamping parts) and ± 0.3~± 0.8mm (formed stamping parts). This type of part adopts ordinary punching and bending process, with moderate cost.

Low precision level (ST8~ST11/FT8~FT10): used for non fitting parts (such as bases and support plates), with a tolerance range of ± 0.5~± 1.5mm (flat stamping parts) and ± 0.8~± 2.0mm (formed stamping parts). This type of part uses simple molds (such as open punching molds), which have lower costs.

3. Key dimensional tolerance design: tailored to the characteristics of stamping processes

The key dimensions of stamped parts (such as aperture, bending angle, bending height) need to be designed separately according to their process characteristics:

Punching size (aperture, groove width): The dimensional accuracy of the punched part mainly depends on the punching gap (8%~12% of the material thickness) and the sharpness of the mold edge. For example, a low carbon steel hole with a diameter of 5mm (material thickness of 1mm) adopts A-level precision (GB/T 15055-2021), with a tolerance of ± 0.1mm; if it is a stainless steel hole (material thickness of 1.2mm), due to the high hardness of the material, the tolerance needs to be relaxed to ± 0.15mm.

Bending dimensions (bending height, angle): The dimensional error of bent parts mainly comes from rebound (material elasticity recovery), and the mold angle needs to be adjusted through rebound compensation. For example, a 90 ° bent part (made of 2mm thick low-carbon steel) has a rebound angle of approximately 2 °~3 °, so the mold angle needs to be designed to be 87 °~88 ° to compensate for rebound; The tolerance for bending height is usually ± 0.2~± 0.5mm (medium grade), which needs to be adjusted according to the bending radius (≥ 0.6 times the material thickness).

Forming dimensions (stretching depth, flange diameter): The dimensional accuracy of the stretched part depends on the material elongation (such as low carbon steel elongation ≥ 30%) and the edge pressure (to prevent wrinkling). For example, a cylindrical tensile component (diameter of 50mm, depth of 30mm, material thickness of 1mm) has a flange diameter tolerance of ± 0.3mm (FT5 grade) and a depth tolerance of ± 0.4mm (FT6 grade).

4. Geometric tolerance supplement: Control shape and position errors. Stamped parts are prone to shape errors (such as flatness and roundness) or position errors (such as hole displacement and perpendicularity) due to mold wear and uneven material flow. Geometric tolerance supplement control is required

Flatness: Used to control the surface flatness of flat panel components (such as mobile phone battery covers), usually requiring ≤ 0.1mm/m (precision grade) and ≤ 0.3mm/m (medium grade).

Roundness: Used to control the roundness of shaft components (such as snap in columns), usually requiring ≤ 0.05mm (precision grade) and ≤ 0.1mm (medium grade).

Verticality: Used to control the verticality of assembly surfaces (such as brackets and bodies), usually requiring ≤ 0.1mm (precision level) and ≤ 0.2mm (medium level).

Hole position error: Used for hole type parts (such as bolt connection holes), the tolerance of the hole center distance is ± 0.1~± 0.3mm (medium grade), and the hole edge distance (from the hole edge to the part edge) needs to be ≥ 1.5 times the material thickness (such as 1mm thick plate, hole edge distance ≥ 1.5mm).

5. Rebound compensation: Solving the key problem of stamping parts. Rebound is an inherent defect of stamping parts (material elasticity recovery), which needs to be compensated through mold design or process adjustment:

Angle rebound compensation: For bent parts, the rebound angle θ can be calculated using the formula: θ=E ⋅ r σ s ⋅ t, where σ s is the material yield strength (MPa), t is the material thickness (mm), E is the elastic modulus (MPa), and r is the bending radius (mm). For example, a low carbon steel (σ _2=235MPa, E=206GPa) bent part (t=2mm, r=4mm) has a rebound angle of about 1.1 °, so the mold angle needs to be designed as 88.9 ° (90 ° -1.1 °).

Shape rebound compensation: For complex curved parts (such as automotive panels), CAE simulations (such as Abaqus, Dynaform) need to be used to predict the rebound amount, and then the mold surface can be adjusted iteratively (such as adding protrusions or indentations) to compensate for rebound.

6. Process control: The tolerance fluctuations of key stamping parts that reduce tolerance fluctuations mainly come from process parameters (such as punching clearance, edge pressure, mold temperature), which need to be controlled through process optimization:

Punching clearance: Excessive clearance can easily lead to burrs (height ≥ 10% of material thickness), while insufficient clearance can cause mold wear (such as blade cracking). For example, for 1mm thick low-carbon steel, the punching gap is taken as 0.08~0.12mm (8%~12% of the material thickness).

Edge pressure: The edge pressure of the stretched part should be moderate (such as 2mm thick low carbon steel, edge pressure ≥ 80MPa), to avoid wrinkling (uneven material flow) or cracking (excessive material stretching).

Mold temperature: For high-strength steel (such as boron steel), the mold temperature should be controlled at 80-100 ℃ to avoid material cold work hardening (which may cause difficulties in subsequent processing).

7. Case study: Tolerance design of car door inner panel Taking the car door inner panel (material: low carbon steel, thickness 1.5mm) as an example, explain the tolerance formulation process:

Functional requirements: The inner panel of the car door should be assembled with the door frame, and the hole positions should be aligned with the door lock and hinge. The flatness should meet the painting requirements (≤ 0.3mm/m).

Tolerance level selection: Assembly hole (connected to hinge): ST5 level (flat stamping part), tolerance ± 0.2mm; Bending height (edge of door inner panel): FT6 level (formed stamping part), tolerance ± 0.4mm; Flatness: ≤ 0.3mm/m (medium level).

Rebound compensation: The bending angle is 90 °, the rebound angle is about 1.5 °, and the mold angle is designed to be 88.5 °.

Process control: Punching gap of 0.12~0.18mm (8%~12% of material thickness), edge pressure of 100MPa (to prevent wrinkling).

8. Precautions

Unmarked tolerance: Non critical dimensions should be marked with unmarked tolerance (such as level m in GB/T 15055-2021) to avoid overly complex drawings.

Mold wear: After continuous production for 5000 times, the punching diameter may increase by 0.02~0.05mm, and the mold needs to be regularly corrected (such as grinding the cutting edge).

Testing timing: The parts need to be measured 24 hours after forming (eliminating shrinkage), under environmental conditions of 23 ± 2 ℃ and 50 ± 5% humidity.

Cost control: For every one level increase in tolerance (such as ST5 → ST4), the cost increases by 30%~50%, and excessive design should be avoided (such as marking ST3 level on non mating surfaces).

The process of formulating tolerances for stamped parts can be summarized as follows: functional requirements → selection of standards → selection of tolerance levels → design of key dimensions → supplementation of form and position tolerances → rebound compensation → process control → inspection and verification. The key is to balance accuracy and cost, ensuring that parts meet assembly and usage requirements while reducing production costs through "standard specifications+rebound compensation+process optimization".