Standard Operating Procedure (SOP) for Shearing, Punching, and Press-Brake Bending: Process Accuracy Specifications

[Haifeng Process Standards Quick Reference]

Accuracy in sheet-metal cutting and forming for the frames, tank supports, and pouring-platform base plates used in Haifeng polyurethane equipment establishes the accuracy of the complete machine. Every 0.1 mm/m of straightness error in sheared stock can increase weld-fit-up gaps by more than 0.15 mm, raise welding distortion by 20%–30%, and ultimately put the servo metering-pump mounting-base flatness out of tolerance during final assembly. Every 0.5° of bend-angle error can increase the fit-up gap between a tank support plate and flange by 0.3 mm, directly reducing the pass rate of tank airtightness tests.

Key process parameters at a glance:

1. Scope

1.1 Applicable operations

This SOP applies to the following sheet-metal operations for Haifeng polyurethane equipment:

2. Pre-Job Preparation and Machine Accuracy Verification

2.1 Check technical documents and materials

2.2 Verify machine accuracy

2.3 Verify die and tool accuracy

3. Structured Sheet-Metal Workflow

3.1 Shearing and blanking

3.1.1 Adjust blade clearance

Blade clearance is a key parameter affecting cut-face quality. Excessive clearance causes tearing and burrs; insufficient clearance accelerates blade wear and increases cutting force.

Adjustment method: Set the initial clearance between the upper and lower blades to 0.5 mm. Starting at one end, adjust with a feeler gauge so that clearance is consistent along the full blade length, then return the blade to the target clearance. After adjustment, turn the flywheel by hand to cycle the blades once and recheck the clearance with a feeler gauge.

3.1.2 Shearing parameters

3.1.3 Inspect shearing accuracy

3.1.4 Allowance for weld shrinkage

Based on measured data from Haifeng welding processes, reserve the following process allowances when shearing blanks:

3.2 CNC press-brake bending

3.2.1 Bend-sequence principles

A sound bend sequence is essential for accuracy, reduced die wear, and avoiding workpiece interference.

1. Short sides before long sides: When all four sides require bending, form the short sides first, then the long sides, to simplify locating and reduce interference.

2. Outside to inside: Progress from the outer perimeter toward the center to prevent internal features from obstructing external bends.

3. Local features before general bends: Form special features such as notches and raised sections first, then perform standard bends.

3.2.2 Bending parameters and springback compensation

Common materials for polyurethane-equipment sheet-metal parts are Q235B and 304 stainless steel, which have markedly different springback characteristics.

3.2.3 Deflection compensation

During bending, the ram and worktable deflect under bending force, causing the bend angle at the center to be larger than at the ends (“boat-shaped” deformation). Haifeng requires a mechanical crowning device on the press brake (wedge-type or double-inclined-surface type). It creates a controlled crown in the worktable and a compensation curve along its length that matches the actual deflection profile.

3.2.4 Inspect bending accuracy

3.3 Punch-press punching

3.3.1 Set punching clearance

Punching clearance is one of the most critical stamping parameters. It determines the dimensional accuracy, cut-face quality, and die life of the punched part.

3.3.2 Inspect hole accuracy

3.4 Deburring and surface finishing

3.4.1 Deburring accuracy

Burr height on sheet-metal mating faces and weld-fit-up faces must be ≤0.05 mm. After deburring, use a deburring tool or belt sander to form a 0.1–0.3 mm × 45° chamfer.

4. Key Parameters for Sheet-Metal Machining Accuracy

5. Common Problems and Process Pitfalls

5.1 Excessive sheared-stock straightness error increases weld distortion

Symptom: Diagonal measurements of a welded frame are out of tolerance, and final assembly reveals that the servo metering-pump mounting base is not flat enough.

Root cause: Sheared-stock straightness exceeds 0.5 mm/m, creating uneven gaps during weld fit-up and an imbalanced distribution of welding heat input. Frame fabrication for foaming equipment can suffer from poor weld accuracy, with local rewelding, cutting, or grinding needed during final assembly. The cause often traces back to inadequate straightness control during blanking.

Corrective actions:

1. Inspect the first blank in every batch with a straightedge and feeler gauge. If out of tolerance, immediately adjust shear-blade clearance and hold-down force.

2. Mark the straightness result on each blank. Out-of-tolerance parts must not proceed to welding.

3. Recheck blank straightness before weld fit-up and confirm it is within 0.3 mm/m.

4. Reserve weld-shrinkage allowance during blanking (0.5 mm per transverse seam and 2–3 mm/m per longitudinal seam).

5.2 Incorrect springback compensation causes out-of-tolerance bend angles

Symptom: Bend-angle deviation exceeds ±1°, the tank support plate does not fit the flange closely, and the airtightness test fails.

Root cause: 304/316L stainless steel springs back much more than Q235B carbon steel. Without adequate compensation, springback makes the finished angle too small. A traditional approach is repeated manual trial bending based on operator experience to find the upper ram travel that meets the angle tolerance. This requires substantial labor, lowers productivity, and generates scrap.

Corrective actions:

1. Build a springback-compensation database by material, sheet thickness, and bend radius; trial-bend the first article of each batch.

2. For 304 stainless steel, use 3°–5° of overbend; for 316L, use 3.5°–5.5°.

3. Monitor bend angles with a bend-angle tester and adjust compensation immediately when out of tolerance.

4. After bending tank support plates, inspect their fit to the flange with a dedicated checking fixture.

5.3 Incorrect punching clearance causes poor hole walls and excessive burrs

Symptom: Punched holes in an electrical-cabinet mounting plate have visible tearing; burrs exceed 0.1 mm and mounting bolts do not insert smoothly.

Root cause: Punching clearance is excessive (more than 10% of sheet thickness), increasing the fractured zone and reducing the bright-shear zone. Clearance is a critical stamping parameter: it determines dimensional and cut-face quality and has a major effect on die life.

Corrective actions:

1. Set per-side clearance strictly to 5%–8% of sheet thickness; use the lower range of 5%–7% for 316L stainless steel.

2. After each die change, measure punch-to-die clearance with a feeler gauge and confirm it is uniform.

3. Check every hole with a plug gauge after punching. Stop immediately and inspect die wear if a hole is out of tolerance.

4. Deburr holes with excessive burrs using a chamfering tool, then reinspect them.

5.4 Press-brake deflection makes the center angle of long parts too large

Symptom: On parts longer than 2000 mm, the center bend angle is larger than at the ends, causing “boat-shaped” deformation and excessive straightness error.

Root cause: The ram and worktable deflect under bending force. The ram’s downward deflection interacts with the worktable’s upward deflection, making the center bend angle larger. If crowning is set incorrectly or is insufficient, center-angle deviation on a long part can exceed 1°.

Corrective actions:

1. Enable the crowning device for bends longer than 1500 mm.

2. Set compensation by worktable length and bending force, then verify with a trial bend.

3. Use a bend-angle tester to measure multiple points along the part (at least three: both ends and the center).

4. If compensation is insufficient, correct it by adding compensation shims or adjusting the wedge position.

5.5 Material-thickness deviation causes bending-dimension variation

Symptom: Height dimensions of bent parts from the same batch vary by more than ±0.5 mm, and bend-angle consistency CV is out of tolerance.

Root cause: Incoming sheet thickness deviates by more than ±5%. Thickness is a key input when calculating bend allowance. A 0.1 mm thickness deviation changes the unfolded bend length by about 0.15–0.25 mm, depending on bend radius and K-factor. Uneven incoming thickness therefore causes dimensional variation within a batch.

Corrective actions:

1. Measure incoming material thickness at multiple points with a micrometer (at least five points) for every batch and record the results.

2. Do not use material with thickness deviation above ±5% for precision-bent parts.

3. Use the same coil or sheet for bent parts in one batch; do not mix material lots.

4. Enter the measured thickness in the bending program; do not use nominal thickness.

5.6 Worn punch-die edges shift hole positions

Symptom: Hole-position deviation gradually increases from ±0.1 mm to above ±0.2 mm, and mounting holes no longer align during batch assembly.

Root cause: The punch or die cutting edge wears, increasing clearance and making it uneven. Die wear increases punching force and shifts the punch ram’s bottom-dead-center position. Every additional 0.02 mm of punch wear can increase hole-position deviation by approximately 0.03–0.05 mm.

Corrective actions:

1. Check punch-edge wear before each batch; regrind or replace the punch when wear exceeds 0.02 mm.

2. After adjusting clearance, use a dial indicator to check punch-to-die alignment deviation (≤0.01 mm).

3. Check hole position every 5000 strokes. Stop and inspect the die immediately if drift is detected.

4. Maintain a die-life log recording stroke counts and hole-position measurements after each regrind.