Metal Stamping Defects: Identify, Investigate and Verify

Start with the symptom and the first affected operation

Keep an affected sample and a recent accepted reference. Record the drawing revision, material batch, operation, time of first observation and how the part was supported during measurement. Photograph the whole part and the marked defect area.

Separate potentially affected work while its extent is assessed. Inspection or tool access must follow the site's isolation procedures; a photograph cannot establish whether a machine is safely stopped.

• Mark which face, edge or radius is affected and whether the pattern repeats.
• Compare parts before and after the suspected operation, including handling and packing.
• Use the diagram to name the symptom; use the checks below to investigate its cause.

• Metal stamping production process

Which defects can appear at each operation?

The groups below describe observable features and nonconformities, not a closed industry checklist. One part can show several groups; a name such as 'hole position out of tolerance' does not identify a single cause.

These categories can overlap: a pierced edge can look sound until a later flange splits it. Record the first affected operation before selecting a corrective trial.

• Cutting, piercing and trimming: excessive burrs, edge or hole-edge cracks, torn or rough cut faces, size or position errors, missed or incomplete cuts, and distortion of thin blanks. Check the edge immediately after cutting and again after forming. [1]
• Bending, flanging and sizing: outer-bend or expanded-hole cracks, inner wrinkles or folds, released angle error, sidewall curl or twist, and flange-height or profile errors. Confirm the drawing's free or restrained measurement state. [1][3]
• Deep drawing: flange or wall wrinkles, radius or wall splits, local necking or excessive thinning, uneven cup rims (ears), and too little stock after trimming. Rim defects apply only when the part has the relevant rim and trim requirement. [2][5]
• Surface and contact: scores, galling, die marks, dents and embedded particles; some materials also show slip lines or ridging. Find the first stage where the mark appears, including feed, transfer and packing. [4][6]
• Overall shape and fit: flatness or profile error, hole-to-mating-surface mislocation, local bulging, wall curl, twist or assembly interference. Compare parts using the same datums and support. [3]

A — Cracks: locate the starting point

Look for the start of the crack, not only its widest opening. A sidewall or radius split with visible thinning calls for different checks from a crack that starts at a sheared edge. Edge condition and later stretching can interact. [1][2]

Compare local thickness, material grade and condition, blank position and nearby cut edges with accepted parts. For a drawn region, review the path by which material enters it, including lubrication and restraint. For an edge-origin crack, inspect the earlier cut before changing the forming stage.

Engineering may review the cutting condition, radius, material flow or operation sequence according to the evidence. A trial must check that the crack is removed without introducing wrinkles, excessive thinning or a new fit problem.

B — Wrinkles: check support and material flow

Wrinkles are waves associated with compressive material flow or insufficient support. Record where they first appear: the flange, sidewall or a locally unsupported region. Also check whether they are present before forming or introduced by later handling. [2]

Compare blank placement, contact and support, lubrication coverage and the way material draws in around the part. An uneven pattern can help locate an imbalance, but appearance alone cannot identify the setting to change.

More restraint can reduce wrinkling while increasing stretching and the risk of splitting. After a reviewed trial, check the complete shape, local thickness and crack-prone regions as well as the original waves.

For a drawn cup, also check whether the rim has ears or the blank falls short of the trim outline. These depend on the part and blank shape; a published holder force or thinning value is not a general setup instruction. [5]

C — Springback: compare the same measurement state

Springback is elastic recovery when forming loads and constraints are removed. It can change a bend angle, curve a wall or twist a part. Material properties, work hardening, geometry and the loading history influence the result; extra press force is not a universal correction. [3]

Measure accepted and affected parts from the same datums, with the same supports and in the state required by the drawing. Pressing a flexible part flat for inspection can hide a free-state problem. If it changed after trimming, include that operation in the investigation.

Compare material batches and verified tool conditions before reviewing compensation or sizing. Recheck hole positions, assembly fit and the required free or constrained condition after any change.

D — Scoring and galling: follow the contact path

Scoring is a surface groove; galling involves adhesion and material transfer at sliding contact. They can occur together. A line on a part can also come from feed equipment, a conveyor or packaging, so locate the first stage where it appears. [4]

Compare the direction and repeated position of marks with contact surfaces. Under safe inspection conditions, check for adhered material, particles, damaged surfaces and inconsistent lubrication. Do not label every scratch as a rough die.

Restore the verified cleaning or lubrication condition where evidence supports it; tooling repair needs an engineering assessment. Inspect several subsequent parts and the affected contact area to see whether the marks recur, and confirm that surface and dimensions still meet the requirements.

Slip lines or bending ridges on some sheet grades can come from incoming material condition. Do not classify every line as die scoring; compare an unformed sample and the first formed part. [6]

E — Burrs: inspect the cut edge, not only its height

A burr is a raised lip associated with a cut edge, usually on the exit side. Its location and size can vary around the contour. Tool wear or damage, clearance, alignment and material variation can contribute. Burr height alone does not describe all cut-edge damage or later edge formability. [1]

Mark the worst locations and compare both sides and the cut-face appearance with a known-good part. Check the agreed measurement method and the material thickness before reviewing the cutting edge and alignment. A burr on one section of the contour needs localized investigation.

Do not apply one clearance percentage or burr limit to every part. After an approved correction or deburring operation, recheck hole size, remaining edge geometry and areas that affect fitting, wiring or contact. Removing a sharp lip does not repair a crack underneath it.

A sheared edge may show rollover, a burnished zone, a fracture zone and a burr zone. Their presence alone does not fail a part. Assess edge damage against the drawing, contact or wiring use, and any later edge forming. A crack that starts at this edge calls for checking the cut as well as the later stretch. [1]

Separate incoming material from later damage

Coil set, crossbow or edge wave can be present before the sheet reaches the press. Keep an unformed sample and compare it with parts after the first operation; changes in thickness or material condition can also affect forming and dimensions. [6]

Plating peel, paint loss, weld defects and packing scratches need their own process trace. Die wear, changing clearance, feed offset and poor lubrication are possible contributors or process conditions, not a root cause established by a photograph.

Use the pattern across parts to narrow the checks

A sudden change after a material or setup change suggests checking that transition first. A trend that develops during a run makes wear, buildup and changing process conditions worth checking. Neither pattern proves the cause.

The same mark at the same location across parts points toward a repeated operation or contact. Marks appearing only after transfer or packing require a handling check. Keep batch and sequence information so these comparisons are made between equivalent parts.

Verify a correction before releasing affected work

Define the acceptance criteria and the trial conditions before changing a reviewed factor. Keep the material, measurement method and other relevant conditions comparable; record what changed and the trial sequence. Only qualified personnel should adjust or inspect tooling under the required safe procedures.

Recheck the original defect and other features the change can influence. Then monitor repeated parts for recurrence using the agreed control plan. One improved sample does not establish stable production or determine the full extent of an affected batch.

Record the decision, responsible reviewer and disposition of affected parts. Rework, sorting or release must follow the agreed requirements; do not silently widen a tolerance to pass a trial.

Before volume production, close the trial against the agreed drawing and material revisions, record the disposition of affected parts, inspection method and approver. If the buyer requires first article inspection (FAI), PPAP or other records, agree the scope in advance; these are not automatic deliverables for every stamping job.

• Define acceptance: mark critical edges, mating holes, appearance faces and whether dimensions are measured free or restrained.
• Locate the first affected operation: retain affected and recently accepted parts, log batch and sequence, and isolate possibly affected work under site procedure.
• Record each reviewed adjustment: qualified personnel document the changed factor, material lot, trial order and results while keeping other conditions comparable.
• Check the whole part and repeated output: inspect the original defect plus affected holes, angles, edges and fit; monitor recurrence using the agreed control plan.

Example: a better angle can still be a failed part

Teaching example: a drawing requires a 90° ± 1° angle and a 10.00 ± 0.10 mm hole. After a corrective trial, the angle measures 90.5°, but the hole measures 10.16 mm, using the agreed measurement conditions.

The angle is within 89°–91°, while the hole exceeds its 10.10 mm upper limit. That trial part fails the hole requirement, even though the original angle problem improved. Continue the investigation and verify both features together. These are illustrative values, not measured factory results.