Key takeaways
- Most stamping defects are tool or strip problems, not operator problems.
- A slug that stays in the die can destroy a station in one stroke, so slug detection is a process control, not a housekeeping task.
- Galling is a friction and lubrication problem that leaves a visible trail before it becomes a dimensional failure.
- A crack at a bend usually starts at a sheared edge, which means the trim and the bend belong in the same design review.
Why stamping defects cluster
A progressive die is a sequence of operations that share one strip. A problem at one station changes the material that arrives at the next, so defects tend to appear in groups rather than one at a time. A slug left in a pierce station can mark the strip, damage the next punch and produce a bent part that looks like a forming problem.
The practical approach is to read the defect as a message about the process. Where is it on the part? Is it on one station or across the strip? Does it appear at start-up, after a tool change or after a run of hours? The answer narrows the cause faster than a list of possible faults.
The four families below cover most of the scrap a stamping supplier sees. Each has a different signature, a different first check and a different fix.
Slugs and slug pulling
A slug is the piece of scrap punched out of a hole. It should leave the die through a chute or a drop hole. When it does not, it can be pulled back up on the punch, sit on the strip and be pressed into the next part. The result is a mark, a dent, a cracked punch or a broken die.
The usual causes are a worn punch, a poor die clearance, a lack of stripping force, a shallow drop hole or a drop hole that has become blocked with oil and debris. A magnetic punch can also hold a steel slug.
The first check is the slug path, not the punch. Look at the drop hole, the chute and the scrap bin. Then check the punch edge and the stripper. If the slug is being pulled up, the fix is usually a change in punch geometry, a stripper with more force or a slug detection sensor.
Slug detection is worth the cost on a high-volume tool. A sensor that stops the press when a slug is present prevents a damaged station and a batch of suspect parts. Without it, the first sign is often a broken punch and a long stoppage.
Galling and pick-up
Galling is the transfer of metal from the strip to the tool. It appears as a rough, smeared or shiny patch on the part, usually on a drawn wall, a bend or a coin face. Left alone, it grows, raises friction, scores the part and changes the dimensions.
The cause is a combination of pressure, sliding and poor lubrication. Soft materials such as copper and aluminium gall more readily than steel. A rough tool surface, a tight clearance and a lack of lubricant all make it worse. Stainless steel work-hardens and can gall even with careful lubrication.
The first check is the tool surface and the lubricant. Look at the station where the mark appears, not the station before it. A polished tool, a coating such as titanium nitride or a change of lubricant often fixes the problem. A temporary fix is to increase lubrication, but that can hide a tool surface issue.
Galling also changes the part. A built-up edge on a punch reduces the hole size, and a built-up edge on a form tool changes the bend radius. If the dimension has moved, check for pick-up before adjusting the tool.
Miss-hits and misfeeds
A miss-hit is a stroke where the strip is not in the correct position. The press closes on a partial part or on the carrier, and the result can be a bent tool, a sheared pilot, a distorted strip or a part with a missing feature.
The causes are a feed error, a short feed, a pilot that did not engage, a strip that has buckled, or a sensor that failed to detect the position. A miss-hit is often preceded by a small feed variation that nobody noticed.
The first check is the feed and the pilots. Measure the feed length, check the pilot engagement and look for strip distortion at the infeed. If the tool has a miss-hit detector, check that it is working and that it is set to stop the press rather than just alarm.
A miss-hit is a tool-safety event. Even if the part looks usable, the tool may have been overstressed. Inspect the station before restarting, and quarantine every part made since the last good check.
Cracks at bends and sheared edges
A crack at a bend usually starts at the outside of the bend, where the strain is highest. The crack may be visible as a line, a roughened area or a complete split. Under a magnifier, the origin is often a burr or a micro-crack at a sheared edge.
The causes are a bend radius that is too tight for the material and temper, a burr on the outside of the bend, a grain direction that is unfavourable, or a material that has work-hardened. High-strength steel, stainless steel and hard tempers are more sensitive than mild steel.
The first check is the bend radius against the material. Then check the burr direction: if the burr is on the outside of the bend, it acts as a stress raiser. A trim that leaves the burr on the inside, or a deburr before bending, reduces the risk.
The fix may be a larger radius, a different temper, a change in grain direction or a stress relief after forming. If the part is already tooled, a radius change is a die change, so the design review should catch it before the tool is cut.
A defect reference for the press
The table below is a first-response guide. It is not a substitute for the PFMEA or the tool room diagnosis, but it helps the press team ask the right question before the tool is pulled.
| Defect | Signature | Likely cause | First check |
|---|---|---|---|
| Slug | Dent, mark or pierced hole with scrap pressed in | Worn punch, blocked drop hole, weak stripper | Slug path and drop hole |
| Galling | Smeared or shiny patch, rough surface | Friction, poor lubrication, rough tool | Tool surface at the station |
| Miss-hit | Missing feature, bent carrier, damaged pilot | Feed error, pilot failure, strip buckle | Feed length and pilot engagement |
| Bend crack | Line or split at the outside of a bend | Tight radius, burr on the outside, hard temper | Radius and burr direction |
| Burr height | Sharp edge above the limit | Worn punch or die, wrong clearance | Punch and die edge |
Record the station, the strip position and the time of the first good part before adjusting the tool.
Containment and corrective action
Containment comes first. Stop the press, quarantine the parts made since the last good check and identify the boundary. Then sort the suspect parts against the drawing, not against the sample that looks acceptable.
The corrective action should address the cause, not the symptom. Adjusting the tool to close a dimension without finding out why it moved will produce the same defect again. A useful record states the defect, the station, the cause, the action, the verification and the date.
Verification means running the process after the change and checking the feature that failed. It also means checking the neighbouring features, because a tool adjustment often moves something else. A change to a bend station can alter the developed length and the hole position downstream.
Design choices that prevent the defect
Some defects are designed in. A hole close to a bend, a sharp internal corner, a bend radius below one material thickness or a burr on a critical face all raise the risk before the tool is built. The DFM review is the place to catch them.
The other lever is the tool specification. Slug detection, a positive stripper, a polished form surface, a suitable coating and a maintenance schedule are not optional extras on a high-volume tool. They are the difference between a stable process and a press that stops every shift.
Finally, keep the process window. A tool that is adjusted at every shift to chase a dimension is a tool that is not understood. If the same defect returns, the answer is usually a design or tool change, not another adjustment.
Frequently asked questions
What causes slug pulling in a stamping die?
A slug is pulled back up when the punch holds it, usually because of a worn punch edge, insufficient stripping force, a magnetic punch or a blocked drop hole. The fix is to improve the slug path, increase stripping force or change the punch geometry, and to add slug detection on a high-volume tool.
How can I tell galling from a tool mark?
Galling is transferred metal, so it looks smeared and tends to grow over a run. A tool mark is a repeatable impression from the tool surface. If the mark changes with lubrication or with the number of strokes, galling is more likely.
Why do stamped parts crack at the bend?
The usual causes are a bend radius that is too tight for the material, a burr on the outside of the bend, an unfavourable grain direction or a hard temper. Check the radius and the burr first, then the material condition.
What should we do with parts made before a miss-hit was found?
Quarantine every part made since the last confirmed good check and sort them against the drawing. A miss-hit can damage the tool, so inspect the station before restarting and record the boundary of the suspect lot.
Sources and further reading
- ISO 2768-1:1989, General tolerances, linear and angular dimensions
- VDA 6.3, Process audit, questions and evaluation
- Qumek tool trials and defect reviews for progressive-die stampings
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