Case Study A : Solving Head Cracking in a Custom 304 Stainless Steel Screw
Project Background
A customer requested a custom 304 stainless steel screw with an oversized head. Compared with an ordinary screw of the same shank diameter, this part required considerably more material to flow into the head during cold forming.
The customer required:
* Consistent head diameter and height
* A fully formed drive recess
* No visible cracks, folds or surface damage
* Stable dimensions during repeat production
* Reliable thread quality after thread rolling
The part was planned for multi-station cold heading, followed by thread rolling, cleaning and final inspection.
The Initial Problem
During the first tooling trial, fine radial cracks appeared around the outer edge of the head on some parts. The cracks were mainly found after the final heading operation.
We inspected samples taken from different forming stages. No cracks were found on the cut blanks or early preforms. The cracks appeared only after the head reached its final diameter.
This indicated that the problem occurred during final head forming rather than during thread rolling. Therefore, the thread-rolling dies were not the primary cause.
Cause Analysis
Our engineering team reviewed the material, tooling and forming process.
1. Excessive deformation in the final station
Too much head volume was formed during the final blow. Material was forced to flow outward too quickly, producing concentrated stress around the head edge.
2. Work hardening of 304 stainless steel
304 stainless steel becomes harder and stronger as it is cold-formed. Because the material had already been deformed in previous stations, its ability to continue flowing was reduced during the final operation.
3. Unsuitable preform shape
The original preform did not distribute enough material toward the outer head area. As a result, the final heading die had to move too much material in one operation.
4. Head-to-shank transition
The transition between the head and shank was too abrupt. This increased local stress and restricted material flow during final forming.
5. Tooling surface and lubrication
The critical forming surfaces were also checked. Excessive friction could further restrict material flow and increase the risk of cracking.
Process Improvement
Instead of simply increasing machine pressure, we modified the forming process.
Progressive head forming
The total deformation was redistributed across multiple forming stations. More of the head shape was created during the preforming stages, reducing the amount of deformation required in the final station.
Preform redesign
The preform profile was adjusted to place material closer to the required final head shape. This allowed the material to flow more evenly toward the outer edge.
Transition optimization
The head-to-shank transition was improved to reduce local stress concentration while remaining within the customer’s drawing requirements.
Tooling improvement
Critical die surfaces were repolished, and the tooling alignment was checked. The lubrication condition was also adjusted to reduce friction during material flow.
Blank consistency
Wire diameter, cut-off length and blank weight were checked to prevent excessive or insufficient material from entering the die cavity.
Second Trial and Verification
After the tooling and process adjustments, a second trial batch was produced.
Samples were collected from each forming stage to confirm that the head developed progressively. The completed parts were inspected for:
* Head-edge cracks
* Incomplete head filling
* Head diameter and height
* Drive recess dimensions
* Head-to-shank concentricity
* Thread dimensions after rolling
* Surface damage after cleaning
No visible head cracks were found in the confirmed trial samples. The head filled more evenly, and dimensional variation was reduced.
A pilot production batch was then completed to verify that the improvement remained stable during continuous machine operation. Final inspection included dimensional sampling and optical sorting for visible defects.
Final Result
By redistributing deformation across the forming stations and optimizing the preform and tooling design, we solved the head-cracking problem without changing the required material or basic product structure.
The improved process achieved:
* No visible head cracks in the approved samples
* More consistent head formation
* Stable drive-recess dimensions
* Acceptable thread quality after rolling
* A repeatable process suitable for batch production
This project demonstrates our ability to analyze cold-heading defects and develop custom stainless steel fasteners according to customer drawings.
Contact
Reach out for custom stainless steel fasteners
Email : china@wubiscrew.com
WhatsApp : +86 13758446714
Location : No. 1889, Binhai 1st Road, Longwan District, Wenzhou, Zhejiang Province, China