In high-precision stamping of large annular components, one recurring headache is the formation of creases or fold marks when forming T-shaped segments that must later be joined into a full circular blank. A common workaround—using multiple V-dies or progressive bending stations—often leaves witness lines on the material surface, which become unacceptable for downstream applications like motor laminations or clutch plates. The die set discussed here tackles this by splitting the circular geometry into individual T-pieces, then using a single-action press with a specially ground punch and die insert that transitions the blank from flat to a pre-curved arc in one hit. The key is controlling the material flow rate: the punch radius is set to 1.2 times the material thickness (for 1.5 mm SPCC steel, that’s a 1.8 mm corner radius), and the die clearance is held at 0.05 mm per side, which prevents localized thinning and eliminates the “springback fold” that typically appears at the segment junction.

From a mold engineering standpoint, the real innovation lies in the segmented insert design. Instead of a one-piece die cavity, the lower die uses four interchangeable hardened inserts (SKD11, HRC 58–60) that form the T-slot profile. Each insert is dowel-pinned and shimmed to allow fine adjustment of the gap between adjacent T-pieces—this gap is critical, as it must be 0.03–0.05 mm wider than the material thickness to avoid pinching during the final coining stage. The upper punch is ground with a 0.2° draft angle on the vertical walls, and a nitrogen gas spring (5 kN) is mounted in the stripper plate to hold the strip down until the punch fully retracts. In production trials, this setup achieved a flatness deviation under 0.08 mm across a 300 mm diameter assembled ring, with zero visible creases on the surface. Cycle time was 18 strokes per minute, and tool life exceeded 120,000 hits before the first regrind—largely because the segmented inserts can be individually replaced without dismantling the whole die set.

For shops that run mixed-batch jobs, this approach also simplifies changeover. The die base is standardized to a 200 mm × 250 mm bolster plate, and the T-segment inserts can be swapped in under 15 minutes using a quick-release clamp system. One caution: the strip width must be held to ±0.02 mm, otherwise the T-piece edges will not align properly during the circular assembly step, leading to uneven welding gaps. Also, lubrication matters—use a low-viscosity stamping oil (ISO VG 10) applied via roller coater, because heavy grease tends to trap fine debris and cause scoring on the insert faces. If you’re evaluating similar tooling for your own production line, pay close attention to the punch-to-die clearance and the insert hardness; both are the difference between a clean T-piece and a rejected one. For more detailed die design calculations or to compare supplier quotes on segmented stamping dies, visit MoldWorld at www.moldw.com—they carry a wide range of technical resources and vetted tooling vendors.