Coil Stock Optimization: Strip Width, Nesting and Scrap Rate
Short answer: in progressive die stamping the material you pay for is the strip, and the part usually occupies only 55–75% of it — the rest becomes edge scrap, bridges between parts, pilot holes and the skeleton. Two rules of thumb set the layout: strip width equals the widest part dimension plus two edge allowances of roughly 1.5–2× material thickness (minimum about 1.5 mm), and feed pitch equals the part length plus a bridge of 1.5–2× thickness. Tighten the layout and a typical 35% scrap rate can drop toward 20%, which on a high-volume part is real money — often more than the quoted press hour.
Coil stock is the quiet half of stamping cost. Buyers negotiate die cost, press rate and tolerance, then sign a price that is mostly the metal they never see again. Material utilization in progressive stamping typically lands between 55% and 75%, meaning a quarter to nearly half of every coil ends up as skeleton. This article covers the layout math — strip width, pitch, bridges, nesting — and where scrap goes, so you can read a quotation the way a die designer does.
Strip Width: The First Number in the Layout
Strip width is the coil dimension that defines everything downstream — the die width, the feed, the press, and the largest slice of your material cost. For a part fed lengthwise along the strip, the minimum width is the widest point of the part across the strip direction, plus two edge allowances. Those edges must hold the strip together between stations and carry it through the feed, so the allowance is not decoration.
A practical starting rule is 1.5–2× material thickness per edge, with a floor of about 1.2–1.5 mm on thin stock where handling forces dominate. Thin, flimsy stock needs proportionally more edge to keep the skeleton stiff; thick stock can run relatively tighter because the skeleton is strong. The die designer will also add width for pilots (register holes punched in the scrap or in a carrier) and for any part features that must be reached from the strip edge. If your supplier quotes a strip width, ask how it was derived — a strip 5 mm wider than the theoretical minimum at 0.8 mm copper across 10 million parts is not a rounding error, it is tonnes of metal.
Pitch, Bridges and the Feed Dimension
Pitch is the distance the strip advances per stroke, which sets the die station spacing and the feed length. The minimum pitch is the part length in the feed direction plus a bridge between consecutive parts — again typically 1.5–2× thickness, minimum about 1.2 mm. The bridge exists because the cut between parts happens on the previous stroke; the skeleton must survive that cut and carry the next part into position. Undersized bridges break during feeding and stop the press; oversized bridges are pure scrap.
| Stock thickness (t) | Edge allowance per side (typical) | Bridge between parts (typical) | Notes |
|---|---|---|---|
| t ≤ 0.3 mm | 1.0–1.5 mm | 1.0–1.2 mm | Fragile skeleton; often needs a carrier web |
| 0.3–1.0 mm | 1.2–1.5 mm | 1.2–1.5 mm | Common terminal and contact range |
| 1.0–2.5 mm | 1.5–2.0 mm (≈1.5–2× t) | 1.5–2.0 mm | Brackets, frames, structural parts |
| over 2.5 mm | 1.5–2× t | 1.5–2× t | Thick stock skeleton is strong; proportions shrink |
Pilots add another pitch consideration. If the die registers on pilot holes in the skeleton, those holes occupy strip width and their spacing must divide the pitch cleanly. If the part itself must be registered from a pilot in a carrier — common for stamped terminals and contacts that stay on a reel through plating — the carrier web becomes part of the layout and the customer often pays for it twice: once as material, once as the carrier that ships with the parts. That is legitimate when the carrier is functional for downstream assembly, and pure waste when it is not. Ask whether parts can be delivered loose or in tubes instead of on a carrier if you do not need reel-fed assembly.
Nesting: Where Layout Skill Saves Real Money
Nesting is the art of arranging the part outline so parts share cut lines and the skeleton is minimized. A rectangle lays out simply; real parts have odd outlines, tabs and holes that a skilled designer can interlock. The standard tricks: rotate the part so its longest axis follows the strip (this usually reduces width more than it increases pitch), turn alternate parts 180° so one part's notch fits the next part's bump, and share a single cut line between adjacent parts where the geometry allows — the classic "two parts, one cut" layout. Mirrored pairs (left-hand and right-hand versions of a bracket) often nest at near-zero extra width.
Multi-up layouts push the same logic further: running two or four parts per stroke multiplies press output but does not multiply material use proportionally, because edge allowances and some bridges are shared. The correct comparison is not parts per stroke but material per part. A single row at 70% utilization beats a double row at 55% on material cost, even if the double row doubles the press speed — which is why high-speed terminal dies are often single-row with very tight pitch, running at 800–1,500 SPM rather than two rows at 400 SPM.
| Layout approach | Typical utilization | When it wins |
|---|---|---|
| Single row, single part | 60–75% | Most parts; simple, robust die |
| Single row, mirrored pair | 65–80% | LH/RH pairs, interlocking outlines |
| Multi-up (2–4 per stroke) | 55–70% per part | Small parts where press speed is the bottleneck |
| Tight single row + high SPM | 65–80% | Terminals, contacts at very high volume |
One caution: utilization is not the only cost. A layout that squeezes 5% more material out of the coil but forces a wider press, a weaker skeleton that jams, or a die with fragile sections can cost more in downtime than it saves in metal. The optimum balances material utilization against die robustness and press compatibility — the same judgment that separates a die that runs for weeks from one that runs for hours. Layout, station sequence and tolerances interact, which is why progressive die design is treated as one engineering problem rather than a drawing exercise.
Where the Scrap Goes and What It Costs
Scrap in a progressive die has five destinations. Edge scrap is the strip outside the part outline; bridges are the metal between parts in the feed direction; skeleton is the connected web left after all cuts; slugs are the metal punched out of holes; and pilots are the holes punched for registration. Each is a material cost, and some are recoverable — skeleton and slugs are usually sold as baled scrap, which returns a small credit, while plated or coated offcuts may be worth more or be harder to sell depending on the coating.
| Scrap source | Typical share of strip | Reduction lever |
|---|---|---|
| Edge scrap | 10–20% of width | Tighten edge allowance, rotate part, interlock outlines |
| Bridges | 5–15% of pitch | Minimum bridge consistent with skeleton strength |
| Slug (holes) | 3–10% | Minimize hole area; can be a functional feature elsewhere |
| Pilots and skeleton | 5–10% | Register from part features; avoid oversized pilot webs |
The buyer-side lever is simpler than the die-side one: specify the function, not the strip. If you send a die shop a part with an open interior that will be discarded, ask whether the opening can be reduced or the cutout used for a second part. If you accept parts on a carrier for plating, confirm the carrier width is priced and whether loose parts are an option. Material grade also interacts with layout — stamping material selection affects minimum bend radius, springback and burr behavior, which can force bridges and edges larger than the pure strength math would suggest.
Scrap Rate Math and How to Read a Quote
Scrap rate is simply 1 minus (part area ÷ strip rectangle area per part), where the strip rectangle is width × pitch. If a part's finished area is 320 mm² and it occupies a 20 mm wide × 20 mm pitch layout (400 mm²), utilization is 80% and scrap is 20%. Most progressive layouts land at 25–40% scrap; well-optimized high-volume layouts reach 15–25%; awkward deep-nested parts with big cutouts can exceed 50%. When a quote looks cheap per part, check the assumed strip width — a fat strip hides cost in the material line; when it looks expensive, check utilization before you blame the press rate.
Material price, thickness tolerance and width tolerance are the other coil variables. Coil stock arrives slit to width, and the slitting tolerance plus camber (the sideways bow of the strip) eat into your layout assumptions — a strip that wanders by 0.3 mm across a feed length can clip a die section that was designed for a straight strip. Buying standard coil widths and slitting locally, or buying pre-slit width that matches the layout, is a supply decision your stamping supplier usually owns; it is worth asking whether they slit in-house or buy slit, because that affects both cost and delivery lead time for your orders.
What to Send So the Supplier Optimizes for You
To get an optimized layout rather than a default one, give the die shop the complete picture: the drawing with the grain-direction requirement noted (bending across the rolling direction behaves differently), the annual volume and the order cadence, the material grade and temper, and whether parts must stay on a carrier. Volume matters because layout optimization is tooling work — a 5% utilization gain on a 1-million-part-year job is worth real die engineering; on a 20,000-part job it is not. Grain direction matters because a layout that rotates the part to save metal may rotate it across the rolling direction and change bend behavior — the trade must be explicit.
That is the full loop: strip width, pitch, nesting and scrap rate are one optimization, and the part's functional requirements are the constraints around it. At BQUQ in Dongguan we run the layout review as part of every progressive die quotation, so the strip width on your quote is the one your part actually needs, not the default one. Send the drawing, material and annual quantity to sc@bquq.com and we will show you the layout — with the utilization number — inside 12 working hours.
Frequently Asked Questions
Q: What is a good scrap rate for progressive die stamping?
A: Most progressive layouts run 25–40% scrap. Well-optimized high-volume layouts reach 15–25%. Utilization is 1 minus the scrap rate — a part that uses 75% of its strip rectangle is doing well. Compare utilization per part, not parts per stroke.
Q: How do I calculate the strip width for my stamped part?
A: Take the widest part dimension across the strip direction and add two edge allowances of roughly 1.5–2× material thickness, minimum about 1.2–1.5 mm. Add any carrier or pilot web width. Pitch is part length plus a 1.5–2× thickness bridge.
Q: Should parts be delivered on a carrier strip or loose?
A: Use a carrier only if your assembly is reel-fed or parts need handling through plating. A carrier is material you pay for twice — as strip and as shipping web. Loose parts in tubes or trays often cut total cost if your process accepts them.
Q: Does rotating a part in the strip affect quality?
A: Yes. Bending across the rolling direction behaves differently from bending along it, and grain direction affects springback, minimum radius and strength. If the layout rotates the part, confirm the grain-direction requirement is still met — that is a design trade, not just a material-saving trick.
Q: Can reducing scrap rate ever cost more than it saves?
A: Yes. An ultra-tight layout can force a wider press, a fragile skeleton that jams the die, or thin die sections that break — downtime that dwarfs the metal saved. The optimum balances material utilization, die robustness and press compatibility.
Related Resources
- Progressive die design guide: station layout, pilots and strip design as one engineering problem.
- Stamping materials guide: grades, tempers and how material choice constrains the layout.
- Stamped terminals and contacts: high-speed single-row layouts in practice.
- About BQUQ: ISO9001-certified source factory in Dongguan running stamping, CNC, springs and heat sinks.
- Contact us: send your drawing, material and volume for a quote within 12 working hours.
Authored by the BQUQ Engineering Team. BQUQ is an ISO9001-certified source factory in Dongguan, China, running CNC machining, metal stamping, custom springs, heat sink and collet lines under one roof. Send drawings to sc@bquq.com or WhatsApp +86 13713157787 for a quote within 12 working hours. www.bquq.com
