Metal Stamping in China: Dies, Tolerances, and the Volume Math

Metal Stamping · High-Volume Forming in China Metal Stamping in China: Dies, Tolerances, and the Volume Math Stamping turns coil stock into a finished metal part in one press stroke, at a per-part cost nothing else touches — once you pay for the die. Here is how progressive, transfer, and compound stamping work, what tolerances they honestly hold, and the DFM rules that decide whether your part stamps cheap or fights the tool. Request a quoteDFM rules What metal stamping is Metal stamping is a cold-forming process that turns flat sheet or coil stock into a finished part using a hardened die in a mechanical press. The die carries the geometry, the press applies the tonnage, and every stroke produces one part — hundreds to thousands of strokes a minute. That is the entire economic point: high fixed cost in the tool, near-zero variable cost in the part. Its neighbors frame the trade. Sheet metal fabrication cuts and bends flat stock with cheap, flexible tooling — right for prototypes and low volume. CNC machining cuts a solid block with no tooling, right for complex geometry and tight tolerance at low volume. Stamping sits at the other end of the volume curve: the die costs real money and takes weeks, but once it exists, the part drops out faster and cheaper than either. The trade is geometry — a stamping must release from a two-piece die, so undercuts and deep internal features are out. The design artifact that matters is the strip layout — how the flat part nests along a coil and which station performs which operation. It, not the part drawing alone, sets the die cost, scrap rate, and the tolerance the part can hold. A supplier who shows it before quoting understands the process. Progressive, transfer, and compound: the three methods "Stamping" is a family, and the method changes the cost, speed, and possible geometry. The three you will specify are progressive, transfer, and compound. Progressive stamping feeds a continuous strip through a die with multiple stations. Each stroke advances the strip one pitch, and every station performs one operation — pierce, then form, then bend, then cut off. One part drops on every stroke, so a 60-stroke-per-minute press makes 60 parts a minute. This is the workhorse for small and medium flat parts at high volume: brackets, contacts, clips, terminals, and shields. It is also the most tooling-intensive, because the stations must stay aligned to a few hundredths of a millimeter across the whole strip. Transfer stamping moves individual blanks from station to station with a mechanical transfer system rather than a carrier strip. Each blank can be rotated, flipped, and reoriented, which opens up deeper draws and larger panels a strip would hold back. It runs slower than progressive, but the die is often cheaper, since the stations are separate tools on a common bedplate. Compound stamping does several operations in a single stroke at one station — classically blanking the outline and piercing the holes together. It suits only flat parts, but is the most accurate method for them, because every hole is cut in the same stroke that defines the outside edge, so hole-to-outline position is set by the die, not by strip feeding. MethodFeedBest forSpeedTooling ProgressiveCarrier strip, station to stationSmall/medium flat parts, high volumeHighest (one part per stroke)Most complex, most expensive TransferIndividual blanks, mechanical transferLarge parts, deep draws, reorientationModerateSeparate station tools, often cheaper CompoundSingle station, one strokeFlat parts needing precise hole-to-edge positionModerateSimple, single-station die The choice follows part size, draw depth, and volume — and it is rarely yours alone. A competent die shop recommends the method from the strip layout and the annual quantity, then prices the die accordingly. The stamping process, step by step Every stamped part is built from a small set of operations, each with its own tolerance, tooling surface, and failure mode. Knowing them is how you read a die quote and spot where a part will drift. Blanking. The flat outline is cut from the strip. Blanking defines the outer profile and the scrap rate — nest parts tightly, because every millimeter of unused coil is cost you paid for. Piercing. Holes, slots, and cutouts are punched through with a punch and die button. Piercing is where burr and hole-position tolerance live, and where the die wears first, because punch and die must hold a controlled clearance or the hole tears instead of shears. Bending. A straight fold is formed along a line. Bending follows the same springback and bend-radius physics as a press brake, but the angle is locked into the die, so it repeats stroke after stroke — at the cost of a tool change if wrong. Drawing. The material is stretched into a three-dimensional shape by pulling the blank over a punch while a blank holder controls metal flow. Drawing is the hardest operation: too little blank-holder pressure and the flange wrinkles, too much and the wall tears. Deep parts draw in stages. Forming. Everything that shapes without cutting or a sharp bend — embossing a rib, coining a thickness, flanging an edge. Forming adds stiffness cheaply, but each formed feature is another station and another wear surface in the die. A progressive die is simply this list chained along a strip, one operation per station. The order is not arbitrary: pierce first, form before blank-off, and keep the part tied to the strip until the last station so it stays located. Materials: steel, stainless, aluminum, copper Four families cover nearly all stamped parts. The choice follows corrosion, strength, weight, and conductivity — and each forms differently, so the material is a DFM decision. MaterialCommon gradesBest forWhat to watch Low-carbon steelSPCC, CRS, DC01Brackets, clips, chassis, structural partsRusts — needs plating or powder coat Stainless steel301, 304, 316Corrosion, springs, food and medicalSpringback; work-hardens; harder on the die Aluminum5052, 6061, 3003Light weight, shielding, drawn cans and cups6061 cracks on tight bends — use 5052 for forming Copper alloysC110, brass C260, beryllium copper, phosphor bronzeContacts, terminals, springs, RF shieldsSoft and gummy; burr control; beryllium cost Cross-check any grade against the ASM Handbook before committing a drawing — formability limits and work-hardening behavior are material science, not opinion. Low-carbon steel is the workhorse: cheap, formable, weldable, but it corrodes, so it ships plated or coated. Stainless is the corrosion and spring answer — 301 in spring temper for flexing contacts, 304 for general corrosion, 316 for chloride exposure — but it work-hardens as it forms, so tight bends cost harder tool steel. Aluminum splits by grade: 5052 forms, 6061 is strong but cracks on a tight radius, 3003 deep-draws. Copper alloys carry current and spring energy: C110 for busbars, brass for terminals, beryllium copper and phosphor bronze for contacts that flex millions of cycles. The full material-selection guide. Tooling: the die is the whole cost The number that dominates a stamping program is the die. It is a precision assembly of hardened tool steel — D2 or SKD11 for punches and die blocks, carbide inserts where wear is worst — held in a guided die set with springs, strippers, and stations. A simple compound die for a flat washer might cost a few thousand dollars; a multi-station progressive die for a complex connector contact can run into the tens of thousands, plus design and tryout. That cost is front-loaded and fixed, which is why stamping is a volume decision. Per-part cost is tiny — a press stroke, a few cents of material, and the amortized share of the tool — so unit price collapses as quantity rises. The crossover where stamping beats CNC and fabrication lands, for a small flat part, in the low thousands to tens of thousands. Below that, you are paying for a die you have not used. Two tooling facts matter at quoting. First, tool life: a well-built progressive die runs hundreds of thousands to millions of strokes before resharpening, so the tool is an asset you own — confirm who owns it and where it is stored. Second, revisions: changing the part after the die is cut means cutting steel again, so freeze the geometry before you tool it. The same volume math applies to die casting. Tolerances: what stamping can honestly hold Stamping is more precise than people assume and less precise than a machined callout demands. A competent die holds roughly ±0.05–0.10 mm on pierced-hole position and blanked profiles in thin material, loosening to ±0.10–0.25 mm as the material thickens and features are formed rather than cut. Bend angles hold within a degree or so; draw depth and flange length are the loosest of all. The loosest number is not the die's fault — it is the material's. Sheet stock carries a mill thickness tolerance, and every bend and draw amplifies it, because the bend allowance and blank-holder pressure are set for a nominal thickness. When a feature genuinely needs machining-grade accuracy, you have three honest options: fineblanking, which shears near-machined edges in a triple-action press; coining, which compresses a feature to precise thickness; or machining the feature after stamping. The last is where in-house CNC earns its keep: the stamping holds the shape and volume, the mill holds the tenths. DFM: bend radius, hole distance, burr, and thickness Most stamping rework comes from four rules broken at the drawing stage. They cost nothing to respect and a full die revision to ignore. Bend radius. The inside radius has a floor set by the material — roughly one material thickness (1T) for mild steel, 1.5–2T for stainless and 6061 aluminum, less for soft copper. Bend sharper and the outer surface cracks. Keep one radius so the die needs fewer forming surfaces. Hole-to-edge distance. Keep a hole at least 1.5–2 material thicknesses from a cut edge, or the thin web distorts and tears under the punch — the single most common first-article failure. Holes near bends. Keep holes at least 2.5× material thickness plus the bend radius away from a bend line, or the hole stretches into an oval as the bend pulls material past it. Minimum hole diameter. A pierced hole should be at least one material thickness in diameter — 1.5T is comfortable — because a punch smaller than the stock is fragile and the exit side tears. Smaller holes are a secondary machining operation. Burr. Piercing always leaves a burr on the side opposite the punch. Specify the burr direction (in or out), and budget a burr height around 5–10% of material thickness. If unacceptable, add deburring or tumbling, or fineblank. Thickness and grain. Design to the nominal thickness the mill actually delivers, and bend across the rolling grain rather than parallel to it, which concentrates stress along the weakest path — especially in stainless and hard aluminum. None of these rules add cost; they remove it. A part drawn with sensible radii, clearances, and a stated burr direction drops straight into the die; the same part drawn without them spends a cycle being re-tooled. The complete DFM guide. Finishes: plating, powder, and passivation Stamped parts are rarely shipped bare, and the finish is set by the base metal: steel is plated or coated, aluminum anodized, copper plated, stainless passivated. Pick the finish before the die is cut, because some finishes need a hole or tab to hang from. Zinc plating is the sacrificial default for steel, usually with a chromate passivation on top. Nickel and tin plating cover copper — tin for solderability and low contact resistance, nickel as a barrier and wear layer. Specify rack or barrel plating up front: barrel is cheaper but tumbles parts, rack holds orientation and finish quality. Powder coat gives steel a thick, any-color film that hides minor flaws — the default for visible brackets and enclosures. Anodizing builds an oxide layer into aluminum — Type II for cosmetics and color, Type III hardcoat for wear — but it grows the surface a few microns, so tight fits need allowance. Passivation is not a coating; it restores the passive chromium-oxide film on stainless. For electrical parts, plating thickness and porosity become functional specifications, not cosmetics. The full surface-finish reference. Stamping vs CNC machining vs sheet metal fabrication Three processes compete for the same metal parts, and the decision comes down to three questions: what volume, what geometry, what tolerance. Stamping wins at high volume for thin, flat, or shallow-drawn parts. The die is expensive and geometry is limited to what a two-piece tool can release, but per-part cost is the lowest and repeatability is excellent. It loses at low volume and while the design is moving. CNC machining wins for solid geometry, complex internal features, and tolerances in the ±0.01 mm range, with no tooling and no minimum. It loses on per-part cost and material waste. Sheet metal fabrication wins for low-to-medium volume of folded parts, because tooling is near-free and lead time is days. It loses at volume, where a press brake cannot match a stamping press on speed or piece price. The practical pattern: fabricate or machine through EVT → DVT → PVT while the design moves, then tool a stamping die once the geometry freezes and the forecast justifies it. A partner that runs both paths tells you where the crossover sits for your part, instead of selling you the one process it owns. Sheet metal fabrication in detail. Why China for stamping China's manufacturing belts run the densest clusters of die shops and stampers anywhere. In the Pearl River Delta — Dongguan and its surrounding towns — the die makers, coil suppliers, heat treaters, and platers sit minutes apart, so a stamping house can quote a die, cut it, and deliver first articles from the same district without importing a single component. That density changes the economics three ways. First, tooling is cheap and fast: a progressive die that costs weeks and a premium in a Western toolroom is a same-district order in the Delta, where the die steel, wire EDM, grinding, and tryout press are all next door. Second, labor depth: die designers and press operators have run your exact part family thousands of times, turning a good die into a great one. Third, finishing is shared: the plater, powder-coat line, and anodizer are specialists serving every stamper, so a small shop delivers a finished part without owning any of those lines. The honest caveat applies to every process in China: the cluster holds world-class shops and shops that cut corners, often quoting similar prices. In stamping the risk concentrates in die quality — soft steel, skipped heat treatment, or a strip layout that drifts tolerance — and in material substitution, where a cheaper grade replaces the one on the drawing. The fix: request the strip layout and a measured first article, confirm the steel grade, and hold the partner to documented quality. Vetting a supplier without flying out. Nex-G: stamping through vetted partners, QA-managed Nex-G's own floor is EMS and CNC under one roof in Dongguan Hengli — a 6,800 m² facility staffed by more than 100 people and running since 2006. Metal stamping is sourced through a vetted partner network and QA-managed by Nex-G, which matters: the stamped part comes back to a shop that can machine to the same drawing, inspect to the same standard, and integrate it into the same build the electronics live in. The anchor is the quality system behind the sourcing. Nex-G holds ISO 9001, IATF 16949 (build-to-print — design excluded under clause 8.3), and ISO 14001, with URS certification to 2027. SPC runs to a Cpk target of ≥1.67 on critical characteristics and a floor of ≥1.33, backed by real data. That discipline is applied to sourced stamping: the strip layout is reviewed before the die is cut, the die is approved from a measured first article, and incoming parts are inspected against the drawing before they enter your build. In-house, 80+ CNC machines from Mazak, Brother, TSUGAMI, and Sodick hold ±0.005 mm on milled and turned features and ±0.002 mm on grinding and wire EDM — the capacity that picks up any stamped feature needing a machined tolerance. The service runs no MOQ, carries programs from EVT → DVT → PVT, and quotes lead times of 3, 7, and 30 days by scope. One partner manages the stamping, the machining, and the board, so the fit is checked at the bench, not at your dock. See the full capability. Frequently asked questions When does stamping beat CNC or sheet metal fabrication?At volume. Below a few thousand pieces, CNC or fabrication wins because there is no die to pay for and the design can still move. Above the crossover, the die amortizes and stamping delivers the lowest per-part cost and best repeatability. How much does a stamping die cost?A simple compound die can run a few thousand dollars; a multi-station progressive die for a complex contact or bracket can run into the tens of thousands or more, plus design and tryout. The cost is front-loaded, which is why the die should be priced from a strip layout, not a guess. What tolerances can I realistically hold?Plan for ±0.05–0.10 mm on pierced-hole position and blanked profiles in thin material, loosening to ±0.10–0.25 mm in thicker stock and on formed features. For a machining-grade feature, fineblank, coin, or machine it after stamping. Progressive or transfer — how do I choose?Progressive for small-to-medium flat parts at the highest volume, where the carrier strip keeps everything aligned and one part drops per stroke. Transfer for larger parts, deeper draws, and shapes that need reorientation. Compound for flat parts where hole-to-outline position is critical. What minimum bend radius should I specify?Roughly one material thickness for mild steel, 1.5–2T for stainless and 6061 aluminum, and less for soft copper. Going sharper risks cracking the outer surface. How do I control burr on stamped parts?Piercing always leaves a burr on the side opposite the punch, typically 5–10% of material thickness. Specify the burr direction, and if unacceptable, add deburring or tumbling, or fineblank for a near-machined edge. Do you make stampings in-house?Nex-G sources metal stamping through a vetted partner network and QA-manages it in-house — strip-layout review, die approval from a measured first article, and incoming inspection under the same ISO 9001 / IATF 16949 / ISO 14001 system and SPC discipline that runs the CNC floor. CNC and EMS are in-house, so stamping, machining, and electronics integrate under one quality gate. Sourcing stamped metal parts for your build?Send the drawing and the annual volume — we will review the strip layout, confirm the die cost and the crossover, and quote the stamping with the machining and electronics under one quality gate.Request a quote Related articlesDie Casting vs CNC MachiningCNC Machining Cost in ChinaThe Complete Guide to Materials Selection