"Selecting Coil Stock: Gauge, Temper and Flatness"
Short answer: coil stock is chosen on four specs — gauge (thickness), temper (strength and hardness), flatness (including camber), and edge condition — and they are not interchangeable. Gauge sets strength and how the part forms; temper controls springback and force tolerances; flatness decides whether a progressive die can feed and locate accurately; edge condition affects die wear and burr height. Specify thickness to about ±0.01 mm, name the temper by standard designation, and call out flatness in mm per 1000 mm. Getting these right prevents the most common stamping problem: inconsistent parts traced back to inconsistent strip.
Strip is the raw material of every stamped part, and its spec decides how the whole project goes. A die that runs perfectly on one coil can produce scrap on the next if the thickness, temper or flatness drifts. Buyers who understand coil stock get better parts, fewer die problems and more honest quotes, because they can specify what actually matters.
The Four Specs That Define Coil Stock
Every coil order should pin down four things.
1. Gauge and thickness tolerance — the nominal thickness and how much it may vary.
2. Temper or grade — the mechanical state of the metal, expressed by standard designation.
3. Flatness and camber — how level and straight the strip is along its length.
4. Edge condition and width — slit edge quality, burr, and coil width tolerance.
Miss any one and you leave a variable the die has to absorb. A die can tolerate small variation, but every tolerance you skip becomes a source of variation in the finished part.
Gauge and Thickness Tolerance
Thickness is the most powerful variable in stamping. It sets part strength, bend radius, springback, and whether the die can shear cleanly. Thickness variation directly becomes part-to-part variation, because a sheared or formed part inherits the strip's thickness.
| Nominal thickness | Typical mill tolerance | Practical stamping note |
|---|---|---|
| 0.10 mm | ±0.005 mm | Very thin; feeding and flatness are critical |
| 0.20 mm | ±0.008 mm | Common for contacts and shields |
| 0.50 mm | ±0.02 mm | General brackets and clips |
| 1.00 mm | ±0.03 mm | Structural parts |
| 2.00 mm | ±0.05 mm | Heavy brackets, busbars |
| 3.00 mm | ±0.08 mm | Heavy-gauge, slower presses |
Specifying a tighter band than the mill can hold raises cost with little gain; instead, design the part so normal thickness variation does not break function. Ask the factory what thickness window the die can run before you finalize the drawing.
Temper: What It Controls
Temper describes how hard and strong the metal is, set by rolling and heat treatment. It is expressed as designations like H14, H02, or T4/T6 for aluminum, or by tensile grade for steel.
| Temper state | Formability | Springback | Typical use |
|---|---|---|---|
| Annealed / soft (O) | Excellent | Low | Deep drawing, tight bends |
| Quarter hard (H02) | Good | Moderate | General stamping, contacts |
| Half hard (H14) | Fair | Higher | Parts needing stiffness |
| Full hard (H18) | Poor | High | Flat springs, high-force clips |
Temper and gauge work together. A spring clip's force comes from both its thickness and its temper, so the two tolerances stack. If force tolerance is tight, control both tightly — or accept a wider force band. For contact and clip parts, temper directly affects retention force, which is why we flag it on every drawing review.
Flatness and Camber
Flatness is the spec buyers most often forget and most often regret. A coil with poor flatness causes feeding jams, poor location at the pilot, and parts that are not flat after cutoff. It shows up as inconsistent hole positions and bends that vary from the start to the end of a coil.
Two numbers matter:
- Flatness: maximum deviation from a flat plane, typically stated as mm per 1000 mm of length.
- Camber: sideways curvature along the strip, which steers the strip off-center and stresses pilots.
| Quality level | Flatness (mm per 1000 mm) | Camber | Suits |
|---|---|---|---|
| Precision strip | ≤ 2 | ≤ 2 mm per 1000 mm | Progressive dies, contacts |
| Standard strip | ≤ 5 | ≤ 5 mm per 1000 mm | General parts |
| Mill edge, rough | > 5 | > 5 mm per 1000 mm | Loose-tolerance parts |
For progressive dies running fine features, insist on precision flatness. For simple blanks, standard strip is fine and cheaper.
Coil Width, Weight and Edge Condition
Coil width must match the die's strip layout with allowance for the feeder. Width tolerance is typically ±0.1 to ±0.3 mm depending on slitting quality. Edge condition matters: a burred or ragged slit edge wears the die's stock guides and entry, and can introduce cracks that propagate when the part is formed.
Coil weight controls how long the press runs between coil changes. Larger coils mean fewer stops and better productivity, but they need a bigger reel and a feeder that handles the mass without slipping. Slitting method — rotary shear versus a fine slitter — changes edge quality and burr, and that in turn changes the deburring step the finished part may need.
Matching Stock to the Press and Part
The final check is whether the stock is right for how the part is made. High-speed progressive dies want precision flatness, consistent gauge and clean edges. Heavy-gauge parts on slower presses tolerate more variation. Parts that get bent tight want a softer temper; parts that must hold a shape want a harder one.
If material cost dominates, buying the thinnest strip that still meets strength and force requirements saves money on every part. The coil stock optimization guide covers nesting and thickness trade-offs, and the stamping materials guide covers which alloy suits which job. When a strip decision is genuinely unclear, the cheapest move is to ask the factory before the die is cut — a die built for the wrong temper is scrap.
BQUQ stamps precise parts from steel, stainless, aluminum and copper coils in Dongguan, and reviews strip specs against the die on every RFQ. Send your drawing and material callout to sc@bquq.com for a quote within 12 working hours.
Coil Size, Packaging and Handling
Coil dimensions are a logistics choice that quietly affects the press. Coil inside diameter is usually 150, 400 or 508 mm, and it must match the decoiler mandrel or you cannot load the coil. Coil outside diameter and weight are limited by what the decoiler and feeder can carry — lighter coils mean more changeovers, heavier coils mean fewer stops but more handling equipment. Order the coil weight your line can sustain without stressing the feeder.
Packaging protects two things: flatness and surface. A coil stored or shipped poorly takes a set, and a coil with a rusted or scratched face produces scrap regardless of how good the die is. Eye-to-sky and eye-to-wall orientations suit different decoilers, so match the packaging to your equipment before the material ships. Keep coils dry, off the floor, and rotated through stock so older material is used first — steel in particular will flash-rust in humid conditions if it sits.
Handling also matters for safety and tolerance. A dropped or crushed coil can have dented edges and a deformed wrap that feeds badly for the rest of the coil. Good slitting, good packaging and careful handling preserve the flatness and edge condition you paid for at the mill. If your line runs a powered straightener-feeder, tell us the coil weight and bore so the material arrives ready to load rather than needing re-work before the press.
Frequently Asked Questions
Q: What thickness tolerance should I specify for stamped strip?
A: Only call out a band the mill can actually hold: roughly ±0.005 mm at 0.1 mm, ±0.02 mm at 0.5 mm and ±0.05 mm at 2 mm. Tighter than mill capability raises cost without improving the part.
Q: What does temper mean on a material certificate?
A: Temper is the metal's mechanical state — how hard and strong it is after rolling or heat treatment. It is expressed by designations like H02, H14, T4 or T6, and it controls formability, springback and final part force.
Q: Why does flatness matter so much in stamping?
A: Poor flatness causes feeding jams, mislocation at the pilots and parts that are not flat after cutoff. With a progressive die it shows up as hole positions and bends that vary from one coil to the next. Use precision strip for fine work.
Q: How do I pick between soft and hard temper?
A: Choose soft (annealed) tempers when the part needs deep drawing or tight bends. Choose harder tempers when the part must hold force or shape, such as a flat spring or a stiff clip, and accept more springback in the die.
Q: Does coil width need to match the die exactly?
A: It must match the strip layout with allowance for the feeder, typically within ±0.1 to ±0.3 mm. Width and edge quality affect guiding and die wear, so confirm the slitting spec with the factory before ordering.
Related Resources
- Stamping Materials Guide: which alloy and grade suits which stamped part.
- Metal stamping services: progressive-die stamping on steel, stainless, aluminum and copper coils.
- About BQUQ: an ISO9001-certified source factory in Dongguan running stamping, CNC, springs and heat sinks.
- Contact us: send your drawing 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
