Secondary Operations in Stamping: Tapping, Riveting, Forming, Assembly
Short answer: secondary operations are the work done to a stamped part after the die has done what a die can do — tapping threads, riveting or clinching parts together, adding forms the strip cannot support, and assembling multiple components. They add cost per part and tolerance risk, so the design rule is simple: push every operation into the die or the strip where possible, and leave as secondary only what a die genuinely cannot do.
A progressive die is brilliant at cutting and forming sheet metal, but it has limits: it cannot make a thread inside a hole, it cannot join two separate pieces, and it struggles with features that pull material in directions the strip cannot feed. That is the gap secondary operations fill. Understanding which operations belong in the die, which belong after it, and what each costs is how stamped parts stay cheap without sacrificing function.
What a Die Cannot Do — and What Happens Next
Some features are physically impossible or uneconomical inside a stamping die. Threads need a rotating tool; joining two parts needs a second component; deep forms need material that a flat strip cannot supply; and any operation that would take more time than the press stroke is better done off-press. Secondary operations exist for exactly these cases.
| Feature needed | In the die? | If not, the secondary route |
|---|---|---|
| Threaded hole | In-die tapping at high volume | Post-press tapping or thread forming |
| Two parts joined | No — needs second part | Riveting, clinching, staking, welding |
| Deep draw | Yes, with draw stations | Transfer press or hydroform |
| Tight bend near edge | Often yes, with form stations | Separate forming press or slide former |
| Heat treatment | No | Batch furnace after stamping |
| Plating | No (strip plating excepted) | Barrel/rack plating after stamping |
Takeaway: classify every drawing feature as die-capable or not before tooling. Features that need secondary work change the part's cost structure — a tapped hole can double the handling cost of a small terminal — and that should be visible in the quote, not discovered at first article.
Tapping: In-Die or After?
Threads are the most common secondary operation on stamped parts, and the process choice is a volume and tolerance trade. In-die tapping — a tapping head synchronized with the press stroke — forms threads at press speed with no extra handling, but adds die complexity and tooling cost, and suits thinner sheet (typically up to around 2 mm) with small thread sizes. Post-press tapping processes parts through a dedicated tapper, which costs per part but uses standard, cheap tooling.
| Factor | In-die tapping | Post-press tapping |
|---|---|---|
| Speed | Press speed, no handling | Slower, part-by-part feed |
| Die cost | Higher (tapping heads, stations) | Standard die, separate machine |
| Thread size range | Small to medium typical | Wider range |
| Material thickness | Thin sheet typical | Up to thicker stock |
| Best volume | High, 100k+ parts/year | Lower volume, mixed sizes |
Takeaway: in-die tapping wins at high volume because it removes an entire handling step; post-press tapping wins on flexibility and low tooling investment. For a part under 50,000 pieces a year with one tapped hole, the die premium rarely pays — quote both routes before building the tool.
Riveting, Clinching and Staking: Joining Without Fasteners
Many stamped assemblies are really several stamped parts that must become one: a bracket with a nut, a contact welded to a carrier, two enclosure halves joined at the edge. The cheap, reliable routes are mechanical — self-clinching fasteners pressed into the sheet, clinching (joining two sheets by cold-forming them together), staking (spreading a stud or boss to capture a part) and riveting.
| Method | How it joins | Typical use |
|---|---|---|
| Self-clinching nut/stud | Pressed into sheet, displaces metal | Threads in thin sheet |
| Clinching | Two sheets cold-formed together | Enclosures, brackets, no fastener |
| Staking | Stud or boss spread to capture | Terminals into plastic or metal |
| Riveting (solid/self-pierce) | Separate rivet deforms to lock | Heavy joints, high strength |
| Spot welding | Local fusion of the sheets | Steel assemblies, fast and strong |
Takeaway: the joint method decides disassembly, strength and cost. Clinching and staking add no parts and cost almost nothing per joint; rivets and fasteners add parts and cost but allow service access. Specify the joint by its service requirement — permanent or serviceable — and the supplier will pick the cheapest method in that class.
Forming Operations Done Off the Strip
Some forming simply cannot ride on the strip. Parts too large for the die width, forms that need material from several directions, or bends that would collide with neighboring stations are candidates for a secondary forming press or a four-slide (multi-slide) machine. The trade is real: moving forming off the die adds a handling step but can shrink the die and improve utilization of expensive progressive tooling.
| Situation | Die forming | Secondary forming |
|---|---|---|
| Part fits strip, forms simple | Preferred — cheapest per part | Only if die cost too high |
| Large part, one small form | Consider | Yes — small dedicated former |
| Multi-direction forms | Complex slides in die | Multi-slide machines excel |
| Volume below die payback | — | Yes, with simple tooling |
Takeaway: strip forming is always cheaper per part than handling a loose part twice — until the die complexity needed to do the form in-strip exceeds the cost of a second machine. When a drawing has one awkward form on an otherwise simple part, ask for both quotes before assuming in-die is better.
Assembly and the Tolerance Stack
Secondary assembly joins stamped parts to other stamped parts, plastic parts or fasteners, and every joint adds tolerance. Two parts that each hold ±0.1 mm can assemble to ±0.2 mm at the interface, which matters when the assembly is a mechanism — a hinge, a latch, a contact that must close at a specific force. Fixturing and datum strategy decide how much of that stack shows up in the final product.
| Assembly step | Typical tolerance effect | Control |
|---|---|---|
| Clinching two sheets | ±0.1–0.3 mm position | Die-registered tooling |
| Staking a terminal | ±0.1 mm typical | Stake punch datum |
| Self-clinching fastener | Position ±0.1–0.25 mm | Press tooling, sheet prep |
| Riveting | ±0.2 mm+ typical | Fixture, rivet quality |
| Adhesive bonding | Position good, cure risk | Fixture + process control |
Takeaway: design assemblies so the critical interface is controlled by one datum, not by the sum of every part's tolerance. Where a stamped bracket or mount carries another component, the mounting feature tolerance should be the tight one and the rest of the part relaxed.
Deburring, Cleaning and the Invisible Operations
Before plating, welding or assembly, stamped parts usually pass through invisible operations: deburring (mechanical, vibratory or electrochemical), cleaning, and sometimes stress relief. These matter because burrs and oil from the press contaminate every downstream process. Vibratory deburring is cheap but can round sharp functional edges; manual deburring is precise but slow. The control plan should state which edges need deburring and which must stay sharp.
| Operation | Cost class | Risk if skipped |
|---|---|---|
| Vibratory deburr | Low per part | Rounded edges, embedded media |
| Manual/edge deburr | Medium | Labor cost, inconsistency |
| Electrochemical deburr | Higher | Chemistry control |
| Cleaning/degrease | Low | Plating and weld defects |
| Stress relief | Medium | Distortion after forming |
Takeaway: plan deburring by function — sharp edges that must cut or grip stay sharp; edges that carry current or slide against insulation get fully deburred. The drawing should mark both, because an unmarked edge will be deburred by default and cost you a feature you needed.
Cost Logic: Buy the Operation Into the Quote
Secondary operations are where stamping quotes drift from the number you expected, so make them visible at quotation time. When you send a drawing, list the tapped holes, the clinched joints, the plating and the deburring requirements — a factory that prices these at RFQ stage is managing your project; one that discovers them at first article is managing its own margin. At BQUQ, secondary operations on the stamping line — from in-die tapping to sub-assembly of stamped terminals and contacts — are planned with the die layout and documented in the same process flow and inspection reports that ship with each batch under ISO9001.
Email sc@bquq.com or WhatsApp +86 137 1315 7787 with your PDF/DXF/STEP file. An engineer reviews it and replies with price, lead time and DFM notes on working days.
Frequently Asked Questions
Q: What is the most common secondary operation for stamped parts?
Deburring and tapping are the two most common. Deburring follows nearly every stamping run, and tapped holes are the most frequent added feature. Both are low-cost per part when planned, expensive when discovered late.
Q: In-die tapping or post-press tapping — which is cheaper?
At high volume, in-die tapping is cheaper per part because it eliminates a handling step and runs at press speed. Below roughly 50,000–100,000 parts a year, post-press tapping with standard tooling usually wins on total cost.
Q: Can a stamped part have threads without a secondary operation?
Yes, via thread forming or in-die tapping at high volume. Thread forming rolls threads into a pre-pierced hole without cutting, which is fast and strong in ductile sheet. Both options should be discussed at die design, not after.
Q: What is clinching and when should I use it?
Clinching joins two sheets of metal by cold-forming them together locally — no fastener, no heat. Use it for permanent joints in enclosures and brackets where the parts must never come apart and where a fastener would add cost and corrosion risk.
Q: Do secondary operations affect stamped part tolerances?
Yes. Every additional operation adds its own variation, and assembled features stack the tolerances of all parts involved. Keep the functional interface controlled by a single datum and relax everything else to hold assembly accuracy without expensive part tolerances.
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
