Electro-Mechanical Stamping: Motor and Relay Parts

The Snapshot

A commutator segment in a 12 V DC micro-motor runs at 6,000 to 12,000 RPM, and a 0.02 mm burr or step at the segment edge is enough to start the arc that pits the surface and raises contact resistance.
A general-purpose electromagnetic relay is rated for roughly 100,000 electrical operations at full load and up to 10 million mechanical operations; the stamped reed has to hold contact force and spring set across both.
Silver contact material runs 106% IACS conductivity with 1.59 µΩ·cm resistivity; a welded-shut contact on a 10 A relay is usually a contact-area or inrush-current sizing failure.
Electro-mechanical stampings typically hold ±0.01 to ±0.02 mm on the electrically critical dimension, burr under 0.015 mm, and flatness within 0.03 mm on armature plates.

This guide walks the purchasing and engineering decisions behind the small stamped parts that make motors and relays work: commutator segments, relay reeds, armature plates, silver contacts, and conductive strips. These parts look trivial on the bill of materials, but they carry current, bend millions of times, and switch loads on and off. The material grade, the tolerance table, the plating spec, and the process controls below decide whether a product ships for ten years or comes back after 30,000 cycles. If you are sourcing electro-mechanical stampings, the tables give you the numbers to put on the RFQ and the checkpoints to enforce at first article.

Part Classification

In a small electro-mechanical assembly, the stamped precision parts fall into five families. The commutator segment, the relay reed or spring blade, the silver contact, the armature plate, and the conductive strip or terminal. None of these is a big structural stamping. They are thin, tight-tolerance, electrically critical pieces where a 0.02 mm deviation changes performance. Each family fails differently, and the failure mode fixes the material, the tolerance, and the process control you should demand.

Commutator Segments

The commutator is a ring of copper segments that the brushes ride on. In a typical 12 V DC micro-motor spinning at 6,000 to 12,000 RPM, the brush crosses the inter-segment gap roughly every millisecond. Any unevenness at the segment edge, a burr, a step, or a chamfer that is off, produces an arc. That arc is the spark you see and the noise the EMC lab complains about.

Material is red copper, almost always C11000 (Chinese grade T2) for cost, or oxygen-free C10100 when you need the highest conductivity and lowest hydrogen-embrittlement risk. The numbers: C11000 carries about 100% IACS conductivity, which is 58.0 MS/m, with resistivity near 1.72 µΩ·cm at 20 °C. C10100 pushes to 101% IACS. You do not want to substitute a cheaper alloy here. Every point of conductivity lost shows up as heat at the brush interface, and heat accelerates the spark.

Relay Reeds and Spring Blades

A relay reed, or the moving spring blade inside an electromagnetic relay, lives its whole life bending. Open, close, open, close. The rated electrical life of a general-purpose relay at full load is about 100,000 operations. The mechanical life, with no current, can be ten million. The gap between those two numbers is exactly where material and stamping quality earn their money.

The blade is usually phosphor bronze. Grade C5210 (roughly Cu-8%Sn with 0.03 to 0.35% phosphorus) in the hard or extra-hard temper delivers tensile strength around 700 to 750 MPa with conductivity in the 10 to 13% IACS range. You trade conductivity for spring character. C5191 (Cu-6%Sn) is the softer alternative when you need more formability and less spring force. The fatigue limit is what counts: a properly stress-relieved C5210 blade holds a bending stress around 400 to 500 MPa for millions of cycles.

Silver Contacts

The contact rivet on a relay is the part that closes the circuit. Spot-welding of contacts, the failure where the relay welds shut and will not release, comes from the make-and-break arc melting the contact tips together. The contact material is normally silver-based: pure Ag for light loads, AgCdO or AgSnO2 for loads above a few amps because the oxide disperses and resists welding.

Silver's conductivity is 106% IACS, resistivity 1.59 µΩ·cm at 20 °C, the best of any metal. That is why it is on the contact face. But silver is soft and sticks to itself under arcing. The stamped contact bridge or rivet has to be sized so the contact area carries the rated current without the local temperature rise exceeding the material's sticking threshold.

Armature Plates

The armature in a relay or solenoid is a flat stamped plate of soft magnetic material, often cold-rolled low-carbon steel or silicon steel at 0.5 to 1.0 mm, that the coil pulls against the pole piece. It is not about conductivity; it is about magnetic flux and flatness. Any bow changes the air gap, and the air gap cubed governs pull-in force.

Conductive Strips and Terminals

The conductive strip or busbar terminal is the quiet one. Usually C11000 or brass C2600, 0.3 to 0.8 mm thick, it carries the main current path with a connection into a PCB or a crimp. The spec here is current rating versus temperature rise, and the stamping has to keep the cross-section consistent. No necking at a bend, no thinned wall at a pierced hole, because the current follows the metal you actually made, not the drawing.

Material Selection

Material selection for electro-mechanical stampings is a three-way trade between conductivity, mechanical spring behavior, and cost. The table below lists the grades most common in motor and relay programs, compiled from published material standards and supplier data sheets.

Material	Grade	Conductivity (% IACS)	Resistivity (µΩ·cm, 20 °C)	Tensile (MPa)	Typical part
Red copper	C11000 (T2)	~100	1.72	210-300	Commutator segment
Oxygen-free copper	C10100	101	1.71	220-315	High-conductivity commutator
Phosphor bronze	C5210	10-13	13-17	700-750 (hard)	Relay reed / spring
Phosphor bronze	C5191	12-15	11-14	550-650 (hard)	Softer spring blades
Cartridge brass	C2600	28	6.2	370-470 (hard)	Conductive strips, terminals
Beryllium copper	C17200	22-25	7.0-7.8	1,100-1,380 (aged)	High-force relay springs
Silver	Ag 99.9	106	1.59	130-170 (annealed)	Contact face
Silver tin oxide	AgSnO2 88/12	~50	3.3-3.6	180-240	Contact rivets above a few amps
Low-carbon steel	SPCC / DC01	n/a (magnetic)	n/a	270-410	Armature plates, yokes
Silicon steel	50JN600 / M470-50A	n/a (magnetic)	n/a	430-540	Commutator-insensitive magnetic circuits

Copper and Its Substitutes

Copper dominates the current-carrying parts because nothing cheaper beats its conductivity per dollar. The substitution logic runs in one direction: when a designer asks for "copper" without a grade, the default is C11000, and every deviation from that default has a reason attached. Oxygen-free C10100 appears when hydrogen embrittlement matters, for example when the part is brazed or welded after stamping, or when the conductivity spec is written at 101% IACS minimum.

Brass C2600 appears where the part needs stiffness plus decent conductivity, like a PCB terminal that must not bend during insertion. Its 28% IACS is one-third of copper's, so the cross-section must grow to carry the same current. That is the trade to check on every brass substitution: the drawing may look identical, but the temperature rise at rated current will not be.

Phosphor Bronze for Springs

Phosphor bronze is the workhorse spring material because it combines formability, fatigue resistance, and corrosion resistance at a moderate cost. C5210 in hard temper is the standard relay reed; C5191 covers parts that need deeper forming. Beryllium copper C17200 delivers roughly twice the tensile of phosphor bronze after age hardening, which is why it appears in relays that need very high contact force in a small envelope. It costs several times more per kg and adds a heat-treat step, so it is justified only when the force budget cannot be met any other way.

Contact Materials

Contact material choice follows the load. Pure silver handles light-signal switching where the current is below the welding threshold. Above a few amps, AgCdO or AgSnO2 disperses the arc energy and resists welding; AgSnO2 is the RoHS-clean option that has largely replaced AgCdO in new designs. The stamping tolerance on contact thickness is ±0.02 mm because a thin spot is a hot spot.

Soft Magnetic Steels

Armature and yoke plates use low-carbon steel for cost or silicon steel where lower core loss and higher permeability matter, for example in AC solenoids and higher-frequency switching. The stamping requirement is burr control on the pole face: a burr on the mating face creates a high-reluctance air gap and reduces pull force. Annealing after stamping restores the magnetic properties distorted by shearing, but it softens the material, so the sequence has to be specified deliberately.

Electrical Failure Modes

Every motor and relay failure traces back to one of four electrical mechanisms. Understanding them tells you which stamped feature to check first.

Arcing and Spark Erosion

Arcing is the commutator's operating reality. Each brush crossing sheds a micro-arc, and the arc erodes both the segment edge and the brush. The erosion rate scales with current and with the sharpness of the segment edge. A clean, radiused edge distributes the arc; a burr concentrates it into a single bright spark that pits the copper and accelerates wear. That is why the burr spec on commutator segments is tighter than on any other feature of the part.

Contact Welding

Contact welding is the relay's catastrophic failure. When the contacts close, the final approach velocity and the inrush current determine whether the microscopic weld that always forms holds or breaks. Undersized contact area, excessive bounce, or a thin spot in the silver all push the local temperature past the sticking threshold. For a 10 A relay you size the contact face to roughly 2 to 4 mm² of silver; undersize it and the inrush current welds the tips on the first close.

Resistance Drift

Resistance drift is the silent failure. Contact resistance starts low and climbs as oxidation, plating wear, and arc residue accumulate. A relay rated at 100 mΩ maximum contact resistance can start at 15 mΩ and drift upward over life. The stamped features that control this are plating thickness and location, surface finish of the contact face, and the spring force that keeps the contacts pressed together.

Fatigue Fracture

Fatigue fracture is the mechanical failure that shows up in the field as intermittent operation. A reed that loses its stress-relief treatment, or carries a sheared edge with a torn zone, develops a crack that propagates over tens of thousands of cycles until the blade breaks or its force drifts out of set. The crack almost always starts at a stress raiser: a sharp bend radius, a burr, or a die mark.

Tolerance and Drawing Requirements

Electro-mechanical stampings use a tighter tolerance table than structural stampings because the electrically critical features are small. The table below separates standard practice from the precision grade.

Feature	Standard Grade	Precision Grade
Electrically critical dimension	±0.05 mm	±0.01 mm
Contact thickness	±0.03 mm	±0.02 mm
Burr height	≤10% of thickness	≤0.015 mm absolute
Bend angle	±1°	±0.5°
Flatness (armature)	0.05 mm	0.03 mm
Edge condition	deburred	radiused, stress-relieved
Plating thickness	±20% of spec	±10% of spec

What the Drawing Has to Say

If you are specifying these parts, put the numbers that matter on the print. Material grade and temper. Conductivity or resistivity target. Grain direction relative to the bend. Burr height maximum. Edge condition. Plating thickness and location. Flatness and gap. Tolerance on the electrically critical dimension to ±0.01 or ±0.02 mm, and looser on the cosmetic ones. A BOM that says "copper, spring" is how you get a 100,000-cycle relay that dies at 30,000.

Three drawing signals separate a real spec from a placeholder. A conductivity callout, such as 100% IACS minimum on copper, tells the stamper which grade family you mean. A grain-direction note on a spring blade tells the die designer how to orient the strip in the coil. A burr-side note tells the toolmaker where the sheared edge is allowed to be. Without these three, the supplier is guessing at intent.

Tolerance versus Process Capability

A ±0.01 mm callout is only meaningful if the process can hold it. High-speed progressive stamping on a 25 to 80 ton press with positioning repeatability of ±0.005 mm can hold ±0.01 mm on a single feature only when the die is maintained, the coil thickness is controlled, and the press is not drifting thermally. Ask for the capability study on the tightest feature, not for the whole drawing. The answer separates a shop that quotes tolerances from a shop that proves them.

Stamping Process Control

The parts themselves are not exotic. What separates a stamping house that delivers motor and relay parts that survive from one that does not is process control: progressive dies kept in tolerance, in-line burr inspection, stress relief that is actually at the right temperature and time, and plating that is measured, not assumed. When a customer sends us a failed commutator or a welded relay, nine times out of ten the root cause is a tolerance that drifted or a process step that got skipped, not the design.

Progressive Die Design

Commutator segments, reeds, and terminals are progressive die jobs, run on high-speed presses at 150 to 300 SPM on strip up to 650 mm wide. The die design choices that matter for electro-mechanical parts: piloting from the carrier strip, fine-blanked or shaved edges on critical surfaces, bend stations with springback compensation, and in-die deburring where the part geometry allows. A 20-station progressive die for a relay blade is not exotic tooling; it is the standard way these parts are made in volume.

Burr and Edge Control

Two stamping details decide whether the reed survives. First, the bend radius. A sharp 90-degree bend on 0.15 mm phosphor bronze creates a cold-worked neck that becomes a fatigue crack origin. We radius every bend to at least 1.5 times the material thickness and grain-flow the bend so the grain runs along the length, not across it. Second, the edge. A sheared edge with a 0.03 mm burr and a torn zone is a stress raiser; the crack starts there. Deburring and, on critical parts, edge rounding by tumbling or electrochemical finishing takes the stress concentration factor from about 2.5 down toward 1.2.

Stress Relief and Heat Treatment

Stress relief is the step that gets skipped when the schedule slips, and it is the step that decides reed life. Phosphor bronze reeds are stress-relieved after forming to restore ductility and set the spring temper. The temperature and time are alloy-specific; a generic "anneal" cycle can over-soften the blade and drop the contact force below spec. The control plan should name the cycle, the furnace tolerance, and the batch size.

Plating and Surface Finishing

Plating on electro-mechanical parts is functional, not cosmetic. Commutator risers get silver plating at 1 to 3 µm on the riser face; contacts get selective silver or gold only where the circuit wipes; terminals get tin for solderability. The specification should name the coating, the thickness, the location, and the test method. XRF measurement on the part, not on a coupon, is the only way to prove it. For selective plating, the die and the plating mask have to agree on where the coating lands; a half-millimeter shift in the plating window changes contact resistance.

Verification and Testing

Every production lot of motor and relay stampings should get dimensional measurement on the critical features, a conductivity check by eddy current on copper parts, plating thickness by XRF, and a sample teardown for burr and edge condition. For relay reeds we run a fatigue sample to 100,000 cycles on a life tester and confirm the contact force stays in band. The data goes on the first-article report, not in a drawer. That is the difference between a part that passes incoming inspection and a part that passes ten years in the field.

The Test Matrix

Test	What it proves	Method	Frequency
Dimensional CMM	Critical features in tolerance	CMM / optical measurement	First article, then per control plan
Conductivity	Copper grade and temper	Eddy current	Per lot
Plating thickness	Coating spec met on the part	XRF	Per lot
Contact resistance	Wipe performance	4-wire milliohm	Per lot sample
Fatigue life	Reed survives rated cycles	Life tester to 100k cycles	Per qualification
Contact force	Spring set and force in band	Force gage at set gap	Per lot sample
Salt spray	Corrosion resistance of finish	ASTM B117	Per qualification

First Article and PPAP

For automotive and appliance customers, the first-article report should include the dimensional data, material certificates, plating certificates, and the electrical test results above. If the customer works under IATF 16949, the stamped part flows through PPAP like any other production part. The electrical tests are the differentiator: a generic stamping house can measure dimensions, but the conductivity, contact resistance, and fatigue data are what prove the part works as a component.

Failure Case Studies

Welded Relay Contact at 30,000 Cycles

A 12 V relay in a washing machine controller started failing to release after about 30,000 cycles, against a 100,000-cycle rating. The teardown found the contacts welded on the make event with a visible melt spot in the center of the silver face. The contact rivet had been sourced at a smaller face diameter than the drawing called out, and the contact thickness measured 0.03 mm thin on the high side of the dome. The undersized face raised the local current density, and the thin spot raised the resistance, so the interface ran hot enough to weld.

The fix was a contact-area audit at incoming inspection, a ±0.02 mm thickness check on the rivet, and a bounce-time measurement on the relay at final test. The lesson: on silver contacts, the face area and the thickness are the spec, not the silver purity alone.

Commutator Spark Pitting in a Power Tool

A brush motor in a power tool threw visible sparks within 200 hours of use. Inspection of the commutator showed pitting concentrated at one edge of every segment, in a pattern that repeated around the ring. The root cause was a burr left by the segment blanking operation; the burr on each segment acted as a field-emission point that concentrated the brush arc.

The supplier added an in-die deburring station and tightened the burr acceptance from 0.05 mm to 0.015 mm. The spark complaint disappeared at the next field trial. The lesson: on commutators, the burr spec is a functional requirement, not a cosmetic one.

Reed Fatigue Fracture in an Automotive Relay

A relay in an engine bay application failed intermittently around 80,000 operations. The reed showed a fatigue crack starting at the inner bend radius, propagating across the blade over about a third of the width. The bend radius on the production die had been reduced from the drawing value during a tool revision, and the stress-relief cycle had been dropped from the control plan during a line move.

Restoring the 1.5T bend radius and the stress-relief step returned life to the rated 100,000 operations. The lesson: two process deviations, each small, compound into a field failure. The drawing values are the contract; every deviation from them is a risk to be documented.

Supplier Selection

Electro-mechanical stampings need a different supplier profile than structural stampings. Score candidates against this matrix:

Capability	Why it matters
High-speed progressive stamping, 150-300 SPM	Volume economics on small parts
In-line burr and vision inspection	Catches edge defects at press speed
In-house tool room (wire EDM, CNC, grinding)	Die revisions in days, not months
Reel-to-reel selective plating	Functional plating location control
Conductivity and contact-resistance testing	Proves electrical performance, not just dimensions
Fatigue life testing	Qualifies spring parts to rated life
IATF 16949 or ISO 14001 systems	Process discipline for automotive and appliance

Audit Questions

Ask ten questions with numbers attached. What is the current burr spec on your commutator segments? What is your in-line inspection rate, and what does it catch? Where is the stress-relief furnace, and what is its temperature tolerance? How do you measure plating thickness on the part? What was your scrap rate on the last relay-reed program?

What is the capability index on the tightest feature of a similar part? How many die revisions did the last motor-commutator tool need, and why? What fatigue testing do you run on spring parts? How do you control coil thickness variation from lot to lot? What happens when a dimensional drift is found mid-run?

Red Flags

Three answers end the conversation. A burr spec that is looser than 10% of thickness on a commutator. A plating certificate from a coupon instead of the part. And a "we have never had a fatigue failure" claim with no fatigue test data to back it. Electro-mechanical parts fail electrically and mechanically, so the supplier has to test both.

FAQ

What is the difference between C11000 and C10100 copper for commutators?

C11000 is electrolytic tough-pitch copper at about 100% IACS, the cost-effective default. C10100 is oxygen-free copper at 101% IACS, specified when the part is welded or brazed after stamping, or when the drawing demands the highest conductivity class. The conductivity difference is small; the hydrogen-embrittlement resistance is the real reason to upgrade.

Why is phosphor bronze used for relay reeds instead of steel?

Phosphor bronze combines spring behavior, formability, and corrosion resistance in one alloy, and it holds its spring set over millions of cycles. Steel springs are stronger but rust without plating and are harder to form into small blades. Beryllium copper is the upgrade when the force budget demands it.

How tight can a stamped contact tolerance go?

On a high-speed progressive press with ±0.005 mm positioning, a single feature can hold ±0.01 mm when the die is maintained and the coil is controlled. The practical limit for a production part is usually ±0.01 to ±0.02 mm on the critical dimension, with looser values everywhere else to keep the die affordable.

What causes contact welding on relays?

Contact welding is caused by the make-and-break arc melting the tips together. The three contributors are undersized contact area, excessive bounce on close, and thin spots in the silver that run hot. Fix the face area, the bounce, and the thickness spec, and the welding stops.

Should I specify selective or full plating on a terminal?

Selective plating puts precious metal only where the circuit wipes, cutting precious-metal cost by 70-90% versus full coverage. Full plating is simpler and covers edges, but it pays for metal that never touches a contact. For reel-to-reel parts, selective plating is the standard answer.

Next Step: Send the Drawing

If you are building motors, relays, or any electro-mechanical device and the stamped parts are the weak link, or you just want them right the first time, send us your drawing and material spec. We will review it for the failure modes above, quote the progression tooling, and run the first article with full dimensional, conductivity, and contact-resistance reporting. Thirty years of getting these small parts wrong-proof is what we do. Request a quote and include the drawing; the DFM review is free and the first-article data package is standard.