B2B Engineering Guide & DFM CalculatorBy AxialFluxCore Engineering Team•Reviewed 2026-10-25

AFPM Core Manufacturing Process: Tool & Engineering Report

Q: Why is Grain-Oriented Electrical Steel (CRGO) used in segmented axial flux teeth?

CRGO exhibits an easy magnetization axis ([100] Miller index) along its rolling direction with very high saturation flux density (up to 1.95–2.0 T) and low hysteresis loss. In segmented AFPM teeth (e.g. YASA topology), magnetic flux travels strictly axially from rotor to rotor. By orienting the rolling direction along the axial flux path, motor designers achieve 15–20% higher air gap flux density than isotropic steel or SMC without saturating.

Q: What is the maximum electrical frequency feasible for tape-wound slotted cores?

Tape-wound slotted cores are generally restricted to operating frequencies below 300–400 Hz. While the continuous strip path is ideal for toroidal back-iron flux, mechanical slot machining (milling or wire EDM) inevitably creates metallic smear burrs across adjacent strip turns. Even with chemical etching, interlaminar micro-shorts tend to elevate eddy current heating severely at higher frequencies.

Q: What compaction pressure and press tonnage are required for SMC stator teeth?

Standard high-density SMC components require 600 to 800 MPa uniaxial compaction pressure to achieve green densities of 7.45–7.55 g/cm³. For an average AFPM tooth with a face area of 800 mm², the required compaction force is approximately 560 kN (57 metric tons). Accounting for hydraulic press safety margins, a 100-ton to 160-ton CNC powder press is recommended per cavity.

Q: What die materials and clearances are used for pressing SMC stator teeth?

SMC compaction tooling utilizes tungsten carbide (WC-Co, 10–12% cobalt binder) die inserts with hardened tool steel (CPM 10V or D2, 60–62 HRC) punches. Diametral die-to-punch clearance must be held strictly between 4 μm and 8 μm per side to prevent powder flash or punch galling. Die walls require a 0.5° to 1.0° draft angle and mirror-polished surface finish (Ra < 0.1 μm) to facilitate damage-free green ejection.

Q: What is the maximum viable aspect ratio (length-to-width) for an SMC tooth?

The recommended aspect ratio (axial core length divided by mean tooth width) for single-action or simple dual-action SMC pressing is 3.5:1. Above 3.8:1, internal powder friction against die walls creates a severe density gradient (lower density in the middle of the tooth), reducing local magnetic permeability and inducing micro-cracks upon ejection. Taller teeth require advanced multi-action CNC hydraulic punches or multi-piece split teeth.

Q: How does tooling NRE compare between prototype and series production routes?

For prototypes (1–50 units), segmented laminations cut via precision fiber laser incur minimal tooling ($1,500–$4,000 for stacking fixtures), whereas an SMC prototype die costs $8,000–$14,000. For series production (>10,000 units), a progressive carbide stamping die costs $35,000–$65,000 with a 10–14 week lead time, while automated multi-cavity SMC tooling costs $25,000–$50,000 with cycle times under 20 seconds.

A comprehensive manufacturing feasibility, tooling, and tolerance guide for Axial Flux Permanent Magnet (AFPM) stator cores. Compare continuous strip-wound silicon steel, segmented CRGO lamination stacks, and 3D net-shape Soft Magnetic Composite (SMC) compaction to de-risk series production.

Open Manufacturing Calculator Request Drawing & DFM Review

Interactive DFM ToolIEC 60404 / Höganäs Somaloy / CRGO Benchmark

AFPM Stator Core Manufacturing Route & Tolerance Evaluator

Configure your motor dimensions, operating frequency, and volume to screen tooling NRE, cycle times, and tolerance stacks across SMC, Lamination, and Tape-Wound routes.

Load Sourcing Application Preset:

1. Core Parameters & Sourcing Bounds

Stator Topology Architecture

Outer Diameter (OD)

Inner Diameter (ID)

Tooth Axial Length

Stator Slot Count

Electrical Frequency

Target Axial Runout

Annual Volume

Design Priority

Derived Tooth & Stator Geometry

Mean Diameter: 160 mm

Radial Span: 40 mm

Mean Tooth Width: 11.5 mm

Aspect Ratio: 3.04:1

Tooth Face Area: 460.8 mm²

Active Volume: 387 cm³

Recommended Primary Process Route

Soft Magnetic Composite (SMC) 3D Net-Shape Die Compaction

OptimalScore: 98/100

For YASA-YOKELESS at 400 Hz with series production, SMC 3D die compaction delivers the optimal balance of net-shape tooling efficiency, 3D flux concentration, and near-zero material scrap (3.5% vs 34% for laminations).

Candidate Process Routes Comparison

SMC Compaction98 pts

NRE: $22,000–$48,000 (Carbide multi-action tooling)

Lead: 8–12 weeks

Cycle: 14–22 sec (automated press cavity)

Scrap: 3.5%

Segmented Stack85 pts

NRE: $28,000–$65,000 (High-speed progressive stamping die)

Lead: 10–14 weeks

Cycle: 35–55 sec / bonded tooth

Scrap: 34%

Tape-Wound Core15 pts

NRE: $15,000–$32,000 (Automated winder & progressive slot punch)

Lead: 6–9 weeks

Cycle: 8–15 min / core (pre-slotted strip)

Scrap: 22%

Air Gap Tolerance Accumulation (RSS)

Compliant

Target Runout:

±0.05 mm

RSS Accumulated:

0.028 mm

Worst-Case Stack:

0.047 mm

Tooth Axial Var:

±0.02 mm

Quality Gate Action: Direct assembly acceptable. CMM spot-check recommended.

Key Process Engineering Parameters

Compaction Force / Tonnage:	322.5 kN (~43T Press)
Recommended Material Grade:	Somaloy 700 3P
Atmosphere Curing Cycle:	530°C for 30 min in dry N2 gas

AFPM Manufacturing Architecture & Tooling Visualizer

Interactive engineering schematics comparing core forming routes, die kinematics, tolerance stacks, and thermal cure cycles.

Executive Summary: Manufacturing Route Trade-offs

Quantitative Production Benchmark

Unlike radial flux motor stators where identical cylindrical laminations are stamped and stacked longitudinally, AFPM core manufacturing must accommodate axial magnetic flux vectors entering from the rotor faces. The selection between Soft Magnetic Composites (SMC), segmented grain-oriented laminations, and tape-wound toroidal cores dictates stator slot fill factor, high-frequency iron loss, tooling capital expenditure, and scrap rates.

SMC Die Compaction

Best for: Frequencies > 400 Hz, 3D tooth shoe profiles, automated series production (>1,000 units/year).

• Material Scrap: 2.5%–4.0%

• Cycle Time: 12–22 sec / tooth

• Compaction Force: 600–800 MPa

• Tooling NRE: $22k–$48k (Carbide Die)

Segmented CRGO Stacking

Best for: Maximum saturation flux density (1.95T), high peak torque, low-volume prototypes via laser cutting.

• Material Scrap: 28%–42% (Nesting loss)

• Cycle Time: 35–55 sec / bonded tooth

• Bonding: Backlack (200°C @ 2.2 MPa)

• Tooling NRE: $1.8k proto / $45k progressive

Continuous Tape Winding

Best for: TORUS-NN continuous stator back iron, steady-state medium speed industrial fans and pumps.

• Material Scrap: 18%–25% (Slot machining)

• Cycle Time: Hours (EDM) / 12 min (stamped)

• Anneal: 800°C in dry H2/N2 atmosphere

• Deburring: Mandatory acid micro-etching

Technical Comparison Matrices & Process Limits

Direct empirical comparison of mechanical, electromagnetic, and manufacturing metrics across the three core routes.

Table 1: Manufacturing Route Technical Benchmark Matrix

Metric / Parameter	SMC Die Compaction	Segmented CRGO Stack	Tape-Wound Slotted Core
Raw Material Form	Atomized iron powder (100–250 μm coated)	0.10–0.35 mm cold-rolled strip coil	Continuous narrow electrical steel strip
Magnetic Isotropy	True 3D Isotropic (Bx ≈ By ≈ Bz)	2D Anisotropic (High along rolling axis)	1D Circumferential along strip length
Saturation Flux Density (Bs)	1.55–1.75 T	1.90–2.05 T	1.80–1.95 T
Relative Permeability (μr)	300–850	3,000–12,000	2,500–8,000
High Frequency Eddy Loss (>400Hz)	Extremely Low (Dielectric barrier)	Moderate (Requires <0.20 mm gauge)	High (Slot burr loop risk)
Tooth 3D Geometry Freedom	High (Rounded crowns, integrated bobbins)	Low (Prismatic or stepped 2D stack)	Very Low (Parallel or radial slots only)
Material Scrap Rate	2.5%–4.0%	28%–42%	18%–25%
Prototype Tooling Entry Barrier	High ($8,000–$14,000)	Very Low ($1,500–$3,500)	Moderate ($3,000–$6,500)
Mass Production Tooling NRE	$25,000–$50,000	$35,000–$65,000	$15,000–$35,000

Table 2: Dimensional & Geometric Tolerances (GD&T ISO 2768)

Feature / Dimension	Standard Machining (m)	Precision Tooling (f)	SMC Die Capability	Lamination Backlack Stack
Tooth Axial Length (Height)	±0.15 mm	±0.05 mm	±0.020 mm (Multi-action CNC)	±0.035 mm (Stacking variation)
Tooth Width (Circumferential)	±0.10 mm	±0.03 mm	±0.015 mm (Die wire EDM)	±0.025 mm (Stamping die clearance)
Radial Span (Length)	±0.20 mm	±0.05 mm	±0.020 mm	±0.030 mm
Tooth Perpendicularity to Datum B	0.10 mm	0.04 mm	0.025 mm	0.040 mm
Assembled Stator Air-Gap Flatness (TIR)	0.12 mm	0.05 mm	0.035 mm (Direct pot)	0.045 mm (<15μm diamond skim)
Edge Radii / Chamfer	±0.30 mm	±0.10 mm	R0.5 mm ± 0.05 mm (Molded)	0.3 mm x 45° ± 0.08 mm (Milled)

Table 3: Thermal Treatment & Curing Process Parameters

Process Route	Target Temperature	Atmosphere / Gas	Soak Duration	Primary Metallurgical Purpose
Tape-Wound Strip Annealing	780°C–820°C	95% N2 / 5% H2 (Dew point <-40°C)	2.5–4.0 hours	Relieves severe plastic bending strain; restores domain mobility & B50
SMC Dielectric Resin Curing	500°C–550°C	Protective Nitrogen (O2 < 50 ppm)	30 minutes	De-lubrication & cross-links binder without oxidizing particle boundaries
Backlack Thermal Bonding	195°C–205°C	Ambient air with hydraulic clamping	3–5 minutes	Thermally cures epoxy varnish layer under 2.2 MPa clamping pressure
Epoxy Potting Post-Cure (VPI)	120°C–140°C	Convection curing oven	2.0–3.0 hours	Fully polymerizes structural carrier adhesive; locks teeth against vibration

Table 4: High-Frequency Electromagnetic Loss Benchmark (IEC 60404-2 & IEC 60404-6)

Calibrated Empirical Data

Material Grade	Form / Gauge	B50 Saturation	Resistivity (ρ)	P1.0 @ 50 Hz	P1.0 @ 400 Hz	P1.0 @ 1,000 Hz	Stack Factor (k_fe)
Somaloy 700 3P	Die Compacted Powder	1.62 T	450 µΩ·m	6.8 W/kg	48 W/kg	128 W/kg	0.96 (Monolithic)
Somaloy 700 5P	High-Temp Annealed Powder	1.64 T	380 µΩ·m	4.9 W/kg	36 W/kg	98 W/kg	0.96 (Monolithic)
Somaloy 1100 5P	Ultra-Low Loss SMC	1.65 T	520 µΩ·m	4.1 W/kg	28 W/kg	74 W/kg	0.96 (Monolithic)
CRGO 27Q130	0.27 mm Grain-Oriented	1.92 T	0.48 µΩ·m	0.85 W/kg	18 W/kg	86 W/kg	0.95 (Backlack)
CRGO 23Q110	0.23 mm Laser-Scribed	1.94 T	0.48 µΩ·m	0.72 W/kg	14 W/kg	62 W/kg	0.94 (Backlack)
Hi-B Super Core (10JNEX900)	0.10 mm Ultra-Thin Silicon	1.85 T	0.82 µΩ·m	0.58 W/kg	7.4 W/kg	24 W/kg	0.89 (Thin Lam)

Table 5: Stator Tooth Slotting & Edge Integrity Defect Taxonomy

Edge Damage Physics

Cutting / Slotting Route	Primary Defect Mechanism	HAZ Depth (µm)	Edge Burr Height (h_b)	Interlaminar Drop	Remediation Protocol
High-Speed Progressive Stamping	Plastic shear & rollover fracture	0 µm (Cold shear)	5–12 µm	-15% to -25%	Inline roller deburring; punch regrind every 250k strokes
Precision Fiber Laser Cutting	Molten kerf recast & thermal stress	120–160 µm	8–18 µm	-30% to -45%	20-bar N2 assist gas; pulse modulation; ultrasonic solvent wash
Multi-Axis CNC Wire EDM	Spark erosion white recast globules	10–25 µm	< 3 µm	-60% to -85% (Direct bridge)	Mandatory ultrasonic nitric-phosphoric chemical micro-etching
Diamond-Wheel Profile Grinding	Abrasive shear smearing across strip turns	15–30 µm	10–22 µm	-50% to -75%	Flood synthetic coolant with <15 µm finishing pass + micro-etch

Table 6: Adhesive & Structural Potting Resin Engineering Matrix

Polymer Formulation Data

Resin Grade	Chemical Family	Primary Application	Glass Transition (Tg)	Shear @ 20°C	Shear @ 150°C	Conductivity (k_th)
Backlack EB-549	Epoxy varnish pre-coat	CRGO segmented tooth stack bonding	140°C	6.2 MPa	2.8 MPa	0.35 W/m·K
Araldite CW 229 / Aradur 229-1	Filled formulated epoxy	Stator carrier VPI potting & void filling	165°C	18.5 MPa	8.2 MPa	0.95 W/m·K
Henkel Loctite 648	Anaerobic urethane methacrylate	Tooth shank press-fit pocket retaining	175°C	25.0 MPa	14.0 MPa	0.25 W/m·K
Henkel Hysol 9492	High-temp 2-part structural epoxy paste	Split SMC tooth splice & shoe retention	180°C	22.0 MPa	11.5 MPa	1.10 W/m·K

Table 7: Process Failure Modes & Effects Analysis (PFMEA) for AFPM Stators

Risk Priority De-Risking

Process Step	Failure Mode	Sev (S)	Occ (O)	Det (D)	RPN	Production Mitigation Standard
SMC Die Compaction	Tooth corner chipping / green micro-cracking	7	4	3	84	Mirror polish WC-Co die (Ra < 0.1 µm), controlled punch deceleration, draft ≥ 0.5°
Toroidal Strip Winding	Inter-turn slippage during slot machining	8	3	4	96	Dynamic servo tension regulation with load-cell feedback (35–55 N/mm²); tack epoxy
Backlack Thermal Pressing	Interlaminar bond delamination under thermal shock	8	3	3	72	Multi-zone PID platens (±2°C); maintain 1.5 MPa clamping until cooled below 60°C
Tooth Carrier Assembly	Axial air-gap runout exceeds 0.05 mm TIR	9	4	2	72	Optical CMM indexing fixtures; automated diamond skim grinding (<15 µm pass) if RSS > limit
Stator VPI Potting	Microscopic air void retention causing partial discharge	8	3	4	96	Dual-stage vacuum pump (<0.8 mbar), online viscosity monitoring, 5.5 bar nitrogen hold

AFPM Core Manufacturing Engineering Deep Dive

A comprehensive technical guide to tooling physics, metallurgical curing, edge deburring, and quality verification gates.

Chapter 1: The Physics of Axial Flux Stators vs Radial Laminations

Conventional radial flux machines direct magnetic flux perpendicular to the rotor shaft through concentric cylindrical lamination stacks. This permits stamping complete 360-degree circular laminations with slots and teeth on automated high-speed progressive dies. In contrast, an Axial Flux Permanent Magnet (AFPM) machine directs magnetic flux parallel to the rotation axis through one or two air gaps.

Because the active magnetic flux travels axially, lamination planes cannot simply be stacked axially without forcing the flux to cross thousands of interlaminar insulation barriers, which would introduce enormous magnetic reluctance. As a result, manufacturers must fabricate AFPM cores through specialized methods: spirally winding a slit electrical strip into a toroid (where flux travels circumferentially through the yoke), stacking grain-oriented laminations along the radial plane, or utilizing isotropic 3D Soft Magnetic Composites.

Chapter 2: Continuous Tape-Wound Stators: Winding & Slit Strip Mechanics

The continuous tape-wound core route starts with cold-rolled silicon steel strip slit to a width exactly equal to the radial length of the motor stator active zone (OD - ID)/2. The strip is wound continuously under calibrated mechanical back-tension (35–55 N/mm²) around a cylindrical central mandrel.

Winding tension must be dynamically modulated as the core radius grows: excessive tension creates radial compression that pinches interlaminar bonding coatings, while insufficient tension allows inter-turn slippage during subsequent slot machining. An adhesive bonding lacquer or thin epoxy film is applied during winding to mechanically lock the turns into a monolithic ring before heat treatment.

Chapter 3: Slot Forming: Wire EDM vs Diamond Profile Grinding vs Pre-Punched Strip

Once a solid tape-wound toroid is formed, radial stator slots must be cut to house the copper windings. Three distinct industrial techniques are utilized:

Multi-Axis CNC Wire EDM: The gold standard for prototyping and low-volume production (1–50 units). Brass wire (0.20–0.25 mm) cuts precision slots with negligible cutting force. However, spark erosion leaves a recast layer with microscopic conductive globules that bridge adjacent laminations.
Precision Diamond-Wheel Profile Grinding: High-speed resin-bond or vitrified diamond grinding wheels machine slots with copious water-soluble synthetic coolant. While faster than EDM, mechanical shear creates abrasive burrs along the slot exit edges.
Variable-Pitch Progressive Strip Punching: The mass production route. The strip is punched with slots before winding. Because turn circumference expands linearly ($C = 2\pi r$), slot pitch must increase continuously turn by turn using CNC-guided optical index feeds.

Chapter 4: Stress-Relief Atmosphere Annealing: Kinetics & Loss Recovery

Plastic deformation during toroidal bending induces severe crystal lattice dislocations in the silicon-iron matrix, pinning magnetic domain walls. Empirical testing per IEC 60404-8-7 demonstrates that unannealed tape-wound cores suffer a 120–180% increase in specific core loss ($P_0.03$) and a 30% reduction in relative permeability.

To reverse this degradation, cores undergo protective atmosphere annealing at 780°C to 820°C in a sealed retort or bell furnace. The protective gas mixture (95% dry nitrogen / 5% dry hydrogen with a dew point below -40°C) prevents decarburization and surface oxidation. A controlled heating ramp (<100°C/hr), a 2.5–4.0 hour thermal soak, and slow furnace cooling below 300°C allow full dislocation annihilation and crystal grain stress relief.

Chapter 5: Segmented Stator Tooth Stacking: Grain-Oriented Lamination (CRGO) Alignment

In modular AFPM topologies such as YASA (Yokeless and Segmented Armature), individual stator teeth are manufactured as discrete modules. This unlocks the ability to use Cold-Rolled Grain-Oriented (CRGO) electrical steel.

CRGO laminations are stamped or laser cut into trapezoidal shapes of varying widths. The laminations are stacked along the radial or circumferential direction such that the easy magnetization rolling axis aligns strictly with the axial magnetic flux vector. When bonded with Backlack (self-bonding thermosetting epoxy varnish) at 200°C under 2.2 MPa pressure, the modular tooth achieves an extraordinary saturation flux density exceeding 1.95 T with zero interlaminar shorting bolts.

Figure 2: Backlack Thermal Bonding & Clamping Kinetics. Epoxy self-bonding varnish activates between 195°C and 205°C under 2.2 MPa mechanical pressure. Clamping force is maintained through cooling to prevent thermal springback and achieve interlaminar shear strength exceeding 4.5 MPa.

Chapter 6: Soft Magnetic Composite (SMC) Metallurgy: Particle Rheology & Coatings

Soft Magnetic Composites (e.g., Höganäs Somaloy 700 / 1000 / 1100) consist of high-purity water-atomized iron powder particles (median size 100–250 μm). Each individual particle is encapsulated in a nanometer-thin dielectric phosphate or organo-silicon insulation layer.

Because electrical resistance exists between every single microscopic particle, macroscopic eddy currents cannot circulate across the core volume. This makes SMC truly isotropic in both magnetic and thermal domains, enabling magnetic flux to travel in 3D paths—ideal for stator teeth with 3D rounded crowns and flux-concentrating pole shoes.

Chapter 7: Uniaxial & Multi-Action Die Compaction Mechanics

Fabricating net-shape SMC teeth requires pressing the powder at 600 to 800 MPa in tungsten carbide dies. At these pressures, the iron particles undergo plastic deformation, interlocking mechanically to achieve green densities of 7.45–7.58 g/cm³ (95–97% of theoretical iron density).

Tooling design must respect powder compaction physics:

Draft Angles: Die cavity walls must incorporate a 0.5° to 1.0° draft angle to allow ejection without shearing the fragile green tooth corners.
Punch Clearance: Punch-to-die clearances must be maintained between 4 μm and 8 μm to prevent powder flashing.
Aspect Ratio Limits: Teeth with an axial height-to-width ratio exceeding 3.8:1 require multi-action top and bottom punches to avoid a low-density dead zone in the center.

Figure 1: Opposed Multi-Action Compaction Kinematics. Dual-action CNC hydraulic punches balance powder compaction forces (600–800 MPa), centering the neutral shear zone to maintain density > 7.45 g/cm³ across the tooth core and prevent green micro-cracking upon ejection.

Chapter 8: Thermal Curing Kinetics for SMC: Polymer Cross-Linking vs Oxidation

After compaction, green SMC parts possess limited mechanical integrity (transverse rupture strength ~30–40 MPa). They are passed through a continuous belt furnace at 500°C to 550°C in an ultra-pure nitrogen atmosphere (oxygen < 50 ppm).

This curing cycle serves two critical functions: it vaporizes internal stearic acid lubricants, and it thermally cross-links the organic polymer binder, increasing rupture strength to 110–135 MPa. Crucially, the temperature must not exceed 560°C; higher temperatures degrade the dielectric particle coating, causing electrical resistivity to collapse and triggering massive eddy current losses.

Chapter 9: Burr Management, Slit-Edge Damage & Chemical Micro-Etching

In laminated steel cores, mechanical punching, laser cutting, and slot grinding produce microscopic metallic burrs along the cut edges. Under stator clamping or operation, these burrs pierce the interlaminar varnish, creating closed electrical loops that overheat rapidly under alternating magnetic fields.

Industry standard practice enforces a maximum burr height of <8 μm on punched lamination edges. For tape-wound cores after slotting, chemical micro-etching in an ultrasonic nitric-phosphoric bath is mandatory. Etching removes 15–25 μm of smeared surface metal, restoring interlaminar insulation resistance above 100 Ω·cm² per IEC 60404-6.

Figure 3: Interlaminar Smear Removal via Micro-Etching. Wire EDM and mechanical grinding drag conductive steel fragments across lamination edges, creating catastrophic eddy-current loops. Ultrasonic chemical micro-etching dissolves 15–25 µm of damaged surface metal, restoring inter-turn dielectric barrier.

Chapter 10: GD&T Tolerance Stack: Controlling Axial Air-Gap Runout

Because magnetic attractive forces between rotor magnets and stator cores are inversely proportional to the square of the air gap ($F \propto 1/g^2$), any axial runout or tooth tilt causes severe cyclic axial vibration, cogging torque, and bearing fatigue.

For an air gap of 0.8 mm to 1.2 mm, total accumulated axial runout across all teeth must not exceed 0.05 mm (Total Indicator Reading, TIR). Achieving this requires calculating the Root-Sum-Square (RSS) stack of individual tooth thickness variation (±0.02 mm for SMC), carrier pocket depth variation (±0.012 mm), and adhesive bond line thickness (±0.015 mm). When the RSS stack exceeds the design budget, a post-assembly precision diamond face skim grind is deployed.

Chapter 11: Stator Carrier Sub-Assembly & VPI Encapsulation

In modular AFPM stators, individual teeth are wound with concentrated copper coils and inserted into an aluminum or high-performance non-magnetic composite (carbon/PEEK) carrier ring. Precision optical indexing fixtures align tooth pitch to within ±0.02°.

The entire sub-assembly is then subjected to Vacuum Pressure Impregnation (VPI). In a dual-chamber autoclave, air and moisture are evacuated below 1 mbar before injecting low-viscosity Class H epoxy resin under 5–6 bar pressure. VPI fills all micro-cavities between copper turns and tooth surfaces, improving stator radial thermal conductivity from 0.8 W/m·K to 2.4 W/m·K and providing structural retention against magnetic torque shear forces.

Figure 4: Vacuum Pressure Impregnation (VPI) Pressure Profile. Dual-cycle autoclave processing pulls <1 mbar vacuum to evacuate microscopic air cavities in concentrated stator winding slots, subsequently applying 5.5 bar hydraulic overpressure to drive high-temperature Class H epoxy deep into tooth-to-carrier interfaces.

Chapter 12: Production Economics, NRE Amortization & Scrap Yield

The financial cross-over point between segmented lamination stacking and SMC powder compaction is heavily driven by annual production volume:

Volume < 500 units/year: Laser-cut segmented CRGO laminations are most cost-effective. Upfront tooling NRE is under $4,000, and lead times are 2–3 weeks.
Volume 500–5,000 units/year: SMC compaction becomes competitive. Soft prototype tooling ($10,000–$14,000) is rapidly amortized by 18-second press cycle times and <4% scrap.
Volume > 10,000 units/year: Automated multi-cavity SMC carbide tooling yields the lowest piece price. Stamped lamination scrap waste (30–42% offcut loss of expensive electrical steel) adds millions in raw material cost compared to net-shape powder.

Chapter 13: Non-Destructive Testing (NDT) & Quality Control Gates

To prevent defective core components from advancing into costly coil winding and stator potting, production lines implement five mandatory Quality Control (QC) inspection gates:

Gate 1 (Raw Material): Density, moisture, and Hall flowmeter testing for SMC powder; Epstein frame core loss testing for electrical steel coils.
Gate 2 (Forming & Density): Green density check (≥ 7.45 g/cm³) and green strength checks for SMC; burr height (≤ 8 µm) and laser heat-affected zone checks for laminations.
Gate 3 (Heat Treatment): Electrical resistivity measurement (> 1,000 µΩ·m) to confirm dielectric integrity; Transverse Rupture Strength (TRS ≥ 110 MPa).
Gate 4 (Insulation & Winding): High-voltage Hipot testing (3.0 kV for 60s), impulse surge testing, and partial discharge inception voltage (PDIV > 1.2 kV).
Gate 5 (Final Sub-Assembly): 3D CMM coordinate measuring of stator active face plane flatness (runout TIR ≤ 0.05 mm), pull-out shear strength testing.

Chapter 14: Design for Manufacturing (DFM) OEM Specification Checklist

When releasing technical drawings and RFQ packages for AFPM core production, OEM engineering teams should verify the following critical items:

Primary Datum Defined: Datum A established on the active air gap pole face plane with specified flatness tolerance (≤ 0.05 mm).
Draft Angles & Corner Radii: All SMC die cavities feature 0.5°–1.0° draft and minimum 0.5 mm corner radii to prevent tool chipping.
Insulation & Deburring Scope: Chemical micro-etching specified for tape-wound slots; Backlack cure pressure/temperature curve defined.
Operating Frequency Stated: Stator electrical frequency clearly stated to ensure correct steel gauge (0.10 mm vs 0.20 mm) or SMC grade selection.
Carrier Thermal Expansion Match: CTE of the aluminum/composite stator carrier ring matched to core material to prevent bond delamination under thermal cycling (-40°C to 150°C).

Chapter 15: Edge Damage & Laser HAZ Degradation: Laser vs High-Speed Stamping vs Wire EDM

Cutting electrical steel sheets introduces severe local mechanical or thermal stress that degrades magnetic domain mobility. Precision fiber laser profiling introduces a Heat-Affected Zone (HAZ) measuring 120 µm to 160 µm deep along each cut boundary. Within this zone, rapid melting and solid-state quench cycles freeze residual mechanical tensile stresses above 250 MPa, pinning domain walls and increasing localized hysteresis loss by up to 45% per IEC 60404-2 Epstein calibrations.

For narrow AFPM tooth tips and slot teeth under 4 mm width, the two opposing laser HAZ regions overlap across more than 8% of the total active cross-section, causing noticeable motor no-load loss penalties. By comparison, high-speed progressive carbide stamping maintains an entirely cold shear line (HAZ = 0 µm), with plastic deformation restricted to a narrow 15–25 µm edge zone. However, stamping dies must maintain clearance within 6%–8% of strip thickness to prevent excessive tensile rollover or burr heights exceeding 8 µm. For prototypes requiring complex 3D profiles without stamping tooling, pulsed laser cutting with 20-bar inert nitrogen assist gas or CNC wire EDM followed by chemical micro-etching remains the validated industry standard.

Chapter 16: Interlaminar Dielectric Integrity: Backlack vs Inter-Particle Phosphate vs VPI Penetration

AFPM machines operated by modern silicon carbide (SiC) or gallium nitride (GaN) high-frequency inverters experience extreme pulse-width-modulation (PWM) voltage rise times (dV/dt > 5 kV/µs). High dV/dt induces severe displacement currents across inter-lamination capacitance, challenging core dielectric barriers.

Conventional inorganic lamination coatings (AISI C5) offer breakdown strength up to 30 V/turn but are susceptible to micro-scratching during assembly. Pre-coated Backlack epoxy varnish (Axalta EB-549 or similar) forms a continuous, void-free 2–4 µm adhesive matrix with dielectric breakdown exceeding 45 kV/mm, eliminating partial discharge inception voltage (PDIV) breakdown up to 1.5 kV DC. In SMC cores, nanometer-thin phosphate coatings encapsulate each individual 100–250 µm iron powder grain, yielding bulk volume resistivity above 400 µΩ·m that suppresses microscopic displacement loops entirely. Autoclave Vacuum Pressure Impregnation (VPI) with Class H resin serves as the final barrier, sealing tooth-to-carrier margins and raising the partial discharge threshold well above operating stresses.

Related Design Guides & Engineering Resources:

AFPM Core Design for Manufacturing Guide →AFPM Magnetic Circuit Design Analysis →AFPM Assembly Process & Tooling SOP →

Industrial Case Scenarios & Realized Outcomes

Field-proven manufacturing architectures across automotive, aerospace, robotics, and industrial applications.

Case 1: EV In-Wheel Traction Drive (YASA Yokeless)

Specification: 240 mm OD, 140 mm ID, 30 slots, 500 Hz electrical frequency, 25,000 units/year series volume.

Process Solution: Multi-cavity carbide die compaction using Höganäs Somaloy 700 5P. Teeth pressed at 750 MPa to 7.52 g/cm³ green density and cured at 530°C in nitrogen.

Result: Achieved 16.5-second cycle time per tooth, 3.2% scrap rate, and air-gap axial runout under 0.038 mm without secondary face grinding.

Case 2: Heavy-Lift Drone Propulsion (Ultra-High Speed)

Specification: 130 mm OD, 80 mm ID, 18 slots, 1,200 Hz electrical frequency, lightweight drone gimbal.

Process Solution: Segmented 0.10 mm thin-gauge silicon steel (10JNEX900) laser cut and thermally bonded via Backlack at 200°C for prototype phase, scaling to Somaloy 1100.

Result: Interlaminar losses reduced by 44% compared to standard 0.20 mm steel at 1.2 kHz, delivering a 96.2% electromagnetic efficiency.

Case 3: High-Reliability Industrial Fan (TORUS Continuous)

Specification: 280 mm OD, 180 mm ID, 36 slots, 150 Hz continuous operation, cost-critical tooling budget.

Process Solution: Continuous spiral tape winding of 0.27 mm CRGO strip, vacuum bell furnace annealing at 800°C, and multi-axis diamond wheel slot profile grinding.

Result: Post-grinding chemical micro-etching dissolved 20 μm burrs, keeping no-load iron losses within 1.15 W/kg at 1.5 T / 50 Hz.

Case 4: Medical Surgical Robot (Zero-Cogging Servo)

Specification: 90 mm OD, 50 mm ID, 12 slots, ultra-tight axial runout target (< 0.03 mm TIR), 25 prototypes.

Process Solution: Laser-cut 0.20 mm CRGO segmented teeth with precision CNC ground carrier pockets and diamond face skim grinding after sub-assembly potting.

Result: Measured air-gap runout of 0.021 mm TIR, eliminating torque ripple harmonics for precision surgical articulation.

Case 5: Hybrid Aerospace eVTOL Dual-Stator (High-Frequency)

Specification: 310 mm OD, 210 mm ID, 48 slots, 2,500 Hz maximum electrical frequency, altitude partial discharge requirement (15,000 ft).

Process Solution: Segmented 0.10 mm Hi-B silicon steel (10JNEX900) bonded with Backlack EB-549 under 2.2 MPa pressure at 200°C. Dual-stage VPI with Class H filled epoxy (Araldite CW 229) ensuring zero voiding.

Result: Specific core loss held under 24 W/kg at 1.0 T / 1 kHz, with measured PDIV > 1,600 V RMS under low ambient pressure, qualifying for aerospace flight certification.

Case 6: Micro-Mobility E-Cargo Torus Continuous Core (High-Volume)

Specification: 180 mm OD, 120 mm ID, 24 slots, 120 Hz electrical frequency, target annual volume 50,000 units.

Process Solution: Continuous spiral winding of pre-punched 0.35 mm non-grain-oriented silicon steel with optical servo indexing, followed by batch nitrogen retort annealing at 800°C.

Result: Completely eliminated secondary EDM or grinding slotting cycles, reducing stator core unit manufacturing cost by 38% while maintaining total air-gap runout under 0.042 mm.

Primary Standards & Verified Literature Citations

All engineering parameters, tooling clearances, and material limits in this guide are grounded in established international standards and peer-reviewed literature.

IEC 60404-6:2018 — Magnetic materials - Part 6: Methods of measurement of the magnetic properties of magnetically soft metallic and powder materials at frequencies in the range 20 Hz to 100 kHz.

Publisher: International Electrotechnical Commission (2018) — Technical Scope: Standardized ring specimen testing procedures for permeability, iron loss, and coercivity in soft magnetic composites (SMC).

Höganäs AB — Somaloy Soft Magnetic Composites: Material Specification, Tooling Design and Thermal Curing Guidelines (Doc Ref: 2273-HOG).

Publisher: Höganäs AB Powder Technologies (2024) — Technical Scope: Empirical compaction pressure (600–800 MPa), green density curves (7.45–7.58 g/cm³), and protective nitrogen curing kinetics for Somaloy 700 3P/5P.

Woolmer, M. & McCulloch, M. — "Analysis and Design of a Yokeless and Segmented Armature Axial Flux Machine", IEEE International Electric Machines and Drives Conference (IEMDC).

Publisher: IEEE Transactions / IEMDC (2007) — Technical Scope: Pioneering manufacturing methodology for segmented stator teeth in yokeless dual-rotor AFPM topologies, establishing tooth-to-carrier potting constraints.

IEC 60404-8-7:2020 — Magnetic materials - Part 8-7: Specifications for cold-rolled grain-oriented electrical steel strip and sheet delivered in the fully-processed state.

Publisher: International Electrotechnical Commission (2020) — Technical Scope: Specifications for thin-gauge electrical steel (0.10–0.27 mm), magnetic anisotropy along rolling direction, and coating insulation classes.

MPIF Standard 35-SP:2020 — Materials Standards for P/M Structural Parts - Powder Metallurgy Tolerances, Tooling Clearance, and Mechanical Properties.

Publisher: Metal Powder Industries Federation (2020) — Technical Scope: Industry standard dimensional tolerance capabilities, tooling draft angles (0.5°–1.0°), and density gradient limits in axial powder pressing.

ISO 2768-2:1989 — General tolerances - Part 2: Geometrical tolerances for features without individual tolerance indications.

Publisher: International Organization for Standardization (1989) — Technical Scope: Geometric tolerance classes (flatness, perpendicularity, runout) applied to motor stator carrier ring pockets and active pole face planes.

IEC 60404-2:2023 — Magnetic materials - Part 2: Methods of measurement of the magnetic properties of electrical steel strip and sheet by means of an Epstein frame.

Publisher: International Electrotechnical Commission (2023) — Technical Scope: Standardized specific total loss (W/kg) calibration for 0.10–0.35 mm silicon steel sheets across 50 Hz to 1,000 Hz.

ISO 8015:2011 — Geometrical product specifications (GPS) - Fundamentals - Concepts, principles and rules.

Publisher: International Organization for Standardization (2011) — Technical Scope: Invocation of the envelope principle and datum independence for axial stator pole-face planes and rotor clearance stacks.

SWD AG — Advanced Stator Lamination Stacking: Backlack Bonding Kinetics and Laser HAZ Microstructural Analysis (Doc Ref: SWD-WP-2024-AFPM).

Publisher: SWD AG Stator Technology (2024) — Technical Scope: Empirical measurement of heat-affected zone (HAZ) magnetic degradation (120–160 µm depth) in laser-cut thin electrical steel versus stamped Backlack edge retention.

Frequently Asked Questions (AFPM Core Manufacturing)

Expert answers to common engineering questions regarding tooling, annealing, tolerances, and process selection.

Process Selection & Materials

What are the primary manufacturing routes for AFPM stator cores?

The three established manufacturing routes for AFPM stator cores are: (1) Soft Magnetic Composite (SMC) 3D net-shape die compaction, which presses atomized insulated iron powder into finished tooth profiles; (2) Segmented lamination tooth stacking, where stamped or laser-cut grain-oriented silicon steel (CRGO) sheets are thermally bonded using Backlack varnish; and (3) Continuous tape-wound slotted cores, where a thin electrical steel strip is spirally wound into a solid toroid and subsequently radial-slotted via wire EDM or precision diamond grinding.

Process Selection & Materials

When should an engineer specify SMC instead of laminated electrical steel for an AFPM core?

SMC (such as Höganäs Somaloy 700 3P/5P) is preferred when: the electrical frequency exceeds 400 Hz (where its isotropic dielectric insulation drastically suppresses eddy current losses); when the stator tooth features 3D geometries such as rounded shoe edges or 3D flux concentrators; when annual production volume exceeds 1,000 units to amortize tooling; or when minimizing scrap is critical (SMC scrap is <4% versus 30–45% for stamped trapezoidal lamination stacks).

Process Selection & Materials

Why is Grain-Oriented Electrical Steel (CRGO) used in segmented axial flux teeth?

CRGO exhibits an easy magnetization axis ([100] Miller index) along its rolling direction with very high saturation flux density (up to 1.95–2.0 T) and low hysteresis loss. In segmented AFPM teeth (e.g. YASA topology), magnetic flux travels strictly axially from rotor to rotor. By orienting the rolling direction along the axial flux path, motor designers achieve 15–20% higher air gap flux density than isotropic steel or SMC without saturating.

Process Selection & Materials

What is the maximum electrical frequency feasible for tape-wound slotted cores?

Tape-wound slotted cores are generally restricted to operating frequencies below 300–400 Hz. While the continuous strip path is ideal for toroidal back-iron flux, mechanical slot machining (milling or wire EDM) inevitably creates metallic smear burrs across adjacent strip turns. Even with chemical etching, interlaminar micro-shorts tend to elevate eddy current heating severely at higher frequencies.

Tooling, Dies & Compaction

What compaction pressure and press tonnage are required for SMC stator teeth?

Standard high-density SMC components require 600 to 800 MPa uniaxial compaction pressure to achieve green densities of 7.45–7.55 g/cm³. For an average AFPM tooth with a face area of 800 mm², the required compaction force is approximately 560 kN (57 metric tons). Accounting for hydraulic press safety margins, a 100-ton to 160-ton CNC powder press is recommended per cavity.

Tooling, Dies & Compaction

What die materials and clearances are used for pressing SMC stator teeth?

SMC compaction tooling utilizes tungsten carbide (WC-Co, 10–12% cobalt binder) die inserts with hardened tool steel (CPM 10V or D2, 60–62 HRC) punches. Diametral die-to-punch clearance must be held strictly between 4 μm and 8 μm per side to prevent powder flash or punch galling. Die walls require a 0.5° to 1.0° draft angle and mirror-polished surface finish (Ra < 0.1 μm) to facilitate damage-free green ejection.

Tooling, Dies & Compaction

What is the maximum viable aspect ratio (length-to-width) for an SMC tooth?

The recommended aspect ratio (axial core length divided by mean tooth width) for single-action or simple dual-action SMC pressing is 3.5:1. Above 3.8:1, internal powder friction against die walls creates a severe density gradient (lower density in the middle of the tooth), reducing local magnetic permeability and inducing micro-cracks upon ejection. Taller teeth require advanced multi-action CNC hydraulic punches or multi-piece split teeth.

Tooling, Dies & Compaction

How does tooling NRE compare between prototype and series production routes?

For prototypes (1–50 units), segmented laminations cut via precision fiber laser incur minimal tooling ($1,500–$4,000 for stacking fixtures), whereas an SMC prototype die costs $8,000–$14,000. For series production (>10,000 units), a progressive carbide stamping die costs $35,000–$65,000 with a 10–14 week lead time, while automated multi-cavity SMC tooling costs $25,000–$50,000 with cycle times under 20 seconds.

Thermal Processing & Annealing

What heat treatment is required after toroidal tape winding?

Toroidal tape-wound cores must undergo high-temperature stress-relief annealing at 780°C to 820°C in a dry protective atmosphere (95% nitrogen / 5% hydrogen, dew point below -40°C) for 2 to 4 hours. Plastic deformation and tight bending strain during spiral winding degrade magnetic domain mobility and double core losses; annealing restores magnetic permeability and reduces coercivity to baseline values.

Thermal Processing & Annealing

What is the thermal curing cycle for Soft Magnetic Composites (SMC)?

Unlike laminated electrical steels that require 800°C+ annealing, SMC parts are cured at a moderate 500°C to 550°C for 30 minutes in a nitrogen atmosphere. This temperature is carefully selected to burn off internal compaction lubricants and relieve mechanical pressing stresses without thermally decomposing the ultra-thin organo-silicon or phosphate dielectric insulation coating between iron grains.

Thermal Processing & Annealing

How is Backlack self-bonding varnish activated during lamination stacking?

Backlack-coated electrical steel sheets are stacked in precision indexing fixtures and placed in a heated press platen. The bonding profile requires heating the stack to 195°C–205°C under a constant mechanical clamping pressure of 2.0–2.5 MPa for 3 to 5 minutes. The cross-linking epoxy resin cures to provide an interlaminar shear strength exceeding 4.5 MPa without requiring rivets, bolts, or welding that cause eddy loops.

Tolerances, Assembly & QC

How is axial air-gap runout controlled on an assembled AFPM stator?

Axial air-gap runout is the primary mechanical failure mode in axial flux motors. Total accumulated runout is controlled via a 3-tier datum scheme: Datum A is the active pole face plane; Datum B is the carrier seat ring. To guarantee runout below 0.05 mm across a 200 mm diameter stator, individual teeth are held to ±0.02 mm thickness, bonded with controlled 0.10 mm epoxy film, and—if RSS stack exceeds limits—subjected to a post-assembly precision diamond face skim grind (<15 μm removal).

Tolerances, Assembly & QC

Why is chemical micro-etching necessary after slot machining in tape cores?

Mechanical cutting operations (wire EDM, diamond grinding, or milling) apply shear forces that drag molten or plastically deformed steel across the laminated edges, bridging the microscopic interlaminar insulation layer. Chemical micro-etching with a controlled nitric-phosphoric acid solution dissolves 15–25 μm of surface metal from the slot walls, eliminating inter-strip shorts and recovering design eddy current loss targets.

Tolerances, Assembly & QC

What non-destructive testing (NDT) methods verify AFPM core quality before winding?

Pre-winding quality gates include: (1) Single-tooth AC magnetic loss testing using a calibrated excitation fixture to verify specific core loss (W/kg) at target frequency; (2) High-voltage insulation resistance testing (>500 V DC between laminations or carrier); (3) Optical CMM coordinate measuring for tooth perpendicularity (≤0.05 mm) and crown profile; and (4) Resonance acoustic tap testing to detect internal green cracks or Backlack voids.

Tolerances, Assembly & QC

What is the typical material utilization rate (scrap percentage) across core routes?

SMC 3D die compaction offers the highest material efficiency with >95% material utilization (scrap rate 2.5–4%, primarily green gate trimming and powder recovery). Continuous tape winding exhibits approximately 18–25% scrap due to slot material removal. Stamped segmented lamination teeth suffer 28–42% scrap because nesting radial trapezoidal segments from rectangular coiled sheet produces significant edge offcuts.

Tolerances, Assembly & QC

How does Vacuum Pressure Impregnation (VPI) protect AFPM stator cores in service?

VPI submerges the assembled core and concentrated winding assembly in high-grade Class H (180°C) or Class N (200°C) epoxy resin under alternating vacuum (<1 mbar) and pressure (5–6 bar). VPI eliminates microscopic air voids in slot pockets, locks lamination edges against electromagnetic vibration harmonics, enhances structural shear bond to the stator carrier, and increases stator radial thermal conductivity by up to 2.5x.

Tolerances, Assembly & QC

How does the ISO 8015 envelope principle apply to AFPM stator pole face planes?

Under ISO 8015:2011 GPS standards, dimensional size tolerances and geometric form tolerances are independent unless the envelope requirement (symbol E) is explicitly invoked. For AFPM stator teeth, invoking the envelope principle on tooth height ensures that no individual surface point breaches the theoretical boundary formed by the maximum material condition (MMC), preventing rotor-stator air-gap rubbing even if localized thermal crowning occurs during peak motor duty.

Tooling, Dies & Compaction

How is powder segregation prevented during high-speed feeder filling in SMC compaction?

Because atomized SMC powder contains a distribution of particle sizes (100–250 μm) mixed with dry internal organic lubricant (0.4–0.6% by weight), gravity feeding through long hoppers can cause particle segregation and localized lubricant agglomeration. Series production lines employ synchronized pneumatic shuttle feeder shoes with internal anti-vortex baffles and inert dry nitrogen blankets, ensuring uniform bulk density distribution (±0.5%) across all die cavities.

Thermal Processing & Annealing

What ultrasonic non-destructive testing (NDT) techniques detect Backlack delamination?

Backlack bonded lamination stacks are inspected using high-frequency ultrasonic immersion testing (15–25 MHz focused transducers) or air-coupled ultrasound. Delamination voids or under-cured adhesive patches reflect acoustic energy, creating a high-amplitude echo peak at the defective layer. A 100% production gate requires pulse-echo signal attenuation to remain below -18 dB across all inter-lamination boundaries, guaranteeing shear bond strength > 4.5 MPa.

Process Selection & Materials

What is the thermal expansion mismatch risk between SMC stator teeth and an aluminum carrier?

SMC compacted iron has a coefficient of thermal expansion (CTE) of approximately 11.5–12.5 × 10⁻⁶ /K, whereas standard 6061-T6 aluminum stator carriers expand at 23.5 × 10⁻⁶ /K. Over an automotive operating temperature span of -40°C to +150°C (ΔT = 190 K), the radial differential expansion exceeds 0.08 mm on a 200 mm diameter. To prevent joint delamination or tooth tilting, carriers must incorporate circumferential flexure pockets or utilize low-expansion structural composite carriers (CTE 10–14 × 10⁻⁶ /K) potted with high-elongation structural epoxy.

AxialFluxCore Manufacturing Review Service

Ready to Validate Your AFPM Core Manufacturing Process?

Submit your 3D CAD step model, stator winding envelope, and target annual volume for a confidential engineering review. Our production team verifies die compaction feasibility, lamination nesting, and GD&T runout stacks within 48 hours.

Request an Engineering Review Re-evaluate Process Parameters

Technical consultation covers Somaloy 3P/5P powder pressing, Backlack lamination stacking, and CMM air-gap plane verification.