In precision motor manufacturing, burr height is far more than a cosmetic concern. A burr on a stator or rotor lamination — no matter how small — can compromise stacking factor, increase eddy current losses, damage insulation coatings, and in the worst case, create electrical short circuits between laminations. For electric motor applications, especially thin-gauge silicon steel used in EV traction motors, controlling burr height is one of the most direct measures of stamping process health.
Many motor manufacturers struggle with burr-related quality issues without realizing that burr formation is a predictable, controllable outcome of die design, material selection, manufacturing precision, and maintenance discipline. This guide breaks down the root causes of excessive burr and provides actionable strategies to achieve consistent, burr-free motor lamination stamping — whether you’re running a tool steel compound die for prototypes or a tungsten carbide progressive die for high-volume production.
1. What Is Burr and Why It Matters in Motor Laminations
A burr is a small, raised edge or ridge of material that forms at the cut edge of a stamped part when the metal fractures under shear stress. During motor lamination stamping, burrs typically form on the punch side of the strip as the punch pushes through the silicon steel and separates it from the die.
While all stamping operations produce some burr, excessive burr height creates multiple problems:
- Reduced stacking factor: Burrs prevent laminations from packing tightly together. A stack of laminations with 0.020mm burrs may lose 1–3% of its theoretical magnetic core density, directly reducing motor efficiency.
- Increased interlaminar eddy currents: Burrs can bridge the insulation coating on adjacent laminations, creating a path for eddy currents to flow between sheets. This generates additional heat and lowers motor performance.
- Insulation damage: Sharp burrs can scratch or pierce the thin insulation layer during stacking or pressing, creating local short circuits.
- Accelerated die wear: Excessive burr is often a symptom of worn tooling. Ignoring it accelerates punch and die degradation.
- Downstream processing problems: Burrs interfere with winding (especially hairpin insertion), welding, and gluing processes.
Industry standards for burr height
For high-quality motor laminations, acceptable burr height is typically:
| Material Thickness | Max Recommended Burr Height |
|---|---|
| 0.20–0.27mm (EV thin-gauge) | ≤ 0.010mm |
| 0.30–0.35mm | ≤ 0.015mm |
| 0.50mm | ≤ 0.020mm |
A general rule of thumb is that burr height should not exceed 5% of material thickness. For a 0.25mm lamination, that’s just 0.0125mm — less than half the diameter of a human hair. Achieving and maintaining such tight burr control requires a systematic approach across die design, manufacturing, and maintenance.
2. The Root Causes of Excessive Burr
Burr height is influenced by several interacting factors. Identifying the root cause is the first step toward effective correction.
2.1 Improper cutting clearance
The single most influential parameter is the clearance between punch and die. If the clearance is too small, the material tears rather than shears cleanly, producing a secondary shear zone and higher burr. If the clearance is too large, the material flows into the gap before fracture, creating a large rollover and burr. For silicon steel, the optimal clearance is typically 3–5% of material thickness per side.
2.2 Worn cutting edges
As punches and die inserts wear, their edges become rounded. A rounded edge cannot concentrate shear stress effectively, forcing the material to bend and stretch rather than cut cleanly. This results in progressively increasing burr height. Regular inspection and regrinding are essential.
2.3 Inadequate guidance and alignment
If the punch and die are not perfectly aligned — due to worn guide posts, loose bushings, or poor die set parallelism — the clearance becomes uneven around the punch circumference. This uneven clearance leads to localized high burr on one side of the lamination slot or perimeter.
2.4 Material variability
Silicon steel with inconsistent thickness, varying insulation coating thickness, or edge camber from slitting can all contribute to burr variation. Thicker-than-spec material may locally reduce clearance and cause high burr; thinner material may increase clearance and produce ragged edges.
2.5 Excessive stamping speed
While high-speed stamping improves productivity, it also generates frictional heat at the cutting interface. This heat can soften the cutting edges of tool steel punches and increase adhesive wear, accelerating burr formation. For carbide tools, thermal stress can cause micro-chipping if speed exceeds the tool’s design limits.
3. Optimal Die Clearance for Burr-Free Stamping
3.1 The clearance-burr relationship
Stamping clearance directly determines the ratio of shear, fracture, and tear zones on the cut edge. A proper clearance produces:
- Clean shear zone (about 1/3 of material thickness)
- Smooth fracture zone
- Minimal rollover
- Minimal burr
3.2 Recommended clearance values for silicon steel
| Material Thickness | Recommended Clearance per Side (5% t) | Full Die Clearance (both sides) |
|---|---|---|
| 0.20mm | 0.010mm | 0.020mm |
| 0.25mm | 0.0125mm | 0.025mm |
| 0.30mm | 0.015mm | 0.030mm |
| 0.35mm | 0.0175mm | 0.035mm |
| 0.50mm | 0.025mm | 0.050mm |
These values serve as starting points. Fine-tuning should be done through trial stamping, measuring burr height, and adjusting the die. For extremely thin EV-grade steel, some die makers use clearances as low as 3% to minimize burr, but this increases punch wear and requires carbide tools to maintain edge integrity.
3.3 How to verify clearance uniformity
Uniform clearance around every punch is just as important as the nominal value. This requires:
- Precision wire EDM cutting of die openings and punch profiles with ±0.001mm accuracy
- Jig grinding for final sizing of critical dimensions
- CMM inspection to verify clearance after assembly
A die built with standard machining tolerances may have clearance variation of ±0.005mm — enough to cause measurable burr differences around a single slot. This is why our Precision Manufacturing process includes jig grinding and CMM verification on every motor core die, ensuring clearance uniformity within ±0.002mm.
4. Material and Coating Strategies to Reduce Burr
4.1 Tool material selection
The cutting edge material has a direct impact on how long burr height remains within specification.
- Conventional tool steel (D2, DC53): Adequate for low-volume and prototype stamping, but edges wear quickly on silicon steel, leading to burr escalation after 10–20 million strokes.
- Powder metallurgy tool steel (ASP23, VANADIS 4): Finer carbides and higher hardness extend sharp-edge life to 30–50 million strokes with proper maintenance.
- Tungsten carbide (WC-Co): The ultimate choice for burr control in high-volume motor production. Sub-micron grain carbide with 6–8% cobalt maintains a razor-sharp edge for 100 million strokes or more, keeping burr height remarkably consistent. See our dedicated Tungsten Carbide Stamping Die page for more details.
4.2 Surface coatings
PVD coatings reduce friction and adhesive wear, helping the cutting edge stay sharp longer:
- TiCN: High hardness, excellent for abrasive silicon steel; extends edge life by 20–40% on tool steel.
- DLC (Diamond-Like Carbon): Ultra-low friction prevents material pickup; ideal for thin-gauge, adhesive-coated steel.
- TiAlN: Provides thermal stability for high-speed stamping, protecting edges from heat-related softening.
Coatings are particularly effective when applied to carbide punches, further delaying the onset of burr growth. We covered coating systems in depth in our article 5 Proven Ways to Extend Motor Lamination Die Life Beyond 100 Million Strokes.
4.3 Deep cryogenic treatment
For tool steel components, deep cryogenic treatment at −185°C after quenching and before tempering transforms retained austenite to martensite and precipitates fine carbides. This enhances wear resistance and dimensional stability, keeping clearances stable and burr height low over longer runs.
5. Precision Manufacturing and Die Alignment
5.1 The role of wire EDM and jig grinding
Cutting clearance depends on both the punch and die being manufactured to tight tolerances. In modern motor core die construction:
- Wire EDM (slow wire): Achieves profile accuracy of ±0.001mm and surface roughness Ra 0.2–0.3 µm.
- Jig grinding: Refines critical holes and profiles to sub-micron tolerances, correcting any EDM recast layer that could affect edge quality.
- Surface grinding: Maintains punch and die plate flatness within 0.002mm across the die face.
These processes, carried out in a climate-controlled environment, minimize dimensional variation that would otherwise translate into uneven clearance and localized burr.
5.2 Guidance and die set precision
Even a perfectly machined die will produce high burr if the punch deflects during stamping. Key elements include:
- Ball-bearing guide posts and bushings: Provide near-zero play over the full stroke, maintaining alignment at high speed.
- Guided strippers: The stripper plate itself is guided on the guide posts and fitted with hardened, replaceable guide inserts around each punch. This supports the punch tip and prevents lateral movement during cutting.
- Die set parallelism: The upper and lower die shoes must be parallel within 0.005mm across the bolster area to prevent uneven die height.
A well-designed Motor Core Progressive Die integrates all these features as standard, ensuring that clearance uniformity is maintained not just at die tryout but throughout millions of strokes.
5.3 Thermal management
At 400–600 strokes per minute, frictional heat can cause local thermal expansion of punches and dies, momentarily reducing clearance and increasing burr. This is especially critical in long, slender punches for rotor slots. Mitigation measures include:
- Internal or external air cooling directed at the cutting zone
- Thermal-stable die materials (carbide’s low thermal expansion helps)
- Optimized punch design to reduce heat generation (e.g., shorter punch length, larger cross-section where possible)
6. Maintenance Practices for Consistent Burr Control
6.1 Burr-height-based regrinding schedule
The most reliable way to keep burr within tolerance is to measure it regularly and regrind the tool when it approaches the limit. A typical schedule for a progressive die might be:
| Die Type | Initial Burr (new tool) | Regrind Trigger | Regrind Amount |
|---|---|---|---|
| Tool steel progressive die | 0.010mm | 0.018mm | 0.03–0.05mm |
| Carbide progressive die | 0.005mm | 0.012mm | 0.03–0.08mm |
For example, on 0.25mm EV-grade silicon steel, a new carbide die might produce burr of 0.005mm. The tool is reground when burr reaches 0.012mm, restoring it to 0.005–0.008mm. This proactive approach prevents scrap and maximizes die life.
6.2 Controlled regrinding
Regrinding must remove the minimum material needed to restore a sharp edge. Heavy grinding generates heat, risks cracking, and wastes valuable tool life. Recommended regrind depth per cycle is 0.03–0.08mm, using diamond wheels on carbide and flood coolant.
6.3 Inspection checklist during maintenance
Every time the die is pulled for maintenance, the following should be checked:
- Edge condition: Use a microscope or profile projector to inspect all cutting edges for chipping, rounding, or built-up edge.
- Clearance verification: Spot-check clearance at several locations with feeler gauges or optical measurement.
- Guide component wear: Check guide post/bushing play; replace if play exceeds 0.005mm.
- Stripper plate condition: Verify insert hardness and fit; replace worn guides.
- Chip and slug evacuation: Ensure slug holes are clear and air blow channels are functioning.
Our Quality Control department follows these procedures on every die we service, and we provide customers with detailed inspection reports so they can maintain the same discipline in their own toolroom.
7. Measuring and Inspecting Burr
7.1 Common measurement methods
- Profile projector (optical comparator): Projects the edge profile at 20–50× magnification, allowing visual measurement of burr height against a calibrated screen.
- Laser scanning micrometer: Provides non-contact, high-speed measurement across the entire perimeter, useful for 100% inspection in high-volume lines.
- Stylus profilometer: Traces the surface to measure burr height and roughness; best for laboratory verification.
- Microscope with digital camera: Captures edge images for documentation and SPC analysis.
7.2 SPC (Statistical Process Control)
For high-volume EV motor production, burr height should be tracked with SPC charts. Measure 5–10 parts at regular intervals (e.g., every 2 hours) and plot the average and range. A rising trend indicates edge wear and triggers scheduled regrinding before the burr exceeds specification. This data-driven approach is central to achieving consistent quality and is something we encourage all our customers to adopt.
8. Case Example: Achieving <0.010mm Burr on 0.25mm EV Silicon Steel
Consider a typical EV traction motor stator lamination stamped from 0.25mm non-oriented silicon steel. The die is a carbide progressive die with auto-stacking stations running at 400 SPM.
Key specifications:
- Tungsten carbide grade: sub-micron WC with 6% Co
- Coating: DLC on all cutting punches
- Cutting clearance: 0.012mm per side (4.8% t)
- Guide system: ball-bearing posts with guided stripper
- Regrind interval: every 40–50 million strokes, 0.05mm removal
Result: Burr height measured 0.005–0.008mm at die tryout, remaining below 0.010mm for over 80 million strokes before the first regrind. Lamination stacking factor exceeded 98.5%, and interlaminar eddy current losses were negligible.
This is not a hypothetical scenario — it reflects the capability we have delivered to customers in the EV & Traction Motor sector. With the right combination of design, materials, and maintenance, burr-free stamping at high volume is entirely achievable.
9. Conclusion: Burr Control Is a System
Achieving and maintaining low burr height in motor lamination stamping is not a single adjustment or a one-time fix. It is a system that encompasses:
- Optimal die clearance (3–5% of material thickness)
- Premium tool materials (carbide for high volume)
- Appropriate surface coatings
- Precision manufacturing and rigid guidance
- Proactive, data-driven maintenance
When all elements work together, burr height becomes a stable, predictable quality parameter — not a source of daily frustration.
Ready to reduce burr and improve your motor core quality? Whether you need a new Motor Core Progressive Die engineered for ultra-thin silicon steel, or you’re looking to upgrade an existing tool with carbide inserts and DLC coatings, our team can help. Send us your drawing and specifications and receive a detailed technical proposal within 48 hours.
