Key takeaways
- Laminations are stamped from thin electrical steel, so the material grade and coating affect both the magnetic loss and the stamping behaviour.
- The profile is a set of magnetic and mechanical decisions: yoke width, tooth width, slot shape, air gap and keyway.
- Burr direction matters because a stack is built from one orientation, and burr height affects the stack length, the core loss and the shaft fit.
- Stacking method changes the core: interlocking is fast, welding and riveting are strong, and adhesive or loose stacks suit different volumes.
Why motor laminations are stamped
A laminated core is built from thin steel sheets, each insulated from the next, so that eddy currents cannot circulate freely through the core. The sheets are stamped because the profile is a flat shape with slots, teeth and a bore, and a progressive die can produce it at high speed with a repeatable edge. Laser cutting can make the same shape, but stamping is usually the volume process.
The stamping is only one part of the chain. The steel grade, the insulation coating, the die condition and the stacking method all change the performance of the finished core. A buyer who compares quotes on piece price alone is comparing only the cutting step, not the magnetic part.
The same press and die can also make rotor plates, which carry the shaft, the magnets or the cast aluminium cage. Rotor plates often have tighter requirements on the bore and the shaft fit, because the rotor rotates and the air gap is small.
Electrical steel: grade, thickness and coating
Motor laminations are usually made from non-oriented electrical steel, a low carbon steel with silicon added to raise resistivity and reduce core loss. The silicon content is typically in the range of about 1 to 3.5 per cent. More silicon lowers loss but makes the steel harder and more brittle, which affects blanking, burr and die wear.
Thickness is a trade-off. Thin strip, around 0.35 mm, reduces eddy current loss at high frequency but costs more per kilogram and needs more strokes per stack. Thicker strip, around 0.50 or 0.65 mm, is common for general industrial motors. The drawing should state the grade, the thickness and the coating, not just the thickness.
The insulation coating is part of the specification. It must survive the stamping, the stacking and any annealing or baking step, and it must not break down at the edges. A coating that is too thick reduces the stacking factor, because more of the stack is insulation and less is steel. A coating that is too thin can short between laminations and raise the loss.
The certificate is not the specification
A mill certificate gives the grade and the mechanical properties, but it may not give the coating class or the core loss at the finished thickness. Ask for the property that matters to the motor, not only the steel designation.
The lamination profile
The stator profile starts with the bore and the air gap. The air gap is kept small to reduce the magnetising current, but it must be large enough to clear the rotor under tolerance and thermal growth. The tooth width carries the flux, and the slot area carries the copper. A wider tooth reduces saturation but takes slot area; a narrower tooth gives more copper but can saturate at the root.
The yoke is the return path around the outside of the slots. It needs enough section to carry the flux without saturating, and enough stiffness to hold the stack and the frame. The slot shape decides the winding, the insulation and the tooling: a parallel slot is easier to wind, a tapered slot gives more room near the air gap.
Mechanical features sit on top of the magnetic ones. A keyway or a notch locates the stack in the frame. Interlock features hold the laminations together. Mounting holes and clearance holes pass through the stack. Every hole removes magnetic material, so each one should be justified by a function, not inherited from a previous design.
Die clearance, burr and the stack
The die clearance sets the edge: a tight clearance gives a cleaner shear and a smaller burr, but it increases tool wear and stripping load. A larger clearance reduces wear but leaves a bigger rollover and burr. For laminations, the burr is not only a handling issue. A burr on one side of every sheet creates a stack that is not flat, and the burrs can contact each other and create an electrical path between laminations.
Burr direction is controlled by the die and by the way the strip is fed. If every sheet is stacked in the same orientation, the burrs sit on the same face and the stack can lean. Some stacks are built with alternate sheets turned over, which balances the burr but changes the interlock and the coating contact. The drawing or the stack specification should say which orientation is required.
Die wear is a production variable. A die that was sharp at the first trial will produce a larger burr after a few hundred thousand strokes. The inspection plan should include a burr check at a defined interval, not only at the first article. If the burr limit is tight, the die needs a maintenance schedule that keeps it inside the limit.
Stacking methods and what they change
A stack can be held together by interlocking features punched into the sheet, by welds along the outside, by rivets or through-bolts, by adhesive between sheets, or by loose stacking for a later process such as casting or clamping. Each method leaves a different mark on the core.
Interlocking is fast and needs no consumables. A small tab is pushed from one sheet into a recess in the next, so the sheets grip each other. The interlock creates a local short circuit and a mechanical stress point, and the displaced metal changes the stack length. It is common in high-volume stators and rotors where the core is not annealed after stacking.
Welding gives a strong stack and a good path for handling, but the weld nugget creates a magnetic short and can distort the bore. Riveting is similar. Adhesive bonding gives a continuous bond with no magnetic short at the outside, but it needs a controlled cure and a clean surface. Loose stacking gives the best magnetic path but needs a fixture or a casting process to hold the core.
| Method | How it holds | Effect on the core | Typical use |
|---|---|---|---|
| Interlock | Tabs pressed into the sheet below | Local short and stress at the tab | High-volume stators and rotors |
| Welding | Weld along the outside diameter | Strong stack, magnetic short at the weld | Large cores and frames |
| Riveting | Rivets through the stack | Mechanical joint, local short at the rivet | Medium and large cores |
| Adhesive | Glue between coated sheets | No outside short, needs a controlled cure | Cores with low loss targets |
| Loose stack | Clamp, frame or casting | Best magnetic path, no joint in the stack | Cores cast or clamped later |
The best method depends on the loss target, the mechanical duty and the volume, not stacking speed alone.
Rotor plates and the shaft fit
The rotor plate carries the shaft and, in many designs, the magnets or the squirrel cage. The shaft fit is usually the tightest call on the part. A bore that is too loose lets the rotor move and changes the air gap; a bore that is too tight can crack the laminations during pressing. The fit should be specified with a tolerance and a press force, not only a nominal diameter.
Rotor plates often need a skew, either by rotating each sheet as it is stacked or by stamping a skewed slot. Skew reduces cogging torque and noise, but it complicates the interlock and the stack fixture. If the drawing calls for skew, the stacking method has to allow it.
Balance is another rotor requirement. The stamping can be symmetrical by design, but the stack, the shaft and the magnets can still be out of balance. The rotor drawing should say whether the balance is corrected after assembly, because that decides how much material can be removed from the plates.
What to inspect on a lamination
The first article should include the bore, the outside diameter, the slot positions, the tooth width, the keyway and the stack length. The burr height and the coating condition should be recorded. If the part is interlocked, the pull-apart force and the stack length after interlocking are useful checks.
For a motor programme, the electrical checks matter as much as the dimensions. Core loss, insulation resistance between laminations and the stacking factor are the properties that connect the stamping to the motor performance. These tests are usually done on a sample stack, not on every sheet, and the sampling plan should be agreed at the start.
Traceability should link the stack to the steel lot and the die maintenance state. If a motor fails a loss test, the ability to trace the lamination back to a coil and a die run is what makes the investigation possible. That record is a normal part of a PPAP pack for an automotive or appliance programme.
Tooling and volume
A lamination die is a precision tool. It has many stations, tight clearances and a large number of parts per stroke, and it is usually built for a long life. The tooling cost is high, but it is spread over a large volume, so the piece price can be low. At low volume, laser cutting or a smaller die may be more sensible.
The volume also decides the stacking method. Interlocking is fast and suits high volume. Adhesive bonding has a cure time and suits a lower volume or a loss-critical design. Loose stacking needs a downstream process, so it only makes sense if that process already exists.
When comparing quotes, ask for the strip layout, the number of outs, the material grade and coating, the stacking method and the burr limit. Those five items explain most of the price difference between two lamination suppliers.
Frequently asked questions
What steel is used for motor laminations?
Most motor laminations use non-oriented electrical steel with about 1 to 3.5 per cent silicon, in thicknesses such as 0.35, 0.50 or 0.65 mm. The grade, thickness and insulation coating should all be specified, because each affects core loss and stamping behaviour.
Does burr direction matter in a lamination stack?
Yes. If every sheet is stacked the same way up, the burrs sit on one face and the stack can lean or short between sheets. The stacking specification should say whether the sheets are alternated or kept in one orientation.
Is interlocking bad for core loss?
An interlock creates a local magnetic short and a stress point, so it can raise loss compared with a loose or bonded stack. The effect depends on the design and the number of interlocks. A loss-critical core may justify a different stacking method.
How tight should the rotor bore be?
Tight enough to hold the shaft without movement, but not so tight that pressing cracks the laminations. Specify the fit, the press force and the inspection method together, and check the bore after stacking because interlocking can change it.
Sources and further reading
- IEC 60404-8-4, Cold-rolled non-oriented electrical steel strip and sheet
- ASTM A677/A677M, Standard specification for nonoriented electrical steel
- Qumek lamination die, stacking and inspection notes
Qumek Engineering Desk
Engineering team
The engineering team behind Qumek drawings, DFM notes, tolerances and inspection standards. Every guide is reviewed against the drawings and processes used on live programmes.
Reviewing a lamination drawing for tooling?
Send the profile, the steel grade, the stack method and the annual volume. The engineering desk will comment on the die, the burr call and the stacking route.
Send a drawing