Design Considerations for Lamination Bonding
09/27/2024
For engineers working with magnetic steel laminations, a secondary OD or ID grinding pass is often the difference between a good core and a great one. That single operation controls runout, slip fit concentricity, and ultimately the efficiency and noise level of the finished motor.
One of the most common questions design teams face is deceptively simple: “How much stock should we leave before grinding?”
Lasertech’s cross functional trials, which combined bonding lab data, machine shop experience, and design engineering input, point to a not to exceed value of 0.015 inches of total stock. Below is why that guideline works, when it shifts, and the hidden costs of drifting too far from it.
The 15-Thousandths Rule: Proven by Finish Data and Bond Tests
Leaving no more than 0.015 inches for removal gives grinders enough material to true up minor warpage, adhesive squeeze out, and tolerance stackups. In Lasertech’s finish trials, this allowance helped consistently achieve final tolerances while keeping cycle times efficient.
Bond pull tests further confirmed that adhesive strength remained intact, showing heat input from grinding stayed below the epoxy’s softening threshold. When stock allowances climb higher; cycle times stretch, wheel wear increases, and localized heat check micro cracks can form which potentially undermines insulation.
When Less Is (and Isn’t) More
Can you grind with less than 0.015 inches? Yes, if the lamination design includes a strong back iron buffer.
Back iron acts as both a heatsink and structural support. The thicker it is, the less the lam flexes under wheel pressure, which allows stock allowances as low as 0.008–0.010 inches without creating chatter marks.
Thin back iron, by contrast, makes the lam behave more like a bell under the grinder. Vibrations couple directly into the bond line and can propagate micro delamination. In those cases, leaving more stock is the safer choice.
Quick guideline:
- Back iron less than 0.060 in. → Leave 0.015–0.020 in.
- Back iron 0.060–0.125 in. → 0.012–0.015 in. works well
- Back iron greater than 0.125 in. → You can reduce to 0.008–0.012 in., but always validate with grind trials
Bond Integrity Matters
Grinding removes steel but also transmits heat through the adhesive layer. Epoxies commonly used for lamination bonding begin softening between 110 and 130 °C. Excessive infeed rates or dull wheels can spike temperatures beyond that threshold, leading to micro voids or even bond failures that only surface later as vibration or elevated core losses.
Lasertech’s process window controls wheel face temperature below 90 °C by pairing the 15 thousandths rule with strong coolant flow and conservative feeds (3-4 ipm on thin laminations).
Design-for-Grind Checklist
To set suppliers and machinists up for success:
- Declare OD/ID stock directly on the print. Ambiguity forces guesswork and often results in over grinding.
- Specify back iron thickness alongside stock allowance so stiffness context is clear.
- Call out surface finish and TIR targets, as they influence dressing frequency and wheel choice.
- Note adhesive type and cure temperature. High temperature phenolics tolerate heavier passes than low temperature epoxies.
ROI: Faster Qualification, Less Scrap, Better Efficiency
Following a stock allowance guideline rooted in real grind data shortens the learning curve at PPAP. Fewer rejected cores mean faster qualification, less downtime from wheel changes, and improved flux density from perfectly concentric stacks.
Clients who adopted Lasertech’s 0.015 inch baseline reported machining cycles reduced by 12 percent and scrap reduced by 18 percent across the first three production runs.
Validate Before You Scale
Need to confirm your own grind allowance? Lasertech’s machine shop offers complimentary pilot runs that correlate surface finish, bond pull strength, and back iron stiffness. Reach out to schedule a sample trial and establish a process window that scales smoothly from prototype through production.
