One of the most frequent reasons sheet metal chassis fail assembly inspection is incorrect flat pattern unfolding. When metal is bent on a CNC press brake, the outer surface stretches in tension while the inner surface compresses. The neutral axis, where no length change occurs, shifts inward, requiring precise bend deduction calculations.
What Is the K-Factor?
The K-factor is the ratio of neutral axis depth to total sheet thickness (t). For standard air bending on modern CNC press brakes:
- Mild Steel (1.5 mm to 3.0 mm): Standard K-factor typically falls between 0.38 and 0.42.
- Stainless Steel (304/316): Higher yield strength results in a K-factor around 0.44 to 0.46.
- Aluminium (5052-H32): Softer alloy with high ductility, typically using a K-factor of 0.40.
Rules for Flange Length and Hole Placement
To avoid scrap and damaged press brake tooling, observe these minimum dimensions:
- Minimum Flange Length: A flange must be long enough to bridge the V-die opening during bending. As a general rule, minimum internal flange length should equal at least 4 times the sheet thickness plus the inside bend radius.
- Distance from Hole to Bend: Holes placed closer than 2.5 times sheet thickness from the bend line will stretch and deform out of round. If close proximity is required, create the hole as a post-bend secondary drilling operation or add a rectangular relief slot along the bend line.
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FamilyMake's engineering team validates your 3D CAD models, verifies flat pattern developments against available machine tooling, and manufactures tight-tolerance enclosures on schedule.
