Sheet MetalDesign Guide
A sheet metal part is one flat blank, cut and folded. Design around that: bend radius, relief, hole placement, and the K-factor that makes the flat pattern come out right. These are the rules our DFM review checks.
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Design rules
Everything about a sheet metal part traces back to one fact: it starts as a single flat blank that gets cut, then folded on a press brake. Design with that process in mind and parts form clean the first time.
- 01
Minimum bend radius
Inside radius at least equal to material thickness. Too tight and the outside surface cracks or thins.
- 02
Bend relief
At a flange edge, add relief cuts at least one thickness deep so the brake does not tear the adjacent material.
- 03
Hole-to-bend distance
Keep holes at least 2.5 times thickness plus the bend radius from the bend, or they deform when formed.
- 04
Hole-to-edge distance
At least 2 times thickness from any edge so holes stay round and edges stay flat.
- 05
Minimum flange length
At least 4 times thickness plus the radius, so the flange has enough material to seat in the brake.
- 06
Uniform thickness
A part is cut from one flat sheet, so the whole part is one gauge. Design around a single thickness.
K-factor and bend allowance
When metal bends, the outside stretches and the inside compresses. Somewhere between them is the neutral axis, a line whose length does not change. The K-factor tells you where that line sits, as a fraction of the material thickness. It usually falls between 0.3 and 0.5 depending on the material and how it is bent.
This matters because the flat blank cannot just be the sum of the finished dimensions. It has to account for how much the metal stretches through each bend. That correction is the bend allowance:
Add the bend allowances to the flat sections and you get the true blank length. Get the K-factor wrong and every formed dimension drifts. We develop the flat pattern from the K-factor that matches the material and our brake tooling, so the part comes off the press on print.
Steel gauge chart
Gauge is a legacy way of specifying steel thickness. Lower number, thicker material. Aluminum and stainless use their own gauge tables, so we recommend calling out the decimal thickness on the drawing to avoid ambiguity.
| Gauge (steel) | Thickness | Metric |
|---|---|---|
| 24 ga | 0.024" | 0.61 mm |
| 22 ga | 0.030" | 0.76 mm |
| 20 ga | 0.036" | 0.91 mm |
| 18 ga | 0.048" | 1.21 mm |
| 16 ga | 0.060" | 1.52 mm |
| 14 ga | 0.075" | 1.90 mm |
| 12 ga | 0.105" | 2.66 mm |
| 10 ga | 0.135" | 3.42 mm |
Nominal values for standard steel sheet. Actual mill thickness varies within tolerance.
Common questions.
As a rule, keep the inside bend radius at least equal to the material thickness. Softer materials like 5052 aluminum and mild steel tolerate tighter bends; harder tempers and thicker gauges need a larger radius to avoid cracking on the outside of the bend.
K-factor is the ratio that locates the neutral axis inside a bend, the line that neither stretches nor compresses. It typically runs 0.3 to 0.5 depending on material and bend method. It matters because the flat blank has to be cut longer or shorter than the sum of the finished dimensions to account for how the metal stretches. We develop the flat pattern from the actual K-factor so formed parts land on print.
Bend allowance is the arc length of the neutral axis through the bend: BA = angle (in radians) x (inside radius + K-factor x thickness). Add the bend allowances to the flat sections to get the true blank length. In practice our CAD and brake tooling handle this; the K-factor is the input that makes it accurate.
Keep holes at least 2.5 times the material thickness plus the bend radius away from the bend line. Closer than that and the forming operation pulls the hole out of round or into an oval. If a hole must sit near a bend, we can pierce it after forming, at added cost.
Cold and hot rolled steel, stainless 304 and 316, aluminum 5052 and 6061, galvanized steel, copper, brass, and spring steel, in gauges from 0.020" to 0.250". Material is sourced to ASTM specs with certs available on request.
A STEP file of the 3D part plus a flat DXF is ideal, with a 2D drawing calling out bends, hardware, tolerances, and finish. The DXF of the flat pattern lets us quote cutting time directly.
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