How to Design Sheets in Sheet Metal for Mechanical Equipment

• 1
  Key takeaways
• 2
  Choosing Material and Thickness for Sheets in Sheet Metal
• 3
  Setting Bend Radius, K-Factor, and the Flat Pattern
• 4
  Hole Placement, Slots, and Features Near Bends
• 5
  Tolerances, Datums, and What Sheet Metal Can Hold
• 6
  Finishes, Hardware, and Assembly Checks Before Release
• 7
  Step by Step: From Concept to Released Flat Pattern
• 8
  When Sheet Metal Fits the Part and When Machining Does

The first decision on any sheet part is the material grade, and it should come from the service environment, not from what is in the rack. For a machine guard or enclosure, 6061-T6 aluminum at 1.5–3.0 mm gives a good stiffness-to-weight ratio and bends cleanly. For a bracket that sees coolant or washdown, 304 stainless at 1.0–2.0 mm is the default. For structural frames under vibration, 5052 or 5083 aluminum takes forming better than 6061 and resists stress cracking at the bend.

Thickness drives everything downstream: bend radius, minimum flange length, and hole size. A rule that holds across most press-brake work is minimum flange length equals 4× material thickness plus bend radius. On a 2.0 mm sheet with a 2.0 mm inside radius, that is 10 mm of flange. Go shorter and the punch has nothing to grip, so the bend curls or the flange pulls away from the die.

Grain direction matters more than most drawings admit. Sheet is rolled, so its ductility runs along the rolling direction and drops across it. If you bend a 90° flange parallel to the rolling direction on a tight radius, you raise the risk of cracking, especially in 6061-T6 and 7075. Where the part allows it, orient bends 45° to the rolling direction, or specify a larger inside radius. We mark the rolling direction on the flat pattern so the brake operator can follow it.

Do not over-specify thickness for stiffness you can get from a formed rib or a hem. Going from 1.5 mm to 2.5 mm raises weight by two thirds and can push the part off a standard brake. A single stiffening rib or a joggled edge often recovers the same rigidity at lower mass and lower cost.

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  6061-T6Good stiffness, bends need a generous radius. Common for guards and covers.
• 2
  5052 / 5083Best forming aluminum. Use where bends are tight or repeated.
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  304 / 316LCorrosion and washdown. 316L for chloride or medical wash.
• 4
  1018 / A36Weldable frames and base plates. Needs paint or plating.

The flat pattern is a calculation, not a guess. When sheet bends, the inside surface compresses and the outside stretches. Somewhere between them is the neutral axis, and its position is described by the K-factor. For air bending of mild steel, K-factor typically sits between 0.33 and 0.45 depending on the ratio of inside radius to thickness. For aluminum it trends lower, for stainless slightly higher. If your CAD library uses a single K-factor for every material, your flat patterns will be off by 0.2–0.8 mm per bend, and that error compounds on a part with six bends.

The inside bend radius should be at least equal to material thickness for most grades, and 1.5× thickness is safer for 6061-T6 and 7075. A radius tighter than thickness forces the outer fibers past their elongation limit and you get micro-cracks on the outside of the bend. Those cracks are not always visible, but they show up later as fatigue failures on a machine that vibrates.

Bend deduction and bend allowance are the two numbers your CAD system needs. Bend allowance is the arc length of the neutral axis through the bend. Bend deduction is the difference between the sum of the two outside flange lengths and the flat length. Confirm both against the shop's tooling before you release the drawing. Press-brake tooling differs between shops, and a flat pattern built for one die set can be a few tenths off on another.

Set up a bend table in your CAD template with the K-factor per material and per thickness range you actually use. It takes an afternoon. It saves rework on every sheet part that follows, and it lets the fabricator quote from your flat without re-deriving it.

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  Minimum inside radius1× thickness for mild steel, 1.5× for 6061-T6.
• 2
  K-factor range0.33–0.45 for air bending, material dependent.
• 3
  Minimum flange4× thickness plus inside radius.
• 4
  Bend reliefAdd a relief slot where a bend meets a wall.

Holes and slots weaken the local section, and a bend next to a hole concentrates stress. Keep the edge of any hole at least 1.5× material thickness from the start of the bend radius, and 2.5× if the hole is on a load path. A 6 mm hole on a 2.0 mm sheet should sit no closer than 3 mm from the bend tangent line. Violate that and the hole goes oval during forming.

Slots behave differently from round holes. A slot running perpendicular to the bend line will close up or open depending on which side of the neutral axis it sits. If you need a slot near a bend for adjustment or cable routing, run it parallel to the bend line and keep the end radius at half the slot width. Square-ended slots are a crack initiation site and a punch breakage risk.

For tapped features on thin sheet, do not tap into 1.0 mm material and expect it to hold a bolt. Use a pressed-in clinch nut, a weld nut, or a PEM-style fastener. Clinch nuts need a hole sized to the manufacturer's spec, usually within ±0.05 mm, and they need clearance behind the sheet for the installation tool. Add that clearance to your model, or the assembly will not go together.

Corner reliefs matter on formed boxes. Where two bends meet at a corner, the material has to go somewhere. Without a relief, it tears or wrinkles. A corner relief of 1× thickness wide and 1× thickness deep is a reasonable starting point, and the relief should extend past the bend tangent line.

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  Hole to bend1.5× thickness minimum, 2.5× on loaded parts.
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  Slot directionParallel to bend line, end radius = half slot width.
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  Threaded featuresClinch or weld nuts, not tapped 1.0 mm sheet.
• 4
  Corner relief1× thickness wide and deep at intersecting bends.

A press brake is not a machining center. Formed sheet holds angular tolerances of about ±1° per bend and linear tolerances of ±0.2–0.5 mm on formed dimensions, depending on part size and thickness. If your drawing calls ±0.05 mm on a formed flange, the shop will either reject the drawing or quote it as a machining operation. Be explicit about which dimensions are critical and which are reference.

Pick datums that the fabricator can actually touch. A bend line is a poor primary datum because its position depends on the flat pattern and the tooling. A machined edge or a punched hole is a good datum. If a hole pattern must align with a mating machined plate, dimension the hole pattern from a single datum hole and let the bend dimensions float.

Hole-to-hole tolerance on a turret punch or laser is typically ±0.1 mm, which is fine for most covers and brackets. Where you need tighter, plan a secondary machining operation after forming. We routinely machine a bore or a sealing face to ±0.005 mm on a part that was first laser cut and formed, and that combination is often cheaper than machining the whole part from plate.

Flatness after welding is the hard one. A welded sheet frame will pull, often 1–3 mm over a 1,000 mm span. If flatness matters, either design in a post-weld machining pass, specify a stress relief, or add adjustable feet and shim points. Do not assume a welded frame comes out flat.

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  Angular±1° per bend is realistic on a press brake.
• 2
  Formed linear±0.2–0.5 mm, size and thickness dependent.
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  Punched holes±0.1 mm hole to hole on laser or turret.
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  Welded flatnessAssume 1–3 mm pull per meter unless relieved.

Finish choice affects fit. Anodizing builds 5–25 μm per surface depending on the type, and hardcoat anodize can add more. If a hole is a press fit or a bearing seat, mask it or size it undersize before anodizing. Powder coating is thicker, often 60–120 μm, and it will close a 0.2 mm clearance gap. Call out masked areas on the drawing, not in an email.

Hardware insertion sequence matters. Clinch nuts, standoffs, and studs should be installed after forming but before finish, unless the finish process damages them. Plating baths and anodize tanks attack some fastener platings. If the hardware must be installed after finish, the holes need masking and the assembly needs a torque spec.

Before release, run a short checklist: every bend has a radius and direction; every hole is clear of bend tangents; the flat pattern matches the shop's K-factor; hardware has clearance for the installation tool; the finish is specified per surface, not per part; and the drawing calls out the material grade, temper, and thickness. A ten-minute review here saves a week of rework.

If the part is a one-off prototype, ask the shop for a DFM review before cutting material. We return a quotation and a free DFM analysis within 12 hours, and production can start within 24 hours. For a part that will run 10,000 units, the same review pays back many times over in tooling and scrap avoided.

• 1
  Anodize build5–25 μm per surface. Mask press fits.
• 2
  Powder coat build60–120 μm. It closes small clearances.
• 3
  Hardware timingInstall after forming, before finish where possible.
• 4
  Release checkRadius, hole clearance, K-factor, finish, material callout.