Sheet metal fabrication workflow
Plan a sheet-metal part from flat pattern and cutting through bending, hardware, welding, finishing and inspection.
Choose the sheet-metal route around the finished assembly
A flat bracket, folded enclosure and welded frame may all start as sheet, but they have different cut, bend, hardware, weld, finish and inspection requirements.
Define the bend geometry
T is sheet thickness and R is the inside radius. The outside corner has radius R + T; mating parts must clear that real shape.
Keep features out of the forming zone
Dimension the hole edge from the bend tangent, then confirm the required clearance for the actual die opening and material. A center distance alone can hide an overlap.
| Part need | Likely route to review | Controlling question |
|---|---|---|
| Flat plate or bracket | Laser or punch cutting, then edge break and finish | Do holes, slots, edge condition and flatness meet the assembly? |
| Folded enclosure | Cut, press-brake bends, hardware, finish and assembly | Can the flat pattern, bend sequence and tool access produce the intended box? |
| Welded frame or box | Cut and form pieces, fixture, tack/seam/stitch weld, inspect and finish | What datums and distortion controls keep the assembly square? |
| Mixed metal assembly | Sheet components plus purchased hardware or machined interfaces | Which surfaces locate, which surfaces carry load and which parts need separate inspection? |
Design the flat pattern before you decorate the solid model
The final folded shape is made from a developed blank. Material thickness, bend radius, bend allowance, grain direction, reliefs and feature positions all affect the formed result.
Define the bend geometry
T is sheet thickness and R is the inside radius. The outside corner has radius R + T; mating parts must clear that real shape.
Keep features out of the forming zone
Dimension the hole edge from the bend tangent, then confirm the required clearance for the actual die opening and material. A center distance alone can hide an overlap.
- Use one material and thickness intentionally. Changing thickness changes bend allowance, stiffness, hardware compatibility, weight and the flat pattern.
- Keep a bend strategy visible. Mark bend directions, preferred grain direction and surfaces that must remain cosmetic or accessible.
- Place features away from bends. Holes and slots near a bend can distort or move. Use the chosen supplier’s feature-distance guidance and qualify critical positions.
- Add relief where the material needs it. Bend reliefs and corner treatment can prevent tearing, bulging or interference at intersecting flanges.
Treat bend radius and flange length as manufacturing features
A press brake forms a bend with tooling, so the inner radius, flange length, bend angle and tool access must work together. Supplier tables are starting points, not universal material laws.
| Feature | Specify or check | Failure mode |
|---|---|---|
| Inside radius | Material, thickness, tooling and the required final geometry | Cracking, distortion, an unavailable tool or a radius that changes the assembly. |
| Flange length | Minimum length for the selected die, thickness and bend | The flange cannot be formed or loses support in the tooling. |
| Bend angle | Functional angle, allowable variation and inspection datum | Springback or accumulated angle error changes the enclosure or bracket fit. |
| Feature near bend | Hole/slot distance and whether the feature crosses the bend zone | Elongated holes, distorted edges or poor hardware seating. |
| Bend sequence | Order of operations and tool clearance | A later bend collides with the tool or traps the part. |
Add hardware where assembly needs a repeatable thread or standoff
PEM hardware, rivets, weld nuts, captive fasteners and through bolts each change the flat pattern, tool access, material compatibility and inspection plan.
Leave material around the hole
Choose the insert or fastener first. Its geometry and installation method set the pilot hole, boss wall and edge distance.
Match the installation method
Heat-set, ultrasonic, press-fit and bonded inserts are different processes. Do not apply a thermoplastic heat-set rule to cured thermoset resin.
Schematic illustrations; not to scale. Confirm the design for the chosen material, process and final use.
| Choice | Useful when | Review |
|---|---|---|
| Pressed-in hardware | A repeatable thread or standoff belongs in a sheet component | Hole size, material/thickness compatibility, edge distance, installation direction and pull-out. |
| Weld nut or stud | The assembly needs fixed hardware and heat can be controlled | Weld distortion, access, coating sequence and evidence of the weld process. |
| Rivet or clinch joint | A permanent or serviceable mechanical joint suits the assembly | Tool clearance, joint stack-up, hole quality and final flushness. |
| Through bolt | Standard hardware, service access or high clamp control matters | Nut access, bearing area, washer clearance and the assembly sequence. |
Weld the assembly around datums and distortion control
Welding can join shapes that cannot be formed in one press-brake operation, but heat, fixture design, sequence and access affect squareness, flatness, appearance and downstream finishing.
Leg and throat are different dimensions
For an ideal flat-faced, equal-leg 90° fillet, throat a = z / √2, about 0.707 × leg z. This geometry does not establish the required weld strength.
State the acceptance question
Dimensions, surface-breaking defects and leakage are different checks. Select inspection methods and acceptance criteria for the actual joint and service.
Choose seam, tack, stitch or another joint treatment from strength, sealing, appearance and distortion requirements. Identify which surfaces must remain datum or cosmetic after welding, and provide access for the torch, fixture and inspection.
Define the assembly fixture and tack sequence before promising a final dimension. A weldment can be nominally correct in the CAD model and still move during heat input. If a surface must be machined after welding, include that operation and its datum chain in the release package.
Hot-work safety requirements apply to the fabrication environment. The product drawing still needs its own joint, acceptance and inspection requirements.
Sequence finish, masking and assembly around the final use
Powder coat, paint, plating, anodize, passivation, brushing and edge treatment have different substrates, thicknesses, appearance and masking needs.
Measure from the functional interface
Locate critical holes from the mounting face and a shared origin. Long chains of intermediate dimensions accumulate variation.
Inspect the delivered condition
A welded, coated or hardware-installed assembly can differ from its cut components. Mark which dimensions apply after the final operation.
| Question | Why it matters | Call out |
|---|---|---|
| Does hardware go in before or after finish? | The coating can cover threads, change contact and alter grounding. | Masking, installation sequence, touch-up and acceptance. |
| Which faces are cosmetic? | Grain, weld marks, edge break and coating variation are visible only on the defined faces. | Cosmetic zones, sample approval and allowed texture/colour variation. |
| Which surfaces locate or seal? | Finish build and weld distortion can change fit. | Final dimension after finish, masked faces and inspection condition. |
| Does the environment require corrosion evidence? | Substrate, pretreatment and coating system control durability. | Material, finish system, thickness or test requirement and record. |
Inspect the features that make the assembly work
Sheet metal tolerances accumulate across bends and assembly steps. Choose datums and checks that reflect the assembled function, not only the flat blank.
| Stage | Check | Reason |
|---|---|---|
| Flat blank | Cut profile, holes, slots, edge condition and material identity | Errors here propagate into bends and hardware. |
| Formed part | Bend angle, flange length, hole-to-bend and overall envelope | Confirms the part can assemble before finish adds another variable. |
| Hardware | Part number, seating, thread, alignment and retention | Hardware is a functional feature, not a decorative add-on. |
| Welded assembly | Datums, squareness, flatness, weld acceptance and distortion | The joined assembly can move even when individual blanks were correct. |
| Finished part | Cosmetic faces, coating, masked features, fit and documentation | Acceptance belongs to the delivered state. |
For detailed general rules, use the Sheet metal design guide; this workflow focuses on the handoffs between operations.
Frequently asked questions
What are the main steps in sheet metal fabrication?
A typical route is material selection, flat-pattern development, cutting, deburring or edge treatment, bending/forming, hardware insertion, welding or riveting when required, finishing, assembly and inspection. The exact sequence depends on the part and its acceptance requirements.
Should holes be made before or after bending?
Often before bending when the flat pattern and feature distance support it, but the correct answer depends on access, distortion, tooling and tolerance. A feature near a bend may need a different sequence or post-form operation.
When should sheet metal parts be welded?
Welding is useful when the geometry, strength, sealing or assembly cannot be achieved efficiently with one formed part and hardware. Include fixture, weld sequence, distortion control, finish and inspection in the decision.
Does powder coating change sheet-metal dimensions?
It adds a coating layer and can affect fit, threads, grounding and masked interfaces. State the final condition, masking and inspection requirements rather than treating finish as a cosmetic afterthought.
What should a sheet-metal RFQ include?
Send the formed and flat geometry when available, material and thickness, bend or weld requirements, hardware, finish, cosmetic faces, quantity, critical datums, inspection documents and packaging or assembly needs.
Sources and further reading
- Protolabs: sheet metal fabrication design guidelinesOfficial guidance on bend radii, feature distances, hardware, welding and finishes; supplier-specific limits must not be generalized.
- Protolabs: sheet metal fabrication toolkitCovers bend radii, flange length, hardware and tolerances as a design review framework.
- Protolabs: sheet metal assembliesAssembly options including welding, riveting, hardware insertion and formed features.
- OSHA: welding, cutting and brazing overviewSafety and regulatory reference for hot-work operations; it does not define a product acceptance specification.
Quote the formed part, hardware and finished assembly as one route.
Upload your CAD and drawing with the material, quantity, finishing and critical dimensions. Confirm the manufacturing route before production.