SOLIDWORKS Design Approach · Part 13
Sheet metal parts and welded structures can be modelled with ordinary features. They should not be. Both carry manufacturing parameters that a generic solid cannot express, and both need to be unfolded or cut-listed before anything can be made.
Positioning
Nothing in this class of parts is beyond the reach of features, curves and surfaces. The point is efficiency and correctness. A sheet metal part modelled as a set of extrusions will look right and will not unfold; a welded frame modelled as an assembly of individual bars will look right and will not produce a cut list. Both are created far more effectively through dedicated modelling techniques, and both belong to the fabricated rather than the machined world — which is the only real thing they have in common.
Material context
Sheet metal is thin flat stock that can be cut and bent into a wide range of shapes. Thickness varies considerably — broadly from around 1 mm up to about 6 mm, which is a little under a quarter of an inch. Anything substantially thinner is regarded as foil or leaf; anything thicker is plate. It is one of the fundamental forms in metalworking, and the list of everyday products made from it is long: aircraft wings, car bodies, medical tables, building roofs, brackets, chassis, enclosures, panels, channels, hinges, razor blades and signage.
Punching, rolling, embossing, stamping, braking, notching, shearing, spot welding, insertion and finishing. Numerical control programming is routinely used to drive the cutting and punching stages.
Aluminium, steel, brass, copper and stainless dominate industrial work; silver, gold, nickel, platinum and titanium appear in specialist applications.
Audio-visual, electronics, fibre optics, medical, security, semiconductor and telecommunications equipment are all built substantially from folded sheet.
Sheet arrives as flat sheets or as coiled strip, the coils formed by passing flat sheet through a roll slitter. Thickness is expressed as gauge, running roughly from 30 gauge down to 8 gauge — and the higher the gauge number, the thinner the sheet. Gauge is also material-dependent, so a gauge number without a material is an incomplete specification.
The controlling parameters
Two aspects separate it from ordinary part modelling.
Gauge, bend radius, bend angle, relief type and the K-factor are not cosmetic. They determine whether the modelled part can actually be produced on the available press brake, and they must be carried in the model rather than noted on a drawing.
Before anything is cut, the developed length of the blank has to be known. That calculation depends on how the material behaves through the bend, which is what the K-factor describes.
When sheet is bent, material on the outside of the bend stretches and material on the inside compresses. Somewhere between them lies a neutral plane whose length is unchanged. The K-factor is the position of that neutral plane expressed as a fraction of the material thickness, measured from the inside surface. It is a property of the material, the thickness, the bend radius and the forming method, and it is the single number that converts a folded geometry into a correct flat blank. Get it wrong and every folded part comes out the wrong size in one direction — consistently, which at least makes the error easy to diagnose.
K-factor tables and default bend allowances are starting points, not answers. Different presses, tooling sets and material batches produce measurably different results. Production shops establish their own values by folding test coupons and measuring them, and any model handed to a fabricator should be checked against their table rather than the software default.
Toolset
The primary sheet metal feature is the flange, which behaves much like an extrusion but carries the bend parameters with it. Three types exist:
Around those sit the features that make folded parts practical: sketched bends, jogs, hems, closed and welded corners, corner reliefs, tabs, lofted bends, cross breaks, vents, forming tool impressions and the unfold/fold pair that allows a flat-state operation to be performed part-way through a folded model.
Sheet metal parts behave differently in the feature tree. Creating a base flange generates not one node but three, and understanding what each holds removes most confusion:
Four routes
| Method | How it works | Choose it when |
|---|---|---|
| Native sheet metal features | Start with a base flange and add sheet metal features until the part is complete. | Designing a folded part from scratch. The most controlled route and the default choice. |
| Convert a solid body | Model a solid as usual, then nominate a fixed face and the bend edges. The system infers the rip edges and produces the folded part. | The form was conceived as a solid, or arrived from another system. |
| Convert a shelled solid | Shell the solid to wall thickness, then insert bends at the corners. | Enclosures and boxes whose overall envelope is the governing dimension. |
| Model in the flat state | Create the developed blank first, then add bends to fold it up. | The blank is fixed by nesting or by an existing cutting programme and the folded form must follow from it. |
If the constraint is the finished envelope, model folded. If the constraint is material utilisation and the blank must nest efficiently on standard sheet, model flat. Choosing the wrong starting state produces a part that satisfies one constraint and fails the other by an amount nobody discovers until the quotation.
Second class
Welding joins metal or thermoplastic components by melting the joint and adding a filler material that mixes with the parent material; on cooling, a strong joint results. Welded structures are everywhere: building frames, vehicle exhaust systems, campfire grills, trailer dollies, house gutters, heat exchangers and condensers.
Processes include gas welding, arc welding, spot welding, resistance welding and solid state welding, among others. They share one requirement — an energy source capable of melting the joint and the filler — and differ in how that energy is generated. Gas welding burns a fuel gas to produce a flame; arc processes use an electric arc; resistance processes use ohmic heating at the interface. Supporting equipment runs from machines, cylinders, spot welders, cutters and torches through to grinders, filler metals, compressors, cutting guides, clamps, extraction hoods and personal protective equipment.
A welded frame is created as a single multi-body part rather than as an assembly of individual bars. That is deliberate: the members share a common skeleton, the joints must be trimmed to one another, and the deliverable is a cut list rather than a bill of materials. Modelling it as an assembly puts every one of those things in the wrong place.
A 2D or 3D sketch establishing the skeleton — the centrelines that the members will follow.
Apply a cross section to each run of the skeleton. Conceptually, adding meat to the bones.
Add the joints by type, trimming members to one another, and stiffen with gussets where required.
Apply end caps where open sections would otherwise be exposed, welded onto the open ends of the structure.
Weldment features fall into three groups. Base features generate the structural members from the skeleton. Weld-specific features produce weld beads, gussets and end caps. Generic features — trim and extend, split, chamfer — adapt ordinary operations to multi-body work. The whole set exists so that a cut list can be generated automatically, listing each member's profile, length, angle cuts and quantity.
Documentation
Weld symbols are how joint type and parameters are specified on an engineering drawing, and both ISO and ANSI symbol sets are in use. A symbol is constructed from an arrow pointing at the joint, a horizontal reference line, a symbol placed above or below that line to indicate which side the weld is on, and supplementary information — size, length, pitch, contour, finish and field-weld or all-round indicators.
The symbol families cover butt welds in their several preparations, fillet welds, plug and slot welds, spot and seam welds, back and backing welds, surfacing, and edge and corner flange welds.
Supplementary symbols state whether the completed weld face is to be flat, convex or concave, and by what method that contour is achieved.
A symbol below the reference line applies to the arrow side of the joint; above the line applies to the other side. Symbols may also be shown inverted, with the identification line above, to suit the drawing office convention.
The two systems overlap heavily but not completely. State which standard the drawing follows, in the title block, on every sheet.
A weld symbol that omits size is an instruction to the fabricator to guess, and fabricators guess conservatively — usually by welding more than required, which adds cost, distortion and heat-affected zone. Size every structural weld. Where a continuous weld is not required, say so with length and pitch rather than leaving it implied.
Series
The SOLIDWORKS Design Approach series works through computer aided design as an engineering discipline, from first principles to manufacture.
AI answers can contain mistakes — double-check anything important.