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Sheet metal fabrication covers a family of processes, not a single one. A part that looks simple in CAD can require multiple sequential operations: cut to profile, punched for holes, bent to geometry, tapped for fasteners, and finished. Choosing the right process at each step, and designing the part to work with those processes, is where the engineering judgment matters.
The Core Sheet Metal Fabrication Processes
Laser Cutting
Laser cutting uses a focused beam to cut profiles, holes, slots, and internal features from flat sheet. It is the dominant process for prototype and low-volume sheet metal work because it requires no tooling: the cutting path lives in a DXF or CAM file, and design changes cost nothing beyond updated software. Laser cutting achieves tolerances of +/-0.1 mm on flat features and produces clean edges on steel, stainless steel, aluminum, brass, and copper up to approximately 12 mm thick. RPM Fast’s laser cutting process overview covers material-specific limits and edge quality details.
Press Brake Bending
Press brake bending forms flat blanks into three-dimensional geometry by pressing the sheet against a die with a punch. It is how enclosures, channels, brackets, and flanges get their shape after laser cutting. Bend accuracy depends on material springback, thickness variation, and the tooling radius. A typical working tolerance is +/-0.5 degrees on bend angle and +/-0.5 mm on formed dimensions. Tighter tolerances are achievable with secondary fixturing or secondary bending operations but add cost and time.
Waterjet Cutting
Waterjet uses a high-pressure abrasive stream to cut any material: metals, composites, glass, and thick stock beyond laser capacity. The key advantage is no heat-affected zone, which matters for heat-sensitive alloys. For most steel and aluminum under 12 mm, laser cutting is faster and more cost-effective.
Punching and Stamping
Punching uses a hardened die to shear holes and features through sheet metal at high speed. It is efficient for high-repeat hole patterns in production volumes but requires tooling investment beyond standard sizes. For prototype quantities, laser cutting handles the same features without tooling cost.
Welding and Assembly
Multi-piece assemblies incorporate welding as a final step. TIG welding produces clean joints with minimal distortion on stainless steel and aluminum. MIG welding is faster for mild steel at lower cosmetic requirements. Correct flange widths and clearances for welding access must be designed into the CAD before fabrication.
Process Comparison
The table below summarizes the five core sheet metal processes across the factors most relevant to engineering decisions.
| Process | Kerf / Precision | Material Range | Edge Quality | Best Thickness | Tooling Cost |
|---|---|---|---|---|---|
| Laser Cutting | +/-0.1 mm | Steel, SS, Al, brass | Excellent | 0.5-12 mm | None |
| Waterjet | +/-0.15 mm | Any material | Smooth, no HAZ | 1-200 mm | None |
| Plasma Cutting | +/-0.5-1 mm | Conductive metals | Moderate; HAZ | 6-50 mm | None |
| Press Brake | +/-0.5 deg | All sheet metals | N/A (bending) | 0.5-10 mm | Low-moderate |
| Punch / Stamp | +/-0.1 mm | Steel, Al, SS | Good; slight burr | 0.5-6 mm | Moderate-high |
Laser cutting and press braking cover the majority of prototype and low-volume sheet metal work. Waterjet becomes relevant for materials above 12 mm, heat-sensitive alloys, or non-metallic materials. Punching and stamping earn their place at production volumes where tooling cost is recovered across thousands of identical parts.
DFM Rules for Sheet Metal
Sheet metal has specific geometry constraints that are enforced by the physics of bending and cutting, not by convention. Parts that violate these rules either cannot be made as drawn or require expensive workarounds.
1. Minimum Bend Radius
The inside bend radius must be at least equal to the material thickness for most mild steel and aluminum alloys. For stainless steel and harder aluminum grades like 7075, the minimum inside radius is 1.5 to 2 times the thickness. Going below minimum bend radius causes cracking on the outside of the bend. This is the single most common DFM issue on sheet metal drawings and is entirely preventable with correct callouts.
2. Hole and Slot Clearance from Bends
Holes and slots placed too close to a bend line will distort when the bend is formed. The minimum distance from the edge of a hole to the bend line should be at least 1.5 times the material thickness plus the bend radius. Slots oriented perpendicular to a bend line are particularly vulnerable. If a hole must be close to a bend, it should be placed after bending by drilling or punching the formed part, though this adds an operation.
3. Consistent Wall Thickness
Sheet metal fabrication works from a single sheet gauge. Unlike casting or machining, you cannot vary wall thickness within the same part without welding or bonding separate pieces. If a design requires localized stiffening, ribs can be formed into the sheet by stamping or the part can be designed with folded flanges. Adding a different gauge section requires separate fabrication and joining.
4. Bend Relief Notches
When a bend line runs near a corner, a bend relief notch prevents tearing at the bend termination. The notch width should be at least the material thickness and the depth should reach the bend tangent line. Omitting bend relief causes tearing or distortion at corners during forming.
5. Flange Length Minimums
A flange that is too short cannot be clamped securely in the press brake, leading to inaccurate bend angles. The minimum flange length is typically three to four times the material thickness, with 6 mm as a practical floor for most setups.
Sequencing Operations for Prototype Parts
For prototype and low-volume sheet metal parts, the typical operation sequence is: laser cut the flat blank, deburr, bend, drill or tap any post-bend holes, clean, and finish. Secondary operations like thread inserts (PEM fasteners), welding, or powder coating are added after the primary forming steps. Getting this sequence right in the design phase prevents rework later. RPM Fast’s sheet metal fabrication service supports the full sequence from flat blank through finishing for prototype and production quantities.
A common sequencing error is specifying tight tolerances on features that land on opposite sides of a bend. The flat blank tolerance is tight; the formed tolerance after bending is wider. If assembly depends on post-bend feature positions, drawing tolerances must reflect what the forming process can hold.
Frequently Asked Questions
What is the most common sheet metal fabrication process?
Laser cutting and press brake bending are the most common sheet metal fabrication processes for prototype and low-volume work. Laser cutting handles profile cutting and hole features with high precision and no tooling cost. Press brake bending forms flanges, channels, and enclosure geometries from flat-cut blanks. Most sheet metal parts require both processes in sequence.
What is the minimum bend radius for sheet metal?
The minimum inside bend radius for most sheet metals is approximately equal to the material thickness. For mild steel, a 1:1 ratio of inside radius to thickness is standard. Harder alloys like stainless steel and 7075 aluminum require a larger minimum radius, typically 1.5 to 2 times the thickness, to avoid cracking on the outer bend surface. Going below minimum bend radius causes material cracking and is a common DFM issue on sheet metal drawings.
What tolerances can sheet metal fabrication achieve?
Laser-cut profiles typically hold +/-0.1 mm on flat features. Bent dimensions are less precise due to springback variation and material batch differences; +/-0.5 mm on formed dimensions is achievable with good fixturing, and +/-1.0 mm is more realistic without it. Hole-to-hole position on a flat blank can be held to +/-0.1 mm. Formed hole-to-hole or feature-to-feature dimensions across a bend are typically +/-0.5 to +/-1.0 mm.
How long does sheet metal fabrication take for prototype parts?
For simple laser-cut and bent parts with standard finishes, lead times of 3 to 7 business days are typical for prototype quantities. Parts requiring welding, thread inserts, or specialty surface treatments add 1 to 3 days. Complex assemblies or tight tolerance requirements extend timelines further. RPM Fast delivers prototype and low-volume sheet metal parts with short lead times as part of its multi-process manufacturing capability.
Designing Parts the Process Can Build
Sheet metal fabrication rewards designs that work with the process constraints. Parts that respect minimum bend radii, adequate hole clearances, bend relief notches, and tolerances appropriate to formed dimensions come off the machine as drawn.
RPM Fast is ISO 9001:2015 certified and reviews sheet metal designs for DFM issues as part of the quoting process. To start a project, request a quote from RPM Fast with your CAD files and we will return DFM feedback and pricing within 1 to 2 business days.
