Sheet Metal Fabrication

Sheet metal fabrication is the process of transforming flat metal sheets into useful parts and assemblies through cutting, bending, forming, and joining. The products of sheet metal work are everywhere - HVAC ductwork, electrical enclosures, roofing, automotive body panels, equipment guards, kitchen equipment, and architectural features. This guide provides the comprehensive knowledge you need to start producing accurate, professional sheet metal work on your first day in a fabrication shop.

Safety in the Sheet Metal Shop

Sheet metal work presents specific hazards that you must respect:

• Sharp edges - The most common injury in sheet metal work is cuts from raw edges and burrs. Always wear cut-resistant gloves (ANSI A4 or higher) when handling sheet metal. Deburr all cut edges before passing parts to the next operation.
• Pinch points - Shears, brakes, and press brakes have crushing/shearing forces that can amputate fingers instantly. Never reach into the point of operation. Keep hands at least 4 inches from the blade or punch. Use hold-down devices, back gauges, and material supports. Never bypass safety guards or light curtains.
• Eye protection - Safety glasses at all times in the shop. Full face shield when grinding, plasma cutting, or performing any operation that produces sparks or flying debris.
• Hearing protection - Sheet metal work is loud. Shearing, hammering, and grinding all produce noise levels above 85 dB. Wear earplugs or earmuffs.
• Lifting - Sheet metal is heavy. A 4x8 foot sheet of 16-gauge steel weighs about 80 pounds. Use team lifts, suction cup lifters, or overhead cranes. Never carry large sheets alone, especially in wind.
• Flying chips and splinters - Metal chips from drilling, notching, and nibbling are razor-sharp and hot. Keep your work area clean and sweep up chips regularly.

Materials and Gauges

Sheet metal thickness is measured in gauge numbers. The gauge system is counterintuitive: higher gauge numbers mean thinner metal.

Common Materials

• Mild steel (cold-rolled, CRS) - General-purpose fabrication. Good formability, easy to weld, readily available. Rusts if uncoated. Common gauges: 16 (0.060") to 24 (0.024").
• Galvanized steel - Mild steel with a zinc coating for corrosion resistance. Standard for HVAC ductwork, roofing, and outdoor applications. Common gauges: 22 (0.030") to 26 (0.018"). Note: welding galvanized steel produces toxic zinc fumes. Use ventilation and respiratory protection.
• Stainless steel (304 and 316) - Corrosion-resistant. Used in food service equipment, medical equipment, chemical processing, and architectural work. Harder to form and cut than mild steel. Work-hardens when bent or machined. Requires stainless-specific tooling to prevent contamination.
• Aluminum - Lightweight, corrosion-resistant, and easy to form. Used in aerospace, transportation, decorative work, and electronics enclosures. Softer than steel, so it scratches easily. Use protective film or paper interleaving during handling. Common alloys: 3003 (general forming), 5052 (structural, better corrosion resistance), 6061 (structural, heat-treatable).
• Copper and brass - Used in decorative work, electrical enclosures, and specialized equipment. Soft, easy to form, expensive.

Gauge Reference (Steel)

Gauge	Thickness (inches)	Thickness (mm)	Weight (lbs/sq ft)
10	0.135	3.43	5.625
12	0.105	2.66	4.375
14	0.075	1.90	3.125
16	0.060	1.52	2.500
18	0.048	1.21	2.000
20	0.036	0.91	1.500
22	0.030	0.76	1.250
24	0.024	0.61	1.000
26	0.018	0.46	0.750

Cutting Methods

Hand Tools

• Aviation snips - The most basic sheet metal cutting tool. Available in three types, color-coded for easy identification:
  • Yellow handle - Straight cuts
  • Red handle - Left-curving cuts (blade cuts to the left)
  • Green handle - Right-curving cuts (blade cuts to the right)
  • Use the correct snips for the direction of your cut. Using the wrong snips distorts the workpiece. The waste piece curls away, not the good piece.
• Throatless bench shear - A hand-operated shear mounted on a bench. Can make straight and curved cuts on light-gauge material (up to about 16 gauge). No distortion on either side of the cut.
• Hand seamer - Bends narrow flanges and tabs on light-gauge sheet metal. Not a cutting tool but essential for forming work at the bench.

Power Cutting Tools

• Power shear - Electric or pneumatic handheld tool that cuts straight and gently curved lines quickly. Leaves a clean edge with minimal distortion. Works on material up to about 14 gauge.
• Nibbler - Punches out small crescent-shaped chips to cut any shape, including tight curves and interior cutouts. Does not distort the workpiece. Leaves a slightly rougher edge than a shear.
• Plasma cutter - Uses an ionized gas arc to cut metal. Fast, clean cuts on material from thin sheet to 1-inch plate. Requires compressed air (60-80 PSI) or specialized gas. Set amperage based on material thickness. A 45-amp plasma cutter handles up to 1/2" mild steel. Keep the tip at the correct standoff distance (typically 1/16" to 1/8" from the surface). Move at a speed that allows the arc to cut completely through. If sparks are blowing back toward you, you are moving too fast.
• Turret punch press - A CNC machine that punches holes and shapes from sheet metal using interchangeable tooling. Used in production shops for repetitive work.
• Laser cutter - CNC-controlled CO2 or fiber laser for precision cutting. Produces extremely clean edges with tight tolerances. The standard in modern fabrication shops.
• Waterjet cutter - Uses a high-pressure water stream with abrasive garnet to cut any material without heat distortion. Used for materials sensitive to heat (heat-treated metals, composites).

Machine Shearing (Squaring Shear)

The squaring shear (power shear or hydraulic shear) is the primary tool for making straight cuts on full sheets.

• Capacity - Rated by the maximum thickness and width it can cut. A typical shop shear cuts up to 10-gauge mild steel at 4 to 10 feet wide.
• Back gauge - An adjustable stop behind the blade that sets the width of the cut piece. Set the back gauge to the required dimension using the scale on the machine or by measuring with a tape.
• Hold-downs - Hydraulic or mechanical clamps that hold the sheet against the table during the cut. Make sure the material is seated firmly against the back gauge and hold-downs before cutting.
• Blade clearance - The gap between the upper and lower blades. Set clearance to approximately 5-8% of material thickness. Too little clearance causes rough edges and excessive blade wear. Too much causes burrs and bending.
• Operating procedure - Feed the sheet against the back gauge, ensure hands are clear of the blade (minimum 4 inches), activate the hold-downs, and cycle the blade. Never reach under the blade for any reason.

Layout

Layout is the process of marking measurements, bend lines, and cutting lines on flat sheet metal before fabrication begins. Accurate layout is the foundation of accurate parts.

Layout Tools

• Scribe - A hardened steel point used to scratch precise lines on metal surfaces. More accurate than a marker but harder to see. Use for close-tolerance work.
• Fine-point marker or paint pen - Easier to see than scribe lines. Use for general shop work.
• Steel rule - A precision ruler marked in 1/64" or 0.5mm increments. The essential measuring tool.
• Combination square - A steel rule with a sliding head for marking 90-degree and 45-degree angles. Also functions as a depth gauge and marking gauge.
• Dividers - Used to scribe arcs, circles, and transfer dimensions.
• Protractor or angle gauge - For marking and measuring angles other than 90 and 45 degrees.
• Trammel points - Adjustable points mounted on a beam for scribing large circles and arcs.
• Chalk line - For marking long straight lines on full sheets.

Layout Best Practices

• Always measure from a factory edge or a known reference point. Never measure from a sheared edge unless you have verified it is square.
• Mark bend lines clearly and indicate the direction of the bend (up or down) and the bend sequence (which bend gets made first).
• Account for bend allowance in all dimensions. A common beginner mistake is to lay out the flat pattern at the finished dimensions without accounting for the material consumed in the bend.
• Mark all holes, notches, and cutouts before bending. It is almost always easier to punch or drill holes while the part is flat.
• Double-check all dimensions before cutting. "Measure twice, cut once" is not just a saying - it saves material, time, and frustration.
• Use a center punch to mark the center of all drill holes. The punch mark prevents the drill bit from wandering.

Bending Fundamentals

Bending is the most common forming operation in sheet metal work. Understanding what happens to the material during a bend is essential for accurate work.

Bend Terminology

• Bend radius - The radius of the inside surface of the bend. Specified on the drawing or determined by the tooling. Tighter radii stress the material more and can cause cracking, especially on harder materials.
• Bend angle - The angle of the bend measured from the flat, unbent position. A 90-degree bend means the two flanges are perpendicular.
• Bend line - The line on the flat pattern where the bend will be made.
• Flange - The flat portion of the part that extends beyond the bend.
• Inside dimension (ID) - The distance measured on the inside of the bend from the bend apex to the end of the flange.
• Outside dimension (OD) - The distance measured on the outside of the bend.
• Minimum flange length - The shortest flange that can be formed without the tooling losing control of the metal. Rule of thumb: minimum flange = 2 times the material thickness plus the bend radius.

What Happens During a Bend

When sheet metal is bent, the outer surface stretches and the inner surface compresses. Between them is the neutral axis, a plane that neither stretches nor compresses. The neutral axis is not at the center of the material thickness. For air bending, the neutral axis is located at approximately 0.33 to 0.44 of the material thickness from the inside surface. This location is described by the K-factor.

Bend Allowance

Bend allowance (BA) is the arc length of the neutral axis through the bend. It represents the amount of flat material that is consumed in forming the bend.

Formula: BA = (pi / 180) x bend angle x (bend radius + K-factor x thickness)

Where:

• Bend angle is in degrees
• Bend radius is the inside bend radius
• K-factor is typically 0.33 for air bending, 0.44 for bottoming/coining
• Thickness is the material thickness

Example: A 90-degree bend in 16-gauge steel (0.060") with a 1/16" (0.0625") inside radius and K-factor of 0.33:

BA = (3.14159 / 180) x 90 x (0.0625 + 0.33 x 0.060)
BA = 0.01745 x 90 x (0.0625 + 0.0198)
BA = 1.5708 x 0.0823
BA = 0.129"

Bend Deduction

Bend deduction (BD) is the amount subtracted from the total outside dimensions to get the flat pattern length.

Formula: BD = 2 x setback - bend allowance

Setback (SB) = tan(bend angle / 2) x (bend radius + thickness)

For a 90-degree bend: SB = bend radius + thickness (since tan(45) = 1)

Practical Flat Pattern Development

For most shop work, rather than calculating every bend:

1. Measure a test bend. Bend a piece of scrap of the same material and thickness. Measure the leg dimensions and the flat length consumed by the bend. This gives you a reliable bend deduction for that material, thickness, and tooling combination.
2. Use a bend chart. Most shops have charts posted on the brake that list bend deductions for common material thicknesses and die openings.
3. Use CAD/CAM software. Modern design software (SolidWorks, AutoCAD, Inventor) calculates flat patterns automatically when you specify the material and K-factor.

Brake Press Operation

A press brake bends sheet metal by pressing it between an upper tool (punch) and a lower tool (die).

Press Brake Components

• Ram - The moving upper beam that carries the punch. Hydraulic or electric drive.
• Bed - The stationary lower beam that holds the die.
• Back gauge - A motorized stop behind the die that positions the workpiece for the bend location. Set the back gauge to the dimension from the bend line to the edge of the flange.
• Punch - The upper tool that pushes the metal into the die. Available in various angles and nose radii.
• Die - The lower tool with a V-shaped opening. The die opening width determines the inside bend radius and the tonnage required.

Types of Bending

• Air bending - The punch pushes the sheet partially into the die without bottoming out. The bend angle is determined by the depth of the punch stroke. This is the most common method because one set of tooling can produce a range of angles. The inside bend radius is approximately 15 to 20% of the die opening width.
• Bottom bending (bottoming) - The punch pushes the sheet fully to the bottom of the die. Produces a more precise angle than air bending but requires more tonnage and dedicated tooling for each angle.
• Coining - The punch applies enough force to plastically deform the material at the bend, producing a very precise angle with minimal springback. Requires 3 to 10 times more tonnage than air bending. Rarely used except for precision work.

Die Selection

Die opening width is the most important tooling selection decision.

Rule of thumb: Die opening = 8 times material thickness for material up to 10 gauge. For heavier material, use 10 to 12 times thickness.

• Narrower dies produce tighter radii and sharper bends but require more tonnage and can mark the material.
• Wider dies produce larger radii and gentler bends with less tonnage and less marking.

Press Brake Operating Procedure

1. Review the drawing - Identify all bends, dimensions, tolerances, bend sequence, and material specification.
2. Select tooling - Choose the punch and die based on bend angle, inside radius, material type, and material thickness.
3. Install tooling - Clamp the punch and die securely. Verify alignment.
4. Set the back gauge - Program or manually set the back gauge to position the workpiece for the first bend.
5. Set the stroke depth - Determine the punch depth for the required angle. On CNC brakes, program the angle. On manual brakes, use a test bend and adjust.
6. Run a test part - Always bend a test piece from the same material before running production. Measure the angle with a protractor or angle gauge. Measure the flange dimensions. Adjust back gauge and stroke as needed.
7. Bend sequence - Follow the specified bend sequence. General rule: make inside bends first, then outside bends. This prevents flanges from interfering with the back gauge or tooling.
8. Safety - Keep hands out of the pinch point at all times. Use the foot pedal with two-hand controls or light curtain safety systems as required by your shop. Never reach between the punch and die. Use material supports for large or heavy workpieces.

Springback

When the punch retracts after a bend, the material springs back slightly toward its original flat position. The amount of springback depends on:

• Material type - Stainless steel and high-strength steel spring back more than mild steel. Aluminum has moderate springback.
• Material thickness - Thinner material springs back more.
• Bend radius - Larger radii produce more springback.

To compensate for springback, overbend slightly. If you need a 90-degree bend and the material springs back 2 degrees, bend to 88 degrees. CNC brakes can be programmed to compensate automatically.

Joining Methods

Riveting

• Blind (pop) rivets are the most common in sheet metal work. Drill the holes, insert the rivet, and pull with a rivet gun until the mandrel snaps.
• Size the rivet: diameter should be about 3 times the material thickness. The rivet should protrude through both pieces by at least 1.5 times the rivet diameter.

Spot Welding (Resistance Welding)

• Two electrodes squeeze the overlapping sheets together and pass a high current through the contact point, melting a small spot (nugget) that fuses the sheets.
• Fast, clean, and no filler required. Common in automotive and appliance manufacturing.

Seaming

• Sheet metal seams (Pittsburgh lock, snap lock, standing seam) mechanically interlock the edges of two sheets without fasteners or welding.
• Standard for HVAC ductwork and roofing.

Soldering and Brazing

• Used to join sheet metal where a water-tight or gas-tight seal is needed (gutters, copper roofing, decorative metalwork).

Tips from Experienced Fabricators

• "Check your blank dimensions before bending." Once the part is bent, you cannot uncut it. Verify that your flat pattern is correct by measuring the blank before the first bend.
• "Make your first bend the one closest to a reference edge." This gives you the most accurate positioning against the back gauge.
• "Use a hemming die for safe edges." A hem folds the raw edge of the sheet back on itself, eliminating the sharp edge and adding stiffness to the flange.
• "Grain direction matters." Sheet metal has a grain direction from the rolling process at the mill. Bending across the grain produces a smoother bend with less risk of cracking. Bending parallel to the grain on tight radii can cause cracking, especially on harder alloys and stainless steel.
• "Keep your shop clean." Metal chips, scrap, and debris on the work table scratch parts, jam tooling, and cause measurement errors. Clean your workspace after every job."

Key Takeaways

• Always wear cut-resistant gloves and safety glasses when handling sheet metal.
• Match the cutting method to the material, thickness, and required accuracy.
• Account for bend allowance in every flat pattern. Use test bends to verify your calculations.
• Select the correct die opening for the material thickness and desired bend radius.
• Follow the correct bend sequence to avoid tooling interference.
• Keep hands out of pinch points on shears and press brakes at all times.