Sheet Metal Fabrication

Sheet metal work turns flat stock into ductwork, enclosures, guards, panels, kitchen equipment and roofing. The tools range from a pair of snips to a CNC laser, but the job is always the same: cut the right shape, bend it in the right order and join it. Sheet is thin, so small errors show, and raw edges cut.

What You Will Be Able to Do

• Read gauge numbers and estimate the weight of a sheet.
• Choose a material for a job.
• Pick a cutting tool and use it safely.
• Lay out a part and plan bends that will not crack.
• Choose a way to join the finished parts.

Handling Sheet Safely

Fresh cut edges are one of the biggest hazards in the trade. They can cause deep cuts through ordinary gloves.

• Wear cut-resistant gloves when you handle sheet, and safety glasses at all times.
• Deburr edges before you pass a part along.
• Plan your lifts. A big sheet is heavier and more awkward than it looks, and it can catch the wind or slip through your hands. Get help, or use a lifter.
• Wear hearing protection around shears, grinders and hammering.
• Keep your hands out of the point of operation on shears, brakes and punches, and never bypass a guard. Lock out a machine before you clear a jam or change tooling.
• Sweep up chips and offcuts. They are sharp, and they scratch parts.

Gauge and Material

Reading Gauge

Gauge is a number for thickness. The higher the gauge, the thinner the metal. The thickness for each number depends on the metal, so 16-gauge steel, stainless and aluminum are not the same. For critical work, measure with a caliper or micrometer instead of relying on the number.

You may also see SWG on specifications. It stands for Standard Wire Gauge, a British system with different thicknesses, so check which one you have.

Worked example: A 4 by 8 foot sheet has 32 square feet. At 2.5 pounds per square foot, a 16-gauge steel sheet weighs about 80 pounds. A 10-gauge sheet weighs about 180 pounds, so it is not a one-person lift.

Common Materials

Warning: Welding, cutting or brazing galvanized steel makes zinc oxide fume, which causes metal fume fever. Use local exhaust, and use respiratory protection if your employer requires it.

Stainless needs special handling. It work-hardens, meaning that it gets harder as you form it. It also springs back more than mild steel, and it picks up iron from steel tools. Keep tools for stainless clean and separate, so that the surface does not rust later.

Cutting

Hand Tools

Aviation snips are color-coded by the way they cut.

Choose the pair so that the waste curls away and the good piece stays flat. Handle colors are a common standard, but check the label, as a few brands differ.

Other hand tools include a throatless bench shear, for straight or curved cuts on light gauges, and a hand seamer, for bending small flanges.

Machine Tools

Using a Squaring Shear

A squaring shear, also called a guillotine shear, makes the long straight cuts that start most jobs.

1. Check that the material is within the rated capacity.
2. Set the back gauge to the required width, and confirm it with a tape on a test piece.
3. Set blade clearance, if your shear has an adjustment. About 5 to 8 percent of material thickness is a common starting point. Too little wears the blades and gives a rough edge, and too much leaves burrs and bends the sheet.
4. Slide the sheet in against the gauge, so that it sits flat.
5. Move your hands clear, then cycle the shear. The hold-downs clamp the sheet before the blade drops.

Warning: Never reach under the guard or behind the blade, and keep your feet away from the pedal until you are ready to cut. Shear blades can take off fingers.

Layout

Layout is marking the sheet before you cut or bend it.

• Start from a known edge. Use a factory edge or a verified square edge.
• Use the right marker. A scribe gives a fine line, but it scratches the surface. A fine marker is better on finished surfaces and coated metal. Never scribe a bend line on the outside of a bend, because the scratch can start a crack.
• Mark bend lines with direction and order. Show up or down, and which bend comes first.
• Allow for the bend. The flat blank is not the sum of the outside dimensions. Your blank has to be calculated from bend deductions or a shop bend chart.
• Mark holes and notches while the part is flat. That is almost always easier than working on a formed part.
• Center punch before drilling. The small dimple stops the drill bit from wandering off the layout point.
• Leave space between nested parts. When parts are nested on one sheet for a CNC laser or plasma cutter, the gaps prevent heat distortion, give the tool clearance and keep the sheet stable.

Common layout tools include a steel rule, combination square, dividers, trammel points and a chalk line.

Bending

A press brake bends sheet between a punch and a V-shaped die. A finger brake, also called a box and pan brake, has removable clamping segments, so that you can bend the fourth side of a box and clear flanges that are already up.

Air Bending and Springback

In air bending, the punch presses the metal partway into the die without bottoming it out, and the depth sets the angle. It is the most common method. As a starting point on mild steel, 10 gauge or thinner, the die opening should be about 8 times the material thickness. Coining gives the most precise bend angle and the least springback, but it needs far more force, commonly quoted as 3 to 10 times the tonnage of air bending.

Springback is the slight opening of the angle after the punch lifts, as the metal recovers. You overbend slightly to make up for it. Stainless and other high-strength materials spring back the most.

Bends That Will Not Crack

Grain direction runs the way the sheet was rolled at the mill. It matters most on harder alloys and tight radii.

Hems

A hem is a 180 degree fold that doubles the metal back on itself. On a customer-facing edge, it removes the sharp edge and stiffens the flange. A closed hem lies flat, and an open hem leaves a small gap.

Bend Order

Plan the order before you start. As a general rule, make inside bends first and outside bends last. If you do the outside bends first, formed flanges can hit the tooling or the back gauge on later bends.

Joining

Sizing a Rivet

A common rule is a rivet diameter about 3 times the thickness of the sheet. The rivet's grip range must match the total thickness of the sheets. Drill a clean hole of the size listed for the rivet, and deburr it. Keep rivets away from the edge, and do not mix metals that will corrode each other.

Spot Welds

In resistance spot welding, two electrodes clamp overlapping panels and pass a high current through them, which melts a small nugget between the sheets. There is no filler. It is fast, which is why it suits sheet panels in production.

Common Mistakes

• Handling raw edges bare-handed. Sheet edges slice skin in a moment.
• Laying out at finished dimensions. The part comes out too long.
• Using a steel brush on stainless. Iron left on the surface rusts.
• Bending tight radii along the grain. The outside of the bend can crack.
• Skipping relief notches. Corners tear or bunch up.
• Bending in the wrong order. You can end up with a part that cannot be finished.

Check Your Understanding

A job calls for 18-gauge steel, and your rack has 14-gauge and 20-gauge. Which one is closest, and can you swap?

20-gauge is the closer one, and 14-gauge is much thicker, since higher numbers mean thinner metal. But do not swap without approval, because the print sets the material.

You need to cut a curve to the right in light sheet. Which snips do you reach for?

Green-handled snips. Check the label, as the color code is standard but a few brands differ.

A tight bend in 6061 aluminum cracked on the outside. What are two things you could change?

Use a larger inside radius, and bend across the grain. A softer alloy such as 3003 may work if the print allows.

A customer complains that the edge of a panel is sharp and flimsy. What feature could fix both problems?

A hem. Folding the edge back on itself takes away the sharp edge and stiffens the flange.

Key Takeaways

• Higher gauge means thinner metal, and gauge varies by material.
• Fresh edges cut, so wear gloves and deburr.
• Choose your cutting tool by the shape, volume and heat limits of the job.
• Lay out from a known edge, and allow for the bend in your flat pattern.
• Keep the inside radius at least equal to the thickness, and bend across the grain.
• Plan your bend order before you make the first bend.