Press Brake Operation & Setup

A press brake bends sheet and plate by forcing a punch into a V-shaped die. The same ram that folds steel can crush a hand, so a good operator treats setup and safety as one job. If you can pick the right die, confirm the tonnage, sequence the bends and prove the first part, you can run most work a shop will hand you.

What You Will Be Able to Do

• Compare air bending, bottom bending and coining.
• Choose a V-die from the material thickness.
• Calculate air-bend tonnage and check it against the machine.
• Sequence bends and use the backgauge without stacking errors.
• Correct springback, and change tooling safely.

Three Ways to Bend

Air bending presses the sheet partway into the V without touching the bottom. It is the most common press brake method. One punch and die can make a range of angles just by changing ram depth, and it needs the least force.

Bottom bending drives the sheet down until it sits against the die walls. A die slightly sharper than the target angle, such as 88 degrees for a 90 degree bend, allows for springback. The angle is more repeatable, but tooling wears faster and each angle needs a matching tool.

Coining forces the punch tip into the metal itself, thinning it at the bend. Springback nearly disappears. It takes so much force that it is limited to thin material or short parts on most machines. Coining is not the same thing as bottom bending, even though the terms get mixed up.

Choosing a V-Die

The V-die opening is your first decision on every job. A common rule is:

Example: 14-gauge mild steel is 0.075 inch thick. 8 x 0.075 = 0.600 inch. The nearest standard die is 0.625 inch (5/8 inch), so use that.

A die that is too narrow needs more tonnage, leaves tool marks and can crack the outside of the bend. A die that is too wide gives a larger radius and less control. Shop charts may vary these rules for your tooling.

Inside Radius

In air bending, the V-die sets the inside radius, not the punch tip. A good estimate on mild steel is:

Inside radius = V-die opening divided by 6

A 0.625 inch die gives about 0.104 inch, and a 1 inch die gives about 0.167 inch. If the print calls for a specific radius, choose the die to hit that radius.

Calculating Air-Bend Tonnage

Every machine and every tool has a limit. Go over it and you can crack tooling, damage the beam or blow a seal. For air bending mild steel:

Tons per foot = (T x T x 575) / V

Total tons = (T x T x L x 575) / V

T is material thickness in inches, V is the die opening in inches and L is the bend length in feet. The 575 is a constant for mild steel.

Worked example 1: 10-gauge mild steel (0.135 inch), 60 inches long, in a 1 inch V-die.

1. T x T = 0.135 x 0.135 = 0.0182
2. L = 60 / 12 = 5 feet
3. 0.0182 x 5 x 575 = 52.4
4. 52.4 / 1 = about 52 tons

Worked example 2: 16-gauge mild steel (0.060 inch), 48 inches long, in a 1/2 inch V-die.

1. T x T = 0.0036
2. L = 4 feet
3. 0.0036 x 4 x 575 = 8.28
4. 8.28 / 0.5 = about 16.6 tons

For other materials, multiply by a factor. Use your machine or tooling maker's chart for exact values.

Also check the tooling's rated load in tons per foot. A short, heavy bend on a small section of the bed can overload the tooling even when the total is under the machine's rating. Keep loads near the center of the machine when you can, and follow the maker's rules for off-center work.

If the beam pops, groans or visibly deflects under the ram, stop. Back off and recheck your tonnage and tooling before you go on.

Setting Up a Job

1. Read the print. Note the material, thickness, radius, angles and tolerance.
2. Pick tooling. Choose the V-die and a punch that will clear any flanges.
3. Calculate tonnage. Check it against the machine and tooling.
4. Install tooling. Follow the change procedure below.
5. Set the backgauge. Check that it moves freely.
6. Enter the program. On CNC machines, load the program or enter the thickness, die and angle.
7. Test the safeguards. Check your light curtain or laser guard, and your two-hand controls.
8. Clean the die. A chip left in the V marks every part after it, and it can cause uneven angles or tool damage.
9. Bend a first article. Use the same material, measure it, and tweak the program for springback before you run production.

Run a first article on every job, each new lot and each material change. Steel from two different mills can spring back differently.

Changing Tooling

Your employer's lockout procedure and the machine manual come first, but the usual order is:

1. Stop the machine and lock it out.
2. Block the ram down, or secure it so that it cannot move. Remember that hydraulic machines can store energy.
3. Remove the old tooling.
4. Install the new tooling, and seat it fully.
5. Remove the block and clear the area.
6. Check stroke depth and tonnage limits, then cycle slowly before you run.

Some modern machines have a setup mode for this job. Use it the way the maker instructs, and never reach in under a ram that is under power.

Backgauge and Bend Sequence

The backgauge is the stop behind the die that locates each bend.

• Gauge from a flat edge, not a formed flange. Each bend has a small error. If you gauge from a bent flange, that error gets added to the next bend, and tolerances stack.
• Check that the part sits squarely against the fingers. Use at least two fingers on wide parts.
• Sequence bends so that the part clears the machine. Formed flanges can hit the punch or beam.

Bending a Tray

On a four-sided tray with the flanges up, bend the short inside flanges first, then the long outside flanges last. If you bend the long flanges first, they can clash with the upper beam on the inside bends, and you may not be able to finish the part.

Other sequence rules:

• Bend inside flanges before outside ones.
• Look at Z-bends and hems early. After some bends, a part can no longer sit flat against the gauge.
• Walk through the sequence on paper, or with a dry run, before your first live stroke.

Springback

After the ram lifts, the metal relaxes and the angle opens. That is springback, and on mild steel after an air bend, it is usually 1 to 3 degrees.

To correct it, overbend past the target by increasing the ram depth. If a bend reads 92 degrees and you want 90, add a small correction to the depth and bend another test piece. Some CNC brakes measure the angle during bending and correct it automatically.

Programming

A CNC press brake can be set up two ways:

• Offline programming works out sequence, tool layout and backgauge positions from a 3D model. The operator loads the program, stages the tools and runs a first article.
• On-machine programming means you enter the thickness, die, angle and length, and the control works out the ram depth. It is quick for simple parts.

Good operators can do both, and they can check a program against the print.

Staying Clear of the Crush Zones

OSHA covers press brakes under its general machine guarding rule, 29 CFR 1910.212, and it treats ANSI B11.3 as the consensus standard for safeguarding them.

Common safeguards work together:

Test them at the start of every shift, and never defeat them. Taping down a button or muting a guard just to save time is a common cause of serious injuries.

Stay out of these zones:

• Between punch and die. Never reach in during a stroke, even on a slow cycle.
• Around the die to steady a part. Support long parts from the outside, and use a fixture, magnets or a helper instead of holding the bend point.
• Behind the backgauge while it moves. The fingers can crush a thumb against the rear stop.
• In the path of a whipping part. A long sheet swings up fast as it bends. Keep your head and chest clear, and hold your supports loosely.

Heavy parts need a helper or a lift. Do not wear loose gloves or sleeves near the dies.

Troubleshooting

Crowning adjusts the bed so that the ram and bed stay parallel under load. Without it, long bends can come out with a more open angle in the middle.

Common Mistakes

• Skipping the tonnage check. A chart is quick, but you should be able to run the numbers.
• Treating the first article as optional. Every lot can behave differently.
• Gauging off a formed flange. Errors stack up.
• Holding the part near the bend. The sheet moves fast, and hands get pulled in.
• Leaving chips in the die. Each part gets a mark.

Check Your Understanding

You need to bend 1/8 inch mild steel, 72 inches long. You pick a 1 inch die. About how much tonnage do you need?

0.125 x 0.125 = 0.0156. Multiply by 6 feet and by 575, then divide by 1, to get about 54 tons.

A 90 degree bend in mild steel comes out at 92 degrees. What is happening, and how do you fix it?

That is springback. Increase the ram depth to overbend slightly, then check another test piece.

You are bending a four-sided tray. A coworker starts with the long sides. What problem can that cause?

The long flanges can hit the upper beam on the remaining bends, and the part may not be finishable. Bend the short inside flanges first.

Halfway through a job, the machine groans and the bed seems to flex. What do you do?

Stop, back off and recheck your tonnage and tooling. Do not keep running.

Key Takeaways

• Air bending covers most work, and ram depth sets the angle.
• Start with a V-die at about 8 times the thickness, and adjust for thin sheet and heavy plate.
• Inside radius is about the die opening divided by 6.
• Confirm tonnage with T x T x L x 575 / V before you bend.
• Gauge from flat edges, and bend inside flanges before outside ones.
• Prove a first article, and test your safeguards every shift.