CNC Knowledge

The movement cycle of the CNC flexion process and the hydraulic control form

A press brake does not bend a flange in one motion. It moves through five distinct stages, and the hydraulics decide what happens in each. This guide walks the CNC flexion process stage by stage, with the pressure settings, speed ranges and valve behavior that keep a bend repeatable.

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5-stage cycleProportional valve control±0.005 mmRam and bed parallelism

CNC Knowledge: The movement cycle of the CNC flexion process and the hydraulic control form

On this page

1
Key takeaways
2
Rest state and the rapid descend stage in the CNC flexion process
3
Form and stamping: where the hydraulic control form sets the bend
4
Decompression, dwell and return stroke
5
What the control loop and the oil are telling you
6
Setting up one bend cycle, step by step
7
Cycle stages at a glance

Quick read

Key takeaways

Five stages, not oneRest, descend, form, decompress, return. Each stage has its own valve and pressure state.

Fast approach, slow contactGravity handles the approach. The proportional valve takes over about 6 mm above the sheet.

Forming speed caps accuracyOil supply from the pump limits stamping speed. Faster is not better on thick plate.

Dwell holds the angleA short hold at bottom dead center lets stressed material flow before release.

Release is where angles driftDecompression that is too fast snaps the ram back and springs the flange.

Stage 1-2

Rest state and the rapid descend stage in the CNC flexion process

At rest, the ram sits above the upper dead point and the hydraulic circuit holds it there. The rod-side cavity is sealed by a recirculation-off valve, so the ram cannot creep down under its own weight while the operator loads the sheet. If that seal leaks, the ram drifts a few tenths of a millimeter overnight and the first bend of the morning comes out short. Check the valve seat during scheduled maintenance, not after a scrap run.

The descend stage starts when the control releases the seal. Gravity does most of the work. On a 100-tonne brake the ram can fall at 100 to 150 mm/s with the pump only supplying makeup oil, which is why approach speed is set by a flow control rather than by pump displacement. Keep the approach fast. Time spent creeping down from the top dead point buys nothing and heats the oil.

As the ram falls, the rodless cavity pulls a vacuum and the rod-side cavity pushes oil back to tank through a proportional steering valve. That valve generates a back pressure which brakes the ram. Without it the ram arrives at the sheet at full speed and the tooling takes the shock. Set the braking ramp so the ram is down to roughly 10 to 20 mm/s by the time the punch is 6 mm above the sheet.

The 6 mm figure is a working number, not a law. Heavy plate and large V-dies tolerate a shorter approach, thin sheet and small dies need more. The rule we use: switch from fast to slow at 6 mm, then let the proportional valve bring the ram to contact speed before the punch touches metal.

1
Seal checkVerify the recirculation-off valve holds the ram for 8 hours without visible drift.
2
Braking rampTarget 10-20 mm/s at 6 mm above the sheet, not at contact.
3
Oil temperatureKeep 35-50 °C. Cold oil raises back pressure and slows the approach.

Stage 3

Form and stamping: where the hydraulic control form sets the bend

Contact begins the forming stage. The rodless cavity is now pressurized and the pump supplies the flow that drives the punch into the die. Stamping speed is limited by how much oil the pump can deliver, and it is trimmed by a proportional valve in series with a directional valve. On a typical 100-tonne machine, forming speed runs 5 to 10 mm/s in the working stroke. Push it to 20 mm/s and you trade angle consistency for cycle time.

The directional valve does a second job. It keeps the two sides of the ram synchronized and governs where the ram stops, which is the lower dead point. On a synchronized brake the two cylinders are cross-linked through the valve so that one side cannot lead the other. A 0.05 mm mismatch across a 2,000 mm bed twists the ram and shows up as a taper in the flange.

Tonnage is not fixed by the part alone. It depends on material, thickness, bend length and die opening. A rough shop formula is 1.33 × tensile strength × thickness² / V-opening, per meter of bend. For 3 mm 304 stainless in a 24 mm V-die that lands near 75 tonnes per meter. Set the pressure relief above that figure but not far above it. Excess tonnage bows the bed permanently.

Crowning is the other half of the form stage. Under load the bed and ram deflect in the middle, so the flange in the center of a long part comes out with a larger angle than the ends. A CNC crowning system or a set of shims compensates. If you bend 2,000 mm parts and never touch crowning, your angle tolerance is probably not what you think it is.

1
Forming speed5-10 mm/s is a safe band for most carbon and stainless work.
2
Relief settingSet 10-15% above calculated tonnage, not at machine maximum.
3
CrowningRe-check compensation whenever bend length or material changes.

Stage 4-5

Decompression, dwell and return stroke

When the ram reaches the lower dead point, the control holds it there for a short dwell. During the dwell the material under the punch continues to yield and the bend angle creeps toward the target. This is where springback is partly paid off. Dwell times of 0.2 to 1.0 s are common. Very thick plate or high-strength steel may want 1 to 2 s. Too short a dwell and the flange opens after release; too long and you add cycle time for nothing.

Decompression follows. The rodless cavity is bled to tank through a dedicated decompression valve, slowly at first. If you dump that oil in one step, the stored energy in the frame and the punch releases as a shock, the ram kicks upward, and the flange springs open. The pressure drop should follow a ramp over 100 to 500 ms depending on tonnage. On a 200-tonne machine, a hard dump can move the ram 0.5 mm in a few milliseconds.

After decompression the directional valve shifts and the ram returns to the top. Return speed is high, often 100 mm/s or more, because nothing is in contact. Two things matter here. First, the return should not overshoot the top dead point hard enough to shake the tooling loose. Second, the pump should not be starved during return on a machine running a fast cycle, or the next approach starts late.

The full cycle on a small part can run 4 to 8 seconds. On a 4,000 mm bend with a long dwell it can run 20 seconds or more. When cycle time matters, look at the approach and return stages first. The forming and dwell stages are usually the ones you should not touch.

1
Dwell band0.2-1.0 s for most work; 1-2 s for thick or high-strength plate.
2
Decompression ramp100-500 ms, scaled with tonnage.
3
Return shockCheck for tooling movement if the ram slams at the top.

Control and diagnosis

What the control loop and the oil are telling you

Most modern brakes run a closed-loop position control on the ram. The proportional valve is the actuator, the linear encoder or the cylinder pressure sensor is the feedback, and the controller adjusts valve current to follow the programmed profile. When a bend drifts, the question is whether the command changed or the response did. Log the valve current against ram position for one cycle and compare it with a good cycle from the same setup.

Pressure and position tell different stories. A rising valve current with position lagging means the load went up, which points to material or die change. Position tracking fine but tonnage high means the relief setting is low or the die opening is wrong. A flat current with the ram not moving means the valve is stuck or the pump is starved. Read both channels before you touch a setting.

Oil temperature affects every stage. At 25 °C the oil is thick, back pressure is high, and the approach is slow. At 60 °C the oil thins, internal leakage rises, and the ram may not hold position at the bottom. Keep the hydraulic unit between 35 and 50 °C with a chiller or a heater, and check the tank level and filter differential weekly. Dirty oil wears the proportional valve spool, and a worn spool shows up as inconsistent approach speeds before it shows up as a bad angle.

For parts that will be bent in production, we usually ask for the material, thickness, bend length, die opening and target angle before quoting. From those numbers we can estimate tonnage, flag whether crowning is needed, and say whether the cycle will run on a standard or a large-frame brake. Sheet metal parts we run alongside our machined work, so the bend stage and the milling stage share one inspection plan.

1
Log firstCapture valve current and ram position for one full cycle before adjusting.
2
Oil window35-50 °C, filter differential checked weekly.
3
Setup dataMaterial, thickness, bend length, die opening, target angle.

How to set up

Setting up one bend cycle, step by step

Follow this order on the machine. Changing steps out of order is the usual cause of a drifting angle.

1
Confirm the die and material dataMatch the V-opening to the thickness: roughly 8× thickness for carbon steel, 10× for stainless. Record material, thickness and bend length in the program.
2
Calculate tonnage before you set pressureUse 1.33 × tensile strength × thickness² / V-opening per meter. For 3 mm stainless in a 24 mm V-die, about 75 tonnes/m. Set the relief 10-15% above that.
3
Set the approach and braking pointsApproach at 100-150 mm/s. Start braking 6 mm above the sheet so the ram is at 10-20 mm/s at contact. Shorten the gap slightly for thick plate.
4
Set forming speed and lower dead pointForm at 5-10 mm/s. Program the lower dead point from the target angle, then fine-tune with a test bend rather than with the tonnage dial.
5
Add dwell and decompressionDwell 0.2-1.0 s, longer for thick or high-strength plate. Decompression ramp 100-500 ms. If the flange springs open on release, lengthen the ramp before you change the angle.
6
Run a test bend and measureMeasure the flange angle and the bend line position. Check the two ends and the center of a long part. Crowning error shows as a center-to-end angle difference.
7
Log the cycle and lock the programSave valve current and position traces. Protect the program. On the next run, compare the new trace with the saved one before you adjust anything.

Reference

Cycle stages at a glance

Typical values for a 100-200 tonne press brake. Adjust to your machine and tooling.

Stage	Speed / time	Hydraulic control	What goes wrong
Rest	Ram held above top dead point	Rod cavity sealed by recirculation-off valve	Ram creeps down, first bend short
Descend	100-150 mm/s approach	Gravity feed, back pressure from proportional valve	Contact too fast, tooling shock
Form	5-10 mm/s stamping	Pump flow plus proportional and directional valves	Taper from ram mismatch, bed bow
Dwell	0.2-1.0 s hold	Pressure held at lower dead point	Angle opens after release
Decompress	100-500 ms ramp	Rodless cavity bled through decompression valve	Ram kickback, flange springs open
Return	100 mm/s or more	Directional valve shifts, pump refills	Tooling shake, pump starvation

Get the cycle right and the angle follows

Five stages, one hydraulic circuit. Tune the approach, the forming speed, the dwell and the decompression ramp in that order, and most bend problems disappear without touching the tonnage dial.

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FAQs

Common questions on the movement cycle

Why does the ram brake 6 mm above the sheet instead of touching down slowly from the top?

Approach speed and contact speed want different things. A fast approach saves cycle time and does not affect the bend, because nothing is in contact. Braking early costs a fraction of a second and gives the proportional valve time to settle the ram at 10-20 mm/s before the punch meets metal.

The 6 mm figure is a starting point. Thick plate with a large V-die can brake later. Thin sheet with a small die should brake earlier, because the punch tip is easier to damage.

How do I know if the tonnage setting is too high?

Two signs. The bed or ram deflects more than the crowning system can compensate, so the center of a long bend comes out with a wider angle than the ends. Or you hear the pump loading hard at the bottom of the stroke and the frame flexes visibly.

Set the relief 10-15% above the calculated tonnage for the job, not at the machine maximum. Excess tonnage does not improve the angle and it does permanent damage to the bed over time.

Should I lengthen the dwell or change the lower dead point to fix an open angle?

Try the dwell first, up to about 1 s. The material is still yielding at the bottom, so a longer hold lets the angle close without extra tonnage.

If the angle is still open after that, move the lower dead point. Changing tonnage to close an angle is the wrong lever: it loads the frame and does not address springback.

The flange springs open right after the ram releases. What is the cause?

Most often decompression is too fast. The stored energy in the frame releases as a shock and the ram kicks up, which pulls the flange open. Lengthen the decompression ramp toward 500 ms and check again.

If the ramp is already long, check for hydraulic oil leakage past the decompression valve or air in the rodless cavity. Both make the pressure drop uneven.

Does oil temperature really change the bend?

Yes, mainly through viscosity. Cold oil raises back pressure and slows the approach, so the braking distance changes. Hot oil thins and internal leakage rises, so the ram may not hold position at the lower dead point.

Keep the unit at 35-50 °C. If your first bends of the morning differ from afternoon bends, log the oil temperature alongside the angle before you adjust the program.

Can one program run the same part on two different brakes?

Only after re-checking the cycle. Approach and return speeds, decompression ramp, crowning and lower dead point are machine-specific. The material and die data transfer, the hydraulic settings do not.

Run one test bend on the second machine, compare the valve current trace with the original, and adjust the ramp and dead point before releasing the job to production.

Send us the bend data and we will check the cycle

Material, thickness, bend length, die opening and target angle are enough for us to estimate tonnage, flag crowning and tell you whether the part fits a standard or large-frame brake.

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