Stop-to-Cut Logic in Roll Forming Machines — Hydraulic Shear PLC Sequence & Fault Diagnostics

Introduction — What “Stop-to-Cut” Means in Roll Forming

In many roll forming machines, particularly medium-speed roofing and cladding lines, panels are cut using a stop-to-cut hydraulic shear system.

In this method, the machine briefly stops when the required panel length is reached. The hydraulic shear then cuts the material before the line accelerates again to continue production.

Stop-to-cut systems are commonly used in machines producing:

• roofing panels
• wall cladding panels
• trim profiles
• light gauge sections

These systems are simpler and less expensive than flying shears but require precise PLC timing to maintain production accuracy.

Understanding the PLC logic controlling stop-to-cut systems is essential for diagnosing cutting faults and improving production reliability.

Key Components of a Stop-to-Cut System

A hydraulic stop-to-cut system consists of several mechanical and automation components.

Typical components include:

• hydraulic shear frame
• hydraulic cylinder
• hydraulic power unit
• position sensors or limit switches
• PLC control system
• encoder length measurement system

The PLC coordinates these components to perform accurate cutting operations.

Encoder-Based Cut Trigger

The stop-to-cut process begins when the panel reaches the programmed length.

The PLC measures strip movement using an encoder system.

As the strip moves through the machine, the encoder generates pulses.

The PLC counts these pulses to determine the strip length.

When the pulse count reaches the programmed cut length, the PLC prepares to initiate the stop-to-cut sequence.

Typical Stop-to-Cut PLC Control Sequence

A typical PLC stop-to-cut sequence consists of several stages.

Stage 1 — Length Detection

The PLC continuously monitors encoder pulses during production.

When the measured length reaches the programmed value, the PLC sets a cut request flag.

This flag prepares the machine for the cutting operation.

Stage 2 — Machine Deceleration

The PLC commands the main drive to slow down.

The VFD decelerates the machine using the programmed ramp profile.

Gradual deceleration prevents strip tension problems.

The PLC monitors encoder feedback to confirm that the machine has stopped.

Stage 3 — Shear Ready Confirmation

Before triggering the shear, the PLC verifies several interlocks.

Typical interlocks include:

• shear in home position
• hydraulic pressure available
• safety guards closed
• machine fully stopped

If any condition is not satisfied, the PLC blocks the cutting operation.

Stage 4 — Shear Activation

Once the machine is fully stopped and all interlocks are satisfied, the PLC activates the hydraulic shear.

The PLC energizes a solenoid valve that sends hydraulic pressure to the cylinder.

The cylinder drives the shear blade downward to cut the panel.

Stage 5 — Shear Down Position Confirmation

Sensors confirm that the shear has reached the fully down position.

These sensors may include:

• limit switches
• proximity sensors
• hydraulic pressure sensors

The PLC waits for confirmation before proceeding to the next stage.

Stage 6 — Shear Return Stroke

After cutting, the PLC commands the hydraulic valve to reverse.

The hydraulic cylinder retracts and returns the shear blade to the home position.

Sensors confirm that the shear is fully raised.

Stage 7 — Machine Restart

Once the shear returns to the home position, the PLC resets the cut flag.

The main drive accelerates back to production speed.

The machine continues producing the next panel.

Stop-to-Cut Timing Considerations

Accurate timing is critical in stop-to-cut systems.

Several timing factors must be considered.

These include:

• drive deceleration time
• hydraulic shear response time
• blade travel time
• strip movement inertia

The PLC may compensate for these factors by triggering the stop slightly before the exact panel length.

This compensation ensures the final panel length remains accurate.

Hydraulic System Integration

The hydraulic system provides the force needed to cut the panel.

Typical hydraulic components include:

• hydraulic pump
• oil reservoir
• pressure regulator
• solenoid control valves

The PLC controls the solenoid valves that direct hydraulic flow to the shear cylinder.

Hydraulic pressure must remain stable for consistent cutting performance.

Safety Interlocks for Hydraulic Shears

Hydraulic shears present significant safety hazards.

For this reason, PLC safety interlocks are critical.

Typical safety interlocks include:

• emergency stop circuits
• safety guard switches
• hydraulic pressure monitoring
• shear position monitoring

If any safety device is triggered, the PLC stops the machine immediately.

Common Stop-to-Cut Faults

Several problems may occur in stop-to-cut systems.

Fault 1 — Incorrect Panel Length

Incorrect panel lengths may occur if:

• encoder calibration is incorrect
• shear timing compensation is incorrect
• strip slippage occurs

These problems usually require recalibration of the length measurement system.

Fault 2 — Shear Not Triggering

If the shear does not activate, possible causes include:

• missing shear ready signal
• hydraulic pressure faults
• PLC logic interlock faults

Checking PLC input signals often helps identify the issue.

Fault 3 — Shear Not Returning

If the shear blade does not return after cutting, possible causes include:

• hydraulic valve failure
• cylinder seal problems
• PLC output faults

Hydraulic pressure and valve operation should be inspected.

Fault 4 — Machine Stops but No Cut Occurs

This issue may occur if the PLC stops the machine but fails to trigger the shear.

Possible causes include:

• missing encoder trigger signal
• PLC logic error
• faulty shear position sensor

Reviewing the PLC sequence helps identify the problem.

Troubleshooting Stop-to-Cut Systems

When troubleshooting stop-to-cut faults, technicians should check several areas.

Important checks include:

• encoder pulse signals
• PLC cut trigger logic
• hydraulic pressure levels
• shear position sensors

Observing the machine sequence during operation often reveals the fault.

Commissioning Stop-to-Cut Systems

Commissioning a stop-to-cut shear requires careful testing.

Typical commissioning steps include:

1 verifying encoder length measurement
2 testing drive deceleration timing
3 testing hydraulic shear movement
4 calibrating cut timing compensation

Multiple test panels should be produced to verify length accuracy.

Preventative Maintenance for Hydraulic Shears

Routine maintenance improves shear reliability.

Recommended inspections include:

Monthly checks:

• inspect hydraulic hoses and fittings
• verify blade condition

Quarterly inspections:

• check hydraulic oil level and quality
• test PLC interlocks

Regular maintenance reduces cutting failures.

Benefits of Stop-to-Cut Systems

Stop-to-cut shear systems provide several advantages.

These include:

• simpler mechanical design
• lower equipment cost
• reliable cutting performance
• easier maintenance

For medium-speed roll forming machines, stop-to-cut systems remain widely used.

FAQ — Stop-to-Cut Hydraulic Shear Control

What is stop-to-cut in roll forming machines?

Stop-to-cut is a cutting method where the roll forming machine briefly stops so a hydraulic shear can cut the panel.

Why do machines stop before cutting?

Stopping the machine allows the shear to cut accurately without needing to synchronize with moving strip.

What triggers the shear in a stop-to-cut system?

The PLC triggers the shear when encoder-based length measurement reaches the programmed panel length.

Why might the shear fail to cut?

Possible causes include missing PLC signals, hydraulic pressure problems, or sensor faults.

How is panel length accuracy maintained?

The PLC uses encoder pulse counting and timing compensation to ensure accurate panel length.

Are stop-to-cut systems used on high-speed machines?

No. High-speed machines typically use flying shears instead of stop-to-cut systems.