Flying Shear “Window” Logic Pattern for Roll Forming Machines — Capture → Predict → Fire

Introduction — Why Flying Shear Logic Is Different from Stop-to-Cut Systems

In many roll forming machines the strip must continue moving while the cut occurs. Stopping the entire forming line for each cut would dramatically reduce production speed.

To solve this, manufacturers use flying shear systems.

A flying shear carriage accelerates to match the strip speed, performs the cut while moving with the material, and then returns to its home position.

For this to work reliably, the PLC must trigger the cut at exactly the right moment. If the cut occurs too early or too late, several problems can occur:

incorrect panel length
shear collision with material
incomplete cuts
mechanical stress on the carriage system.

Because of this, flying shear systems rely on a precise PLC programming method known as the window logic pattern.

The standard logic structure is:

Capture → Predict → Fire

This pattern ensures the shear fires only when the strip reaches the correct location and the carriage is ready to synchronize with the strip speed.

Understanding the Flying Shear System

A typical flying shear system includes several key components.

These include:

servo-driven carriage
encoder tracking strip movement
hydraulic or servo cutting head
linear rails or guides
PLC motion control logic.

The carriage accelerates to match strip speed, moves with the strip while the cut occurs, and then decelerates back to its starting position.

The PLC must coordinate all of these actions.

Why Window Logic Is Needed

A flying shear cannot fire at an exact mathematical instant. Real machines require a small tolerance window because of:

servo acceleration time
encoder resolution
PLC scan time
mechanical delays.

The PLC therefore defines a trigger window.

If the strip position enters this window while the shear carriage is ready, the cut command is issued.

This ensures reliable operation even at high production speeds.

Step 1 — Capture the Current Encoder Position

The first stage of the pattern is capture.

The PLC reads the encoder position of the strip as it moves through the machine.

This encoder may be mounted on:

a measuring wheel contacting the strip
the main drive shaft
the servo feed system.

The encoder provides continuous position feedback.

The PLC tracks this position in real time.

Determining the Target Cut Position

Before prediction begins, the PLC must know where the cut should occur.

The target cut position is determined from:

panel length setting
accumulated strip movement
cut compensation offsets.

Example:

Panel length: 6000 mm

Next cut position:

6000 mm
12000 mm
18000 mm

Each time a panel is cut, the next target position is calculated.

Step 2 — Predict the Shear Trigger Point

The next stage is prediction.

Because the shear requires time to accelerate and complete the cut, the PLC cannot trigger exactly at the final cut location.

Instead, the PLC calculates a pre-trigger position.

This position accounts for:

servo acceleration time
blade travel time
control system delay.

Example:

Target cut position: 6000 mm
Compensation offset: 80 mm

Trigger position:

5920 mm

When the encoder reaches 5920 mm, the PLC prepares to fire the shear.

Predictive Timing Calculation

Prediction often depends on strip speed.

At higher speeds, the shear must trigger earlier to compensate for mechanical delays.

Many systems calculate a dynamic offset using:

Strip speed × shear response time

Example:

Strip speed: 40 m/min
Response delay: 100 ms

Distance traveled during delay:

66.7 mm

The PLC subtracts this value from the target cut position to determine the trigger point.

Step 3 — Define the Trigger Window

Instead of triggering at one exact encoder value, the PLC defines a window range.

Example:

Trigger point: 5920 mm
Window tolerance: ±5 mm

Valid trigger range:

5915 mm to 5925 mm

If the encoder position enters this range and the system is ready, the shear command is issued.

This prevents missed triggers caused by scan timing variations.

Step 4 — Fire the Shear

When the encoder enters the trigger window and all conditions are satisfied, the PLC commands the flying shear carriage to begin its cycle.

Typical actions include:

1 servo accelerates carriage
2 carriage matches strip speed
3 shear blade cuts the material
4 carriage decelerates and returns home.

This entire cycle occurs while the strip continues moving through the machine.

Shear Readiness Conditions

Before the PLC allows a shear trigger, several conditions must be verified.

Typical readiness conditions include:

carriage at home position
servo drive ready
no shear faults
hydraulic pressure available
safety systems active.

If any condition is not satisfied, the PLC blocks the cut command.

Window Logic Safety Checks

To prevent unintended cuts, the PLC must include several safety checks.

These checks ensure that the shear fires only once per panel.

Typical logic includes:

cut request flag
trigger window active
cut command issued only once
cut request cleared after firing.

This prevents multiple triggers while the strip remains within the window.

Handling Missed Windows

Occasionally the shear may fail to trigger within the window.

Possible causes include:

servo drive not ready
incorrect speed prediction
PLC timing delays.

If the encoder passes the window without triggering the cut, the PLC must detect the error.

Typical response includes:

generating a fault alarm
stopping production
requiring operator intervention.

Flying Shear Motion Synchronization

During the cut cycle, the carriage must match the strip speed.

Servo control ensures the carriage moves at the same velocity as the strip.

If synchronization fails, the cut may be inaccurate.

The PLC monitors:

servo speed feedback
carriage position
synchronization status.

This ensures the shear maintains correct alignment with the strip.

Returning the Carriage to Home

After the cut completes, the carriage must return to its starting position.

The PLC commands the servo drive to decelerate and move back to home.

Home sensors confirm that the carriage has returned safely.

Only after the carriage is ready can the next cut occur.

Encoder Resolution and Window Size

Encoder resolution influences the size of the trigger window.

High-resolution encoders allow very precise cut timing.

Low-resolution encoders may require a slightly larger window to compensate for pulse spacing.

Choosing the correct encoder resolution is important for high-speed flying shear systems.

Speed Changes and Dynamic Compensation

Strip speed may vary during production.

Flying shear systems must adjust prediction calculations dynamically.

The PLC continuously monitors strip speed and adjusts the trigger offset accordingly.

This ensures accurate cut positioning even if production speed changes.

Troubleshooting Flying Shear Window Logic

Several problems can occur in flying shear systems.

Missed Cuts

Often caused by incorrect prediction timing or window size.

Double Cuts

Usually caused by missing one-shot logic in the trigger sequence.

Incorrect Panel Length

May result from incorrect encoder scaling or compensation offsets.

Technicians should verify encoder readings and timing calculations when diagnosing these problems.

Commissioning Flying Shear Systems

During commissioning, engineers must carefully tune the window logic.

Typical steps include:

1 verifying encoder scaling
2 measuring mechanical shear response time
3 calculating initial trigger offset
4 running test panels at low speed
5 increasing speed gradually while checking cut accuracy.

Fine adjustments may be required to achieve precise panel lengths.

Preventative Maintenance for Flying Shears

Regular maintenance helps maintain cutting accuracy.

Recommended checks include:

Monthly inspections:

inspect shear blade condition
verify carriage alignment.

Quarterly inspections:

verify encoder calibration
check servo drive performance.

Maintaining these components ensures stable flying shear operation.

Benefits of Window Logic Control

Using the capture → predict → fire pattern provides several advantages.

These include:

accurate panel lengths
stable high-speed production
reduced mechanical stress
predictable PLC behavior.

For high-performance roll forming machines, proper flying shear window logic is essential.

FAQ — Flying Shear Window Logic

What is flying shear window logic?

Flying shear window logic defines a range of encoder positions where the PLC is allowed to trigger the cutting cycle.

Why is prediction required in flying shear systems?

Prediction compensates for the time required for the shear carriage to accelerate and complete the cut.

What happens if the PLC misses the trigger window?

The machine may produce an incorrect panel length and the PLC typically generates a fault condition.

Why must the shear trigger only once per panel?

Without proper one-shot logic, the PLC could issue multiple cut commands while the strip remains within the window.

How does strip speed affect flying shear timing?

Higher speeds require earlier trigger offsets to compensate for mechanical response time.

What sensors are used in flying shear systems?

Encoders provide strip position feedback, while additional sensors monitor carriage position and readiness.