Flying Shear Control Logic in Roll Forming Machines — Timing, Position, Phase & Speed Matching
Introduction — What Is a Flying Shear in Roll Forming?
High-speed roll forming machines cannot stop the line every time a panel needs to be cut. Stopping the machine repeatedly would severely reduce production speed.
To solve this problem, many modern roll forming machines use a flying shear cutting system.
In a flying shear system, the cutting carriage accelerates to match the strip speed, performs the cut while moving with the strip, and then returns to its starting position.
This allows the machine to continue running without stopping.
Flying shears are commonly used in:
- high-speed roofing panel lines
- metal deck roll forming machines
- structural purlin lines
- automotive roll forming systems
Because of the high speeds involved, flying shear systems require precise PLC and motion control logic.
Main Components of a Flying Shear System
A flying shear system consists of several mechanical and automation components.
Typical components include:
- shear carriage
- linear rails and guide system
- servo drive or servo motor
- hydraulic or mechanical shear blade
- encoder feedback system
- PLC or motion controller
These components must operate in perfect synchronization with the strip movement.
Key Requirements for Flying Shear Operation
Flying shear control requires four critical elements.
These include:
- precise position tracking
- speed synchronization with the strip
- correct timing window for cutting
- controlled return motion
If any of these elements are incorrect, the cut may occur at the wrong position or damage the material.
Encoder-Based Position Tracking
Flying shear systems rely on encoders to track the strip position.
The encoder may be mounted on:
- the main drive shaft
- a measuring wheel contacting the strip
The PLC continuously counts encoder pulses to determine how far the strip has moved.
This allows the PLC to predict when the next cut position will occur.
Cut Length Detection
The operator sets the desired panel length through the HMI.
The PLC calculates the required encoder pulse count corresponding to this length.
When the encoder pulse count reaches the programmed value, the PLC prepares to initiate the flying shear cycle.
However, the cut cannot occur instantly.
The shear carriage must first accelerate to match the strip speed.
Flying Shear Acceleration Phase
When the cut trigger point approaches, the PLC commands the servo system to accelerate the shear carriage.
The carriage accelerates along the machine rails until it reaches the same speed as the strip.
This process is called speed matching.
Speed matching ensures that the shear blade moves with the strip during the cut.
Without speed matching, the blade would drag across the material and damage the panel.
Phase Lock with Strip Movement
Once the carriage reaches the strip speed, it must also match the strip position.
This synchronization is called phase lock.
Phase lock ensures that the shear carriage remains aligned with the strip position during the cut.
Encoder feedback is used to maintain this synchronization.
The servo drive continuously adjusts the carriage speed to stay locked with the strip movement.
Timing Window for Cutting
The PLC defines a cutting window during which the shear blade may activate.
The cutting window is determined by:
- carriage position
- strip speed
- blade stroke time
If the blade activates outside the correct window, the cut will be inaccurate.
The PLC ensures the blade activates only when all timing conditions are satisfied.
Shear Blade Activation
Once the carriage speed and position match the strip, the PLC activates the shear blade.
This may be done using:
- a hydraulic cylinder
- a servo-driven blade system
The blade moves downward and cuts the strip while the carriage continues moving at strip speed.
Because the carriage and strip are synchronized, the cut occurs without disturbing the strip.
Carriage Return Cycle
After the cut is completed, the shear carriage must return to its starting position.
The PLC commands the servo system to decelerate the carriage.
Once the carriage slows down, it reverses direction and returns to the home position.
The carriage must complete this return cycle before the next cut position occurs.
Typical Flying Shear PLC Sequence
A typical flying shear sequence includes several stages.
Stage 1 — Length Monitoring
The PLC monitors encoder pulses and calculates strip length.
Stage 2 — Cut Preparation
When the cut position approaches, the PLC prepares the shear carriage for motion.
Stage 3 — Carriage Acceleration
The servo motor accelerates the carriage to match strip speed.
Stage 4 — Phase Lock
The carriage synchronizes with the strip position.
Stage 5 — Blade Activation
The shear blade cuts the strip.
Stage 6 — Carriage Deceleration
The carriage slows down after the cut.
Stage 7 — Return to Home
The carriage returns to its starting position.
Speed Matching Calculations
Speed matching requires accurate calculation of strip velocity.
The PLC determines strip speed using encoder pulse frequency.
For example:
If an encoder generates 2000 pulses per meter and the machine runs at 40 meters per minute:
Encoder pulse rate = 80,000 pulses per minute
The servo controller uses this information to match the carriage speed with the strip speed.
Motion Control Systems Used in Flying Shears
Flying shear systems often use advanced motion control hardware.
Common control architectures include:
- PLC with integrated motion control
- PLC with servo drives
- dedicated motion controllers
Servo drives provide precise control of carriage position and speed.
Safety Systems for Flying Shears
Flying shear systems operate at high speed and require robust safety systems.
Typical safety features include:
- emergency stop circuits
- carriage position monitoring
- blade position sensors
- safety guard interlocks
The PLC continuously monitors these systems to ensure safe operation.
Common Flying Shear Faults
Several problems may occur in flying shear systems.
Fault 1 — Incorrect Panel Length
Incorrect panel length may occur if:
- encoder calibration is incorrect
- servo speed matching is inaccurate
- cut trigger timing is incorrect.
Fault 2 — Carriage Cannot Catch Strip Speed
If the carriage cannot accelerate fast enough, the cut may occur late or not at all.
Possible causes include:
- servo motor tuning problems
- mechanical friction in the rails.
Fault 3 — Phase Lock Loss
If encoder feedback is unstable, the carriage may lose synchronization with the strip.
This can cause inaccurate cuts.
Fault 4 — Carriage Return Too Slow
If the carriage does not return quickly enough, the next cut position may be missed.
This often occurs when the machine speed is increased.
Troubleshooting Flying Shear Systems
Technicians troubleshooting flying shear problems should check:
- encoder feedback signals
- servo drive parameters
- carriage mechanical movement
- PLC motion control logic
Monitoring motion control data during production can help identify the problem.
Commissioning Flying Shear Systems
Commissioning a flying shear system requires careful tuning.
Typical commissioning steps include:
1 verifying encoder scaling
2 tuning servo acceleration profiles
3 setting cut timing windows
4 testing carriage return timing
Multiple test cuts should be performed at different speeds.
Preventative Maintenance for Flying Shears
Routine maintenance improves reliability.
Recommended inspections include:
Monthly checks:
- inspect carriage rails
- verify encoder mounting
Quarterly inspections:
- inspect servo motor couplings
- test blade position sensors
Proper maintenance ensures stable cutting performance.
Benefits of Flying Shear Systems
Flying shear systems provide several advantages.
These include:
- continuous high-speed production
- accurate panel length control
- reduced production downtime
- improved manufacturing efficiency.
Because of these benefits, flying shears are widely used in high-speed roll forming lines.
FAQ — Flying Shear Control Logic
What is a flying shear in roll forming machines?
A flying shear cuts panels while the strip continues moving, allowing the machine to operate without stopping.
Why must the shear carriage match the strip speed?
Speed matching prevents the blade from dragging across the strip during cutting.
What is phase lock in flying shear systems?
Phase lock synchronizes the shear carriage position with the strip position during the cut.
How does the PLC know when to cut?
The PLC uses encoder pulse counting to determine when the strip reaches the programmed panel length.
What causes flying shear timing errors?
Common causes include encoder faults, servo tuning problems, or incorrect PLC timing parameters.
Why must the carriage return quickly after cutting?
The carriage must return to its starting position before the next cut position occurs.