Double Shears or Missed Cuts on a Roll Forming Machine — One-Shot Logic, Signal Bounce & Sensor Issues

Introduction — Why Double Cuts or Missed Cuts Happen

In roll forming production, the shear must activate exactly once for every panel length. If the shear fires twice or fails to cut completely, production problems occur immediately.

Typical results include:

• two panels cut instead of one
• panels stuck together
• panels with incorrect lengths
• scrap material generated
• stacker jams.

These problems usually originate from three main areas:

• PLC one-shot logic errors
• sensor bounce or noise
• position detection problems.

Understanding how these issues affect the shear control sequence helps technicians identify the root cause quickly.

Understanding the Shear Trigger Process

The PLC determines when to fire the shear based on strip position or machine sequence conditions.

A typical sequence may follow these steps:

1 encoder measures strip movement
2 PLC detects the target cut length
3 PLC generates a cut request
4 shear valve or motion command is issued
5 blade cuts the material
6 PLC resets the cut request.

For correct operation, the cut command must be issued only once per panel.

If the PLC triggers the command more than once, double cuts can occur.

Symptoms of Double Shears

Typical symptoms include:

• two cuts occurring close together
• very short panels followed by longer panels
• stacker receiving panels in unexpected sizes
• machine alarms related to shear sequence errors.

These symptoms often indicate that the PLC is generating multiple cut commands.

Symptoms of Missed Cuts

Missed cuts occur when the shear fails to activate when it should.

Typical signs include:

• strip continues moving past the target length
• extremely long panels produced
• stacker jams due to oversized panels.

Missed cuts often originate from trigger logic or sensor failures.

Root Cause Category 1 — Missing One-Shot Logic

One of the most common programming mistakes is failing to use proper one-shot logic.

A one-shot ensures that a command is issued only once when a condition becomes true.

Without a one-shot, the PLC may repeatedly trigger the shear command during multiple scan cycles.

Example:

Target length reached → condition remains true for several scans → shear triggered repeatedly.

This can produce multiple cuts.

How One-Shot Logic Works

One-shot logic detects the transition of a signal from false to true.

Instead of triggering continuously, the command activates only once when the condition first becomes true.

Typical one-shot applications include:

• cut triggers
• punch triggers
• stacker drop commands.

Using one-shot logic prevents repeated commands during a single event.

PLC Scan Cycle Effects

PLC programs run continuously, scanning logic many times per second.

If a condition remains true for several scan cycles, outputs may be triggered repeatedly unless controlled carefully.

Example:

PLC scan time = 10 milliseconds.

If a cut condition remains true for 50 milliseconds, the PLC may process the condition five times.

Without one-shot protection, this can produce multiple commands.

Root Cause Category 2 — Sensor Bounce

Mechanical sensors sometimes produce unstable signals.

This phenomenon is known as signal bounce.

Instead of switching cleanly from off to on, the signal may oscillate several times rapidly.

This can cause the PLC to interpret multiple triggers.

Sensor bounce commonly occurs with:

• mechanical limit switches
• poorly mounted proximity sensors
• worn sensor brackets.

Effects of Sensor Bounce

When sensor bounce occurs, the PLC may see multiple transitions.

Example signal pattern:

OFF → ON → OFF → ON → OFF → ON.

If this signal controls a shear trigger, the PLC may issue several cut commands.

Preventing Sensor Bounce

Several methods can reduce bounce effects.

Input Filtering

The PLC can delay acceptance of a signal until it remains stable for a short period.

Example:

Signal must remain active for 20 milliseconds before being accepted.

Debounce Timers

A timer can prevent repeated triggers for a defined period after the first activation.

Mechanical Improvements

Improving sensor mounting and replacing worn components can eliminate bounce at the source.

Root Cause Category 3 — Encoder Position Errors

If the encoder signal that measures strip movement becomes unstable, the PLC may miscalculate strip position.

This can cause multiple cut triggers or missed cuts.

Possible encoder problems include:

• loose measuring wheel
• electrical noise on encoder cables
• damaged encoder connectors.

Technicians should inspect encoder hardware when shear timing becomes unreliable.

Electrical Noise in Sensor Signals

Roll forming machines contain many high-power electrical devices such as:

• VFD drives
• hydraulic solenoids
• servo drives.

These devices can generate electrical noise that interferes with sensor signals.

If sensor wiring is poorly shielded or grounded, noise may produce false signals.

Shielded cables and proper grounding reduce this problem.

Incorrect Trigger Window Logic

Flying shear systems often use a trigger window to allow slight variation in encoder timing.

If the window is too wide, the PLC may detect multiple trigger opportunities.

If the window is too narrow, the PLC may miss the trigger entirely.

Proper tuning of the trigger window is essential for reliable cutting.

Hydraulic Shear Cycle Problems

Sometimes double cuts occur due to mechanical or hydraulic issues.

Possible causes include:

• sticky hydraulic valves
• delayed cylinder movement
• blade rebound after cutting.

These issues may cause the PLC to believe the first cut did not occur, leading to a second command.

Hydraulic system inspection may be required.

Troubleshooting Procedure

Technicians should follow a structured troubleshooting process.

Step 1 — Observe the Cut Event

Determine whether the problem is a double cut or a missed cut.

Step 2 — Check PLC Cut Trigger Logic

Verify that one-shot logic is implemented correctly.

Step 3 — Inspect Sensor Signals

Look for unstable or bouncing sensor inputs.

Step 4 — Verify Encoder Stability

Check encoder wiring and signal quality.

Step 5 — Inspect Hydraulic Components

Ensure the shear cylinder and valves operate smoothly.

Testing the System at Low Speed

Running the machine at reduced speed can help isolate problems.

At lower speeds, technicians can observe:

• sensor behavior
• shear movement
• PLC trigger timing.

This often makes it easier to identify the root cause.

Preventative Maintenance

Several maintenance practices help prevent double cuts or missed cuts.

Recommended actions include:

• inspecting sensors regularly
• cleaning measuring wheels
• maintaining hydraulic valves
• checking encoder cable connections.

Preventative maintenance reduces the likelihood of intermittent cutting errors.

Advanced PLC Diagnostics

Modern PLC systems often provide tools for analyzing signal behavior.

Engineers can monitor:

• encoder pulse frequency
• sensor signal transitions
• cut trigger logic states.

These diagnostics can reveal subtle timing problems.

Production Impact of Cutting Errors

Double cuts or missed cuts can significantly affect production efficiency.

Problems may include:

• increased scrap material
• reduced production speed
• stacker jams
• increased machine wear.

Resolving cutting problems quickly helps maintain production quality.

Benefits of Correct Shear Trigger Logic

Proper shear trigger logic provides several benefits.

These include:

• consistent panel lengths
• reliable machine operation
• reduced downtime
• improved product quality.

For high-speed roll forming machines, reliable cut control is essential.

FAQ — Double Cuts and Missed Cuts

Why does my roll forming machine cut twice?

This often occurs when PLC logic lacks one-shot protection or when sensor signals bounce.

What causes missed cuts?

Missed cuts may occur due to encoder signal errors, narrow trigger windows, or incorrect PLC logic.

What is one-shot logic?

One-shot logic allows a command to activate only once when a condition changes from false to true.

Can sensor bounce cause double cuts?

Yes. Rapid switching of unstable sensor signals may trigger multiple commands.

How can I test the shear system safely?

Running the machine at slow speed helps observe the cut sequence and identify signal problems.

Can electrical noise affect shear triggering?

Yes. Poor shielding or grounding may cause false sensor signals in high-power industrial environments.