Case Study: CNC Safety System Issue-to-Resolution

1. Toshulin VTL SKJ 16

Key Issues

• No labeled wiring and incomplete electrical drawings, making relay identification extremely difficult.
• Initial relay chosen for Run/Jog mode selection was incorrect, causing both modes to be influenced when attempting bypass/interrupt.
• Excessive troubleshooting occurred because the initial relay was assumed to be correct for too long.

Resolution

• After extensive trial-and-error, the correct control relay was found that allowed the addition of a bypass relay tie-in without affecting other functions.
• A reliable tie-in point was identified: holding the Stop PB prevented table rotation in both Run and Jog, making it a suitable circuit to interrupt.

2. Gray VTL SKJ 16

Key Issues

• Electrical prints were incorrect due to refurbishing with a different PLC and VFD.
• New PLC had no visible indicators, making signal tracing difficult.
• Machine no longer had a jog mode, impacting safety logic design.
• HMI-based tie-in buttons were solid-state and could not be intercepted.
• Operator needs required that setup be performed with guarding open and table allowed to rotate, requiring a safe but flexible solution.

Resolution

• Stopping characteristics were evaluated and E-Stop PB performance was deemed acceptable for risk reduction.
• A temporary jumper was installed at operator request, but a proper risk-reducing bypass design was implemented:
  • Supervisor-controlled keyed bypass selector switch (prevents unauthorized use).
  • Handheld, momentary, spring-return PB serving as a “hold-to-run” safety device during setup.
• Final logic:
  • Run Mode: Safety relay in series with E-Stop; guarding must be closed to run; opening guarding stops machine.
  • Bypass Mode: Safety relay bypassed only when keyed switch is on and handheld PB is actively held.
  • Loss of handheld PB input reopens the E-Stop circuit, requiring a full reset.

 

3. Bullard Cutmaster 33077

Key Issues

• I/O indicator lights showed no change until after a start command, hiding the mode of operation.
• Table would coast for ~30 seconds when the E-Stop was used—too long for practical stopping control.
• HMI tie-in buttons were solid-state and not suitable for interruption.
• VFD stop commands were non-standard, and documentation did not match real configuration.
• No accessible PLC stop command signal or configuration data.

Resolution

• Wires traced directly to the VFD; online VFD info suggested stop command locations, but control scheme was not standard 3-wire or 4-wire.
• Multiple signal combinations were tested until:
  • A method was found to stop motion,
  • Initiate a drive fault when necessary, and
  • Achieve proper Run and Jog modes.
• A safety-relay PB was added at operator station.
• Final configuration allowed safe control of both Run and Jog despite undocumented drive control architecture.

 

 

Overall Analysis: Why These Challenges Occurred

Common Root Causes

1. Inaccurate or missing electrical documentation
   • Machines had undocumented modifications, refurbishments, or PLC/VFD retrofits.
   • Prints did not match the machines—signals, terminals, and logic were inconsistent.
2. Solid-state / HMI-based controls
   • Many OEM buttons were HMI soft keys or solid-state I/O that cannot be safely intercepted.
3. No clear indication of PLC mode or stop logic
   • Difficulty determining Run/Jog state before issuing a command.
   • No visible indicators on updated PLCs.
4. Non-standard or unknown VFD control wiring
   • VFDs using unconventional stop logic.
   • Lack of access to VFD configuration parameters.
5. Safety limitations during setup
   • Operators needed rotation with guarding open, requiring engineered reductions of risk (not full elimination).
6. Mechanical stopping behavior
   • Long coast times on some machines made typical E-Stop tie-ins ineffective or unsafe.

 

 

How to Determine Tie-In Issues BEFORE Development

To prevent time-consuming “hunt and test” troubleshooting, future CNC and VTL safety upgrades should evaluate the following before quoting or designing.

A. Identify Mode Selection & Stopping Methods

• Document all modes: Run, Jog, Setup, Indexing, Manual, etc.
• Determine how each stop command works:
  • Hardwired stop?
  • PLC stop instruction?
  • VFD stop input?
  • Cycle pause or slow-stop?
• Understand stopping category needed (Stop Category 0, 1, or 2).

 

B. Determine the Signaling Method

For each command button:

• Is it hardwired or solid-state?
• Is it momentary or maintained?
• Is it NO or NC?
• Does the PLC receive an input signal?
• Is there a physical terminal to intercept or reroute?

Avoid relying on:

• Soft HMI buttons
• Solid-state signals without accessible I/O
• Interrupting outputs (creates complex/unsafe behavior)

 

C. Validate Availability of Accessible Tie-In Points

Look for:

• Discrete PLC inputs
• Discrete VFD stop circuits
• Hardwired relay logic
• Safety relay or control relay terminals

Evaluate whether:

• The signal can be interrupted without affecting unrelated modes.
• Alternative signals (like Stop PB on Toshulin) exist that influence all modes safely.

 

D. Determine What Programming Access Exists

• Can PLC logic be modified?
• Can VFD parameters be accessed?
• Can HMI buttons or modes be edited?

If not, only hardwired solutions are possible, which may limit safety upgrade options.

 

E. Mechanical Stopping Characteristics

• Measure coast times.
• Determine if mechanical braking or controlled stopping is required.
• Equivalent stopping performance may influence where tie-ins can be placed.

 

F. Evaluate the Setup Requirements

• Does the operator need rotation with guarding open?
• If so:
  • Require supervisor-controlled bypass.
  • Require hold-to-run enabling devices.
  • Limit speed and torque during setup.

This prevents unsafe “temporary jumpers” from becoming permanent.

High-Level Conclusion

Across all three CNC/VTL machines, the main challenge was the lack of matching electrical documentation, combined with undocumented retrofits and solid-state controls that prevented straightforward safety relay tie-ins. Each machine required custom troubleshooting, incremental testing, and adaptive engineering to identify reliable, safe, and operator-acceptable control points.

The solutions ultimately implemented:

• Provided safe stopping control,
• Allowed required setup flexibility, and
• Ensured compliance with functional safety principles while working within the limits of older or modified equipment.

Establishing a consistent pre-assessment methodology—focused on mode identification, command signal type, wiring accessibility, controller communication, and stopping behavior—will significantly streamline future CNC safety upgrades.