A Bridgeport Series 1 CNC with a BOSS 8 control displayed an axis limit error at startup, would not reach the expected home-axis state, and would not jog after axis enable changed from red to green. The confirmed fault was a loose Wire 41 connection that removed power from the X- and Y-axis limit-switch circuits. Restore that circuit before investigating drive hardware or phase-converter behavior.
Recognize the Failure Pattern
| Observation | Diagnostic meaning |
|---|---|
| Axis limit error appears at startup | Inspect the limit-switch circuit, including its power path, rather than assuming an axis is physically on a switch. |
| Axes are clear of the limit switches | A loose connection, broken conductor, or unpowered switch circuit can still produce the limit indication. |
| Axis-enable button changes from red to green, but no axis moves | The command is being accepted at the panel, but this does not prove that the drives have enabled. |
| Drive chokes do not produce their audible sound | This supports checking whether the drives actually energized. |
Confirmed Root Cause
Wire 41 was loose. The open connection removed power from the X- and Y-axis limit switches, causing the BOSS 8 control to report an axis error even though the axes were not mechanically resting on their limit switches. Tightening the connection restored machine operation.
This distinguishes an electrical limit-chain failure from a genuine overtravel condition. Do not force an axis or attempt an undocumented button combination until the limit inputs have power and their wiring continuity is verified.
Trace the Limit-Switch Circuit
- De-energize the machine using the site-approved electrical isolation procedure before tightening connections or testing continuity.
- Inspect each limit switch and its cabling for loose terminals, broken conductors, shipping-related movement, or sticky residue that prevents reliable operation.
- Inspect Wire 41 and its terminal connection specifically. The documented repair was tightening this loose wire.
- Using the maintenance wiring diagram, identify the rear-cabinet terminal strip and follow the numbered conductors through the limit circuit.
- After restoring power under qualified electrical procedures, trace the 24 VDC circuit across successive terminals. The point where voltage stops identifies the open switch, conductor, or connection.
The evidence identifies Wire 41 as the failed conductor but does not establish that Wire 41 itself carries 24 VDC. Confirm its function and test points from the machine-specific schematic.
Check Drive and Control Readiness
If the limit circuit is intact but the drives still do not enable, check all fuses, including fuses located behind the motor drives. Check the 24 VDC supply and the logic supply feeding the FMDC board. The FMDC is mounted on the back of the white swing-out door at the lower right of the large cabinet.
The FMDC has eight LEDs along its top edge. Two green LEDs at one end blinking at dissimilar rates were identified as the indication that the board booted and was communicating with the rest of the control. Treat this as a readiness check, not proof that the limit wiring and drive power circuits are healthy.
The machine in this case used a 7.5 HP rotary phase converter. Phase-converter concerns and transformer configuration were considered, but they were not the confirmed cause of the axis limit error. If supply configuration remains suspect, compare the transformer connections with the voltage-configuration tables in the wiring schematics rather than inferring correct tap settings from a single measurement.
Verify the Repair
- Confirm that Wire 41 and the other limit-circuit terminals are mechanically secure.
- Verify that power reaches the X- and Y-axis limit-switch circuits using the schematic and numbered terminals.
- Start the control and confirm that the axis limit error clears.
- Enable the axes and verify that the drives energize.
- Enter jog mode and confirm controlled movement toward the normal homing sequence. Stop if the control still reports a limit or if any axis binds.
If the panel continues to show only a four-digit error, the FMDC K3 monitor connection can provide real-time errors, an error log, and board status through a PC. Decode any displayed value using the BOSS 8 maintenance documentation; the evidence does not provide the digit definitions.
Frequently Asked Questions
Why does a BOSS 8 show an axis limit error when no axis is on a limit switch?
An open or unpowered limit-switch circuit can look like an active limit condition. In the documented case, loose Wire 41 removed power from the X- and Y-axis limit switches.
Where should I trace the BOSS 8 limit-switch voltage?
Use the maintenance wiring diagram and follow the numbered wires along the left-to-right terminal strip in the rear cabinet. Trace the 24 VDC circuit until the voltage disappears, then inspect the associated switch, wire, and terminal.
How can I tell whether the BOSS 8 FMDC board booted?
Inspect the eight LEDs along the top of the FMDC board. Two green LEDs at one end blinking at dissimilar rates indicate that it booted and is communicating, but this does not verify limit-circuit power or drive enable.
