Troubleshooting CNC Control Problems on Deep Hole Drills

Systematic Approach to CNC Control Faults

CNC control problems can stop production for hours or days if you chase symptoms without a plan. I have developed a systematic approach over 15 years of maintaining deep hole drilling machines with Fanuc, Siemens, and Mitsubishi controls. The approach works across all brands.

I always start with the power supply. A control that will not power up usually has a blown fuse or tripped breaker. I check the main power input at the disconnect switch, then trace through the transformer to the control power supply module. A loose connection on the transformer tap can cause intermittent shutdowns that are hard to reproduce.

Power Supply Problems

The control power supply is the most common failure point I see. These units run 24/7 and the internal capacitors degrade over time. When a power supply starts to fail, the symptoms include random alarms, flickering display, and intermittent spindle speed fluctuations.

I check the DC output voltages with a multimeter. A 24 VDC supply should read 24.0 V plus or minus 0.5 V. If I see 23.0 V or lower, the power supply is failing. I replace it before it fails completely. I keep spare power supplies on the shelf for each control type in the shop.

Spindle Drive Faults

If the spindle will not start, I check the safety interlock circuit first. The machine will not start the spindle if a door interlock is open or the coolant pressure is below the minimum threshold. Most deep hole drilling machines require coolant pressure above 10 bar (145 psi) before the spindle can engage.

I check the interlock status on the control diagnostic screen. On a Fanuc control, I go to the PMC diagnostic screen and look for the interlock signal. If the signal is 0, the interlock is active and I trace which safety device is holding the spindle.

Spindle drive faults show an alarm code on the control. Common alarms include:

• Overcurrent: The spindle motor draws more current than the drive rating. This usually indicates a mechanical bind in the spindle, a failing motor bearing, or a short in the motor winding.
• Overvoltage: The DC bus voltage exceeds the drive limit. This happens during rapid deceleration when the regenerative braking circuit cannot absorb the energy fast enough.
• Encoder loss: The drive loses position feedback from the spindle encoder. I check the encoder cable for damage and reseat the connectors.
• Overtemperature: The spindle motor thermistor has tripped. I let the spindle cool for 30 minutes and check the cooling fan.

Axis Drive Faults

Axis drive faults on deep hole drilling machines are typically related to the feed axis that drives the tool or workpiece. The axis drive module generates an alarm code that I read on the control display.

I maintain a log of alarm codes for each machine. For example, on a Fanuc Alpha series drive, alarm 9 means the DC link fuse is blown. Alarm 2 means overvoltage. Alarm 8 means the motor has exceeded its maximum speed, which can happen if the encoder cable is disconnected.

When I get an axis overcurrent alarm, I check the mechanical condition before replacing the drive. I turn the ballscrew by hand with the motor disconnected. If the ballscrew turns stiffly, the problem is mechanical, not electrical. I have seen shops replace 3000 USD drive modules only to find that a damaged ballscrew was causing the overcurrent.

Alarm 434 on the Z-axis (the deep hole feed axis) is particularly common on deep hole drilling machines. The Z-axis takes the heaviest cutting load and operates near its limits for hours at a time. If the alarm appears after 30 minutes of running and clears after the machine cools down, the cause is usually mechanical — tight gibs, a worn leadscrew bearing, or insufficient lubrication. I check the lubrication lines first, then the leadscrew preload.

Common Alarm Code Reference

I keep this reference sheet posted on every machine. It covers the alarms I see most frequently across Fanuc and Siemens controls.

I treat alarm 408 on Fanuc and F7900 on Siemens with extra caution. Both can be caused by simple connector issues, but both can also indicate a failing drive module. I always check the cheapest fix first — reseating cables and measuring supply voltages — before ordering replacement parts.

Lubrication System Faults

Lubrication faults are especially common on deep hole drilling machines because coolant mist gets into the lubrication system and dilutes the oil. The machine alarms with a lubrication fault, but the root cause is often coolant contamination rather than a failed pump.

I follow a five-minute diagnostic protocol for lubrication alarms:

1. Check the sight glass for fluid level and look for coolant contamination (milky oil indicates coolant ingress).
2. Test the pressure sensor with a multimeter or hand pump.
3. Measure the pump motor current draw against the nameplate rating.
4. Disconnect the main supply line at the distribution block and check flow rate into a graduated container.
5. Verify the controller parameters — Fanuc PMC timer values or Siemens machine data for lube interval.

If coolant contamination is the issue, I drain and replace the lubrication oil and inspect all seals on the lubrication tank. A simple oil change fixes the majority of lubrication alarms on deep hole drilling machines.

Communication and Cable Issues

Communication errors between the CNC control and the servo drives are usually caused by loose or damaged cables. The fiber optic cables used on Fanuc and Siemens systems are sensitive to contamination on the optical faces. I clean the fiber optic connectors with a lint-free swab and isopropyl alcohol.

For serial communication errors (RS-232 or RS-485), I check the baud rate and parity settings. The control and the device must match exactly. I have wasted hours troubleshooting a communication error that was caused by a 9600 vs 19200 baud rate mismatch.

I reseat all connectors as part of my troubleshooting process. A connector that looks secure can have a broken retention clip that allows it to vibrate loose. I use a small zip tie to secure any connectors that have broken clips.

Control Parameter Backup Procedure

I back up control parameters after every machine installation and after any approved parameter change. This practice has saved me days of troubleshooting time.

Fanuc backup procedure:

1. Press the SYSTEM key, then the PARAM soft key.
2. Insert a USB drive or connect a PC via RS-232.
3. Press the OUTPUT soft key to write all parameters to the drive.
4. Also back up the PMC program from the PMC diagnostic screen.
5. Store the backup file with the machine serial number and date in the filename.

Siemens backup procedure:

1. Insert the CF card into the control slot.
2. Navigate to the backup menu from the start screen.
3. Select Save NC Data and confirm.
4. The control writes all machine data, tool data, and programs to the card.

Standard backup schedule:

• After machine installation or commissioning
• After any parameter change approved by engineering
• After a control software update
• Quarterly for all production machines

On Fanuc controls, I use the parameter compare function (system key, then parameter soft key) to compare current parameters against the backup. Siemens controls store the original parameters in the CF card slot. If the comparison shows differences, I investigate what changed and whether it was intentional.

Control Maintenance Schedule

I perform these tasks on a regular schedule to prevent problems before they stop production.

For more information on CNC controls, see my guide on comparing CNC controls for deep hole drilling.

Key Takeaways

• Always check the power supply output voltage first when troubleshooting control problems.
• Mechanical binding often causes electrical drive faults — check the mechanics before replacing drives.
• Keep a log of alarm codes for each machine brand in the shop — reference Fanuc alarm 408 and Siemens F7900 as recurring issues.
• Lubrication faults on deep hole drilling machines are often caused by coolant contamination — check the oil for cloudiness before replacing the pump.
• Back up control parameters after every installation and approved change; use the parameter compare function to detect unauthorized changes.
• Clean fiber optic connectors with isopropyl alcohol when troubleshooting communication errors.
• Replace control backup batteries annually and clean cooling fans quarterly.
• For more information, see comparing CNC controls for deep hole drilling.