Axis position errors in deep hole drilling are not just CNC alarms — they cause real damage. A position error during drilling can drive the feed axis into the part at the wrong rate, causing drill breakage. A position error during retract can pull the chip tube out of the spindle. Every axis error must be treated as a process safety issue.
Position Error Types
Error Classification
| Error Type | CNC Alarm (Typical) | Description | Severity |
|---|
| Following error | Position deviation too large | Actual position lags commanded position beyond tolerance | High — machine stops |
| Standstill monitoring | Axis moved while stopped | Uncommanded axis movement detected | Critical — safety issue |
| Overshoot | Excess error at stop | Axis passes commanded position and cannot correct | Moderate |
| Backlash error | Position repeatability error | Axis position differs depending on direction of approach | Moderate |
| Thermal drift | Position changes with temperature | Axis position shifts as machine warms up | Low to moderate |
| Encoder error | Position feedback loss | No position signal from encoder | Critical — machine stops |
Following Error Details
| Parameter | Description | Acceptable Range | Alarm Threshold |
|---|
| Following error (steady feed) | Position lag during constant feed | 0.01–0.10 mm (depends on feed rate) | 0.20 mm (typical) |
| Following error (rapid traverse) | Position lag during rapid move | 0.05–0.50 mm | 1.00 mm (typical) |
| Standstill error | Position drift when stopped | < 0.005 mm | 0.010 mm |
| Overshoot after stop | Position recovery after feed stop | < 0.005 mm | 0.015 mm |
Common Causes
Mechanical Causes
| Cause | Symptom | Diagnosis Method | Corrective Action |
|---|
| Binding guideway | Following error on one axis, increases with travel | Manual axis movement — feel for resistance | Lubricate, scrape, or adjust gibs |
| Worn ball screw | Following error in one direction only | Measure backlash, check for play | Replace or preload ball screw |
| Coupling loose | Random position errors | Check coupling set screws | Tighten or replace coupling |
| Linear guide damage | Following error at specific positions | Feel for bumps during manual traverse | Replace damaged guide section |
| Chip under way cover | Following error at one position | Visual inspection of way covers | Clean and repair way covers |
| Hydraulic counterbalance fault | Vertical axis drift | Check hydraulic pressure | Repair hydraulic system |
Electrical Causes
| Cause | Symptom | Diagnosis Method | Corrective Action |
|---|
| Encoder contamination | Random position errors | Check encoder signal with oscilloscope | Clean or replace encoder |
| Encoder cable damage | Intermittent position loss | Visual inspection, continuity check | Replace cable |
| Servo drive fault | Axis will not move | Check drive status LED, fault code | Replace drive |
| Motor tachometer fault | Speed oscillation | Monitor speed feedback | Replace motor |
| Loose encoder connector | Intermittent errors | Push on connector while monitoring alarm | Tighten or replace connector |
| Electrical noise | Random, non-repeatable errors | Check cable shielding, grounding | Improve grounding, add ferrites |
Contamination Causes
| Contaminant | Effect | Prevention | Corrective Action |
|---|
| Coolant on encoder | Signal loss, erratic position | Encoder seal, positive pressure | Replace encoder, improve sealing |
| Chips in way covers | Binding, abnormal load | Cover maintenance | Clean way covers, repair damage |
| Oil on brake | Brake slip on vertical axis | Proper seals | Clean brake surfaces |
| Debris on scale feedback | Position errors (linear scale machines) | Scale pressurization | Clean scale, replace seal |
Diagnostic Procedures
CNC Diagnostic Check
| Step | Action | Information Obtained |
|---|
| 1 | Check alarm history | Recent position error events |
| 2 | Read following error display | Current position lag value |
| 3 | Read position deviation on all axes | Compare normal vs error condition |
| 4 | Check servo status display | Drive ready, torque, speed |
| 5 | Read encoder position feedback | Actual vs commanded position |
| 6 | Check temperature of axis motors | Overheating indicator |
| 7 | Compare axis load display | Binding or overload indication |
Mechanical Diagnostic Check
| Step | Action | Detail |
|---|
| 1 | Jog axis manually at slow speed | Feel for binding, roughness |
| 2 | Jog axis full stroke in both directions | Check for consistent resistance |
| 3 | Measure backlash | Dial indicator against axis — move in both directions |
| 4 | Check ball screw preload | Axial play at screw end |
| 5 | Check guideway lubrication | Oil flow at all lubrication points |
| 6 | Check way covers for damage | Remove covers, inspect underneat |
| 7 | Check coupling tightness | Set screws, keyway fit |
Corrective Adjustments
Backlash Compensation
| Step | Action | Detail |
|---|
| 1 | Measure actual backlash | Dial indicator, approach from both directions |
| 2 | Enter backlash value in CNC parameter | Typically parameter 1851 (Fanuc) or equivalent |
| 3 | Verify correction | Re-measure backlash |
| 4 | If backlash > 0.05 mm after compensation | Mechanical wear — replace ball screw before compensation limit |
Following Error Monitoring Adjustment
| Parameter | Adjustment Direction | Effect |
|---|
| Position loop gain (PGN) | Increase | Lower following error, higher stability risk |
| Position loop gain (PGN) | Decrease | Higher following error, more stable |
| Feed forward gain | Increase | Lower following error during acceleration |
| Torque limit | Increase | Allows higher torque to overcome resistance — masks mechanical problems |
Preventive Measures
| Measure | Frequency | Purpose |
|---|
| Check axis lubrication | Daily | Prevent guideway binding |
| Inspect way covers | Weekly | Prevent chip ingress |
| Measure backlash | Monthly | Detect ball screw wear early |
| Check coupling tightness | Monthly | Prevent loose coupling errors |
| Clean encoder and scale | Every 6 months | Prevent contamination-related errors |
| Check motor cooling fans | Monthly | Prevent motor overheating |
| Review position error alarm log | Monthly | Detect patterns before they cause failures |
FAQ
What causes axis position errors on deep hole drilling machines?
The most common causes are: mechanical binding (chip ingress into way covers, inadequate lubrication, worn guideways), loose or worn couplings between the servo motor and ball screw, encoder contamination or cable damage, and incorrect servo gain parameters. For vertical axes, hydraulic counterbalance system faults are a common cause of position drift.
How do I diagnose a following error on the feed axis?
Start by checking the CNC alarm display — note the axis and the amount of following error. Jog the axis manually at slow speed — feel for binding or roughness. Check guideway lubrication — dry ways cause increased friction and following error. Check the ball screw coupling — a loose coupling is a common cause. If the axis moves smoothly, check encoder signal and servo drive condition.
What is the difference between following error and position deviation?
Following error is the difference between the commanded position and the actual position during motion — it is a normal condition that exists whenever an axis is moving. Position deviation is the term used when the following error exceeds the programmed tolerance and triggers an alarm. A following error of 0.02–0.05 mm during feed motion is normal; a deviation of 0.20 mm or more triggers an alarm.
How do I fix backlash in the feed axis of a deep hole drilling machine?
First, measure backlash with a dial indicator — approach a reference point from both directions and read the difference. Enter the measured value as backlash compensation in the CNC parameter. If backlash is less than 0.05 mm, compensation is usually sufficient. If backlash exceeds 0.05 mm, the ball screw or nut is worn and should be replaced — compensation cannot correct large mechanical wear.
Can coolant contamination cause axis position errors?
Yes — coolant contamination is a common cause of axis position errors. Coolant can enter encoder housings through failed seals, causing erratic position signals. Coolant mixed with way oil creates a sticky residue that increases guideway friction. Chips carried by coolant into way covers cause binding. Machine tool builders put significant effort into protecting axis components from coolant — and when those protections fail, position errors follow.
Axis position errors are never just "electrical" or "mechanical" — they are system problems that require systematic diagnosis. Check the simple things first (lubrication, way covers, couplings), use CNC diagnostics to confirm findings, and repair the root cause — not the symptom. This article reflects industry practice as of 2026.
