How to Diagnose the Real Cause of Turning Accuracy Problems
Hardinge machines are known for precision collet systems and tight concentricity. So when runout increases or part accuracy drifts, the natural question is:
Is the collet worn — or is the spindle developing instability?
Misdiagnosing this can lead to unnecessary spindle service — or worse, ignoring early spindle wear.
This guide walks through how to separate collet issues from spindle-related problems.
First: Understand What Runout Means
Runout is radial deviation from true centerline rotation.
In turning applications, even small increases in runout can affect:
- Surface finish
- Roundness
- Diameter consistency
- Tool life
- Repeatability
Because Hardinge systems are highly sensitive, small mechanical changes show up quickly.
Common Signs of Collet-Related Runout
Collet wear or damage often produces:
- Consistent runout regardless of RPM
- Visible wear marks inside the collet
- Grip inconsistencies
- Tool slipping under load
- Improvement when swapping to a new collet
If runout decreases immediately after replacing the collet, the spindle is likely stable.
Common Signs of Spindle-Related Runout
Spindle instability often shows up differently:
- Runout increases at higher RPM
- Heat rises during longer cycles
- Finish quality gradually declines
- Multiple collets show similar runout
- Stable when cold, worse when warm
Spindle-related issues are frequently RPM-sensitive or thermal in nature.
Diagnostic Steps
Step 1: Measure at the Spindle Nose
Use a dial indicator to measure:
- Spindle nose runout
- Internal taper runout
If excessive runout is present before installing tooling, the spindle may require evaluation.
Step 2: Swap Collets
Install a known-good collet.
If runout improves significantly, the original collet was likely the cause.
If runout remains unchanged, continue diagnosing.
Step 3: Check RPM Sensitivity
Run at:
- Low RPM
- Normal RPM
- Higher RPM
If runout or vibration increases with speed, spindle-related instability is more likely.
Step 4: Monitor Thermal Behavior
If runout increases after warm-up, bearing preload or thermal growth may be affecting stability.
Collet wear alone typically does not cause heat-related drift.
Why Spindle Preload Matters
Hardinge turning spindles rely on precise bearing preload to maintain stiffness.
As preload shifts due to wear:
- Radial stiffness decreases
- Micro-movement increases
- Runout increases under load
Preload degradation often appears gradually.
When It’s Likely the Collet
- Immediate improvement after replacement
- No RPM sensitivity
- No heat increase
- No vibration change
When It’s Likely the Spindle
- RPM-sensitive instability
- Heat increase during production
- Consistent runout across multiple collets
- Gradual finish degradation
- Narrowing stable speed range
Why Early Diagnosis Matters
Running a spindle with growing instability can lead to:
- Bearing failure
- Taper wear
- Shaft damage
- Expanded rebuild scope
Replacing collets repeatedly without improvement often delays necessary spindle service.
Final Thought
Hardinge collet systems are extremely precise — but they depend on spindle stability.
If runout changes with speed or temperature, the spindle may be signaling early wear.
If replacement collets solve the issue immediately, tooling was likely the cause.
Accurate diagnosis protects both uptime and repair cost.