Dental Milling Process Issues: Chipping, Fit, and Cracking — Causes and How to Fix Them

2026-07-21

Table of Contents

•When the Machine Is Fine but the Restoration Isn't
•When the Machine Is Fine but the Restoration Isn't
•1. Bur Slipping or Failing to Grip During Cutting
•2. Chipping on Zirconia Cuts
•3. Bur Breakage During Cutting
•4. Bur Breakage During Cutting
•5. Crown Fits Too Tight
•6. Surface Lines, Hairline Cracks, or Fracture
- •6a. Surface Lines on the Inner Crown Surface
- •6b. Hairline Cracks or Fracture (In the Lab or In the Mouth)
•When You Can't Solve It Alone, After-Sales Is What Matters

When the Machine Is Fine but the Restoration Isn't

Not every problem on a milled restoration is a machine fault. Chipping, an internal gap, a crown that won't seat, surface lines, hairline cracks — these usually trace back to bur selection, calibration, cutting strategy, design parameters, material shrinkage, or the sintering process, not the mill's hardware. This article walks through six of the most common issues in that category, each broken down into Problem, Cause, and Solution, so you can work out where in the workflow the problem actually started.

When the Machine Is Fine but the Restoration Isn't

1. Bur Slipping or Failing to Grip During Cutting

Problem: The bur comes loose or falls out of the chuck partway through a cutting job.

Cause:

Worn chuck — the three-prong collet has been in service a long time and the inner wall is worn.
Insufficient air pressure — pressure drops or becomes unstable during a tool change or mid-run, reducing chuck clamping force. Milling equipment requires a standard air pressure of ≥6.5 bar.
Positioning ring misalignment — the bur's positioning ring has shifted out of place.

Solution:

Remove the chuck with the proper tool, clean and re-lubricate it, then reinstall — and check that air pressure matches the manufacturer's spec at the same time.
If the positioning ring has shifted, calibrate it against a manufacturer-supplied reference bur before resuming use. If the ring itself is damaged, replace it.
After all checks are complete, recalibrate the machine and confirm the measured parameters are correct before starting work.

2. Chipping on Zirconia Cuts

Problem: Visible chipping appears along the margin or connector after milling a zirconia restoration.

Cause: This one has more variables than most, so it's worth listing all of them before troubleshooting:

Worn or damaged bur — overly thin margins in the design data accelerate wear. Burs from the same batch can still wear at different rates depending on cumulative cutting volume and run time.
Uncalibrated machine — chipping that persists after switching to a new bur usually points to a calibration deviation.
Mismatched cutting strategy or material — if calibration checks out and chipping continues, the cutting strategy settings or material compatibility need review.
Material defect — if the cutting strategy is confirmed correct but chipping appears after switching material, the material batch itself needs investigating.
Excessive undercut in the design — the undercut exceeds what the mill or the bur's working limit can handle.
Software fault — an error in the CAM software or the mill's control system produces an abnormal toolpath.

Solution: Work through the causes in this order, rather than guessing:

Check the bur first. Most labs don't change materials often, so start with the tool — replace it immediately if it's worn or damaged.

Build calibration into your routine maintenance schedule. Catching small deviations during regular checks avoids blind troubleshooting that can damage the machine — this class of equipment should only be maintained by trained personnel.

Once the bur is ruled out, work through the remaining layers in order: calibration data, then cutting strategy, then material batch — running a test cut with a known-good batch of material if needed to rule out a material issue.

3. Bur Breakage During Cutting

Problem: The bur snaps during the cutting process.

Cause:

Insufficient bur toughness — toughness is exhausted after extended cutting, and the bur breaks on its own.
Insufficient air pressure — unstable or momentarily low pressure during a tool change or mid-run reduces chuck clamping force, letting the bur loosen and break under uneven load.
Exceeding the machining limit — an oversized undercut in the design exceeds the mill's processing limit.
Excessive cutting depth — an error in the toolpath data sets cutting depth too deep, causing the bur to interfere with the ceramic block and break.
Unstable cutting strategy — disorganized or unstable toolpath parameters cause sudden swings in cutting load.
Overly deep developmental grooves in the design — groove depth exceeds the bur's working limit. This is most common with the smallest fine burs (R0.3).

Solution:

Follow standard operating procedure — know the machine's working capacity and stay within spec to avoid subjecting the bur to unexpected force.
Check bur quality first. If widespread breakage appears right after switching bur suppliers, confirm the new batch meets quality specs before looking elsewhere.
Verify before cutting — check the design data's undercut depth and developmental groove depth against the machine's and bur's rated limits.

4. Bur Breakage During Cutting

Problem: The bur snaps during the cutting process.

Cause:

Insufficient bur toughness — toughness is exhausted after extended cutting, and the bur breaks on its own.
Insufficient air pressure — unstable or momentarily low pressure during a tool change or mid-run reduces chuck clamping force, letting the bur loosen and break under uneven load.
Exceeding the machining limit — an oversized undercut in the design exceeds the mill's processing limit.
Excessive cutting depth — an error in the toolpath data sets cutting depth too deep, causing the bur to interfere with the ceramic block and break.
Unstable cutting strategy — disorganized or unstable toolpath parameters cause sudden swings in cutting load.
Overly deep developmental grooves in the design — groove depth exceeds the bur's working limit. This is most common with the smallest fine burs (R0.3).

Solution:

Follow standard operating procedure — know the machine's working capacity and stay within spec to avoid subjecting the bur to unexpected force.
Check bur quality first. If widespread breakage appears right after switching bur suppliers, confirm the new batch meets quality specs before looking elsewhere.
Verify before cutting — check the design data's undercut depth and developmental groove depth against the machine's and bur's rated limits.

5. Crown Fits Too Tight

Problem: The crown is difficult or impossible to fully seat, with excessive resistance on the model or in the mouth.

Cause:

Design or cement-space parameter set too small.
Excessive scanning error, distorting the model data.
Bur damage reducing cutting accuracy.
Incorrect zirconia shrinkage ratio setting.
Hardware error — excessive backlash in the spindle, lead screws, or other components.
Crystallization temperature too high, causing the restoration to deform.

Solution:

Check the design side first, rather than defaulting to a bur problem. Adjust parameters based on crown morphology: smaller parameters for a shallow crown with a large convergence angle, larger parameters for a tall crown with a small convergence angle, and scale up further as the number of abutment teeth increases.
For crystallization-temperature-related issues, check compliant parameters directly with the material supplier.
Run the routine checks in parallel — scanning accuracy, bur condition, and machine backlash — to rule out hardware error.

6. Surface Lines, Hairline Cracks, or Fracture

This covers two related but distinct patterns: surface lines that appear on the inner crown surface during milling, and cracks or fractures that show up later — either still in the lab or after the restoration is in the mouth.

6a. Surface Lines on the Inner Crown Surface

Problem: Visible ridge-like lines appear on the intaglio (inner) surface of the crown after milling.

Cause:

Restoration margin too thin.
Shoulder margin not smooth enough.
Cross-sectional area of the connector (in a bridge) too small.
Occluso-gingival distance of the restoration too small.

Solution:

Coordinate with the treating dentist — the shoulder should be polished during tooth preparation, keeping the restoration margin at a minimum thickness of 0.6 mm.
Optimize the design — increase lingual wall thickness and connector thickness to prevent lines caused by insufficient structural strength.

6b. Hairline Cracks or Fracture (In the Lab or In the Mouth)

Problem: A crack develops in the restoration during fabrication, or the restoration fractures after cementation.

Cause:

Reduced machine stability, causing excessive cutting vibration.
Bur damage concentrating cutting stress.
Improper technique when separating the restoration from the block, applying unnecessary force.
Impact or external force on the restoration.
Improper drying process.
Sintering ramp-up rate too fast, generating thermal stress that causes cracking.

Solution:

Keep up machine maintenance — replace worn burs promptly, calibrate the machine on a regular schedule, and follow a routine maintenance program.
Handle restorations with care — reduce spindle speed and pressing force when separating a restoration from the block.
Control the sintering process strictly to the ceramic manufacturer's sintering curve. Once the furnace reaches 1530°C and has held sufficiently, let the temperature drop back into the compliant range before removing the restoration — never force removal before the target temperature is reached or while the restoration is still hot.

When You Can't Solve It Alone, After-Sales Is What Matters

Some of these issues won't resolve with a single fix, because the actual cause sits across more than one step — a design parameter combined with a material batch, or a calibration drift combined with a cutting strategy. Working through it usually means cross-checking data with your design software and confirming parameters with your material supplier, not just adjusting one setting and re-cutting.

If you've worked through the checks above and still can't pin down the cause, or the machine itself needs attention — recalibration, a hardware repair — going to after-sales is faster and safer than continuing to troubleshoot blind. Our after-sales support includes:

Online support — responsive help for day-to-day questions
Live video guidance — a technician sees your machine in real time and walks you through the fix
Annual on-site inspections — a technician checks equipment condition on a regular schedule, catching issues before they become failures
Proactive parts supply — we help stock wear parts like burs and seals in advance, so you're not waiting on a shipment during unplanned downtime

Contact After-Sales Support

Dental Milling Machine Troubleshooting: Common Faults and How to Fix Them