Failure mode analysis of PVD coated hobs

1. Factors affecting tool life
Tool material, hardness
PVD coating materials and quality
Workpiece material hardness and metallurgical structure
Cutting speed and feed
Cutting fluid types and proper use
Scrambling method
Hobbing Tool Clamping
Maximum Wear Criteria

2. Wear criteria
The maximum allowable tool wear should be established and the tool should be resharpened promptly after the tool wear reaches the standard. Recommended values are as follows (for reference only):
The allowable wear of carbide tools is 0.10 mm, and 0.15 mm is commonly used in practice.
Typical wear on powder metallurgical HSS is 0.2mm.
Typical wear on conventional HSS is 0.3-0.4mm

3. Signs of excessive wear and tear
Increase in machine power or a sharp drop in power
Increased machine vibration
Increased noise during cutting
Excessive heat is generated, increasing the temperature of the workpiece and the tool.
Deterioration of workpiece surface roughness
Workpiece size overruns
Increased burr on the tool cutting surface along the workpiece
Excessive tool wear measured in real life with a magnifying glass
Knives chipped, hand feel the edge of the uneven scraping feeling
Chip accumulation and poor chip removal, chip tumors, sticky chips

4. Schematic diagram of the failure mode of the hobs

Wear (normal wear vs. premature wear)

Boundary wear on top and side edges
edge and corner wear
Side and rear edge wear (grooves)
Crescent pits (wear on the front edge surface)
Chipping, micro-chipping

abnormal wear

Tooth fracture
Cracks caused by grinding the front edge surface
Oversized crescent pits
Cracks at stress concentration points
accumulated debris

4.1 Boundary wear of top and side edges (normal wear)

Tool damage pattern

One type of side wear isWear through PVD coatingThe substrate is also abraded. Different applications allow for a normal amount of wear.

Possible causes

Cause 1: Normal wear and tear is bound to happen, it's just a matter of whether the service life meets the set targets.

Cause 2: Adequacy of tool material, wear resistance of PVD coating.

Cause 3: Burrs from tool resharpening, not thoroughly removed before PVD coating

Possible solutions>

• Option 1: Reduce the speed and thus the temperature.
• Option 2: Use of more wear-resistant tool substrates and PVD coatings.
• Option 3: Control tool resharpening to minimize and mitigate burrs; PVD coating pre-treatment to remove burrs thoroughly.

4.2 Crescent Pit Wear

Tool damage pattern

Oversized crescent depressions close to the edge can result in a fracture-prone edge or poorer workpiece surface quality.

Possible causes

Cause 1Excessive cutting pressure, high temperatures, front edge corrosion, high chip thicknesses

Cause 2: Inadequate selection of tool materials, PVD coatings and high temperature resistance

Possible solutions

Option 1: Reduce speed and thus temperature

Option 2: Reduced feed to minimize chip thickness

Option 3: Positive rake angle on the front face

Scenario IV: Use of more heat resistant tool substrates and PVD coatings with better heat resistance (e.g. ALTiN, CrAlN, etc.)

4.2.1 Grooved wear on the rear edge surface

Tool damage pattern

Severe strains on the rear edge surface, generally one or two layers of composite flaking strains

Possible causes

Cause 1: Extra-long cutting beyond tool life, or chipping that is not detected in time to continue machining?

Cause 2: caused by a buildup of chips in the chipformer that can't be discharged?

Cause 3: Tool resharpening without full repair of the last wear?

Cause 4: Workpiece not clamped

Possible solutions

Option 1: Increase the amount of tampering

Option 2: Reduce the number of tool tampering cycles

Option 3: see chipping wear

Scenario IV: See Cutting Buildup

Option 5: Evaluating the amount of hob resharpening

Option 6: Increase workpiece clamping force

4.3 Chipping

Tool damage pattern

Small to medium-sized chipping of the cutting edge

Possible causes

Cause 1: Tool material is too hard and brittle

Cause 2: Workpiece material too hard

Cause 3: Insufficient rigidity and vibration during cutting

  Possible solutions

Option 1: Selection of tool materials for better strength

Option 2: Edge passivation after tool resharpening and before coating

Option 3: Improvement of tool strength during heat treatment

Scenario IV: Reduce chip thickness by decreasing the feed

Option 5: Inspection of tooling, addition of fixing brackets

4.4 Broken edges, broken teeth

Tool damage pattern

Large chipped edges, or whole teeth broken off.

Possible causes

Cause 1: Excessive shock loads
Cause 2: Machine collisions, malfunctions, workpiece slippage
Cause 3: Pre-use handling damage
Cause 4: Extension of crescent pits to the edge leading to edge breakage
Cause 5: Cracking during resharpening
Cause 6: Stress concentration

Possible solutions

Option 1: Reduce speed to reduce temperature and feed to reduce chip thickness.
Option 2: Reduced tool rake angle for lower cutting resistance
Option 3: Use of tool materials with better impact resistance
Scenario IV: Enhancement of regrinding process control
Option 5: Rounded transitions to stress concentrations in the joints

4.5 Chip buildup in the chipformer

Tool damage pattern

Chips from the workpiece accumulate, get stuck in the chipformer, and stick to the front edge or the back of the tooth.

Possible causes

Cause 1: Inadequate chipformer size
Cause 2: Trapezoidal chipformer jammed with swarf
Cause 3: Poor surface quality at the front and back of the chipformer
Cause 4: Insufficient coolant or air-cooled air flow
Cause 5: The amount of chips discharged is greater than the capacity of the chip conveyor.

Possible solutions

Option 1: Grinding the back of the chipformer to increase the size of the chipformer or to improve the surface roughness.
Option 2: Transition the trapezoidal cross-section of the chipformer with a rounded arc.
Option 3: Adjust the position and flow of the coolant nozzle or air nozzle
Scenario IV: Increase cutting time by decreasing feed rate
4.6 Chip Tumor at the Edge

   Tool damage pattern

The workpiece material sticks and cold welds to the cutting edge of the tool and forms a new cutting edge, and the irregularity of the new cutting edge causes poor surface quality of the workpiece.

There is a risk of chipping the tool edge when the chip-accumulating tumor spalls off.

Possible causes

Cause 1: Workpiece materials are soft and sticky
Cause 2: Insufficient tool back angle
Cause 3: Insufficient coolant flow or wrong type

Possible solutions

Option 1: Increase cutting speed
Option 2: Use of PVD coatings to reduce the coefficient of friction on tool surfaces
Option 3: Changing workpiece material or normalizing to increase workpiece hardness
Scenario IV: Increase rear corners, evaluate dynamic rear corners
Option 5: Increase the front corner angle of the front blade
Option 6: Use of anti-sticky coolant