{"id":970,"date":"2020-07-03T11:20:49","date_gmt":"2020-07-03T03:20:49","guid":{"rendered":"https:\/\/www.naxau.cn\/?p=970"},"modified":"2021-04-01T15:13:00","modified_gmt":"2021-04-01T07:13:00","slug":"failure-mode-analysis-of-pvd-coated-hobs","status":"publish","type":"post","link":"https:\/\/www.naxauam.com\/en\/failure-mode-analysis-of-pvd-coated-hobs\/","title":{"rendered":"Failure mode analysis of PVD coated hobs"},"content":{"rendered":"

\"Hob<\/p>\n

1. Factors affecting tool life<\/strong>
\nTool material, hardness
\nPVD coating materials and quality
\nWorkpiece material hardness and metallurgical structure
\nCutting speed and feed
\nCutting fluid types and proper use
\nScrambling method
\nHobbing Tool Clamping
\nMaximum Wear Criteria<\/p>\n

2. Wear criteria<\/strong>
\nThe 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):
\nThe allowable wear of carbide tools is 0.10 mm, and 0.15 mm is commonly used in practice.
\nTypical wear on powder metallurgical HSS is 0.2mm.
\nTypical wear on conventional HSS is 0.3-0.4mm<\/p>\n

3. Signs of excessive wear and tear<\/strong>
\nIncrease in machine power or a sharp drop in power
\nIncreased machine vibration
\nIncreased noise during cutting
\nExcessive heat is generated, increasing the temperature of the workpiece and the tool.
\nDeterioration of workpiece surface roughness
\nWorkpiece size overruns
\nIncreased burr on the tool cutting surface along the workpiece
\nExcessive tool wear measured in real life with a magnifying glass
\nKnives chipped, hand feel the edge of the uneven scraping feeling
\nChip accumulation and poor chip removal, chip tumors, sticky chips<\/p>\n

4. Schematic diagram of the failure mode of the hobs<\/strong><\/p>\n


\nWear (normal wear vs. premature wear)<\/p>\n

Boundary wear on top and side edges
\nedge and corner wear
\nSide and rear edge wear (grooves)
\nCrescent pits (wear on the front edge surface)
\nChipping, micro-chipping<\/p><\/blockquote>\n

abnormal wear<\/strong><\/p>\n

Tooth fracture
\nCracks caused by grinding the front edge surface
\nOversized crescent pits
\nCracks at stress concentration points
\naccumulated debris<\/p><\/blockquote>\n

4.1 Boundary wear of top and side edges (normal wear)<\/strong><\/p>\n

<\/p>\n

Tool damage pattern<\/strong><\/p><\/blockquote>\n

One type of side wear isWear through PVD coating<\/strong>The substrate is also abraded. Different applications allow for a normal amount of wear.<\/p>\n

Possible causes<\/strong><\/p><\/blockquote>\n

Cause 1<\/strong>: Normal wear and tear is bound to happen, it's just a matter of whether the service life meets the set targets.<\/p>\n

Cause 2<\/strong>: Adequacy of tool material, wear resistance of PVD coating.<\/p>\n

Cause 3<\/strong>: Burrs from tool resharpening, not thoroughly removed before PVD coating<\/p>\n

Possible solutions><\/strong><\/p><\/blockquote>\n

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

    4.2 Crescent Pit Wear<\/strong>
    \n<\/p>\n

    Tool damage pattern<\/strong><\/p><\/blockquote>\n

    Oversized crescent depressions close to the edge can result in a fracture-prone edge or poorer workpiece surface quality.<\/p>\n

    Possible causes<\/strong><\/p><\/blockquote>\n

    Cause 1<\/strong>Excessive cutting pressure, high temperatures, front edge corrosion, high chip thicknesses<\/p>\n

    Cause 2<\/strong>: Inadequate selection of tool materials, PVD coatings and high temperature resistance<\/p>\n

    Possible solutions<\/strong><\/p><\/blockquote>\n

    Option 1<\/strong>: Reduce speed and thus temperature<\/p>\n

    Option 2<\/strong>: Reduced feed to minimize chip thickness<\/p>\n

    Option 3<\/strong>: Positive rake angle on the front face<\/p>\n

    Scenario IV<\/strong>: Use of more heat resistant tool substrates and PVD coatings with better heat resistance (e.g. ALTiN, CrAlN, etc.)<\/p>\n

    4.2.1 Grooved wear on the rear edge surface<\/strong>
    \n<\/p>\n

    Tool damage pattern<\/strong><\/p><\/blockquote>\n

    Severe strains on the rear edge surface, generally one or two layers of composite flaking strains<\/p>\n

    Possible causes<\/strong><\/p><\/blockquote>\n

    Cause 1<\/strong>: Extra-long cutting beyond tool life, or chipping that is not detected in time to continue machining?<\/p>\n

    Cause 2<\/strong>: caused by a buildup of chips in the chipformer that can't be discharged?<\/p>\n

    Cause 3<\/strong>: Tool resharpening without full repair of the last wear?<\/p>\n

    Cause 4<\/strong>: Workpiece not clamped<\/p>\n

    Possible solutions<\/strong><\/p><\/blockquote>\n

    Option 1<\/strong>: Increase the amount of tampering<\/p>\n

    Option 2<\/strong>: Reduce the number of tool tampering cycles<\/p>\n

    Option 3<\/strong>: see chipping wear<\/p>\n

    Scenario IV<\/strong>: See Cutting Buildup<\/p>\n

    Option 5<\/strong>: Evaluating the amount of hob resharpening<\/p>\n

    Option 6<\/strong>: Increase workpiece clamping force<\/p>\n

    4.3 Chipping<\/strong><\/p>\n

    <\/p>\n

    Tool damage pattern<\/strong><\/p><\/blockquote>\n

    Small to medium-sized chipping of the cutting edge<\/p>\n

    Possible causes<\/strong><\/p><\/blockquote>\n

    Cause 1<\/strong>: Tool material is too hard and brittle<\/p>\n

    Cause 2<\/strong>: Workpiece material too hard<\/p>\n

    Cause 3<\/strong>: Insufficient rigidity and vibration during cutting<\/p>\n

    \n
    \n
    \n
    \u00a0\u00a0<\/strong>Possible solutions<\/strong><\/section>\n<\/section>\n<\/section>\n<\/blockquote>\n

    Option 1<\/strong>: Selection of tool materials for better strength<\/p>\n

    Option 2<\/strong>: Edge passivation after tool resharpening and before coating<\/p>\n

    Option 3<\/strong>: Improvement of tool strength during heat treatment<\/p>\n

    Scenario IV<\/strong>: Reduce chip thickness by decreasing the feed<\/p>\n

    Option 5<\/strong>: Inspection of tooling, addition of fixing brackets<\/p>\n

    4.4 Broken edges, broken teeth<\/strong><\/p>\n

    <\/p>\n

    Tool damage pattern<\/strong><\/p><\/blockquote>\n

    Large chipped edges, or whole teeth broken off.<\/p>\n

    Possible causes<\/strong><\/p><\/blockquote>\n

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

    Possible solutions<\/strong><\/p><\/blockquote>\n

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

    4.5 Chip buildup in the chipformer<\/strong><\/p>\n

    <\/p>\n

    Tool damage pattern<\/strong><\/p><\/blockquote>\n

    Chips from the workpiece accumulate, get stuck in the chipformer, and stick to the front edge or the back of the tooth.<\/p>\n

    Possible causes<\/strong><\/p><\/blockquote>\n

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

    Possible solutions<\/strong><\/p><\/blockquote>\n

    Option 1<\/strong>: Grinding the back of the chipformer to increase the size of the chipformer or to improve the surface roughness.
    \nOption 2<\/strong>: Transition the trapezoidal cross-section of the chipformer with a rounded arc.
    \nOption 3<\/strong>: Adjust the position and flow of the coolant nozzle or air nozzle
    \nScenario IV<\/strong>: Increase cutting time by decreasing feed rate
    \n4.6 Chip Tumor at the Edge<\/strong><\/p>\n

    <\/p>\n

    \n
    \n
    \n
    \u00a0 \u00a0Tool damage pattern<\/strong><\/section>\n<\/section>\n<\/section>\n<\/blockquote>\n

    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.<\/p>\n

    There is a risk of chipping the tool edge when the chip-accumulating tumor spalls off.<\/p>\n

    Possible causes<\/strong><\/p><\/blockquote>\n

    Cause 1<\/strong>: Workpiece materials are soft and sticky
    \nCause 2<\/strong>: Insufficient tool back angle
    \nCause 3<\/strong>: Insufficient coolant flow or wrong type<\/p>\n

    Possible solutions<\/strong><\/p><\/blockquote>\n

    Option 1<\/strong>: Increase cutting speed
    \nOption 2<\/strong>: Use of PVD coatings to reduce the coefficient of friction on tool surfaces
    \nOption 3<\/strong>: Changing workpiece material or normalizing to increase workpiece hardness
    \nScenario IV<\/strong>: Increase rear corners, evaluate dynamic rear corners
    \nOption 5<\/strong>: Increase the front corner angle of the front blade
    \nOption 6<\/strong>: Use of anti-sticky coolant<\/p>","protected":false},"excerpt":{"rendered":"

    1. Factors affecting tool life Tool material, hardness PVD coating material and quality Workpiece material hardness and metallurgical structure Cutting speed and feed Cutting fluid type 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