VMC Cutting Tool Geometry and Considerations

 

VMC Cutting Tool Geometry and Considerations

The precise geometry of cutting tools is paramount in Vertical Machining Centers (VMCs) to achieve optimal material removal, surface finish, and tool life. Each tool type is designed with specific geometric features to perform its intended operation efficiently.  

Cutting Tool Geometry

1. Face Mill: Face mills are primarily used for machining flat surfaces. Their geometry focuses on efficient chip evacuation and good surface finish.  

• Axial Rake Angle: The angle between the tool face and a plane perpendicular to the axis of rotation, measured in the axial direction. Positive axial rake angles reduce cutting forces and promote smoother cutting.
• Radial Rake Angle: The angle between the tool face and a radius passing through the cutting edge, measured in the radial direction. Positive radial rake angles improve chip flow and reduce heat generation.  
• Lead Angle (or Entering Angle): The angle between the main cutting edge and the workpiece surface. A larger lead angle (e.g., 45-degree or 90-degree) influences chip thinning and impact on the insert.  
• Clearance Angle (Relief Angle): The angle between the relief surface of the tool and the machined surface of the workpiece. Ensures that only the cutting edge contacts the workpiece, preventing rubbing.  
• Corner Radius/Wiper Flat: A rounded edge at the corner of the insert (corner radius) or a small flat section (wiper flat) helps improve surface finish.  

2. End Mill: End mills are versatile tools used for a variety of operations including profiling, slotting, and pocketing.  

• Helix Angle: The angle at which the cutting edges are twisted around the tool axis. A higher helix angle (e.g., 30-45 degrees) provides smoother cutting action, better chip evacuation, and reduced cutting forces. Lower helix angles are stronger for harder materials.  
• Flute Count: The number of cutting edges (flutes) on the tool. More flutes provide a better finish and are stiffer, but reduce chip space. Fewer flutes offer more chip space for roughing.
• Rake Angle: Similar to face mills, positive rake angles are common for efficient cutting.
• Clearance Angle: Ensures the non-cutting part of the tool does not rub against the workpiece.  
• Core Diameter: The diameter of the tool's core, which affects its strength and chip evacuation.  

3. Drill: Drills are designed to create cylindrical holes.  

• Point Angle: The angle between the cutting edges at the tip of the drill. Common angles are 118 degrees for general purpose drilling and 135 degrees for harder materials (which offers better centering and reduces thrust force).
• Lip Relief Angle: The angle behind the cutting lips that provides clearance as the drill penetrates the material.
• Helix Angle: Similar to end mills, this angle influences chip evacuation and thrust force. Standard helix angles are common, while high helix drills are used for softer materials and low helix for harder materials.  
• Chisel Edge: The non-cutting edge at the very center of the drill point. Its length and web thickness influence the thrust force.
• Margin: A narrow land along the cutting edge that provides stability and guides the drill in the hole.  

4. Countersink: Countersinks create conical holes for screw heads or to deburr holes.  

• Included Angle: The total angle of the conical tip, typically 60, 82, 90, or 120 degrees, matching the angle of the screw head.
• Flute Count: Typically 1 to 6 flutes. More flutes offer a smoother finish and less chatter.
• Rake and Clearance Angles: Designed to provide efficient cutting and chip evacuation for the conical shape.

5. Tap: Taps are used to cut internal threads.  

• Thread Form: Defined by parameters like pitch, major diameter, minor diameter, and pitch diameter, adhering to standards like ISO Metric, Unified National, etc.
• Chamfer (Taper) Length: The number of threads at the start of the tap that are ground with a lead. This helps in starting the tap and distributes the cutting load. Common chamfers include taper (8-10 threads), plug (3-5 threads), and bottoming (1-2 threads).
• Flute Count: The number of flutes influences chip evacuation and strength. Straight flutes are common, while spiral flutes (for through holes) and spiral point (for blind holes) are used for better chip control.  
• Rake Angle: Positive rake for free-cutting materials, negative rake for harder materials.
• Relief Angle: Provides clearance behind the cutting edge.  

6. Finish Bore: Finish boring tools are designed for achieving high precision and surface finish in existing holes.  

• Single Point or Two-Point: Can be single-point for ultimate precision or two-point for faster material removal with good finish.
• Rake Angle: Often positive for efficient chip formation and reduced cutting forces.
• Clearance Angle: Critical for preventing rubbing and maintaining dimensional accuracy.
• Tool Nose Radius: A small radius at the cutting edge is crucial for achieving good surface finish and extending tool life.
• Adjustability: Many finish boring tools are adjustable for precise diameter control.  

7. Reamer: Reamers are used to enlarge and finish existing holes to precise dimensions and surface finishes.  

• Chamfer Angle: The lead-in angle at the front of the reamer that guides it into the hole and performs the initial cutting.
• Land: The narrow strip behind the cutting edge that provides support and guides the reamer.
• Margin: A narrow land along the cutting edge, similar to drills.
• Flute Count: Typically more flutes (e.g., 6-12) than drills for better stability, roundness, and surface finish.
• Helix Angle: Straight flutes are common, but spiral flutes are used for better chip evacuation and smoother cutting in some materials.
• Back Taper: A slight reduction in diameter towards the shank to prevent rubbing.

Insert Holding Methods and Types

1. Face Mill Insert Holding Methods: Face mill inserts are almost exclusively held mechanically due to the high cutting forces and need for precision indexing.

• Clamp Type: A top clamp presses down on the insert, holding it securely against the pocket.
• Screw Type: A screw passes through a central hole in the insert and threads into the tool body. This provides a very secure hold and allows for smaller tool bodies.
• Wedge Clamp: A wedge is driven behind the insert, forcing it against the pocket walls.
• Lever Lock: A lever mechanism expands within the insert hole, clamping it in place.

2. End Mill Insert Types: While solid carbide end mills are common, insertable end mills offer versatility and cost-effectiveness for larger diameters and certain applications.  

• Square Shoulder Inserts: Used for 90-degree shoulders, slotting, and profiling. Common geometries include APKT, ADMX, etc.
• Ball Nose Inserts: Feature a full radius for 3D contouring and surfacing.  
• High Feed Inserts: Designed with a specific geometry that allows for very high feed rates with a small depth of cut, ideal for roughing.  
• Button Inserts (Round Inserts): Used for profiling and roughing applications where strength and chip thinning are advantageous.
• Roughing Inserts: Robust inserts designed for high material removal rates.  
• Finishing Inserts: Inserts with geometries optimized for surface finish.

3. Drill Insert Types: Insertable drills are widely used for efficient hole making, especially in larger diameters and specific materials.

• U-Drill Inserts (SPT/SPG/SPM/SPX etc.): Designed for specific pockets in U-drills, which utilize two inserts (one outer, one inner) to cut the hole. These inserts have specific chip breaker geometries.
• Indexable Drill Inserts (SumoDrill/CoroDrill etc.): Inserts with specific geometries designed for various materials and drilling conditions, often featuring advanced chip breaker designs.  
• Solid Carbide Inserts (Brazed or Exchangeable Head): While not "insertable" in the traditional sense, some drills use brazed carbide tips or exchangeable solid carbide heads for specific applications.

Insert Cutting Edge Geometry

Insert cutting edge geometry refers to the specific features of the cutting edge itself, influencing chip formation, heat generation, and tool life.  

• Chip Breaker: Grooves, steps, or other features on the rake face of the insert designed to curl, break, and control the chips. Different chip breaker geometries are optimized for various materials (e.g., tough, brittle, gummy) and cutting conditions (e.g., roughing, finishing).  
• Honing/Chamfer: A small radius or chamfer applied to the cutting edge for strength and to prevent micro-chipping. A stronger edge (larger hone or chamfer) is better for interrupted cuts or tough materials, while a sharper edge is better for lower cutting forces and better finish.  
• Wiper Geometry: A flat section or a larger radius on the trailing edge of the insert (often found on face mill inserts) to improve surface finish by acting like a wiper.
• Corner Radius (RE): The radius at the corner of the insert. It strengthens the cutting edge, influences surface finish, and distributes cutting forces.  
• Relief Angle: The angle of the flank surface below the cutting edge that prevents rubbing. Different relief angles (e.g., 7 degrees for C-style, 11 degrees for D-style inserts) are standardized.
• Rake Angle: The angle of the rake face, influencing cutting forces and chip formation. Can be positive, negative, or neutral.  

Cutting Parameters

Cutting parameters are critical in determining machining efficiency, tool life, and surface finish.  

1. Cutting Speed (Vc):

• Definition: The speed at which the cutting edge passes over the workpiece material, typically measured in meters per minute (m/min) or surface feet per minute (sfm).  
• Formula: $V_c=\frac{\pi \times D\times N}{1000}$ (for mm/min) or $V_c=\frac{\pi \times D\times N}{12}$ (for inches/min)  
  • D: Tool diameter (mm or inches)
  • N: Spindle speed (RPM)
• Effect: Directly impacts cutting temperature, tool wear, and machining time. Higher cutting speeds generally lead to higher material removal rates but also higher temperatures and faster tool wear.  

2. Feed Rate (f or Vf):

• Definition: The distance the cutting tool advances into the workpiece during one revolution (for rotating tools) or per minute.
  • Feed per tooth (fz): For multi-flute tools like end mills and face mills, it's the distance the tool advances for each tooth.
  • Feed per revolution (fn): For tools like drills, reamers, and boring bars, it's the distance the tool advances for each full rotation.
  • Feed per minute (Vf): The linear speed of the tool relative to the workpiece, measured in mm/min or inches/min.
• Formulas:
  • $V_f=f_z\times Z\times N$ (for end mills/face mills)
  • $V_f=f_n\times N$ (for drills/reamers/boring bars)
  • Z: Number of teeth/flutes
  • N: Spindle speed (RPM)
• Effect: Influences chip thickness, surface finish, and cutting forces. Higher feed rates generally lead to higher material removal rates but can degrade surface finish and increase cutting forces.  

3. Depth of Cut (ap and ae):

• Definition: The amount of material removed by the cutting tool in a single pass.
  • Axial Depth of Cut (ap): The depth measured parallel to the tool axis (e.g., for slotting with an end mill or down the side of a workpiece with a face mill).  
  • Radial Depth of Cut (ae): The depth measured perpendicular to the tool axis (e.g., for side milling with an end mill or the width of cut for a face mill).  
• Effect: Directly impacts material removal rate, cutting forces, and heat generation. A larger depth of cut increases material removal but also puts more stress on the tool and can generate more heat.  

Tool Wear and Tool Life

Tool Wear: The gradual degradation of the cutting tool's geometry and material due to mechanical and thermal stresses during machining. Common types of tool wear include:

• Flank Wear (Wear Land): Occurs on the relief surface of the tool, behind the cutting edge. It's typically measured as a wear land width (VB) and is the most common and predictable form of wear.
• Crater Wear: Occurs on the rake face of the tool, where the chip flows over the surface. Caused by high temperatures and chemical reactions between the chip and the tool material.  
• Notch Wear: Localized wear at the depth of cut line, often seen in machining work-hardened materials or when chip flow is constrained.  
• Chipping/Fracture: Small pieces of the cutting edge break off, often due to brittle tool materials, excessive cutting forces, or vibrations.
• Built-Up Edge (BUE): Workpiece material adheres to the rake face of the tool, changing its effective geometry and causing poor surface finish. Common in machining soft, ductile materials.  
• Thermal Cracking: Cracks that form due to rapid heating and cooling cycles, especially in interrupted cutting.  
• Plastic Deformation: The cutting edge deforms under high temperatures and stresses, leading to a loss of sharpness.  

Tool Life: The duration for which a cutting tool can effectively perform its intended function before reaching a predetermined wear criterion (e.g., a specific flank wear width, complete failure, or unacceptable surface finish). Tool life is typically measured in terms of:

• Cutting Time: The actual time the tool is in contact with the workpiece.
• Number of Parts: The quantity of parts machined before replacement.
• Volume of Material Removed: The total volume of material removed.

Tool life is a critical factor in machining economics, as it directly impacts production costs and efficiency.  

Relative Effect of Each Cutting Parameter on Tool Life

The relationship between cutting parameters and tool life is often described by the modified Taylor Tool Life Equation:

$V{T}^n{f}^x{a}_p^y=C$

Where:

• V: Cutting Speed
• T: Tool Life
• f: Feed Rate
• ap​: Depth of Cut
• n,x,y: Exponents determined experimentally for specific tool-workpiece combinations.
• C: Constant

The relative effect of each parameter on tool life is generally:

1. Cutting Speed (Vc):

   • Most Significant Effect: Cutting speed has the most dominant impact on tool life. Even a small increase in cutting speed can lead to a substantial decrease in tool life. This is because higher cutting speeds generate significantly more heat at the cutting zone, accelerating wear mechanisms (especially crater wear and plastic deformation).  
   • Impact: Doubling the cutting speed can reduce tool life by a factor of 4 to 10 or more, depending on the tool and workpiece material.

2. Feed Rate (f):

   • Significant Effect: Feed rate has a notable impact on tool life, but less pronounced than cutting speed. Increasing the feed rate increases chip thickness and cutting forces, leading to higher stress on the cutting edge and increased wear.  
   • Impact: Doubling the feed rate might reduce tool life by a factor of 2 to 4. Higher feed rates can also lead to flank wear and chipping due to increased mechanical load.  

3. Depth of Cut (ap):

   • Least Significant Effect: Within reasonable limits, the depth of cut typically has the least impact on tool life compared to cutting speed and feed rate. While a larger depth of cut increases the volume of material removed and overall cutting forces, the localized stress and temperature at the cutting edge are primarily governed by cutting speed and feed per tooth/revolution.
   • Impact: Doubling the depth of cut might reduce tool life by a factor of 1.5 to 2. However, excessive depth of cut can lead to tool breakage or significant deflection.

In summary, when optimizing machining processes for tool life, the focus is primarily on controlling cutting speed, followed by feed rate, and then depth of cut. Understanding and applying these principles are crucial for maximizing efficiency and profitability in VMC operations.