CNC Cutting Tool Materials and Applications

CNC Cutting Tool Materials and Applications

The choice of cutting tool material is paramount for achieving good surface finish, long tool life, and efficient material removal rates.¹ Here's a look at some common materials:

• High-Speed Steel (HSS):
  • Composition: A ferrous alloy containing tungsten, molybdenum, chromium, vanadium, and sometimes cobalt.²
  • Properties: Relatively tough, wear-resistant, can be sharpened easily, and performs well at moderate cutting speeds.³
  • Applications: General-purpose machining, drilling, tapping, milling, and turning, especially for lower production volumes or when machining softer materials like aluminum and some steels.⁴ It's also used for form tools and applications where complex geometries are needed.⁵
• Carbides (Cemented Carbides):
  • Composition: Composite materials made of hard carbide particles (like tungsten carbide, titanium carbide, or tantalum carbide) bonded together with a metallic binder (usually cobalt or nickel).⁶
  • Properties: Very high hardness and wear resistance, can withstand higher cutting speeds and temperatures than HSS. Different grades exist, optimized for various materials and cutting conditions.⁷
  • Applications: Widely used for turning, milling, drilling, grooving, and threading of a broad range of materials, including steels, cast irons, non-ferrous metals, and even some superalloys. Coated carbides offer even better performance.⁸
• Ceramics:
  • Composition: Non-metallic inorganic compounds like aluminum oxide (alumina), silicon nitride, or sialon.
  • Properties: Extremely high hardness and wear resistance, excellent hot hardness (retains hardness at high temperatures), and good chemical inertness.⁹ However, they are generally less tough than carbides and more brittle.
  • Applications: High-speed machining of cast iron, hardened steels, superalloys, and abrasive materials. Often used for finishing operations where high surface quality is required.¹⁰
• Cermets (Ceramic-Metallic Composites):
  • Composition: Composites combining ceramic materials (like titanium carbide or titanium nitride) with metallic binders (like nickel or cobalt).
  • Properties: Offer a good balance of hardness, wear resistance, and toughness, often bridging the gap between carbides and ceramics. They have good resistance to built-up edge.
  • Applications: Finishing and semi-finishing of steels, stainless steels, and cast irons at moderate to high cutting speeds.
• Polycrystalline Diamond (PCD):
  • Composition: Synthetically produced diamond crystals bonded together.
  • Properties: Exceptionally high hardness and wear resistance, very high thermal conductivity. However, it's brittle and cannot be used for machining ferrous materials due to a chemical reaction at high temperatures.
  • Applications: Machining highly abrasive non-ferrous materials like aluminum alloys with high silicon content, copper, brass, composites, and plastics.
• Cubic Boron Nitride (CBN):
  • Composition: Synthetically produced crystals of boron and nitrogen.
  • Properties: Second only to diamond in hardness, excellent hot hardness and chemical stability.¹¹ Suitable for machining ferrous materials.
  • Applications: Machining hardened steels (above 45 HRC), cast irons, superalloys, and hard facing alloys at high cutting speeds.

Cutting Tool Geometry and Insert Holding Methods

Let's break down the geometry and holding methods for the specific operations you mentioned:

1. Internal and External Turning

• Cutting Tool Geometry:
  • Nose Radius (r): The radius at the cutting edge tip. Influences surface finish and tool strength. Smaller radius for better finish, larger for higher feed rates and strength.
  • Lead Angle (or Approach Angle, $\kappa$): The angle between the major cutting edge and the direction of feed. Affects chip thickness, cutting forces, and vibration.
  • Side Cutting Edge Angle (${\gamma }_s$): The angle between the side cutting edge and the shank of the tool.
  • End Cutting Edge Angle (${\gamma }_e$): The angle between the end cutting edge and the shank of the tool.
  • Back Rake Angle (${\alpha }_b$): The angle between the tool face and a plane perpendicular to the cutting direction.¹² Influences chip flow and cutting forces. Positive rake for softer materials, negative for harder materials.
  • Side Rake Angle (${\alpha }_s$): The angle between the tool face and a plane parallel to the cutting direction.¹³
  • End Relief Angle (${\theta }_e$): The angle between the end flank and the machined surface. Prevents rubbing.
  • Side Relief Angle (${\theta }_s$): The angle between the side flank and the machined surface. Prevents rubbing.
• Insert Holding Methods:
  • Clamping (Top Clamp): A screw or lever mechanism presses the insert against the tool holder.¹⁴ Provides good stability and chip evacuation. Common for general turning.
  • Lever Lock: A lever applies pressure to the insert, often with a pin or wedge for secure locking. Suitable for heavier cuts.
  • Pin Lock: A pin inserted through a hole in the insert and secured in the holder. Offers good accessibility.
  • Screw-on: The insert is directly screwed onto the tool holder. Simple and rigid.
  • Wedge Clamp: A wedge is driven in to secure the insert. Provides strong clamping force.

2. Grooving

• Cutting Tool Geometry:
  • Groove Width (w): The width of the cutting edge, determines the width of the groove.
  • Groove Depth Capability: The maximum depth the tool can cut.
  • Side Clearance Angles: Angles on the sides of the insert to prevent rubbing against the groove walls.
  • Front Clearance Angle: Angle on the front of the insert.
  • Rake Angles (Front and Side): Influence chip formation and cutting forces.¹⁵ Often neutral or slightly positive.
• Insert Holding Methods:
  • Top Clamp: Similar to turning, a clamp holds the insert from the top.
  • Screw-on: The insert is directly screwed onto a specialized grooving tool holder.
  • Dovetail Clamping: The insert has a dovetail shape that fits into a matching slot in the holder, providing excellent stability for radial forces.

3. Threading

• Cutting Tool Geometry:
  • Thread Profile Angle: The angle between the thread flanks (e.g., 60° for metric and unified threads).
  • Pitch (p): The distance between adjacent thread crests.
  • Nose Radius: Radius at the thread crest and root.
  • Clearance Angles: Angles to prevent rubbing on the thread flanks.
• Insert Holding Methods:
  • Top Clamp: Common for external threading.¹⁶
  • Screw-on: Often used for internal threading bars and some external threading holders.
  • Indexable Cartridges: Threading inserts are often held in cartridges that can be easily indexed or replaced on the main tool holder.¹⁷

4. Face Grooving

• Cutting Tool Geometry:
  • Similar to external grooving, but the tool is designed to cut grooves on the face of the workpiece.¹⁸
  • Groove Width: Determines the width of the face groove.
  • Groove Depth Capability: The maximum radial depth.
  • Side Clearance Angles: Important to prevent rubbing as the tool moves radially.
  • Front Clearance Angle: Angle on the leading edge.
  • Rake Angles: Influence chip formation.¹⁹
• Insert Holding Methods:
  • Top Clamp: A common method for face grooving tools.
  • Screw-on: Direct screw mounting for stability.

5. Drilling

• Cutting Tool Geometry (for indexable insert drills):
  • Point Angle ($\beta$): The angle between the two main cutting edges at the drill point. Influences chip formation and centering. Common angles are 90°, 118°, 135°, etc.
  • Lip Angle (or Chisel Edge Angle): The angle of the chisel edge relative to the cutting lips.²⁰ Affects thrust forces.
  • Helix Angle ($\lambda$): The angle of the flutes relative to the drill axis. Influences chip evacuation.
  • Rake Angle: Angle on the cutting lips.²¹
  • Clearance Angle: Angle behind the cutting lips to prevent rubbing.
• Insert Holding Methods:
  • Screw-on: Inserts are typically screwed directly into pockets on the drill body.²² Different insert shapes are used for the center and periphery of the drill to optimize cutting action.

Insert Cutting Edge Geometry

The cutting edge itself has crucial features:

• Honing (or Edge Preparation): A controlled rounding or chamfering of the cutting edge. Increases edge strength and reduces chipping, especially for interrupted cuts or harder materials.²³ Common types include:
  • T-land with Honing: A flat land followed by a radius.
  • S-land: A negative land.
  • Radius Honing: A simple rounding of the edge.
  • Chamfer: An angled flat surface.²⁴
• Chipbreaker Geometry: Features on the insert face designed to control chip formation and evacuation.²⁵ Different designs are optimized for various materials and cutting conditions (e.g., flat top, with a groove, with protrusions). They help break long, stringy chips into smaller, manageable pieces.
• Coating: Thin layers of hard, wear-resistant materials (like TiN, TiCN, Al₂O₃, PVD diamond, etc.) applied to the insert surface to improve hardness, reduce friction, and increase tool life.²⁶ The choice of coating depends on the workpiece material and cutting conditions.

This overview provides a foundational understanding of CNC cutting tool materials, their applications, the geometry involved in various machining operations, insert holding methods, and the crucial aspects of the cutting edge geometry. The specific selection of tool material, geometry, and holding method will depend heavily on the workpiece material, the desired surface finish, the required tolerances, the machine tool capabilities, and the overall economics of the machining process.