What is Turning in CNC Machining?(magnesium vs titanium Lisa)

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Turning is a machining process used to create rotational, symmetrical parts by removing material from the outside diameter of a workpiece. Turning is performed on a lathe, which holds the workpiece while it rotates at high speeds. A cutting tool is fed into the workpiece, removing material in the form of chips to create the desired shape.
Turning is one of the most common and oldest machining processes, dating back thousands of years. It is an essential process for manufacturing round parts and shapes like cylinders, cones, spheres, shafts, and disks. Modern CNC lathes automate turning operations for precision, efficiency, and repeatability.
How Does Turning Work?
The basic turning process on a lathe involves rotating the workpiece at high speeds while gradually feeding a single-point cutting tool into the material. The cutting tool removes excess material as chips, leaving behind the desired shape. Here are the main components and steps involved:
- Lathe: The lathe holds and rotates the workpiece at controlled speeds and feeds. CNC lathes are programmed with machining instructions.
- Chuck: The chuck, typically three- or four-jaw, grips the workpiece and centers it along the lathe axis of rotation. Collet chucks provide even tighter concentricity.
- Cutting tool: Made of hard materials like high-speed steel or carbide, the cutting tool cuts against the workpiece to remove material. Common tool shapes are square, triangle, and round.
- Tool post: The tool post holds and positions the cutting tool in relation to the workpiece. The tool can be fed manually or under CNC control.
- Tailstock: The tailstock applies pressure to the workpiece from the opposite end for added turning support and precision.
- Coolant: Coolant is applied to reduce heat and flush away chips. Common coolants are oils, water-based fluids, and air.
- Feed rate: The distance the tool travels into the spinning workpiece per revolution. The optimal feed rate depends on tool shape, workpiece material, and surface finish requirements.
- Depth of cut: The depth, or radial distance, that the tool penetrates the workpiece per pass. Several passes spread the total depth of cut.
- Speeds/Feeds: The cutting speed and feed rates are set based on the workpiece material, tool material, tool shape, depth of cut, and more. Rigid machine setups allow for faster speeds and feeds.
CNC Turning vs Manual Turning
Both CNC and manual lathes perform the turning process, but there are some key differences:
- CNC turning is automated by programmed machine code. Manual turning relies on human control and expertise.
- CNC offers more repeatability, precision, and consistency. Manual allows for adjusting on the fly but depends on the operator.
- CNC turning is faster and can run unattended. Manual turning is slower but gives the operator more control.
- CNC machines can perform complex geometry and external/internal features. Manual turning is limited to simpler geometries.
- CNC requires more upfront programming knowledge. Manual leverages hands-on operator skill.
- CNC turning is better suited for complex, high-volume production. Manual turning can be used for small batches and one-offs.
- The initial CNC machine investment is higher. Manual lathes are more affordable for small shops.
For complex parts turned in large volumes, CNC turning is usually the better choice. Manual turning may be preferable for simpler parts produced in smaller quantities.
Turning Operations
Different turning operations are possible depending on the features required in the workpiece:
- Facing: Machining a flat surface on the face of the part, perpendicular to the axis of rotation. Performs an ideal locating surface.
- Straight turning: Machining the outside diameter of a cylindrical workpiece to a straight dimension. The most common turning operation.
- Taper turning: Machining a tapered or angled diameter on the workpiece. Done by offsetting the tool or tailstock.
- Grooving: Cutting narrow grooves or recess into the workpiece surface. Performed to create seals, parting lines, and threading relief.
- Parting: Separating a finished workpiece from the excess stock by cutting entirely through it. Parting tools have thin blades.
- Drilling: Removing material to create deep, circular holes using a rotating drill tool. Performed on the center axis of the workpiece.
- Boring: Enlarging or smoothing an existing hole in the workpiece. Requires a single-point boring tool.
- Threading: Using a tool to cut helical threads on the inner or outer workpiece diameter. Many thread forms are possible.
- Knurling: Creating a textured crosshatch pattern on the workpiece surface using a knurling tool. Provides an easier grip on handles.
- Form turning: Shaping irregular external contours using a CNC-controlled form tool. The tool mimics the shape of the profile.
- Internal turning: Machining the inner surfaces and diameters of the workpiece. Requires small, specialized tools to reach the internal dimensions.
Workholding for Turning
Proper workholding is critical for accurate and safe turning operations. The workpiece must be securely clamped to avoid slipping and maintain precise concentricity. Here are common workholding methods:
- Three-jaw chuck: The most common. Three jaws clamp the workpiece by its outside or inside diameter. Limited accuracy but quick to load.
- Four-jaw chuck: Uses four independent jaws to clamp on all sides. Allows for precision clamping of irregular shapes. Time-consuming to indicate in.
- Collet chuck: Collects offer the most accuracy and clamping force. Available in a range of sizes and styles to match different workpieces.
- Faceplate: A large steel plate that mounts directly to the spindle. Workpieces can be bolted or welded to the faceplate for rigid turning of complex shapes.
- Mandrel: A tapered shaft that fits into the lathe spindle. Small parts can be loaded onto the mandrel for turning. Requires tailstock support.
- Between centers: The workpiece is supported between the headstock drive center and tailstock live center for no clamping force. Common for long, thin parts.
- Steady rest: Provides additional support for long, slender workpieces during turning operations. Prevents bending and vibration.
Proper workholding selection minimizes workpiece deflection and runout. This allows for ideal turning precision, finish, and safety.
Turning Tool Materials
The cutting tool material impacts tool life, cutting speeds/feeds, and finish in turning applications. Common tool materials include:
- High-Speed Steel (HSS): Durable all-around material for both roughing and finishing. Low cost but lower speeds than other materials.
- Cobalt Steel: Similar properties as HSS with higher heat resistance allowing faster speeds. More brittle than HSS.
- Carbide: Extremely hard with high heat/wear resistance for high speeds/feeds. More expensive and brittle. Risk of chipping.
- Ceramics: The hardest and most heat/wear resistant tool material. Used for high production applications at very high speeds. Brittle with risk of fracturing.
- Cubic Boron Nitride (CBN): Second hardest material optimal for cutting steels/cast irons without coolant. Low friction and heat generation. Very expensive.
- Diamond: The ultimate turning tool material. Extreme hardness/heat resistance allows ultra-high speeds and longest tool life. Used for non-ferrous materials and carbon composites.
The ideal tool material pairs cost, tool life, cutting performance, and workpiece finish based on the application requirements. Beyond material, tool coating technology also improves turning performance.
Turning Applications
Turning produces rotational, axisymmetric components across many applications, including:
- Automotive: Crankshafts, camshafts, wheel hubs, drive shafts, axle pins, pulleys, bushings, and engine valves.
- Aerospace: Turbine disks, thrust bearings, landing gear components, fuselage sections, and engine shafts.
- Medical: Bone screws, replacement joints, surgical instruments, implants, and prosthetics.
- Defense: Missiles, artillery shells, submarine components, gun barrels, and mine hardware.
- Fluid power: Hydraulic cylinders, compressor rotors, pump shafts, valves, and pressure vessels.
- Oil/gas: Drill bits, drill collars, casings, drill pipes, wellhead parts, and extraction hardware.
- Construction/mining: Bucket teeth, conveyor rollers, sieves, shovels, wheels, gear shafts, and crushers.
- Power generation: Turbine rotors, nuclear fuel rods, generator shaft and couplings, valves, pressure vessels.
Turning remains a fundamental manufacturing process used across every industry to produce strong, precision rotational parts from metal and plastic. Advancements in CNC technology continue to drive more automation, intelligence, and innovation in turning and machining. CNC Milling CNC Machining