What is a Turning? An Overview of Turning Operations in CNC Machining(tin melting point Atwood)

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Turning is one of the most common and important operations performed on CNC machining centers. It involves rotating a workpiece while a single-point cutting tool is fed linearly against it to cut away material. This produces cylindrical forms and contours on the workpiece that have rotational symmetry about a central axis.
Turning operations are performed on lathes or turning centers, which are designed specifically for this purpose. By controlling the movements and cutting parameters precisely using CNC (Computer Numerical Control), modern CNC lathes can turn complex geometries with excellent accuracy and surface finishes.
Here is an overview of turning operations:
The Fundamentals of Turning
Turning uses a cutting tool with a single sharp cutting edge that is fed against the rotating workpiece. The cutting edge removes material as chips, generating the desired shape. The surface generated by turning has a constant cross-section along the rotation axis of the workpiece. This makes turning ideal for fabricating rotational, cylindrical and conical forms.
The workpiece is held firmly on the machine using a chuck, collet or between centers. As it rotates, the cutting tool follows a programmed path parallel or at an inclined angle to the axis of rotation. By coordinating the tool’s linear and transverse motions precisely, intricate profiles can be machined. The tool’s positioning and depth of cut determine the workpiece’s final dimensions.
Turning is done on the external surface of the workpiece. Internal turning operations such as boring and drilling use similar single-point cutting action but with the cutting tool positioned inside a bore or hole. Grooving, threading, facing and parting-off are specialized turning operations done to generate specific features on the machined parts.
The Main Turning Operations
The basic turning operations performed on CNC lathes and turning centers include:
- Rough Turning: Also called roughing, it involves removing the bulk of material from the raw workpiece to produce a shape close to the final contour. Large depths of cut and high feed rates are used for high metal removal rates. Rough turning leaves a small amount of stock material for the next operation.
- Finish Turning: Also known as finishing, it involves light cuts to remove the remaining stock left by rough turning. A sharp cutting tool, small depth of cuts and low feed rates are used to achieve the final dimensions, tight tolerances and desired surface finish. Multiple finishing passes may be required.
- Facing: Machining a flat surface perpendicular to the axis of rotation. This is done at the beginning of part fabrication to ensure the workpiece face is smooth and sits properly against chuck jaws or centers.
- Boring: An internal turning operation to enlarge and finish holes and bores to specifications. The cutting tool moves in and out of the hole to remove material and achieve the desired internal diameter.
- Grooving: Turning a groove or recess of required width and depth on the workpiece surface. This is done using a specially shaped turning tool or a grooving tool.
- Parting Off: Severs a machined part from the bar stock or parent material by making a transverse cut across the diameter. Parting-off tools make this final cross-cut to free the finished part.
- Threading: Using a threading tool to cut helical threads and grooves on the workpiece surface. Various thread forms (metric, UNC, etc) can be produced via CNC coordinated movements.
- Drilling: Making a hole in the centerline of a rotating workpiece using a twist drill or drill bit. Performed on lathes using the drilling or live tooling capabilities.
- Taper Turning: Producing conical or tapered diameters by turning the workpiece inclined at an angle to the cutting tool’s motion. The tool feeds parallel to the rotated tapered surface.
- Form Turning: Machining complex rotational contours and special profiles using CNC program-controlled interpolation of the cutting tool and tool tip motions.
Benefits of Turning on CNC Lathes
Modern CNC lathes offer several benefits for turning operations:
- High Accuracy and Repeatability: CNC allows precisely programmed positioning and movements which can be repeated consistently over production runs. Parts can be turned with tolerances within 0.001 inches.
- Faster Processing: CNC automation increases production rates and reduces cycle times significantly compared to manual turning. Quick tool indexing enables faster processing.
- Intricate Profiles: Complex shapes can be produced using coordinated CNC motions that would be impossible to do manually. CAD/CAM integration facilitates this further.
- Reduced Setups: CNC allows performing multiple operations in a single setup, reducing changeover times and inaccuracy. For example, grooving and threading can be done after the turning pass.
- Safer Operation: The operator is removed from direct contact with the cutting. Risk of accidents is reduced.
- Cost Advantage: CNC turning has higher initial investment but lower per-part production costs at mid and high volumes. Skilled labor requirement is also reduced.
Turning Tools Used on CNC Lathes
A variety of single-point standard and special cutting tools are used on CNC lathes:
- Round Shank Insert Tools: Have interchangeable inserts with the cutting edges. Ideal for high production turning. Brazed carbide inserts with PVD coatings for hardness are commonly used.
- Double-Sided Inserts: Have two usable cutting edges on the insert, essentially doubling tool life. Cost-effective for large production runs.
- Boring Bars: Long overhang tools for internal boring, with replaceable inserts. Come in various styles for different boring applications.
- Threading Tools: Have specially ground thread profiles to generate threads, including Acme, UN, metric, buttress and other forms. Carbide inserts provide durability.
- Grooving Tools: Narrow cutting tools for groove cutting. Allow grooving in a single pass or via multiple passes for wide grooves.
- Parting & Cut-Off Tools: Thicker tools for parting operations. Have advanced chip-breaking geometries to facilitate parting-off.
- Form Tools: Custom ground tools for generating specific complex profiles not possible with standard inserts.
Workholding Methods for Turning
Turning requires the workpiece to be held securely but also allow for efficient changeovers between jobs. Different workholding methods are used:
- Chuck: A multi-jaw hydraulic or pneumatic chuck which clamps onto the workpiece. Provides strong and accurate holding. Jaws can be interchanged for different jobs.
- Collet: A sleeve with an internal taper that matches the workpiece’s tapered shank. Pulling the collet into the spindle grips the work tightly. Fast job changeovers.
- Centers: The workpiece rotates between a live center in the spindle and a dead center in the tailstock. Common for long parts that cannot be chucked.
- Faceplates: Large rotary plates that can be bolted or clamped to large workpieces. Allows turning flanges and dishes.
- Mandrels: Shanks inserted into the workpiece bore and then clamped in the chuck. Used to hold small parts with bores or tubes.
In summary, turning is a versatile CNC machining process capable of producing a variety of external and internal cylindrical features. Understanding the turning methods, tools and workholding options allows maximizing the value from CNC lathes and turning centers. With the right approach, precision turned parts can be manufactured quickly, accurately and cost-effectively. CNC Milling CNC Machining