Introduction to Turning Operations on CNC Machines(facing mill Moore)

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Turning is one of the most common machining operations performed on CNC machine tools. It is used to reduce the diameter of a rotary workpiece by removing material from its outside surface. The turning operation can produce cylindrical shapes, tapers, grooves, threads, and complex contours through interpolation of multiple axes.
On CNC lathes, the workpiece is held and rotated on a spindle while a single point cutting tool removes material. The cutting tool is mounted on a tool turret and can be positioned in two axes – X and Z. The X-axis controls the radial depth of cut, moving the cutter perpendicular to the spindle axis. The Z-axis controls the longitudinal feed, moving the cutter parallel to the spindle axis. By precisely controlling the positions and feeds of the tool relative to the rotating workpiece, intricate features can be machined with tight tolerances.
Types of Turning Operations
There are several basic types of turning operations that can be performed on CNC lathes:
- Facing: Machining the end face of a cylindrical workpiece flat and perpendicular to the center axis. This establishes a reference surface and final part length. Facing is done with the cutter fed parallel to the spindle axis.
- OD Turning: Reducing the outside diameter of a workpiece by taking cuts along its length. This can completely turn down material to a smaller diameter or remove a specified amount to achieve a target dimension. The depth of cut is applied radially with the cutter moving perpendicular to the spindle axis.
- Boring: Enlarging the inside diameter of a hole. The boring bar tool is fed radially into the stationary workpiece to incrementally increase the bore’s internal diameter. Boring is used to achieve tight tolerance holes or create internal features like grooves and tapers.
- Taper Turning: Machining a tapered profile by turning the OD while the tool or workpiece is set at an angle offset from perpendicular. The taper angle is programmed based on the desired amount of reduction over a defined length.
- Threading: Cutting external screw threads by feeding the single point tool linearly while synchronizing the rotation of the spindle. The thread type, size, fit, and tolerance are defined in the CNC program.
- Grooving: Cutting narrow, linear grooves into the surface of the workpiece. This is accomplished by positioning the tooltip depth and feeding radially while the part turns. Grooves can be used for sealing surfaces, parting lines, lubrication reservoirs, and decorative patterns.
CNC Lathe Workholding
The workpiece must be securely mounted on the CNC lathe before machining can begin. There are several workholding options available:
- Chuck: A mechanical jaw device that clamps onto the outside of the workpiece. Chucks provide rigid holding and accurate concentricity for bar stock or forgings centered on their external diameters. Different jaw sets can accommodate various sizes, shapes, and materials.
- Collet: A precision machined collar with a tapered inner profile that contracts to grip the workpiece when pulled into a mating tapered spindle socket. Collets provide high runout accuracy well-suited for finished parts and thin-walled tubing.
- Faceplate: A mounting plate that can be bolted onto the spindle nose and uses T-slot clamps, V-blocks, step blocks, and straps to secure workpieces. This allows for off-center and irregular shaped parts to be held.
- Lathe Centers: Live centers and dead centers that the workpiece rotates between. The points make contact with the part along its center axis at each end to provide turning support and resist deflection forces. Centers enable long parts to be completely turned in one setup.
CNC Lathe Cutting Tools
Single point cutting tools ground with precise angles perform the turning, facing, boring, and grooving operations. The most common tool materials are:
- High-Speed Steel (HSS): Durable tools for general machining of carbon steels at moderate speeds and feeds. HSS tools can be sharpened many times.
- Cobalt Steel: Heat resistant high-speed steel tools with 8% cobalt content. Used for high temperature alloys or hard materials where heat dissipation is important. More wear resistant than HSS.
- Carbide: Extremely hard tipped tools made from tungsten or titanium carbide. Best for machining hardened steels, titanium, and nickel-based alloys at high surface speeds. Carbide inserts are indexable and can have multiple cutting edges.
- Ceramic: Advanced ceramic cutting tools made from aluminum oxide. With high heat resistance and hardness, ceramic tools excel at high speed finishing of super alloys, stainless steel, and cast iron. Produces excellent surface finish.
- CBN & PCD: Cubic boron nitride and polycrystalline diamond cutters for the most wear resistant turning of hardened or abrasive materials like chilled cast iron. CBN and PCD deliver extended tool life on hard turning applications.
CNC Lathe Programming
CNC turning operations are controlled through part programs, which provide instructions for all aspects of the machining process. Part programs are written in G-code, the standard numerical control programming language that machines understand.
Key programming considerations for CNC turning include:
- Defining stock dimensions
- Setting spindle speed (S) and feed rates (F)
- Positioning cutter X and Z axes initial locations
- Calling out tool numbers to select cutters
- Specifying cutting depths, passes, and increments
- Controlling coolant or dry operation
- Applying feed direction (conventional or climb)
- Inserting canned cycles for standard operations
- Controlling spindle on/off and direction
- Executing linear and circular interpolation moves
CNC lathes allow fast, flexible, and automated turning to accurately produce parts with excellent repeatability. Programming is simplified through the use of conversational interfaces, CAD/CAM software, and saved canned cycles. CNC turning has enabled high efficiency machining of precision components with short lead times across many industries. CNC Milling CNC Machining