What is a Turning? An Overview of Turning Operations in CNC Machining(snap in design Lyndon)

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Turning is one of the most common and important machining processes used in manufacturing today. It involves rotating a workpiece while a single-point cutting tool moves along the axis of rotation to remove material. This produces cylindrical shapes and often smooth surface finishes. Turning operations are performed on specialized machine tools called lathes that hold the workpiece and provide the rotational motion.
In CNC (computer numerical control) machining, turning operations are programmed into a CNC machine's controller. The machinist can then execute the turning program which commands the precise movements and cutting parameters. CNC automation makes modern turning incredibly accurate and consistent compared to manual lathe work. Understanding what a turning is and how it works is key for anyone involved in manufacturing.
How Does Turning Work?
Turning uses a cutting tool with a single sharp cutting edge to slice away material as the workpiece spins. The cutter is fed linearly or along an angle into the rotating workpiece to make cuts. The depth of cut, feed rate, and cutting speed can be precisely set to control the process. As material is removed by the cutting tool's movement, a cylindrical shape is formed.
Common types of turning operations include:
- Facing - Machining the end surface of a cylindrical workpiece. This forms a flat reference surface.
- Straight turning - Continuously machining the outside diameter of a cylindrical workpiece. This reduces the diameter and creates the finished surface.
- Taper turning - Machining a tapered contour onto the cylindrical surface. The diameter gradually reduces from one end to the other.
- Grooving - Cutting narrow, straight grooves into the surface. Often used to create seals, parting lines, and threading.
- Threading - Cutting helical screw threads into or onto the workpiece. Multiple passes create precisely formed threads for fasteners or lead screws.
- Boring - Enlarging or smoothing existing holes in the center of the workpiece. Requires a single-point boring bar tool.
- Drilling - Creating a hole in the center or end of a workpiece with a rotating drill bit tool.
- Form turning - Using a shaped cutter to machine complex contours, profiles, and special forms.
The majority of turned parts are external cylindrical or cone shapes. However, internal turning operations like boring and threading are also very common. Parts like shafts, sleeves, pins, bushings, and pulleys are often manufactured on CNC lathes.
Key Components of a CNC Lathe
CNC lathes contain several important components that enable high-precision turning:
- Headstock - Holds the rotating spindle where the workpiece is mounted. Provides power for workpiece rotation.
- Tailstock - Holds tooling for internal turning operations like drilling, boring, and threading. Can slide to accommodate different length workpieces.
- Tool turret - Holds multiple cutting tools and quickly indexes between them for different turning operations. May have live tooling for milling.
- Tool post - Holds a single-point turning tool rigidly for exterior turning passes.
- Carriage - The movable platform that holds the tool post or tool turret. Controls linear movement and cutting feed rates.
- CNC controller - Computerized control system that guides the machine tool motions and turning sequence. Converts programmed code into movements.
- Coolant system - Provides cutting fluids to the tool-workpiece interface to cool, lubricate, and remove chips.
- Chuck - A specialized workholding device that clamps onto one end of the workpiece and centers it precisely for rotation.
- Steady rest - Supports the free end of long, thin workpieces during turning operations to reduce deflection.
When these components are programmed and operated together, intricate turning operations can be completed quickly, accurately, and automatically through CNC code.
Primary Turning Tool Materials
The most common tool materials used for cutting tools in CNC turning operations include:
- High-Speed Steel (HSS) - General purpose tools with good wear resistance and ability to cut many materials. Used for roughing cuts.
- Cobalt Steel - Similar properties to HSS but can withstand higher temperatures. Used for high-speed turning applications.
- Carbide - Very hard, heat and wear resistant tools made of tungsten carbide. Allows high cutting speeds and closer tolerances. Used for finishing.
- Ceramics - Advanced ceramic cutting tools like silicon nitride or aluminum oxide. Withstand very high machining temperatures for long tool life. Used to machine abrasive alloys.
- Cubic Boron Nitride (CBN) - Second only to diamond in hardness. Allows high-speed turning of hardened steels and cast irons. More affordable than diamond tools.
- Diamond - Hardest known material. Used for turning non-ferrous alloys and graphite. Very sharp cutting edge but expensive and brittle.
The right tool material balances cost, tool life, and workpiece finish based on the turning application. Many CNC machines will utilize a selection of different tool materials in their turrets to optimize the entire machining process.
Turning Process Parameters
When programming a CNC turning operation, the machinist can precisely define these key parameters:
- Cutting Speed (Vc) - The surface speed at which the work material moves past the cutting tool, measured in feet per minute (FPM) or meters per minute (MPM). Affected by spindle RPM.
- Feed Rate (f) - The linear or radial rate of infeed by the cutter, measured in inches per revolution (IPR) or millimeters per revolution (MPR). Determines finish and cutting time.
- Depth of Cut (doc) - Thickness of the layer being removed by a turning pass. Measured radially on the workpiece or axially for facing cuts.
- Cutting Tool Geometry - The shape, angles, and dimensions of the cutting tool tip. Affects cutting forces, chip flow, accuracy, and surface finish.
- Cutting Fluids - Coolants or lubricants applied to reduce heat and improve surface finish. Depends on workpiece material, operations, and desired results.
Optimizing these parameters allows turning to be completed efficiently while achieving dimensional accuracy, part quality, and reasonable tool life. This is simplified by CNC programming.
G and M Codes for CNC Turning
CNC turning machines are programmed using G and M codes. Here are some of the most common codes used:
- G00 - Rapid traverse to quickly position the cutter
- G01 - Linear interpolation for feeding the cutter during cuts
- G02/G03 - Circular interpolation for tapered cuts or grooving
- G96 - Spindle speed command by Constant Surface Speed (CSS)
- G97 - Spindle speed command by revolutions per minute (RPM)
- G99 - Return to feed per revolution from feed per minute
- M00 - Program stop for operator inspection
- M03/M04 - Start spindle rotation clockwise/counterclockwise
- M08/M09 - Coolant on/off
Many additional codes are used for turning cycles, repetitive operations, tool changes, diameter/radius modes, and more. These simple codes are programed into sequences that control entire turning processes when executed on a CNC machine.
Setting Up and Running a CNC Turning Job
The basic workflow for completing a CNC turning operation is:
1. Review engineering drawings and specifications of part features.
2. Select workpiece and fixture based on part dimensions, features, and materials.
3. Choose appropriate cutting tools for each operation and setup tools.
4. Write CNC program code defining all motions and turning cycles. May use CAM software to generate.
5. Set up workpiece in the chuck or collet and select first tool.
6. Run first trials at conservative parameters. Check for errors or collisions.
7. Adjust feed rates, depth of cuts, and other variables to optimize cycle time and quality.
8. Run production job at proven parameters. Periodically measure samples to confirm accuracy.
9. Deburr and finish parts as needed. Inspect for defects against tolerances.
10. Record runtimes, tool wear, and other data for continuous improvement.
With practice and the right CNC turning procedures in place, machinists can repeatably produce precision turned components, operate cost-effectively, and meet production demands.
Advantages of CNC Turning
Some of the major benefits of CNC turning compared to manual turning include:
- Improved accuracy and repeatability from precise programmed tool paths. Can hold extremely tight tolerances.
- Ability to machine complex geometries and contours through CNC interpolation capabilities.
- Quicker cycle times and increased productivity through optimized cutting parameters.
- Reduced skilled labor requirements and operator involvement. Simplified setup through stored programs.
- Flexible production volume capabilities from single runs to high-volume output.
- Ability to run unmanned for lights-out production. Greater machine utilization.
- Safer operation and consistent quality once programs are proven out and automated.
- Advanced data reporting for process control and improvement.
For these reasons, CNC turning centers continue to expand in manufacturing facilities, production shops, and machine shops around the world. Mastering CNC turning fundamentals is an essential manufacturing skill set. CNC Milling CNC Machining