Introduction to Turning in CNC Machining(metal for machining Judith)

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Turning is one of the most common and important machining processes used in manufacturing. It involves rotating a workpiece while a single point cutting tool removes material to create cylindrical shapes. Turning can be performed manually using a lathe, but is more commonly done using computer numerical control (CNC) lathes and turning centers.
CNC turning has many benefits over manual turning. It allows for highly accurate and repeatable machining, as well as the ability to produce complex parts unattainable with manual methods. CNC machines are programmed with CAD/CAM software that converts part designs into code the machine can interpret to cut the proper shape. Operators can then set up the machine and tools required and let it run untended to complete the turning operations.
Types of Turning in CNC
There are several different types of turning operations possible on CNC machine tools:
- Outside diameter (OD) turning - The most common type, this involves removing material from the external surface of a rotating cylindrical workpiece. The cutting tool moves linearly along the axis of the part to cut away material to obtain the desired diameter. Through programmed movements of the tool, precise diameters, tapers, grooves, and contours can be machined.
- Inside diameter (ID) turning - Also known as boring, the cutting tool moves along the internal surface of a cylindrical part to enlarge the diameter or produce an internal form. This requires the tool sit on a separate axis from the rotating part.
- Facing - Machining the face of a rotating part to create a flat surface perpendicular to its axis. This is an important preparatory step for many turned parts.
- Undercutting - Forming recessed areas like grooves, slots, and threads inside part diameters. Special tooling and programming techniques allow cutting away material underneath surfaces.
- Parting/cutoff - Separating a finished part from its source stock by cutting completely through it. This is the final operation in completed turned parts.
- Turning of non-cylindrical forms - Using intelligent programming and advanced tool setups, CNC turning machines can also produce non-round forms like polygons, ovals, and freeform shapes. Secondary operations may be required to complete intricate geometries.
Turning Methods
There are a few primary methods and strategies used in CNC turning:
- Longitudinal turning - The classic method where the cutting tool follows the axis of the workpiece to reduce diameters and cut forms. The majority of turning operations utilize this method.
- Face turning - Machining occurring with the cutting tool perpendicular to the rotation axis of the part. This allows facing of surfaces and cutting of radial features.
- Contour turning - Producing non-cylindrical forms using careful control of the tool movement in relation to the workpiece rotation. Complex curves and shapes can be generated via this method.
- Turning with driven tools - Uses a rotary cutting tool mounted in the turret alongside stationary tools. This permits complex machining with a single setup, like milling and drilling holes or shapes within a turned part.
- Hard turning - Using very hard cutting tool materials to machine hardened workpieces over 45 Rockwell C hardness. Avoids separate grinding steps in bearing and gear production.
- Multitasking turning - Performing multiple machining operations in one setup on CNC lathes with live tooling, sub-spindle, and Y-axis capabilities. Milling, drilling, and off-center turning can be combined with normal turning.
Tooling for CNC Turning
A variety of single point cutting tools are used for the different turning operations and materials. High speed steel, carbide, ceramic, diamond, and cubic boron nitride tools are commonly used. The most important characteristics are hardness to withstand high temperatures and pressures, toughness and wear resistance. Tool geometry varies based on the operation - roughing uses a stronger positive rake angle while finishing emphasizes a sharper cutting edge.
Insert style tools with indexable carbide inserts are the most common for production turning. These allow changing just the insert instead of the entire tool when worn or broken. Brazed carbide tips and replaceable tool bits are other common options. Special boring bars, thread turning tools, grooving tools, and custom tools may also be used. Proper tool selection helps maximize material removal rates and surface finish while minimizing cost per part.
Turning Materials
CNC turning can effectively machine a very wide range of materials. The most commonly machined materials include:
- Steels - Low carbon, alloy, tool, and stainless. Steel is versatile, strong, and among the easier materials to turn. Carbide tooling is most common.
- Aluminum - Non-ferrous metal valued for its light weight and corrosion resistance. Due to its softness, higher feed rates and speeds are used. Tools require sharp cutting edges.
- Titanium - An exotic metal with high strength-to-weight ratio and corrosion resistance. More challenging to cut so sharp edges and positive tool rakes are preferable. High heat generation must be controlled.
- Plastics - Softer synthetics like nylons, acetals, PTFE that machine easily. Avoid melting from frictional heat. Require sharp positive geometry tools and chip control.
- Composites - Materials like carbon and glass fiber reinforced plastics. Abrasive fibers wear tools quickly. Low elastic modulus calls for light depths of cut.
- Precious metals - Soft high value metals like gold and silver. High production rates are possible but buildup on insert cutting edges must be minimized.
Turning Tips, Techniques, and Best Practices
Here are some tips for effective CNC turning operations:
- Select suitable tooling and optimal feeds and speeds. This improves tool life and productivity. Consider the workpiece hardness and machinability.
- Utilize high pressure coolant. This controls heat, flushes away chips, and improves surface finishes.Reduce depths of cut for better accuracy and inserts lasting longer.
- Employ dynamic roughing strategies. Lower radical depths of cut and more efficient chip thinning improves material removal rates over 50%.
- Include finish passes at conservative parameters. This reduces vibration, deflection, and other accuracy robbers for precision finishing.
- Program approach and retract paths that avoid contacting the workpiece. Collisions can break inserts and scrap parts.
- Monitor tool wear. Replacing inserts before catastrophic failure prevents machining problems and damage.
- Deburr parts as needed. Loose bits in assemblies can cause issues. Chamfer sharp edges if A burr-free part helps prevent cuts and safety hazards during handling and operation.
By following good practices and taking advantage of CNC capabilities, you can achieve high-quality turned components quickly, accurately, and cost-effectively. The versatility of CNC turning makes it ideal for prototyping one-off parts as well as high volume production machining.
Turning is an essential process for manufacturing precision cylindrical and contoured workpieces. Utilizing CNC turning centers and lathes with proper tooling and programming techniques makes the process incredibly versatile and productive. Companies can meet demands for complex turned parts that would be infeasible to produce manually. CNC turning will continue growing in popularity and capability as the technology advances. The possibilities are nearly endless for what this fundamental machining process can accomplish. CNC Milling