What is CNC Turning?(metal cast iron Bertha)

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CNC turning is a machining process that uses computer numerical control (CNC) to automate the turning of parts on a lathe. CNC turning allows for precise and consistent machining of cylindrical and conical parts with complex geometries and tight tolerances. The CNC lathe is one of the most versatile machine tools used in manufacturing today.
How CNC Turning Works
In CNC turning, the cutting tool moves while the workpiece rotates. The cutting tool follows program instructions to remove material and create the desired part features and dimensions. Here is an overview of how CNC turning works:
- The workpiece, typically a cylinder, is clamped in a chuck or collet on the spindle of the CNC lathe. The spindle rotates the workpiece at a defined speed.
- The turret holds multiple cutting tools and indexes them into position for machining operations. Common tool types are turning, facing, boring, grooving, and threading tools.
- The computer controls the movements of the turret and cutting tools according to a CNC program. Cutting parameters like feed rate, depth of cut, and spindle speed are also programmed.
- As the workpiece rotates, the cutting tool feeds horizontally and/or vertically based on the programmed instructions. This allows material to be removed to achieve the desired diameters, lengths, grooves, threads, and other features.
- Coolant is applied to the cutting area to reduce heat, improve tool life, and flush away chips. The turret can also be equipped with a tailstock for supporting long workpieces.
- Once complete, the finished part is unloaded. The turret and spindle can then be set up for the next part to repeat the process.
CNC Turning Capabilities
Modern CNC lathes offer a vast range of turning capabilities:
- Turning - Producing straight, tapered, grooved, and contoured cylindrical surfaces. Single point turning tools and inserts are used.
- Facing - Machining the ends of parts using facing tools and inserts. This evens out ends, creates flat surfaces, and makes parts the correct length.
- Boring - Enlarging holes to precise diameters with boring bars or radial tool bits. Boring also improves hole roundness and finish.
- Grooving and Threading - Cutting grooves, threads, and tapers with form tools, grooving inserts, and 60° threading inserts.
- Drilling and Tapping - Performing center hole drilling and tapping using drilling heads or live tooling. This eliminates secondary operations.
- Milling - Addition of a milling turret expands the CNC lathe into a multifunctional "turning center". Milling operations like slotting, keyway cutting, profiling, and contouring can be performed.
- Grinding - CNC lathes can be fitted with grinding attachments for in-process grinding of diameters, faces, tapers, and other features.
CNC turning centers with multiple spindles and turrets can also perform simultaneous machining on multiple parts to boost production rates. This covers the basics of how CNC turning works and its capabilities. Next we'll look at the benefits of CNC turning.
Benefits of CNC Turning
There are many advantages to using computer numerical control for turning operations:
- Consistency - CNC automates turning, removing variability in manual operations and improving consistency across parts. Once programmed, the CNC lathe reliably machines parts within tight tolerances.
- Accuracy - CNC lathes provide more consistent accuracies of 0.005 inches or less compared to 0.010 to 0.015 inches for manual lathes. The increased precision reduces scrap rates.
- Complexity - CNC allows turning intricate components with eccentric diameters, grooves, tapers, threads, and contours that would be infeasible to machine manually.
- Efficiency - CNC turning is highly efficient and produces parts in one automated setup. The lack of manual intervention also improves safety. CNC multi-spindle machines further boost throughput.
- Flexibility - Changeovers between parts are quick with CNC. The program can be rapidly altered to machine new parts. Skilled operators can also make edits at the machine to optimize programs.
- Documentation - The CNC system records the exact toolpath the part program follows. This digital record ensures parts can be accurately replicated even years later.
- Automated Inspection - In-process inspection probes check part dimensions. The CNC program can automatically compensate to keep parts in tolerance. Post-process laser inspection is also facilitated.
- Deburring and Finishing - CNC lathes can deburr, polish, and finish parts in the same setup using deburring tools, brushes, and abrasive surfaces. This reduces secondary processing.
- Training - With a shortage of skilled machinists, CNC turning centers are easier to operate after basic programming training. The manual skill requirement is reduced.
The pros of CNC turning demonstrate it is an ideal choice for producing precision turned parts in manufacturing.
CNC Turning Operations
The basic types of turning operations that can be performed on a CNC lathe include:
- Facing - Uses a standard facing tool to create a flat surface on the end of a cylinder or workpiece. Defines the part length.
- Straight Turning - Uses turning inserts to reduce the diameter of a rotating workpiece to a specified dimension. Controls the finished diameter.
- Taper Turning - Turning operation that rotates the compound slide to an angle to cut a tapered diameter rather than a straight diameter.
- Grooving - Cutting grooves of various widths and depths along the diameter using grooving inserts. Used to cut lubrication channels.
- Undercutting - Forms an undercut diameter by feeding the tool radially across the part. Used to create a relief for Orings.
- Threading - Cutting standard or custom threads using 60° threading inserts. Single or multi-start threads can be produced.
- Drilling - Performs center hole drilling using twist drills held in tool holders or tailstock. Eliminates pre-drilling.
- Boring - Enlarges an existing hole to a specific diameter and finish using single point boring bars. Improves hole size and finish.
- Reaming - Finishes hole location and size to tight tolerances using adjustable or fixed reamers. Produces high hole precision.
- Tapping - Cutting internal threads by tapping holes using a floating tap holder. Often combined with drilling to simplify processing.
- Knurling - Creates diamond-shaped or straight line patterns on the diameter by displacing and work hardening the material. Provides a grip surface.
More complex turned parts combine multiple turning operations and use custom tooling. The programming determines the sequence of operations.
CNC Turning Machines
There are several types of CNC lathe machines used for production turning:
- Vertical Turning Lathe - Oriented vertically with the spindle facing horizontally. Used for turning very large diameter parts.
- Horizontal Turning Lathe - The standard horizontal orientation with part held between centers or in a chuck. Most versatile general purpose.
- Chucking Machine - Designed for high production chucking work. Features a hydraulic chuck for rapid part loading.
- Bar Machine - Feeds bar stock through the spindle and turns it into finished parts via cutoff. Highly efficient.
- Multi-Spindle Lathe - Multiple spindles allow completing multiple parts in one operation. Produces complex parts at high volumes.
- Turning Center - CNC lathe with live tooling, C-axis, and milling capabilities. Performs turning and milling in one setup.
Entry-level CNC lathes have a single spindle and turret. Mid-range lathes add a second spindle, second turret, C-axis, and live tooling. High-end CNC turning centers have multiple spindles with integrated automation and robotic part handling.
CNC Lathe Components
Key components of a CNC turning center include:
- Headstock - Holds the main spindle which rotates the workpiece. Typically has a high torque motor.
- Chuck - Clamps and centers the workpiece in the spindle. Types include 3-jaw, hydraulically actuated, and collet chucks.
- Tailstock - Located opposite the headstock and supports long workpieces with a live center. Can also hold tooling.
- Turret - Carousel style tool holder that indexes turning tools into position for machining. Multiple tool stations allow flexibility.
- Tool Holders - Hold and set the radial and axial position of turning inserts and boring bars for precise cutting.
- Ballscrews - Translate rotary motion into linear motion to move the turret and cutting tools along set axes.
- Guideways - Support and guide linear motion of the turret, tools, and tailstock. Keep motion precise and vibration-free.
- Control System - Computer and CNC controller that takes direction from the program to command movements and functions.
- Chip Conveyor - Removes chip debris from the machining area through the interior of the machine. Keeps the work area clear.
- Coolant System - Provides lubricating and cooling fluid to the cutting area. Essential for reducing heat and flushing chips.
Understanding these core components assists with programming, operating, and maintaining CNC turning machines.
CNC Turning Advantages
There are several compelling advantages that make CNC turning a superior manufacturing process compared to manual turning:
Precision - CNC turning machines offer far greater precision and repeatability than manual lathes. Programmed tool paths and speeds optimize finish and accuracy. Parts can be machined to tolerances under 0.001".
Speed - CNC allows much faster metal removal rates than manual turning. Combined with automated loading, parts can be completed in seconds versus minutes. Cycle times are significantly reduced.
Complexity - Intricate parts with precision diameters, tapers, grooves, and threads can be machined via CNC. Components previously requiring multiple setups are completed in one turning operation.
Efficiency - CNC turning produces finished parts in one automated cycle. The lack of manual intervention boosts productivity and efficiency while reducing labor costs.
Consistency - The computer control eliminates variability in output. Once proven, CNC programs churn out identical parts cycle after cycle. Scrap and defects are minimized.
Flexibility - CNC allows quick changeovers between parts. Programs can be selected and tooling swapped out to accommodate new jobs. Customization is easy.
Integration - CNC turned parts can be automated via robotic part loading and unloading. Post-process inspection and finishing can also be integrated to create a lean cell.
Training - While skilled CNC programmers are needed, machine operators can be trained faster than manual machinists. The skill requirement is reduced.
For medium to high volume production runs, CNC turning is the clear choice. The precision, speed, efficiency, and flexibility of CNC far outweigh manual turning methods.
CNC Turning Process
The basic CNC turning process consists of four key stages:
1. Design - Part geometry is modeled in CAD software. The model defines all dimensions, tolerances, finishes, and features. CAM software then converts this into toolpaths.
2. Set Up - The workpiece is securely clamped in the chuck. Tool holders are loaded with proper inserts per the program. The turret is indexed to the starting position.
3. Production - The operator starts the program and the CNC turning cycle begins. All tools automatically machine the part based on the programmed operations and parameters.
4. Inspection - Completed parts are verified to meet specifications. Measurements are taken using gauges, calipers, or CMMs. Tool offsets may be adjusted to refine tolerances.
Most CNC turned parts follow a similar high-level workflow. However, there are several distinct approaches to programming the CNC machine:
- Manual Programming - Machine G-code is written line-by-line using G-code language. Very time intensive but maximizes control.
- CAD/CAM - CAM software converts CAD models into toolpaths then into G-code. Much faster than manual programming.
- Conversational - Simplified shop floor programming using the control interface and fill-in-the-blank prompts. Good for low complexity parts.
- Teach Mode - Programming where the operator "teaches" the machine toolpaths at the machine using a pendant. Simple parts can be taught.
Whether parts are programmed offline or at the machine, CNC turning automates the manufacturing process after initial creation of the part program.
CNC Turning Tools
A wide selection of cutting tools are used for the various CNC turning operations:
Turning Inserts - Made of carbide, ceramic, or CBN. Achieve fine surface finishes and closer tolerances. Multiple cutting edges provide economy.
Boring Bars - Single point boring bars for internal boring. Can be adjustable or fixed, and indexable or solid carbide.
Threading Tools - 60° inserts for external threading. May have chipbreaking geometry and coatings for easy chip evacuation.
Grooving Tools - Inserts shaped for groove cutting. Provide good chip control when machining narrow grooves.
Form Tools - Custom profile tools for complex contours or shapes not possible with standard inserts. Require special tool grinders.
Drills - Provide drilling ability on the CNC lathe. Reduce separate drilling setup. May be straight or indexable insert drills.
Reamers - Used to accurately size and finish holes. Available in adjustable and fixed styles. Deliver precision hole sizing.
Taps - Allow internal threads to be directly tapped on the CNC lathe. Save secondary tapping operations.
Knurling Tools - Hardened steel wheels that indent patterns into the workpiece diameter through displacement and cold working.
Correct selection of tooling is critical for productive CNC turning. The appropriate inserts, geometries, and coatings must be matched to the material and operation. Tool holders also play a key role in properly presenting tools.
CNC Turning Materials
CNC lathes can machine a very wide range of materials from plastics to exotic alloys. Common materials include:
- Titanium - Difficult to machine but used for high strength aerospace applications. Require rigid setups and slow speeds.
- Stainless Steel - Challenging material due to work hardening. Low cutting speeds and feeds plus high pressure coolant are used.
- Tool Steel - Very hard material machined for mold cavities and dies. Requires high edge strength tooling and cobalt-enriched inserts.
- Aluminum - Softer but gummy material. Requires sharp tools, positive rake, and chipbreaker geometries. High speeds are employed.
- Brass - Excellent machining properties. Makes an ideal first time CNC turning material. Carbide inserts handle high speeds and feeds.
- Plastics - Easy to machine and inexpensive. Does not wear tooling significantly. Produces stringy chips that must be managed.
- Cast Iron - Rough but machinable material for many industrial components. Nodular cast iron cuts easier than grey varieties.
- Inconel - Exceptionally hard superalloy that requires rigid setups. Abrasive and work hardens. Low speed and feed advised.
Careful programming and tool selection helps overcome the unique machining traits of each material. Harder alloys often benefit from using the latest coated carbide or ceramic grades.
Production CNC Turning
For production scale manufacturing, CNC turning centers are optimized in several ways:
- Bar Feeders - Automatically feed bar stock into the spindle for continuous production. Permits lights out operation.
- Guide Bushings - Support bars in the headstock and prevent whipping during long overhang turning. Critical for bar feeds.
- Sub-Spindle - Opposite side spindle allows complete machining in one pass. Grab part after main operations and finish rear end.
- Automation - Robotic part handling systems to load and unload finished parts for untended production. In-line secondary processes can also be automated.
- Multi-Spindle - Machines with multiple spindles working simultaneously for maximum output. Produce complex components at high volumes.
- Multi-Turret - Addition of a second turret doubles the number of available tools. Permits aggressive roughing by a dedicated turret.
- Chip Conveyors - Quickly evacuate chips outside the work area. Critical for high volume swarf generation common in production turning.
For critical processes, in-process verification checks part quality during the run. Automated SPC data collection confirms the process remains in control. These innovations maximize the production capabilities of CNC turning.
In summary, CNC turning is a versatile and automated machining process that utilizes CNC-controlled machine tools to efficiently and accurately produce precision turned parts. The combination of consistency, speed, accuracy, and flexibility makes CNC turning ideal for manufacturing industries where high productivity and precision are paramount. CNC Milling CNC Machining