What is CNC Turning?(steel vs titanium strength York)
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CNC turning has revolutionized the production of round parts like shafts, bearings, gears and pulleys. It enables fast, accurate and repeatable machining of complex geometric shapes that would be impossible to produce manually. CNC turning is widely used across many industries including automotive, aerospace, medical, energy and electronics.
How Does CNC Turning Work?
A CNC turning center consists of a computer control unit, servo motors, spindle and cutting tools. The cutting tools are positioned on a turret that can slide back and forth horizontally across the workpiece and interchange tools automatically. The turret may have multiple tool holders so several operations like facing, turning, grooving, threading etc. can be performed in a single setup.
The operator loads a program into the CNC control which interprets it into commands for the servo motors to position the turret and coordinates the rotation and movement of the cutting tool and spindle. As the workpiece rotates, the cutter moves across it in synchronization to remove material precisely as instructed in the program. Coolant is used to keep the cutting tool from overheating. The program specifies the feed rate, spindle speed, depth of cut and tool path required to generate the component.
CNC turning centers can be programmed directly at the machine using G-code or by using CAM software to generate the CNC code from 3D models. CAM allows complex programming of non-cylindrical shapes. The program is simulated to visually verify the tool path before running the actual cycle on the machine.
Types of CNC Turning
There are two main types of CNC turning methods:
1. CNC Lathe Turning:
This uses a single point cutting tool to shape the external diameter of the workpiece as it rotates. Operations like facing, turning, tapering, grooving, threading etc. can be performed by moving the turret linearly across the workpiece. Lathes with twin spindles or dual turrets can machine parts from both ends simultaneously, reducing cycle times.
2. CNC Turning Centers:
They use a rotating turret with multiple cutting tools to machine complex shapes and angles. The turret indexes between positions bringing different tools like drills, boring bars, OD/ID grooving tools, taps etc. into operation. Turning centers provide greater flexibility and are ideal for medium to high production jobs. They allow complete machining in one setup by combining turning, boring, facing and milling operations.
Applications and Benefits of CNC Turning
CNC turning is used to produce parts that have rotational symmetry and require a high degree of dimensional accuracy and repeatability. Typical applications include:
- Automotive: Engine valves, pistons, turbocharger housings etc.
- Aerospace: Engine components like turbine disks, shafts, bearings, fasteners
- Medical: Bone screws, surgical power tools, implants
- Oil and gas: Valve bodies, couplings, drill components
- Electronics: Connectors, insulators, semiconductors
The key benefits of CNC turning include:
- Higher productivity: CNC turning is much faster than manual methods. Complex profiles that took hours can be produced in minutes.
- Consistency: Every workpiece is identical as programmed. Human errors are eliminated.
- Accuracy and repeatability: Modern CNC machines can hold tolerances within 0.005 mm for dimensional precision and repeatability.
- Multi-axis control: Simultaneous, coordinated movement allows contouring and profiling.
- Automation: Reduces labor costs and allows unmanned production. Multi-part production with tool changes can run 24/7.
- Flexibility: Quick changeovers between jobs. Machine can turn a flange followed by a gear without setup changes.
- Intricate shapes: Complex geometries like eccentric diameters, internal undercuts etc. can be programmed easily.
- No special skills: Operator only needs basic CNC programming and setup skills. Specialized manual machining expertise is not required.
- Safety: The operator is detached from the machining process. No manual adjustments needed while the machine runs.
The Main CNC Turning Operations
The most common machining operations that can be performed on CNC turning centers include:
Facing: A square shoulder is machined across the face of the part using a facing tool or the nose of a roughing tool. This creates a uniform surface at right angles to the axis of rotation. Facing is done at the start of the turning cycle.
Turning: It is used to accurately reduce the outside diameter to the required dimension and finish by feeding the tool along the axis of the workpiece. The depth of cut is small, usually 0.1-0.25mm.
Taper turning: A tapered diameter is machined by coordinated movement of the turret and tailstock. The tailstock can be set at an angle or programmed to move in synchronization with the turret.
Boring: An internal diameter is enlarged using a boring bar with inserts to boring size within close limits. Boring bars can machine blind holes with a very small diameter to length ratio.
Grooving: Narrow grooves or undercuts are machined using OD/ID grooving tool holders and inserts. Used to create slots, seals and oil retention areas.
Threading: External and internal threads are cut using custom ground thread turning inserts. Single or multi start threads can be produced.
Drilling: Deep center holes can be accurately drilled using rotating tool holders. Avoiding drill chuck runout improves hole position tolerance.
Tapping: Threads can be tapped after drill holes to create threaded holes. Rigid tapping without clutch reduces tap breakage risk.
Knurling: A diamond shaped pattern is rolled onto the surface using a knurling tool. Used to create an easier grip on hand tools and levers.
Parting and Cut-off: Deep narrow grooves are machined to separate finished parts from the raw material bar stock. Parting tools include inserts with chip breakers.
Form turning: Complex profiles are generated using synchronized multi-axis interpolation of the turret with specially shaped inserts.
CNC Turning Process Cycle
A typical CNC turning cycle includes the following sequence of operations:
1. The workpiece is center drilled to create a starting hole for drilling and turning operations.
2. Facing cut to square the face and establish a uniform reference surface
3. Center hole is drilled and reamed to prepare for turning
4. Rough turning pass using a round carbide insert to remove bulk of material
5. Finish turning pass at final dimensions along with chamfers and radii
6. Grooving for seals and undercuts
7. Boring to enlarge holes to size along with threading and tapping
8. OD/ID threading using thread turning inserts
9. Taper turning by coordinated turret and tailstock movement
10. Knurling or form turning operations if required
11. Parting off workpiece from the remaining bar stock
12. Secondary operations like milling, engraving on the backside etc.
CNC turning centers allow all these operations to be performed in a single setup without moving the workpiece. Automated tool changers and probing reduce non-cut time and human intervention. This results in faster cycle times and maximum productivity.
Programming CNC Turning Machines
CNC turning machines are programmed using G-code, a standard language that controls all machine movements and functions. Each command is specified in a block that defines things like:
- Motion commands (G00, G01, G02 etc.)
- Feed rates and spindle speeds
- Turning cycles and canned cycles
- Tool selections and tool paths
- Coordinate positions
- Coolant control
Here is a simple G-code program example to turn a diameter:
N01 G54 (Select coordinate system)
N02 G00 X50 Z10 (Rapid traverse to start point)
N03 S1000 M03 (Spindle speed 1000 rpm ON)
N04 G96 S250 (Set surface speed 250 m/min)
N05 G01 X40 F0.2 (Linear interpolation X40 at 0.2 mm/rev feed)
N06 X30 (Turn next diameter)
N07 G00 X40 (Rapid traverse to return start point)
N08 M05 (Spindle OFF)
N09 G28 (Return to home position)
N10 M30 (Program end and rewind)
Modern CNC systems also allow graphical programming using CAD/CAM software. Complex profiles are drawn in CAD and then tool paths are generated through CAM modules. The post-processor converts CAD/CAM tools paths into G-code for the turning machine. Graphics prove easier than G-code for technicians.
Setting Up CNC Turning Machines
Before running a CNC turning production job, the machine must be set up properly:
- The workpiece is indicated and centered accurately in the chuck
- Cutting tools are loaded into the turret in sequence of operation
- Tool offsets are measured and entered into the tool table
- Part zero or datum point is established by touching off tools
- Coolant nozzles are positioned properly towards the cutting edge
- Trial run conducted on sample workpiece to confirm program
- If required, touch probe is used to digitize the raw profile for generating offsets
- Fixtures like steady rests, follow rests are installed to support long slender parts
Proper setup and indicating ensures there is minimum runout between the rotating workpiece and the tool tip. This is crucial for achieving tight tolerances and fine surface finishes.
Cost Considerations for CNC Turning
The major costs involved in CNC turning include:
- Machine tool: Turning centers range from basic 3-axis machines to highly advanced multi-axis mills turn machines with live tooling, Y-axis etc.
- CNC control system: More advanced models have better torque, acceleration, precision, and features.
- Tooling and fixtures: Cost depends on inserts grades, tool holders required and workholding fixtures. Quick change tooling helps cycle times.
- Software: Need to budget for CAD/CAM programming software and annual licensing costs.
- Training and labor: CNC operators need both machining and programming skills.
- Maintenance: Schedule preventive maintenance and replacement of worn parts. Need service contracts for major repairs.
- Floor space: CNC turning machines have a larger footprint than conventional lathes.
- Utilities: Require consistent electricity supply and machine cooling systems.
For small batch production, it is advisable to outsource turning operations to a specialty shop since they can split costs over many customers. For mass production, investing in CNC machine tools lowers per-part costs significantly.
CNC turning has made modern machining highly accurate and efficient. With computer controlled movement and cutting parameters, it has automated the production of precision turned parts. Continued advances in CNC technology like smarter machine controls, multi-tasking capabilities and Industry 4.0 connectivity will further boost productivity and precision. CNC Milling CNC Machining