Introduction to Turning in CNC Machining(nitride coating Lynn)

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Turning is one of the most common and important machining processes used in manufacturing. It involves rotating a part while a single point cutting tool is moved parallel to the axis of rotation to cut away material. Turning can be used to create cylindrical parts, reduce diameters, face surfaces, and produce various geometric features. It is an extremely versatile process that can produce parts with excellent dimensional accuracy and surface finishes.
In computer numerical control (CNC) machining, turning operations are programmed into a CNC machine using G-code. The CNC machine reads the G-code and precisely controls the motions of the cutting tool and part to execute the programmed turning operations. CNC turning centers provide automated control over turning parameters like feed rate, depth of cut, spindle speed, and more. This results in high productivity, consistency, and accuracy.
Types of Turning Operations
There are several basic types of turning operations that can be performed on a CNC lathe:
- Facing: Machining the end surface of a part to produce a flat surface perpendicular to the axis of rotation. This levels the part and ensures a reference surface for other operations.
- OD Turning: Machining the external cylindrical surface of a part by moving the cutter along the axis of rotation. This reduces the diameter to specific dimensions. Multiple passes are made to achieve the final diameter.
- ID Turning: Machining the internal cylindrical surface of a part, like boring a hole. The cutter moves along the axis of rotation inside the part. This enlarges the diameter to specific dimensions.
- Taper Turning: Machining a tapered surface by turning at an angle to the axis of rotation. The cutter moves at a slight angle to the axis to generate a tapered diameter.
- Grooving: Machining a groove or recess in the part by moving the cutter perpendicular to the axis of rotation. The groove may be straight, tapered, or contoured.
- Threading: Machining internal or external screw threads using single point threading tools. The tool moves in synchronization with the rotation to cut the thread profile.
- Drilling: Using a rotating drill bit that moves along the axis of rotation to create or enlarge holes in the part. Holes can be drilled straight or at an angle.
- Boring: Enlarging an existing hole to increase its diameter and improve precision. Requires a boring head that can offset the cutter from the axis of rotation.
CNC Turning Process
The basic CNC turning process involves four key stages:
1. Setup: The workpiece is securely clamped in a chuck or fixture on the CNC lathe. The cutting tools are loaded into tool holders and positioned in the turret. The tool offsets are entered to establish each tool’s position.
2. Programming: G-code is written based on the print specifications. This defines the turning operations, cut depths, feeds, speeds, and other parameters. Cam software can also be used to generate turning toolpaths.
3. Machining: The CNC program is run and the machine performs the sequence of turning operations. Coolant is applied to the cutting area for lubrication and chip removal. The turret indexes each tool as needed.
4. Completion: The finished part is unloaded. Quality checks are conducted to ensure specifications are met. Any necessary post-processing operations may also be completed.
Turning Machine Configurations
There are several common CNC turning machine configurations:
- Vertical turning centers: Have a vertical spindle and the workpiece is clamped to a rotating chuck. Best for small to medium parts.
- Horizontal turning centers: Have a horizontal spindle with the workpiece mounted to a rotating chuck. Ideal for heavy, larger parts. Provides excellent chip control.
- Chucking machine: Designed for high production turning of small, round parts. Uses a 3-jaw or collet chuck to grip parts.
- Bar machine: Holds bar stock and feeds it through for turning then cuts finished parts off. Highly productive for small parts.
- Multi-function machines: Combine turning capabilities with milling or other operations, enabling complete part machining in one setup.
- Swiss-style machines: Use sliding headstock and rotating guide bushing for enclosed machining of small, complex parts. Excellent accuracy.
Turning Operations G-Code
G-code is the programming language used to control CNC turning machines. Here are some common G-codes for turning operations:
- G00 - Rapid traverse to a position
- G01 - Linear interpolation motion at specified feed rate
- G02 - Circular interpolation clockwise
- G03 - Circular interpolation counter-clockwise
- G96 - Constant surface speed control ON
- G97 - Constant surface speed control OFF
- G99 - Return from canned cycle
- G28 - Return to machine home position
- M03 - Start spindle clockwise rotation
- M04 - Start spindle counter-clockwise rotation
- M05 - Stop spindle rotation
- M06 - Tool change
- F - Feed rate
- S - Spindle speed
Canned cycles are also used to simplify programming of common turning operations:
- G76 - Threading cycle
- G77 - Outer diameter turning cycle
- G78 - Inner diameter turning cycle
- G79 - Constant dwell grooving cycle
- G92 - Threading cycle
Turning Tooling
A variety of single point cutting tools are used for turning operations:
- Round Shank Tool Bits: Made of carbide, HSS, or ceramic. Brazed to a steel shank. Many styles for roughing, finishing, threading, grooving, and parting.
- Indexable Insert Tooling: Carbide inserts are mechanically clamped to a tool body. Provides economical use of the carbide. Wide range of insert shapes.
- Boring Bars: Long overhang tools for internal boring. Can be single point or indexable insert tools. Guided or non-guided styles.
Tool holders securely grip and position the cutting tools in the CNC turret. Common tool holders include:
- Static Tool Holders: Non-rotating holders for external turning and facing tools. Different styles for round and square tools.
- Driven Tool Holders: Rotating holders for drills, taps, milling cutters, and other driven tools. Enable live tooling on CNC lathes.
- Boring Bars: Specialized tooling for internal boring. Fine adjustment capabilities.
Coolants and cutting fluids are critical for turning operations. They prolong tool life, improve surface finish, and aid in chip formation and removal. Common types include water-soluble oils, synthetic coolants, and vegetable-based oils. Coolant is applied via nozzles, flood coolant systems, or through the machine spindle.
Turning Application Examples
Here are some examples of parts produced through CNC turning:
- Vehicle Parts: Brake rotors, axles, drive shafts, wheel hubs
- Industrial Parts: Gears, rollers, pulleys, bushings, valves, fittings
- Aerospace: Engine components, hydraulic fittings, landing gear parts, fasteners
- Medical: Implants, surgical instruments, needles, tubing
- Hardware: Nuts, bolts, screws, spindle shafts, linkage components
With the versatility of modern CNC turning centers, many parts can be completely machined in one setup. Multi-function machines allow turning, milling, drilling, tapping, and more. This enables high productivity and minimizes secondary operations.
Turning is an essential machining process that is widely used to produce precision cylindrical parts. CNC automation has made turning efficient, consistent, and incredibly accurate. With proper cutters, tool holders, G-code programs, and coolant delivery, CNC turning produces quality parts for almost any industry. Continued advancements in tooling options, chip control, multi-function capabilities, and automation will further increase the capabilities of CNC turning in the future. CNC Milling CNC Machining