Introduction(cnc paper cutter Marjorie)
- source:WEINBERG CNC Machining
In CNC (computer numerical control) turning, the motions of the cutting tool are controlled by a computer program that dictates the feed rate, depth of cut, spindle speed and other parameters. CNC turning is highly automated and precise, allowing complex parts to be machined efficiently and accurately.
What is Turning in CNC machining?
Turning uses a single point cutting tool to shape the outside diameter of a rotating workpiece. The workpiece is held and rotated by the chuck of a lathe, while the cutting tool is fed horizontally or parallel to the axis of rotation to remove material. By precisely controlling the movements and location of the cutter, complex geometric shapes can be machined.
The turning process can produce cylindrical shapes, tapers, grooves, threads and contoured surfaces. Typical workpieces machined by turning include shafts, rods, pins, spindles and axles. The diameter that can be turned is limited by the distance between the lathe centers and the size of the chuck. Very long thin parts can be turned using an attachment such as a steady rest.
Types of Turning in CNC
There are several variations of the turning process:
- Outside diameter (OD) turning - machining the external surface of a rotating cylinder
- Inside diameter (ID) turning - machining the internal surface of a rotating cylinder
- Facing - machining the flat face of a rotating workpiece, usually to make it perpendicular to the center axis
- Taper turning - producing a tapered part by turning at an angle to the center axis
- Grooving - cutting a slot or recess around the diameter of a workpiece
- Threading - cutting screw threads by precisely coordinating axial movement with rotation
- Boring - enlarging or smoothing pre-existing holes in a workpiece
- Parting - cutting off a completed part from the remaining material
The basic sequence for a turning operation is:
1. The workpiece is loaded into the lathe chuck or collet and centered. For long parts, a steady rest may support the free end.
2. The cutting tool is selected and loaded into the tool post at the proper height and angle. Carbide inserts with specialized geometries are commonly used.
3. The spindle speed (rpm) and feed rate are set based on the workpiece material, tool material, tool geometry, depth of cut and desired surface finish. Faster speeds require coolant to prevent overheating.
4. The tool is fed towards the rotating workpiece to begin the cut. For roughing cuts, a large depth of cut and fast feed rate are used. Finishing cuts use a smaller depth of cut and slower feed rate. Multiple passes are made until the desired dimensions are achieved.
5. For threading, the tool coordinates axial travel with the rotation to accurately cut the thread shape. Threading dials help synchronize the movements.
6. The tool is retracted, the chuck is opened, and the finished part is removed. For mass production, an automated parts catcher is often used.
7. The cutting tool may be replaced or re-sharpened as needed. Worn tools can negatively impact part tolerance and finish.
Turning Machines Used in CNC
The most common type of CNC turning machine is the CNC lathe. Variants of CNC lathes include:
- Vertical lathes - The workpiece is held vertically rather than horizontally. This allows machinig very large diameter parts.
- Chucking machines - Designed for high volume production. Automatically loads and unloads workpieces. Can be single or multi-spindle.
- Bar fed machines - Feed bar stock through the spindle and machine parts sequentially without manual loading. Allows unattended operation.
- Turret lathes - Use multiple cutting tools mounted on a programmable turret to machine complex parts in one setup.
- Precision lathes - Achieve very high tolerances and fine surface finishes. Ideal for machining precision engineered components.
- CNC turning centers - Highly versatile for combined turning and milling operations on complex parts.
- Gang tool lathes - Multiple cutting tools can be engaged with the workpiece simultaneously for faster machining of complex contours.
CNC controllers adjust the operating parameters and precisely control the movements and positioning of the cutting tools during the turning process. Backlash compensation improves precision when reversing direction. Modern CNC lathes provide extremely high accuracy and repeatability for precise, automated turning operations.
Turning Operations Used in CNC
Some of the common turning operations that can be performed under CNC include:
- Straight Turning - The most basic operation for machining straight outside diameters.
- Taper Turning - Produces tapered diameters by offsetting the tool at an angle from the centerline.
- Profiling - Machining contoured shapes by interpolating the tool along multiple axes.
- Grooving - Cutting grooves and recesses into the workpiece surface. Useful for seals, parting lines and decorations.
- Threading - Single or multi-start threads can be cut by coordinating the rotation and linear motion.
- Boring - Enlarging the inside diameters of holes, or making the holes precise by fine boring.
- Drilling - Spotting holes in the centers of workpieces, or drilling radial holes. Performed with live tooling.
- Tapping - Cutting internal screw threads with a tap mounted in the turret.
- Knurling - Creating crosshatched patterns on the surface to aid grip. Done with a knurling tool.
- Parting Off - Cutting completed parts from the bar stock remnant for automated production.
- Face Grooving - Grooving/threading on the face of a part, facilitated by live tooling.
The wide range of turning operations possible on modern CNC lathes makes them highly versatile for machining precision parts. Complex components can be completed in a single setup.
Advantages of CNC Turning
There are many benefits of using CNC operated lathes and turning centers:
- Higher productivity - CNC automates turning operations for faster cycle times and around-the-clock capability.
- Improved quality - CNC turning machines offer better repeatability and precision. Parts can be made within tighter tolerances.
- Operator safety - The operator does not need to directly monitor the cutting process reducing hazardous manual interventions.
- Reduced setup time - CNC allows quick changeover between jobs. Cutting programs can be stored and reused.
- Integration with automation - CNC turning centers can utilize automated workpiece handling, tool changing and parts catching.
- Multi-axis capabilities - Modern CNC lathes allow live tooling, cross drilling, milling and other multi-axis functions.
- Intricate geometry - Complex part geometry can be produced using programmed interpolation of motions that would not be possible manually.
- Unattended operation - CNC allows turning operations to be run untended, enabling lights out production.
- Less skilled labor - Intricate parts can be produced without highly skilled machinists due to the automation capabilities.
- Consistent quality - CNC eliminates variations in output and greatly reduces scrap rates compared to manual turning.
With so many advantages, it's easy to see why CNC turning has become an essential manufacturing process for machining precision parts to high levels of productivity and quality. Continuing advances in CNC technology promise even greater benefits in the future.
Turning is a fundamental machining process that is greatly enhanced by the precision and automation capabilities of computer numerical control systems. CNC turning is used across manufacturing industries to produce machined parts of various shapes, sizes and complexities. Understanding what CNC turning entails is key for engineers, operators, programmers and anyone involved in manufacturing technology and processes. With its many benefits and continuously improving capabilities, CNC controlled turning will remain an integral production technique for the future. CNC Milling