What is a Turning Operation in CNC Machining?(removing chrome plating Sigrid)

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Turning is one of the most common machining processes used in manufacturing. It involves rotating a cylindrical workpiece while applying a cutting tool to the surface to remove material. The resulting cylindrical shape is characterized by its diameter, surface finish, concentricity, and other attributes. Turning operations are commonly performed on lathes or turning centers which provide the rotational motion and cutting forces needed for turning.
In CNC (computer numerical control) machining, turning operations are programmed and executed by computer controls rather than manually. CNC allows turning intricate and complex parts with tighter tolerances and increased automation compared to manual turning. Understanding what a turning operation entails and how it works is key for programmers, operators, and anyone involved in CNC-based manufacturing.
How Turning Works
The turning process utilizes a rotating cylindrical workpiece and a tool bit that traverses across the work to cut away material. The cutting forces are applied perpendicular to the workpiece's axis of rotation, causing rounded shavings to come off the surface. The two main components needed are the rotating workpiece (secured in a chuck or collet) and the cutting tool which must be set at the proper depth and feeds for the desired material removal.
The workpiece rotates at a constant speed while the tool follows a particular path to cut the desired shape. The tool path can be guided manually by hand controls or programmed into a CNC machine. Typical CNC turning centers provide one or two spindles for mounting workpieces, an automatic tool changer, and programmable axes of motion.
Types of Turning in CNC
There are several variations of turning processes available with CNC:
- Outside diameter (OD) turning - The most common turning operation, OD turning is used to reduce the external diameter of a cylindrical workpiece. The work rotates while the tool traverses horizontally across the part.
- Inside diameter (ID) turning - Also called boring, ID turning removes material from the internal diameter of a workpiece. The cut is performed with an insert or boring bar inside the bore.
- Facing - Used to create a flat surface on the end face of a cylinder. The tool moves perpendicular to the rotating axis to cut off material.
- Taper turning - Produces a tapered shape by turning at an angle offset from perpendicular. The tool or workpiece is offset from parallel to machine tapers.
- Grooving/parting - Narrow grooves and parting cuts are made with a specially shaped tool. Grooving is performed on the OD or ID, while parting cuts completely separate the workpiece into two parts.
- Threading - Threading tools cut spiral threads onto a cylinder through synchronized rotational and linear movement. Multiple passes are made to cut full thread depth.
- Drilling - Performed on certain CNC lathes to add center holes or drilling operations inline with turning. Live tooling or backside tools are used to drill axial holes in the workpiece.
- Form turning - Complex shapes, profiles, and contours can be turned using specially shaped form tools. The program coordinates linear and rotational axes to machine the form.
Turning Operations Described
Outside Diameter (OD) Turning
The most common type of turning, OD turning is performed on the external surface of a cylindrical workpiece. The purpose is usually to reduce the diameter of a workpiece to precise dimensions and surface finish requirements. A cutting tool is fed perpendicular into the rotating outside diameter while traversing the part longitudinally. The depth of cut, feed rate, speed, and tool path all contribute to the final shape.
Multiple passes are typically required to remove all the required material. The tool moves further radially with each pass until the desired diameter is reached. Depth of cut, feed rates, and cutting speed determine the rate of material removal. Fine finishing passes leave optimal surface finish and high dimensional accuracy. OD turning allows cylindrical parts to be sized with relatively tight tolerances. It is commonly used on castings, forgings, and bar stock to produce parts like shafts, rolls, pistons, rods, and axles.
Inside Diameter (ID) Turning
Also referred to as internal turning or boring, ID turning removes material from the internal diameter of tubular workpieces or pre-bored holes. An insert tool or boring bar is fed radially into the ID of the rotating workpiece to enlarge and finish bored holes and internal cavities. The same variables apply as with OD turning. The main difference is the cutting forces are directed radially outward rather than inward.
ID turning produces precise hole sizes and surface finish characteristics on the interior diameters of parts. Finishing and sizing bored holes is one of the most common ID turning operations. Machining allowances are often left on holes for finishing as the last step. ID turning also enables threading, grooving, and other features to be added to internal surfaces. Care must be taken to avoid chatter and vibration issues which can damage boring bars. Proper selection of boring bar material and geometry is key for successful ID turning.
Facing Turning
Facing is a turning operation that creates a flat face on the end of a cylindrical workpiece. It is performed by feeding the tool perpendicular to the rotating work axis. The tool moves crosswise along the face to remove material until the desired face length is achieved. Facing is often one of the first steps machined on the workpiece to create a reference surface. This establishes a square starting point for further turning operations.
In CNC turning, facing can be performed using a standard turning tool. For repetitive facing work, a special tool holder called a facing tool bit or facing head is often utilized. The inserts have larger nose radii optimized for facing rather than turning. Facing is also useful for squaring rough surfaces, sizing lengths, and preparing surfaces for secondary operations like parting or grooving. Along with OD/ID turning, facing is an essential operation for most turned components.
Taper Turning
While the above operations produce straight cylindrical features, taper turning is used to machine parts with angled sides. By offsetting the tool at an angle, material can be removed to create tapers. There are two main methods:
- Feeding at an angle offset from perpendicular to the workpiece. The tool followed a skewed path rather than being fed straight into the part.
- Setting the workpiece at an angle using a taper turning attachment. This tilts the part so the tool can still feed perpendicularly against the angled work surface.
In both cases, the angle of the tool or work determines the resulting taper angle. More tool offset and tilt creates a steeper taper angle. Programming coordinates and calculating the proper feed angles requires trigonometric functions. CNC controls greatly simplify programming taper turning with their ability to control skewed axes automatically.
Common examples of tapered turning include tool shanks, morse tapers, and various conical workpieces. The ability to turn precise tapers avoids the need for secondary grinding or forming operations. This makes CNC turning an ideal process for machining taper geometries.
Grooving and Parting Off
While turning produces cylindrical forms, grooving and parting operations cut different special profile shapes. In grooving, a tool with a slender profiling insert is used to machine grooves, radii, and recesses into the work diameter. Grooves can be turned on the OD, ID, or face of the workpiece. Typical grooving inserts have a rectangular, triangular, or semi-circular shape. Programming coordinates position the tool laterally across the blank where the groove is required.
Parting tools have inserts specifically designed for parting off work. They are much wider than standard inserts to support parting forces. The tool feeds radially into the rotating work until it cuts through the entire diameter. This separates the workpiece into two parts resulting from a single turned bar. Parting is commonly performed last to cut finished parts from the raw stock after all features are complete. Parting and grooving tools provide turning capabilities beyond just cylindrical shapes.
Thread Turning
Threading is one of the most common features cut on turned parts. Threads provide assembly and motion capabilities for mating with threaded fasteners, printers, adjusters, and other threaded components. Acme, worm, and various metric/UN threads can be produced by turning. The process requires coordinated rotation of the work with longitudinal movement of the tool following the thread helix path.
The CNC control coordinates both the synchronized spinning and linear travel. Infeed depth is gradually increased over multiple passes until full thread depth is achieved. Thread turning produces internal and external threads in a single setup. Challenges include calculating the right feed rates, dealing with tool deflection, and avoiding chatter marks. Thread turning tools utilize ground thread inserts that are shaped to cut the thread form. Overall, threading is simplified, accelerated and standardized through CNC turning compared to manual methods.
Drilling Turning Operations
Some CNC lathes include live tooling capabilities for drilling and milling operations in addition to turning. This enables drilling cross holes and machining other features into the face or OD of the workpiece. Holes are drilled with rotating tools mounted in tool turrets or tailstocks. Tool changers swap these out automatically with turning tools.
Backworking is a technique where rotating tools are mounted behind the part to drill axial holes into the back side. Deep holes can be added without the need to change setups or machines. Live drilling improves throughput by reducing secondary operations previously done on separate machines. Turning operations and drilling/milling can be combined in one integrated CNC program. Live tooling is the main feature enabling single-setup, multi-process turning centers.
Form Turning Complex Shapes
While basic turning produces cylindrical surfaces, CNC programming enables the creation of far more complex contours and form shapes. This is known as form turning or contour turning. The tool follows 2D or 3D programmed paths to cut unique profiles into the workpiece. Diamonds, squares, tear drops, and various irregular forms can be produced via form turning.
Complex geometries often require specially shaped form tool inserts. The program coordinates movement and offsets between multiple axes to traverse the tool along the exact path. Form turning is commonly used for parts like blisk blades, medical implants, gears, cams, and other parts that are naturally difficult to machine. The CNC's capabilities for controlled, multi-axis movement is what makes form turning with intricate tool paths possible.
Programming Turning Operations
Whether simple shafts or complex parts, all CNC turning is programmed via CAM software. The basic process is:
1. Create 3D model of the finished part geometry
2. Define stock dimensions where material is in its raw form
3. Select tooling including inserts, tool holders, and any live tooling
4. Program tool paths defining how each tool will move to machine features
5. Specify depths, feeds, speeds, offsets and other operating parameters
6. Generate CNC code (G & M codes) that the machine will run
7. Load program onto machine control and setup workholding fixtures
8. Execute turning cycles and make any needed tool adjustments
9. Inspect finished components for accuracy
Programming considers the entire workflow from stock to finished part. Every tool motion and machining feature is defined in the CAM plan. Simulations help visualize the plan before running on the actual machine. The automated nature of programming and tool changing reduces manual work compared to traditional turning methods.
Advantages of CNC Turning
- Tighter tolerances and better surface finishes vs. manual turning
- Faster cycle times with optimized tool paths and parameters
- Quick changeover between jobs with stored programs and tools
- Minimal setups due to live tooling, backworking, etc.
- Reduced manual labor through automation
- Ability to turn complex profiles not possible manually
- Simplified integration with CAD/CAM processes
In summary, the full range of turning and auxiliary operations can be leveraged through CNC. Completed machined components come off ready for assembly or finishing rather than requiring extensive manual rework as with manual lathes. The technology continues advancing with smarter machine tools, controls, and software that further improve precision, efficiency and automation. Turning on modern CNC machines offers vastly expanded capabilities versus traditional methods. CNC Milling CNC Machining