What is CNC Turning?(material hardness Ingemar)
- source:WEINBERG CNC Machining
How CNC Turning Works
In CNC turning, the workpiece is rotated while a single point cutting tool moves linearly to remove material. The linear and rotational axes are controlled precisely by the CNC system to achieve the desired dimensions and surface finish. Here are the key steps in CNC turning:
1. The operator uploads the CNC program into the machine control. This program contains the commands for the toolpath and cutting parameters.
2. The workpiece is loaded into a collet or chuck which is mounted onto the CNC lathe's spindle. The spindle provides the rotational motion.
3. The tool turret indexes to bring the correct cutting tool into position. Common tool types are roughing, finishing, grooving, threading, drilling, and boring tools.
4. The machine applies coolant to lubricate and reduce heat at the cutting interface. Coolant also flushes away chips.
5. The spindle rotates the workpiece at the programmed speed while the tool feeds towards it. The tool follows the toolpath in the CNC program, removing material as it moves.
6. Multiple tools can be automatically changed to perform different operations in the same setup. For example, roughing, finishing, grooving, and parting off.
7. Once complete, the finished part is unloaded. Quality checks are performed to ensure accuracy.
Advantages of CNC Turning
There are several key advantages of CNC turning compared to manual turning:
- Higher accuracy and better repeatability. The CNC system can precisely control dimensions and surface finish.
- Faster production time due to automation of toolpath movements. Complex parts can be machined efficiently.
- Ability to produce complex geometries using multi-axis turning centers. Eccentric and non-round shapes are possible.
- Quick changeover between jobs. CNC programs can be easily modified for new parts.
- Less skilled labor required. The CNC system guides the cutting process rather than manual operator skill.
- Safer working environment. The operator does not need to directly interface with the machine during cutting.
- Reduced waste and material costs via optimized toolpaths. Machining parameters can be easily adjusted.
CNC Turning Operations
Here are some of the common turning operations that can be performed on CNC lathes:
- Facing - Machining the face of the workpiece to create a flat reference surface.
- Rough turning - Removing the bulk of material using deeper, faster cuts with a roughing tool. Leaves excess stock for finish turning.
- Finish turning - Uses a finer feed rate and shallower depth of cut to achieve the final dimensions and surface finish.
- Chamfering - Beveling sharp edges for deburring and safety. Done with a chamfering or grooving tool.
- Grooving - Machining grooves or recesses using a form tool or insert. For light cuts only.
- Cut-off - Parting the finished workpiece off the excess barstock using a specially ground parting tool.
- Drilling - Creating holes axially using a rotating twist drill or boring tool.
- Boring - Enlarging existing holes to accurate diameters using single point boring bars.
- Threading - Cutting external or internal screw threads using dedicated threading tools.
- Taper turning - Creating linear tapers by offsetting the tool position or by manipulating the axes.
- Knurling - Rolling or indenting a crosshatch pattern onto the surface to create a grip texture.
- Radiusing - Forming a rounded edge instead of a sharp corner to remove stress concentrations.
CNC Turning Machine Configurations
CNC lathes come in several common configurations:
- Vertical turning centers - The spindle is oriented vertically. Allows chucked workpieces up to 25,000 lbs.
- Horizontal turning centers - The spindle is horizontal. Most common configuration. For smaller workpieces.
- Chucking machine - Uses a 3, 4, or 6 jaw chuck for holding. Lower rigidity than a collet.
- Bar fed machine - Feeds barstock through the spindle to allow unattended production. Good for high volumes.
- Multi-axis turn mill - Combines a lathe and milling axes. Allows complex milling operations.
- Multi-spindle - Multiple cutting heads for production turning of small, identical parts. Very fast.
- Swiss-style lathe - Uses sliding headstock to feed barstock through the spindle. Good for small, complex parts.
CNC Turning Workholding Methods
Proper workholding is critical for accurate and safe CNC turning. Common workholding methods include:
- 3-Jaw Chuck - The most popular. Self-centering. Good for irregular shapes. Limited rigidity compared to collets.
- 4-Jaw Chuck - Independent jaws for special workpieces. Must be manually aligned. Highest rigidity.
- Collet Chuck - Precision hardened collets with taper interface. Extremely rigid with excellent TIR. For round stock only.
- Faceplate - Large flat plate that can be mounted to the spindle. For mounting unusually shaped work.
- Steady rest - Support that contacts the workpiece to prevent deflection under cutting forces. Used for long thin parts.
- Follower rest - A trailing support that prevents deflection during internal boring operations on small diameters.
- Mandrel - A precision ground shaft used to hold bushings or sleeves for internal grinding or turning.
- Centering - Supporting work between centers. Requires a precisely machined center hole and point.
CNC Turning Operations
Here are some examples of common parts produced by CNC turning:
- Engine crankshafts - Machined with very tight tolerances. Multi-diameter and eccentric. Highly automated for volume production.
- Transmission shafts - Hardened steel shafts with splines, gears, and bearing surfaces. Require multiple tools.
- Nozzle bodies - Contoured shapes with internal bores. Made from corrosion resistant alloys.
- Rollers - Cylindrical rollers with bored center holes. Made in large quantities for conveyors and guides.
- Pistons - Aluminum pistons for engines and pumps. Complex profiles machined in a single setup.
- Wheels - From automotive wheels to casters. Turning produces the outside diameter and face.
- Pulleys - Used to transfer rotary motion via belts. Require precise tolerances and surface finishes.
- Bushing and bearings - Made to tight tolerances from bronze and babbitt alloys. Some with oil ports.
- Fasteners - Bolts, screws, and nuts. High speed production turning of simple solid stock.
- Valve bodies - Internal fluid passages made to direct flow. Require boring, threading, and contouring.
As seen from these examples, CNC turning can produce a diverse range of rotational parts to meet virtually any production requirements. The key is applying the right tooling, planning, and programming to maximize the advantages of CNC.
CNC Turning Tooling
Correct selection of tooling is critical for productivity and part quality in CNC turning. Here are the most common tool types:
- Roughing - Designed for heavy material removal. Made from durable carbide with chipbreaker geometries.
- Finishing - Sharper cutting edge for fine surface finish. Positive rake angle and highly polished.
- Threading - Ground with precise thread form profiles. Carbide inserts or HSS/carbide tool bits.
- Grooving/cutoff - Narrow insert for cutting grooves and parting off. Requires high pressure coolant.
- Boring - Replaceable or fixed insert tools for internal boring. Single or multi-point designs.
- Drilling - Replaceable carbide insert drills and solid carbide drills. For through and blind hole drilling.
- Form/profile - Shaped inserts for contouring and profiling. Diamond inserts for non-ferrous materials.
- Tool holders - Precise interface between tool and turret. Provides rigidity and accuracy. Many styles.
- Inserts - Indexable carbide inserts with special substrate and coatings for tool life. Many geometric styles.
Optimizing toolpath strategies, speeds and feeds, and tool selection allows CNC turning to be performed productively and accurately. The versatility of CNC tooling enables a wide range of part features.
CNC Turning Software
CNC turning requires CAM software to generate the machine code instructions the CNC system executes. CAM software processes 3D models into toolpaths optimized for turning. Here are the main steps in CAM programming for CNC turning:
1. Import 3D model - The solid model defines the required part geometry. Most CAD files can be imported.
2. Define stock setup - Stock dimensions are set as well as chuck and bushing definitions.
3. Select tools - All required tools are selected from the tool library to be used in programming.
4. Generate toolpaths - The CAM software calculates roughing, finishing, grooving, threading, and drilling toolpaths.
5. Set feeds and speeds - Cutting parameters are set based on tool data, material, and operation type.
6. Simulate and verify - The programmer simulates the toolpaths to visually check for errors before posting.
7. Post process - The CAM file is converted into a CNC machine code file like G-code. It's then loaded on the CNC.
Popular CAM packages for CNC turning include Mastercam, GibbsCAM, Esprit, EdgeCAM, and SolidCAM. Powerful toolpath algorithms make programming efficient. Simulations ensure program accuracy.
CNC Turning Materials
CNC lathes can machine a very wide range of materials from plastics to exotic alloys. Here are some of the most commonly machined materials:
- Aluminum - Popular for parts requiring good strength-to-weight ratio and corrosion resistance.
- Steel - Low carbon steel is easily machined. Alloy steel and tool steel offer greater strength.
- Stainless steel - Corrosion resistant with excellent high temperature properties. More difficult to machine.
- Titanium - Extremely strong and light. Used in aerospace applications. Challenging to cut and requires high pressure coolant.
- Brass - Good machinability and corrosion resistance. Used for decorative applications. Creates long stringy chips.
- Bronze - Better frictional properties than brass. Used for bushings and bearings. Prone to work hardening.
- Plastics - Engineering plastics like Delrin and nylon are frequently turned. Produce small stringy chips.
- Cast iron - Relatively easy to cut but abrasive. Used where vibration damping is required.
In general, softer, low carbon materials like aluminum and mild steel are the easiest to turn. Harder alloys require rigid setups and sharp tooling to achieve good part quality and tool life.
CNC Turning Finishes and Tolerances
A key advantage of CNC turning is the ability to produce very tight dimensional tolerances and fine surface finishes. Some examples include:
- Fine boring - Capable of holding 0.0002" tolerance for a slide fit. Requires rigid setup.
- Threading - Can produce Class 3A threads holding 0.0001" tolerance on the pitch diameter.
- Grooving - Fine groove widths down to 0.015" wide are possible depending on insert size.
- Surface finish - Can achieve Ra 2-4 microinch with the right tooling and light cuts.
- Geometric tolerances - CNC machines can reliably hold parallelism, runout, and position tolerances to 0.001".
- Concentricity - Precision spindle bearings combined with a stiff setup allows 0.0005" TIR or better.
With the right programming techniques, tooling, and machine setup, CNC turning can achieve similar tolerances and finishes as grinding. Multiple operations can also be performed in a single setup.
In summary, CNC turning is an extremely versatile machining process capable of producing precision rotational parts from simple geometries to complex components. Automating the turning process with CNC control provides numerous benefits including reduced cycle times, increased accuracy and repeatability, and the ability to easily change over between different parts. With skilled operators and programmers, CNC turning is a highly efficient manufacturing process suitable from low to high production volumes. CNC Milling