What is CNC Turning?(remove chrome from wheels Jim)

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CNC turning is a machining process that uses computer numerical control (CNC) to automate the turning of cylindrical parts. It is a form of subtractive manufacturing where material is removed from a workpiece to create the desired final shape. CNC turning is commonly used to produce parts that have rotational symmetry and require high precision.
How CNC Turning Works
In CNC turning, the workpiece is held in a spindle which rotates at high speeds. As the workpiece rotates, a single point cutting tool moves in towards the workpiece to remove material. By precisely controlling the movement and position of the cutting tool, complex shapes and geometries can be machined with a high level of accuracy and repeatability.
The basic CNC turning process involves three main stages:
1. Clamping - The raw material, usually cylindrical stock, is securely clamped in the chuck or collet of the CNC lathe. Proper clamping prevents vibration and slippage during machining.
2. Rough cutting - In the roughing stage, the majority of the material is removed quickly to achieve the approximate dimensions. Deeper and faster cuts are taken in order to maximize material removal rate.
3. Finishing - Light cuts are taken in the finishing stage to achieve the final dimensions, surface finish, and tolerances specified in the engineering drawing. Slower feed rates and speeds are used to ensure accuracy and desired surface finish. Coolant is often applied during finishing to control chip formation, temperature, and lubrication.
Types of CNC Turning Operations
Common CNC turning operations include:
- Facing - Machining the face of the workpiece to provide a smooth reference surface and accurate length.
- Straight turning - Machining the outside diameter of a cylindrical workpiece to a required dimension and finish.
- Taper turning - Producing a tapered diameter by offsetting the tool or tailstock.
- Grooving - Cutting internal or external grooves for sealing, gripping, or snap rings.
- Undercutting - Forming an enlarged internal recess to permit welding flush to the outside surface.
- Threading - Cutting external or internal screw threads to mate with nuts or bolts.
- Drilling & boring - Creating center holes or machining internal cavities with boring bars.
- Parting/cutoff - Separating the finished part from the stock by cutting through the entire diameter.
CNC Turning Machine Configurations
There are several common configurations of CNC turning machines:
- Vertical turning centers - The workpiece rotates vertically on its end in the spindle with cutters positioned radially. Allows machining very large diameters.
- Horizontal turning centers - The most common configuration where the workpiece rotates horizontally with the cutter approaching from the side. Very versatile for medium to large lot production.
- Chucking machines - Designed for high production turning of small, repetitive parts. Excellent accuracy by using hydraulic or pneumatic chucks.
- Single spindle machines - Can only machine one part at a time but offer maximum flexibility. Useful for turning large complex parts.
- Multi-spindle machines - Allow simultaneous turning of multiple parts for high volume production. Limited to shorter parts with simple geometries.
- Swiss-style lathes - Use sliding headstocks to feed bar stock through the spindle allowing complex machining in a single setup. Ideal for very small precision parts like medical components.
CNC Turning Tools & Tool Holders
The common types of cutting tools used for CNC turning include:
- Round inserts - Simple and robust indexable inserts with multiple cutting edges. For general turning, facing, grooving, and parting operations.
- Carbide bars - Solid carbide bars ground to specific profiles for turning, profiling, and boring. For optimum surface finish and complex forms.
- Diamond inserts - Inserts with a polycrystalline diamond coating for hard turning applications without coolant. Provides excellent tool life in hardened steels.
- Boring bars - Specialized bars for internal boring. Available in single or adjustable tip styles to reach deep bores.
- Form tools - Custom ground tool steel cutters for specialized profiles not possible with standard inserts.
Tool holders securely grip and position the cutters in relation to the workpiece. Common types include ISO standard turning holders, coolant-fed holders, and specialty holders for grooving, parting, and boring bars. The turret holds multiple tool holders for quick indexing to the cutting position.
CNC Turning Workholding
Proper workholding is critical for accurate and efficient CNC turning. Common workholding methods include:
- 3-Jaw Chucks - The most versatile method. Self-centering or independent jaws allow gripping of different sizes. May deform thin-walled parts.
- Collets - Accurately hold and center bar or tubular stock. Available in different styles and high precision versions.
- 4-Jaw Chucks - Manual or self-centering chucks that provide maximum gripping for irregular stock. Require indication to get centered.
- Faceplates - Hold work against a faceplate using clamps or adhesives. Allows off-center turning and elimination of jaw marks.
- Steady rests - Support long slender parts during machining to prevent deflection and vibration under cutting forces.
- Tailstocks - Support the free end of the workpiece and allow live center turning for concentric accuracy.
Advantages of CNC Turning
Some of the key advantages of CNC turning include:
- High precision and accuracy - CNC machines can hold tolerances within 0.001" for diametral and positional accuracy.
- Excellent repeatability - CNC automation ensures each part is machined exactly the same. Margins of error are virtually eliminated.
- Fast setups -Saved tool offsets, zero positions, and canned cycles reduce changeover time between jobs.
- Improved surface finishes - Programmable speeds/feeds and rigid construction allow for fine finishes and low Ra values.
- Complex geometry capabilities - The programmable toolpath allows practically any turned profile within the kinematic limits of the machine.
- Safer operation - The enclosed environment and automated handling reduces operator interface. No manual cranking of the chucks/turrets.
- Reliable and consistent - CNC eliminates operator fatigue, lapses in judgment, and variances between parts.
- Increased productivity - CNC turning produces parts much faster compared to manual turning. More parts per shift and reduced labor costs.
With the precision, speed, and flexibility of CNC turning, many industries have adopted CNC lathes as an integral part of their manufacturing processes. The automotive, aerospace, medical, and electronics industries all utilize CNC turning to produce vital components accurately and efficiently. As technology continues to advance, CNC machining will become even more capable and widespread. CNC Milling CNC Machining