Lathe Turning: Workholding, Machines, Operations & Plastics
Turning Fundamentals
Turning is one of the most common and versatile cutting technologies, primarily performed using a lathe.
Workholding Devices
Jaw Chuck
No additional support needed.
- Self-centering: Suitable for cylindrical or polygonal parts.
- Independent jaws: Require exact positioning, increasing time and cost.
Face Plates
Used for parts with irregular profiles.
Pneumatic or Hydraulic Chucks
Operated by pneumatic or hydraulic pressure.
Magnetic Chucks
Centering can be complex; cannot hold non-magnetic metals. Saves time in workpiece placement.
Jaw Chuck and Center
One side is held by a chuck, and the other is supported by a live center. The support rotates to minimize friction.
Between Centers
The part is held between two centers. Rotation is produced using a lathe dog, which requires setup time.
Steady and Follow Rests
- Steady Rest: Used for supporting long workpieces during internal operations.
- Follow Rest: Used for supporting long workpieces during external operations, moving with the tool.
Mandrel
Used for holding hollow workpieces for internal or external concentric machining operations.
Expansion (Collets)
Workpiece is fixed using an elastic collet, leveraging its deformation capability. Significant time can be spent setting up.
- Elastic Collets: Grip bars by applying pressure. Low setup time, typically used for small diameters.
Lathe Machine Types
Note: The following points relate to each machine type: 1-Machining Time, 2-Idle Time, 3-Setup Time, 4-Typical Series Size.
Bench, Center, Engine Lathe
Manual operation. The same part holding device can be used for different parts.
- 1. Important
- 2. Important
- 3. Small
- 4. Unitary or up to 10 parts
Face Lathe
Used for turning large and heavy parts.
- 1. Important
- 2. Important
- 3. Important
- 4. Unitary, small series
Vertical Lathe
Used for turning very large, heavy parts with large diameters.
- 1. Important
- 2. Important
- 3. Important (due to weight)
- 4. Unitary, small series
Turret Lathe
Semiautomatic with automatic tool change. Several tools can work simultaneously. Initial setup time is longer until adapted.
- 1. Less than bench lathe
- 2. Less than bench lathe
- 3. Important initially (all tools must be adjusted)
- 4. 25 parts or more
Copy Lathe
Tool movements are based on a template. Cannot perform internal operations. One tool works at a time; tool change is manual.
- 1. Less than bench lathe
- 2. Less than bench lathe
- 3. Less than turret lathe
- 4. 5 parts or more
Single Spindle Automatic Lathe
Features automatic movements, part feeding, and changing. Can use different tools simultaneously.
- 1. Less than bench lathe
- 2. Zero
- 3. Important
- 4. 1000 parts or more
Multi-Spindle Automatic Lathe
Similar to single spindle, but 6 to 8 parts can be produced simultaneously. Spindles rotate independently.
- 1. Low
- 2. Zero
- 3. Important
- 4. 10,000 parts or more
Turning Center (CNC Lathe)
Numerical Control Lathe. Available in horizontal and vertical configurations (vertical for large pieces). Features automatic movement and tool change. One tool operates at a time. The same part holding device can be used for different parts.
- 1. Less than bench lathe
- 2. Small
- 3. Slightly important
- 4. 10 to 1000 parts
Machining Operations
External Operations
- Turning: Single-point tool moves axially along the workpiece.
- Facing: Single-point tool moves radially across the end of the workpiece.
- Conical or Taper Turning: Achieved using the compound rest set at an angle.
- Grooving: Creates a groove on the outer diameter (OD) or face. Care must be taken not to introduce too much stress.
- Parting or Cutoff: Similar to grooving, used to separate a part from the stock. Chip evacuation is a critical factor.
- Form Turning: Uses a cutting tool with a complex shape. The tool itself is complex and expensive, but the feed motion is linear (e.g., creating a curved profile).
- Contour or Profiling Turning: Uses a simple tool shape, but the feed motion is complex to create contours (e.g., shaping a handle).
- Chamfering: Eliminates sharp edges for safety, assembly, or tolerance requirements.
- Undercutting: Creates a recess to facilitate subsequent operations (like threading or grinding). Often requires tools with round cutting edges.
- Knurling: A unique lathe operation that forms a pattern on the surface without cutting material (e.g., for grip on handles).
- Threading: Requires a precise relationship between the spindle speed and the axial feed rate to create threads.
Internal Operations
- Boring: Enlarges a previously drilled hole. The tool moves using the longitudinal carriage.
- Facing: Machines an internal surface perpendicular to the rotational axis.
- Grooving: Tool movement is controlled using the cross-slide to create internal grooves.
- Taper or Conical Turning: The compound rest may be turned on the cross-slide to machine internal tapers at any angle.
- Center Hole Drilling: Creates a special hole to accommodate a lathe center for later support.
- Drilling: A common operation, often performed by mounting a drill bit in the tailstock.
- Reaming: A finishing operation, typically following drilling, to achieve precise hole diameter and smooth finish.
- Threading: Can be done with a single-point tool or by tapping. Tapping is often a manual operation with the part stationary.
Plastics Overview
Properties
Mechanical Properties
High elasticity, low mechanical strength, low hardness, high deformation capacity, high impact absorption.
Physical Properties
Good thermal and electrical insulators, low melting point, low density, significant shape changes at high temperatures.
Chemical Properties
Very good corrosion resistance, loss of properties under sunlight (UV degradation), some types can degrade in high oxygen atmospheres, generally impossible to mix different types of polymers, recycling can be difficult.
Types of Polymers
Thermoplastics
Internal structure does not change during heating and shaping. Account for over 70% of industrial production. Can be repeatedly melted and cooled while retaining their shape. Generally have low melting points, are flexible, and soft. Examples include PVC and PE (used for electrical insulation).
Thermosets
Undergo a permanent change in molecular structure (curing process) during heating. Once cured, they cannot be remelted. They are strong materials suitable for high-temperature applications, offering longer life and strength. Examples include polyester and epoxy resins.
Elastomers
A specific type within thermosets, characterized by high elasticity and a mesh-type molecular structure. Examples include silicone and natural rubber.
Common Manufacturing Processes
- Injection Molding
- Extrusion
- Thermoforming
- Blow Molding
- Compression Molding
- Transfer Molding
- Rotational Molding