Around Machining Processes: Turning Operations and Lathe Types
ITEM 4: AROUND MACHINING PROCESSES
Production of Round Parts
Small parts: Screw hinge mount gafas.
Large parts: Rollers, rolling rings of wind generators.
Method of Work
Work part rotation. Turning raw billet. Turning of parts made with other processes (casting, forging, etc.).
The tool moves by removing material and forming the desired shape.
Major Movements
Feed Rate (f) [mm/rev]
Is the displacement of the tool during a swivel around the workpiece.
Depth of Cut (d) [mm]
Half of the difference between the diameter prior to machining and obtained after machining.
Cutting Speed (Vc) [m/min]
Speed at the point where the piece is being machined. Linked to the spindle speed and the diameter of the piece. Vc constant if the diameter is constant.
Forces in Turning
Cutting force (Fc): In the direction tangential to the cutting surface. Can be calculated through the cutting power.
Thrust force (Ft): Also called a forward force. Force tending to push the tool and away the plate.
Radial force (Fr): Tends to avert the tool of the workpiece.
Geometry Tools
Angle of Attack
Important to control the flow of chips and resistance of the tool tip.
– Positive angles improve cutting operation by reducing forces and temperatures.
– A small angle at the tip of the tool can cause splintering and fail prematurely.
Lateral Angle
More important than the posterior angle. Angles centered around ± 5º.
Cutting Edge Angle
It affects the formation of chips, the resistance of the tool. Amendment to the cutting forces. It affects the surface finish of the workpiece.
Relief Angle (ranging from ± 5º)
Controls the interference and friction. If the tool-piece friction is high the tip of the tool can chip. If small flank wear can be excessive.
Nose Radius
It affects the part surface finish and strength of the tip of the tool. Small radii increase roughness and decrease tool resistance. Large radii can cause the tool to rattle (Chatter).
Material Removal Rate (MMR)
Volume of material removed per unit time (mm3/min).
– For each round, eliminating a layer of material in a ring.
– The ring volume is the product of cross-sectional area (f)(d) and the average circumference of the ring (PI * Dprom).
Roughing Cuts and Finishing Operations
It is usual first perform one or more roughing cuts with high f and high Vc. Such cuts are followed by finishing operations.
Materials, Cutting Feeds and Speeds of Tools
Selection of tools based on Vc and f.
Cutting Fluids
Normally the application of fluid improves the machining operation. There is a tendency towards dry machining.
Components of the Lathe
1. Bancada
Supports the main components of the lathe.
2. Carro
Slides along the guide-assembly. Consists of cross slide, tool holders and tablero.
Corredera Transverse: Radial movement in and out.
Top: Motion machine equipped with manual and machining of the car and slide transversal.
3. Head
Fixed to the bench. Supplying energy to a spindle at different speeds of rotation.
4. Counterpoint
Can slide along the guides. Supports the other end of the piece of work.
5. Progress Bar and Screw Guide
Provides moving the car and slide transversal.
6. Lathe Specifications
Flip: Ø max. of the workpiece that can be machined.
Dot pitch max: Distance between head and counterpoint.
Length bancada.
7. Clamping Devices and Accessories (Tools)
Hold the workpiece securely.
Claw plate (chuck):
– Equipped with three or four jaws.
– Clamping jaws can allow internal and external clamping.
– Soft Jaws: Machined to give the desired shape. They adapt to irregularities in the workpiece.
8. Chucks Center
Are placed inside the hollow workpiece or tubular. Hold parts that require machining on both ends (solid chuck, multiple chuck, conical chuck).
9. Mouthpiece
Split-tapered bushing, longitudinal type:
– Nozzle insertion.
– Nozzle thrust.
– The contract radially tapered surface of the nozzle segments holding the work surface.
– They fit well and hold most of the circumference.
– Used in parts with maximum diameter 2.5 cm.
10. Trawl Plato
Parts are subjected to irregular work. They have slots and holes through which screws subject the piece.
Basic Operation
1. Turning
Workpieces straight, tapered, curved or slotted. Example: axles, spindles and pins.
2. Facing
Flat surface at the end of the workpiece perpendicular to the axis. Useful for parts that are assembled with other components.
3. Grooving
Slots at the end of the workpiece in the axis perpendicular to rotation.
4. Court with a Tool (Profiles)
Produce axis symmetrical shapes for functional or aesthetic purposes.
5. Boring or Perforated
Enlarged cylindrical cavity made in the pipeline.
Drilling: Produces internal holes.
Slots: Produces an annular hole or cavity.
Reasons for boring: Better dimensional accuracy, superficial bucking.
6. Parting Off (Cut)
Cut a piece fully to the axis of rotation.
7. Threading
Produces external or internal threads.
8. Knurling
Produces regular roughness on cylindrical surfaces.
Types of Lathes
Turning-Machines
They are placed on a workbench or table. Power is low. Usually hand-fed. Machining of small parts.
Special Purpose Lathe
Work pieces up to 2 m in diameter by 8 lengths. Capacity of 450 kW. Example: Guns of arms, rolling mill rolls.
Copying Lathe
The cutting tool follows a path equal to the contour of the template.
They have been replaced by Numerical Control lathes.
Automatic Lathes
Fully automatic: The parts are moved and removed automatically.
Semi-automatic: Progress and withdrawal are done by an operator.
Machines for Bars
Known as automatic screw machines.
High-speed machining for the production of screws and threaded parts.
All operations are performed automatically.
After each part or screw is machined, the bar automatically advances through the spindle hole and is then cut.
Turret Lathes (Gun)
Performs multiple cutting operations: Turning, boring, threading, facing.
Various cutting tools can be mounted on the turret.
Computer-Controlled Lathes
Movement control of the machine and tool by computer numerical control (CNC).
They have good dimensional accuracy and repeatability.