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Material removal Processes Turning and Hole Making (Drilling, Drills, and…
Material removal Processes Turning and Hole Making
The Turning Process
Majority of turning operations use simple single-point cutting tools, which is a right-hand cutting tool
Important process parameters have a direct influence on machining processes and optimized productivity
Tool Geometry
Rake angle control both the direction of chip flow and the strength of the tool tip
Relief angle controls interference and rubbing at the tool–workpiece interface
Roughing and Finishing Cuts
First practice is to have one or more roughing cuts at high feed rates and large depths of cut
Little consideration for dimensional tolerance and surface roughness
Followed by a finishing cut, at a lower feed and depth of cut for good surface finish
Introduction
Turning is performed at various:
Rotational speeds, N, of the workpiece clamped in a spindle
Depths of cut, d
Feeds, f, depending on the workpiece materials, cutting-tool materials, surface finish, dimensional accuracy and characteristics of the machine tool
Lathes and Lathe Operations
Introductions
Lathes are considered to be the oldest machine tools
Speeds may range from moderate to high speed machining
Simple and versatile
But requires a skilled machinist
Lathes are inefficient for repetitive operations and for large production runs
Lathe Components
Bed
Supports all major components of the lathe
Carriage
Slides along the ways and consists of an assembly of the cross-slide, tool post, and apron
Headstock
Equipped with motors, pulleys, and V-belts that supply power to a spindle at various rotational speeds
Tailstock
Slide along the ways and be clamped at any position, supports the other end of the workpiece
Feed Rod and Lead Screw
Powered by a set of gears through the headstock
Lathe Specifications
Max diameter of the workpiece that can be machined
Max distance between the headstock and tailstock centers
Length of the bed
Workholding Devices and Accessories
Workholding devices must hold the workpiece securely
Chuck is equipped with three or four jaws
Three-jaw chucks used for round workpieces
Four-jaw (independent) chucks used for square, rectangular, or odd-shaped workpieces
Power chucks are used in automated equipment for high production rates
Chuck selection depends on the type and speed of operation, workpiece size, production and dimensional accuracy requirements and the jaw forces required
A collet is a longitudinally-split, tapered bushing
Used for round workpieces and shapes like square or hexagonal
It can grips the entire circumference of the part, suitable for parts with small cross sections
Face plates are used for clamping irregularly shaped workpieces
Mandrels used to hold workpieces that require machining on both ends or on their cylindrical surfaces
Types of Lathes
Boring and Boring Machines
Introduction
The cutting tools are mounted on a boring bar to reach the full length of the bore
Boring bars have been designed and built with capabilities for damping vibration
Large workpieces are machined on boring mills
A vertical boring mill is similar to a lathe, has a vertical axis of workpiece rotation
In horizontal boring machines, the workpiece is mounted on a table that can move horizontally in both the axial and radial directions
Design Considerations for Boring
Through holes should be specified
Greater the length-to-bore-diameter ratio, the more difficult it is to hold dimensions
Interrupted internal surfaces should be avoided
Drilling, Drills, and Drilling Machines
Introduction
Holes are used for assembly with fasteners, for design purposes or for appearance
Hole making is the most important operations in manufacturing
Drilling is a major and common hole-making process
Drills have high length-to-diameter ratios, capable of producing deep holes
Drills are flexible and should be used with care in order to drill holes accurately and to prevent breakage
Drills leave a burr on the bottom surface upon breakthrough, necessitating deburring operations
Twist Drill
The most common drill is the conventional standard- point twist drill
The geometry of the drill point is such that the normal rake angle and velocity of the cutting edge vary with the distance from the center of the drill
Main features of this drill are:
Point angle
Lip-relief angle
Chiseledge angle
Helix angle
Drills are available with a chip-breaker feature ground along the cutting edges
Other drill-point geometries have been developed to improve drill performance and increase the penetration rate
Thrust Force and Torque
Thrust force acts perpendicular to the hole axis
Excessive thrust force can cause the drill to break, distort the workpiece and cause the workpiece to slip into the workholding fixture
The thrust force depends on:
Strength of the workpiece material
Feed
Rotational speed
Drill diameter
Drill geometry
Cutting fluid
Drilling Practice
Drill does not have a centering action, tends to “walk” on the workpiece surface at the beginning of the operation
A small starting hole can be made with a center drill
before drilling
Or the drill point may be ground to an S shape which has a self-centering characteristic and produces accurate holes with improved drill life
Drilling Machines
Drilling machines are used for drilling holes, tapping, reaming and small-diameter boring operations
The most common machine is the drill press
The types of drilling machines range from simple bench type drills to large radial drills
The drill head of universal drilling machines can be swiveled to drill holes at an angle
Numerically controlled three-axis drilling machines are automate in the desired sequence
using turret
Drilling machines with multiple spindles (gang drilling) are used for high-production-rate operations
Reaming and Reamers
Introduction
Reaming is an operation used to:
Make existing hole dimensionally more accurate
Improve surface finish
Most accurate holes in workpieces are produced by:
Centering
Drilling
Boring
Reaming
For even better accuracy and surface finish, holes may be burnished or internally ground and honed
Hand reamers have a tapered end in the first third of their length
Machine reamers are available in two types: Rose reamers and Fluted reamers
Shell reamers are used for holes larger than 20 mm
Expansion reamers are adjustable for small variations in hole size
Adjustable reamers can be set for specific hole diameters and therefore are versatile
Tapping and Taps
Internal threads in workpieces can be produced by
tapping
A tap is a chip-producing threading tool with multiple cutting teeth
Tapered taps are designed to reduce the torque required for the tapping of through holes
Bottoming taps are for tapping blind holes to their full depth
Collapsible taps are used in large-diameter holes
Tapping may be done by hand or with machines:
Drilling machines
Lathes
Automatic screw machines
Vertical CNC milling machines
One system for the automatic tapping of nuts is shown
