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Lathes

Lathes

 

 

Lathes

      Definition

Lathe is a machine, which removes the metal from a piece of work to the required shape and size

     Common types of lathes: 

Engine Lathe:

The most common form of lathe, motor driven and comes in large variety of sizes and shapes. 

Bench Lathe:

A bench top model usually of low power used to make   precision machine small work pieces.

Tracer Lathe:

A lathe that has the ability to follow a template to copy   a shape or contour.
Automatic Lathe:

The lathe in which the work piece is automatically fed and removed without use of an operator.  Cutting operations are automatically controlled by a sequencer of some form.

Turret Lathe:

The lathes which have multiple tools mounted on turrent either attached to the tailstock or the cross-slide, which allows for quick changes in tooling and cutting operations.

Computer Controlled Lathe:

Highly automated lathes, where cutting, loading, tool changing, and part unloading are automatically controlled by computer coding.

 The figure (1) shows Photographic view of Engine Lathe

           
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Fig (1) Engine Lathe

Centre lathe – constructional features

  • Head stock
  • Tail stock
  • Bed
  • Carriage
  • Feed rod
  • Lead screw
  • Feed change gear box

Lathe specifications

  • Distance between centers
  • Swing over the bed
  • Swing over the cross slide
  • Horse power of the motor
  • Number of speeds
  • Number of feeds                                                 

Lathes and Lathe Operations

 

  • Lathes are the oldest machine tools
  • Lathe Components

 

  • Bed: supports all major components
  •  
  • Carriage: slides along the ways and consists of the cross-slide, tool post, apron

 

  • Headstock – Holds the jaws for the work piece, supplies power to the jaws and has various drive

Speeds

  • Tailstock – supports the other end of the work piece
  • Feed Rod and Lead Screw – Feed rod is powered by a set of gears from the headstock

 

LATHE BED

 

  • The bed is the base of the lathe and supports all the major components of lathe.
  •  Lathe bed material made of grey cast iron , to resist deflection and absorb vibrations during cutting

 

Carriage Feed

  •   Longitudinal Feed or “Turning” - The tool is fed along the work.
  • Cross Feed or “Facing” – The tool is fed across the work.

 

Tail Stock:

It’s like a stationary drill press

It is centered with your work piece

For drilling use a drill chuck that fits your bits

Jam the drill chuck into the tail stock

To remove the chuck turn the tail stock back to zero and the chuck should pop out

Cutting Tools

 

There are basically two types of cutting tools:

  • Single point (e.g. turning tools). ( fig .2 )

 

  • Multiple point (e.g. milling tools).

 

Fig (2) shows single point cutting tool

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Fig ( 2)
Various lathe operations

  • Turning – produces straight, conical, curved, or grooved work pieces

 

  • Facing – produces a flat surface at the end of the part
  • Boring – to enlarge a hole

 

  • Drilling - to produce a hole
  • Cutting off – to cut off a work piece

 

  • Threading – to produce threads
  • Knurling – produces a regularly shaped roughness

 

Fig (3) shows  different types of lathe operations

 

lathes

Fig (3) Types of Lathe operations

 

 

 

 

Work holding Devices for Lathes:

 Many different devices, such as chucks, collets, faceplates, drive plates, mandrels, and lathe centers are used to hold and drive the work while it is being machined on a lathe.

 

Work pieces can be held by various methods

  • Work piece mounted between centers

 

  • Work piece mounted within a single chuck
  • Work piece mounted within a collet

 

  • Work piece mounted on a faceplate

 

Three Jaw chuck: It usually has three jaws, the jaws are moved simultaneously within the chuck (fig.4).

Four Jaw chuck: This is independent chuck generally has four jaws , which are adjusted individually on the chuck face by means of adjusting screws(fig.5).

Magnetic chuck: Thin jobs can be held by means of magnetic chucks.

Face plates: The face plate is used for irregularly shaped work pieces that cannot be successfully held by chucks or mounted between centers (fig.6).

Mandrels: A work piece which cannot be held between centers because its axis has been drilled or bored and which is not suitable for holding in a chuck or against a faceplate is usually machined on a mandrel.

Collet chuck :  Collet chuck is used to hold small work pieces.

 

 

 

 

3 JAW CHUCK                                4 JAW CHUCK          

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Fig.(4)                                                           Fig.(5)

 

  FACE PLATE

 

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Fig,.6 Face plate
Formulas:

Cutting speed (V) = ΠDN/1000 M/min

Depth of cut (D) = (D1-D2)/2 mm

Where D1=original diameter and D2 = final diameter of the work piece    

Metal Removal Rate (MRR) = Π x D x d x f       mm3

In terms of cutting speed (V in mm/min), MRR=1000 x V x d x f

Where D represents original diameter of the work piece in mm
Where N represents revolution per minute (rpm)

  Where d represents depth of cut in mm

  Where f represents feed in mm/rev

   Taper Turning
Tan α = (D1- D2)/2L   where α = angle of taper

    D1= major diameter in mm
D2= minor diameter in mm
L= Length of taper in mm
The conicity K of the taper is defined as K= (D1- D2)/L

 

 

 

 Lathe simple problems

 

Problem -1

A mild steel rod having 50 mm diameter and 500 mm length is to be turned on a lathe. Determine the machining time to reduce the rod to 45 mm in one pass when cutting speed is 30 m/min and a feed of 0.7 mm/rev is used.

Solution
Given data: D = 50 mm, Lj = 500 mm
v = 30 m/min, f = 0.7 mm/rev
Substituting the values of v and D in

V = ΠDN/1000 M/min
Required spindle speed as: N = 191 rpm

Problem -2

Determine the angle at which the compound rest would be swiveled for cutting a taper on a work piece having a length of 150 mm and outside diameter 80 mm. The smallest diameter on the tapered end of the rod should be 50 mm and the required length of the tapered portion is 80 mm.
Solution
Given data: D1 = 80 mm, D2 = 50 mm, Lj = 80 mm (with usual notations)
tan a = (80-50) / 2´80       or      a = 10.620

The compound rest should be swiveled at 10.62°

 

           
Reference

  • work shop Technolgy by Hajra choudry
  •  Advances in Manufacturing Technology C.J Thomas

 

Website

  •  www. engbasics.com
  •  www. efunda.com

 

 

 

 

 

 

 

(Part-A)

  1. What do you mean by lathe?

2.   What are all the various operations can be performed on a lathe?

  1. What are all the principle parts of the lathe?
  2. Difference between three jaw chuck and four jaw chuck
  3. State the various parts mounted on the carriage?
  4. write  four types of tool post
  5. What is an apron?
  6. State any two specification of lathe?
  7. List any four types of lathe?
  8. What do you mean by semi-automatic lathe?
  9. State the various feed mechanisms used for obtaining automatic feed?
  10. List any four holding devices?
  11. What are the different operations performed on the lathe?
  12. State any two specifications of capstan lathe & turret lathe?
  13. Compare the advantage of capstan lathe & turret lathe?
  14. Define tooling?
  15. What are the three stage of a tool-layout?
  16. What are the different drives used in copying lathe?
  17. What are the components that can be turned on a copying lathe?
  18.  Define automatic lathes.

(Part –B)

  • Explain the different components of a lathe with neat sketch.
  • What are the different operations that can be performed on a lathe? also , mention the tools required for each operation
  • What are the different machining operations that can be performed on a lathe? Explain any five in detail.
  • Explain the different work holding devices used in a lathe?
  • write short notes on
    • universal scroll chuck                        b. magnetic chuck
  • collet chuck                           d. centers

6. Explain the following operations performed on lathe with suitable sketch.
a. turning
b. facing
c. grooving
d. knurling
d. thread cutting
7. Explain the different methods of holding the work piece in a lathe with suitable sketches.

 

Source: http://www.srmuniv.ac.in/downloads/lathe.doc

Web site to visit: http://www.srmuniv.ac.in

Author of the text: indicated on the source document of the above text

 

The lathe is a machine tool used principally for shaping articles of metal (and sometimes wood or other materials) by causing the workpiece to be held and rotated by the lathe while a tool bit is advanced into the work causing the cutting action. The basic lathe that was designed to cut cylindrical metal stock has been developed further to produce screw threads, tapered work, drilled holes, knurled surfaces, and crankshafts. The typical lathe provides a variety of rotating speeds and a means to manually and automatically move the cutting tool into the workpiece. Machinists and maintenance shop personnel must be thoroughly familiar with the lathe and its operations to accomplish the repair and fabrication of needed parts.

TYPES OF LATHES

Lathes can be divided into three types for easy identification: engine lathes, turret lathes, and special purpose lathes. Small lathes can be bench mounted, are lightweight, and can be transported in wheeled vehicles easily. The larger lathes are floor mounted and may require special transportation if they must be moved. Field and maintenance shops generally use a lathe that can be adapted to many operations and that is not too large to be moved from one work site to another. The engine lathe (Figure 7-1) is ideally suited for this purpose. A trained operator can accomplish more machining jobs with the engine lathe than with any other machine tool. Turret lathes and special purpose lathes are usually used in production or job shops for mass production or specialized parts, while basic engine lathes are usually used for any type of lathe work. Further reference to lathes in this chapter will be about the various engine lathes.
lathes

ENGINE LATHES

Sizes

The size of an engine lathe is determined by the largest piece of stock that can be machined. Before machining a workpiece, the following measurements must be considered: the diameter of the work that will swing over the bed and the length between lathe centers (Figure 7-1).
Categories
Slight differences in the various engine lathes make it easy to group them into three categories: lightweight bench engine lathes, precision tool room lathes, and gap lathes, which are also known as extension-type lathes. These lathe categories are shown in Figure 7-2 Different manufacturers may use different lathe categories.
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Lightweight
Lightweight bench engine lathes are generally small lathes with a swing of 10 inches or less, mounted to a bench or table top. These lathes can accomplish most machining jobs, but may be limited due to the size of the material that can be turned.
Precision
Precision tool room lathes are also known as standard manufacturing lathes and are used for all lathe operations, such as turning, boring, drilling, reaming, producing screw threads, taper turning, knurling, and radius forming, and can be adapted for special milling operations with the appropriate fixture. This type of lathe can handle workpieces up to 25 inches in diameter and up to 200 inches long. However, the general size is about a 15-inch swing with 36 to 48 inches between centers. Many tool room lathes are used for special tool and die production due to the high accuracy of the machine.

GAP OR EXTENSION-TYPE LATHES

Gap or extension-type lathes are similar to toolroom lathes except that gap lathes can be adjusted to machine larger diameter and longer workpieces The operator can increase the swing by moving the bed a distance from the headstock, which is usually one or two feet. By sliding the bed away from the headstock, the gap lathe can be used to turn very long workpieces between centers.

LATHE COMPONENTS

Engine lathes all have the same general functional parts, even though the specific location or shape of a certain part may differ from one manufacturer The bed is the foundation of the working parts of the lathe to another (Figure 7-3).
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The main feature of its construction are the ways which are formed on its upper surface and run the full length of the bed.
Ways provide the means for holding the tailstock and carriage, which slide along the ways, in alignment with the permanently attached headstock
The headstock is located on the operator's left end of the lathe bed. It contains the main spindle and oil reservoir and the gearing mechanism for obtaining various spindle speeds and for transmitting power to the feeding and threading mechanism. The headstock mechanism is driven by an electric motor connected either to a belt or pulley system or to a geared system. The main spindle is mounted on bearings in the headstock and is hardened and specially ground to fit different lathe holding devices. The spindle has a hole through its entire length to accommodate long workpieces. The hole in the nose of the spindle usually has a standard Morse taper which varies with the size of the lathe. Centers, collets, drill chucks, tapered shank drills and reamers may be inserted into the spindle. Chucks, drive plates, and faceplates may be screwed onto the spindle or clamped onto the spindle nose.
The tailstock is located on the opposite end of the lathe from the headstock. It supports one end of the work when machining between centers, supports long pieces held in the chuck, and holds various forms of cutting tools, such as drills, reamers, and taps. The tailstock is mounted on the ways and is designed to be clamped at any point along the ways. It has a sliding spindle that is operated by a hand wheel and clamped in position by means of a spindle clamp. The tailstock may be adjusted laterally (toward or away from the operator) by adjusting screws. It should be unclamped from the ways before any lateral adjustments are made, as this will allow the tailstock to be moved freely and prevent damage to the lateral adjustment screws.
The carriage includes the apron, saddle, compound rest, cross slide, tool post, and the cutting tool. It sits across the lathe ways and in front of the lathe bed. The function of the carriage is to carry and move the cutting tool. It can be moved by hand or by power and can be clamped into position with a locking nut. The saddle carries the cross slide and the compound rest. The cross slide is mounted on the dovetail ways on the top of the saddle and is moved back and forth at 90° to the axis of the lathe by the cross slide lead screw. The lead screw can be hand or power activated. A feed reversing lever, located on the carriage or headstock, can be used to cause the carriage and the cross slide to reverse the direction of travel. The compound rest is mounted on the cross slide and can be swiveled and clamped at any angle in a horizontal plane. The compound rest is used extensively in cutting steep tapers and angles for lathe centers. The cutting tool and tool holder are secured in the tool post which is mounted directly to the compound rest. The apron contains the gears and feed clutches which transmit motion from the feed rod or lead screw to the carriage and cross slide.

CARE AND MAINTENANCE OF LATHES

Lathes are highly accurate machine tools designed to operate around the clock if properly operated and maintained. Lathes must be lubricated and checked for adjustment before operation. Improper lubrication or loose nuts and bolts can cause excessive wear and dangerous operating conditions.
The lathe ways are precision ground surfaces and must not be used as tables for other tools and should be kept clean of grit and dirt. The lead screw and gears should be checked frequently for any metal chips that could be lodged in the gearing mechanisms. Check each lathe prior to operation for any missing parts or broken shear pins. Refer to the operator's instructions before attempting to lift any lathe. Newly installed lathes or lathes that are transported in mobile vehicles should be properly leveled before any operation to prevent vibration and wobble. Any lathes that are transported out of a normal shop environment should be protected from dust, excessive heat, and very cold conditions. Change the lubricant frequently if working in dusty conditions. In hot working areas, use care to avoid overheating the motor or damaging any seals. Operate the lathe at slower speeds than normal when working in cold environments.

SAFETY

All lathe operators must be constantly aware of the safety hazards that are associated with using the lathe and must know all safety precautions to avoid accidents and injuries. Carelessness and ignorance are two great menaces to personal safety. Other hazards can be mechanically related to working with the lathe, such as proper machine maintenance and setup. Some important safety precautions to follow when using lathes are:

  • Correct dress is important, remove rings and watches, roll sleeves above elbows.
  • Always stop the lathe before making adjustments.
  • Do not change spindle speeds until the lathe comes to a complete stop.
  • Handle sharp cutters, centers, and drills with care.
  • Remove chuck keys and wrenches before operating
  • Always wear protective eye protection.
  • Handle heavy chucks with care and protect the lathe ways with a block of wood when installing a chuck.
  • Know where the emergency stop is before operating the lathe.
  • Use pliers or a brush to remove chips and swarf, never your hands.
  • Never lean on the lathe.
  • Never lay tools directly on the lathe ways. If a separate table is not available, use a wide board with a cleat on each side to lay on the ways.
  • Keep tools overhang as short as possible.
  • Never attempt to measure work while it is turning.
  • Never file lathe work unless the file has a handle.
  • File left-handed if possible.
  • Protect the lathe ways when grinding or filing.
  • Use two hands when sanding the workpiece. Do not wrap sand paper or emory cloth around the workpiece.

 

 

Source: http://faculty.ksu.edu.sa/hossainy/Book1/Chapter%207.doc

Web site to visit: http://faculty.ksu.edu.sa

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