Unit 5: Lathe and Drillingmachines
Elements of Mechanical Engineering.
Unit 5: Lathe and DrillingMachines.
Cutting tools:- Tools which are used to separate/remove material stock from the work piece are known as cutting tools. Ex hack saw, chisel etc.
Machine tools: -Machine tools are power driven cutting tools or machines which enable the removal of excess stock of material from the work piece.
Relative Motion: - change in position of a object ( fixed/moving) with respect to the other object (fixed/moving)is called as relative motion.
Lathe:-
- Lathe is one of the most widely used machine tools in metal cutting work.
- Lathe is usually used for machining (metal removal) the job which is rotated and a cutting tool is fed to cause the cutting action.
- Lathe is generally used for machining cylindrical jobs.
Working principle of Lathe
The work piece is hold firmly in the work holding device called the chuck and is supported by the dead center as shown in the fig.above
The chuck is rotated at a particular speed by some mechanisms; the cutting tool is moved against the rotating work piece to facilitate the removal of material.
Schematic diagram of Lathe Machine.
1. Bed: The lathe bed provides a heavy rigid frame on which all the main components like headstock, tail stock and carriage are mounted. It comprises inner and outer guide rails that are precision machined. bed is made out of cast iron alloy.
2. Headstock: It is mounted in a fixed position at the left end on the bed. It carries a hollow spindlewhich can rotate at different speeds.
The spindle carries a work-holding device which in turnrotates the job to be machined. The spindle speed is controlled by a stepped cone pulley and back
gear arrangement both of which are housed inside the headstock.
3. Tailstock: It is mounted at the left end on the bed and can slide towards the headstock. It canbe fixed at any position on the bed to suit the length of the work to hold a lathe center, drill bit or other such tool. The tailstock is used for drilling,
reaming, tapping, threading and centering parts.
4. Carriage: It moves on the guide ways of the lathe bed. A cutting tool is fixed in the tool post, the movement of which is guided by the carriage. The carriage is equipped with levers, clutchesand gears that control its movement either by hand or by power feed. It carries the following
units.
• Saddle: It is the main part or the base of the carriage which slides along the bed ways. It supportscross-slide, compound rest and tool post. Generally, it uses a hand wheel to move alongthe bed but it can also be power operated through the apron mechanism.
• Cross slide: It is mounted on the saddle and slides in a direction perpendicular to the axis ofthe spindle
• Compound rest: It is mounted on the top of the cross-slide. It can swivel to any desired
angle in a horizontal plane. It is operated by a handwheel through the compound rest.
.Tool post: It is fixed on top of the compound rest and mounts the tool holder, in which the cutting tools are clamped.
. Apron is attached to the front of the carriage and bears the mechanism which controls the movement of the carriage and the cross slide.
. Legs: The whole machine is supported on two legs made out of cast iron. The leg on left side houses the electric motor and the headstock, the right side leg houses tailstock and other parts.
Lathe Operations
The lathe is the most versatile general-purpose machine tool. It is capable to perform a largevariety of operations .
Following are a few operations that can be performed on a lathe:
1. Turning: Turning is a lathe operation in which the cutting tool removes metal from the outsidediameter of a workpiece. In other words, reduction in the diameter of the workpiece is calledturning.
Depending upon the job requirement, the turning operation may be of various types
such as
a.Straight turning
b.Stepped turning
c.Taper turning
d.Contour turning.
2. Facing: Facing is the process of removing metal from the end of a workpiece to produce a flat surface.
In other words a reduction in length of the workpiece is called facing. The cutting tool is fedinto the workpiece in a direction perpendicular to its axis
a. Straight Turning b. Step Turning
Taper Turning Knurling
Grooving Chamfering Thread cutting
Drilling Boring
Parting Off: It uses a cutting tool to cut-off the workpiece at a specific length.. It is normally usedto remove the finished end of a workpiece from the bar stock(raw material) that is clamped in the chuck.
Grooving: It is an operation of reducing diameter of the workpiece in the form of a neck or a narrowgroove of desired shape, may be a round, a square or a V shape.
Chamfering: It is a process of cutting the sharp edge at the end of a job to produce a beveled edgeat a desired angle.
Drilling: It is a lathe operation by which an axial hole is created in a solid workpiece. It is donewith the help of a special multi-point tool called a drill.
Reaming: It is an operation by which the surface of a drilled hole is finished. It is done with thehelp of a special multi-point tool called a reamer.feed (0.5-2 mm ).
Boring: It is an operation by which a drilled hole is enlarged in diameter. It is done with the helpof a single-point cutting tool called a boring bar.
Knurling: It is a special operation by which a smooth-turned surface is made rough by impressinga diamond shaped or straight line pattern. It is done for the purpose of creating a better gripping.surface.
Knurling is performed with special metal wheels called a knurling tool.
Thread Cutting: It is a process of cutting helical grooves of a standard thread form. It may be oftwo types, namely, external thread cutting and internal thread cutting.
(tapping,taperboring,undercutting,milling,counterboring,grooving,filling,polishing,spinning,spring winding& forming)
Taper Turning
Taper turning is the process of generating a conical surface.
Taper turning on a lathe is done by feeding the cutting tool at an angle to the axis of rotationthe workpiece.
A taper is a uniform increase or decrease in the diameter of the workpiece along its length.
There are four basic methodsof turning tapers on a lathe:
• by form tool;
• by compound rest swivel;
• by tailstock set-over;
• by taper turning attachment.
Determination of angle of taper:
Consider a cylindrical workpiece of length L and diameter D.as shown in fig below It is required to reduce the diameter D to d uniformly over the length L. The amount of taper in the workpiece is usually specified asconicity. It is denoted by letter K.
Where D- dia at the larger end, d-dia at the smaller end, L= length of workpiece
Taper Turning by Swiveling the Compound Rest
>The work piece is rotated on lathe axis .The cutting toolmounted on the compound rest isattached to a circular base which can be swiveled and locked at any desired taper angle as shown in Fig.
Compound rest may be swiveled at either side of the lathe. When thecompound rest set at the desired taperangleis moved against the workpiece the cutting tool generates
the required tapered surface on theworkpiece.
As the compound tool resthasonly limited movement, this method can generate only short tapers.
Taper Turning by Taper Turning Attachment
>Here a taper attachment which isset at an angle to the axis of rotation ofthe workpiece. guidethe tool in a straight path tocut both internal and external tapers as shown inFig. >The workpiece held betweenthecenters and is alignedtothe lathe axis.
The attachment consists of bracket or frame connected to therearend of the lathe bed.
A guide barhavinggraduations in degrees may beswiveledeither side of the zero.
Graduationon a pivot mounted over the bracket as shown in fig.
The cross slide is made free fromlead screw and its rear end is attached to theguide block by means of a bolt.
When the longitudinal feed is given by moving the carriage, the guide block slides overguide bar set at half the taper angle. The tool mounted on the cross slide follows the angular path and generates the required taper.
Taper Turning by Tailstock Set-over
Here the workpiece is held between the live center and the dead center of the tailstock. But the tailstock is offset at an angle equal to half the taper angle as shown in Fig.
The cutting tool is fed parallel to the lathe axis. And the desired taper is produced.
Taper Turning by a Form Tool
Here a taper tool of broad nose having straight cutting edge is used to produce the desired taper as shown in Fig.
>since this method produces taper only for short length it is not widely used.
Specification of Lathe:
A lathe is mainly specified by the maximum size of the work that it can handle.
Other important specifications include the following
1.The largest diameter of the workpiece, commonly called the swing.
2. The maximum distance between headstock and tailsstock centers.
3. power of the main drive, that is, motor;
4. range of spindle speeds;
5. range of feeds;
6. space occupied by the machine
Drilling is an operation of producing holes in a solid metal/non metals by means of a revolving tool called drill or twist drill.
The machines employed for drilling holes are called drilling machines. The drilling machine .Drilling machine is the one of the essential machine tool used in fabrication, production shop and tool rooms, for machining/producing holes.
The machine consists of a spindle which provides rotary motion to the drillingtool(twist). The spindle may also produce motion to table on which the work rests.
TYPES OF DRILLING MACHINES
Drilling machines are manufactured in various sizes and varieties to suit different types
work. They are classified as follows.
l) Portable drilling machine
2) Sensitive or bench drilling machine
3) Up-right drilling machine
4) Radial drilling machine
5) Gang drilling machine
6) Multiple-spindle drilling machine
7) Deep-hole drilling machine
Portable Drilling Machine:
It is small drilling machine designed to drill small holes up to 18 mm diameter. The machine can be carried to the jobs which normally cannot be brought to the workshop, such as concrete selling slabs,walls,heavy machinery, and fabrication sites.
It is very light in weight and can be conveniently handled. Fig above shows a portable hand held drilling machine.
Sensitive or Bench-Type Drilling Machine
The drilling headalong with the column is mounted and clamped on a bench. The workpiece can beclamed to the worktable or on the base with the help ofT-slots. The worktable can be moved up anddown along the vertical column and clamped at any desired height.
The drill is mounted in a drill chuck which is rotated by the drive system of the machine.
The chuck is moved up and down by handle with the help of a rack-and-pinion mechanism.
Sensitive drilling machine is designed for drilling small holes, usually less than 12 mm,
diameter. The name "sensitive" is used because the machinist can "feel" the action of the drill as it goes into the work.
Following are its main parts:
• Vertical column: It is the basic structure to hold the other parts.
• Drilling head: This has a box-type structure. It houses the power drive and gear boxes'
change of speed and feed.
• Spindle: It holds and provides rotary motion and feed motion to the drill.
• Table: It has T-slots on the top surface. The job is clamped on the table using a drilling
Jig or machine vice.
Upright/Pillar Drilling Machine
This machine is quite similar to the benchdrilling machine but of little larger size and higher capacity and is fixed on the floorfoundation.
This drilling machine is ableto take larger drills.
Radial Drilling Machine:
Themachine consists of a vertical column mounted on a large base.
It supports a radial arm which canswing about the column axis and move vertically up and down. The radial arm carries a drill headwhich can slide in its guideways and clamped at the desired position. A motor is mounted on thedrill head to rotate the drill spindle. A feed lever on the drill head guides the feed motion.
A combination of these movements helps to locate the drill at any desired position. This type of drilling machine is used in tool rooms and in large scale manufacturing units.
Other drillings machines
1. Deephole drilling machines
2. Universal drilling machines
3. Gang drilling machines
4. Multispindle drilling machine
5. CNC drilling machines.
Drilling
It is a process of creating or originating a hole in a solid workpiece using a tool called Twist drill.
Reaming:
Reamingis the operation of finishing an already drilled hole using a reamer. A reamer which matches the diameter of the already existing hole is fitted in to the machine spindle and reaming operation is carried over which is similar to drilling operation except higher spindle speed in reaming .The material removed is very less and the drilledhole surfaces are finished with high accuracy. The accuracy of a reamed hole is around ±0.005 mm.
Boring
Boring is the operation of enlarging an alreadydrilled hole using a boring tool. The boring tool is fixed to the boring bar which in turn fits into the machine spindle. The speedof rotation of a boring tool is lesser than the speed of rotationof a corresponding reamer. Holes with accuracy of± 0.00 125 mm can be obtained by this operation.
Counter-Boring
It is a special operation performed on drilling machine for enlarging a limited portion of the hole. The bottom surface of the enlarged hole is flat. It is generally used when a bolt or cap head screw or a nut is required to sit below the level of a workpiece's surface.
Counter-boring is done with the help of a special tool called a counter-bore. The counter-bore is guided in drilled hole with the help of a pilot fixed at its lower end.
Counter-Sinking:
`
It is a process of making a conical hole to a limited depth at the entrance tocylindrical hole. A counter-sunk hole allows sitting the conical head of a bolt or a screw below surface of the workpiece. It is cut with the help of a tool called a counter-sinking tool.
Counter sink cutters are manufactured with six common included angles which are 60°, 82°, 90°, 100,110° or 120°.
Tapping:
It is process of thread cutting inside a drilled or a bored hole using a tool called ta]
A special attachment containing a reversing mechanism is required for tapping.
Specification of drilling machine
I) The size of the working table and capacity of the machine.
2) Maximum size of the work piece on the particular machine.
3) Maximum diameter of drill on the particular machine.
4) Number of spindles on the particular machine.
5) Length of arm and column diameter must be specified for radial drilling machine.
Dept. of Mechanical Engineering, Srinivas School of Engineering, Mangalore.