An operator on lathe should be
well conversant with the correct name of various parts of lathe. The following
are the main parts of the lathe machine:
(a) Bed (b) Tail stock (c) Head stock (d) Carriage or Saddle (e) Cross slide (f) Compound slide (g) Tool post (h) Apron (i) Lead screw (j) Half nut (k) Feed shaft (l) Feed gear box (m) Tumbler gear (n) Back gear mechanism
Bed
The bed gives support to all the
mountings of the lathe. It may be casting of Nickel alloy cast iron and is
carefully seasoned, machined and scrapped. Seasoning of bed is done to avoid
distortion due to cooling stresses developed during solidification of the bed
after casting.
Bed ways or guide ways are
machined at the top of the bed for supporting and guiding the saddle and
tailstock. Lathe beds are of two types, flat and prismatic or inverted ‘V’ type
with included angle of 900. Flat ways give large bearing surface with
corresponding reduction in wear. The’ V’ type guide ways give better guide and
ensure proper alignment. The modem lathes have a combination of flat and ‘V’
type ways to give combined advantages of flat and ‘V’ ways (Refer Fig 2.12).The
common bed castings are fine grained with hardness of 200 + 10% B.H.N. Some
additional steps taken to improve wear resistance of lathe beds are:
(a) Chilling of castings
(b) Increasing hardness of the wearing surface by flame hardening.
(c) Superimposing separately
hardened steel slide ways over the top of the bed casting.
Tail Stock
It is a moving part and can be
fixed at any desired position on the bed. Its body is made of cast iron. It is
made in two halves. The lower half (base) is fixed on the bed and the upper
half is fitted on the base. The base can move along the bed ways. The upper
half can be adjusted sideways on both sides, while offsetting the centre for
taper turning between the centres. Adjusting screws are used for adjusting the
tailstock for offsetting. One screw is loosened while the other screw is
tightened for setting the tailstock centre to one side.
Head Stock
The headstock is that part of the
lathe which serves as a hosing for the driving pulleys and gears, provides
bearing for the machine spindle and keeps the later in alignment with the bed.
It consists of the following main parts:
(a) Cone pulley/gears
(b) Back gears and back gear
lever
(c) Main spindle
(d) Feed reverse lever
The main shaft of the headstock
is called spindle. It is a hollow shaft. The front end of the spindle is called
the nose and it carries a dog plate or a chuck to hold the job. Cone pulley or
gears are used to change the speed of spindle. Back gear is used to transmit
the heavy power with reduced speed.
Spindle
It is a hollow cylindrical shaft
and revolves in two bearings, fixed one each at the front and rear ends of the
head stock. Long metal rod can pass through to other side of the head stock.
The spindle end facing tail stock is called spindle nose. Spindle nose has
Morse taper to accommodate the lathe centre or collet chuck. It has threaded
portion to accommodate face plate or chuck.
Carriage or Saddle
The carriage is located on the
bed between the head stock and the tailstock. It slides along the bed guide
ways and can be located on the bed at any desired position by tightening lock
screw. It can move manually by hand wheel or by power feed. It consists of
following main parts:
(a) Saddle. It is part of carriage which slides along the bed ways and
supports the cross slide, compound rest and tool post.
(b) Cross Slide. It is mounted on the saddle and moves cross wise by a
handle or by means of power feed. The facing operation is done by cross slide.
(Refer Fig 14)
(c) Compound Rest. It is mounted
on the cross slide. It has a graduated circular base, which is called swivel
plate. Upper sliding part moves parallel to the bed and the lower part can be
swiveled to the desired angle. It is used for turning tapers, conical shapes
etc.
(d) Tool Post. It is fitted on the top of the compound slide for fixing
the tool or the tool holder. Four types of tool posts are commonly used. They
are:
(i) Round Tool Post. It has a cylindrical
body with a slot into, which the tool is inserted as shown in figure. The bolt
tightens the tool. This tool post is generally used for light work because the
tool is not held with sufficient rigidity.
(ii) Open Side Tool Post. In this tool post the tool may be clamped
more rigidly. In this tool post the height of the cutting edge can be adjusted
by means of rocker. The tool is clamped by two strong bolts.
(iii) Square Tool Post. It is also known as
square turret or four way tool post. It can
accommodate four tools, which can work one after the other by rotating the tool
post. It saves time of removing and setting the tools.
(iv) Quick Change Tool Post. The quick
change tool post is mounted on the top slide, and the tool holders can be
clamped within seconds by turning the lever. This works on an eccentric locking
device giving high rigidity to the tool holder. The tool height can be adjusted
by loosening the square head screw, the tool post can be moved up or down as
per the requirement. Once the height is adjusted, it can be retightened the
screw.
(e) Apron. It is a rectangular box, which is suspended towards lead
screw below the saddle. It has a gear train inside and a half nut. The half nut
lever is located outside. All feed levers and feed clutch also housed in it. In
short we can say that the apron contains the mechanism of controlling tool
feeds and thread cutting.
Lead Screw
The lead screw is a long threaded
shaft used as a master screw, and is brought into operation only when threads
have to be cut. In all other times the lead screw is disengaged from the gear
box and remains stationary. (Refer Fig 19)
Half Nut
It is also called split nut. For
thread cutting a precise longitudinal power traverse of the carriage is
obtained by engaging half nut to the lead screw. Half nut is housed in the
apron. (Refer Fig 20)
Feed Shaft
The feed shaft is a long and
accurately finished shaft supported at both ends in the bearing situated at
either end of the lathe bed. This is also passing through the apron parallel to
the lead screw.
It is provided to effect the
automatic movement to the saddle and cross slide. It gets the drive from the
machine spindle through a feed box. It has got a long keyway running the entire
length of the rod which carries a gear in the apron. This gear rotates when the
feed shaft is given power and transmits the motion to another gear in mesh with
it and having worm shaft integral. By operating a lever on the apron the worm
gets engaged with a worm gear which in turn drives another gear. This gear is
in mesh with a rack provided on the side of the lathe bed and when in
operation, the rotating gear travels along the rack and carriage.
Tumbler Gear Mechanism
Tumbler gears are used to change the
direction of rotation of the lead screw and the feed shaft. The saddle and the
cross slide can be given automatic movement to the desired side. The mechanism
is an assemble of gears i.e. two gears incorporating the tumbler mechanism and
the third one the stud gears assemble on the quadrant plate.
Back Gear Mechanism
The back gears are used for
effecting reduction in spindle speeds, thereby facilitating a wider range of
speeds. The back gears are mounted on an eccentric shaft which is operated by
means of a hand lever known as back gear engaging lever. The back gear is
disengaged by shifting lever. When bull gear pin is engaged the cone pulley
gear, bull gear and the spindle will rotate as one body. For reducing the
spindle speed by back gears, take out pin to disengage the bull gear from cone
pulley engage back gear with the gear through lever. This way when cone pulley
will rotate the gear will rotate and this will rotate back gear. This is mostly
used while cutting the threads.
Feed Gear Box
The feed or quick-change gear box
serves the purpose for changing the speeds of feed rod and lead screw. By
changing the speed of feed rod and lead screw the change in the automatic feed
can be achieved. To extend the cutting process over the entire surface of the
work piece, the tool may be moved with varying speeds in relation to the revolution
of the work. The feed movement to the cutting tool is imparted either by hand
or power on lathes, which do not have a feed gear box, feeds are obtained by
changing the gears. This method involves lot of time and is inconvenient also.
The feed of the tool along the axis of the lathe spindle is called longitudinal
feed and that perpendicular to spindles axis is called cross feed. The feed of
tool influences the surface finish on the work. A coarse feed results in poor
finish and vice-versa.
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