About the Lathe
THE METAL LATHE
The lathe is a machine tool in which a piece of metal, can be held and
rotated while it is being shaped by a cutting toot. The Lathe is capable of
producing cylindrical, conical, helical and spherical shapes. This type of lathe
is often referred to as a centre lathe. Small machines are usually mounted on a
bench or a stand are sometimes called bench lathes.
THE
BED
The diagram above shows a sectioned view of the lathe bed which is well ribbed
cast iron. The top of the bed is machined to form 'ways' upon which the saddle
and tailstock slide. Most lathes have ' flame hardened' ways to resist ware and
maintain accuracy through the life of the machine.
THE
HEADSTOCK
The headstock is bolted securely to the 'inner ways' on the left hand end
of the bed. The headstock provides a method of holding and rotating the work
piece to be machined. It has several distinct parts which perform important operations.
These parts include such operations as providing different turning speeds, a
set of gears which enable automatic movement of the saddle along the ways and a
mechanism for automatic thread cutting.
CARRIAGE
The carriage is the part of the lathe that supports and controls the movement
of the cutting tool. The main parts of the carriage are the saddle and the
apron. The apron is fastened to the saddle and hangs in front of the bed. The
apron contains the hand wheel and lever for hand feed, and a clasp nut for automatic
feed and thread cutting. The cross slide is fitted for cross feed of the cutting
tool and is operated by hand feed.
THE
TAILSTOCK
The tailstock is made up of 2 castings. One rests on the ways and may be clamped
to the bed in any position so that work can be performed on jobs of different
lengths. The other casting is fitted to the lower one and has a sideways movement
that can be used for taper turning. The tailstock spindle is tapered to take
large tapered drills and drill chucks etc. The spindle is moved in and out by a
hand wheel and screw.
THE
COMPOUND SLIDE
The Compound Slide rest is mounted on top of the cross slide and it has swivel
base that rotates through 360 degrees. The compound slide holds the tool post
which is moved by a feed screw.
The compound slide is also used to control the path of the cutting tool
when short tapers are being turned in the lathe. The diagram below shows the compound
set to 30 degrees. This means that the cut will be made at 30 degrees to the
axis of the lathe.
TOOL
POST
The tool post is a device for firmly clamping the cutting tool or tool holder
to the lathe. The tool post is fixed to the top of the compound slide. Below is
a standard tool post and the square tool post which can hold 4 different cutting
tools.
THREE
JAW CHUCK
The three Jaw Chuck is 'self-centering’. The jaws all move together and are
always the same distance from the centre. This chuck is designed to hold round
or hexagonal work. The movement of the jaws is controlled by a scroll plate
which is rotated by a chuck key. The drawing below shows a 3 jaw self-centering
chuck.
FOUR
JAW CHUCK
The 4 jaw chuck has jaws that can be moved independently of each other.
This chuck is used to hold irregular or odd shaped work. It requires careful
setting up. It holds work firmer and work can be made to run perfectly true with
the aid of a dial indicator.
TOOL
HOLDERS
There are straight, left, right styles of tool holders. The straight is for
general turning, the right hand for facing operations and for turning work
close to tailstock and the left hand for turning work close to the headstock.
LATHE
TOOLS
The cutting tools or bits commonly used in M.D.T. workshops are made from
high speed steel. They are available in 5 mm to 25 mm square section and 50 mm
to 150 mm lengths.
The shape of the cutting tool is very important. The tool must be ground so
that it is sharp enough to force its way into the work but must retain
sufficient material behind the cutting edge so that it can withstand the
pressure on it when it is cutting.
The following diagram illustrates the angles and clearances that should be considered
when grinding a tool for a specific job.
CENTRES
The headstock and tailstock spindles are bored out to a standard morse
taper to receive headstock and tailstock centres whose shanks are also morse
tapers. The points are ground to an angle of 60 degrees to fit the countersink
section of a centre drill.
The tailstock centre supports the right hand end of the work. The work
rotates on the bearing surface of this centre so it must be hardened and well
lubricated.
Live centres are used in the tailstock and have built - in roller or ball
bearings which enable the point of the centre to rotate with the work.
CENTRE
DRILL
Work that is to be supported by a centre must be drilled to fit the centre
and to fit the bearing surface. This is usually done with a centre drill which
is a combination drill and countersink drill. The cone angle matches the lathe
centres.
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