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Processes
- Turning
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process of turning takes place on a lathe – one of the oldest
and most versatile mechanical tools. Lathes have been used for
centuries for shaping materials in concentric circles – from
the first basic foot operated lathes used for turning wooden
bowls to the latest CNC machines used for turning hundreds of
metal components to the same minute tolerances. For this section
we will concentrate on electrically driven, manually operated
metal turning lathes –although much of the information is relevant
to all lathes. |
As mentioned earlier,
the lathe is a particularly versatile tool, it is capable of
a variety of tasks – from simply using a cutter to produce a
flat, perpendicular end on a piece of bar, to threading work
and winding springs.
The basic principal
of the lathe is simple; the work is held in a revolving chuck,
and can be turned to a reduced diameter using a cutting tool
mounted on the bed of the lathe. The cutting tool can be moved
along and across the bed (x and y axes) in order to cut along
the length of the work or across it. The tool is moved via two
wheels that are wound in or out to move the tool holder over
the bed of the lathe.
Most lathes have
a tailstock at the opposite end of the machine to the headstock
(where the chuck is held) the tailstock is mounted directly
in line with the centre of the chuck, and can be moved along
the bed (towards or away from the chuck) as it sits on longitudinal
rails.
The two main functions
of the tailstock are: to hold a drilling chuck, so that perfectly
concentric holes can be drilled in the work, and to hold a centre
which engages with the end of long pieces of work, preventing
them from running off centre while being machined. As well as
these two main functions, the tailstock can be used for other
tasks, such as holding taps, or providing a square base for
dies to brace against during threading work. It can be deliberately
moved off centre to a desired degree to give a taper to turned
work. This application of the tailstock is limited as it causes
the work to ‘flex’ when revolving.
This photo in the
gallery shows a medium sized lathe, which is a good example
of a manually operated metalworking lathe. Other lathes may
have variable speeds controlled by external levers, this model’s
speed is altered through a conventional internal gear system.
The Basic
Cutting Tool:
Lathe cutting tools are made from a
range of materials. The choice is dictated by several factors:
Type of material to be turned, surface finish required, cutting
speed, cost of tooling. The common material for general applications
is hardened alloy steel, the alloying elements usually being
tungsten, chromium, vanadium, cobalt and carbon. The most commonly
used alloy steel is called High Speed Steel or (HSS), it contains
18% tungsten, 4% chromium, 1% vanadium. These make the steel
harder, tougher and more resistant to wear and breakage.
There are several
key features of the shape of the cutting tool, which govern
its cutting characteristics and efficiency. These features,
the rake and clearance angles are detailed in this diagram showing
the interaction between the work and the cutting tool:
The angle of either of these key areas is important for the
cutting tool to produce a clean and smooth cut on the workpiece.
If the clearance angle is too little, the tool will rub the
work – causing excess heat in the components and the possibility
of the two bonding together, or the cutting tool becoming softened.
If the clearance angle is too great the tool will be weaker
than necessary –and will be more susceptible to breaking.
If the rake angle is too little the tool will not cut well,
and will rub against the work causing a poor and uneven finish.
If the rake
angle is too large the tool will dig into the work, overheating
it, causing a poor finish and increasing the chance of breakage.
Other Cutting tools
The basic cutting tool described above is used for the majority
of machining tasks; facing work off, turning areas of reduced
diameter etc – but there are some situations where this type
of cutter cannot be used.
Cutting
threads on a lathe requires the
tool to form the shape of the thread while being moved along
the work. The shape of a standard cutter does not have the correct
profile needed to cut the correct angles. These specialist cutting
tools are known as form tools.
Parting
off is the process of cutting
through the work on the lathe; as opposed to using a hacksaw
– the cut is of better quality and leaves the end of the work
square to the length. The parting off tool is a relatively thin
(around 3mm) tool, with a cutting edge on the end as it is designed
to go across into the work as opposed to running along its length.
Lathe Speeds
As with drilling, the speed at which the material is cut affects
the quality of the cut itself, and the life and usability of
the tool. Again,
similarly to drilling, harder materials require lower speeds,
and softer materials can be worked at higher speeds. The speed
at which the cutter is moved over the surface of the work is
important as well; if it is moved too quickly the tool will
over heat and possibly break, and the finish of the work will
be poor.
Work Holding
As has been mentioned, the work is held in a spinning chuck.
The most common chuck has three self-centering jaws mounted
120o from each other that are all wound towards or
away from the centre of the chuck together, to grip the work.
All work carried out on a three jaw chuck will be concentric
with the original bar’s dimensions.
Alternatively,
if the work must be machined off centre, or the work is not
of cylindrical shape (square section for example) a four jaw
chuck is used. This has four jaws mounted at 90
degrees
to
each other
that are moved individually to give the workpiece the correct
placement.
Although not strictly
to do with workholding, the Morse taper system should be highlighted
as it is important in the operation of both drilling machines
and lathes. The Jacob’s chuck used in drills usually has a Morse
tapered shank where it interfaces with the machine itself; this
is also how tools are attached to the tailstock on the majority
of lathes. The system works by having a tapered shank on the
tool, which interfaces with an identically tapered hole in the
drill of tailstock – friction holds the two together against
the turning forces of the machine. Sometimes the shank also
has two flats ground at the end to form a tongue which engages
in a slot in the housing to further prevent rotary movement.
Lathe tools in the
gallery shows a variety of tools with Morse tapered shanks:
from left to right Jacobs’ chuck (with key) revolving centre,
and dead centre.
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