Turning - Tornalama .: Makina Mühendisi

Single point metal cutting

In metal cutting, a wedge-shaped tool is used to remove material from the workpiece in the form of a ‘chip’. Two motions are required: the ‘primary
motion’, e.g. the rotation of the workpiece in a lathe; and the ‘secondary motion’,e.g. the feed ofa lathe tool.

Single-point tools are used for turning, shaping, planing, etc., and multi-point tools are used for milling, etc. It is necessary to understand the forces acting on the tool and their dects on power requirement, tool life and production cost.
In the following tables of tool forces and formulae
specific power consumption, metal removal rate, tool
life, etc., are given. A graph shows the tool life plotted
against cutting speed for high-speed steel, carbide and
ceramic tools.

Cutting tool forces
Tool forces vary with cutting speed, feed rate, depth of
cut and rake angle. Force may be measured experimentally
by using a ‘cutting tool dynamometer’ in
which the tool is mounted on a flexible steel diaphragm
and its deflections in three planes measured by three
electrical transducers. Three meters indicate the force,
typically of 25 N up to, say, 2000 N. Graphs show
typical characteristics.
Symbols used:
F, =cutting force (in newtons)
F, = radial force (in newtons)
F,=feed force (in newtons)
Resultant force on tool in horizontal plane

P Let:= work diameter (mm)

= number of revolutions per minute

Turning
plastics - depth of cut, feed, and cutting sped
Turning of
Cutting speed (m min-
’)
Material
Throw-
Depth of away
cut Feed Brazed carbide
Condition (mm) (mm rev-
Thermoplastics, polyethylene, Extruded,
’) HSS carbide tip4 0.25 50 145 160
polypropylene, TFE moulded
fluorocarbon or cast
modified acrylic moulded
High-impact styrene, Extruded,
4 0.25 53 160 175
or cast
Nylon, acetals and
- 4 0.25 50 160 175
Polystyrene Moulded or
4 0.25 18 50 65
Soft grades of thermosetting Cast,
4 0.25 50 160 175
polycarbonate
extruded
plastic moulded
or
plastic moulded
Hard grades of thermosetting Cast,
filled4 0.25 48 145 160
or filled
HSS,
Typical standard times for capstan and turret lathe operations(s) Operation 3 Engage feed 1.5
Change feed
3 Feed to bar stop 3.5
Index tool post
3.5 Chuck in, 3-jaw chuck 4.5
There are many types of lathe tool, the principal ones
being: bar turning; turning and facing; parting-off
facing; boring; and screw cutting. Some are made from
a bar of tool steel, others with high-speed steel tips
welded to carbon steel shanks and some with tungsten
carbide tips brazed to a steel shank. A tool holder with
interchangeable tips can also be used.
5.2.9 Lathe-tool nomenclature and
Tool features
setting
For cutting to take place the tool must have a ‘front
clearance angle’ which must not be
tool is weakened. There must also be a ‘top rake angle’
to increase the effectiveness of cutting. The value of this
angle depends on the material being cut. Typical
values are given in the following table.
Lathe-tool nomenclature and
Tool features
setting
There are many types of lathe tool, the principal ones
being: bar turning; turning and facing; parting-off
facing; boring; and screw cutting. Some are made from
a bar of tool steel, others with high-speed steel tips
welded to carbon steel shanks and some with tungsten
carbide tips brazed to a steel shank. A tool holder with
interchangeable tips can also be used.
5.2.9 Lathe-tool nomenclature and Tool features
setting
For cutting to take place the tool must have a ‘front
clearance angle’ which must not be so large that the
tool is weakened. There must also be a ‘top rake angle’
to increase the effectiveness of cutting. The value of this
angle depends on the material being cut. Typical
values are given in the following table.
Other features
In addition to front clearance and top rake, there are
side clearance and side rake.
improves cutting and reduces wear.
Symbols used:
4-top rake angle
A small nose radius
a
=front clearance angle
/3
= wedge angle
S
=plan relief or trail angle
E
= plan approach angle
8
=true rake angle
y
= true wedge angle
1
=side clearance angle
$
=side rake angle.
Tool
setting
The tool must not be set too high or too low, or
inclined at an angle. The effects are shown in the figure.
Parting-off tool
This is used for ‘parting-off the workpiece from bar
stock held in
clearance’ on both sides as well as ‘side clea’rance’. The
tool is weak and must be used with care. It must be set
on or slightly above centre. If set even slightly below
centre the work will climb onto the tool before
parting-off.
a chuck. Note that there is ‘body
Above centre: tool tends to rub.
Below centre: work tends to climb over tool.
Inclined upwards: tool rubs.
Inclined downwards: work tends to drag tool in.
so large that the
Time Time Operation
Change speed
high-speed steels.of plastics

d-depth of cut (mm)

Cutting power,