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Cutting temperature

The cutting temperature is vital, particularly
when its high, it effects both the tool and the work being output. A large
section of heat is taken away by the chips. This is not a major concern as the
chips are not used. The possible effects of high cutting temperatures are that
the tool will be likely to wear out much faster.  There will be some sort of flaking and on the
cutting edge because of the thermal shocks. The elevated temperatures will also
cause build up formation. The thermal aspect that occurs on the cutting tool
during machining is a traditional concern since cutting metals are associated
with elevated temperatures in the cutting zone. The high cutting temperatures
cause hardness change, metallurgical transformation, or even chemical
composition change due to work done in deforming and in overcoming sliding
friction between tool, workpiece, and chip. Hence, they have profound
consequences for tool life, dimensional and form accuracy, and surface
integrity of the product. For end milling, the cutting temperature on the tool
is particularly crucial since the tool during end milling generally undergoes
thermal shock in each revolution. The fast heating and cooling of the tool at
work are associated with considerable temperature variations in cutting edge;
furthermore, the heat generation during chip formation does not flow easily
through the workpiece and chip.

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Tool materials

Tool materials play a huge role in material
cutting and tool wear. The progression of high speed steels to carbide and
moving further onto ceramics and other durable material. From the 1960’s the
development of The
use of coatings, particularly titanium nitride, allows high-speed steel tools
to cut faster and last longer. titanium nitride provides a high surface
hardness, resists corrosion, and it minimizes friction. In industry today,
carbide tools have replaced high-speed steels in most applications. These
carbide and coated carbide tools cut about 3 to 5 times faster than high-speed
steels. A higher percentage of cobalt binder increases strength, but lowers the
wear resistance. Carbide is used in solid round tools or in the form of
replaceable inserts. Every manufacturer of carbide tools offers a variety for
specific applications. The proper choice can double tool life or double the
cutting speed of the same tool. Shock-resistant types are used for interrupted
cutting. Harder, chemically-stable types are required for high speed finishing
of steel. More heat-resistant tools are needed for machining the super alloys, like
Inconel and Hastelloy.

 

 

 

https://me-mechanicalengineering.com/cutting-tool-materials/

 

 

 

Work material

 

The work material must correspond with the
tool. This is because the tool has to a correct material, so it doesn’t destroy
the work material. The
cutting tool materials must be harder than the material which is to be cut, and
the tool must be able to withstand the heat generated in the metal-cutting
process. Also, the tool must have a specific geometry, with clearance angles
designed so that the cutting edge can contact the workpiece without the rest of
the tool dragging on the workpiece surface. The angle of the cutting face is
also important, as is the flute width, number of flutes or teeth, and margin
size. To have a long working life, all the above must be optimized, plus the
speeds and feeds at which the tool is run.

Contact stresses

The influence of shear stresses on the
physical processes proceeding on the surface of contact of a cutting tool with
a chip removed has been investigated. 
The mechanisms of appearance of contact layers and the dependence of the
lengths of the elastic and plastic parts of these layers on the normal and
shear stresses in the contact zone have been considered.  It was established that a natural “white”
layer formed in the process of cutting plays a protective role and, therefore,
decreases the rate of wear of the tool.

When metal is cut, the cutting force acts
mainly through a small area of the rake face, which is in the contact with the chip
and thus is known as the tool chip interface. Therefore, it is of interest   in  
cutting   force   determination   and considerations of tool wear to determine
the contact conditions at the tool chip interface. To comprehend the contact process
at the tool chip interface, the following should be analysed he contact pressure
(normal and shear stresses) distribution, the      temperature distribution, and the
parameters   of relative motion. Many
studies have been aimed at obtaining a better understanding of the conditions at
the Tool chip interface through studying the distribution of the normal and shear
stress at this interface. A variety of experimental techniques including photo elastic
tools, split tool   dynamometer,
transparent tool for the direct observation of the tool chip interface, metallurgical
examination of ‘quick stop’ chip section experimental slip line field method
and others have been developed.

 

 

 

 

 

 

 

 

 

 

Cutting
conditions

 

For each operation, decisions must be made
about machine tool, cutting tool(s), and cutting conditions These decisions
must give due consideration to work part machinability, part geometry, surface
finish, and so forth. Cutting conditions: speed, feed, depth of cut, and
cutting fluid

Cutting tool wear condition monitoring is
an important technique that can be useful especially in automated cutting
processes and unmanned factories to prevent any damage to the machine tool and
workpiece. In any metal cutting operation, one of the major hurdles in
realizing its complete automation is that of the cutting tool-state prediction,
where tool-wear is a critical factor in productivity. Cutting tool condition
monitoring can help in on-line realization of the tool wear, tool breakage, and
workpiece surface roughness. A good cutting tool condition monitoring system
should be characterized by (a) fast detection of impact or collisions, (i.e.,
unwanted movement between tool and workpiece, or tool and any other component
of the machine tool), (b) tool chipping (cutting edge breakage), and (c)
gradual tool wear (crater and flank) caused by abrasion due to friction between
cutting tool and workpiece (flank wear) and cutting tool and chip (crater
wear).

Fluids

Cutting fluids are used in metal machining
for a variety of reasons such as improving tool life, reducing workpiece
thermal deformation, improving surface finish and flushing away chips from the
cutting zone. Practically all cut in fluids presently in use fall into one of
four categories:

Straight
oils are non-mollifiable and are used in machining
operations in an undiluted form. They are composed of a base mineral or
petroleum oil and often contains polar lubricants such as fats, vegetable oils
and esters as well as extreme pressure additives such as Chlorine, Sulphur and
Phosphorus. Straight oils provide the best lubrication and the poorest cooling
characteristics among cutting fluids.

Synthetic
Fluids contain no petroleum or mineral oil base and
instead are formulated from alkaline inorganic and organic compounds along with
additives for corrosion inhibition. They are generally used in a diluted form
(usual consent ration = 3 to 10%). Synthetic fluids often provide the best
cooling performance among all cutting fluids.

Soluble Oil Fluids form an
emulsion when mixed with water. The concentrate consists of a base mineral oil
and emulsifiers to help produce a stable emulsion. They are used in a diluted
form (usual concentration = 3 to 10%) and provide good lubrication and heat
transfer performance. They are widely used in industry and are the least
expensive among all cutting fluids.

Semi-synthetic
fluids are essentially combination of synthetic and
soluble oil fluids and have characteristics common to both types. The cost and
heat transfer performance of semi-synthetic fluids lie between those of
synthetic and sol able oil fluids.

 

Cutting tool shape

The tool cutting edge angle significantly
affects the cutting process because, for a given feed and cutting depth, it
defines the uncut chip thickness, width of cut, and thus tool life. The physical
background of this phenomenon can be explained as follows: when or decreases,
the chip width increases correspondingly because the active part of the cutting-edge
increases. This results in improved heat removal from the tool and hence tool
life increases. For example, if the tool life of a high-speed steel (HSS) face
milling tool having or = 60° is taken to be 100% then when or = 30° its tool
life is 190%, and when or = 10° its tool life is 650%. An even more profound effect
of or is observed in the machining with single-point cutting tools. For
example, in rough turning of carbon steels, the change of or from 45° to 30°
sometimes leads to a fivefold increase in tool life. The reduction of art,
however, has its drawbacks. One of these is the corresponding increase of the
radial component of the cutting force, which reduces the accuracy and stability
of machining particularly when the machine, tool holder and workpiece fixture
are not sufficiently rigid. Rake angles come in three varieties: positive, zero
(sometimes referred to as neutral) and negative,

Cutting tools for metal cutting have many
shapes, each of which are described by their angles or geometries.

Every one of these tool shapes have a
specific purpose in metal cutting. The primary machining goal is to

achieve the most efficient separation of
chips from the workpiece. For this reason, the selection of the right

cutting tool geometry is critical. Other
chip formation influences include:

• the workpiece material

• the cutting tool material

• the power and speed of the machine

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