Manufacturing Processes – Chips Formation

Machining : Machining is a semi-finishing or finishing process essentially done to impart required or stipulated dimensional and form accuracy and surface finish to enable the product to

•  fulfill its basic functional requirements
•  provide better or improved performance
•  Render long service life.
• Machining is a process of gradual removal of excess material from the preformed blanks in the form of chips.

The type of chip produced when cutting the metal depends on the material being machined and the cutting conditions at the time. These conditions may involve;

ü   the tool,

ü   the rate of cutting,

ü   the type or condition of the machine .

ü   the use or absence of a cutting lubricant.

The types of chip are described in the following order.

a)      Continuous

b)      Discontinuous.

c)      Built-up edge

d)     Serrated or segmented

v  Chip has two surfaces: one that is in contact with the tool face (rake face) and the other from the original surface of the workpiece.

a) Continuous Chip:

The continuous chip leaves the tool as a long ribbon and is usually formed with

ü  Ductile material such as mild steel, copper and Aluminum.

ü   At high cutting speed.

ü   At high rake angles.

ü   Decreasing the cut.

ü   Using a cutting lubricant. 

(b) Discontinuous Chips

The discontinuous chip leaves the tool as small segments of metal and results from. See figure 4.

ü   Brittle workpiece material.

ü   Workpiece materials that contain hard inclusion and impurities.

ü   Very low or very high cutting speeds.

ü   Large depth of cuts.

ü   Low rake angles.

ü   Lack of an effective cutting fluid.

      (c) Built-up Edge Chips

The built-up edge associated with the continuous chip. It is the formation of material layers gradually deposit on the tool tip. The layers build-up and welded to the tool face under the heavy pressure and heat generated at the tip of the tool face.

ü  As it becomes larger, the BUE becomes unstable and eventually breaks up.

ü   Poor finish from the cut due to high friction and cutting pressure.

ü   A thin stable BUE reduces wear by protecting the rake face of the tool.

         (d) Serrated Chips

There are semi continuous chips. Metals with low thermal conductivity and strength that decrease sharply with temperature, such as titanium, exhibit this behavior. The chips have a saw tooth like appearance. It is also known as segmented or non-homogeneous chips.

v  Chips Breaking

·         Long continuous chip are undesirable.

·         Chip breaker is a piece of metal clamped to the rake surface of the tool which bends the chip and breaks it.

·         Chips can also be broken by changing the tool geometry, thereby controlling the chip flow.

·         CBs increase the effective rake angle of the tool and, consequently, increase the shear angle.

o   Need and purpose of chip-breaking :

 Continuous machining like turning of ductile metals, unlike brittle metals like grey

Cast iron, produce continuous chips, which leads to their handling and disposal

problems. The problems become acute when ductile but strong metals like steels are

machined at high cutting velocity for high MRR by flat rake face type carbide or

ceramic inserts. The sharp edged hot continuous chip that comes out at very high speed

ü  becomes dangerous to the operator and the other people working in the vicinity

ü  may impair the finished surface by entangling with the rotating job

ü  creates difficulties in chip disposal.

Therefore it is essentially needed to break such continuous chips into small regular pieces for

• safety of the working people

• prevention of damage of the product

• easy collection and disposal of chips.

Chip breaking is done in proper way also for the additional purpose of improving

machinability by reducing the chip-tool contact area, cutting forces and crater wear of

the cutting tool.