Mechanism of Chip Formation | Mechanics of Machining

Mechanism of Chip Formation | Mechanics of Machining

Mechanism of chip formation in 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 form of the chips is an important index of machining because it directly or indirectly indicates :
• Nature and behaviour of the work material under machining condition
• Specific energy requirement (amount of energy required to remove unit volume of work material) in machining work
• Nature and degree of interaction at the chip-tool interfaces.

The form of machined chips depend mainly upon :
• Work material
• Material and geometry of the cutting tool
• Levels of cutting velocity and feed and also to some extent on depth of cut
• Machining environment or cutting fluid that affects temperature and friction at the chip-tool and work-tool interfaces.

Knowledge of basic mechanism(s) of chip formation helps to understand the characteristics of chips and to attain favourable chip forms.

Mechanism of chip formation in machining ductile materials

During continuous machining the uncut layer of the work material just ahead of the cutting tool (edge) is subjected to almost all sided compression as indicated in Fig. Compression of work material (layer) ahead of the tool tip.

Figure: Compression of work material (layer) ahead of the tool tip

The force exerted by the tool on the chip arises out of the normal force, N and frictional force, F as indicated in Fig. Compression of work material (layer) ahead of the tool tip.

Due to such compression, shear stress develops, within that compressed region, in different magnitude, in different directions and rapidly increases in magnitude. Whenever and wherever the value of the shear stress reaches or exceeds the shear strength of that work material in the deformation region, yielding or slip takes place resulting shear deformation in that region and the plane of maximum shear stress. But the forces causing the shear stresses in the region of the chip quickly diminishes and finally disappears while that region moves along the tool rake surface towards and then goes beyond the point of chip-tool engagement. As a result the slip or shear stops propagating long before total separation takes place. In the mean time the succeeding portion of the chip starts undergoing compression followed by yielding and shear. This phenomenon repeats rapidly resulting in formation and removal of chips in thin layer by layer. This phenomenon has been explained in a simple way by Piispannen [1] using a card analogy as shown in Fig. Piispanen model of card analogy to explain chip formation in machining ductile materials.

In actual machining chips also, such serrations are visible at their upper surface as indicated in Fig. Piispanen model of card analogy to explain chip formation in machining ductile materials. The lower surface becomes smooth due to further plastic deformation due to intensive rubbing with the tool at high pressure and temperature. The pattern of shear deformation by lamellar sliding, indicated in the model, can also be seen in actual chips by proper mounting, etching and polishing the side surface of the machining chip and observing under microscope.

Figure: Piispanen model of card analogy to explain chip formation in machining ductile materials

The pattern and extent of total deformation of the chips due to the primary and the secondary shear deformations of the chips ahead and along the tool face, as indicated in Fig. Primary and secondary deformation zones in the chip, depend upon[1] Piispannen V., “Theory of formation of metal chips”, J. Applied Physics, Vol. 19, No. 10, 1948, pp. 876.
• work material
• tool; material and geometry
• the machining speed (VC) and feed (so)
• cutting fluid application

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Figure: Primary and secondary deformation zones in the chip

The overall deformation process causing chip formation is quite complex and hence needs thorough experimental studies for clear understanding the phenomena and its dependence on the affecting parameters. The feasible and popular experimental methods [2] for this purpose are:
• Study of deformation of rectangular or circular grids marked on the side surface as shown in Fig. Pattern of grid deformation during chip formation [2] Bhattacharya, A.., “Metal cutting – Theory and Practice”, Book, New Central Book Agency, Kolkata.
• Microscopic study of chips frozen by drop tool or quick stop apparatus
• Study of running chips by high speed camera fitted with low magnification microscope.

Figure: Pattern of grid deformation during chip formation

It has been established by several analytical and experimental methods including circular grid deformation that though the chips are initially compressed ahead of the tool tip, the final deformation is accomplished mostly by shear in machining ductile materials.

However, machining of ductile materials generally produces flat, curved or coiled continuous chips.

Mechanism of chip formation in machining brittle materials

The basic two mechanisms involved in chip formation are
• Yielding – generally for ductile materials
• Brittle fracture – generally for brittle materials

During machining, first a small crack develops at the tool tip as shown in Fig. Development and propagation of crack causing chip separation due to wedging action of the cutting edge. At the sharp crack-tip stress concentration takes place. In case of ductile materials immediately yielding takes place at the crack-tip and reduces the effect of stress concentration and prevents its propagation as crack. But in case of brittle materials the initiated crack quickly propagates, under stressing action, and total separation takes place from the parent workpiece through the minimum resistance path as indicated in Fig. Development and propagation of crack causing chip separation.

Figure: Development and propagation of crack causing chip separation

Machining of brittle material produces discontinuous chips and mostly of irregular size and shape. The process of forming such chips is schematically shown in Fig. Schematic view of chip formation in machining brittle materials.

Figure: Schematic view of chip formation in machining brittle materials

Geometry and characteristics of chip forms

The geometry of the chips being formed at the cutting zone follow a particular pattern especially in machining ductile materials. The major section of the engineering materials being machined are ductile in nature, even some semi-ductile or semi-brittle materials behave ductile under the compressive forces at the cutting zone during machining.

The pattern and degree of deformation during chip formation are quantitatively assessed and expressed by some factors, the values of which indicate about the forces and energy required for a particular machining work.

Chip reduction coefficient or cutting ratio

The usual geometrical features of formation of continuous chips are schematically shown in Fig. Geometrical features of continuous chips’ formation.

The chip thickness (a2) usually becomes larger than the uncut chip thickness (a1). The reason can be attributed to
• compression of the chip ahead of the tool
• frictional resistance to chip flow
• lamellar sliding according to Piispannen

Figure: Geometrical features of continuous chips’ formation

The significant geometrical parameters involved in chip formation are shown in Fig. Geometrical features of continuous chips’ formation and those parameters are defined (in respect of straight turning) as:

The role of rake angle and friction at the chip-tool interface on chip reduction coefficient are also schematically shown in Fig. Role of rake angle and friction on chip reduction coefficient.

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Figure: Role of rake angle and friction on chip reduction coefficient

Chip reduction coefficient, ζ is generally assessed and expressed by the ratio of the chip thickness, after (a2) and before cut (a1) as in equation 5.1.
But ζ can also be expressed or assessed by the ratio of [3] Kronenberg, M., “A new approach to some relationships in the Theory of Metal Cutting”, J. Applied Physics, Vol.6, No. 6, 1945.
• Total length of the chip before (L1) and after cut (L2)
• Cutting velocity, VC and chip velocity, Vf

Considering total volume of chip produced in a given time,

Shear angle: It has been observed that during machining, particularly ductile materials, the chip sharply changes its direction of flow (relative to the tool) from the direction of the cutting velocity, VC to that along the tool rake surface after thickening by shear deformation or slip or lamellar sliding along a plane. This plane is called shear plane and is schematically shown in Fig. Shear plane and shear angle in chip formation.

Shear plane: Shear plane is the plane of separation of work material layer in the form of chip from the parent body due to shear along that plane.
Shear angle: Angle of inclination of the shear plane from the direction of cutting velocity [as shown in Fig. Shear plane and shear angle in chip formation].

Figure: Shear plane and shear angle in chip formation

Equation 5.10 depicts that with the increase in ζ, shear angle decreases and vice-versa. It is also evident from equation (5.10) as well as equation (5.4) that shear angle increases both directly and indirectly with the increase in tool rake angle. Increase in shear angle means more favourable machining condition requiring lesser specific energy.

Cutting strain: The magnitude of strain, that develops along the shear plane due to machining action, is called cutting strain (shear). The relationship of this cutting strain, ε with the governing parameters can be derived from Fig. Cutting strain in machining.

Figure: Cutting strain in machining

Due to presence of the tool as an obstruction the layer 1 has been shifted to position 2 by sliding along the shear plane.
From Fig. Cutting strain in machining,

Built-up-Edge (BUE) formation

Causes of formation

In machining ductile metals like steels with long chip-tool contact length, lot of stress and temperature develops in the secondary deformation zone at the chip-tool interface. Under such high stress and temperature in between two clean surfaces of metals, strong bonding may locally take place due to adhesion similar to welding. Such bonding will be encouraged and accelerated if the chip tool materials have mutual affinity or solubility. The weldment starts forming as an embryo at the most favourable location and thus gradually grows as schematically shown in Fig. Scheme of built-up-edge formation.

Figure: Scheme of built-up-edge formation

With the growth of the BUE, the force, F (shown in Fig. Scheme of built-up-edge formation) also gradually increases due to wedging action of the tool tip along with the BUE formed on it. Whenever the force, F exceeds the bonding force of the BUE, the BUE is broken or sheared off and taken away by the flowing chip. Then again BUE starts forming and growing. This goes on repeatedly.

Characteristics of BUE

Built-up-edges are characterized by its shape, size and bond strength, which depend upon:
• work tool materials
• stress and temperature, i.e., cutting velocity and feed
• cutting fluid application governing cooling and lubrication.

BUE may develop basically in three different shapes as schematically shown in Fig. Different forms of built-up-edge.

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Figure: Different forms of built-up-edge

In machining too soft and ductile metals by tools like high speed steel or uncoated carbide the BUE may grow larger and overflow towards the finished surface through the flank as shown in Fig. Overgrowing and overflowing of BUE causing surface roughness.

Figure: Overgrowing and overflowing of BUE causing surface roughness

While the major part of the detached BUE goes away along the flowing chip, a small part of the BUE may remain stuck on the machined surface and spoils the surface finish. BUE formation needs certain level of temperature at the interface depending upon the mutual affinity of the work-tool materials. With the increase in VC and so the cutting temperature rises and favours BUE formation. But if VC is raised too high beyond certain limit, BUE will be squashed out by the flowing chip before the BUE grows. Fig. “Role of cutting velocity and feed on BUE formation” shows schematically the role of increasing VC and so on BUE formation (size). But sometime the BUE may adhere so strongly that it remains strongly bonded at the tool tip and does not break or shear off even after reasonably long time of machining. Such detrimental situation occurs in case of certain tool-work materials and at speed-feed conditions which strongly favour adhesion and welding.

Figure: Role of cutting velocity and feed on BUE formation

Effects of BUE formation

Formation of BUE causes several harmful effects, such as:
• It unfavourably changes the rake angle at the tool tip causing increase in cutting forces and power consumption
• Repeated formation and dislodgement of the BUE causes fluctuation in cutting forces and thus induces vibration which is harmful for the tool, job and the machine tool.
• Surface finish gets deteriorated
• May reduce tool life by accelerating tool-wear at its rake surface by adhesion and flaking

Occasionally, formation of thin flat type stable BUE may reduce tool wear at the rake face.

Types of chips and conditions for formation of those chips

Different types of chips of various shape, size, colour etc. are produced by machining depending upon
• type of cut, i.e., continuous (turning, boring etc.) or intermittent cut (milling)
• work material (brittle or ductile etc.)
• cutting tool geometry (rake, cutting angles etc.)
• levels of the cutting velocity and feed (low, medium or high)
• cutting fluid (type of fluid and method of application)

The basic major types of chips and the conditions generally under which such types of chips form are given below:

* Discontinuous type

of irregular size and shape

– work material – brittle like grey cast iron

of regular size and shape

– work material ductile but hard

and work hardenable

– feed – large

– tool rake – negative

– cutting fluid – absent or inadequate

* Continuous type
                Without BUE:     Work material

Cutting velocity

Feed

Rake angle

Cutting fluid

– ductile

– high

– low

– positive and large

– both cooling and lubricating

                Without BUE:     Work material

Cutting velocity

Feed

Cutting fluid

– ductile

– medium

– medium or large

– inadequate or absent

* Jointed or segmented type
                                               Work material

Cutting velocity

Feed

Tool rake

Cutting fluid

– semi-ductile

– low to medium

– medium to large

– negative

– absent

Often in machining ductile metals at high speed, the chips are deliberately broken into small segments of regular size and shape by using chip breakers mainly for convenience and reduction of chip-tool contact length.

 

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