By Dr. Tushar Banerjee Assistant Professor Production

ByDr. Tushar BanerjeeAssistant Professor

Production & Industrial EngineeringNIT Jamshedpur

Manufacturing Processes-IISubject Code: PI-1502

Unit-II

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Mechanism of Chip Formation

(ductile, brittle)

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Mechanism of Chip Formation in Ductile Materials

Fig. 1. Compression of work material ahead of tool tip (Ref: LM-05)

Mechanism of chip formation in ductile material is yielding

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Fig. 2. Piispanen’s Model of card analogy to explain chip formation in machining ductile materials(Ref: LM-05)

Mechanism of Chip Formation in Ductile Materials

Fig. 3. Primary and secondary deformation zonesin the chip (Ref: LM-05)

The pattern and extent of totaldeformation of the chips due toprimary deformation of the chip aheadof the tool face and secondary sheardeformations of the chip along the toolface as shown in Fig. 3, depend on thefollowing factors.• Work material• Tool (material and geometry)• Cutting velocity (Vc) and feed (So)• Cutting fluid application

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Mechanism of Chip Formation in Brittle Materials

Fig. 4. Development and propagation of crack causing chip separation in brittle material(Ref: LM-05)

Mechanism of chip formation in brittle material is fracture

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Mechanism of Chip Formation in Brittle Materials

Fig. 5. Schematic view of chip formation in machining brittle materials (Ref: LM-05)

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Geometry and Characteristics of Continuous Chip Formation

Fig. 6. Geometrical features of continuous chip formation (Ref: LM-05)

Fig. 6

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…………………… (1)

…………………… (4)

…………………… (2)

…………………… (3)

Geometry and Characteristics of Continuous Chip Formation

eq. (4)

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Fig. 7. Role of rake angle and chip-tool interface friction on chip-reduction coefficient(Ref: LM-05)

Geometry and Characteristics of Continuous Chip Formation

Fig. 7

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Shear Angle

Fig. 8. Shear plane and shear angle in chip formation (Ref: LM-05)

8

8

11

…………………… (6)

(6)

(6)

…………………… (5)

…………………… (7)

(6) (4)

(8)

Shear Angle

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Cutting Strain

Fig. 9. Cutting strain (Ref: LM-05)

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9

…………………… (8)

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Built-up Edge (BUE) Formation

Fig. 10. Scheme of built-up edge (BUE) formation (Ref: LM-05)

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Built-up Edge (BUE) Formation10

Fig. 11. Overgrowing and overflowing of BUE (Ref: LM-05)

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Built-up Edge (BUE) Formation

Fig. 12. Role of cutting velocity and feed on BUE formation (Ref: LM-05)

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Classification of Chips

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Classification of Chips

Fig. 13. (a) Discontinuous and (b) continuous chips

(a) (b)

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Shear Plane and Shear Zone Theories

So far it is considered that chip is formed abruptly along a shear plane only. Butpractically it is not feasible, because, then change in velocity from cutting velocity (Vc) tochip flow velocity on rake surface (Vf) and plastic strain from almost nil to high valuewould have been in no time resulting in infinite retardation and strain rate. Practically,the shear deformation occurs over a zone (instead of across a plane) as indicated in Fig.14. However, the thickness of the shear zone comes to be very low, about 20 μm at highspeed machining. Due to this shear plane theory is conveniently used for general study.

Fig. 14. (a) Shear plane and (b) shear zone theories

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Orthogonal and Oblique Cutting

Fig. 15. Chip flow along orthogonal direction (Ref: LM-06)

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Orthogonal and Oblique CuttingReasons for chip deviation from orthogonal direction, i.e. reasons for oblique cuttingare as follows.

Fig. 16. Chip flow deviation due to presence of inclination angle (Ref: LM-06)

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Orthogonal and Oblique Cutting

Fig. 17. Chip flow deviation due to RCE(Ref: LM-06)

Fig. 18. Chip flow deviation due to tool-noseradius (Ref: LM-06)

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Chip-Tool Contact Length

In machining ductile materials, Ce becomes smaller than Cp and gradually almost disappearswith the increase in cutting velocity. The nature and length of chip-tool contact affects(i) Magnitude and pattern of cutting forces(ii) Cutting temperature

Fig. 19. Chip-tool contact in machining

Chip-tool contact length (C) refers to the length of contact of chip with the tool rake surfacein the direction of chip flow as shown in in Fig. 19. The nature and length of contactbetween the flowing chip and the tool at its rake surface are important aspects and playimportant role in machining. Chip-tool contact length usually comprises of two portions i.e.length of plastic contact (Cp) and length of elastic contact (Ce) i.e. C = Cp + Ce ……………… (9)

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From fig. 20, for orthogonal cutting, we have

Fig. 20. Evaluation of chip-tool contact length

…………………… (10)

Chip-Tool Contact Length

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ReferencesText Books:1. ‘Machining & Machine Tools’ by A. B. Chattopadhyay, Wiley India2. ‘Metal Cutting-Theory & Practice’ by Amitabha Bhattacharyya, New Central

Book Agency

Reference Books:1. ‘Metal Cutting Principles’ by Milton C. Shaw, Oxford University Press2. ‘Manufacturing Science’ by A. Ghosh & A. K. Mallick, East-West Press Pvt. Ltd.

Online Resources:1. https://nptel.ac.in/courses/112/105/112105127/2. https://nptel.ac.in/courses/112/105/112105126/