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GATE Mechanical Engineering Short Notes on Types of Chips in Machining process

Shot notes for GATE Mechanical Engineering on Type of chips formation during the machining operation:

(1) Discontinuous Chips

  • The material ahead of the tool edge fails with a brittle fracture along the shear zone. This produces small fragments of discontinuous chips. Since the chip breaks up into small segments, the friction between tool and chip reduces, thus resulting in a better surface finish. These kind of chips are easier to collect, handle and dispose off. 
  • Conditions for development of discontinuous chips: (a) Low cutting speed, (b) Small feed, (c) Brittle and non ductile materials like brass, CI castings, titanium etc., (d) Small rake angle, (e) Large uncut chip thickness
  • Effect of discontinuous chips: (a) Less tool life, (b) More power consumption, (c) Good surface finish, (d) No lubrication or cutting fluid.

(2) Continuous Chips

  • These kind of chips are produced when more ductile materials are used for the machining process. 
  • Due to large plastic deformation possible with ductile materials longer continuous chips are produced. 
  • This type of chip is most desirable since it represents stable cutting conditions resulting in good surface finish. 
  • But there is a chance of deteriorating the surface finish, as the chips coil up in a spiral fashion.
  • The spiralling of the chips are avoided by attaching to the tool face, a chip breaker. The main function of the chip breaker is to reduce the radius of curvature of the chip and thus break it. 
  • Conditions favouring continuous chip formation: (a) Ductile and soft material, (b) Small uncut thickness, (c) High cutting speed, (d) Large rake angle, (e) Suitable cutting fluid.
  • Effect of continuous chip: (a) Higher tool life, (b) Low coefficient of friction, (c) Steady forces, (d) less power consumption, (e) Surface finish may be poor if the chips hit on the finished part of the work piece.

(3) Continuous chips with Built Up Edge (BUE)

  • When machining ductile materials, conditions of high local temperature and extreme pressure in the cutting zone along with the friction may cause work material to get welded to the cutting edge of the tool forming the built up edge. 
  • Once BUE is formed, successive layers of the work piece are then added until these are broken under high pressure. 
  • BUE changes its size, as at first, it increases followed by decrease and then again increase. This is the cause of vibrations and poor surface finish. 
  • In a way BUE protects the tool cutting edge, but it changes the tool geometry. 
  • BUE can be reduced and eliminated by increasing the cutting speed, increasing the rake angle and using a suitable cutting fluid. 
  • Conditions for formation of BUE: (a) Stronger adhesion between tool and work piece, (b) Small rake angles, (c) Large uncut thickness, (d) Low cutting speed, (e) No cutting fluid.
  • Effects of BUE: (a) High power consumption, (b) Poor surface finish, (c) High coefficient of friction, (d) Increased tool life.

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