EFFECT OF CUTTING VELOCITY ON CHIP THICKNESS IN TURNING OPERATIONME-261 METAL MACHINING AND MACHINE TOOLS

MADE BY: SUYASH AGARWALCLASS : B.TECH PART II, MECHANICAL ENG. IIT(B.H.U) VARANASIROLL NO. : 14135087 (BATCH 2, GROUP 3)DATE: 22 JANUARY 2016

INTRODUCTIONIn any machining, material is gradually removed in form of chips. Machining of different work materials produces different types and pattern of chips. Even a given work material shows a wide variation in the chip form under different machining conditions. The features and characteristics of machining chips widely vary depending upon1. Work material- brittle, ductile etc.2. Type of machining- continuous, interrupted etc.3. Cutting tool- material and geometry4. Levels of the process parameters- cutting velocity, feed and also depth of cut5. Machining condition- dry or wet, type and method of application of cutting fluid that affects cutting temperature, friction and tool wear.The two basic mechanism that accomplish chip formation are yielding (generally for brittle materials) and brittle fracturing (for brittle materials). Machining of brittle materials produces discontinuous chips and mostly of irregular shape and size. However, most of the engineering materials behave ductile in machining. During machining, the uncut layer of work material just ahead of the cutting tool (edge) is subjected to almost all sided compression. The force exerted by the tool on the chips arises in the form of normal force and frictional force. Due to such compression, shear stress develops and grows within that compressed region, in different magnitude, in different directions. When the value of the shear stress reaches or exceeds the shear strength of that work material in the deformation region, yielding or slip begins resulting in shear deformation in that region and initiating of separation in the form of a small crack along the plane of maximum shear stress. The slip or shear stops propagating before the total separation takes place. The succeeding portion of the work material starts undergoing compression followed by yielding and shear. This phenomenon repeats rapidly resulting in formation and removal of chips in thin layers. The lower surface becomes smooth due to further plastic deformation for intensive rubbing with tool at high pressure and temperature.

The chip thickness becomes larger than the uncut chip thickness because of the compression of the chip ahead of tool, frictional resistance to chip flow and lamellar sliding of chip segments. The angle that the tool makes with respect to the vertical from the workpiece is called the back rake angle.

(The chip forming shear process. defines the onset of shear or lower boundary. defines the direction of slip due to dislocation movement.)

EXPERIMENTAIM: To study the effect of chip thickness by varying cutting velocity in turning operations.MATERIALS REQUIRED: Mild steel bar as a work piece, HSS tool, digital Vernier calliper.THEORY: In turning, a work piece is rotated about its axis as cutting tool is fed into it, shearing away unwanted material and creating the desired part. Undesired materials comes out in form of chips. In cutting and abrasive processes, the cutting edge penetrates into the workpiece material, which is thus plastically deformed and slides off along the rake face of the cutting edge. This is called chip formation. Three key parameters determine productivity and part quality. These parameters are:1. Cutting speed2. Feed rate3. Depth of cutThe cutting speed is the speed of the work as it rotates past the cutting tool.

In continuous chip formation the chip slides off along the rake face at a constant speed in a stationary flow. Continuous chip formation is promoted by a uniform, fine-grained structure and high ductility of the workpiece material, by high cutting speeds and low friction on the rake face, by positive rake angles and a low un-deformed chip thickness. With continuous chip formation, built-up edges can occur. They are formed by particles of the workpiece material, which adhere to the rake face and to the cutting edge. These particles have been subject to high deformation and have been strain-hardened. Built-up edges influence the cutting edge geometry. They generally facilitate chip formation (lower forces). When they move off, they can drag along workpiece particles (adhesive wear). Sometimes strain-hardened parts of the built-up edges are integrated into the newly formed workpiece surface. Therefore, the formation of built-up edges is generally undesirable. However, it does not occur at higher cutting speeds and resulting higher temperatures in the chip formation zone, because there is no strain-hardening if the recrystallization temperature is exceeded during the deformation process.

PROCEDURE: 1. Fix the tool into the chuck and tool into tool post.2. Perform centring operation.3. Set the machining parameters.4. Start turning operation, as the material is removed in the form of chips, start collecting them in a piece of paper.5. Change the cutting velocity and repeat 46. After collecting adequate sample, check for thickness of chips with the help of digital Vernier calliper.7. Record the observations in the following table.

RESULT: In a turning operation on a mild steel bar using a high speed steel cutting tool, it is observed that the increase in cutting velocity leads to the reduction in chip thickness.PRECAUTION: 1. Centering should be done properly2. Chip should be collected properly. (As chip temperature is high)3. Digital Vernier calliper should be handled with care.