Presentation on theme: "MT-284 MANUFACTURING PROCESSES INSTRUCTOR: SHAMRAIZ AHMAD MS-Design and Manufacturing Engineering Topic: Turning and Surface Finish."— Presentation transcript:
MT-284 MANUFACTURING PROCESSES INSTRUCTOR: SHAMRAIZ AHMAD MS-Design and Manufacturing Engineering Topic: Turning and Surface Finish Processes
Todays Lecture Forces in Turning process Friction and heat sources in cutting Surface Finishing processes – Electroplating – Honing – Lapping – Super finishing – Polishing – Buffing
TURNING OPERATION Turning operation : the work piece is rotated and a single edge cutting tool removes a layer of material as it moves parallel to axis of rotation. Turning is traditionally carried out at Lathe machine. The starting material is generally a work piece generated by other processes such as casting and extrusion etc
CUTTING FORCES The force acting on the tool in orthogonal cutting are shown in fig. The cutting force Fc acts in the direction of the cutting speed, V, and supplies the energy required for the machining operation. The thrust force F t acts in the direction normal to the cutting velocity. The two forces produces the resultant force R, which can be resolved into two components acting on tool (Friction force F resisting the flow of chip on rake face, and Normal force N perpendicular to F) The Resultant force R of F and N is oriented at an angle β, called the friction angle. Another force Fs, shear force that causes shear deformation and Fn normal to Fs. The shear stress is t=Fs/As.
The cutting force is shown to be: The ratio of F to N is the coefficient of friction µ, at the tool chip interface. The angle is known as the friction angle β. The coefficient of friction is shown as:
SHEAR ANGLE RELATIONSHIPS M.E Merchant relationship is based upon: – The shear angle adjust itself so that is cutting force is minimum – Maximum shear stress occurs in shear plane. As the rake angle decreases and/or as the friction at the tool chip interface increases, the shear angle decreases and thus the chip become thicker.
TEMPERATURES IN CUTTING In cutting, nearly all of energy ( approx 98%) dissipated in plastic deformation is converted into heat that in turn raises the temperature in the cutting zone. – There are severe temperature gradients in the cutting zone. CAUSES: FIGURE Typical temperature distribution in the cutting zone. Temperature generated in shear plane is a function of the specific energy for shear and the specific heat of the material. The temperature rise at the tool-chip interface is also a function of the coefficient of friction. Flank wear is an additional source of heat, caused by rubbing of the tool on the machine surface.
Temperature Effect During cutting rise in temperature is very important, because: – Adversely affect the strength, hardness and wear resistance of the tool. – Cause dimensional changes in the part being machined, making control of dimensional accuracy difficult. – Can induce thermal damage to the machined surface, adversely affecting its properties and service life – The machine tool itself may be subjected to temperature gradient.
Temperature in Cutting The temperature in metal cutting can be reduced by: application of cutting fluids (coolants). change in the cutting conditions by reduction of cutting speed and/or feed; selection of proper cutting tool geometry (positive tool orthogonal rake angle).
SURFACE FINISHING Surface finishing may be defined as any process that alters the surface of a material or aesthetic or functional purposes. Why Surface Finishing Processes: Surface finishing can hide any number of faults in a casting. Surface finishing can improve the aesthetic appeal of a casting by changing its gloss, shininess and color which increases sales value. It can improve corrosion resistance and tailor surface properties.
1. ELECTROPLATING First use reported almost 200 years ago. It may be defined as the process wherein an electric current is carried across an electrolyte and in which a substance is deposited at one of the electrodes. Electrolysis is possible because solvents, water in particular, have the ability to ionize substances dissolved in it. These ions are electrically charged and are attracted to oppositely charged electrodes where they are neutralized by the charges on these electrodes. The cathode product is the deposition of metal and the anodic product most often is the dissolution of metal.
Electroplating This is a process by which a thin layer of metal is deposited on the surface of an electrically conducting part. The part is used as a cathode, and the depositing material forms the anode. The electrodes are dipped in a solution of the appropriate salt, such that on application of voltage, the metal from the anode is dissolved into the solution, and deposited on the cathode. A simple example of this process is copper plating using CuSO4 solution, using Copper anodes.
Electroplating Electroplating Process
Uses of Electroplating Electroplating is one of the means available to the surface finisher to apply a metal coating to a metallic Electroplating is used to deposit a very wide range of pure metals and alloys for se in decorative, functional and jewellery applications. Nickel/chromium composites (usually called chrome plating), copper, brass an alloy), bronze (an alloy) and zinc are used for decorative applications.
2. HONING Honing is a abrasive finishing process performed by a honing tool, which contains a set of three to a dozen and more bonded abrasive sticks. The sticks are equally spaced about the periphery of the honing tool. They are held against the work surface with controlled light pressure, usually exercised by small springs. The honing tool is given a complex rotational and oscillatory axial motion, which combine to produce a crosshatched lay pattern of very low surface roughness
Honing It produces surface finish of about 0.1 μm with crosshatched surface characteristics which improves the function and service life. Honing is low-speed operation, 25 to 95 m/min while grinding is high speed operation. A cutting fluid must be used in honing to cool and lubricate the tool and to help remove the chips. A common application of honing is to finish the holes. Typical examples include bores of internal combustion engines, bearings, hydraulic cylinders, and gun barrels.
3. Lapping In lapping, instead of a bonded abrasive tool, oil- based fluid suspension of very small free abrasive grains (aluminum oxide and silicon carbide, with typical grit sizes between 300 and 600) called a lapping compound is applied between the work piece and the lapping tool. The lapping tool is called a lap, which is made of soft materials like copper, lead or wood. The lap has the reverse of the desired shape of the work part. To accomplish the process, the lap is pressed against the work and moved back and forth over the surface. Lapping is sometimes performed by hand, but lapping machines accomplish the process with greater consistency and efficiency.
Lapping is used lo produce optical lenses, metallic bearing surfaces, gages, and other parts requiring very good finishes and extreme accuracy.
20 Lapping Abrading process used to remove minute amounts of metal from internal or external surfaces surface Reasons for lapping –Increase wear life of part Improve accuracy and surface finish Improve surface flatness Provide better seals and eliminate need for gaskets
4. SUPERFINISHING Super finishing is a finishing operation similar to honing, but it involves the use of a single abrasive stick. The reciprocating motion of the stick is performed at higher frequency. Also, the grit size and pressures applied on the abrasive stick are smaller. A cutting fluid is used to cool the work surface and wash away chips.
Super finishing In super finishing, the cutting action terminates by itself when a lubricant film is built up between the tool and work surface. Thus, super finishing is capable only of improving the surface finish but not dimensional accuracy. The result of these operating conditions is mirror like finishes with surface roughness values around 0.01 μm. Super finishing can be used to finish flat and external cylindrical surfaces.
5. POLISHING Polishing is a finishing operation to improve the surface finish by means of a polishing wheel made of fabrics or leather and rotating at high speed. The abrasive grains are glued to the outside periphery of the polishing wheel. Polishing operations are often accomplished manually. Polishing is often used to remove contamination of instruments, remove oxidation Polishing and Buffing terms are generally used together.
6. BUFFING Buffing is a finishing operation similar to polishing, in which abrasive grains are not glued to the wheel but are contained in a buffing compound that is pressed into the outside surface of the buffing wheel while it rotates. As in polishing, the abrasive particles must be periodically replenished. As in polishing, buffing is usually done manually, although machines have been designed to perform the process automatically.
Polishing is used to remove scratches and burrs and to smooth rough surfaces while buffing is used to provide attractive surfaces with high luster.