Different type of metal surface removing process

Super finishing is an abrashing process, efficient in surface refining of cylindrical, flat, spherical and cone shaped parts. it is not primarily a dimension changing process but mainly used for producing finished surface of fine quality. Only a slight among of stock is removed (Average 0.002 to 0.02 mm on a disc). process of honing involves two motions whereas super finishes requires three to five or even more.

Honing:- The honing operation is used for finishing the inside surface of a hole. Here, abrasives in the form of sticks are mounted on a mandrel which is then given a reciprocating movement (Along hole axis) superimposed on a uniform rotary motion. The grit size normally varies from 80-600 mesh. Because of the nature of the path of the abrasive grits on the surface of the work, a random cross-marked surface finish (desirable for lubrication) is obtained. Depending on the work material, the honing speed may vary from 15-60 m/min, and the honing pressure lies in the range 1-3 N/mm2 . in special cases, material up to 0.5 mm may be removed by honing. The tolerance and finish achieved in this operation are of the order of 0.0025 mm.



Lapping:- It is an abrading process for refining surface finish and the geometrical accuracy of flat cylindrical and spherical surfaces. It is a process of removing surface roughness, tool marks, surface cracks from grinding, slight distortions and other minor defects from previously operation. It improves surface quality and close fit between mating parts. Lapping consists of use of loose grain abrasive flours mixed with oil. Abrasive size ranges from 120 to 1200 mesh. Lap is generally made of material softer than the work material.


Polishing (Flexible Grinding):- In polishing operation, the smoothness on a surface of the resident wheels of wood, belt, leather, canvas or fabric or attached to the bolts operating over resilient wheels.



What is the difference between honing and lapping operations:-

              Honing is an abrasive machining process in which cutting speed is much lower than that of grinding. It is primarly used to size and finish bored holes or remove common error left by cutting tools. The honing tool consists of a set of Al2O3 or SiC bonded abrasives called stones mounted on a metal mandrel. The shares ans stones on uniformly spaced to produces even cutting forces. Material removal is using small typically about 0.127 mm or less. Because of low cutting speed in the process. Heat and pressure are less. As a result there is no distortion of the work pieces and the process produces a nearly perfect diameters. The tool is inserted into the box and adjusted to wear against the walls. The work is geven a slow reciprocating motion as the mandrel rotates, thus generating a straight and round hole. A cutting fluid is used to flush away chips and to keep temperatures low. kerosene is generally used as a cutting fluid.

                Honing is also done for external cylindrical or flat surfaces and to remove sharp edge an cutting tools and inserts. Typical tolerance that can be maintained are 0.005 mm for the diameter and 0.005 mm for roundness and straightners. The soft materials such as silver and brass as well as using hard materials. Including the hardest alloys, carbides and ceramics can be honed. The use of microy stalline CBN is especially advantageous for honing soft or medium hard materials Honing machine can be either horizontal or vertical. Horizontal honing machines are used for long holes such as canon or riffle barrels. Honing can also be done on many hard products include I.C. engine cylinders, bearings, gun berrels, ring gauges, piston pins and shafts.


Lapping:- Lapping is an abrasive finishing operation used on flat, cylindrical, spherical or specially formed surface in contact with lap. The lap is relatively softer material in which abrasive particles are embedded on its surface. Alternatively, loose abrasives may be carried between the lap and the work surface through carriers such as oil, grease, or water. The type of abrasive and carrier are dependent on the materials to be lapped. soft materials are lapped with aluminium oxide and hard materials with diamonds or silver carbide grit. As the lap is rubbed against the work, the abrasive particles in the lap result materials from the surface of work piece. The lap is usually a soft process metal such as cast iron or copper. It can be even leather or cloth.



               Since the abrasive particles are using smaller, hence very limited amount of metal removal is possible, usually less than 0.03mm. It is a slow process which is used only to produce geometrically true surfaces, or to correct minor surface imperfections or to obtain very flat and smooth surfaces. External accuracy is both tolerance and geometry are the hall marks  of lappings. Materials of almost any hardness can be lapped.
     
              various types of lapping machines are available. In its most common industrial form the lap is a ratating table and the parts being lapped have a planetary movement resulting in a uniform surfaces such as spherical objects and glass lenses, the lap has to be of three dimensional shape. since lapping is essentially a finishing operation. Parts should not be far from the exported size and geometry before lapping.

No comments