HONING

These are some operations by which a product receives the final machining stage or finishing operation before final dispatch. These processes or methods remove a very small amount of metal, and hence the surface finish obtained is specified in the ranges of micro finishes. Honing is one of these operations.

HONING

The process of finishing ground surfaces to a high degree of accuracy and smoothness with abrasive blocks applied to the surface under a light controlled pressure and with a combination of rotary and reciprocating motions.

Honing is a controlled, low-speed sizing and surface finishing process in which stock is abraded by the shearing action of a bonded abrasive honing stick. In honing, simultaneous rotating and reciprocating action of the stick results in a characteristic cross-hatch lay pattern. Because honing is a low-speed operation, metal is removed without the increased temperature that accompanies grinding and thus any surface damage caused by heat is avoided. Honing uses a special tool, called a honing stone or a hone, to achieve a precision surface. The hone is a composed of abrasive grains that are bound together with an adhesive

In addition to removing stock, honing involves the correction of errors from previous machining operations. These errors include

• Geometrical errors such as out-of-roundness, waviness, bell mouth, barrel, taper, rainbow, and reamer chatter
• Dimensional inaccuracies
• Surface character (roughness, lay pattern, and integrity)

Honing corrects all of these errors with the least possible amount of material removal; however, it cannot correct hole location or perpendicularity errors. The most frequent application of honing is the finishing of internal cylindrical holes. However, numerous outside surfaces also can be honed. Gear teeth, valve components, and races for antifriction bearings are typical applications of external honing.

Process

The tool reciprocates through the bore, the pressure and the resulting penetration of grit is greatest at high spots and consequently the waviness crests are abraded, making the bore straight and round. After leveling high spots, each section of the bore receives equal abrading action. The hole axis is usually in the vertical position to eliminate gravity effects on the honing process; however, for long parts the axis may be horizontal

Advantages of Honing

1. It is characterized by rapid and economical stock removal with a minimum of heat and distortion.
2. It generates round and straight holes by correcting form errors caused by previous operations.
3. It achieves high surface quality and accuracy.

Honing Machines

For the production of few parts, honing may be performed on drill presses or engine lathes on which arrangements can be made for simultaneous rotating and reciprocating motions. The stroking can be done manually or powered depending on the equipment capabilities. On the other hand, the production honing is done with machines built for the purpose. These vertical machines are available in a wide range of sizes and designs. Some horizontal machines operate by manual stroking. In power stroking, the WP is usually held stationary in a rigid fixture, while the hone is rotated and hydraulically powered for stroking, which is considered beneficial for heavier WPs.

Machining Parameters

Parameters affecting the performance of honing process are:

1. Rotation speed the choice of the optimum surface speeds is influenced by:
2. Material being honed—higher speed can be used for metals that shear easily.
3. Material hardness—harder material requires lower speed.
4. Surface roughness—rougher surfaces that mechanically dress the abrasive stick permit higher speed.
5. Number and width of sticks in the hone—speed should be decreased as the area of abrasive per unit area to hone increases.
6. Finish requirement—higher speed usually results in finer surface finish

Tool Specifications
In addition to the variables involved in the honing process, the most important influence on the work result are the specifications of the honing stones, i.e. grain type, grain size, type of bond, hardness and treatment.