01-06-2012, 12:58 PM
Finishing processes
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FINISHING OPERATIONS
Introduction
As the name of this group of abrasive operations suggests, their objective is to achieve superior surface
finish up to mirror-like finishing and very close dimensional precision. The finishing operations are
assigned as the last operations in the single part production cycle usually after the conventional or
abrasive machining operations, but also after net shape processes such as powder metallurgy, cold
flashless forging, etc.
The finishing processes discussed in this section include honing, lapping, superfinishing, polishing, and
buffing. The typical surface finishes for these operations are presented in the figure. Also presented for
comparison are surface roughness values for fine grit size grinding.
Honing
Honing is a 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.
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 workpiece 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 workpart. To accomplish the process, the lap is pressed against the
work and moved back and forth over the surface in a figure-eight or other motion pattern, subjecting
all portions of the surface to the same action. Lapping is sometimes performed by hand, but lapping
machines accomplish the process with greater consistency and efficiency.
Superfinishing
Superfinishing 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 and smaller amplitudes. 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.