07-11-2012, 05:15 PM
Abrasive flow machining (AFM): An Overview
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ABSTRACT
Abrasive Flow Machining (AFM) was developed in 1960s as a method to deburr, polish, and radius difficult to
reach surfaces like intricate geometries and edges by flowing a abrasive laden viscoelastic polymer over them.
Based on the application, three different types of machines have been reported i.e, one way AFM, two way AFM
and orbital AFM. Because of simplicity in analyzing the physics, analysis of AFM process always refers to two
way AFM. It uses two vertically opposed hydraulic cylinders, which extrude medium back and forth through
passage formed by the workpiece and tooling. Abrasion occurs wherever the medium passes through the highly
restrictive passage. The key components of AFM process are the machine, tooling and abrasive medium.
Process input parameters such as extrusion pressure, number of cycles, grit composition and type, tooling and
fixture designs have impact on AFM output responses (such as surface finish and material removal). AFM is
capable to produce surface finish (Ra) as good as 0.05 µm, deburr holes as small as 0.2 mm and radius edges
from 0.025 mm to 1.5mm. AFM has wide range of applications in industries such as aerospace, medical,
electronics, automotive, precision dies and moulds as a part of their manufacturing activities. For better surface
integrity, texture and its performance, continuous developments are taking place for modifying the existing AFM
process technology and AFM machine configuration. To overcome some of the draw backs such as low finishing
rate and inability to correct the form geometry, researchers have proposed various versions of AFM machines
abbreviated as M-AFM, DBGAFF, CFAAFM, spiral polishing and R-AFF.
INTRODUCTION
Abrasive flow machining (AFM) was developed by
Extrude Hone Corporation, USA in 1960. There are
three types of AFM machines that have been reported
in the literature: one way AFM [1], two way AFM [2]
and orbital AFM [3]. Commonly used AFM is Two-way
AFM in which two vertically opposed cylinders extrude
medium back and forth through passages formed by
the workpiece and tooling as shown in Fig.1.
AFM is used to deburr, radius and polish difficult
to reach surfaces by extruding an abrasive laden
polymer medium with very special rheological
properties. It is widely used finishing process to finish
complicated shapes and profiles. The polymer
abrasive medium which is used in this process,
possesses easy flowability, better self deformability
and fine abrading capability. Layer thickness of the
material removed is of the order of about 1 to 10 µm.
Best surface finish that has been achieved is 50 nm
and tolerances are +/- 0.5 µm. In this process tooling
plays very important role in finishing of material,
however hardly any literature is available on this () of
the process. In AFM, deburring, radiusing and
polishing are performed simultaneously in a single
operation in various areas including normally
inaccessible areas. It can produce true round radii
even on complex edges.
Classification of AFM machine
As mentioned earlier, AFM machines are classified
into three categories: one way AFM, two way AFM
and orbital AFM. A brief discussion of the same is
given below.
One way AFM process
One way AFM process [1] apparatus is provided with
a hydraulically actuated reciprocating piston and an
extrusion medium chamber adapted to receive and
extrude medium unidirectionally across the internal
surfaces of a workpiece having internal passages
formed therein, as shown in Fig.2. Fixture directs the
flow of the medium from the extrusion medium
chamber into the internal passages of the workpiece,
while a medium collector collects the medium as it
extrudes out from the internal passages. The
extrusion medium chamber is provided with an access
port to periodically receive medium from the collector
into extrusion medium chamber.
Two-way AFM process
Two way AFM machine [2] has two hydraulic cylinders
and two medium cylinders. The medium is extruded,
hydraulically or mechanically, from the filled chamber
to the empty chamber via the restricted passageway
through or past the workpiece surface to be abraded
(Fig.1). Typically, the medium is extruded back and
forth between the chambers for the desired fixed
number of cycles. Counter bores, recessed areas and
even blind cavities can be finished by using restrictors
or mandrels to direct the medium flow along the
surfaces to be finished.