26-12-2012, 01:46 PM
Effect of Machining Feed on Surface Roughness in Cutting 6061 Aluminum
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ABSTRACT
The general manufacturing objective during the fabrication of automotive components, particularly through
machining, can be stated as the striving to achieve predefined product quality characteristics within equipment,
cost and time constraints. The current state of the economy and the consequent market pressure has forced
vehicle manufacturers to simultaneously reduce operating expenses along with further improving product
quality. This paper examines the achievability of surface roughness specifications within efforts to reduce
automotive component manufacture cycle time, particularly by changing cutting feeds. First, the background
and attractiveness of aluminum as a lightweight automotive material is discussed. Following this, the
methodologies employed for the prediction of surface roughness in machining are presented. The factors
affecting surface roughness as well as practical techniques for its improvement through optimizing machining
parameters are discussed next. Emphasis is placed on portraying the dominance of feed on surface quality over
other controllable machining parameters, thus substantiating the motivation for this study. Controlled milling
experiments show the relationship between feed and surface quality for 6061 aluminum, and the results are used
to recommend machining practices for cycle time reduction while maintaining quality requirements.
INTRODUCTION
Aluminum is the second most abundant metallic element and the most abundant structural metal in the earth’s
crust. It is mostly extracted by the chemical refinement of bauxite using the Bayer process to form aluminum
oxide (alumina) from which (99.9% pure) aluminum extracted by the Hall-Heroult method. It is commercially
available as wrought or cast in the form of ingots, bars, sheets, etc.
Aluminum is a slivery white metal especially noted for its density: about a third that of steel. The oxide layer
formed on freshly machined aluminum insulates it against further attack thus providing good corrosion
resistance. It has good electrical and thermal conductivities as well as good ductility and malleability. Also, it
can be surface finished within a wide range of values. Some of the limitations of aluminum are lower strength at
elevated temperatures, limited formability and relatively higher cost compared to steel. Aluminum is widely
used in the food and chemical industry, in metallurgical applications, the electrical industry, for structural
applications, cryogenic applications, etc., and of course extensively in the transportation industry.
ALUMINUM AS A LIGHTWEIGHT AUTOMOTIVE MATERIAL
When targeting the reduction of the mass of automotive components by replacing heavier steel components
with lightweight materials, there are a number of candidate materials that fit the requirements. Three of these
common lightweight engineering materials include: aluminum, magnesium and titanium. Some of the most
relevant material properties of these three materials are compared in Table 1 along with the properties of steel
that is used in automotive components.
TOTAL LIFE COST MODEL
The tractive force supplied to propel the vehicle works to overcome three main resistive forces, namely: the
rolling resistance of the tires due to material hysteretic loss, the aerodynamic force, and the inertial force due to
the vehicle acceleration. A mass-energy relationship can be written over standard driving cycles as in Eq. (1).
ALUMINUM IN THE AUTOMOTIVE INDUSTRY
Vehicle manufacturers have increasingly turned to aluminum to improve fuel economy, safety and performance.
As a result, aluminum has surpassed iron as the second most used automotive material worldwide (behind
steel). In North America, the current average aluminum content in passenger cars and trucks is 324 lbs and a
comparison with other major countries is show in Figure 1. Currently, over forty 2009 North American vehicles
contain over 400 pounds of finished aluminum: a list is show in Figure 2.
PROCESSING OF ALUMINUM
MACHINABILITY
Machinability is considered as the ease with which a material can be machined and it is customary to speak of it
as a material property. Although there is no physical quantity to rate machinability, it can be quantified as a
combination of the machinability index, chip formation characteristics, tool wear, cutting forces acting on the
tool, material removal rates, achievable surface finish, etc. Usually good machinability translates to a
combination of cutting with minimum energy, minimum tool wear and good surface finish.
The machinability of aluminum is considered to be very good and by some definitions, excellent. In the
automotive industry, it is a direct measure of the quality of a product which affects manufacturing cost.
Additionally, it influences surface friction, the ability of holding lubricant, light reflection, electrical and
thermal contact resistances, etc. The desired roughness value and the relevant process to achieve it are usually
specified for each individual part in the automotive industry [7].
MACHINING ECONOMICS
The fundamental idea of machining economics is simply to obtain the lowest possible cost per part that is
manufactured while maintaining the quality standards of the product. A fundamental cost model [20] for
machining a part is given by Eq. (2).
SURFACE ROUGHNESS FROM AN AUTOMOTIVE STANDPOINT
Surface roughness and integrity are of prime importance for machined automotive components in terms of
aesthetics, tribological considerations, corrosion resistance, subsequent processing advantages, fatigue life
improvement as well as precision fit of critical mating surfaces. Hence, the achievement of a predefined surface
finish for automotive components directly translates to product quality and hence the motivation for this study.
The natural metallic surface finish of aluminum is usually aesthetically satisfactory without further polishing.
The natural protective oxide layer that is formed is transparent and does not affect the appearance of the
material surface. A wide variety of surface textures can be created from rough to mirror smooth. The hue and
color can be affected chemically, and paints, enamel and other surface coating can be applied with ease without
adverse effects or deterioration [24].
FEED, SPEED AND DEPTH OF CUT
In general, it is found that surface roughness increases with an increase in the feed rate and depth of cut and a
decrease in cutting speed. Roughness is found to reduce drastically up to a particular critical value of surface
speed which is attributed to the reduction in size of the built up edge. At this speed, when the effect of the built
up edge is considered negligible, the profile of the cutting edge of the tool (pointed or curved) gets imprinted on
the work surface, and the surface roughness from this point on depends on the feed rate. A larger depth of cut,
or in other words a larger chip cross-sectional area adversely affects surface finish though it is usually not
significant until it is large enough to cause chatter. Note that the effect of increased feed is more pronounced on
surface finish than the effect of an increased depth of cut. Thus, measures for improving machining productivity
(increasing feed and depth of cut) work against achieving better surface quality.