24-09-2012, 01:59 PM
RESEARCH ON TOOL LIFE IN MICRO END MILLING AS RELATED TO
WORKPIECE QUALITY CRITERIA
1WORKPIECE QUALITY CRITERIA.pdf (Size: 166.17 KB / Downloads: 31)
ABSTRACT
In this work a tool life study for micromilling is
developed, based on practical quality criteria in the
microworkpiece machined, such as the surface roughness
and the workpiece dimensional accuracy. It is observed
an increase in all these variables as the machining time
increases. These variables are significantly influenced by
tool wear and thus they are much related to tool life. The
evolution of the different variables is strongly affected by
the cutting speed, causing more rapid tool deterioration
and a quick reaching of the critical quality values. A great
change in machining forces is observed when cutting
speed increases as well. All these evidences are consistent
with Taylor’s Law for the conventional machining.
INTRODUCTION
The miniaturization of devices is demanding
nowadays the production of mechanical components
with manufactured features in the range of a few to a
few hundred microns in fields that include optics,
electronics and medicine. Specific applications include
microscale fuel cells, fluidic microchemical reactors
requiring microscale pumps, micromoulds, and many
more [1]. Some examples of micromachined features
and parts are shown in Fig. 1. These applications require
very tight tolerances, and both functional and structural
requirements. Many of these pieces have to be metallic
and micromilling is one of the most suitable methods to
manufacture them.
EXPERIMENTAL PROCEDURE
Machining tests were performed in a KERN
Micromilling and Microdrilling Machine with an up to
160.000 r.p.m. spindle speed and a Computerized
Numerical Control with 0.1 μm resolution. Micromilling
Machine is shown in Fig. 2. Micromilling was carried
out in Aluminium 7075 fulfilling ISO recommendations
about Tool Life testing in Milling [8]. A planning
operation was performed on the pieces to get the mill
weared by intervals and to measure the surface
roughness. Tests were performed with 0.8÷3 mm
diameter mills with 2 flutes. In order to assess the
machining accuracy thin walls were made in which the
thickness was used as an index for precision.
TOOL LIFE CRITERIA
A summary of the results for tool life from the
different points of view indicated before is going to be
given in this section.
Surface roughness. This is a very important
parameter to control in micromachining since a very
little roughness value in micropieces is usually very
high as compared with the dimensions of the piece and
could cause dangerous effects in sliding surfaces and
assembly problems. The criterion used in this work is
that the pieces machined must have a value of Ra
smaller than 1 μm to consider that the tool is still
operative. Thus, tool life is the time of machining till the
measured roughness reaches the critical value pointed
out before. This value can be varied according to the
quality needs of the piece. However, the model of
roughness increase extracted from this study can be
used whichever tool life criterion is taken.
TOOL WEAR
Mills wear can be observed and measured by means
of photographs taken with a digital camera which
includes a length reference for each zoom used. It can
be observed a different type of wear depending on the
cutting conditions used. In some cases can be measured
as for the tip wear or flank wear, but in the case of builtup
edge material added is thoroughly extended by the
mill surfaces. Mill wear for different cutting conditions
can be observed in Fig. 8 and 9.
The wear observed in the first case is caused by
built-up edge. Some of the workpiece material was
sticked to the mill and therefore a rough rake surface
can be observed. In the second case, a clear flank wear
can be seen along the end of the edge. By comparing the
weared area with the reference length, a value for the
wear can be obtained and related with the machining
time.
CONCLUSIONS
A tool life surveying as a function of cutting speed
according to a surface finish criterion and workpiece
accuracy is presented. All variables related with the
machining are strongly affected by the cutting speed,
which causes rapid tool deterioration and therefore a little
tool life. This model is correlated with flank wear values
and machining forces so that it can be used in production
planning of machined micropieces. In this initial
surveying no significant differences between conventional
machining and micromachining are noticed, except for
changes in machining forces with cutting conditions.