Biomedical Engineering Reference
In-Depth Information
Drilling has not been researched to the same extent as turning and milling
processes. This is due to the fact that drilling tools have a more complex
geometry compared to milling and turning tools - chisels create a ploughing
action, cutting lips have varying rake angles, chip thickness changes along the
edge etc. All this creates a number of problems when modeling the process.
Chatter in drilling often involves large amplitude torsional vibrations, while
they are not present in the cases of milling or turning. When analyzing
problems of chatter in drilling, several models have been investigated
throughout the literature: torsional - axial model, bending model and the
combination of axial and bending models. Apart from the different models,
several different influential factors have to be taken into consideration like:
friction, chisel edge, margin engagement, grinding errors, misalignment of the
tool, drill flank, pilot hole size, length of the drills, etc.
As suggested in reference [7], there are two different approaches for
detecting chatter used throughout the literature. The first approach is the
Frequency Domain Method (FDM), which is based on the appearance of
increase in frequency or equivalent signals used like acceleration or sound,
with the onset of chatter. The second approach covered in literature is the
Time Domain Method (TDM) based on on-line determination of a time series.
Both of these techniques still encounter certain problems in determination of
the proper chatter onset threshold especially if the cutting conditions show
variability. The analytical part of research is covered in a number of papers
that deal with analysis of stability and bifurcations of non-linear differential
equations [8]. Stability curves obtained by these analyses give a significant
increase in efficiency of cutting operations, and the boundary conditions of
chatter onset still remain extremely important. Determination of whether the
bifurcation is subcritical or supercritical is considered to be crucial.
2.
I
NVESTIGATION OF
C
HATTER IN
D
RILLING
Ema and his colleagues started some of the first work on the issues of
chatter problems in drilling in 1986 and 1988, [9-11], analyzing the issues of
chatter by keeping the tool stationary and feeding it into the rotating
workpiece. These papers investigated the effect of the drill geometry, chisel
edge, drill flank, and pilot hole. They have showed that the whirling
vibrations, which are a result of the regeneration effect at the major cutting
edges, result in distorted polygonal holes with an odd number of sides, which
was later investigated by Bayly and his team (Figure 2). They showed that the
