The architecture of drilling machines and bits has improved
significantly over the past few years, facilitating increased small
drilled hole productivity.
In the not so distant past, 0.20 mm mechanically drilled holes were
considered state-of-the-art. In the past few years, however, a great
deal of effort has been devoted to small hole drilling to improve the
drill hole quality, drill life and hole location accuracy. Micro
drilling has prompted machine manufacturers and drill bit suppliers to
change basic product designs to accommodate this important and
indispensable technology. Today, drilling 0.20 mm in a two-panel stack
is almost a routine occurrence, and many manufacturers make
high-quality holes using micro drilling techniques in full production.
When drilling small, deep holes, there are three main goals: accuracy,
hole quality and productivity. In this process, having the right
machine and an optimum drill bit for the challenge are critical
components for success.
The drill bit is perhaps the most important factor in any drilling
process. Even if all the other segments of the drilling operation are
perfect, the outcome would be less than optimum if a damaged or
improper drill bit is used. Drill bits are designed with three basic
needs in mind. The bit needs to be rigid to address the critical aspect
of registration through the hole and to ensure straight holes, from top
to bottom through the stack. It also needs strength to withstand the
forces generated during drilling and finally, it must be maintained to
provide a sharp cutting edge over the life of the tool.
The main area of work in drill bit geometry is mostly centered on web
thickness (
FIGURE 1). By reducing a bit’s web thickness, a number of
positive elements are created. More chips can be removed during
drilling, preventing the bit from snagging. It reduces the drill bit’s
“crush” zone, the area of the bit working more like a punch than a
cutter. A larger web also requires more force to be exerted on the bit
for drilling.
Choosing the proper flute length for a job is as important as choosing the correct feed and speed.
If a tool with a flute which is too long is used, the risks include:
unnecessary drill breakage due to the reduced strength of an extended
flute drill; accuracy problems due to the drill bit’s tendency to
wander more because of the added length; and reduced productivity due
to reduced feed rates to compensate for the first two problems.
If a tool with a flute which is too short is used, the risks include:
incomplete drilling due to the bit being too short; and poor hole wall
quality due to the bit overheating and its inability to remove chips
from the hole.
Drill breakage can also be caused by the bit snagging on the chips that
haven’t been removed. The overall flute length should be the thickness
of the material to be drilled plus 20% for chip clearance at the top of
the stack (
FIGURE 2).
Entry and backer material plays an important role in micro drilling.
The entry materials must provide flatness and a smooth surface for the
drill bit to make contact with, allowing the bit to enter the board
stack as straight as possible. The material should be hard, in order to
prevent entry burrs, which can be pushed back into the hole creating
subsequent problems later on in the process. It also needs to have a
consistent thickness and composition to ensure consistency in hole wall
quality.
The entry and backer material should not increase drill wear, as this
can impact hole wall quality, as well as the number of holes a drill
bit can produce. In addition, reducing the number of hits before
resharpening or changing tools increases costs. The entry and backer
materials should minimize drill temperature to reduce smearing and
drill bit wear, and the materials selected should be designed to not
deposit back into the hole.
There are a number of different backup and entry materials available.
According to the particular requirements of the job to be executed, the
manufacturer has to choose which is the best, taking into account the
characteristics of the material as mentioned.
Pits and dents in the panel surfaces can also be problematic, surface
imperfections can deflect a small diameter drill bit. Newer drilling
machines offer many options in stacking, registration and autoloading,
but in all cases, stacks must be clean, free of burrs and have no
spaces between the panels.
Most problems associated with the drill bit can be attributed to poor
handling techniques. People handle bits many times before drilling.
Providing the maximum protection for small bits is certainly a matter
of considerable importance for the user, this is especially true in the
light of the extreme fragility of many small drill bits which can be
broken by a careless touch. The reject rate due to routine handling is
quite high.
A great deal of effort has been devoted to new packaging methods that
eliminate handling of small bits. Cooperation between some drilling
machine constructors and drill bit suppliers has resulted in a revised
packaging design, consisting of a plastic strip which contains 10 tools
and is designed to be directly inserted into the drilling machine’s
tool changer magazine, eliminating handling.
In order to achieve proper feed and table speed, many variables have to
be taken into account, that’s why the tool parameters are never
perfect. Every drill room has a different opinion when it comes to the
importance of bit life, hole quality, drill wander and hit rate.
Optimum feed and speeds for small hole production greatly depend on the
drilling machine’s performance and drill bit design. Based on the
experience of experts and on the results of many tests, some parameters
are well known. Tool life and hole quality, for example, are mostly
subordinated to the drill bit peripherical speed in surface feet per
minute (SFM), as shown in
FIGURE 3.
Many changes have been made to drilling equipment in the last few
years. The principal areas where attention has been directed are:
- Spindle speed
- Spindle collet run-out
- Pressure foot
- Axes positioning and stability
Since abandoning ball bearing spindles, where the maximum speed was
80,000 RPM, the performance
of air-bearing spindles has greatly helped
the micro drilling process, and today, spindles running at more than
250,000 RPM are available.
From a machine standpoint, run out is the greatest contributor to drill
breakage and accuracy problems. A big help in the drastic reduction of
spindle run out has been the elimination of collars on the drill bit.
This solution offers various benefits from a technical (
FIGURE 4) and
economic point of view.
Since spindles behave differently at different speeds, it is very
important to see what is happening when the actual drilling speed is
achieved. A solution has been found in an accurate laser station which,
when directly installed on drilling machines, is able to perform in
real time, checking tool height, diameter and dynamic run-out in a few
seconds.
The elimination of the collar has facilitated the reduction of the
pressure foot insert (
FIGURE 5). In fact, with large hole pressure foot
inserts, there is a tendency for thin panels to lift during the
drilling cycle (“oil can effect “), but with small inserts, the
pressure foot force acts on a very limited area around the hole to be
drilled, thus avoiding the oil canning. In addition, a better vacuum
concentration is created for a fast and improved chip evacuation,
critical for small hole drilling.
A smooth, accurate axis positioning system is an important aspect of
any drilling machine. An axis with overshoot or with positioning
difficulty will lead to accuracy problems, as well as reduced
productivity. By applying new technical solutions and with the
reduction of the mass in motion, high speed and acceleration can be
achieved.
In order to reach fast and accurate axis positioning, the addition of
linear motors on XY axis and the change to low mass on axis has been
found beneficial. These improvements have increased small hole drilling
productivity and quality possible.
Much of the work has now been done to improve the architecture of
drilling machines, making a significant increment of small hole
productivity possible. Eventual problems have to be faced starting with
the process engineering approach; quality, productivity and process
control are aspects that cannot be treated independently.
PCD&F
Pietro Zulli is an applications engineers with Pluritec Industries;
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