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More Machining Technical Articles
How To Machine Magnesium? Machining Magnesium - Key Pointers
Magnesium has been used in manufacturing notebook computer frames, video cameras, digital cameras, PDAs and other
consumer electronics products because of its high strength to weight ratio. When magnesium is alloyed with aluminum,
the resultant material is very light and strong, and easily machinable.
The main concern in machining magnesium alloy is the danger of fire ignition when dry cutting. Fire may
occur when the melting point of the alloy (400-600 degrees Celsius) is exceeded during machining. The
small chips and fine dust generated during cutting are also highly flammable and pose a serious fire
risk if not properly handled.
There are several points to note when machining magnesium:
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Firstly, use a lower cutting speed when compared to cutting aluminum. The workpiece temperature
goes up with an increase in cutting speed and also smaller undeformed chip thickness. In other
words, the slower the machining speed and the larger the chips, the lower the workpiece
temperature will be. Due to this reason, some companies have modified woodworking tools
for machining magnesium so as to achieve larger chips and lower fire hazard. The cutting tools used
should have relief and clearance angles that are sufficiently large to prevent unnecessary
cutting tool-workpiece friction, thus lowering the heat generated during the cutting process.
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Secondly, keep the machining center clean. Cleaning the machining centers regularly and
storing the magnesium chips correctly are important aspects of machining magnesium. Keep
a container of cast iron chips near by when machining magnesium, If fire occurs, smother
the fire with the cast iron chips.
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Thirdly, if coolants are necessary for high speed machining, do not use water-based lubricants. Instead use a light
mineral oil, or a water-soluble cutting fluid such as Castrol Hysol MG specially formulated for
machining magnesium. Some companies in Japan use semi-dry machining via a misting system.
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Fourthly, monitor the workpiece temperature during machining. Experiments were carried out using
thermocouples mounted into the workpiece to monitor the workpiece temperature during machine. Dry
cutting of magnesium alloy thin walls was achieved using cutting speed of 440m/min for roughing and 628m/min for
fine finishing.
Despite the fire hazards, as competition from overseas low-cost production bases intensifies, and magnesium becomes increasingly used
in electronics products, most machining job shops could very well find
machining of magnesium a niche worth pursuing.
Author Ken Yap is a director of Suwa Precision Engineering Pte Ltd in Singapore and represents
metal stamping, precision machining, miniature precision balls, gears and PCB manufacturers from Suwa,
also called "The Oriental Switzerland" in Japan due to its Swiss resemblance for rich watch-making
industry, its mountainous terrain and its precision component making industry.
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Why Does The Tool Bit Break Easily In Micro Milling? Analysis of
Micro Milling Tool Breakage Problem
Micro milling is one of the three common micro cutting techniques used in micro machining.
In micro milling, the tool bit with diameter as small as 0.1mm is held in a high speed spindle
rotating at 20,000 to 150,000 rpm, and used to mill steel, brass and aluminum with depth of cut at
about 30 microns and feed rates of 120mm/m to 240mm/m to provide surface quality finishes as good as
0.2 microns.
While micro milling has been successfully applied in manufacturing bio-medical components, embossing dies
and micro encoders, the breakage of the tool bit has been identified by many users as a teething problem.
Why does the tool bit break so easily in micro milling as compared to conventional milling?
There are 3 main reasons:
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Firstly, when metal is removed by machining, there is a substantial increase in the specific energy
required as the chip thickness decreases. This means that in the case of micro machining, as the
chip gets thinner with smaller depths of cut, the micro tool bit will be subject to greater resistance
when compared to conventional machining. It is as if the workpiece material becomes harder during micro
machining. This resistance force is strong enough to exceed the bending strength limit of the tool bit even
before the tool experiences any significant wear, and leads to the breakage of the tool bit. One way to
prevent this is to make the chip thickness smaller than the edge radius of the tool bit.
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Secondly, a sharp increase in cutting forces and stress from chip clogging during the micro milling
process would cause the tool bit to break. In most micro milling operations using miniature micro
tool bit with two cutting edges, each cutting edge removes the chips from the machining area only
within half a rotation. However, if chip clogging occurs, the cutting forces and stresses will
increase beyond the bending strength limit of the tool bit within a few tool rotations, and the tool
bit will break. Some users prefer high speed steel tool bits as these are very much more flexible and
tolerate clogging better than carbide tool bits.
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Third, the tool bit tends to lose its cutting edge due to built-up edge and cannot machine
efficiently. As the workpiece starts to push on the tip of the tool bit, the tool bit will
deflect slightly. The increase in tool deflection and the stress generated by the milling
with every rotation will eventually cause the breakage of the tool bit. This process is also
called extensive stress-related breakage.
In view of the above phenomena occurring in micro milling, most micro milling machines are sold
with sensors to measure the forces acting on the tool bit, and advanced CAM software to predict
the chip load throughout the micro machining process. In this way, precision manufacturers seeking a
niche in micro milling could try to keep their machines running smoothly with minimal machine downtime.
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The diamond tool is commonly used in micro-machining as it can withstand the micro hardening
of the workpiece surface during micro-machining. This micro-hardening creates enough resistance
to break the tool bit easily in micro milling, but not a diamond tool. Micro-machining using
diamond tool could be performed at high speeds and generally fine speeds to produce good surface
finish such as mirror surfaces and high dimensional accuracy in non-ferrous alloys and abrasive
non-metallic materials.
However, if a diamond tool were to be used to cut steel, one of the most common engineering materials
used in industries, the diamond tool will face severe tool wear. While diamond only softens at 1350
degree Celsius and melts at 3027 degree Celsius, and is also the hardest material in the world, it has
a weakness. Diamond succumbs to graphitization, which means that it will change its crystal structure to
graphite crystal structure at 200 degree Celsius in the presence of a catalyst metal such as carbon steel
and alloys with titanium, nickel and cobalt.
There have been various attempts to improve the tool life of the diamond tool while cutting steel
so as to improve the efficiency and profitability of this operation. Such processes include micro-cutting
the steel workpiece in a carbon-rich gas chamber as well as a cryongenically cooled chamber. However, these
methods require costly equipment modification and restrict direct supervision of the micro-cutting process.
The latest breakthrough came when the diamond tool was subject to ultrasonic vibration during micro-cutting.
It has been shown that a diamond tool subject to ultrasonic vibration can cut the steel well enough to produce
a mirror surface finish with acceptable tool life. The ultrasonic vibration at the diamond tool tip allows
the tool face to cool down considerably during the cutting process and delays the chemical reaction between
the diamond tool and the steel workpiece. As a result, the diamond tool life is increased by a few hundred
times.
For example, a single crystal diamond tool with feedrate 5 micron/revolution, cutting speed zero to
5m/min and depth of cut 10 micron was attached to a ultrasonic vibration generator so that the diamond
tool tip vibrated about 4 microns while it was used to cut stainless steel. The mirror surface finish of
the cut steel surface was measured at 8 nm Ra!
With more and more machining companies moving into the niche micro machining field, such ultrasonic
vibration assisted cutting can only help the progressive company to achieve process leadership and
innovative differentiation.
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