By Kent Swanson,
MTI, Inc.
Following a rough dicing operation, secondary grinding and
lapping processes have been traditionally required to achieve final part
size and geometry of precision magnets. However, these secondary operations
may be eliminated through proper selection of dicing blades and processes.
Additionally, the application of multi-blade dicing gang technology will
further improve throughput and efficiency.
Abrasive Selection
Due to the hardness of most magnetic materials, diamond abrasive blades
are commonly used to machine ferritic, ceramic and rare earth materials.
The exception is alnico, which must be machined with a CBN abrasive because
a chemical reaction occurs between diamond and the alnico constituents
at elevated temperatures in the grinding zone, causing rapid blade wear.
Blade Bond Selection
Resin-bond superabrasive blades are generally the "softest"
of the available bond types. Their free-cutting and user-friendly characteristics
make them a good choice for new process development. They're also an excellent
choice for applications in which surface finish is important.
Limitations of resin-bonds include relatively high blade wear and instability
in thin-blade operations. Due their inherent lack of stiffness, resin-bonds
become limited in applications requiring cut widths less than 0.004 inches.
Sintered metal-bonds represent the next step in bond hardness, strength
and stiffness. While they may be more difficult to true and dress than
a resin-bond, they generally yield the best economics in terms of consumable
cost. Due to their higher strength and stiffness, they are available in
thicknesses down to 0.003 inches.
Electroformed nickel blades posses the greatest strength and stiffness
properties of all bond types, and are available in thicknesses as low
as 0.001 inches.
E-Core ferrite material that underwent slicing
& grinding operations, used in electronics applications.
Trapezoidal latch magnets, rare earth that underwent
slicing and form grinding processes, used on hard
disk drives, and actuator magnets, rare earth, that
were sliced from a solid blank, also used on
hard disk drives
Blade Exposure
The aspect ratio between blade exposure and thickness is a key parameter
in precision dicing operations, since it has a direct influence on cut
accuracy and quality. These ratios are also important because they affect
kerf loss, and therefore part yield from a given block of material.
Fortunately, some generalizations may be made with regard to aspect ratio
by expressing it as a function of blade stiffness, or bond type. 1A8 and
1A1-style resin-bond blades should be limited to aspect ratios no greater
than 12 to 1. 1A1R resin-bond rim, and all metal-bond styles should not
exceed a ratio of 18 to 1, while electroformed Ni blades may be accurately
implemented at ratios up to 30 to 1.
Blade Mounting
Every attempt should be made to eliminate axial (side-to-side) runout
when mounting the blade on the arbor or hub. All contact surfaces should
be thoroughly cleaned prior to assembly. Once assembled, blade runout
may be checked with a simple mechanical or electronic indicator, either
on the machine spindle, or offline between bench centers. Ideally, axial
runout should not exceed 0.0003 inches. Excessive runout may be often
corrected by loosening the assembly and rotating the blade(s) to a new
position. This action will cancel non-parallelism that exists in each
component.
Blade Preparation - Truing and Dressing
The most accurate cuts are achieved by a square blade profile with minimal
axial and radial runout. Although it is difficult to "true"
the sides of a thin dicing blade to eliminate axial runout, an effective
truing operation will simultaneously square the blade profile, and eliminate
any radial runout. A square profile will minimize axial deflection, while
elimination of radial runout guarantees that cutting loads will be evenly
distributed around the full periphery of the blade.
Truing is typically achieved through the use of either a conventional
abrasive stick or rotary abrasive wheel. Although simply feeding the blade
through an abrasive stick will true a blade in time, rotary truing is
a much more efficient and effective process. An abrasive stick tends to
radius the blade as it removes bond material, whereas a rotating abrasive
wheel cylindrically grinds the blade to generate the optimal square profile.
Rotary truing can be accomplished either offline on a specially designed
cylindrical grinder such as the MTI 1030, or on the dicing machine for
greater accuracy. Motorized online units, such as that on the MTI NSX-250
dicing system, have the capability to eliminate radial blade runout due
hub/spindle clearances. Additionally, an online system allows the truing
operation to be incorporated automatically into dicing cycle.
Since it desirable to focus wear on the blade and not on the truing wheel,
the truing wheel should be run at the highest attainable speed. Infeed
rates should be limited to 0.000050 inches to 0.0002" per pass, with
a traverse rate of approximately 15 ipm.
Before a dicing blade is ready for use, it must undergo a final conditioning,
or "dressing" procedure. Dressing is a controlled process targeted
at removing only the bond matrix, leaving the diamond or CBN particles
sharp and exposed.
Although similar to the truing operation, dressing is a much less aggressive
process. The blade is passed through the dressing stick at a depth corresponding
to that of the actual cut. Blade speed/feedrate should be approximately
2000 sfm / 4 to 10 ipm. While the total number of dressing passes depends
upon the grit and bond hardness of the blade, generally a total of five
linear inches is acceptable for fine-grit resin-bonds, while coarser-grit
metal-bonds may require up to 40 inches of dress. The grit size of the
dressing stick should be approximately two steps coarser than that of
the blade.
Multi-blade Gang Assemblies
Gang blade assemblies offer dramatic increases in process throughput.
However, there are a few issues that deserve special consideration for
successful implementation of gang technology: the geometry of each component
in the assembly is critical, since thickness and parallelism tolerances
may be cumulative. Additionally, it is preferable to use dicing machines
similar to the MTI NSS-350 and MSS-816, that have been specifically designed
with the rigidity and power required to withstand the high loads generated
by gang dicing operations.
Dicing Processes
A single-pass at full depth is normally recommended for optimal cut geometry.
Blade speed should be between 6000 and 9000 sfm. Cutting feedrate depends
upon a wide variety of blade and application parameters, and is best determined
empirically. Therefore, it is best to begin at a slow feedrate, i.e. 1
to 4 ipm, then increase as process parameters allow. Some useful and easily
measured parameters include spindle load, cut straightness and edge chipping.
It is also certainly beneficial to monitor these parameters over time.
Cut geometry will degrade and spindle loads will increase as blades become
dull and profiles begin to radius. Although a retruing and dressing sequence
would eliminate the problem, integrating a periodic dressing procedure
into the dicing cycle can minimize the frequency of such complete reconditioning.
Manufacturing Technology, Inc. (MTI) was founded in 1979 and is known worldwide as a manufacturing company dedicated to the design and production of ultra-precision machine systems for slicing, slotting, dicing, grinding and lapping operations. These machine tools process material from alumina to zirconia as well as, ceramics, unfired ceramics, glass, composites, rare earth magnets, silicon, ferrous and non ferrous alloys and more.
Kent Swanson has 20 years experience in the Precision Grinding Industry
and has spent seven years at MTI as the company’s R&D manager.
He can be reached at kswanson@mtionline.com.