1. 1
Study on the Optimization and Process Modeling
of the Rotary Ultrasonic Machining of Zerodur
Glass-Ceramic
by
JAMES DANIEL PITTS
B.S. (University of California, Davis) 2012
THESIS
Submitted in partial satisfaction of the requirements for the degree of
MASTER OF SCIENCE
in
Mechanical and Aeronautical Engineering
in the
OFFICE OF GRADUATE STUDIES
of the
UNIVERSITY OF CALIFORNIA
DAVIS
Approved:
__________________________________
Kazuo Yamazaki (Chair)
__________________________________
Bahram Ravani
__________________________________
Masakazu Soshi
Committee in Charge
2014
2. 2
“Don't be too timid and squeamish about your actions. All life is an experiment. The
more experiments you make the better.” ― Ralph Waldo Emerson
3. iii
Abstract
Rotary ultrasonic machining (RUM), a hybrid process combining ultrasonic machining
and diamond grinding, was created to increase material removal rates for the fabrication
of hard and brittle workpieces. The objective of this research was to experimentally
derive empirical equations for the prediction of multiple machined surface roughness
parameters for helically pocketed rotary ultrasonic machined Zerodur glass-ceramic
workpieces by means of a systematic statistical experimental approach.
A Taguchi parametric screening design of experiments was employed to systematically
determine the RUM process parameters with the largest effect on mean surface
roughness. Next empirically determined equations for the seven common surface quality
metrics were developed via Box-Behnken surface response experimental trials. Validation
trials were conducted resulting in predicted and experimental surface roughness in
varying levels of agreement.
The reductions in cutting force and tool wear associated with RUM, reported by previous
researchers, was experimentally verified to also extended to helical pocketing of Zerodur
glass-ceramic.
5. v
Acknowledgements
I would like to acknowledge and express my sincere appreciation to the many colleagues,
professors, friends, and family members that have been instrumental in, not only this
research, but the completion of my graduate studies. Firstly, I would like to thank
Professor Kazuo Yamazaki, director of the UC Davis Intelligent Manufacturing and
Mechatronic Laboratory (IMS-M Lab), the Precision Manufacturing Center, and Machine
Tool Technologies Research Foundation (MTTRF) Berkeley Institute. Professor
Yamazaki’s tireless support has provided me with the overwhelming amount of resources
utilized throughout my undergraduate and graduate manufacturing activities.
Professors Bahram Ravani and Masakazu Soshi have given me, as committee members,
advisers, and instructors, countless insights that continue to be instrumental to my course
work, master’s research, and professional life as an engineer.
Dr. Masahiko Mori, whose unyielding support to machine tool education, research, and
development have provided me, and countless other young engineers, the opportunity to
fully realize and expand the capabilities of manufacturing technologies.
I would also like to thank the many past and present members of the UC Davis IMS-M
Lab.
6. vi
Table of Contents
Abstract..........................................................................................................................................iii
Dedication...................................................................................................................................... iv
Acknowledgements......................................................................................................................... v
Table of Figures ............................................................................................................................xii
1 Introduction............................................................................................................................. 1
1.1 A Brief History of Hard and Brittle Machining.................................................................. 1
1.1.1 Hard and Brittle Material Removal: Birth of Tool Making ........................................ 1
1.1.2 Early Advanced Materials and Sharpening Methods.................................................. 3
1.1.3 Optics in Antiquity...................................................................................................... 4
1.1.4 Quartz to Manufactured Glass .................................................................................... 5
1.1.5 Increased Accuracy; Automatic Production Methods................................................. 6
1.1.6 From Perspicillium to Ganymede ............................................................................... 8
1.1.7 From Galileo to Hubble .............................................................................................. 9
1.2 Zerodur Glass-Ceramic......................................................................................................11
1.2.1 Advanced Properties of Modern Materials............................................................... 12
1.2.2 Engineering Applications of Zerodur........................................................................ 12
1.3 Industrial Need for Improved Zerodur Machining ........................................................... 16
1.3.1 Rotary Ultrasonic Machining.................................................................................... 17
1.4 Research Objective ........................................................................................................... 17
7. vii
1.4.1 Benefits of This Research ......................................................................................... 18
1.5 Chapter Summary ............................................................................................................. 18
2 Fundamentals of Rotary Ultrasonic Machining.................................................................... 19
2.1 History; From USM to RUM............................................................................................ 19
2.1.1 History of Ultrasonic Machining .............................................................................. 19
2.1.2 From USM to RUM.................................................................................................. 21
2.1.3 RUM Material Removal Mechanisms ...................................................................... 22
2.1.4 RUM Tooling............................................................................................................ 25
2.2 Previous RUM Research................................................................................................... 27
2.2.1 RUM Process Parameters ......................................................................................... 28
2.2.2 RUM Process Outcomes........................................................................................... 30
2.3 Chapter 2 Summary .......................................................................................................... 31
3 Experimental Methodology .................................................................................................. 32
3.1 Research Overview ........................................................................................................... 32
3.2 Helical Pocketing.............................................................................................................. 33
3.3 Experimental Materials..................................................................................................... 34
3.3.1 Statistical Modeling and Validation Stock Material ................................................. 36
3.4 Machine Tool and Ultrasonic Systems.............................................................................. 36
3.4.1 US20 Specifications.................................................................................................. 38
3.4.2 Actor® Ultrasonic Tool Holder ................................................................................ 38
3.4.3 Through Spindle Coolant.......................................................................................... 39
18. xviii
Figure 138: easySONIC Gage Study Report .............................................................................. 170
19. 1
1 Introduction
The following chapter provides an introduction to the history, materials, and applications
of hard and brittle machining. From this starting point the objectives and possible benefits
of this research are presented.
1.1 A Brief History of Hard and Brittle Machining
It is easy for a modern observer to discount the current state of technological
sophistication as isolated from those of antiquity, however, recent archeological
discoveries provide new insights into the origins of all current manufacturing processes
and advanced hard and brittle material applications. The following subsections are
provided to explain the essential benefits to the entirety of human existence made
possible by advanced materials and their applications.
1.1.1 Hard and Brittle Material Removal: Birth of Tool Making
Hard and brittle materials have been sought after and utilized throughout human history
due to their superior characteristics and abundance. For the overwhelming majority of
mankind’s existence, hard and brittle materials have been essential to survival [1].
Naturally occurring solid aggregates, mineraloids, and volcanic glasses served as the
starting points for all cutting and shaping processes as well as base work-piece materials.
Thus far, the oldest reliably dated examples of stone tools, known as the Odowan toolkit,
were created 2.6 million years ago [2], [3]. Current research has provided evidence of
early humanity’s astounding level of process complexity with respect to tool making and
item construction. Percussive and other pressure-based reduction techniques, commonly
20. 2
referred to as knapping, served as an efficient means for shaping hard and brittle
materials.
Figure 1: Impact-Based Lithic Reduction
Knapping is a means of lithic reduction in which controlled fracturing of a hard and
brittle workpiece’s surface in order to remove unwanted material or to create usage
flakes. As seen in
Figure 1, the removed material, known as the flake, is extracted from the core material.
Generally, localized fractures created during the knapping process is initiated through the
use of an impacter or pressure-based hand tool possessing suitable levels of hardness and
fracture toughness.
Due to their razor-sharp cutting edges, flakes were initially removed from flint, chert, and
other conchoidal fracturing materials for use as simple cutting tools [2]. The exact
physical principle of conchoidal fracture is not fully described and therefore relies on
21. 3
empirically determined norms. Conchoidal fracturing typically refers to the phenomenon
producing Hertzian cone cracks. First described by the German physicist Heinrich Rudolf
Hertz, as a result of his investigation of wave-front propagation through various media,
Hertzian cone cracks leave visible bulbs of percussion on the fracture plane by means of
a force propagated through a brittle,
Figure 2: Hertzian Cone crack in glass [4]
amorphous, or cryptocrystalline solid [5]. The applied force thereby enables full or partial
removal of the cone material, producing the structures seen in Figure 2.
Although highly effective for bulk material removal, early percussion-based shaping
technologies could not produce finely controlled cutting edges. Larger tools such as hand
axes were produced through the removal of successive exterior flakes in order to create a
core element possessing the required shape. Under prolonged usage the cutting edges of
these early axes would be removed by fracturing and the axe would either need to be re-
knapped or discarded.
1.1.2 Early Advanced Materials and Sharpening Methods
As seen in the modern manufacturing industry, early man sought out advanced materials
and processes in order to produce weapons, tooling, structures, and ornamental objects.
The limitations of flint and other widely available hard and brittle materials lead early
22. 4
toolmakers to seek out more advanced cutting tool substrates and stock materials. One of
the most highly praised Stone Age materials was obsidian, a naturally occurring volcanic
glass formed when extrusive igneous rock is rapidly cooled to form a hard and brittle
material of amorphous structure.
Like standard stone tool base materials, obsidian fractures conchoidaly but with no
preferred planes of weakness and could therefore be readily shaped by well-established
knapping methods. Obsidian is still to this day highly prized for its ability take a highly
sharpened edge. Recent investigation into obsidian-based cutting has produced cutting
blades sharper than those possible with steel; however, edge sharpness cannot be retained
as well as steel blades. Early stone and glass tools required repeated sharpening.
Motivated by this need for the maintenance and repair of tool cutting edges, early tool
users established re-sharpening techniques. In its most simple form, early tool users could
simply rub their tools on gritty rocks in order to re-sharpen them. This basic form of
material removal can be considered early grinding [6]. Grinding is the commonly
accepted name for a machining process in which hard abrasive particles are utilized as the
cutting medium. As today, grinding techniques were heavily relied upon in the re-
sharpening of tooling and the production of finely polished hard and brittle objects.
1.1.3 Optics in Antiquity
The first lenses were created in the Near East or Eastern Mediterranean [7]. Recent
archeological findings have proven that at least 3,500 years ago the Minoan era occupants
of the Greek island of Crete possessed the ability to create basic magnification with
quartz optics. Two single crystal quarts lens measuring 15 and 8 mm in diameter and
dating to the Archaic Greek period were discovered during a 1983 excavation of a cave in
23. 5
central Crete and are known as the Idaean Cave lenses. These lens are considered in
context with other lenses found in the Palace of Knossos dating from 1400 B.C. [8]. The
larger of the two, seen in Figure 3, has the ability to magnify clearly up to seven times.
Albeit with considerable distortion, magnification of up to twenty times is also possible.
The need for quality optics was not simply limited to magnification. So called burning-
glasses were produced to start fires with sun light by around 3500 B.C. [7].
Figure 3: Piano-convex crystal lens. Archaeological Museum, Herakleion [7]
These ancient lenses likely had far greater capabilities than demonstrated by the surviving
examples. Chemical analysis of the surrounding shows that chemical etching may have
diminished the surface quality of the lenses during their extended burial.
1.1.4 Quartz to Manufactured Glass
Today we think of glass as the natural material of choice for the production of optics but
this is not the case. With an index of refraction of 1.54, crystal quartz is a better material
24. 6
for lenses than common glass at only 1.46. [8]. Quartz is an abundant mineral in the
Earth’s continental crust; however, naturally occurring high quality single crystals, large
enough to be fashioned into lenses are exceedingly difficult to find in quantities required
for any level of mass production. The scarcity of high quality crystal helped to motivate
the invention of glass manufacturing.
The earliest evidence of the manufacture of glass dates back to at least Egyptian times,
but it was 1st
century A.D. Romans who first manufactured glass objects at a large
enough scale to enable the use of glass for common household items [7], [9]. In early
1854 archeological excavations, in the Roman city of Pompeii, uncovered a 65mm
diameter glass lens along with several polished stones it what was termed “The House of
the Engraver” [9]. It stands to reason that for trades responsible for the creation of small
and intricately featured products, in which many years of training and practice are
required to become a practicing master craftsman and instructor, would more than outlast
the normal time span of optimal human vision. The demand for precision and accuracy at
small scale in trades such as engraving and gem cutting are commonly agreed upon as a
source for the development of optical magnification solutions.
1.1.5 Increased Accuracy; Automatic Production Methods
The optical elements previously discussed possessed relatively good optical quality, and
provided evidence of the utilization of abrasive machining techniques dating back several
millennia. Although there is some evidence of primitive lathes being used during their
manufacture, these ancient lenses were still reliant on manual shaping methods in their
construction and therefore likely suffered from protracted production times, limited
repeatability, and low quality. Evidence of geometric deficiencies due to manual grinding
25. 7
methods can be seen in the Idaean Cave lenses. Both lenses have what has been
interpreted as deep cutter marks on their perimeters. These errors in manufacture are in
part responsible for the reduction of totally clear magnification found above seven times.
The manufacture of early optical products required only simple machines. Turning is one
of the most basic machining processes in which material removal is achieved the use of a
stationary cutter acting upon a rotating workpiece. The origin of turning dates back to the
ancient Egyptians around 1300 BCE with the invention of the two-person lathe, as seen
in Figure 4.
Adapted From [10]
Figure 4: Proposed Method of Ancient Lens Fabrication
S. Kalpakjian and S. R. Schmid, the authors of the authoritative paper in the subject,
describe the process early optics makers could have utilized to produce lens like those
found in the Idaean Cave as follows:
“An approximately shaped blank having the desired curvature is first mounted on
the end of the shaft with sealing wax. Using an abrasive medium on the surface of
the blank, the end of a round, hollow, cylindrical metal or wooden rod is held in
26. 8
contact with the surface, continuously rotated about its axis, and moved over the
surface. An alternative method would be to rotate the tube rapidly with a bow
drill and rotate the axle slowly by hand. These methods generate perfectly
spherical surfaces if continued until the entire surface contacts the end of the
tube. [10]”
With these primary methods, device makers helped to empower great advancements in
science made during the European Renaissance, between the 14th and 17th centuries
throughout Europe.
1.1.6 From Perspicillium to Ganymede
In the summer of 1609, Galileo Galilei (1564-1642), the Italian physicist, astronomer,
mathematician, philosopher, and engineer visited Venice. Venice, and the neighboring
island of Murano was Galileo’s early source for polished optics [11], [12]. During his
time in Venice, Galileo became fascinated by the new invention of a Dutch spectacle-
maker by the name of Hans Lippershey [13]. Lippershey called his invention the
perspicillium which consisted of a tube capped at each end by polished glass lenses.
Galileo quickly recreated a perspicillium with ten times the magnification of
Lippershey’s device and renamed it the telescope.
Equipped with his telescope, Galileo went on to make some of the greatest discoveries in
observational astronomy. He is credited with the discovery of Jupiter’s moons, also
known as the Galilean moons: Io, Europa, Ganymede, and Calisto [11]. On display at the
Istituto e Museo di Storia della Scienza is one of Galileo’s original lens, as seen in Figure
27. 9
5. The 38mm objective lens is now cracked, but it was used to make many observations
from 1609-1610.
Taken at the Istituto e Museo di Storia della Scienza Florence Italy 2012
Figure 5: Mounted Objective Lens of Galileo’s Telescope
1.1.7 From Galileo to Hubble
The telescope is often considered to be one of the prototypical scientific instruments[14].
It wasn’t until the invention of optical instruments, in the late European Renaissance, that
the fog of thousands of years of conjecture and dogma could begin to be cleared allowing
humankind’s understanding of the universe and its position in it to be realized.
Throughout countless iterative improvements, optical systems have continued to prove
their preeminent position as scientific instruments.
28. 10
Humankind ceaselessly endeavors to expand its sight outward. This expansion of
capability has required the creation of ever increasing telescope sizes. A common saying
in astronomy is that size does matter. Galileo’s simple 38 mm objective lens has evolved
over the course of 400 plus years into truly gigantic segmented reflecting telescopes, the
largest of which measures 10.4 m in diameter. Magnification is often cited as a
telescope’s primary specification; however, as we attempt to observe objects farther and
fainter than ever before, telescope size continues to increase due to the fact that the larger
the telescope the greater the amount of light it can collect. In order to find ever distant
and fainter objects require the creation of larger and larger telescope mirrors. Like the
lens before them, modern telescope mirrors require ultra-precision hard and brittle
manufacturing techniques to achieve near flawless image creation.
Due to the fact the current mirror construction methods limit monolithic construction to
approximately 8 m in diameter, most large telescopes rely on multiple mirror segments
combined in such a way as to act as a single mirror surface [15]–[17].
29. 11
Source: http://salt.camk.edu.pl/firstlight/salt9.jpg
Figure 6: Southern Africa Large Telescope
Figure 6 depicts the segmented main mirror of the 9.2 meter Southern Africa Large
Telescope (SALT). SALT consists of 91 identical hexagonal mirrors each of which
measure 1 meter in diameter. Manufactures of optical systems and the material sciences
have had to continually improve in order to produce the extreme size and precision of
modern telescopes and related systems.
1.2 Zerodur Glass-Ceramic
Zerodur glass-ceramic is a lithium aluminosilicate glass-ceramic developed by Schott AG
(Schott AG, Mainz, Germany) to have a remarkably low coefficient of expansion
(±0.007E−7/K). Composed of 70 to 78% high-quartz micro-crystallites from 30 to 50
(nm) in size, Zerodur has both an amorphous and a crystalline component [18]. When
subjected to heat, the quartz micro-crystals contract and the glass components expand
[16]. When the mixture of quartz, glass, and other minor components is carefully
30. 12
balanced during production, thermal expansion can be finely tuned. By controlling
thermal effects at the materials level, the need for peripheral thermal control systems or
design considerations in significantly decreased
1.2.1 Advanced Properties of Modern Materials
Due to their superior materials properties such as customizable thermal coefficients of
expansion, chemical stability, high internal quality, high wear resistance under high
temperature, etc., advanced glass-ceramics are supplanting more traditional materials in
critical industrial, scientific, and aerospace applications [19]–[23]. Regrettably, the very
same superior characteristics that make advanced ceramics ideal for many advanced
applications often result in their relatively low machinability and therefore lead to
increased machining costs compared to more traditional engineering materials [22], [24]–
[26].
1.2.2 Engineering Applications of Zerodur
The extraordinary thermal expansion properties and ability to be produced in large
batches make Zerodur an ideal substrate material for a large number of extreme precision
optical and scientific instruments. This propensity for application is best summarized as
follows:
Most applications take advantage of the negligibly small coefficient of thermal
expansion and its homogeneity over the entire volume. This property provides for
stability in shape and volume if the piece is exposed to temperature changes and
temperature gradients. This behavior is a requirement, in particular, for mirror
31. 13
substrates in precision reflective optics supports, frames, or scales and gauges.
[27]”
1.2.2.1 Telescope Mirror Substrates
For the more than 400 years since their invention, telescopes of ever increasing sizes and
capabilities continued to be created in an effort to see farther and clearer than humankind
has before [15]. Significant improvements in the material sciences and production fields
have enabled modern glasses and glass-ceramics to further enhance the scope and
performance of many devices in the optical, aerospace, and precision manufacturing
industries.
The object seen in Figure 7 is an optical support structure made of a single piece of
Zerodur approximately 1.2 meters in diameter. The unique design of this work piece is
due to the need for extremely light weight, and stiff structures. The process by which a
component’s mass is reduced to a minimum is referred to as light weighting. In this
process, the maximum amount of material is removed while maintaining structural
stability. This optimized structure requires an extensive amount of material to be
removed resulting in extended cycle times.
These protracted operations, in turn, result in extremely high operational costs for
manufacturers. Unfortunately, the very process of machining these delicate structures,
through the cutting forces generated, can cause critical fracture and therefore total loss of
the work piece. This fragility requires that process parameters be effectively managed to
32. 14
mitigate the threat of critical fracture and reduce the occurrence of defects in brittle work
pieces [28].
Figure 7: Ultra-Lightweight Zerodur Optical Support; Schott AG
1.2.2.2 Aerospace Avionics
In many modern aerospace applications, gyroscopes are highly critical elements required
for the determination of a body’s inertial reference and therefore making precise position
measurements possible. Typically, to help ensure safe operation, the 3-D-position of an
air or spacecraft is determined through the use of a minimum of three gyroscopes.
Reliable pitch and yaw measurement is critical for safe flight, thus all modern aircraft
utilize gyroscope-based attitude indicators to provide highly accurate and precise
methods of orientation measurement. Although all planes are equipped with compasses,
gyroscope-based heading indicators provide a reliable means of measuring an aircraft’s
33. 15
direction in situations such as acceleration or turning. Minute changes in a gyroscope
positioning can greatly affect its accuracy. The use of Zerodur as a base material helps to
minimize thermal expansion effects on gyroscope accuracy and precision.
Figure 8: Zerodur Gyroscope Housing, IMTS 2014 DMG MORI Ultrasonic Demo
Figure 8 is an example of a Zerodur gyroscope body created as a machining
demonstration by DMG MORI for the 2014 International Manufacturing Technology
Show (IMTS). With its near-zero coefficient of thermal expansion, Zerodur is uniquely
suited to outperform traditional materials in the “challenging demands with respect to
temperature and pressure resistance.”[29]
1.2.2.3 Semiconductor Fabrication
Modern semiconductor fabrication and the ever advancing computational capabilities it
enables, depends on the effective resolution of micro-lithographic processes. In micro-
lithographics the desired semiconductor structures are projected onto silicon wafers in
order to expose lithograph chemical reactions responsible for the construction of
34. 16
microchip element constriction. Zerodur mirrors are routinely relied upon to produce
resolution capabilities of 500 line pairs per mm allowing resolution of line widths of
approximately 1µm [27]. This method has reached wide-spread utilization, as can be seen
at the Japanese camera companies Cannon and Nikon, where Zerodur-based micro-
lithographic systems are commonly relied upon.
1.3 Industrial Need for Improved Zerodur Machining
Glass ceramics have traditionally been machined using various grinding methods. For
extreme precision applications, basic grinding processes require excessive tool
maintenance and decreased material removal rates (MRR) to ensure dimensional
tolerance, reduce the occurrence of critical fracture, and optimize surface quality
resulting in relatively low productivity and protracted production times. Additionally,
basic grinding methods suffer from limited work piece complexities and therefore may
require multiple work piece setups and fixturing solutions. These limitations necessitate
larger capital, labor, and tooling costs, resulting in much greater overall machining costs.
The “cost of machining can be as high as 90% of the total cost” for many hard and brittle
high precision workpieces [30].
The limited machinability of many hard and brittle materials provide a powerful incentive
to the development of manufacturing processes that minimize production time and cost.
Casting and other net shaping techniques can be employed; however the lead time for
model and form creation limit process flexibility and require extensive capital
investments that may not be suitable for small batch manufacturing. Additionally, for
extreme precision components, net shaping methods often do not produce the required
35. 17
surface quality or dimensional tolerance dictating the need for some level of finishing
operations.
1.3.1 Rotary Ultrasonic Machining
Rotary ultrasonic machining (RUM) is a hybrid machining process combining ultrasonic
machining (USM) with diamond impregnated grinding. When coupled with the dexterity
of modern 5-axis machining centers, RUM has recently matured into a robust and
effective method of producing complex machined features in materials that were
previously considered too costly to utilize.
RUM operational parameters like vibration frequency, amplitude, feed rate, and spindle
speed, have only recently been experimentally investigated and are, as of yet, not fully
understood. Thus far, there are only a limited amount of publications utilizing the most
recent RUM machines. The advanced capabilities of modern RUM machine tools further
compound the need for improved process knowledge. The application of RUM to provide
optimized surface quality and MRR is of primary concern throughout this research.
1.4 Research Objective
The objective of this research was to twofold:
Primary: experimentally derive empirical models for the prediction of machined surface
roughness parameters for RUM helically milled Zerodur glass-ceramic pockets by means
of a systematic statistical experimental approach.
Secondary: To experimentally investigate and quantify possible reductions in cutting
force and tool wear associated in the helical pocketing of Zerodur glass-ceramic.
36. 18
1.4.1 Benefits of This Research
This work will provide effective RUM strategies to improve the machined surface quality
of Zerodur glass-ceramic workpieces while maintaining a high material removal rate.
Secondarily, this research will advise the design and implementation of future RUM
manufacturing operations in both industrial and academic settings. An expanded ability to
precisely predict and manipulate the surface roughness will allow for an expanded use of
RUM.
1.5 Chapter Summary
In order to develop a more comprehensive understanding of a manufacturing process, it is
often necessary to possess a more than cursory grasp of its history and founding
principles. The purpose of this chapter is to provide both the historical prospective and
contemporary requirements that make Zerodur glass ceramic components integral parts of
many extreme precision engineering applications.
The need for improved methods for the machining of Zerodur and other similar hard and
brittle materials was presented; the remaining sections of this chapter outlined the
objectives and benefits of this research.
37. 19
2 Fundamentals of Rotary Ultrasonic Machining
2.1 History; From USM to RUM
This section provides a general overview of the hard and brittle machining techniques
that superseded the invention of RUM.
2.1.1 History of Ultrasonic Machining
Ultrasonic machining (USM) is an early method of shaping hard and brittle materials, and
is widely considered the most frequently used method [23], [31]. R. W. Wood and A. L.
Loomis are credited with the first work on high powered piezoelectric ultrasonic
oscillators in 1927 [26], [31]–[33]. First patented in 1945 by members of the Cavitron
Corporation, USM provided a means by which hard and brittle materials could be
machined into complex geometries not readily possible with conventional methods [34].
As seen in Figure 9, material is removed primarily by the hammering action of a high-
frequency, low-amplitude oscillating metallic tool in conjunction with abrasive slurry.
Typically suspended in an aqueous solution, abrasives are pumped between the tool and
workpiece, resulting in material removal by brittle fracture.
38. 20
Figure 9: Schematic of USM Process
2.1.1.1 Review of USM Advantages and Disadvantages
The following subsection provides a general overview of the advantages and
disadvantages of USM utilization over traditional machining methods such as grinding
and drilling.
Advantages of USM
Complex freeform features in brittle materials can be machined with a single tool.
USM does not require workpiece conductivity.
No heat affected zones have been found resulting from USM.
No chemical or electrical change to a workpiece’s surface.
Disadvantages of USM
Low relative material removal rate.
High tool wear due to slurry and tool surface interaction.
USM is not suitable for deep hole creation; depth to diameter ratio is limited to about
3:1.
Diminished edge accuracies due to unwanted slurry-wall interaction.
39. 21
2.1.2 From USM to RUM
RUM was initially invented in order to overcome the disadvantages of USM on the
machining of deep holes in uranium glasses and other hard and brittle materials. Percy
Legge of the Harwell U.K. Atomic Energy Authority developed the RUM process in
which no slurry is required [35]. Figure 10 is an image of Legge’s prototype machine.
One of the main motivations for the development of RUM was the mitigation of lengthy
process times in the creation of nuclear glasses utilized in the atomic energy sector [12],
[13]. Experimental results have shown that the machining rate obtained from RUM is
about 10 times higher than that from USM under similar conditions [19].
Figure 10: Initial RUM Prototype; Legge 1964 [35]
40. 22
The invention of RUM enabled improved surface quality, hole accuracy, lower tool
pressures, and increased capability to machine deep holes [23], [31], [36], [37]. RUM is a
hybrid process combining USM-like tool vibration and diamond-impregnated tooling.
Tool vibration frequencies greater than 20 kHz, in conjunction with spindle rotation, are
typically utilized. Oscillation amplitudes are typically in the range of 5-10µm. Figure 11
illustrates the combination of axial ultrasonic and rotary movement present in RUM and
their respective zones of primary material removal.
Figure 11: RUM Tool Actuation Diagram
This nontraditional tool-movement presents a particular challenge for the development of
analytical material removal models and there is still no scientific consensus or complete
model.
2.1.3 RUM Material Removal Mechanisms
When combined with 5-axis machining centers, RUM enables a wide spectrum of
previously difficult-to-machine materials to be used in the creation of complex
workpieces. The RUM process is inherently complex due to the combination of multiple
41. 23
simultaneous interactions between the tool, workpiece, removed material, and supplied
coolant. Previous research has shown that RUM possesses multiple forms of material
removal, each of which is described in the following subsections.
2.1.3.1 RUM Material Removal Mechanisms
To date there is no commonly agreed upon material removal mechanism model in which
brittle fracture, ductile, abrasive flow, and cavitation are included.
Brittle fractures are created on the workpiece at the axial face of tool, as a result of
repeated impact between a tool’s many abrasive grains and the machined surfaces. These
impacts result in Hertzian crack formations, as seen in Figure 12. Once subsurface cracks
have been created, any of the material removal processes are capable of dislodging the
resulting fracture material. The subsequent removal of material affected by impact-based
fracturing constitutes the primary form of material removal in both RUM and USM
processes.
Figure 12: Example of Hammering in RUM
42. 24
As seen in Figure 13 and analogous to conventional diamond impregnated grinding,
abrasion is the secondary form of material removal found in RUM. Tool vibration has
been found to not produce significant MRR increases at the tool’s lateral face [30], [38].
Figure 13: Example of Abrasion in RUM
Through the superposition of hammering and abrasion, a hybrid removal process referred
to as extraction, has also been investigated [30].
Abrasive flow is a process in which abrasive particles, suspended in flowing fluid remove
surface material of a workpiece through a combination of impact and abrasion. Abrasive
flow is commonly considered a tertiary form of material removal and is enabled through
the use of high pressure through spindle coolant, as seen in
Figure 14. Due to the fact that the amount of material removed by abrasive flow is orders
of magnitude less than hammering or abrasive material removal, it is normally neglected.
Recent investigation has suggested that ultrasonic cavitation may provide an additional,
yet minimal, source of material removal [39]. To date there is no commonly agreed
upon material removal mechanism model in which brittle fracture, ductile, abrasive flow,
and cavitation are included.
43. 25
Figure 14: Schematic View of Coolant-Base Abrasive Follow
2.1.4 RUM Tooling
The majority of tools utilized for RUM consist of abrasive particles suspended in a
bonding material similar in composition to those seen in typical grinding wheels. Any
number of tool geometries and compositions are possible. Hollow tools are required for
the use of through spindle coolant Figure 15 is a selection of common RUM tool sizes
and varieties.
Figure 15: Common RUM Tools
44. 26
2.1.4.1 Tool Bond Material
The fundamental role of a tool’s bond material is to hold abrasive grains together and
therefore provides the structural integrity of the grinding tool. Although a solid diamond
tool could be ideal for many applications, the use of diamond abrasives in a binding
material allows users to independently control overall tool material properties. Desired
properties of the bond material include strength, toughness, hardness, porosity, and
temperature resistance. Three common bonding methods are metal, resin, and
electroplating.
2.1.4.1.1 Metal Bonded Tooling
Metal bonding is suggested for glass machining. Usually bronze is the common material
for diamond and Cubic Boron Nitride (CBN) abrasives.
2.1.4.1.2 Resin Bonded Tooling
Resin bonded tools are best suited for finishing operations due to their generally weaker
bond strength. By enabling tool bond fracture, under excessive loading, a workpiece’s
surface is less prone to be deeply abraded and thus a surface of high relative smoothness
is created. Resin bonded tools have been found to maintain lower relative operational
temperatures leading to a reduction in surface burn and other temperature-based surface
defects.
2.1.4.2 Electroplated
Electroplated tooling lacks the advantage of grain refreshment upon tool wear; however,
their overall cost is less. An example of an electroplated tool of this bond material may be
45. 27
seen in Figure 16. A particular advantage of electroplating is the ability to easily produce
complex tool and abrasive geometries allowing for increased customization of tool
characteristics.
Figure 16: Electroplated Diamond Tools
2.2 Previous RUM Research
Publications referring to RUM first appeared in the mid 1960’s. Since then a number of
papers have been published on its various characteristics, parametric trends and
applications for a number of materials. The use of RUM has been investigated for the
machining of the following materials.
Alumina / Advanced Ceramics ; [21]–[23], [40], [41]
Glasses; [24], [25], [35], [42]–[48]
Graphite; [49]
Potassium Dihydrogen Phosphate; [50]
Magnesia Stabilized Zirconia; [30], [36], [38], [51]
Matrix Composites & CFPD; [52]–[56]
46. 28
Silicon Nitride; [31]
Silicon Carbide; [37]
Zerodur; [57], [58]
2.2.1 RUM Process Parameters
2.2.1.1 Spindle Speed
Spindle speed is a critical controllable factor in any milling process; spindle speed was
investigated throughout multiple stages of this research. A review of relevant literature
revealed that many of the previous RUM researchers were limited by their respective
machine tools specifications, often spindle speeds limited to between 3,000 and 8,000
rpm. Higher rotational speed RUM machine tools have recently enabled increased MRR
and surface quality and therefore have been incorporated during this research.
Spindle speed’s influence in RUM outcomes is as follows:
Surface roughness decreases with increased spindle speed [38], [53] .
Edge chipping has been found to be reduced by increased spindle speed [40].
Spindle speed has been found to have significant effects on cutting forces such that
decreased cutting forces occur with increased spindle speed [21], [53], [59].
MRR increases when spindle speed is increased however not proportionally [19],
[23], [36], [54].
2.2.1.2 Feed Rate
Feed rate is of primary concern in parametric selection due to its effect on MRR, cutting
force and surface quality in the RUM process as follows:
Feed rate can be considered to have the greatest effect on RUM MRR [25]
MRR increases with increasing feed rate [21], [38], [54], [60].
47. 29
Cutting forces increase with increased feed rate [21], [54], [60].
Surface roughness (Ra) decreases with increased feed rate [60].
2.2.1.3 Ultrasonic Amplitude
Axial tool actuation is the primary difference between traditional face grinding and RUM.
Therefore the ultrasonic amplitude has been investigated in previous research with results
as follows:
MRR has been found to increase up to a point with increasing amplitude [19], [22],
[23], [36].
Cutting forces have been found to slightly decreases with in increasing amplitude
[21], [46].
2.2.1.4 Ultrasonic Frequency
Typically frequencies around 18 to 25 kHz, ultrasonic frequency has been found to effect
multiple machining outcomes as follows:
MRR has been found to increase with increasing vibration frequency [19] [23].
Specific tool wear has been reported to increase with increasing frequency [23].
Surface roughness (Ra) increases with increasing frequency [23] [60].
2.2.1.5 Abrasive Grain Size / Type
As with traditional grinding, the grit size of a RUM tool affects process outcomes as
follows:
MRR has been found to increase with increasing abrasive grit size up to an optimum
value [19] [22] [23].
Surface roughness increases to a point, then decreases as grit size is increased [61]
[62] [63].
48. 30
2.2.1.6 Coolant
RUM investigations have found that coolant type and pressure have only a limited effect
on machining outcomes [23], [25], [52].
2.2.2 RUM Process Outcomes
2.2.2.1 Cutting Force
Understanding the cutting forces in a machining operation is essential to mitigate possible
defects such as critical workpiece failure, distortion of fixturing, excessive tool wear, etc.
Major trends in RUM cutting forces are as follows:
RUM has been found to reduce cutting forces when compared to USM and diamond
grinding [21], [45]–[47], [54], [64], [65]
Cutting forces increase with increasing feed rate [21] [38].
MRR has been found to increase with increasing static force [22] [23].
2.2.2.2 Material Removal Rate
An understanding of and a prediction method for MRR are critical for the effective use of
available temporal and capital resources. The following trends for MRR of RUM are as
follows:
RUM has been found to enable greater MRR than USM and grinding [22] [23] [60].
2.2.2.3 Tool Wear
Effective tool usage is critical to ensure the efficient use of available resources. As of yet,
there has been little investigation on diamond impregnated tooling explicitly used for
RUM. The following trends have been found in previous investigations.
49. 31
RUM has been found to produce less tool wear when compared to USM [25], [31],
[64]
RUM has been found to produce less tool wear when compared to grinding alone
[45].
2.3 Chapter 2 Summary
In this chapter the history of RUM has been reviewed. The deficiencies in USM and thus
the motivations for the invention of RUM have been presented along with the commonly
utilized tooling. The results of previous RUM investigations, found during literature
review, were presented. Many research papers have focused on the RUM of various hard-
to-machine materials however; there has been no investigation reported on the helical
pocketing of Zerodur glass ceramic.
50. 32
3 Experimental Methodology
Experimental trials carried out for this research were conducted at both DMG MORI’s
ultrasonic headquarters in Stipshausen Germany and the MTTRF Berkeley Institute in
Berkeley California. The experimental systems used at each of these locations are
detailed in the following chapter and are directly referenced in each experimental
overview, for purposes of clarity, in Chapter 4.
3.1 Research Overview
This research was carried out on a vertical RUM machine tool, equipped with a variety of
systems and peripheral equipment capable of measuring the operational characteristics
and resultant outcomes of a large number of RUM experimental trials. Several custom
fixturing and support elements were fabricated in order to create the experimental system
detailed in this chapter. Figure 17 was created via SolidWorks 2013-14 for both
visualization and use in Computer Aided Manufacturing (CAM) based collision testing.
(Left) Close Up of Machining Area (Right) Overview of System
Figure 17: Primary Experimental Fixturing CAD Rendering
51. 33
The cylinder in the center of the figure represents the Zerodur test material affixed to a
custom fixture designed to be mounted to a cutting force dynamometer, which is in turn
connected to the machine tool via a pneumatic chuck system by way of custom adapter.
Several other fixtures were created for dressing, stock material setup, and data collection
material alignment.
3.2 Helical Pocketing
In order to decrease the machining costs associated with hard and brittle materials, a high
level of MRR must be maintained. Impact-based material removal has been identified by
previous research to be the primary mode removal mechanism [19], [57], [66], [67] .
Impacting only occurs at the axial face of the tool and therefore traditional slotting and
side cutting operations do not benefit from ultrasonic oscillation. Helical milling enables
full engagement of the tool’s axial face and therefore enables full employment of impact-
based material removal. Unlike drilling, helical milling enables the creation of features
much larger than the diameter of the tool as is required in the light weighted Zerodur
optical components discussed previously.
In the helical milling process, a tool gradually moves in the axial direction with a helical
motion as it traverses around a circle, a seen in Figure 18. Helical milling has been seen
to improve RUM outcomes in hard and brittle material during extensive testing at Sauer
Ultrasonic. Additionally, the use of a helical approach helps to decrease the high initial
forces imparted on a tool on entry into the workpiece material. This practice is similar to
the use of ramp feeds in grinding operations.
52. 34
Figure 18: Helical Milling Diagram
Multiple helical toolpaths can be combined to produce pockets of diameter greater than
that of the utilized tooling. A sequence of helical pocketing operations can be slowly
transformed into non-circular shapes in order to create pockets of nearly any shape. All
experimental trials conducted throughout this research consisted of helically milled
pockets in order to model and investigate this method of fabrication for Zerodur
workpieces.
3.3 Experimental Materials
As previously described in Section 1.2, Zerodur glass-ceramic is an ideal choice for
extreme precision application in several industrial and scientific fields because of its
unique characteristics. Due to the fact that there is only one producer and it requires a
complex production process, Zerodur is prohibitively expensive. In order to reduce the
excessive expenditure of available resources, BK7 optical glass was utilized during the
initial familiarization and parametric screening stages of this research. For purposes of
example, Figure 19 and Figure 20 are identical blocks of Zerodur and BK7 respectively,
53. 35
measuring 150 x150 x75 (mm). The Zerodur block costs approximately $5,500 while the
BK7 costs only $300. BK7 is often relied upon as a prototyping substitute for Zerodur
components due to its significantly lower price and its similar RUM machinability. In
preliminary testing, BK7 was found to be more likely to exhibit edge chipping during
machining while producing less tool wear and thus provides a “worst case scenario” for
edge surfaces while minimizing tooling costs.
The physical similarity of these glasses can be seen in their common engineering
parameters, as seen in Table 1 and Table 2 provided by Schott AG. Whenever possible,
BK7 was used in the initial testing phases of each successive experimental trial in order
to ensure the efficient use of laboratory resources. Although lower in price, BK7’s
Figure 19: Zerodur Block Figure 20: BK7 Optical Glass Block
Table 1: Zerodur Physical Parameters
Density 2.53 g/cc
Modulus of Elasticity 90.3 GPa
Poisson’s Ratio 0.240
Knoop Hardness 620
Shear Modulus 34.0 Gpa
Coef. Thermal Expansion 0.007E−7/K
Table 2: BK7 Physical Parameters
Density 2.53 g/cc
Modulus of Elasticity 91.0 GPa
Poisson’s Ratio 0.208
Knoop Hardness 520
Shear Modulus 36.7 Gpa
Coef. Thermal Expansion 86E-7/K
54. 36
3.3.1 Statistical Modeling and Validation Stock Material
Figure 21: Experimental Workpiece, Zerodur Optical Blank
Zerodur optic blanks, as seen in Figure 21, were utilized for Zerodur statistical modeling
and tool wear experimental trials. Polished optical blanks with, 5 (nm) Ra and ¼ λ, were
chosen in order to minimize the presence of surface defects prior to machining that could
have effected machining outcomes. By ensuring stock materials had minimal surface
defects, it could be assumed that all machined surface features were the result of the
experimental process alone.
3.4 Machine Tool and Ultrasonic Systems
A DMG MORI Ultrasonic 20 linear (US20) 5-axis machining center, as seen in Figure
22, was employed for each of the many experimental trials carried out during this
research. The US20 is capable of not only 5-axis RUM, but also High Speed Cutting
55. 37
(HSC) operations and thus provided both fast and effective material removal in a wide
variety of both traditional and advanced materials.
Source: DMGMORI.com
Figure 22: Sauer Ultrasonic 20 linear
During my undergraduate and master’s work with the UC Davis IMS-M Laboratory, I
have been very fortunate to attend several international manufacturing symposia; namely,
IMTS 2010, 2012, 2014, and EMO 2013. Throughout extensive investigation during
these events, no other machining center or system was found to provide RUM capabilities
with the comparable speed, accuracy, and flexibility found in the US20 and other
members of the DMG MORI ultrasonic series of machines. As of the date of this thesis’
creation, there is no other fully integrated ultrasonic milling machine currently available
from any other manufacture. The DMG MORI Ultrasonic line of machines can thus be
considered the preeminent RUM machining solution commercially available at this time.
The capabilities of the US20 provided both benefits and challenges with respect to this
research. Clearly, the ability to machine an ever increasing variety of materials is an
56. 38
excellent benefit, however, due to the system’s novelty there is little to no openly
available academic or industrial research available with respect to the helical pocketing of
Zerodur glass-ceramic to use as a basis of comparison for my research. This lack of
available material was a primary motivation for this research.
3.4.1 US20 Specifications
The US20’s specifications are listed as follows:
5-axis gantry construction
Integrated NC swivel rotary table
2g acceleration in X / Y / Z
X / Y / Z linear driven for little to no backlash
Small footprint; 3,5 m² (37.67 ft.2)
An actively cooled HSK-32/40 spindle.
High speed spindle, up to 42,000 rpm
High contour accuracy
Automated real time feed adaptation
3.4.2 Actor® Ultrasonic Tool Holder
The outward appearance of the US20, and other ultrasonic series equipped machines, is
indistinguishable from a traditional machine tool. The point of their differentiation is the
Actor® Ultrasonic tool holder system and associated frequency generation equipment
developed at DMG MORI’s ultrasonic headquarters in Stipshausen Germany. Ultrasonic
axial tool movements are produced through the application of piezoelectric oscillatory
actuators in the tool holder assembly. An on-machine waveform generator provides the
required drive signals to the tool holder. Through the use of inductive coils in both the
tool-spindle interface and tool holder assemblies, the waveform generator’s signal is
57. 39
imparted to the oscillatory motor. Figure 23 provides the overall tool holder appearance.
The spindle-to-holder interface is a traditional HSK32 tool holding interface.
Figure 23: Actor Ultrasonic Toolholder
3.4.3 Through Spindle Coolant
Reductions in friction, cutting zone temperature, and process forces are often cited as
cutting fluids primary benefits [10]. Additionally, coolant helps to prevent tool jamming
during deep drilling operations. Whenever hollow tooling is feasible, coolant can be
supplied both internally through the cutting tool and externally by nozzles located on or
around a machine’s headstock. Figure 24 provides a view of the Actor® HSK32
ultrasonic tool holder’s through-spindle coolant interface.
58. 40
Figure 24: Actor Ultrasonic Toolholder Internal Coolant Input
3.5 Fixturing Methods
Figure 25: Zerodur Experimental Stock Material Fixturing Example
59. 41
A wax-based fixturing method, as seen in Figure 25, was used to affix experimental stock
material. In addition to the wax-based fixture plates, several other functional elements
were combined through the use of custom fixturing elements. Cutting force is a primary
indicator of proper workpiece loading and process performance. A Kistler 3-Componenet
9257B piezoelectric dynamometer (Kistler Instrument Corp, Amherst, NY, USA) formed
the main body of the experimental fixturing assembly. Many of the experimental system’s
components were designed or chosen to work around or in conjunction with the 9257B
due to its relatively large size compared to the US20’s machining envelope.
Figure 26: Kistler 9257B Schematic [68]
The second design constraint of the experimental fixturing system was the need to
interface with the Erowa ER-029313 pneumatic chuck system, a schematic of which may
be seen in Figure 27 (Erowa LTD, Büron, Switzerland). The Erowa chuck interface is the
60. 42
primary work holding method for the US20. This system provides excellent repeatability
with minimal setup time.
In order to mount the 9257B to the US20’s Erowa chuck, a custom Kistler-to-Erowa
adapter plate was designed and fabricated. Made of polished 25.4 (mm) thick 303
stainless steel, the adapter plate provided a ridged support for the rest of the fixturing
assembly.
Figure 28 is an exploded view of the entire work holding assembly.
Figure 27: Erowa Chuck; ER-029313 [69]
61. 43
Figure 28: Exploded View of Experimental Work Holding Assembly
3.5.1 Glass Fixturing Methods
Unlike traditional ductile materials, the use of compressive fixturing systems such as
vises or clamps can be detrimental to a hard and brittle material. Once a critical
compression or shear threshold has been reached, increased clamping forces will result in
critical fracture and therefore total loss of the workpiece in most situations. For this
reason, stock materials are affixed with waxes, glues, and vacuum systems.
The technique most often employed as an affixing method, and used throughout this
research, is that of adhering stock material with dopping wax. Unlike typical vise
fixturing, wax fixturing prevents stress loading of the part and therefore alleviates the
propagation of critical fractures that can result in total workpiece loss. This method uses
62. 44
wax to affix the stock material to be shaped onto a rod known as a dop. This method is
commonly used by jewelers during the faceting process, as illustrated in Figure 29.
Source: http://gem-sphalerite.com/faceting-process
Figure 29: Gem Stone Dopping Wax Fixturing Example
Specialized equipment, not typically found in a traditional CNC machine shop, is
required for wax fixturing. A sample of the fixturing wax used throughout this research is
shown in Figure 30. Typically used for decorative seals, this wax provides excellent
adhesion in both wet and dry environments. The fixturing wax’s ability to withstand
cutting fluid is critical due to the requirement of high pressure cutting fluids during the
majority of RUM operations.
Figure 30: Fixturing Wax and Heating Plate
63. 45
As seen in Figure 32, a digital heating plate is used to gradually heat up the stock material
and fixture plate to the melting point of the wax, 145°C in the case of the wax used for
this research. Once the workpiece material and fixture are up to temperature, wax is
applied to both the fixture and stock material. Next, these elements are combined and the
assembly is allowed to cool. Particular attention must be paid in order to prevent the
initiation of workpiece fracture due to non-uniform heat distribution. For many hard and
brittle materials, the heating and cooling rates must be very gradual in order to
significantly reduce the risk of crack formation. BK7 glass requires a heating / cooling
rate of 100°C per hour. A digitally controlled convection oven capable of programed
temperature sequences, as seen in Figure 31, was used to minimize the risk of stock
material loss. A digital heat plate, as seen in Figure 32, was used for all Zerodur trials.
Figure 31: Stock Preperation Via Digital Oven
64. 46
Figure 32: Zerodur Stock Materials Preperation Via Digital Hotplate
3.5.2 Wax Fixture
In order to utilize wax fixturing, a series of custom 303 stainless steel fixtures were
designed and fabricated. A schematic and CAD rendering can be seen in Figure 33. Each
experimental fixture was designed to be used in a variety of configurations.
Figure 33: Experimental Fixture Schematic Drawing
65. 47
Source: http://ims.engr.ucdavis.edu
Figure 34: Sodick AQ327L WEDM and Work Area
These connection points and their distribution served as major design criteria for any
applicable fixturing solution. Using M8 bolts, each fixture could be directly mounted to
the 9257B thus providing cutting force measurement capability while enabling relatively
little setup time for each of the many experimental trials required.
In order to allow wax to contact the mating surface of the stock material while ensuring a
flat and uniform profile, an array of plateaus and valleys were added to the fixture’s top
surfaces. These intricate mating surfaces are critical to ensuring a robust connection
between the stock material and the fixture by providing ample surface area for wax
adhesion. These features were created through the use of a 4-axis Wire Electronic
Discharge Machining (WEDM) process. A Sodick AQ327L 4-Axis Wire EDM was used
for the fabrication of all wax fixturing surfaces, (Sodick Co. LTD, Nakamachidai, Japan),
as seen in Figure 33. WEDM was chosen due to its ability to make precise and intricate
geometry of minimal corner radius. The intricately textured mounting structures seen in
66. 48
the accompanying figures were created through the use of only two WEDM operations
separated by a 90 degree automatic index on the AQ327L’s rotary axis.
A Sodick MC430L 3-Axis milling center, as seen in Figure 35, was used to the mill
mounting holes required for connection to the Kistler 9257B. All toolpaths were created
in Esprit 2013, (DP Technologies Corp, Camarillo, CA, USA). These operations were
created separately and then combined in post processing. A total of 20 wax fixtures were
created for both the preliminary testing and final statistical screening, modeling, and
validation phases of this research.
Source: http://ims.engr.ucdavis.edu
Figure 35: Sodick MC430L 3-Axis High Speed Machining Center
3.5.2.1 Fixture Texturization
Once primary machining operations were completed, wax mounting surfaces were
texturized through the use of sandblasting. This texturization increases the bond strength
of dopping wax by creating an increased amount of surface area while simultaneously
eliminating any burs or other unwanted machining artifacts created during milling
operations that could have caused stock material misalignment. The results of
sandblasting may be seen in Figure 36.
67. 49
Figure 36: First 12 Sandblasted Wax Fixtures
3.5.2.2 Fixture Polishing
In order to ensure the highest possible fidelity in the measurement of cutting forces by the
Kistler 9257B, every attempt was made to eliminate gaps or other surface irregularities
between the mating surfaces of the dynamometer and each of the experimental wax
fixtures. Although prepared on a high accuracy milling machine, the back side of the
experimental fixtures, seen in Figure 37, could not be assumed to be perfectly smooth and
were lapped on a granite surface plate with a series of abrasive grit sizes until a highly
polished surface was obtained, as seen in Figure 38.
68. 50
Figure 37: Backside Milling Results Figure 38: Polished Results
3.5.2.3 Experimental Stock Centering Fixture
One drawback of using such a highly polished experimental stock material is the
propensity for stock material misalignment due to unwanted movement during the
fixturing process. This misalignment can occur while the fixturing wax is still in its liquid
phase. Thus experimental stock material alignment was accomplished through the use of
a precision centering fixture. This centering fixture was designed and fabricated, as seen
in Figure 39 and Figure 40, out of 6061 aluminum. The centering fixture provided
increased stock positioning repeatability resulting in a dramatic decrease in result
measurement setup time. A second custom alignment fixture was designed and fabricated
to facilitate a quick yet repeatable means of workpiece alignment during surface analysis
(see section 3.6.2.2 for a full description). All machining operations for the stock
alignment fixture were conducted on the US20.
70. 52
3.6 Surface Quality Measurement
3.6.1 Parametric Screening Surface Measurement
As seen in Figure 41, the Mitutoyo SJ-400 (Mitutoyo Corp, Kanagawa, Japan) was used
to acquire surface roughness measurements. Although used for initial parametric
screening trials this system was located at DMG MORI Sauer’s Stipshausen location and
therefore was not available after primary parametric screening trials carried out at the
Machine Tool Technologies Research Foundation’s Berkeley Institute.
Figure 41: Surface Roughness Measurement System
3.6.2 Statistical Modeling Surface Measurement
A Mitaka MLP2 surface analyzer, (Mitaka Kohki Co, Tokyo, Japan), as seen in Figure
42, was the primary system of surface quality measurement used throughout this
research.
71. 53
Source: http://www.mitakakohki.co.jp
Figure 42:Machined Surface Measurement System
The MLP2 is a Point Autofocus Instrument (PAI) that employs a non-contact surface
texture measurement method known as point autofocusing. By precisely measuring the
distance required to bring the PAI’s laser in focus the relative height of a surface feature
can be determined (ISO/CD 25178-605 2011). A diagram of this system and its major
components is shown in Figure 43. This process can be repeated as the object is moved
laterally via an X Y scanning stage in order to create either a linear or 3D profile of the
surface with high resolution and high accuracy. The point autofocusing has a wide
measuring range in high precision and is not influenced by color or reflection ratios of
workpiece surfaces [70]. Figure 43 is provided in order to better explain basic mechanics
of the autofocus method.
72. 54
Figure 43: Typical Autofucus System [71]
Adapated From [71]
Figure 44: PAI Measurement Example
73. 55
Several peripheral components are required to actuate and log the measurements made by
the autofocus system, as seen in Figure 45.
Figure 45: PAI System Diagram
To minimize the effect of vibrations and minor thermal fluctuation on measurement
accuracy, the MLP2’s measurement systems are located inside a separate measurement
chamber. Figure 46 depicts the measurement area of the MLP2 comprised of the optical
system, a vertical Z-axis, and horizontal X Y-axis scanning stage, and rotary table.
74. 56
Figure 46: Mitaka MLP2 Measurement System
3.6.2.1 Mitaka MLP2 Specifications
Table 3 and
Table 4 provide the MLP2’s autofocus and scanning stage specifications respectively.
Table 3: Autofocus Sensor Specifications
Laser spot diameter 1μm (at 100X )
75. 57
Power ≤ 1mw (Class 2)
Wavelength λ=635nm
Autofocus Repeatability σ＝0.015μm
Table 4: Scanning Stage
Axis Moving range Resolution
X 120mm 10 (nm)
Y 120mm 10 (nm)
Z 130mm 10 (nm)
AF (Autofocus) 40mm 1 (nm)
AZ (Rotary Axis) 360° 0.0002°
3.6.2.2 Mitaka Measurement Custom Fixture
Mitaka MLP2 uses a small vise to affix samples. A specimen can be simply placed on this
vise for measurement; however, due to the large number of experimental trials conducted
throughout this research, an alignment fixture was designed and fabricated, as seen in
Figure 47. Based on the alignment fixture use during wax fixturing, this fixture enables
the repeatable placement of samples with little to no setup or adjustment required. In this
manner, the center positions of each machined pocket were automatically known and
required no adjustment of measurement start positions. In excess of 600 individual profile
measurements were made with the MLP2; the minimal of setup time per sample provided
by the alignment fixture greatly reduced the time required for data collection.
76. 58
(Left) Fixture Mounted to Mitaka MLP2 (Right) Measurement Fixture Usage Example
Figure 47: Mitaka Measurement Fixture
3.7 Surface Profile Methodology
MitakaMap (MMap) is the MLP2’s proprietary interface software. MMap was used to
extract surface profiles, perform corrective mathematical operations, calculate multiple
surface parameters, and perform initial data analysis and evaluation. A set of
measurement operation sequence macros were created in MMap in order to minimize
systematic measurement errors through the use of a standardized process and to expedite
the data collection process. The following sequence of actions was performed for each of
the 750 plus gathered throughout the course of this research. Profile reports were
automatically generated, and numerical information was exported to a CSV file format
for use in Microsoft Excel.
77. 59
Figure 48: Centering Macro Output, MMap
The center of the machined pocket is first located through the use of a centering macro
before surface measurement. MLP2 scans for abrupt changes in surface height within a
set threshold corresponding to the vertical walls of each pocket. The blue and red profiles
seen in Figure 48 represent the X and Y scans respectively of the centering process. From
this data a corrected center position is generated.
The point data generated by the MLP2 is combined to generate a raw surface profile, as
seen in Figure 49. Note the large increase in profile height at the beginning and end of the
measurement profile created as the laser beam approaches the vertical walls of the
machined pocket.
78. 60
Figure 49: Raw Surface Profile
The measurement aberrations seen in the raw profile are eliminated by MMap’s profile
trimming function. Four percent of each profiles start and end portions were trimmed,
resulting in the profile seen in Figure 50.
Profile curve - OP01
0 1 2 3 4 5 6 7 8 9 10 mm
µm
-100
-50
0
50
100
150
79. 61
Figure 50: Extracted Profile Section
As seen in Figure 51, each profile was leveled through the use of MMap’s least squares
leveling function in order to minimize any tilting in the X and Y planes. The use of
custom alignment fixtures in conjunction with least squares leveling dramatically reduce
sample setup time.
Profile curve - Extracted area (92%)
0 1 2 3 4 5 6 7 8 9 mm
µm
-50
-40
-30
-20
-10
0
10
20
30
40
80. 62
Figure 51: Least Squares Leveled Section
Finally a measurement identity card, 9 surface parameters (ISO 4287), and other profile
statistics are presented in an operation report as seen in seen in Figure 52 and Figure 53,
respectively.
Figure 52: Measuremnt Identity Card
Profile curve - Leveled (Least squares method)
0 1 2 3 4 5 6 7 8 9 mm
µm
-50
-40
-30
-20
-10
0
10
20
30
40
Identity card
Name: OP01
Filename: C:UsersMLP2DesktopBB_DOE_75_8.15OP01.am2
Axis: X
Length: 10 mm
Size: 4002 points
Spacing: 2.5 µm
Offset: 35 mm
Axis: Z
Length: 201 µm
Z min: 34488 µm
Z max: 34689 µm
Size: 200510 digits
Spacing: 1 nm
81. 63
Figure 53: Calculated Surface Parameters
3.7.1 Surface Topological Calculation Metrics
The following subsections provide a brief summary and the calculation methods
commonly used to calculate each of the roughness parameters considered throughout this
research. In the scope of this work, roughness is defined as the closely spaced, irregular
deviations on small scale, expressed in terms of its height, width, and distance along a
surface [10]. ISO 4287, the master standard for profile parameters in the ISO GPS
system, was used for all surface roughness measurements. An important distinction must
be made in order to mitigate confusion in the parametric definitions presented later in this
subsection:
Sampling Length: A fixed length sampled from the surface profile curve to find the
characteristics of the surface.
Evaluation Length: The characteristics of the profile curve are evaluated over a fixed
length that includes at least one sampling length. This length is called the evaluation
length. The evaluation length is set to five times the sampling length by ISO standards.
82. 64
Much of the following information was adapted from the following sources: [71]–[74].
3.7.1.1 Arithmetic Average (Ra)
Ra is almost certainly the most widely employed measure of surface quality in
manufacturing. The influence of an irregularity on the measurement value becomes
extremely small, so that stable results can be obtained.
Figure 54: Ra Example [74]
𝑅 𝑎 =
1
𝑛
∑| 𝑦𝑖|
𝑛
𝑖=1
(1)
3.7.1.2 Root Mean Squared (Rq)
Rq, formally known as RMS, is defined as the line height in which an equal amount of
area is created by the roughness profile both above and below. Rq is mainly used in the
United States.
Figure 55: Rq Example [74]
𝑅 𝑞 = √
1
𝑛
∑ 𝑦𝑖
2
𝑛
𝑖=1
(2)
83. 65
3.7.1.3 Mean Height of Profile Irregularities (Rc)
Rc is the mean value of the profile element heights Zt within a sampling length. Rc is
often used to evaluate the “high-grade feel”, adhesion performance, and frictional force.
Figure 56: Rc Example [74]
𝑅 𝑐 =
1
𝑚
∑ 𝑍𝑡𝑖
𝑚
𝑖=1
(3)
3.7.1.4 Maximum Peak (Rp)
Rp is simply the highest point of a surface’s profile. Rp is often used for the evaluation of
frictional force and electrical contact resistance.
Figure 57: Rp Example [74]
𝑅 𝑝 = ( 𝑚𝑎𝑥𝑖) 𝑦𝑖 (4)
84. 66
3.7.1.5 Maximum Valley Depth (Rv)
The inverse of Rp, the maximum valley depth is the lowest point. Rv is often used to
evaluate a surface’s strength and resistance to corrosion.
Figure 58: Rv Example [74]
𝑅 𝑣 = ( 𝑚𝑖𝑛𝑖) 𝑦𝑖 (5)
3.7.1.6 Average Maximum Profiler Height (Rz)
Rz provides an estimate of the overall peak to valley magnitude of a surface and may
serve to predict the thickness of coating needed to completely cover and level a surface.
Rz is often used to evaluation the gloss, luster, surface strength, or surface treatability of
a surface.
Figure 59: Rz Example [74]
𝑅 𝑧 = 𝑅 𝑝 + 𝑅 𝑣 (6)
85. 67
3.7.1.7 Maximum Height of Profile (Rt)
Rt is the difference between the highest peak and lowest valley, and can be thought of as
the amount of material that must be removed to produce a smooth surface. Because Rt is
calculated over the entire measurement length, it can be considered stricter than the
standard Rz.
Figure 60: Rt Example [74]
𝑅𝑡 = 𝑅 𝑝 − 𝑅 𝑣 (7)
3.8 Tooling
6mm Effgen Type: SK-6/4-1-5-FT25-BZ104-6D107-C90 hollow endmill, (Günter
Effgen, GmbH, Idar Oberstein, Germany), as seen in Figure 61, was selected for all
experimental trials. This tool is suited for roughing operations and therefore suited for the
high MRR strategies being investigated throughout this research.
