Finishing operations

Lapping and grinding.

Comparison Between Grinding And Lapping Of

Machined Part Surface Roughness In Micro

And Nano Scale

Micro and Nano surface finish has become an important parameter in

semiconductor, optical, electrical and mechanical industries. The write-up

is a comparison between two traditional finishing processes, grinding

and lapping. Machined parts surface roughness in micro and nano scale

can be measured using two different devises in two different directions

normal and perpendicular to the machining direction. Results will show

that the traditional finishing processes are not suitable for nano scale

surface finish. There is a significant difference between the normal and

perpendicular measured surface roughness in nano and micro scale.

1.0 INTRODUCTION:

Final finishing operations in manufacturing of precise parts are

always of concern owing to their most critical, intensive labour and

least controllable nature. In the era of nanotechnology, deterministic

high precision finishing methods are of utmost importance. The

need for high precision in manufacturing was felt by manufacturers

worldwide to improve interchangeability of components, improve

quality control and longer fatigue life. Taniguchi reviewed the

historical progress of achievable machining accuracy during the last

century. The machining processes were classifieds into three categories

on the basis of achievable accuracy in conventional machining, precision

machining and ultraprecision machining. Ultraprecision machining

are the processes by which the highest possible dimensional accuracy

is achieved at a given point of time. This is a relative definition which

varies with time. It was predicted that by 2000 AD, the machining

accuracies in conventional processes would reach 1 μm, while in

precision and ultraprecision machining would reach 0.01μm (10 nm)

and 0.001μm (1 nm) respectively.

The study of micro and nano surface metrology is becoming common

in industrial and research environments as structures and surface

features become smaller and smaller. Scanning interferometry

is becoming increasingly important in metrology analysis because of

various factors such as ; the possibility of non-destructive measurement

– no sample contact or preparations are required ; its accurate and

quantitative surface characterization; the fast and convenient sample

loading and set-up; the capability of measuring a wide range of

materials; high resolution; highly repeatable measurements; fully

automated measurement – ideal for process control; performing

roughness and step height analysis within a single measurement;

the possibility of surface coating measurement – film thickness and

real surface roughness measurement. It can address many of the

challenging measurement problems that exist when studying samples

at the micro and nano scale. These include the measurement of

critical dimensions, heights, angles, surface roughness, solving etch

rate/time problems, measuring stress gradients, etc.

Roughness is an important parameter for sample properties control.

Various roughness ranges are normally studied in order to define

the overall properties of the surface and one of the limitations to the

analysis is the bandwidth of the measurement method. It is very

important to accurately evaluate the quantities values of surface

roughness, to determine the possibility of their usage and quality

of products, to measure the effective height of surface roughness, a

scanning microscope is used. Comparing the surface roughness

machined with traditional finishing, grinding and lapping in small

scale can help in choosing the finishing methods used in production of

small parts.

The surface roughness in nano scale which is generally measured

with optical microscope/profilometer and the scanned profile of the grinding

and lapping surface show that the surface after grinding process is

smoother than lapping process.

The comparison between the grinding and lapping surfaces gives

a different result in micro and nano, the lapping process is better in

micro while the grinding in nano scale, the cutoff distance in measuring

surface roughness may be the reasons of this result because in nano

scale the cutoff distance is very small i.e. About 2-4 nm while in micro it is about 0.8-1

mm.

2.0 CONCLUSION:

The surface roughness in each case will indicate only qualitative value.

Quantities comparisons is not possible to define in nano scale surface

roughness because it differs from one place to another on the machined

surface. Lapping process gives better surface than grinding in micro

scale, while in nano scale the grinding process is better. Measuring

direction has an effect in the micro and nano surface roughness. It is seen in practice

that the traditional finishing processes it not suitable in nano scale machined part.

https://youtu.be/8YZdE1LuyTs

LAPPING AND POLISHING

1.0: Introduction

Lapping and polishing is a process by which material is precisely removed from a workpiece (or specimen) to produce a

desired dimension, surface finish, or shape. The process of lapping and polishing materials has been applied to a wide

range of materials and applications, ranging from metals, glasses, optics, semiconductors, and ceramics. Lapping and

polishing techniques are beneficial due to the precision and control with which material can be removed. Surface finishes

in the nanometer range can also be produced using these techniques, which makes lapping and polishing an attractive

method for materials processing.

This paper describes some basics about lapping, including equipment setup, typical lapping techniques, and

nomenclature.

2.0: Back to Basics

There are several techniques used for removing material from a particular workpiece (also called specimen in this

discussion). Grinding, lapping, polishing, and CMP (chem.-mechanical polishing) are all techniques used for precise

removal of material. A brief discussion of terms is needed to understand the basics of what is being referred to when

these topics are discussed.

2.1: Grinding

Grinding can be defined as the rapid removal of material from a sample either to reduce it to a suitable size or to

remove large irregularities from the surface. The grinding wheel or plate typically rotates at a high speed (around 200-

1000rpm) and a coarse, bonded abrasive (> 40 m) is used. Grinding is quick and relatively easy process but can

cause deep subsurface damage in delicate materials. Typically grinding is applied to hard metals such as high carbon

steels where rapid removal is essential and subsurface damage is not a critical parameter. For delicate materials the

grinding process must be a balance of material removal and subsurface damage. In many cases it is advisable to

initially cut the specimen with a gentle mechanical method such as a wire saw. A properly prepared wire saw cut

sample can eliminate the grinding process altogether.

2.2: Lapping

Lapping is the removal of material to produce a smooth, flat, unpolished surface. Lapping processes are used to

produce dimensionally accurate specimens to high tolerances (generally less than 2.5 m uniformity). The lapping

plate will rotate at a low speed (<80 rpm) and a mid-range abrasive particle (5-20m) is typically used. Lapping

removes subsurface damage caused by sawing or grinding and produces the required thickness and flatness.

Although the lapping process is less damaging than grinding, there are two regimes of lapping: free abrasive lapping

and fixed abrasive lapping.

Free Abrasive Lapping is when abrasive slurry is applied directly to a lapping plate (e.g. cast iron). This is perhaps

the most accurate method for producing specimens and causes the least amount of damage. Free abrasive lapping is

accurate because of the rigid lapping surface which can be tailored to suit a particular material. Fixed Abrasive Lapping

is when an abrasive particle in bonded to a substrate as with abrasive lapping films and SiC papers. Abrasive lapping

films have various particles bonded to a thin, uniform polyester substrate and are also capable of producing a very flat

surface. SiC papers are much thicker than the film and create the potential for rounded edges on the sample.

Lapping and Polishing

Basics

2.3: Polishing

Polishing is the removal of material to produce a scratch-free, specular surface using fine (<3m) abrasive particles.

Polishing is typically done at very low speeds using either polishing cloths, abrasive films, or specially designed lapping

plates. Polishing with a cloth or lapping plate requires the use of free abrasive, and is a very low damage process when

performed properly. Plate material and cloth material are critical when polishing a particular sample as the properties of

these substrates are important in the final polish quality of the specimen.

Polishing with a lapping plate is a common process used in the case of metals and hard ceramic type materials.

Polishing using copper composite plates or tin / lead lapping plates can produce high quality surface finishes with high

removal rates. In many cases the use of a polishing cloth is required, and thus the selection of a proper polishing cloth

is important. Polishing cloth properties needed depend on the application. If flatness is of primary concern, short nap

cloths (such as Nylon) are used to maintain flatness. When the final surface finish is of primary concern, longer napped

cloths (such as Rayon and Silk) are used. Many cloth materials today combine the best of both worlds, allowing

flatness and surface finish combined to provide maximum performance. Polyurethane pads are commonly used for

final polishing processes and produce excellent flatness and surface finish.

Polishing with abrasive films also produces excellent results. The flatness of the films combined with high removal

rates makes them an attractive alternative to cloth and plate polishing methods.

2.4: Chem-mechanical Polishing (CMP)

Chem-mechanical polishing (CMP) is a technique that combines both chemical and mechanical polishing principles to

achieve uniform removal rates of a highly composite specimen (such as integrated circuit device fabrication). CMP is

typically done using a hard polyurethane polishing pad combined with a slurry of finely dispersed alumina or silica

particles in an alkaline solution. CMP combines the selectivity of chemical polishing with the mechanical removal

properties of standard mechanical polishing techniques. The two combined give excellent selectivity and planarity and

can be tailored to many different materials.

2.5: Abrasive Types

There is a wide selection of abrasives to choose from when selecting a lapping and polishing process. Selecting an

abrasive is dependent upon the specimen hardness, desired surface finish, desired removal rate, lifetime, and price.

There are four basic types of abrasives that are used in lapping and polishing processes: silicon carbide (SiC),

aluminium oxide or alumina (Al2O3), boron carbide (B4C), and diamond (C). All of these abrasives have distinct

properties and are used for different materials and applications.

SiC: SiC is hard and generally has a needle or blocky structure. SiC is used in many applications where

rough lapping is required. It seldom is used for polishing or applications that require smooth surface

finishes.

Al2O3: Al2O3 is relatively hard and has a sharp, angular structure. Alumina is commonly used where fine

surface finishes are required as it breaks down over time and gives excellent surfaces during lapping

and polishing. Alumina is also relatively inexpensive.

B4C: B4C is harder than most other abrasives (excluding diamond) and has a blocky crystal structure. B4C

provides excellent removal rates and is typically used when fast removal with moderate surface quality

is needed.

Diamond: Diamond is the hardest material known and has a sharp, angular structure. Diamond is extremely

useful in lapping and polishing due to its removal rates and surface finishing qualities. Diamond can

produce excellent surface finishes combined with high removal rates.