Mirror Finishing
There is no mystique in producing a fine
mirror finish on bike parts, just a little common sense and the
desire to achieve a special satisfying outcome. Aluminum is a very
dirty material to work with, producing a lot of black dust when it
is being polished, although the final outcome and sense of pride
gained far outways the amount of times you have to wash your hands.
Patience and Commitment, that's all you need.
What is a mirror finish?
If you look at a polished surface magnified
a couple of thousand times it looks like a view of the Rocky
Mountains. Because the peaks and valleys are so small and close
together and the tops have been rounded off from polishing, they
reflect the light and to our eyes the surface appears as smooth as
silk, although it is not.
Absolute flatness is actually impossible to
achieve but we can get very close. Therefore all we are doing when
we polish to a mirror finish is making a series of finer and finer
scratches and eventually rounding off the peaks. This is achieved by
stepping down through the abrasive grades until we achieve the
finish we require. Various methods are employed to achieve a fine
polished surface and can be placed into three main categories:
Abrasive finishing polishing; Buffing or bright finishing; and
Colouring.
The first stage is often described as 'emery
bobbing' or 'scurfing' and involves the use of abrasives which cut
away the surface irregularities and imperfections. This can also be
achieved by the use of abrasive belts, flap wheels, hand rubbing
blocks or abrasive cloths and papers.
The second stage, polishing or buffing,
requires the use of mops and polishing compositions and during this
process a bright, smooth finish is achieved.
The term 'Colouring' is applied to the final
polishing of any metal part when again use is made of mops and
polishing compositions. After all the scratches have been polished
out the metal is finally 'Coloured' to make it clean and brilliant.
For this purpose a lime-based or rouge composition is used on a
finishing mop of soft calico or cotton.
Abrasives
Three main abrasives are now in widespread
use within the industry for metal finishing and these are:
naturally occurring emery; and two man-made abrasives:
aluminum oxide and silicon carbide, aluminum oxide being
the most common.
These abrasives can be obtained in a wide
range of grit sizes, from the finest 'flour' grades of 1200 and 1000
through to the most common grades of 320, 240, 220, 180, 150, 120,
80, 60 and even coarser. Up to 500 grit the grain is sifted through
a series of meshes, the grit size taking its number from the last
mesh it passes through, i.e. the number of holes per linear square
inch. Over 500 grit the grain is measured by the time it takes to
settle in water.
For the purpose of abrasive finishing we
only require grit sizes from 320 to 60.
To obtain maximum life from an abrasive
dressed mop, belt or flap wheel it is essential to use a high grade
abrasive such as aluminum oxide. The cost of dressing mops, making
belts and flap wheels is many times the price of the material used.
It is therefore false economy to use inferior grades.
Fused aluminum oxide and silicon carbide
have short angular grains and are many times sharper and faster
cutting than emery, therefore when making use of these man-made
abrasives it is usual to select a finer grit size than one would use
when using emery. They are suitable when an extremely fast cut is
required and also for polishing abrasive materials such as aluminium
and are absolutely essential when polishing stainless steel.
Abrasives are used in metal finishing in the
following ways:
- On abrasive belts, flap wheels, cloths
and papers, etc.
- On dressed wheels, mops, felt bobs,
etc.
- In grease bonded and lime-based
polishing compositions.
- Loose grains for grit blasting, vapor
blasting and lapping,
- In flexible rubber polishing blocks,
sticks, polishing wheels and mounted point.
Abrasive belts and flap wheels are now
extensively used for preliminary abrasive operations prior to the
polishing of components. is made up of strips of aluminum oxide
coated belt material and bonded circumferentially around a central
resin core which is mounted on a steel shaft. This can be used on
any D.I.Y. electric drill and hey presto you have a portable
abrasive finishing machine which can be taken anywhere there is a
power outlet. The abrasive flap wheel has all the advantages of the
abrasive belt machine but with none of the disadvantages of high
cost, non-portability, size and inability to work in small internal
recesses.
Abrasive Dressed Wheels
The wheels or bobs are dressed with abrasive
grain (emery, aluminum oxide or silicon carbide) using glue or
cement as adhesive.
Felt bobs are the most popular wheel used
commercially, for their long life, although they are quite
expensive. They are made of solid felt, blended to give uniform
texture and resiliency. Felt wheels can be turned to any shape to
suit particular trades or duty, such as grooved bobs for tubular
work, small cones and fingers for jewelry and knife edged.
Finishing and Buffing
The primary function of the polishing
process is to produce a smooth, reflective surface with only a
minimum amount of material being removed.
To do this it is still necessary to use
abrasives within a polishing composition, sharp enough to abrade the
surface to remove marks left by the previous abrasive operations and
impart a bright finish.
The polishing operation generally takes
place in three stages, the first being to remove the marks left from
the abrasive operation.. This is achieved with the use of a stiff
sisal stitched mop and a fast cutting composition: grey or brown
compound. This is then followed by a middle cutting operation using
a white stitched mop with brown or grey polishing compound. Finally
a much softer unstitched calico mop is used and a white, blue or
rouge polishing compound.
Polishing Mops
A wide range of fabrics are used in the
manufacture of polishing mops and these, in order of hardness,
include:
- Cotton - in various qualities
from soft, unbleached cloths to fast cutting, white fabrics
incorporating special dressings, generally known as calico
mops and white stitched mops.
- Sisal - an extremely hard fibre
generally used in conjunction with an abrasive grease based
composition. The type of mop using 100% woven sisal with cloth
interleaves to protect and separate each section is called a
stapol mop.
- Scotchbright™ or Fibral™
- a low density abrasive material provides a simple and
effective method of cleaning materials and for producing a satin
finish. This is a tough, chemically resistant material composed
of tough fibres to which evenly distributed aluminum oxide
abrasive grains are resin bonded. These mops can he used dry
with relatively light pressure. They are available in a number
of grades but generally come in coarse, medium and fine. The use
of too high a mop pressure can result in overheating of the mop
face and cause fusing of the synthetic fibres.
- Leather - these mops are
occasionally used in the traditional trades for the final
finishing of non-ferrous metals. Generally chamois leather mops
are used in the jewelry trade with a rouge composition for the
polishing of gold and silver
