Backing Plate / Pad Motion
Inertial
mass [: is mainly defined by
Newton's law, the all-too-famous F = ma,
which states that when a force F is applied to an object, it will accelerate
proportionally, and that constant of proportion is the mass of that object]
To
determine the inertial mass, you apply a force of F Newton’s to an object,
measure the acceleration in m/s2, and F/a will give you the inertial mass m in
kilograms.
Centripetal and centrifugal force should
not be confused with each other, they are opposites
Centripetal
[: is a force which acts
on a body moving in a circular path and is directed towards the centre around
which the body is moving]
Centrifugal
[: a force, equal and
opposite to the centripetal force, drawing a rotating body away from the center
of rotation, caused by the inertia of the body]
Random
orbital polishers, by design, are unbalanced. You have a large mass (backing
plate/pad/spindle) that is orbiting around a central axis point. The further
the mass is away from the axis, the greater the unbalance, the more intense the
vibration becomes (see
diagram Centripetal Force)
Dynamic
balance [: occurs when the
force on all sizes of the axis are equal and then the tool operates with very
little ‘out of balance’ vibration] the uneven
distribution of mass in a rotating body contributes to the unbalance.
Centre of
gravity [: a point from
which the weight of a body or system may be considered to act. In uniform
gravity it is the same as the centre of mass.
Excessive vibration in
rotating machinery can cause unacceptable levels of noise and substantially
reduce the life of shaft bearings. Hence, the ideal would be to remove all
causes of vibration and run the unit totally smooth. Unfortunately,
in actual practice, this ideal cannot be achieved and some inherent cause of
vibration, or unbalance, will remain.
The best you can do is to reduce this
unbalance to a level that will not adversely affect the bearing life and will
reduce noise levels to an acceptable level. The unbalance is caused by an
effective displacement of the mass centre line from the true axis caused by
some mass eccentricity in the unit.
The benefits of a large orbit or
elliptical offset (15-21 mm) is that more centripetal
force is created as the backing plate orbits, so the backing plate will
rotate more than a similar machine featuring a smaller elliptical offset. A
machine featuring a large stroke delivers increased speed of backing plate
motion using the same orbits per minute.
A large stroke elliptical offset
increases movement of the baking plate / pad, which increases the speed of the backing
because they are travelling a greater distance in the same amount of time. The
resultant speed also increases the development of kinetic energy, therefore
levelling is more consistent.
This type of movement helps to remove residue (oxidized paint, spent abrasives, etc.) more readily than a small elliptical offset and extends the workable life of the pad. This culminates in a very smooth polishing action; making it extremely comfortable to use even when working for prolonged periods.
This type of movement helps to remove residue (oxidized paint, spent abrasives, etc.) more readily than a small elliptical offset and extends the workable life of the pad. This culminates in a very smooth polishing action; making it extremely comfortable to use even when working for prolonged periods.
Vibration
can cause a range of conditions called hand-arm vibration syndrome (HAVS). The
best known is vibration white finger (VWF), but vibration also links to
specific diseases such as carpal tunnel syndrome.
HAVS and VWF usually result
from damage to the fine capillaries in the end of your fingers which reduces
blood flow to the extremities of fingers and hands which take on a 'blanched'
effect appearing white.
Declaration of Vibration Emission
The Supply of Machinery
(Safety) Regulations require that, among other information, suppliers of
machinery must declare the vibration emission of their tools and machines. The
purpose of declaring such information is to allow purchasers and users of tools
and machinery to make informed choices regarding the vibration emission of a
potential purchase
The method of declaring
vibration emission is to apply a standard test to a machine or tool. The purpose
of the standard test is to provide a repeatable and reproducible method of
estimating vibration emission. This will bring the process of emission
declaration in line with the techniques for assessment of human exposure to
hand transmitted vibration as defined in EN ISO 5349:2001 and as called for in
the requirements for quantification of exposure in the Control of Vibration at
Work Regulations (2005).
Surface (Contact) Area
The distance around the circle is a
circumference. The distance across the circle is the diameter (d). The radius
(r) is the distance from the centre to a point on the circle. (π= 3.14), d =
2(r), c = d = 2 (p) r, A = π (r) 2 Even
a minor change in pad diameter makes a big difference in surface area.
A pad should be designed to efficiently
use its surface area. Foam pads that have lines, squares, circles, or dimples
cut out of the pad face, means there is less actual surface area in contact with
the paint surface. Area = π (r2) 6-inch pad area = 18.842 sq.ins, a 4-inch pad
12.46 sq.ins.
Kinetic (Heat) and machine energy
(Speed) and surface pressure applied over a smaller area, which results in faster
correction. A further consideration of pad diameter has to do with distribution
of the machine weight and applied pressure.
Another design parameter that determines
how much surface area actually contacts the paint when using foam pads is the
amount of pores per inch it features (commonly referred to as PPI). More pores,
larger pores, thinner walls between the pores, or how stiff the walls are all
affect how much foam contacts the paint during the buffing process
Block wet sanding (finishing paper and a
sanding block) which ensures a consistent pressure over the surface contact
area, this is the most effective tool for paint defect removal because of its
linear process you abrade the paint surface flat until the defects are removed.
Pad Velocity (Speed)
The larger the pad, the greater the pad
velocity at an identical RPM; V = RPM (Area) A - 6.5-inch = 20.423 sq.ins V=
24,507 inches per minute (IPM) Pad velocity is also substantially increased
with a larger diameter pad, which increase its abrasive ability at the outer
edge; 8-inch =25.136 sq.ins. V= 30,163 RPM
Pressure / Pad Compression
Depending on the types of surface
abrasions you're dealing with, increase pressure is necessary; otherwise most
of the kinetic energy of the machine will be absorbed by the pad (especially
foam) and not transferred to the paint surface.
Just remember that more pressure equals
more aggressive, so be careful around ridges and raised surfaces Maintain the
same pressure and work the product in, it may take three or four passes to
complete before the residue can be removed. Once you see the desired results
move on to the next area, or repeat the process as necessary.
The required pressure applied to obtain
optimum results to adequately compress the pad (50%) and obtain uniform
abrasion is usually in the range of 10 – 15 lbs. (a random orbital buffer
will stall at approximately 20 pounds of applied force) use just enough
pressure to keep the pad rotating at 1-2 rotations per second.
To compress a 6-inch pad 50% requires
you increase the total force by the ratio of its surface areas; Ratio = [Ï€
(radius2)] / [Ï€ (radius2)] = 2.25 as much force, almost 34 pounds).
With the smaller pad you're applying the
same force, at a constant speed but over a smaller, more concentrated area,
which will induce friction and greater abrasion abilities to the polish, both
these abilities require a certain amount of caution as it’s possible to
abrasion burn the paint.
Constant Pressure - Foam Pads have a layer of engineered, instant
rebound foam between the pad and the backing plate. This layer acts as a
cushion or shock absorber between the machine, the operator and the surface
being worked on. It absorbs off-axis motion while maintaining a constant and
uniform pressure on the surface; Lake Country Mfg. Constant Pressure technology
allows even a neophyte detailer to achieve professional-like results.
Foam Pad Size (Area and Applied Pressure)
Different pad sizes can have an impact
on how the buffer breaks down a polish, as it applies its dynamic friction over
less area, control, better manoeuvrability, and how fast you can cover an area.
Smaller pads in general will offer you
more control with any machine polisher, as it can reduce the tendency for the
buffer to hop or skip on the paint. Smaller pads also make it easier to
manoeuvre buffers in tighter areas and closer to trim pieces.
The low profile 5.5 inch buffing pads
pack the same CCS technology and performance into a compact, highly effective
size that works best with dual action polishers and air sanders. Use with a 5 -
inch moulded urethane backing plate for excellent flexibility and balance by
Lake Country (LC) manufacturing
Assuming equal speed, radius and foam
compression (50% - 15 pounds of force applied) the difference between 4- inch
and 6 - inch pads is their different surface area = π (r2) (4-inch = 12.46
sq.ins / 6-inch = 28.26 sq.ins) and therefore surface kinetic (or dynamic)
friction applied and surface pressure applied 4-inch = 3.75 lbs per sq.ins. -
6-inch = 2.5 lbs per sq.ins. Even a minor change in pad diameter makes a big
difference in surface area.
Actual Speed (RPM)
Formula - RPM (C) = V
Speed = Revolutions per minute, C =
circumference (2(π) (r)), V = velocity
Area 7.5 -inch at 1,800 rpm (usable area
6- inches)
Speed - 1,800 (rpm) (6" diameter
times pi = 6 -inch x 3.14) x 18.84 = 33,912.
So, using the same formula, and a
problem of: X (unknown rpm) x 25.12 (8" diameter times pi = 8 x 3.14) =
33,912) then; 33,912 / 25.12 = 1,350 actual speed rpm
Kinetic Friction
[
: when contacting surfaces move relative to each other, the friction between
the two surfaces converts kinetic energy into thermal energy, or heat]
Friction is the force resisting the
relative lateral (tangential) motion of solid surfaces, fluid layers, or
material elements in contact. It is usually subdivided into several varieties:
Dry friction is also subdivided into
static friction between non-moving surfaces, and kinetic friction (sometimes
called sliding friction or dynamic friction) between moving surfaces.
Force diagram
Arrows are vectors indicating directions
and magnitudes of forces. W is the force of weight, N is the normal force, F is
an applied force of unidentified type, and Ff is the force of kinetic friction
which is equal to the coefficient of kinetic friction times the normal force.
Since the magnitude of the applied force is greater than the magnitude of the
force of kinetic friction opposing it, the block is moving to the left.
Heat from Kinetic (or dynamic) Friction
[Energy
in a system may take on various forms (e.g. kinetic, potential, heat, light).
Kinetic friction, or surface resistance induced heat; an often misunderstood
concept of polishing / compounding; abrasives require friction to breakdown,
not heat; heat is just a resultant of friction between two surfaces. Kinetic
friction is required to ‘level’ paint, which is simply the removal of paint
to the lowest point of the paint defect] [1]
Energy in a system may take on various
forms (e.g. kinetic, potential, heat, light). Kinetic friction, or surface
resistance induced heat; an often misunderstood concept of polishing /
compounding; abrasives require friction to breakdown, not heat; heat is just a
resultant of friction between two surfaces, besides the most commonly used
abrasives include varieties of aluminium oxide, silicon carbide, diatomaceous
earth, clay, and silica, to produce enough heat to cause a reduction in size
would harm the paint. Kinetic friction is required to ‘˜level’ paint, which is
simply the removal of paint to the lowest point of the paint defect.
A finishing pad will not provide as much
friction as a cutting foam pad (less surface resistance) although they will both
produce friction induced heat, whereas a wool pad, due to their composition,
creates less friction induced heat but more kinetic friction (due to its
fibrous structure) than most foam pads.
Polishing a paint surfaces transfer’s
kinetic friction induced heat to the paint surface, thermoplastic polymers have
both tensile strength (a linear stress-strain relationship) and elongation
(elasticity) which allow the surface to flex, expand and contract in accordance
to surrounding temperatures, solvents, resins and other ingredients in polishes
will expand causing the paint film surface to expand
As the metal substrate expands the paint
moves with it, due to its elasticity, thereby becoming elongated (thinner) this
is part of the cause of friction induced ‘burn’, you’re applying pressure and
an abrasive to a less dense (thinner’) paint surface, excess friction induced
heat can cause the paint surface to burn, blister, haze, and cause excessive
swirls
Plastic has a much lower rate of thermal
conductivity than metal, so it absorbs heats at a far greater rate.
Polishes and compounds do not need heat
per se for the abrasives to polish a surface, wither they be diminishing or
non-diminishing abrasive, they require both pressure and friction
Kinetic Friction induced heat can cause
a rapid temperature rise; (i.e. initial surface temp 80.oF, friction heat
attained with the polisher stationary and a cutting foam pad at 1,100 RPM for
approx. ten seconds the friction induced heat attained would be around 104.oF)
the paint temperature can be checked by utilizing an instant read-out infra-red
‘gun type’ digital thermometer, paint surface ‘spot’ temperature should be
limited to 110.oF <
In accordance with the Society of
Automotive Engineers (SAE) a localized (spot) temperature of > 115.oF will
cause the urethane clear coat to soften and the foam pad will cause scratching
that is forced deep into the clear coat. (See also the first law of thermodynamics
et al)
Applied Pressure
The pad needs to have an even
distribution of pressure applied to it; depending on the types of surface
abrasions you're dealing with, increase pressure as necessary. Just remember
that more pressure equals more aggressive, so be careful around ridges and
raised surfaces
Maintain the same pressure and work the
product in, it may take three or four passes to complete before the residue can
be removed. Once you see the desired results move on to the next area, or
repeat the process as necessary.
The required pressure applied to obtain
optimum results to adequately compress the pad (50%) and obtain uniform
abrasion is usually in the range of 10 – 15 lbs. (a random orbital buffer
will stall at approximately 20 pounds of applied force) To compress a 6-inch
pad 50% requires you increase the total force by the ratio of its surface areas
Ratio = [Ï€ (radius2)] / [Ï€ (radius2)] = 2.25 as much force, almost 34 psi).
With the smaller pad you're applying the
same force, at a constant speed but over a smaller, more concentrated area,
which will induce an increase in friction and greater abrasion abilities to the
polish / pad combination, both these abilities require a certain amount of
caution as it’s possible to ‘strike through’ (friction burn) the paint.
Polishing Freshly Applied Paint
When a urethane clear coat is sprayed
its outermost surface, measuring a few nanometres in thickness, sustains
microscopic fractures when it comes into contact with air. These fractures are
microns or nanometres in width and thus too small to be seen with the unaided
eye.
Freshly applied paint that in the outgas
stage, is still full of evaporating solvents, and is usually less dense (soft)
despite the additives used (hardener) once a catalyst, kinetic energy (friction
heat) is added, it causes the paint film to expand, temporarily hiding
scratches, this is often the reason for a body-shops bad reputation of
returning vehicles that have sanding scratches in newly applied paint that
should have been removed.
Be cognizant when polishing newly
applied paint the kinetic energy (heat) from a foam pad can also cause solvent
engorgement, which causes the paint film to thin due to the expansion of the
evaporating gases, applied rotational force may also cause the paint to tear
Kinetic friction (heat) is transferred to a solvent (IPA or fresh paint
solvents) causing it to both expand (Charles' law; also known as the law of
volumes) the paint film and soften it.
Automotive paint is classified as a
semi-permeable membrane; it has both tensile strength and elongation
(elasticity) newly painted surfaces are soft and full of out gassing solvents,
resin binders and additives, as well as and water.
Polish contains solvents, which soften
the paint film, kinetic surface friction and applied downward pressure
transfers its energy into heat / torque (force to rotate an object about an
axis), which could result in the alteration of the paint films bond between its
substrate, causing it to delaminate or tear?
The heat makes the gasses expand (pV =
nRT) the expanding gases go through a phase transition (change in density) and
to relive this increased pressure they (a) rupture the paint film surface,
causing small fissures (similar to solvent pop) The heat may cause the gaseous
vapours to expand, but not enough to break through the hardening clear coat.
Once the vapour has evaporated, it may leave a void between the basecoat and
the clear. Therefore you have a cloudy spot where the clear and base is no
longer adhered together. If this is the case, the clear coat will delaminate in
the future.
Once the outgas process is complete
automotive coatings (paint) becomes a semi-solid permeable membrane, Being a
polymer (elastomers) it remains flexible while retaining its tensile strength,
to enable it to expand and contract to follow temperature fluctuations of the
substrate (elongation). Kinetic friction and its associated heat can cause a
rapid temperature rise (i.e. initial surface temp 80.oF, heat attained with a
cutting foam pad at 1,100 RPM for approx. ten seconds is approx. 104.oF) the
paint temperature can be checked by utilizing an instant read-out infra-red
‘gun’ thermometer, paint surface ‘spot’ temperature should be limited to
110.oF <
Applied Pressure
The pad needs to have an even
distribution of pressure applied to it; depending on the types of surface
abrasions you're dealing with, increase pressure as necessary. Just remember
that more pressure equals more aggressive, so be careful around ridges and
raised surfaces
Maintain the same pressure and work the
product in, it may take three or four passes to complete before the residue can
be removed. Once you see the desired results move on to the next area, or
repeat the process as necessary.
The required pressure applied to obtain
optimum results to adequately compress the pad (50%) and obtain uniform
abrasion is usually in the range of 10 - 15 lbs. (a random orbital buffer will
stall at approximately 20 pounds of applied force) To compress a 6-inch pad 50%
requires you increase the total force by the ratio of its surface areas
Ratio = [Ï€ (radius2)] / [Ï€ (radius2)]
= 2.25 as much force, almost 34 psi). With the smaller pad you're applying the
same force, at a constant speed but over a smaller, more concentrated area,
which will induce an increase in friction and greater abrasion abilities to the
polish / pad combination, both these abilities require a certain amount of
caution as it’s possible to ‘strike through’ (friction burn) the paint.
Hardness
So how can a dense (hard) clear coat be so easily scratched?
It’s a matter of physics, not material
density (material hardness). Force acts through a body that has a surface area;
if the surface area is really small while maintaining an equal force, the
pressure becomes astronomical and the object under pressure capable of
penetrating the surface of an otherwise tough material.
Newton's third law of motion [:
when a first body exerts a force F1 on a second body, the second body
simultaneously exerts a force F2 = −F1 on the first body. This means that F1
and F2 are equal in magnitude and opposite in direction]
That’s why a micro fine thread that is
twice as fine as silk and a 100 times finer than a human hair, in an otherwise
soft towel will scratch your paint. And the same reason a mosquito can
penetrate a rhino hide with its proboscis (stinger).
If you press down on your paint finish
with your palm it feels really hard and tough, but that’s because the surface
area of your palm is relatively large and what you’re actually feeling is the
resistance of the steel underneath the paint. Try pressing your thumb nail into
the paint with the same amount of force you used with your palm, if you dare.
Bibliography
1.
Correlation between
vibration emission and vibration during real use - Polishers and sanders. Prepared
by the Health and Safety Laboratory Health and Safety
Executive 2007
I
would like to think that these articles become an asset to anyone who is new to
detailing and to professionals alike, as well as industry experts who seek to
advance their knowledge.
I hope the
above article was informative. By having some understanding of the ‘What’ and
‘Why’ as well as the ‘How’ along with a little science to help you understand
how the chemicals we use react, you can achieve the results you desire.
I would appreciate it if you would share this article as it helps other detailers further their knowledge. Questions and/ or constructive comments are always appreciated.
I would appreciate it if you would share this article as it helps other detailers further their knowledge. Questions and/ or constructive comments are always appreciated.
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