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Grinding Techniques: In Thousands Of Years, The Basics Of Grinding Haven’t Changed

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Grinding is one of the oldest methods known for shaping and sharpening
objects. It was first used in prehistoric times to make weapons
and other tools by rubbing wood, stones, bones and eventually
metal against hard, abrasive rocks.
In thousands of years, the
basics of grinding haven’t changed. We’re still rubbing an abrasive
against metal to change its shape or polish its surface – which
probably explains why many people still call grinding wheels "rocks."
But rocks they aren’t.
A modern grinding wheel is a precision
cutting tool that contains a sophisticated mix of various abrasives
and other ingredients that are carefully blended and processed
to deliver specific cutting qualities. Rebuilders today use a
variety of abrasive wheels rotating at high speeds to grind crankshafts,
camshafts, flywheels, clutch plates, valve faces, stems and seats,
and dozens of other parts.
The basic techniques for grinding
and resurfacing haven’t changed a great deal over the years, but
recently the pace of evolution has quickened as the need for faster
production speeds, higher quality finishes and increased productivity
have taken on increased emphasis and importance. In a highly competitive
market, rebuilders need every advantage they can get, so abrasives
are getting better and better, and more application specific.

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There’s a much broader spectrum of abrasives available than ever
before, including a new generation of blends and superabrasives
that may revolutionize grinding. These new superabrasives might
aptly be called "jewels" rather than rocks because of
their high performance (and price!).

Superabrasives

As tooling speeds increase, along with the demand for increased
productivity, less downtime and higher quality finishes, superabrasives
such as diamond and cubic boron nitride (CBN) may find a growing
range of applications in the aftermarket – at least that’s what
some people are saying.

"We see diamond and CBN emerging as superabrasives because
of their excellent cutting properties," said Jerry Qualiana
of K-Line, Holland, MI. "We’re doing a lot of research and
development work on future abrasives because the aftermarket needs
unique products.

Qualiana says that diamond and CBN could play an increasingly
important role in more grinding applications, as they already
do now in cylinder honing and resurfacing, but the high cost of
these materials is still hard for many rebuilders to justify.
So there hasn’t been much demand for them yet.

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Scott Biesanz at Goodson Shop Supplies, Winona, MN, said superabrasive
grinding wheels are available today, but are considered too expensive
by the typical aftermarket buyer. A typical 28″ diameter, 1″ thick
general purpose crankshaft grinding wheel that costs about $300
(jobber) in a conventional abrasive might cost twice as much in
a superabrasive. "The higher cost of superabrasives can be
easily justified in high volume production and manufacturing applications,
but the typical aftermarket user doesn’t see the cost benefit,"
said Biesanz.

According to Cincinnati Milacron, a manufacturer of grinding abrasives,
superabrasives can provide exceptional wear ability, reduce grinding
time and help eliminate common problems like burn, part-to-part
variations, and excessive downtime due to wheel dressing.

Diamond, the hardest material known, is three times harder than
its conventional abrasive counterpart, silicon carbide. That’s
why a diamond is used to dress a conventional grinding wheel.
Diamond also has a thermal conductivity that’s about six times
that of silicon carbide and aluminum oxide, which permits high
speed grinding speeds without excessive heat buildup in the workpiece.

CBN is second in hardness only to diamond; it is 2.5 times as
hard as its conventional counterpart, aluminum oxide. CBN can
withstand higher temperatures than diamond (2,500° F versus
1,300° F for diamond), and is also a good conductor of heat,
having a thermal conductivity about four times higher than silicon
carbide and aluminum oxide.

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Less expensive than diamond, CBN is a popular superabrasive in
industrial and manufacturing applications today because of its
ability to grind hard steels, superalloys, cast iron and powdered
metal. It is generally superior to both diamond and aluminum oxide
for grinding hard steels (except tungsten carbide, which diamond
handles best), and is used for production grinding of bearing
bores, crankshafts, camshafts and many other parts that require
high quality, precision finishes including fuel injection components.

One of the qualities that has made CBN so popular in such applications
is its ability to operate at higher than normal wheel speeds –
up to 6,500 to 20,000 surface feet per minute (SFPM). Vitrified
CBN wheels show increased grinding efficiency at higher wheel
speeds, yet can also handle speeds as low as 2,500 SFPM in applications
that require small grinding wheels.

The SFPM speed of a wheel can be calculated by multiplying 3.1416
times the diameter of the wheel (in inches) times the rpm and
dividing by 12.

Cincinnati Milacron says CBN really excels in camshaft and crankshaft
grinding, as well as centerless valve grinding. In one case study
the company undertook, changing to CBN abrasive for valve stem
and seat grinding showed a measurable improvement in seat roundness,
stem roundness and straightness. Cycle times were reduced to
six seconds per valve seat and 2.55 seconds for the valve stems.
The need for wheel dressing was also reduced, with an average
of 800 parts per dress on seats and 1,000 parts per dress on stems.
With higher wheel speeds, the parts per dress on stems increased
to 4,000.

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In another case study, a CBN grinding wheel was used to grind
the lobes on camshafts. Changing to the superabrasive reduced
the need to dress the grinding wheel from once every 50 cams to
once every 130 cams. CBN also eliminated any trace of burning
and produced more consistent finish and overall quality.

Blends & bonds

One thing that anyone who has looked through a shop supply catalog
lately will notice is the wide variety of grinding wheels and
abrasives that are available. The days of choosing between a green
(silicon carbide) wheel for grinding cast iron, and a white (aluminum
oxide) wheel for grinding hard steel are gone. Today, there’s
a full spectrum of abrasives for virtually every conceivable need.
Colors range from white to gray to green to black to rose to pepper.

Take valve seat refinishing stones, for example. In one supplier’s
catalogs, there are six different stone types from which to choose:
general purpose gray stones for rough or finish grinding cast
iron seats, ruby stones for general refacing as well as for hardened
seats, blue stones for high performance beryllium seats, pepper
stones for nickel chrome seats, white stones for stellite seats,
and fine grit gray stones for general purpose finish refacing.

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But the color of the wheel is only a small part of the story.
There are a lot of variables that determine the cutting properties
of any given abrasive, including the type of abrasive (aluminum
oxide, silicon carbide, CBN or diamond), the hardness and friability
of the abrasive, the grain or grit size of the abrasive, the type
of bond material that binds the abrasive together (vitrified or
resinoid), the amount of bond in proportion to the abrasive (the
"grade" of the wheel), and the relative spacing of the
abrasive particles with respect to the bond (the "structure"
of the wheel).

On every grinding wheel are manufacturer’s markings that indicate
the type of abrasive, grit size, grade, structure and bond type.
Lower grit numbers indicate coarser grit size while higher numbers
indicate a finer grit size. A finer grit size provides the best
finish and also allows better penetration when grinding hard metals.
Coarse grit is generally better for unhardened metals and for
grinding applications that require large areas of contact between
the wheel and workpiece. Most automotive grinding wheels use grit
sizes between 24 and 80.Grade ratings refer to the amount of bond
material in the wheel, and are designed by letters A through Z.
"A" represents the softest end of the spectrum while
Z represents the hardest wheels. Soft wheels work well on hard
metals, but wear quickly. Hard wheels are better on softer metals,
and generally produce a finer finish and longer life.

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The structure rating of a wheel, which ranges from 0 (most dense)
to 12 (open) indicates the spacing of the abrasive grains in the
wheel. The structure of a given wheel tends to be standard for
a given grit size and grain, but some have more open spacing to
provide cooler, faster cutting action in dry grinding applications.

The color of the wheel depends on the type of abrasive in the
wheel, as well as the impurities mixed with it when it is manufactured.
Pure aluminum oxide is very friable, which makes it well suited
for grinding tool steels and other heat sensitive steels.

There are also semi-friable versions of aluminum oxide. Some may
contain a sodium impurity which gives it a unique pinkish beige
color and helps it fracture more easily. Others may contain a
chromium impurity (pink color) which makes it better suited for
grinding heat sensitive alloy steels and chrome alloys. Combinations
of pure aluminum oxide and various semi-friable versions may offer
improved grinding performance for applications where burning with
pure aluminum oxide is a problem.

According to Steve Rupe, an engineer with Radiac Abrasives, an
abrasives manufacturer in Salem, IL, there’s much greater use
of mixtures than ever before. "The old standard catalog products
are not the answer to today’s grinding needs," said Rupe.
"Customers today want products that are specifically engineered
to their particular application.

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"Abrasive manufacturers are offering more customized products
than ever before to satisfy the demands of the market. By adjusting
the various proportions of different aluminum oxides in a wheel,
we can significantly alter its cutting characteristics."

Rupe said that increasing the percentage of friable aluminum oxide
in a grinding wheel helps keep temperatures down to reduce burning.
It also increases the cutting action of the wheel as well as the
wear. Adding more semi-friable aluminum oxide to the mix improves
durability and wheel life, but the wheel won’t cut as freely or
run as cool. Rupe said that most of the blending that’s being
done is with different grades of aluminum oxide. There aren’t
many instances where silicon carbide is being mixed with aluminum
oxide.

Another area where a lot of work is being done to improve and
customize grinding wheel performance is on the bond. The bond
is the material that holds the abrasive grit together. The bond
itself does no cutting, but holds the grit until it tears away
to expose fresh grit underneath. The secret to good grinding performance,
therefore, is designing the bond so that it holds the grit until
the grit dulls before it allows it to break away.

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If the bond is too strong and holds the grit too long, the wheel
may become dull, glazed, cut poorly and burn the metal. It may
also clog up with debris. On the other hand, if the bond is too
weak, the wheel will wear too quickly resulting in short wheel
life.

The strength of the bond also affects the surface finish on the
metal. A strong bond will produce a finer surface finish while
a weaker bond will leave a rougher finish. So by using various
bond strengths and mixes of abrasives, a wheel manufacturer can
design a wheel to match virtually any application.

Most automotive grinding wheels have a vitrified bond because
it is strong, rigid and can withstand high temperatures, oil and
water. A vitrified bond is a glass-like ceramic material made
of clay, fluxes and other ingredients. During the manufacturing
process, the bond material is carefully mixed with the abrasives
in measured proportions. The resulting paste is then pressed into
a mold (called "greenware"), and fired in a high temperature
(more than 2,300° F) kiln for 24 to 180 hours depending on
the size, density and ingredients in the wheel. The length of
the baking cycle is critical to achieve complete vitrification
and a fully cured bond.

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Resinoid bonds, by comparison, use an organic resin (powdered
phenol formaldehyde) to hold the abrasive particles together.
This type of bond provides better mechanical and thermal shock
resistance than vitrified bonds, and is often used with diamond
abrasives for cut-off grinding, flute, tap and drill grinding.
Resinoid wheels are used primarily for rough grinding or where
wheel speeds are high. But resinoid bonds are limited to low temperature
(less than 350° F) applications. Resinoid bonds can also
soften by prolonged exposure to water.

Metal bonds are also used in certain applications, but primarily
with diamond for special applications like cylinder honing. The
metal bond in this case helps absorb and dissipate heat generated
by the diamond.

Selecting the wheel

One thing most suppliers agree upon is that the automotive market
requires unique abrasives. Generic general purpose abrasives have
their place, but today’s generation of specialized abrasives can
do a far superior job in most cases. But choosing the "right"
abrasive for a particular application takes product knowledge
and a familiarity with what’s available.

"A lot of people who are grinding today are not aware of
all the improvements that have been made in recent years in grinding
technology. These people could easily improve their productivity
by taking a closer look at what’s available and choosing wheels
that are much better suited to their needs," said Radiac’s
Ron Reed."Take crankshaft grinding. We have a new bond system
for our premium 43A line of abrasives that is specifically designed
for this market. The wheel requires less dressing and won’t load
up like an ordinary wheel."

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Jack Roche of Bay State Sterling, a Westborough, MA-based abrasives
manufacturer, said the type of grain that’s used in a grinding
wheel is dictated by the metal you’re grinding. "When grinding
a soft metal, you want a grain that’s less friable. For harder
metals, you want a grain that is more friable. But you also have
to consider the bond and structure of the wheel before you can
make a specific recommendation."

Goodson’s Biesanz says customers want performance at a price,
but don’t always know how to pick the best wheel for a particular
application. "The general rule is that the harder the material
you’re grinding, the softer the wheel should be, he said. "But
when the wheel heats up, the bond goes away and the grain opens
up. The wrong wheel can load up, burn the metal and fishtail.
So we often help customers solve a grinding problem by recommending
a wheel that better suits their needs."

K-Line’s Qualiana says, "Today’s smaller displacement, higher
revving engines have parts that are different from those in the
typical 350 Chevy V8. They have smaller, harder valves that require
a different composition grinding wheel. The same goes for crankshafts.
Steel cranks need a different grinding wheel than ordinary cast
iron cranks.

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"It’s the same with flywheels. Flywheel resurfacing is a
big business today and it’s profitable, too. But you need one
type of abrasive for cast iron and another for steel. So when
choosing a grinding wheel, you have to look at all the variables,
not just the hardness of metal. You have to consider the type
of equipment that’s being used, grinding speeds, surface finish
and so on before you can determine the best wheel to use."

Randy Neal at Atlas Shop Supplies in Norcross, GA, says he sells
predominantly Norton wheels (made by Norton Co., Worcester, MA).
"They must have 200 different combinations of abrasives available,"
said Neal. "We try to target our mix of wheels to what our
customers really want. Industrial users have formulas for determining
an exact abrasive to use, but that’s pretty uncommon in the aftermarket.

"If you were going by technical correctness, you might need
60 different wheels to cover all the different crankshaft applications.
But the typical shop only has four or five different wheels with
little or no variation in grits. That’s real world. So we try
to help them pick a wheel that will give them the best results."

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Common grinding mistakes

Using the wrong type of grinding wheel for a particular application
can cause any of a number of problems as well as reduced wheel
life. But the number one mistake that many users don’t realize
they’re making is failing to dress their grinding wheel properly.

A crankshaft grinding wheel should always be dressed with coolant,
never dry. Diamonds can’t take a lot of heat and can burn or become
dull if no coolant is used during the dressing procedure. Dry
dressing can shorten the life of the diamond by a factor of 10!
Coolant flow should be continuous. Stopping the flow can cause
the diamond to overheat and shatter when the coolant starts flowing
again.

The diamond should be sharp. Once it gets a .030″ flat or a
dome, it needs to be reset or replaced (some suppliers can reset
a diamond to save the cost of having to replace it). Premium grade
dressing diamonds have more facets per stone than standard grade
diamonds. They keep their sharpness longer to provide a better
dress and longer life.

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The diamond also needs to be sized to the wheel that’s being dressed.
For a typical crankshaft grinding wheel, a one-and-one-half to
two carat diamond is generally recommended.

The traverse speed of the diamond across the wheel has a major
affect on wheel finish. Too rapid a speed causes a rough finish
that makes the wheel act more aggressive (cuts faster). Yet a
fast dress can actually help a hard wheel act softer and cut better.
A slow dress has a tendency to close the wheel face and produce
a very smooth surface. Though this may be necessary if you’re
trying to achieve a fine finish on the workpiece, it reduces the
wheel’s cutting ability.

How often the wheel needs to be dressed depends on the type of
grinding that’s being done and the quality of the finish that’s
required. For rough grinding, the wheel usually needs to be dressed
less often. But for high quality finish grinding, more frequent
dressing is a must.

Clues that may indicate a need to redress the wheel include changes
in color, chatter marks on the workpiece, a change in the sound
of wheel while it is grinding, an increase in the power (amps)
needed to drive the wheel, and/or the beginning of a deformation
in the workpiece causing an irregular shower of sparks.

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Dressing a wheel more frequently will obviously shorten its life.
Even so, a wheel should not require more than one or two passes
at a depth of .001″. A deeper dress can actually damage a wheel
by fracturing the bond, causing the wheel to break down more quickly.

Ideally, a crankshaft grinding wheel should be dressed after every
use (especially if sweep grinding). This would, in the opinion
of some experts, eliminate most of the burnish, shape and taper
problems that are frequently encountered by crankshaft grinders.
At the very least, most of these experts recommend dressing the
wheel after grinding two or three crankshafts.

Another mistake that’s often made is using the wrong type of coolant,
or no coolant at all. Virtually every supplier and manufacturer
we interviewed recommended wet grinding over dry grinding. The
primary reason for using coolant is to keep the metal surface
from getting too hot, which can cause burning, cracking, poor
surface finish and distortion (out-of-round). Coolant also helps
flush away swarf to keep the grinding wheel from loading up. Coolant
also helps lubricate the wheel so it provides a more uniform cut,
and rust-inhibitors in the coolant help protect metal surfaces
from corrosion and discoloration.

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Either synthetic or oil-based coolants can be used, but many prefer
synthetics because they won’t spoil over time. It’s important
to make sure the coolant that’s being sprayed on the workpiece
is actually getting between the wheel and metal. A large grinding
wheel can have a surface speed that exceeds 70 mph. At such speeds,
the wheel tends to drag along a surface layer of air that forms
a bubble which blows coolant away from the grinding area. The
use of an air scraper on the wheel ahead of the coolant nozzle
can break up this air bubble to help keep the coolant where it
belongs.

Grinding safety

A variety of publications covering grinding procedures and safety
are available from the Grinding Wheel Institute, Cleveland, OH,
(216) 899-0010. Publications cover mounting procedures, general
safety recommendations, safety requirements and standards for
grinding wheels, even cost reduction principles in grinding.

Some general safety precautions include:

  • Vitrified grinding wheels can be cracked by mishandling, mechanical
    or thermal shock. Because of the high speed at which they rotate,
    cracks may cause a wheel to explode. Wheels should always be inspected
    prior to use. Tapping a vitrified wheel lightly on the side should
    produce a ringing sound. If the wheel doesn’t ring, it’s cracked
    and must be replaced.

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  • Eye protection should always be worn by
    the operator.

  • Wheels are designed to operate within
    certain speed limits. Never exceed the maximum spindle speed rating
    for the wheel.

  • Wheels should be trued and balanced when
    first installed, and rebalanced as needed to compensate for wear.
    If a wheel cannot be rebalanced because of wear, it should be
    replaced.

  • All wheel and machine guards must be in
    place before grinding. The operator should never stand in a direct
    line with the wheel when the equipment is first started.

  • When making contact between the grind
    wheel and part, contact should be made gently without bumping
    or gouging.

  • Grind only on the face of a straight wheel,
    never the side.

  • Never force grind so that the motor slows
    noticeably or the work gets hot.

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