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Are the various colored grinding wheels just a marketing
ploy of the various manufacturers to confuse us wood turners? Isn’t
one grinding wheel as good as another? Sure the stock gray wheels are no
good but why so many various colored grinding wheels? What grade of
grinding wheel is best for wood turners? What grit should we be using? Woodturners are
asking these questions and below are several responses collected from
newsgroups, emails, and various websites. Reprinted here with the kind permission
of the various authors.
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I would be happy to make myself available to you and your peers as a
source for technical information as the need arises.
'FRIABILITY' (Friable) - One of your postings [below] discussed this
term as a measure of how quickly the grain is released from the wheel
(I am
paraphrasing) in respect to the bond. This is only partially true.
To be
more specific, friability refers to a grains ability to fracture under
grinder pressure. Each grain type (of which there are several for
AO and Silicon Carbide, respectively) has a structure which translates
into many characteristics. One of these characteristics is
'tensile strength'. The
grain mentioned in many of your postings referred to White AO.
This is the purest form of AO and therefore is the most 'friable'.
One should always keep in mind that a friable grain does not necessarily
mean that it will act softer than a tougher, less friable grain.
HARDNESS: All wheels, regardless of manufacturer, have a relative
hardness factor associated with the alphabetical scale.
Theoretically speaking, this scale ranges from A-Z. Practically
speaking, this scale ranges from H - M.
Contrary to popular belief, a wheels hardness is not determined by the
hardness of the bond used. Relatively speaking, a wheels hardness
is
determined by the amount of bond, as a percentage, used. Less bond
and more grain equates to a softer wheel. It is true that some
bonds are harder than others, however, the wheels you are using
purchased from stock are all made with general purpose "tool
room" bonds that behave consistently from one manufacturer to
another.
When selecting a wheel, keep in mind that the wheel diameter has a huge
impact on how a specific hardness performs. Someone who may use
6" wheels, for example, may recommend a 'white' AO wheel in a J
hardness for example. However, if you personally use a 7" wheel,
your SFM (Surface feet per minute) is much higher. The faster the
surface of the wheel travels, the harder a certain specification will
act relative to that same specification in a smaller, slower traveling
wheel.
Also, be very careful when attempting to order a wheel from an
alternative
brand whose hardness is similar to the wheel you find works best for
your
application. While we all utilize the same rule, an H hardness
from Norton
(for example) does not equate to an H from Bay State, Camel, Pacific,
etc.
It may, but usually what one manufacturer's H hardness is another
manufacturer's G or I hardness.
Finally, be careful not to ignore new grinding technologies that can be
applied to even the most basic grinding machines (bench grinders).
By and large, the industrial knife sharpening industry has moved away
from AO wheels to 'Plastic' or Epoxy wheels. These wheels cut much
cooler because they break down from heat, acting as a heat-sink by
keeping the thermal energy created by the grinding process from being
transferred to the tool. Vitrified wheels on the other hand (which
is the bond class you are currently most likely using), are really
nothing more than a porcelain and break down from grinding pressure
rather than heat.
As skilled craftsmen and women, you must not ignore the grade of tool
steels used in the manufacture of your tools (there are close to 50).
Knowing this, you can better identify the type of wheel (grain type,
bond, and hardness) that will best respond to the tool. The
Rockwell C ratings of these tool steel varies greatly, and different
grains (other than the white your postings refer to frequently) are
designed with characteristics to
accommodate the grinding characteristic inherent in the various tool
steel.
I hope that this information is helpful and I would welcome anyone with
specific questions to contact me for more information. I would be
more than happy to help when are how I can.
Paul Ketcham
PEKetcham@aol.com
Pacific Grinding Wheel
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Jerry Glaser wrote:
Let me answer your questions from my limited experience
as a user of grinding wheels. Craft Supplies in Utah is selling a grade
of Norton grinding wheel based upon some recommendations from me. After
trying a number of wheels made by several different makers I settled on
the SG grade (blue) made by Norton. This is their best grade. The blue
color is, I suspect, for identification only. All the wheels Norton and
other makers recommend for grinding hardened tool steel use aluminum
oxide grit for the abrasive. The manner in which this grit is made
determines its effectiveness. In the case of the Norton SG wheels, the
grit is made by a relatively new process. In conventional wheels the
grit particles are single crystals, but in the SG wheels each grit is
composed of many, much smaller, grit particles or crystals. As a SG
wheel wears, new sharp grit surfaces are exposed and the wheel retains
much, not all, of its initial sharpness and shape. I found that I can
grind many more tools and faster with the SG wheels before redressing is
needed than with any other grinding wheel I've tried. The time saved is
worth the extra cost to me. You get what you pay for!! A grinding wheel
is composed of the grit particles with a ceramic binder. The whole wheel
is vitrified in a kiln. The porosity is controlled by the addition of
ground powder made from, I'm told, walnut shells, that burn off leaving
voids. For the record: I use an 8" dia. x 1" wide, 46 grit
grinding wheel running at 3000rpm on a belt driven arbor. I occasionally
true it with a multi-stone diamond from Craft Supplies and DRESS it with
a star wheel dresser. The last operation is critical for fast and COOL
grinding of tools. If you let a grinding wheel get glazed it will grind
much slower and get the tool bevel much hotter. I just measured the time
it took to grind the bevel on seven CPM 15V deep gouges starting from a
blank. Using a freshly dressed wheel, the first bevel took 1:22 min. and
the seventh bevel (no redressing of wheel) took 2:38 min. Normally I
would have redressed the wheel after grinding the fourth bevel. I grind
a lot of tools, but a SG wheel lasts me about six months before it wears
down to a six inch diameter.
For more information re grinding wheels try: Norton at http://www.nortonabrasives.com
Click on: Charts, Product Features, etc. Norton is a division of a
French company - De Saint-Gobian. They're world wide and accounts for
such things as Norton wheels made in China. In Nortons rating of
grinding wheels, the BEST are five times better than GOOD. And the best
are the SG grade.
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Roy Hauer wrote:
White colored wheels are generally used on surface grinders, or for
grinding hard steels and alloys (up to say 65+ rc. The general run of
the mill gray colored wheels are aluminum oxide, and this type is
generally found on most bench and pedestal grinders. The white wheel
is also a mix of alum oxide and another material.
Green wheels are normally silicon carbide, and used for grinding
brasses, carbides, and other extremely hard materials.....yes, I said
brass, and they can also be used for aluminum, but the recommended
procedure is wet // coolant grinding when using on alum or brass..
There are also ruby red wheels (not ruby just colored a red or pink,
as well as blue or violet colored. These last two are usually used in
toolrooms and surface grinders, cylindrical and centerless grinding.
You will find that if you grind chisels or any really hard materials
on a bench grinder that the white wheels will work better than the
gray alum ox. I just received over 200.00 worth of wheels from MSC,
and I bought their and always have bought their brand and they have
always been made in North Carolina by Colonial Abrasives. They are
some great wheels. I have had mixed results with wheels produced in
China and sold under the Norton name. They seem to not be as friable,
and glaze up and clog quicker than an American Norton or Colonial or
Carborundum or Camel brand does. I usually shy away from Far East
import wheels for this reason. Camel is a very good brand wheel.
Rutland tool, Penn Tool, MSC and ENCO all sell the above wheels. Don't
pay the 100.00 price as that’s a rip, and US made Norton can be had
for
much less. I assume you have either a 6 or 7 inch bench grinder.
(most common sizes)
Another brand to shy from is what W W Grainger sells, and Craftsman
brands. MSC house brands are Colonial as I stated earlier, and I have
always received Colonial for the last 7 or 8 years when I order them.
Most of the sales clerks in these places are nothing more than order
takers anyhow. Get a copy of MSC or Rutlands big catalog, and read up
on the grades and types. There is a lot of info on grinding wheels
most don't realize.
Don't forget that before mounting, give the wheels a ring check, and
after mounted, allow it to run free without a load for a couple of
minutes, then dress it true......as sometimes arbors and bushings are
not right on the money.
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Steve Tiedman wrote:
When a grinding wheel is easily friable, it means that
the very surface of the wheel, when making contact with the tool in the
sharpening process, will actually wear away easily. The part wearing
away is the "dull" grinding abrasive of aluminum oxide. The
binding agent holding the aluminum oxide particles together is what
crumbles, taking away the dulled particles of aluminum oxide, thus
exposing fresh, sharp particles of aluminum oxide.
This does not mean that you don't have to "dress" the wheels
every few times you grind on them. Dressing the wheels makes a
consistently flat surface, taking out any grooving, and removes any
dulled surface of the wheel material, along with removing metal
particles that are ground into the surface of the wheel.
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David Eisan wrote:
Friable is a new word to me in this context, I had to
look it up,
fri·a·ble
adj.
easily crumbled: Dry soil is friable.
D< L friabilis < friare crumble
-'fri·a'bil·i·ty,
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Russ Fairfield wrote:
"Truing" the grinding wheel makes it round,
concentric with the shaft, and flat across the surface. This means that
a fairly large amount of grit is removed from the wheel.
"Dressing" prepares the surface by exposing sharp new abrasive
particles. Only the outer layer of abrasive particles are removed, and
the old fashioned, low tech, star-wheel dressing tool does this with the
minimum removal of grit from the wheel. It isn't necessary to
"true" the wheel every time you "dress" it. But, if
you are using one of the diamond jigs, you are doing both at the same
time, but some of us remove more from the grinding wheel than is needed.
If you do a lot of tool sharpening (equates to a lot of turning) there
is no
set-up time to using a "dressing wheel", and the grinding
wheels will last
longer because you aren't removing as much from them each time.
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Brian McIntosh wrote:
I would add that dressing might actually be thought of as un-dressing. As you use the wheel to sharpen
tools not only does the sharpness of the grit particles become dulled (not all of them will break off to reveal fresh ones) but some of the
metal from the tool becomes embedded in the surface of the wheel.
You then need to refresh or "un-dress" the wheel to regain a
surface
composed only of fresh grit particles. Devil stones, star wheel
dressers or diamond dressers can be used on your grinding wheels and a
combination of the three will give you best control over the wheel's
condition. Any one is better than none. The single point diamond tools
require a knack of using, (that I have to admit to not having fully
come to terms with), if you are to avoid creating tiny furrows in the
wheel. You can't go far wrong with a star wheel.
One further point as most turners will have other uses for their
grinders you should be aware that the wheel you use for sharpening your
HSS tools shouldn't be used for things like aluminum etc. And my
second one further point is that you will of course be wearing the eye
protection you wear when turning won't you?
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Howard Klepper wrote:
One thing they do not do a good job of is explaining the
different
hardnesses on the H-K scale. As best as I can tell, the abrasive
particles are the same for a given type of wheel (like the 32A or 38A).
The difference is the hardness of the bond. H is softest, K hardest.
An H wheel (I have only had one) is easily grooved, wears fast, and
needs a lot of truing. But a softer wheel runs cooler and glazes less
than a harder wheel. John Jordan recommends J hardness for HSS tools.
8x1 SG wheels are not listed in the MSC catalog, but I think this is
what Jerry Glaser uses, and they probably can be special ordered from
Norton. The blue Oneway wheel is not a Norton--it's made by a Canadian
company whose name I forget (Continental, maybe?)--but the 80 grit blue
Oneway on my grinder has given very good service. On the other side I
have 60 grit white import from Woodcraft. I sharpen gouges on the 80
grit, scrapers on the 60 (I find it raises a better burr). Jerry uses a
46 grit, but he is grinding the profile on his gouges in a production
setting (i.e., speed matters). After the gouge has the desired profile
and you are going to the wheel mainly to sharpen, I think the above
grits are a good choice for most purposes. Some people use finer grits,
but I haven't seen an improvement in cutting and have had the wheels
glaze and burn the tool more easily with finer grits.
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Steve Tiedman wrote:
I had an afternoon turning class with Alan Lacer (of the
AAW Lacer fame) a couple months back and I think he was using a 60 grit
white aluminum oxide wheel for all his general grinding and sharpening.
For heavier reshaping, a courser grit (approx. 36 or so) may be better
suited, but I bet the 60 would work fine for 95% of everything. 80 grit
would also work all right if you were only buying one wheel.
I have 60 and 120 grit white wheels on my 6" grinder (3500rpm +/-).
Although I do use the 120 grit wheel and it produces a nice edge with a
good bevel that has less grinding marks than the 60 grit wheel produces,
this 120 grit wheel also get the tool hotter, quicker. Not a real great
concern with the high speed steels, which are high heat tolerant, but it
is heat none the less. If I had to do it again, and there is no reason
why I couldn't, I'd get the 60 and maybe a 100 grit wheel. The only
reason I got the 60 and 120 grits was because Woodcraft had a sale on
them about a year and a half ago (about $10 each) and I also had to buy
other stuff, and I had a very set budget at that moment. I'm happy with
the wheels I've got. At the rate us turners use up wheels, I ought to
have them on the grinder for years. A friend of mine has reduced his
6" white wheels by maybe 3/16" or so in about 5-6 years of
daily use, and he touches them up with a diamond dresser often. The
secret is using light pressure when grinding, don't force the tool onto
the wheel, let the wheel do the work. It only takes a few seconds to
sharpen on those white wheels, and sharpening jigs cut down the time
spent on the wheel.
If you're only getting one white wheel, get the 60 or 80 grit for all
your grinding and then maybe touch up/hone your skews by hand for a
really keen edge. A medium India slipstone from Woodcraft ($10-15??)
works well for this, and can be used dry, no oil or water. This is what
Alan Lacer uses, and I've been starting to do it with my skews, too.
Sharper edge and no trips to the grinder until I've honed away the
concave skew bevel. 400 and 600 grit silicon carbide sand paper (used
wet) taped to a piece of 1/4" thick glass hones the skew edges
well, also.
Silicon Carbide (gray) wheels are hard, and used with "softer"
steels, like low cost carbon steel turning tools. Aluminum Oxide (white)
wheels are softer, more friable, creating a fresh grinding surface
easier and used on "harder" steels, like the high speed steel
most modern turning tools are made from. Use the gray wheel for
sharpening the lawn mower blade and grinding screw drivers.
p.s. I just checked a few turning books (Keith Rowley, Richard Raffan,
Ernie Conover) and the 60-80 grit white aluminum oxide wheels seem to be
what you want for general grinding and sharpening. Rowley and Raffan
also use the 40 grit (+/-) wheels for heavy grinding and reshaping,
Conover uses the 60-80 grit wheels for "rough grinding" and a
100-120 grit for "finishing grinding". Conover says that there
is never a need to go higher that 150 grit with wheels. It probably
loads the wheel with metal particles fast, making grinding less
effective, and generates a ton of heat on the tool edge.
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Sources for quality grinding wheels and grinding accessories:
Pacific Grinding Wheel http://www.pgw-co.com/
Norton http://www.nortonabrasives.com
MSC Industrial Supply http://www.mscdirect.com
Grainger http://www.grainger.com
Oneway http://www.oneway.on.ca
Crafts Supplies USA http://www.woodturnerscatalog.com
Highland Hardware http://highland-hardware.com
Packard Woodworks http://www.packardwoodworks.com
The Cutting Edge http://www.cutingedgetools.com
Grizzly Imports http://www.grizzlyimports.com
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