What (2"x72") Belts are Best?
I get asked this question a lot. I wish it was easy, but the answer is not so simple.
As knifemakers we use abrasive belts for a many of purposes; grinding steel, sanding wood, brass, synthetics such as G10 and Micarta and acrylics, polishing and honing and the list goes on. So to say that one belt is the best would be really naive. Couple these applications with a dozen manufacturers, testing methods, personal opinions and we have a boggy mire of belt choices. So in an effort to help clear this up, I offer this basic essay covering some terms and applications.
Let's start with the abrasive material itself. These materials are primarily minerals and have a hardness. Mineral hardness is measured on the Mohs scale from 1 being talc, think talcum powder to 10 being diamonds. Hardened steel sits somewhere between 7 and 8 on the Mohs scale, so logically in order to grind steel we want an abrasive with a hardness higher than 7.
Silicon Carbide, Aluminum Oxide, Zirconia Alumina and many others are hard enough to cut steel.
To make a belt, an abrasive material is bonded to a backing. This backing is usually paper, cotton or polyester. The material and thickness of the backing make it suitable for different uses. For example, flexible lightweight belts are good for slack belt uses in shaping handles and rounding items. Heavy backing is great for hogging and coarse grinding.
Common weights of cloth backing
J weight = light and flexible
X weight = moderately heavy, standard thickness
Y weight = heavy, less flex
A heavy Y weight backing is great for tearing through steel, but a poor choice for slack belting a curved handle. For slack belt work, a J weight backing is much more flexible and fits into and around curves much better.
Friability is a measure of the how easily the abrasive materials fracture or break down. It's difficult to find specs on friability, but you should be aware of the term and that you want low friable abrasives for steel. Some materials like ceramic snap off and become sharper again when worked hard.
Some belts are directional. Meaning they are intended to move in only one direction. An arrow on the back of the belt indicates the direction of travel. The absence of any arrow would imply that the belt is bi-directional, meaning it can effectively move in either direction.
Bonding agents are the "glues" that hold the abrasive particles to the backing. Manufactures have their proprietary formulas for bonding. This becomes important if you are working the belts with water. Wet sanding and polishing require waterproof bonding.
Most knifemakers use a 1"x30", a 2"x48" or a 2"x72" sander/grinder. In some cases a maker will use the classic 4"x36". In my experience, the 1"x30" and 2"x72" sizes offer the greatest choice of abrasive options.
Here are some abrasives that we commonly find in the shop, again most of which you will be able to find in the 1"x30" and 2"x72" sizes.
Also known as Corundum or simply referred to as AO. This abrasive is that familiar rusty brown color. In belt form we see just about every size and grit are made with AO. AO belts are generally specified in familiar P grit numbers 60, 120, 400 and so on.
Also know as Carborundum, this abrasive is dark gray or black. Not so common belts, but often seen as wet/dry sandpaper sheets for industrial and automotive applications.
Often called Zircs, these are a good choice for working annealed steel and other metals. Zircs generally have heavier backing and come in lower grits, P36 to 120. I like to use these for rough grinding and profiling before heat treatment. Abrasive life is good value for the money.
Ceramic belts are coated with an extremely hard man-made abrasive. In some cases these ceramics are approaching 10 Mohs (hard like diamonds). Ceramic belts offer long life and are able to abrade very hard materials like hardened steel, titanium and stone.
Structured abrasives are not so much a material, but rather the way the material is arranged on the belt. We often see hatching (cross-cross) patterns, pyramids and other the engineered shapes on man made abrasives. Like ceramic belts, these are able to abrade very hard materials.
These belts are commonly specified in microns and will have A in front of the numbers. e.g. A45. See the Micron to P Grit table for approximate conversions.
Conditioning belts such as Norton's Vortex and 3M's Scotch-Brite are suitable for creating brushed effects on the surface of steel. These range from coarse to very fine. Conditioning belts are expensive, but last a long time given the limited task they perform.
Conditioning belts don't have grit ratings per se, rather they come in coarse, fine, very fine etc.
Cork belts are charged with abrasive compound and used for polishing. The "grit" of the belt depends on the compound applied. Some makers have a green belt (green compound) and a red belt (red compound). These are not Karate belts, rather they keep using the same color compound on the cork to avoid contamination.
Of course all belts get contaminated with steel particles eventually and have to be replaced.
Leather stropping belts are for very fine edge honing. These can be used dry or charged with abrasive compounds.
P Grit and Microns
Most of us are familiar with good old 100 grit sand paper. The grit measure is the FEPA (Federation of European Producers of Abrasives) some times simply called "P" grit. This tells us how coarse or fine the abrasive is. A higher number indicates a finer abrasive. The P grit uses particles that are passed through a screen. A P40 screen has 40 openings per linear inch and the particles that pass through the screen are roughly 1/40th of an inch.
When working with structured abrasives, there is no "grit" per se. Instead, the abrasive coating is of a certain diameter of particles measured in microns. One micron = 0.0000001 of a metre, so the two measurements, one being imperial and the other metric don't lend themselves easily to conversion.
Here is a helpful scale which shows how familiar P grit and microns relate.
Everyone will develop their own techniques and belt usages over time. Here are some tips from my own experience. You may wish to start here and explore different belts, manufacturers and speeds that work best for you.
Hogging, coarse removal, shaping profiles, primary bevel cutting on annealed steel.
Zirconia Alumina - 36 and 60 grit
Finer grinding, post heat treated hardened steel
Ceramic - 60 and 120 grit
Finest grinding, heat treated steel, brushing stainless steel
Brushing stainless steel, coarse polishing
Conditioning belts, coarse to very fine.
The speed of belt travel is typically stated in Surface Feet per Minute (SFM or SF/M). This is how fast the belt is moving and removes the complications of motor RPM, drive wheel size and so on.
Example: Full speed on a grinder with a 1800 RPM motor and 5" drive wheel, means the belt is moving by at 2355 SFM. The same grinder with a 6" wheel will be running at 2826 SFM. Belt manufacturers recommend speeds and are not concerned with your particular grinder setup.
To compute the SFM of your grinder you need to know two things: The motor speed in RPM and the diameter of the drive wheel.
RPM = 1800
D (diameter) = 6"
pi = 3.14
Multiply RPM x D x pi...
1800 x 6 x 3.14 = 33,912
Then divide this number by 12 to convert from inches to feet...
33,912 / 12 = 2826
SFM = 2826
In North America, most AC motors spin at a no-load speed of either 1800 RPM or 3600 RPM. In Europe these are 1500 and 3000 RPMs respectively. Motor nameplate RPM ratings are at loaded conditions and are always less then no-load RPMs.
Here's a handy table I made to help you compute the SF/M for your setup:
For coarse belts, fast speed is okay. For finer belts and polishing belts slower speeds work better. If you have the option to control speed this can be very handy.
What Belts to Buy?
Again this largely depends on what materials you are working with, but a typical starter order for making knives I would recommend:
Zirconia Alumina P36
Zirconia Alumina P60
Zirconia Alumina P120
If you are going to grind your bevels after heat treatment, I'd recommend using a Ceramic P60 instead of the Zirc 60. If you have some extra cash, buy a fine conditioning belt for brushing steel.
I hope that you have found this helpful. Kindly let me know if you have any questions.