We use abrasive belts for a number of tasks as knifemakers, including grinding steel, sanding wood, brass, synthetics like G10 and Micarta, and acrylics, polishing and honing, and so on. But it would be unrealistic to say that one belt is the greatest. We have a boggy mire of belt options when we combine these applications with a dozen manufacturers, testing methods, and personal opinions. So, in an attempt to clarify things, I’ve written a basic essay that covers several terminology and applications.
To begin, consider the abrasive material itself. Minerals with a hardness make up the majority of these samples. On the Mohs scale, mineral hardness ranges from 1 to 10, with 1 being talc (think talcum powder) and 10 being diamonds. Since hardened steel has a hardness of between 7 and 8 on the Mohs scale, we need an abrasive with a hardness of greater than 7.
Silicon Carbide, Aluminum Oxide, Zirconia Alumina, and a number of other materials are durable enough to sever steel.
An abrasive material is bonded to a backing to create a belt. Usually, this backing is made of paper, cotton, or polyester. The backing’s material and thickness make it ideal for a variety of applications. Flexible lightweight belts, for example, work well as slack belts for forming handles and rounding pieces. For hogging and coarse grinding, heavy backing is ideal.
Fabric backing weights that are commonly used
J weight = light and flexible
X weight = moderately heavy, standard thickness
Y weight = heavy, less flex
Slack belting a curved handle with a hard Y weight backing is perfect for tearing through steel, but not so much for slack belting a curved handle. A J weight backing is much more flexible and blends into and around curves much easier for slack belt work.
The tendency of abrasive materials to crack or break down is measured by their friability. Friability specifications are difficult to come by, but you should be aware of the word and know that you want low friable abrasives for steel. When worked hard, certain products, such as ceramic, snap off and become sharper again.
Some belts have a specific route. They are designed to only travel in one direction. The direction of travel is indicated by an arrow on the back of the belt. The lack of an arrow indicates that the belt is bi-directional, which means it can travel in any direction.
Creating a bond
The “glues” that hold the abrasive particles to the backing are called bonding agents. Bonding formulas are proprietary to each manufacturer. This is especially important if you’re working with water on the belts. Waterproof bonding is needed for wet sanding and polishing.
Sizes of belts
A 1″x30″, 2″x48″, or 2 x 72 sanding belts is used by the majority of knifemakers. In certain cases, a manufacturer would use the traditional 4″x36″ size. The 1″x30″ and 2″x72″ sizes, in my experience, provide the most abrasive choices.
Here are some of the most popular abrasives we see in the store, the majority of which are available in 1″x30″ and 2″x72″ sizes.
Aluminum Oxide is a type of oxide of aluminium
AO is also known as Corundum or simply Corundum. This abrasive has a rusty brown hue to it. In the shape of a belt, we see AO in almost every size and grit. AO belts are typically labelled with P grit numbers such as 60, 120, 400, and so on.
Silicon Carbide is a type of silicon carbide that is used
This abrasive, also known as Carborundum, is dark grey or black in colour. Wet/dry sandpaper sheets for industrial and automotive applications are more popular than belts.
These are often referred to as Zircs and are ideal for working with annealed steel and other metals. Zircs have a thicker backing and are available in grits ranging from P36 to 120.