Grinding Wheels 101

A description of grinding wheel characteristics, and some helpful tips on grinding tools. February 14, 2007

This article was reprinted with permission from Wood Tech Tooling,

Wood Tech continues to maintain the industry's widest selection and largest inventory of grinding wheels.

Not all grinding wheels are made equally. Learn how to determine your best value. With all the different cutting tool options available to select from, using the right abrasive is a matter of necessity. It is just as important as the choosing the right knife material! In order to select the best grinding wheel for your particular application, a general understanding of wheel components is necessary.

The combination of abrasive type, abrasive grit size, hardness grade, grain structure and bond type all affect wheel performance. Changes in the amount and type of each of these elements will affect the performance of the wheel . Here are brief descriptions of the elements used to produce wheels.

Abrasive Type
The abrasive grain is what actually removes material from the piece. There are four basic abrasive types:

* Aluminum Oxide (high speed steel applications)

* Silicon Carbide (carbide grinding applications)

* Diamond (carbide grinding applications)

* Cubic Boron Nitride (CBN) (high speed steel applications)

The are a number of different color wheels on the market today: this is not indicative of wheel quality. For example: there are blue colored wheels using conventional blue Aluminum Oxide grain and other blue wheels made with a more expensive Ceramic grain. The ceramic grain offers many operators the ability to grind cooler, faster and with less grinding wheel breakdown. However; most grinders still prefer the more conventional, and less costly, White or Pink wheels for most jobs.

Abrasive Grit Size
Abrasive grains are sized according to an established worldwide standard and are designated by a numerical grit size. The larger the number, the smaller the grain size. Coarse grain size (a smaller number) will increase stock removal rate, but provide less surface finish quality. Fine grit sizes (larger numbers) provide less stock removal, but improve surface quality. Wood Tech uses many wheels with combinations of two or more grit sizes to produce improvements in grinding capabilities.

Hardness Grade
The hardness grade of wheels are designated by letters [A (soft) to Z (hard)]. This grade represents the amount of bond used in the wheel. Generally, harder grades produce a better finish, but will have a tendency to load up faster than softer grades. As always, burning of knife material is to be avoided and a wheel with the correct hardness is vital in producing quality ground knives.

Grain Structure
The grain structure refers to abrasive grain spacing within the wheel. This grain density is identified by a number [1(dense) through 16(open)]. An open structure wheel will be very aggressive, but will not hold its form as well. It will also wear out faster.

A denser wheel will generally provide a better finish, but will generate more heat and slower metal removal. Finding a happy medium is important in producing a hogging wheel that will remove stock quickly and still maintain its form after dressing. It is just as critical in making a finish wheel that holds a small dimension to grind tight corners or small radii.

Bond Type
The grinding wheel bond (the binding agent), is the material which holds the abrasive grain together. Without the right bond, the grinding wheel will literally fly apart. Four major types of bond are: Vitrified (V), Resinoid (B), Rubber (R), and Metal (M). Selecting the best bond for any particular application is based on the following properties:

1. Its ability to allow the abrasive grain to be "pulled out" once the

grinding friction becomes too high due to the grain becoming too dull

to properly cut.

2. The bond's ability to properly hold the grain during the grinding,

wheel dressing, and self sharpening applications.

The vast majority of grinding wheels used in the woodworking industry use an aluminum oxide abrasive type and a vitrified bond. The are a multitude of variations in each category that manufacturers use to fine tune grinding wheel specifications to improve performance. Your individual grinding technique is also a major factor in wheel selection.

Some Helpful Tips to Improve Grinding Results

* Harder knife steels require the use of a softer grinding wheel. Conversely, the softer the knife steel, the harder the grinding wheel must be.

* Slower machine speeds will make a wheel behave softer. Speeding up the machine will have the opposite effect.

* If you primarily use a roughing wheel for all your grinding, you can "trick" the wheel into producing a better finish grind by applying wax into the wheel to clog it up and lowering the back angle by a few degrees. Then increase the Rpm's; this causes the wheel to "buff" rather than grind.

* Wheels dressed thinner than 1/8" (3mm) must be 100 grit or finer to avoid breakage and hold proper radii.

* Maintain the correct coolant ratio. We see this as a common source of grinding related issues. Too lean a ratio will result in rust problems; too rich can lead to health risks. We recommend using a handheld refractometer to assure proper coolant ratios.

* Never exceed the maximum operating speed of the wheel. Keep all guards in place.

* Follow all the Rules and Safety Guidelines from the Grinding Wheel Institute leaflet that we provide with each shipment of grinding wheels.

This article was reprinted with permission from Wood Tech Tooling,