Careful woodworkers understand that choosing the ideal blade for any cutting task involves three primary factors: the type of wood, the type of cut and the type of tool. In this sense, cutting and grinding metal is no different from cutting wood, but instead of a saw blade, you need to choose a bonded abrasive.
Better known as grinding wheels and cutoff wheels, bonded abrasives are classified as such because the abrasive material (grit) is combined with a resin or vitreous binder and heat-formed into a solid wheel or disc (as opposed to a coated abrasive such as sandpaper, which consists of abrasive material applied to a backing).
Any wheel or disc used to cut metal or masonry is correctly described as either a grinding wheel or a cutoff wheel, and there is little need to make distinctions between the types. In either case, you cut metal and masonry by grinding all the way through.
Selecting a wheel
When comparing bonded abrasives, the physical differences between wheels are more important than terminology. You’ll find differences in:
• Composition — Most consumer-market cutoff and grinding wheels contain one of two abrasive substances: aluminum oxide (for metalwork) or silicon carbide (for masonry). Cutoff wheels typically are made with a resinous binder that, when combined with the abrasive, is formed onto a fiberglass net that provides reinforcement. In grinding wheels, the fiberglass net is layered between multiple layers of the abrasive/binder mixture.
• Grit — For bonded abrasives, as for sandpaper, grit refers to coarseness: Smaller numbers denote coarser wheels. A grit number of 24 is standard for cutoff wheels. Grinding wheels range from 10 grit to as fine as 4,000 grit. Consumer-line bench grinders usually come with a coarse (36-grit) and a medium (60-grit) or fine (100-grit) wheel. Tool trivia: The grit number of an abrasive refers to the number of holes per lineal inch in the strainer that’s used to filter out the abrasives for that product.
• Diameter and maximum speed — Every bonded abrasive is built with a maximum rotational speed. This rating, expressed in revolutions per minute (rpm), is related to the diameter of the wheel. Wheels with a smaller diameter must spin faster at the hub than larger wheels to develop optimal cutting speed on the edge. Always compare the diameter and rpm rating of a cutoff or grinding wheel with your tool’s speed before you install it. (The information is printed on the wheel label and on the motor housing of the tool.) If you don’t know the rpm rating of a wheel, don’t use it (see “Safety Tip,” above right).
• Thickness — The most notable difference between grinding wheels and cutoff wheels is thickness. At 3/32 to 1/8 in. thick, cutoff wheels are thinner than their grinding counterparts, which can range from 1/4 in. to 1 in. thick. Thicker grinding wheels cost more but last longer. For precision cutting you can find thin-kerf cutoff wheels (suitable only for smaller angle grinders) as little as 0.04 in. thick. Large-diameter cutoff wheels are thicker.
• Profile — Most wheels for stationary tools and cutoff saws have a solid-disc profile. But wheels for angle grinders come in many profiles. The most common are a solid disc (Type 1) and a depressed-center disc (usually Type 27, but also Type 28 or Type 29). The only noteworthy differences between Type 1 and Type 27 wheels is that the spindle nut that secures the wheel is recessed in the depressed area so you can grind a flat surface. In cutoff work, the depressed-center wheels are better for flush-cutting.
• Color — If you are an engineer, grinding wheel color is an important consideration. For DIYers, it’s not such a big deal. Both aluminum oxide and silicon carbide wheels usually are gray. Aluminum oxide wheels have more of a reddish-brown cast, while silicon carbide wheels are slightly greenish. These common wheels are inexpensive and good for most general-purpose grinding. But you have likely seen blue, white and even pink grinding wheels in tool catalogs. These are of interest mostly to woodworkers who use a bench grinder to sharpen irons, blades, chisels and lathe tools.
Blue wheels get their color from zirconia alumina — an abrasive that’s combined with aluminum oxide to create a wheel that is somewhat more durable and a bit costlier than a gray model. White wheels are made with very pure aluminum oxide abrasive and a vitreous binder. They are billed as faster-cutting and less susceptible to glazing than gray wheels because they are more friable — that is, they crumble more easily to expose new, sharp grit.
Pink wheels are similar to white wheels, but they are more durable because they aren’t as friable. Both white and pink wheels are intended primarily for sharpening tool steel. Either color works fine on high-chrome steel, but medium- to high-carbon steel is less likely to burn if sharpened with a white wheel.
Using bonded abrasives
The power tools a DIYer is most likely to use with a bonded abrasive include cutoff saws, angle grinders, circular saws and bench grinders. Gas-powered masonry saws, sharpening centers and die grinders are also fitted with a bonded abrasive.
When working with bonded abrasives, follow these recommendations:
• Allow the wheel to attain full speed before it touches the workpiece.
• After installing a wheel, test-run the tool for 60 to 90 seconds before touching the workpiece. Make sure the wheel does not vibrate, wobble or show signs of defects.
• Use gentle pressure when touching the wheel to the workpiece.
• Cut with a smooth, even pace. Do not force the cut, but do not proceed so slowly that burning occurs.
• Wear eye and ear protection, gloves, a long-sleeve shirt and any other safety gear recommended by the tool manufacturer.
• Do not exceed cutting capacities specified by the tool manufacturer.
• Never use a dust collection attachment when cutting metal.
• Do not use cutting lubricants with cutoff wheels. Use lubricants with grinding wheels only if specifically endorsed by the tool manufacturer.
Ring Test
Before installing a grinding wheel, test to make sure it has no hidden cracks or defects. Suspend the wheel by inserting a screwdriver through the arbor hole; then rap it gently with a screwdriver handle at several locations. The wheel should produce a clear ringing sound, not a dull or hollow thud, which would suggest that you need to take a closer look at the wheel for problems.
Glossary
Aluminum oxide: grit material used in wheels and blades for cutting metal
Bluing: an unwanted change in steel color caused by overheating
Bonded abrasive: a grinding, shaping or cutting accessory formed by bonding abrasive particles and binder into a solid object
CBN wheel: a sharpening wheel formed from Borazon (cubic boron nitride) and used to sharpen cobalt and high-speed steel tools (but not carbide steel)
Coated abrasive: a sanding or grinding accessory with the abrasive applied to a backing
Cup wheel: a small, cup-shape bonded abrasive (or wire accessory) operated by a die grinder or drill/driver
Dressing: an operation performed on a grinding wheel to revive the surface by removing glazing and buildup.
Friability: the ease with which a wheel crumbles; softer, open-structure wheels are more friable (and sharpen or cut more quickly in most materials)
Grinding point: a small bonded-abrasive shaping/profiling bit designed for a die grinder (but can be used with a drill/driver at maximum speed)
Grit: relative coarseness of the wheel surface (16 to 24 is coarse; 36 to 60 is medium; 80 to 120 is fine; 150 and higher is superfine)
Resinous bond: a method of binding abrasives by combining them with liquefied resins and then pressing and forming to shape
Silicon carbide: grit material used in wheels and blades for cutting masonry
Structure: the amount of space between grains of abrasive material: open (far apart) or closed (closer together)
Truing: an operation performed to balance a wheel that wobbles or is worn unevenly
Vitrified bond: a heat-based method of binding abrasives with sands or clays similar in execution to making glass or vitreous china
Sources
Bosch Power Tools and Accessories
(224) 232-2000
Jepson Inc.
800-456-8665
Rockler Woodworking and Hardware
800-233-9359