What is Grinding?

Grinding takes an abrasive — often attached to a grinding wheel — and uses its many grains to cut a workpiece. Variations on this process are useful for a wide variety of applications.

    On its surface, grinding seems simple: a machine takes a rotating tool (usually a wheel) with abrasive grains and applies it to a workpiece’s surface to remove material. Each grain is its own miniature cutting tool, and as grains dull, they tear from the tool and make new, sharp grains prominent.

    But there are many variations, approaches and considerations for this type of machining, each of which is particularly effective for certain applications with certain materials.

    Principles of Grinding

    In all forms of grinding, three different interactions occur between the abrasive and the machined material. Cutting occurs where the abrasive grain is sufficiently exposed to penetrate the workpiece material and curl a chip, and sufficient clearance exists between the grain, bond and workpiece to flush the chip with coolant or throw it away by wheel action. Plowing takes place when the grain is unable to get enough penetration to lift a chip, instead pushing the material ahead of the abrasive edge. Sliding happens when a lack of cut depth, insufficient clearance or a grit staying on the wheel after dulling results in rubbing or creating slide marks on the workpiece surface. Grinding process control balances these three interactions to achieve the desired parameters.

    These interactions feed into three major commercial grinding processes: rough grinding, precision grinding and ultra-precision grinding. Rough grinding maximizes the metal removed at the cost of surface finish. It primarily sees use in cutting off billets, grinding weld beads smooth and snagging gates and risers from castings. Additional surface finishing passes typically take place afterward — in particular, a “spark-out” pass relieves some of the stress on the machine tool and uses plowing to impart a better surface finish and size tolerance. Precision grinding is a middle-ground between metal removal and part size control, and serves as the basis for creep feed grinding, slot grinding and high-efficiency deep grinding. In ultra-precision grinding, little to no actual cutting occurs, but sliding action from very fine grains rubs the workpiece surface to a high finish. Most surface finishing processes, such as lapping and polishing, are examples of this type of grinding.

    Hundreds of different variables can affect the interaction between the abrasive and the workpiece, but they generally come down to machine tool, work material, wheel selection and operational factors. Balancing these by setting up a part run that fits within the known parameters of all four categories provides a baseline that gradual parameter adjustment can improve.

    Grinding Wheels

    Grinding wheels have two major components: the abrasive grains and the bond. The relative percentages of grain and bond, and their spacing on the wheel, determine the wheel’s structure. Different types of grains work better on different projects, as do different types and “grades” (i.e. strengths) of bond. Broad areas of grinding need coarser grits and softer grades, with smaller areas requiring finer grits and harder grades to withstand the greater unit pressure.

    Straight wheels are the most traditional type of grinding wheel, with the grinding face on the periphery of the wheel. Recessed wheels are variations on this form, featuring a recessed center to fit on a machine spindle flange assembly. The other major type of wheel shape uses a cutting face on the side of the wheel — names for this type of wheel include cylinder wheels, cup wheels and dish wheels, depending on the particular shape. For these wheels, bonded abrasive sections of various shapes, also known as “segments,” are assembled to form a continuous or intermittent side grinding wheel.

    Operational Basics

    Although speeds for grinding wheels and cutting wheels are measured in sfm or smm, wheels are often rated in rpm. It is important never to operate a grinding wheel over its rpm limit — most experts recommend never mounting a wheel on a machine that can exceed the wheel’s limit.

    As speeds increase, each grain cuts and wears less. This emulates a harder grade. Vitrified bonds work up to 6,500 sfm, with organic bonds handling up to around 9,500 sfm. Higher speeds will require specially made grains.

    Work speed defines the speed at which a grinding wheel passes over a workpiece or rotates around a center. High work speeds lower the heat retention and reduce the risk of thermal damage. Both high work speeds and reducing the diameter of the wheel result in increased grain depth of cut, performing like a softer grade wheel.

    Traverse distance, or crossfeed, is the distance a workpiece moves across the face of the wheel. Lowering the traverse distance to no more than one-quarter of the wheel width improves surface finish, but slows down productivity. Increasing the crossfeed to one-half the wheel’s width or above boosts productivity, but lowers surface finish.

    Different types of grinding use different methodologies to determine the work material removal per unit of width, but one consistently useful metric for shops is the grinding gratio, or g-ratio. This is the ratio of volume of work removed to volume of wheel consumed (or, volume of work removed ÷ volume of wheel worn). From a cost standpoint, a higher g-ratio is better.

    Types of Grinding

    Grinding operations come in many types, with this article covering six major types and several of the subtypes within.

    Cylindrical grinding is a common type of grinding in which both the wheel and the workpiece rotate. The workpiece is either fixed and driven between centers, or driven by a revolving chuck or collet while supported in a center. This operation can take place with either traverse movements, where the wheel traverses axially along the part, or plunge movements, where the wheel is thrust into the part. Straight wheels are most commonly used in cylindrical grinding, with common cylindrical grinding machines being plain cylindrical (or roll) grinders, centerless grinders and inside- or outside-diameter grinders. Internal cylindrical grinding does the internal diameter grinding of bores and holes, generating size and concentricity within millionths of an inch. The grinding wheels tend to range in diameter from half an inch to three inches. This small size introduces rapid wear, making CBN and diamond wheels in crush dressable and vitrified form popular for these applications.

    Surface grinding, such as stainless steel grinding, involves grinding a plane surface by feeding the workpiece beneath a rotating grinding wheel. Like cylindrical grinding, it operates in two general formats. The workpiece may travel traversely under the wheel and move back and forth beneath a grinding wheel mounted on a horizontal spindle, or it may move in circles on a rotary table beneath a vertical spindle that cuts on the face of the grinding wheel or grinding segment. Applications for this grinding type may grind a surface flat or introduce grooves by grinding straight channels into the workpiece. While milling can complete these tasks, grinding improves surface finish, has less expensive tooling and allows contours to be dressed into the profile of the wheel — making it much more cost-effective for very hard or abrasive surfaces.

    Centerless grinding creates cylindrical forms at extremely close tolerances. This type of grinding eliminates the need for center holding by supporting the workpiece at three separate points: the grinding wheel, feed wheel and work support blade. Nothing actually clamps the workpiece in place, so each piece flows freely for continuous production (also known as “throughfeed centerless grinding”). The grinding wheel, during ordinary metal grinding, and the feed wheel rotate in the same direction, while the workpiece rotates in the opposite direction between them. The rotation keeps the workpiece down, while the work support blade (slightly angled to raise the workpiece above the centerline for better cylindricity) holds it up. The work support blade should always be at least as long as the grinding wheel is wide. Centerless grinding also comes in three forms. Throughfeed centerless grinding is used on straight cylindrical workpieces without interfering shoulder or projections, and involves the offset axis feed wheel feed the workpiece past the grinding wheel to a discharge position. Infeed grinding (also called plunge centerless grinding) is best when a workpiece has projections, irregular shapes, varying diameters or shoulders, and works best for profiles and multi-diameter workpieces. In this submethod, feed wheels above the grinding wheel feed the workpiece downward, with no lateral movement during grinding. Endfeed centerless grinding grinds conically tapered cylindrical sections like shanks on A and B taper drill bits. Here, the feed wheel, grinding wheel and work blade are set up in a fixed relationship to each other, then two wheels are dressed to a shape matching the end taper of the workpiece and the workpiece is fed from the front of the grinding machine until it reaches an end stop.

    Creep feed grinding is a slow, one-pass operation that makes a deep cut of up to one inch in steel materials at low table speeds between 0.5 and 1 ipm. It is not suitable for conventional grinding machines, but for those which are compatible with it, it offers high productivity and cost effectiveness. Creep feed grinding is a plunge operation with high horsepower requirements, and which also requires a heavy flow of cutting fluid close to the nip to remove chips and cool the work. Continuous dressing at about 20 to 60 millionths per revolution — preferably with a diamond roll — reduces cutting times of fixed machine cutting and keeps the wheel sharp. When a second pass is required, it is typically of no more than 0.002 inch deep to “clean up” the workpiece.

    Snagging is a rough grinding application that removes unwanted metal with little consideration of surface finish. As such, it uses durable straight and straight cup wheels in horizontal and straight shaft grinding machines, although flaring cup wheels are used in right-angle grinders and various round and square-tipped cones and plugs also see use. Typical applications include removing unwanted metal on castings; removing flaws and cracks; removing gates, risers and parting lines; rough beveling; grinding down heavy welds; and preparing surfaces for cleaning or painting.

    Cut-off operations use an abrasive wheel as an alternative to the laser, abrasive water jet, metal saw, friction saw and oxyacetylene or plasma arc torch. A study from Norton Abrasives demonstrated that the abrasive wheel can outperform these other methods with ferrous materials, and that the abrasive wheel is faster and less expensive for nonferrous materials than the common metal saw choice. The abrasive wheel provides more cutting points than a saw, and cuts just as thoroughly at a speed of 2 or 3 miles per minute. Cut-off wheels should run at the highest possible speed, with one horsepower for every inch of wheel diameter. If this proves impossible, use a softer wheel. Production jobs use non-reinforced wheels, with non-reinforced shellac wheels for applications requiring extreme versatility and quality of cut. Reinforced wheels are compatible with portable cut-off, swingframe, locked head push-through and foundry chop stroke operations.

The Basics of Cutting and Grinding discs

Abrasive-cutting processes are widely used to obtain semi-finished products from metal bars, slabs, or tubes. Thus, the abrasive cutting-off process is applied when requiring precision cutting and productivity at a moderate price. Cut-off tools are discs composed of small abrasive particles embedded in a bonding material, called the binder. This work aims to compare the cutting performance of cutting discs with different composition, in dry cutting of steel bars. To do that, disc wear was measured and disc final topography was digitalized in order to determine both disc surface wear patterns and if the abrasive particles bonding into the binder matrix was affected. In addition, X-Ray inspection gave information about the abrasive grit-binder bonding. Therefore, the method here presented allows identifying discs with a superior abrasive-cutting capability, by combining profilometry and tomography to define micrometrical aspects, grit size, and binder matrix structure. Results led to the conclusion that discs with high grit size and protrusion, high grit retention by bond material, and closer mesh of fiberglass matrix binder were the optimal solution.
[font=”IBM Plex Sans”, sans-serif]Plenty of manual cutting applications call for a hand-held grinder and cutting wheel. Cutting sheet metal, sizing a piece for fabrication, cutting out a weld to refabricate it, and cutting and notching in pipeline work are just a few examples of what can be accomplished using a grinder and cutting wheel.[/font]
[font=”IBM Plex Sans”, sans-serif]Resinoid-bonded cutting wheels are a popular choice to achieve these types of cuts because they offer portability and allow you to cut in many different angles and orientations. The bonding agent, in this case resinoid, holds the wheel together so it can cut effectively. The bond wears away as the abrasive grains wear and are expelled so new sharp grains are exposed.[/font]
[font=”IBM Plex Sans”, sans-serif]By following a few best practices, you can extend wheel life, promote safety, and improve productivity and efficiency within the process.[/font]
[font=”IBM Plex Sans”, sans-serif]The Basics of Cutting Wheels[/font]
[font=”IBM Plex Sans”, sans-serif]The main considerations in using resinoid-bonded wheels include the cutting application, the tool being used—such as a right-angle grinder, die grinder, or chop saw—desired cutting action, the material being cut, and space. Wheels typically provide a fast cutting action, long life, and tend to be cost-effective.[/font]
[font=”IBM Plex Sans”, sans-serif]The two main types of resinoid-bonded abrasive cutting wheels are Type 1, which are flat, and Type 27, which have a raised hub. Type 1 wheels generally are used for straight-on cutting on electric or pneumatic right-angle grinders or die grinders and chop saws, among other tools. Type 27 wheels are required when there is some type of interference and the metal cutting disc needs to be raised up from the base of the grinder, but personal preference also plays a role in the decision. They are most commonly used with electric or pneumatic right-angle grinders.[/font]
[font=”IBM Plex Sans”, sans-serif]Resinoid-bonded abrasive cutting wheels are available in various sizes and thicknesses. The most popular range is 2 to 16 inches in diameter, and common thicknesses are from 0.045 in. to 1⁄8 in. Thinner wheels remove less material during the cut.[/font]
[font=”IBM Plex Sans”, sans-serif]Some types of wheels cut faster than others. The abrasive material used in the wheel is one influencer on cut rate and consumable life. Wheels come in several grain options, such as aluminum oxide, silicon carbide, zirconia alumina, ceramic alumina, and combinations of these materials.[/font]
[font=”IBM Plex Sans”, sans-serif]While not as sharp as other grains, aluminum oxide provides toughness and good performance for cutting on steel. Silicon carbide, on the other hand, is a very sharp grain but not quite as tough, making it suitable for cutting nonferrous metals. Zirconia alumina is a self-sharpening, tough, durable grain that holds up well in a range of demanding applications. Ceramic alumina also is designed to self-sharpen as it “breaks” at predetermined points to maintain a consistent cut rate and long life.[/font]
[font=”IBM Plex Sans”, sans-serif]When selecting a resinoid-bonded abrasive wheel, consider that products made with a mixture of zirconia or ceramic alumina with a harder bond typically cost more but offer durability and longer consumable life.[/font]
[font=”IBM Plex Sans”, sans-serif]Make sure to refer to the manufacturer’s recommendations, product descriptions, and RPM ratings to select the proper wheel size and bonded abrasive material for your application. Matching the size and RPM rating of the tool to the size and RPM rating of the wheel is critical for safe and effective usage. Choosing the tool with the greatest amperage or amount of torque while staying within size and RPM requirements of the wheel will increase performance.[/font]
[font=”IBM Plex Sans”, sans-serif]The kind of tool and the tool guard that you use also are factors that play a role in the type of wheel that can be used for an application. A larger-diameter wheel works best if you’re cutting deep into metal or need to cut a piece with a large diameter, for example, because it eliminates the need to rock the wheel back and forth during the cutting process. Look for a wheel with the diameter designed for the size and thickness of material being cut.[/font]
[font=”IBM Plex Sans”, sans-serif]Thin wheels, such as aluminum cutting disc, on the other hand, tend to remove less metal during the cut and have shorter life spans, but provide a quicker cut. There are some exceptions to this as different versions of thin wheels are lasting longer, so be sure to do your research before you make a final decision to ensure the wheel you select maximizes efficiency.[/font]
[font=”IBM Plex Sans”, sans-serif]Specialty cutting wheels are also available that are designed for use with certain materials, such as stainless steel and aluminum.[/font]
[font=”IBM Plex Sans”, sans-serif]Proper Positioning and Other Tips[/font]
[font=”IBM Plex Sans”, sans-serif]In addition to paying attention to designations for RPM rating, size, and material, you should also follow these tips when using resinoid-bonded abrasive cutting wheels.[/font]

  • Use the cutting wheel at a 90-degree angle, perpendicular to the work surface.
  • Apply the proper amount of pressure—not too much, not too little—to allow the cutting wheel to do the work. Always avoid pushing too hard on the wheel, which can cause the grinder to stall or kick back or give you a much less efficient cutting action. It also increases the chances that you will slip or lose control of the tool, which can cause damage or injury.
  • Choose a grinder with the highest torque or amperage available for the application, as this will help the wheel to do more of the work. For example, instead of using a 4.5-in. Grinder cutting wheel on a 6-amp grinder, use a 4.5-in. wheel on a 10-amp grinder. The RPM rating remains the same, but the tool will provide more torque to cut into the metal.
  • Choose a tool and consumables that offer quick, consistent cutting, which typically provides the most efficient performance.
  • Remember, the thinner the cutting wheel, the more susceptible it can be to side loading, which is a term that describes when the wheel bends while moving side to side in the cut. This can turn dangerous if you lean too hard on a wheel, which can cause the wheel to break or jam in the cut. It can also reduce the efficiency of the wheel and increase the cut time.
  • Store the wheel in a clean, dry environment, and avoid placing it in water or mud. This helps minimize environmental effects that could degrade its performance or cause it to crack or wear prematurely. The performance of resinoid bond tends to deteriorate when the wheel is stored for extended periods of time, so be sure to use FIFO (first in, first out) when using wheels.
  • Inspect the wheel and consumable before each use to check for signs of damage or wear. Cutting wheels, including angle grinder cutting discs can become harder to control as they wear down. If you can no longer make a safe cut because the wheel’s diameter is worn so thin, then the best course of action is to replace it.

[font=NexusSerif, Georgia, “Times New Roman”, Times, STIXGeneral, “Cambria Math”, “Lucida Sans Unicode”, “Microsoft Sans Serif”, “Segoe UI Symbol”, “Arial Unicode MS”, serif]A grinding disc is defined by the type of abrasive material, bonding material, grain size, structure of the wheel, and grade of the wheel used for the machining of a component. These factors decide the grinding efficiency of the grinding wheel and surface finish quality of the machined component. A wide range of abrasives are being used in modern era to overcome necessities in machining of various make of components. Abrasives ranging from the economic verses of aluminium oxide to the likes of super-abrasives such as cubic boron nitride and the expensive diamond grains are used for machining as well as surfacing purposes. Over the years, research has depicted that no distinct abrasive material can meet all the requirements of grinding applications. The mechanical and physical properties of a particular abrasive material make it suitable for a certain application.[/font]

Wire Brushes

[font=”Open Sans”, sans-serif]A wheel wire brush is an abrasive tool that has stiff bristles made from a variety of rigid materials designed to clean and prepare metal surfaces. The filaments of wire brushes are small diameter pieces of inflexible material that are closely spaced together as a means for cleaning surfaces that require aggressive and abrasive tools. The means of applying the brush can be either manual or mechanical depending on the type of brush and the surface to be treated.[/font]
[font=”Open Sans”, sans-serif]The short video below explains the manufacturing of a unique type of wire brush called a wire drawn brush, which is a very sturdy and durable brush that is made by a process that ensures filament retention.[/font]