Grinding wheel
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A grinding wheel is an expendable wheel that carries an abrasive compound on its periphery. These wheels are used in grinding machines.
The wheel is generally made from a matrix of coarse particles pressed and bonded together to form a solid, circular shape, various profiles and cross sections are available depending on the intended usage for the wheel. They may also be made from a solid steel or aluminium disc with particles bonded to the surface.
Materials used are generally silicon carbide and diamond with a vitrified bonding agent. In production grinding, a wide array of materials are used. Wheels with different abrasives, structure, bond, grade, and grain sizes are available. The abrasive is the actual cutting material, such as cubic boron nitride, zirconia aluminum oxide, manufactured diamonds, ceramic aluminum oxide, aluminum oxide, and others. The abrasive is selected based on the hardness of the material being cut. The structure of the wheel refers to the density of the wheel (bond and abrasive versus airspace). A less-dense wheel will cut freely, and has a large effect on surface finish. A less dense wheel is able to take a deeper or wider cut with less coolant, as the chip clearance on the wheel is greater. The grade of the wheel determines how tightly the bond holds the abrasive. Grade affects almost all considerations of grinding, such as wheel speed, coolant flow, maximum and minimum feed rates, and grinding depth. Grain size determines the phsyical abrasive size in the wheel. A larger grain will cut freely, allowing fast cutting but poor surface finish. Ultra-fine grain sizes are for precision finish work, where a fine surface finish is required. The wheel bonding agent determines how the wheel holds the abrasives. This affects finish, coolant, and minimum/maximum wheel speed.
The manufacture of these wheels is a precise and tightly controlled process, due not only to the inherent safety risks of a spinning disc, but also the composition and uniformity required to prevent that disc from exploding due to the high stresses produced on rotation.
Grinding wheels are self sharpening to a small degree, for optimal use they may be dressed and trued by the use of grinding dressers. Dressing the wheel refers to removing the current layer of abrasive, so that a fresh and sharp surface is exposed to the work surface. Truing the wheel makes the grinding surface parallel to the grinding table or other reference plane, so the entire grinding wheel is even and produces an accurate surface.
The wheel type (eg:- cup or plain wheel below) fit freely on their supporting arbors, the necessary clamping force to transfer the rotary motion being applied to the wheels side by identically sized flanges (metal discs). The paper blotter shown in the images is intended to distribute this clamping force evenly across the wheels surface.
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[edit] Safety
Safety is extremely important at all times, especially when grinding wheels are involved. They are a mainstay of the engineering and construction industry and the risks involved with them may tend to be taken for granted. Safety awareness and an understanding of the materials and processes involved is essential. Appropriate safety gear must be worn and all safety procedures must be followed. Always err on the side of caution.
Before mounting and balancing a grinding wheel, the wheel must be sounded. Sounding is loosely suspending the wheel by a bit of twine or other material so that it hangs free, and giving the wheel a very light tap with a hard object. Care must be taken not to damage the wheel when sounding. A wheel that is safe to use will ring clearly and solidly, like a bell or tuning fork. A damaged wheel will not make any resonating sound. Damaged wheels must not be used under any circumstances and are best discarded or returned to the manufacturer. This process is critically important for surface and diameter grinding, where frequent handling, storage, and changing of wheels increases the risk of fatal wheel damage.
Contact with a spinning grinding wheel will produce a cut. Because of the heat generated in the grinding process, a burn might also be produced. Wood should never be ground on a wheel, as it can clog the wheel's pores and cause the wheel to burst, with fatal results.
[edit] Types of Grinding Wheels
[edit] Cup wheel
A cup wheel as pictured to the right is predominantly used in Tool and Cutter grinders where orientation of the wheel and a slim profile are required. These wheels are used (and dressed) on the side face and have the advantage of producing a truly flat surface on the side of lathe tools. They are used in jig grinders to produce flat surfaces or counterbores.
[edit] Straight wheel
To the left is an image of a straight wheel. These are by far the most common style of wheel and can be found on bench or pedestal grinders. They are used on the periphery only and therefore produce a slightly concave surface (hollow ground) on the part. This can be used to advantage on many tools such a chisels.
[edit] Cylinder wheel
Cylinder wheels provide a long, wide surface with no center mounting support (hollow). They can be very large, up to 12" in width. They are used only in vertical or horizontal spindle grinders.
[edit] Tapered wheel
A straight wheel that tapers outward towards the center of the wheel. These are stronger wheels than straight wheels, and are used in situations where the wheel may catch on the work surface and fail to prepare and you prepare to fail .
[edit] Straight cup
Straight cup wheels are an alternative to cup wheels in tool and cutter grinders, where having an additional radial grinding surface is beneficial.
[edit] Dish cup
A very shallow cup-style grinding wheel. The thinness allows grinding in slots and crevaces. It is used primarily in cutter grinding and jig grinding.
[edit] Saucer wheel
A special grinding profile that is used to grind milling cutters and twist drills. It is most common in non-machining areas, as sawfilers use saucer wheels in the maintenance of sawblades.
[edit] Diamond wheel
Diamond wheels are grinding wheels with industrial diamonds bonded to the periphery.
They are used for grinding extremely hard materials such as carbide tips, gemstones or concrete. The saw pictured to the right is a slitting saw and is designed for slicing hard materials, typically gemstones.
[edit] Diamond Mandrels
Diamond mandrels are very similar to their counterpart, a diamond wheel. They are tiny diamond rasps for use in a jig grinder doing profiling work in hard material.
[edit] Cut off wheels
Cut off or parting wheels are self-sharpening wheels that are thin in width and often have radial fibres reinforcing them. They are often used in the construction industry for cutting reinforcement steel (rebar), protruding bolts or anything that needs quick removal or trimming. Most handymen would recognise an angle grinder and the discs they use.
[edit] Grinding wheel flanges clamping force
The clamping force of the grinding wheel flanges is an important safety parameter of a grinding operation:
- it must be high enough to drive the wheel without slippage under the most severe operating conditions of the machine.
- it must not apply to the wheel an excessive compression stress which could weaken the wheel.
- it must not distort the flanges.
This clamping force can be calculated by the formula detailed in the EN13218 standard, Annex C.
When flanges are clamped by screws it is essential to know the torque to use when tightening these screws to make sure that the needed clamping force is obtained and that each screw is loaded enough to avoid loosening. For multiple screw flanges this torque calculation is also detailed in the EN13218 standard Annex C.
The design of the flanges and their tightening parameters are their manufacturer's responsibility.
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