Tap a wheel with the end of a screwdriver to test its ring
When a grinding wheel is received in the shop or removed from storage, it should be inspected closely for damage or cracks. Check a small wheel by suspending it on one finger or with a piece of string. Tap it gently with a light nonmetallic instrument, such as the handle of a screwdriver. If the wheel does not give a clear ring, discard it.
All wheels do not emit the same tone; a low tone does not necessarily mean a cracked wheel. Wheels are often filled with various resins or greases to modify their cutting action, and resin or grease deadens the tone. Vitrified and silicate wheels emit a clear metallic ring. Resin, rubber, and shellac bonded wheels emit a tone that is less clear. Regardless of the bond, the sound of a cracked wheel is easy to identify.
Procedure for wheel inspection
Suspend the wheel on a pin or a shaft that fits through the hole so that it will be easy to turn, but do not mouth the wheel on the grinder. If the wheel is too large to suspend, stand it on a clean, hard surface.
Imagine a vertical plumb line up the center of the wheel.
Tap the wheel about 45 degrees on each side of the vertical line, about one or two inches from the wheel’s edge. (Large wheels may tapped on the edge rather than the side of the wheel.)
Turn the wheel 180 degrees so that the bottom of the wheel is now on top.
Tap the wheel about 45 degrees on each side of the vertical line again.
The wheel passes the test if it gives a clear metallic tone when tapped at all four points. If the wheel sounds dead at any of the four points, it is cracked. Do not use it.
The magnetic chuck should never be turned off when the wheel is spinning: if, for some reason, the wheel comes in contact with a part when the chuck is off, it could kick it out, possibly hitting something or someone (at best), or causing a wheel explosion
The spindle should never be turned on unless the wheel cover is fitted into place: if the wheel explodes when the cover is not fitted, nothing will protect you from the fragments
It is recommended that the X-axis stops are located such that they will stop the table before its horizontal travel is exceeded, and that the stop plunger is enabled when grinding
If the table exceeds its horizontal travel, it will hop off its ways, which can potentially cause a wheel crash
Stone parts & workholding fixtures (e.g. the magnetic chuck, a grinding vice) every time the setup is changed (e.g. when a part is removed from the magnetic chuck for measuring)
Wipe mating surfaces (e.g. part face, magnetic chuck face) down to remove grit before clamping
Lap or sand rough (unground) faces before holding them on the magnetic chuck
Magnetic holding force is a function of the surface finish of the mating faces (part face & magnetic chuck face)
The field lines loop back about a quarter of an inch above the magnetic chuck face, so care should be taken that parts thinner than that are held securely: make sure they can't be shifted on the mag chuck before grinding
Work holding force is proportional to the contact area
Tall parts act as a long lever when the grinding forces (tangential to the wheel, parallel to the grinder travel) act on the top
Tall parts are prone to tipping over
They can be blocked in with shorter, wider pieces, but a safer route is to clamp them securely to something that isn't at risk of falling over: either in a grinding vice or to a large square
Everything on the magnetic chuck face should be oriented so that if it does tip over, the part is moved away from the grinding wheel, not into it; otherwise, it will likely result in a catastrophic crash (a wheel explosion) if the part tipswhile grinding
Only ferromagnetic materials can be held directly on the magnetic chuck
Parts made of any other material need to be held using additional workholding fixtures (e.g. a grinding vice) which are themselves held by the magnetic chuck
Dust / grit collection should always be turned on, because you do not want to breathe it
The operator should never put their hand (or any other body part) anywhere near the wheel when it's spinning
Even if the wheel is backed off from the magnetic chuck, the wheel should be turned off before parts are picked up off of the magnetic chuck, in case one of the hand-feed wheels is bumped, potentially causing the table to knock a body part into the spinning wheel
Removing wheel from arbor
Mounting on adapter
Wheel type markings
Every grinding wheel is marked by the manufacturer with a stencil or a small tag. The manufacturers have worked out a standard system of markings.
1, 5, 7
Straight wheels are commonly applied to internal, cylindrical, horizontal spindle, surface, tool, and offhand grinding and snagging. The recesses in type numbers 5 and 7 accommodate mounting flanges. Type number 1 wheels from 0.006" to 1/8" thick are used for cutting off stock and slotting.
Cylinder wheels may be arranged for grinding on either the periphery or side of the wheel.
Tapered wheels take tapered safety flanges to keep pieces from flying if the wheel is broken while snagging.
The straight cup wheel is used primarily for surface grinding, but can also be used for offhand grinding of flat surfaces. Plain or beveled faces are available.
The flaring cup wheel is commonly used for tool grinding. With a resinoid bond, it is useful for snagging. Its face may be plain or beveled.
The chief use of the dish wheel is in tool work. Its thin edge can be inserted into narrow places, and it is convenient for grinding the faces of form-relieved milling cutters and broaches.
The saucer wheel is also known as a saw gummer because it is used for sharpening saws.
Abrasive grains are selected according to the mesh of a sieve through which they are sorted. For example, grain number 40 indicates that the abrasive grain passes through a sieve having approximately 40 meshes to the linear inch. A grinding wheel is designated coarse, medium, or fine according to the size of the individual abrasive grains making up the wheel. The softer and more ductile the material, the coarser the grain size. The larger the amount of stock to be removed, the coarser the grain size. The finer the finish desired, the finer the grain size.
The grade of a grinding wheel designates the hardness of the bonded material. A soft wheel is one on which the cutting particles break away rapidly while a hard wheel is one on which the bond successfully opposes this breaking away of the abrasive grain.
Most wheels are graded according to hardness by a letter system. Most manufacturers of grinding abrasive wheels use a letter code ranging from A (very soft) to Z (very hard). Vitrified and silicate bonds usually range from very soft to very hard, shellac and resinoid bonds usually range from very soft to hard, and rubber bonds are limited to the medium to hard range.
The grade of hardness should be selected as carefully as the grain size. A grinding abrasive wheel that is too soft will wear away too rapidly, the abrasive grain will be discarded from the wheel before its useful life is realized. On the other hand, if the wheel is too hard for the job, the abrasive particles will become dull because the bond will not release the abrasive grain, and the wheel’s efficiency will be impaired.
The harder the material, the softer the wheel. The smaller the arc of contact, the harder the grade should be. The arc of contact is the arc, measured along the periphery of the wheel, that is in contact with the work at any instance. It follows that the larger the grinding wheel, the greater the arc of contact and, therefore, a softer wheel can be used. The higher the work speed with relation to the wheel speed, the milder the grinding action and the harder the grade should be.
The abrasive particles in a grinding wheel are held in place by the bonding agent. The percentage of bond in the wheel determines, to a great extent, the 'hardness' or 'grade' of the wheel. The greater the percentage and strength of the bond, the harder the grinding wheel will be. “Hard” wheels retain the cutting grains longer, while “soft” wheels release the grains quickly. If a grinding wheel is 'too hard' for the job, it will glaze because the bond prevents dulled abrasive particles from being released so new grains can be exposed for cutting. Besides controlling hardness and holding the abrasive, the bond also provides the proper safety factor at running speed. It holds the wheel together while centrifugal force is trying to tear it apart. The most common bonds used in grinding wheels are vitrified, silicate, shellac, resinoid, and rubber.
The majority of grinding wheels have a vitrified bond. Vitrified bonded wheels are unaffected by heat or cold and are made in a greater range of hardness than any other bond. They adapt to practically all types of grinding with one notable exception: if the wheel is not thick enough, it does not withstand side pressure as in the case of thin cutoff wheels.
Silicate bond releases the abrasive grains more readily than vitrified bond. Silicate bonded wheels are well suited for grinding where heat must be kept to a minimum, such as grinding edged cutting tools. It is not suited for heavy-duty grinding. Thin cutoff wheels are sometimes made with a shellac bond because it provides fast cool cutting.
Resinoid bond is strong and flexible. It is widely used in snagging wheels (for grinding irregularities from rough castings), which operate at 9,500 SFPM. It is also used in cutoff wheels.
In rubber-bonded wheels, pure rubber is mixed with sulfur. It is extremely flexible at operating speeds and permits the manufacture of grinding wheels as thin as 0.006 inch for slitting nibs. Most abrasive cutoff machine wheels have a rubber bond.
Bond strength of a grinding wheel is not wholly dependent upon the grade of hardness but depends equally on the structure of the wheel, that is, the spacing of the grain or its density. The structure or spacing is measured in number of grains per cubic inch of wheel volume. The softer, tougher, and more ductile the material, the wider the grain spacing. The finer the finish desired, the closer, or more dense, the grain spacing should be.
Appropriate abrasives per work material
Gray and chilled iron
Brass and soft bronze
Aluminum and copper
Marble and other stone
Rubber and leather
Very hard alloys
Unannealed malleable iron
High speed steels
Annealed malleable iron
Most grinding wheels are made of silicon carbide or aluminum oxide, both of which are artificial (manufactured) abrasives. Silicon carbide is extremely hard but brittle. Aluminum oxide is slightly softer but is tougher than silicon carbide. It dulls more quickly, but it does not fracture easily therefore it is better suited for grinding materials of relatively high tensile strength. The tensile strength of material to be ground is the main factor in the selection of the abrasive to be used.
Abrasive material subtypes
Type / Composition
Dark Aluminum Oxide
Most common grain. Used for Heavy Duty General Purpose
20% Ceramic Aluminum Oxide,
30% Pink Grain,
50% White Grain
Excellent for form and corner holding.
Blue Aluminum Oxide
Grinds fast with excellent cool cutting action and requires minimal dressing.
Red Aluminum Oxide (Ruby)
Harder grain and sharper than PA & AZ wheels good for steels with high level chromium
Pink Aluminum Oxide
General Purpose Grain that is tough but friable. Tool Room sharpening applications.
White Aluminum Oxide
Highly friable grain for fast cool cutting. Good for light grinding on all steels particularly on tool and die steels.
Green Silicon Carbide
Very friable use for carbide grinding applications.
First consider the material to be ground and its hardness. These affect the choice of abrasive and grade or hardness of the wheel.
Aluminum oxide are best for steels, while silicon carbide abrasives are better suited to grinding cast iron, nonferrous metals and nonmetallic materials.
The hardness of the material to be ground also affects choice of the wheel grade or hardness. A harder grade can more easily be used on soft, easily penetrated materials than on hard materials which naturally tend to dull the wheel faster. The softer grades release the dull grains more readily to present new, sharp grains to the work.
Second factor, in selecting a wheel in the amount of stock to be removed and the finish required. These affect the choice of grit size and bond as follows:
A relatively coarse grit size is selected for rapid stock removal without regard for finish as rough grinding; a fine grit should be used where a high finish is desired.
Vitrified bonded wheels are generally used where a commercial finish satisfactory. The organic bonds, resinoid, rubber and shellac, produce the highest finish.
The area of grinding contact between the wheel and the work affects the choice of grit size and grade.
A coarse grit is required when the contact area is relatively large, as in surface grinding with cup wheels, cylinders or segments, to provide adequate chip clearance between the abrasive grains. As area of contact becomes smaller and the unit pressure tending to break down the wheel face becomes greater, finer grit wheels should be used.
As to the grade or hardness, on large area of contact a soft grade will provide normal breakdown of the wheel, insuring continuous, free-cutting action. A harder grade, on the other hand, is needed to stand up under the increasingly higher unit pressure as the area of contact becomes smaller.
The severity of the grinding operation affects the choice of abrasive and grade.
A tough abrasive like 4A Aluminum Oxide should be used for rough, heavy duty grinding of steel.
The milder abrasives like 32 and 38 Aluminum Oxide are best for lighter precision grinding operations on steels and semisteels, while the intermediate 57 and 19 Aluminum Oxide abrasives are used for precision and semiprecision grinding of both mild and hard steels.
The severity of the grinding operation also influences the choice of grade. Hard grade provide durable wheels for rough grinding operations such as snagging, while medium and softer grade wheels can be used for precision type operations which are less severe on the wheel.
The speed at which the grinding wheel is to be operated often dictates the type of bond.
Vitrified bonded wheels should not be used at speeds over 6,500 s.f.p.m. With few exceptions, when the speed exceeds this figure, resinoid, rubber or shellac bonded wheels should be used. Note, the safe operating speed shown on the tag, wheel or blotter must never be exceeded.
The higher the feed rate, the greater the grinding pressure is. If the grinding speed of workpiece must be increased, the feed rate will be increased, then the wear of the wheel will be faster. Therefore a harder grinding wheel is required.
The dresser should be angled backwards, in the direction of the wheel, and slightly left-of-center
Grinding wheels wear unevenly under most general grinding operations due to uneven pressure applied to the face of the wheel when it cuts. Also, when the proper wheel has not been used for certain operations, the wheel may become charged with metal particles, or the abrasive grain may become dull before it is broken loose from the wheel bond. In these cases, it is necessary that the wheel be dressed or trued to restore its efficiency and accuracy. 'Dressing' is cutting the face of a grinding wheel to restore its original cutting qualities. 'Truing' is restoring the wheel's concentricity or reforming its cutting face to a desired shape. Both operations are performed with a tool called a 'diamond dresser'. The diamond dresser is the most efficient for truing wheels for precision grinding, where accuracy and high finish are required. A dresser may have a single diamond or multiple diamonds mounted in the end of a round steel shank. Inspect the diamond point frequently for wear. It is the only usable part of the diamond, and is worn away it cannot dress the wheel properly.
The whole dressing operation should simulate the grinding operation as much as possible. The grinding wheel usually wears more on the edges, leaving a high spot towards the center. When starting the dressing or truing operation, be certain that the point of the dressing tool touches the highest spot of the wheel first, to prevent the point from digging in. Feed the dresser tool point progressively, 0.001 inch at a time, into the wheel until the sound indicates that the wheel is perfectly true. The rate at which you move the point across the face of the wheel depends upon the grain and the grade of the wheel and the desired finish. A slow feed gives the wheel a fine finish, but if the feed is too slow, the wheel may glaze. A fast feed makes the wheel free cutting, but if the feed is too fast, the dresser will leave tool marks on the wheel. The correct feed can only be found by trial, but a uniform rate of feed should be maintained during any one pass.