A bench grinder is a
benchtop type of
grinding machine
A grinding machine, often shortened to grinder, is a power tool (or machine tool) used for grinding. It is a type of machining using an abrasive wheel as the cutting tool. Each grain of abrasive on the wheel's surface cuts a small chip from the ...
used to drive
abrasive wheels
Abrasive Wheels are a punk rock band of the late 1970s – early 1980s. They hailed from Leeds, England and were seldom out of the Independent charts between 1980 and 1984 when the band split. The band were Shonna Rzonca – vocals, Dave Ryan â ...
. A pedestal grinder is a similar or larger version of grinder that is mounted on a
pedestal
A pedestal (from French ''piédestal'', Italian ''piedistallo'' 'foot of a stall') or plinth is a support at the bottom of a statue, vase, column, or certain altars. Smaller pedestals, especially if round in shape, may be called socles. In ci ...
, which may be bolted to the floor or may sit on
rubber feet. These types of grinders are commonly used to hand
grind
A blade's grind is its cross-sectional shape in a plane normal to the edge. Grind differs from blade profile, which is the blade's cross-sectional shape in the plane containing the blade's edge and the centre contour of the blade's back (me ...
various
cutting tools
In the context of machining, a cutting tool or cutter is typically a hardened metal tool that is used to cut, shape, and remove material from a workpiece by means of machining tools as well as abrasive tools by way of shear deformation. The majori ...
and perform other rough grinding.
Depending on the bond and grade of the grinding wheel, it may be used for sharpening cutting tools such as
tool bit
A tool bit is a non-rotary cutting tool used in metal lathes, shapers, and planers. Such cutters are also often referred to by the set-phrase name of single-point cutting tool, as distinguished from other cutting tools such as a saw or water j ...
s,
drill bit
Drill bits are cutting tools used in a drill to remove material to create holes, almost always of circular cross-section. Drill bits come in many sizes and shapes and can create different kinds of holes in many different materials. In order ...
s,
chisel
A chisel is a tool with a characteristically shaped cutting edge (such that wood chisels have lent part of their name to a particular grind) of blade on its end, for carving or cutting a hard material such as wood, stone, or metal by hand, stru ...
s, and
gouges. Alternatively, it may be used to roughly shape metal prior to
welding
Welding is a fabrication (metal), fabrication process that joins materials, usually metals or thermoplastics, by using high heat to melt the parts together and allowing them to cool, causing Fusion welding, fusion. Welding is distinct from lower ...
or fitting.
A
wire brush
A wire brush is a tool consisting of a brush whose bristles are made of wire, most often steel wire. The steel used is generally a medium- to high-carbon variety and very hard and springy. Other wire brushes feature bristles made from brass ...
wheel or buffing wheels can be interchanged with the grinding wheels in order to clean or
polish
Polish may refer to:
* Anything from or related to Poland, a country in Europe
* Polish language
* Poles
Poles,, ; singular masculine: ''Polak'', singular feminine: ''Polka'' or Polish people, are a West Slavic nation and ethnic group, w ...
workpieces. Stiff buffing wheels can also be used when
deburring
A burr is a raised edge or small piece of material that remains attached to a workpiece after a modification process.
It is usually an unwanted piece of material and is removed with a deburring tool in a process called 'deburring'. Burrs are mo ...
is the task at hand. Some buffing machines (buffers) are built on the same concept as bench grinders except for longer housings and arbors with buffing wheels instead of grinding wheels.
Bench grinders are standard equipment in
metal fabrication shops and
machine shop
A machine shop or engineering workshop (UK) is a room, building, or company where machining, a form of subtractive manufacturing, is done. In a machine shop, machinists use machine tools and cutting tools to make parts, usually of metal or plast ...
s, as are handheld grinders (such as
angle grinder
An angle grinder, also known as a side grinder or disc grinder, is a handheld power tool used for grinding (abrasive cutting) and polishing. Although developed originally as tools for rigid abrasive discs, the availability of an interchangeable ...
s and
die grinders).
Operation (including safety standards)
Wheel speed
Wheels come with maximum
RPM
Revolutions per minute (abbreviated rpm, RPM, rev/min, r/min, or with the notation min−1) is a unit of rotational speed or rotational frequency for rotating machines.
Standards
ISO 80000-3:2019 defines a unit of rotation as the dimensionl ...
ratings printed on the label (paper blotter). The grinder's RPM must be equal or lower; "the maximum operating speeds indicated on the wheel's tag must never be exceeded". Greatly overspeeding a grinding wheel makes it explode, which can injure or kill the operator.
Workpiece material suited to wheel grade
The typical wheels on most bench grinders are
vitreous-bond wheels. They work best to do their intended task, but they inherently have a risk of cracking. Grinding wheels designed for steel are not to be used for grinding softer metals, like aluminum. The soft metal gets lodged in the pores of the wheel and later expands with the heat of grinding. This can dislodge pieces of the grinding wheel, causing injury.
Sides of wheel versus periphery
Grinding is prohibited on the side of a typical (disk-shaped) bench grinder wheel, which is designed for grinding on the periphery only. Risk of wheel explosion otherwise threatens. Some tool and cutter grinders have cup-shaped wheels designed to do grinding on the side.
Tool rest and spark arrester placement
The tool rest needs to be tight and within 2 to 3 mm of the wheel (" to "). This prevents the workpiece from slipping down between the wheel and the tool rest. The spark arrestors catch stray sparks.
Grinding is usually done with the workpiece resting on the tool rest. This prevents sudden slips in which the wheel grabs the work momentarily and yanks it out of the operator's hand or pulls the hand toward the wheel.
Wheel guards
The wheel guards serve to intercept the fragments of an exploding wheel, keeping them from killing people or damaging surroundings. This is why running a grinder without the guards is prohibited.
Coolant
Grinding metal on a power-driven grinding wheel quickly heats up the workpiece. Most bench grinders are of the dry type, in which no
cutting fluid
Cutting fluid is a type of coolant and lubricant designed specifically for metalworking processes, such as machining and stamping. There are various kinds of cutting fluids, which include oils, oil-water emulsions, pastes, gels, aerosols (mists) ...
(coolant) is used at the grinding interface, but often the workpiece is recurrently dunked into a pan or pot of water for cooling so as to keep it from getting hot enough to lose its
temper
Temper, tempered or tempering may refer to:
Heat treatment
* Tempering (metallurgy), a heat treatment technique to increase the toughness of iron-based alloys
** Temper mill, a steel processing line
* Tempering (spices), a cooking technique where ...
, burn the operator's hand, or both. Such pots are often mounted just below the grinder for safe and easy reach. For medium-carbon or high-carbon workpieces that are already
hardened and tempered (such as tool bits and drill bits), the dipping of the piece into the water must be frequent enough to avoid very high heat followed by substantial
quenching
In materials science, quenching is the rapid cooling of a workpiece in water, oil, polymer, air, or other fluids to obtain certain material properties. A type of heat treating, quenching prevents undesired low-temperature processes, such as pha ...
, which can easily destroy the existing
heat treatment
Heat treating (or heat treatment) is a group of industrial, thermal and metalworking processes used to alter the physical, and sometimes chemical, properties of a material. The most common application is metallurgical. Heat treatments are also ...
.
Some grinders for knife sharpening duty are of the
wet type, in which the bottom of the wheel runs within a pan of water or other coolant. A tube may also deliver a stream of coolant near the top of the wheel. These grinders are not always called bench grinders, but they are among the class of benchtop grinding machines.
Wheel dressing
Bench grinder wheels sometimes need to be restored to
roundness
Roundness is the measure of how closely the shape of an object approaches that of a mathematically perfect circle. Roundness applies in two-dimensional space, two dimensions, such as the cross section (geometry), cross sectional circles along a c ...
and given a fresh grinding face with newly exposed grains. New wheels have suitable balance when first made, but whether the balanced state will last after the wheel has been mounted, and (especially) after it has been in use for a while, is "quite uncertain". It can seem
counterintuitive
A paradox is a logically self-contradictory statement or a statement that runs contrary to one's expectation. It is a statement that, despite apparently valid reasoning from true premises, leads to a seemingly self-contradictory or a logically u ...
that they could wear as unevenly as they sometimes do, but a slow spin of the resting wheel by hand confirms it by showing the gap between wheel and tool rest varying substantially as the wheel turns. Thus it is normal that wheels sometimes need to be dressed (neatly trimmed), which is achieved by any of several types of
wheel dresser. When bench grinders vibrate excessively, it is usually because the wheels have worn out of round and are thus out of balance. Truing them by dressing usually resolves this problem. Although some consumers might imagine buying a bench grinder and then using it for many years without thinking about wheel dressing, the need for dressing is not so seldom as that. Correcting the imbalance not only reduces the annoyance of vibration (which rattles the wheel guards and shakes the workbench) but also is important to prevent premature failure of the spindle bearings, as heavy vibration beats them excessively.
It is also possible to arrange the weight distribution of the wheels for better balancing. There are several methods of doing this, all being conceptually analogous to
tire balancing with wheel weights. Most bench grinders never have these operations performed, because wheel dressing alone is enough to keep them sufficiently balanced, but these additional methods are not unusual for bench grinders and are quite common for
machine tool
A machine tool is a machine for handling or machining metal or other rigid materials, usually by cutting, boring, grinding, shearing, or other forms of deformations. Machine tools employ some sort of tool that does the cutting or shaping. All m ...
s that do grinding, such as
surface grinders and
cylindrical grinder The cylindrical grinder is a type of grinding machine used to shape the outside of an object. The cylindrical grinder can work on a variety of shapes, however the object must have a central axis of rotation. This includes but is not limited to such ...
s. This is sometimes achieved by drilling a few holes in the steel
flange
A flange is a protruded ridge, lip or rim (wheel), rim, either external or internal, that serves to increase shear strength, strength (as the flange of an iron beam (structure), beam such as an I-beam or a T-beam); for easy attachment/transfer of ...
s that hold the grinding wheels and then finding the angular orientation at which the holes' lack of weight balances out a heavy spot elsewhere on the rotating mass. Another method is a dedicated type of large flange with little weights that can be screwed down wherever needed for balancing (called a balancing flange.
Wheel replacement
Wheels sometimes need replacing. The grinder is unplugged, and then the outer guard is removed. Typically a large nut holds the wheel on the arbor. The
handedness
In human biology, handedness is an individual's preferential use of one hand, known as the dominant hand, due to it being stronger, faster or more Fine motor skill, dextrous. The other hand, comparatively often the weaker, less dextrous or sim ...
(sense) of each nut is assigned to avoid any loosening tendency during grinder use. Thus on a typical grinder the nut is left-handed on the left side of the grinder from the operator's viewpoint, with wheel rotation "toward" the operator (if the wheel could travel), whereas on the other side the nut is right-handed. "In other words, to remove the nut, it must be turned in the direction that the spindle revolves when the wheel is in operation." Holding the wheel against the wrench's turning action is difficult, but usually the wrench is gently tapped with a hammer instead, which neatly taps the nut loose. A large steel flange is on either side of the wheel, then a paper washer (blotter), then the wheel. The paper is conventionally held to be absolutely necessary to prevent the steel flange from bearing directly against the wheel and tending to crack it; trained workers are expected to know and obey this rule. Thus the entire contact area between flange and wheel is covered, either with paper or with compressible coating (rubber or gasket) on the flange. The wheel's inner diameter may meet the arbor precisely or may be larger with a bushing (sleeve) to reduce it. Before the new wheel is mounted it is ring tested, which involves ''gently'' tapping it with a fingernail or tool handle (wood or plastic) to hear its high-pitched ring (comparable to a
bell
A bell is a directly struck idiophone percussion instrument. Most bells have the shape of a hollow cup that when struck vibrates in a single strong strike tone, with its sides forming an efficient resonator. The strike may be made by an inter ...
's ring, although not loud), indicating that it is not cracked. It is rung in various spots to be sure. A "flat" or "hoarse" sound (failing to "sing") indicates a crack. With the wheel having passed the ring test, the bushing (if any), wheel, paper blotters, steel flanges, and nut are assembled, and the nut is tightened, not as hard as possible, but enough to be firmly fastened in place. The guard is reinstalled, and then the grinder can be plugged in again. Dressing may be needed afterward. When the wheel is first started, the operator stands a bit off center, so that if it were to explode, less pathway for injury exists.
References
Bibliography
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{{DEFAULTSORT:Bench Grinder
Grinding machines