Mechanical EngineeringOff-Hand Grinders

Objectives

• State at least five safety rules associated with Off- Hand Grinders

• State and explain types of Off-Hand grinders• State and explain the main parts of Off-Hand

grinders• Select appropriate wheel for material being

ground• Mount wheel in correct position• Dress wheel

Grinding Machine

• Off-Hand Grinders are used for the removal of waste metal and the sharpening of small tools. They are called off-hand because the work is applied to the grinder by hand.

Safety Precautions• Always wear safety

goggles• Cool work piece often• Keep hands clear of

wheel• Do not use rag around

revolving parts• The wheel guard must

always be securely fixed in place

• Belt and pulley guards must always be in place

• Stop all motors before making adjustment

• Test grinding wheel for possible cracks

• Wear a good mask for heavy grinding

• Only one person should operate the machine

Types of Off-Hand Grinders

• Bench Type Pedestal Type

Portable type

Types of Off-Hand Grinders

• Pedestal- they are sturdy and is used for general purpose work. It is mounted on a stand.

• Bench type- they are small and mounted on the bench. It is usually used to sharpen cutting tools.

• Portable type- they are used for general cleaning of metal work and finishing welded jobs.

Parts of the machine• The main parts of the pedestal grinder are

the:BodyGrinding wheelsWork rests andGuards

Body

• The body is made of boxlike construction of cast iron and forms the main component of the machine

Grinding Wheels

• These are used for metal removal and are available in all shapes and sizes. Disc shape wheels are usually used on the pedestal grinding machine. Wheels are also available in many types according to the type of material to be ground, finish required and accuracy.

Guards

• These are installed on the machine to ensure that no part of the body of the operator comes into contact with moving parts.

• They retain chips or fragments of a wheel if it breaks or disintegrates.

• Glass guards are installed over the wheels of some machines. These prevent dust particles from flying into the eyes of the operator

Work Rests

• Work rests are used for supporting and guiding the work. They could be adjusted to facilitate tool grinding. Work rests should be about 2mm from the wheel surface to prevent work from getting between the wheel and the rest.

Manufacture of Grinding Wheels

• Grinding wheel is manufactured by mixing a predetermined amount of abrasive particles with an appropriate bonding agent and baking the mixture.

Abrasive

• These are materials used to cut softer materials. They are used in the manufacture of grinding wheels. They are:

Natural andManufactured

Natural Abrasives

• Emery- this is an impure form of aluminum oxide consisting of crystals of the oxide embedded in a mixture of iron oxide. Emery is 60% pure.

• Corundum- this is an impure form of aluminum oxide consisting of aluminum oxide associated with varying amount of other impurities. Corundum is 90% pure.

Manufactured Abrasives

• Aluminum Oxide- it is produced from bauxite and is very tough. Used for cutting metals with a high tensile strength e.g. high speed steel.

• Silicon Carbide- it is a chemical combination of carbon and silicon, with small additions of coke, salt, sand and sawdust. It is very hard and brittle and used for grinding metals which have a low tensile strength e.g. brass, copper, aluminum, cast iron and tungsten carbide

Bond• This is used to hold the abrasive particles

together to form the wheel. The six common types used to manufacture grinding wheels are:

• Vitrified• Resinoid• Rubber• Shellac• Silicate• Metal bond

Vitrified

• Produced from various kinds of fusible and refractive pottery clays. Possesses high strength and porosity and does not clog very easily. Removes metal rapidly and efficiently. Vitrified bond is the one commonly used for wheels for tool sharpening.

Resinoid

• Made from synthetic resins mixed with abrasive grains. Resinoid wheels can run at higher speeds than vitrified bond wheels. Used for cutting metal bars and tubes, and for work needing a very fine finish

Rubber

• Made from raw rubber and abrasive. Rubber bonded wheels are the strongest of all. They allow a small amount of flexibility, hence they are used as very thin wheels.

Shellac

• Shellac wheels possess a high degree of safety which makes them suitable for operations which require thin wheels, e.g. cutting off harden steel. Used for heavy duty, large diameter wheels.

Silicate

• Used where fine finishes are required, hence they are used for grinding the edges of tools. It is the weakest of all the bonding materials and is easily damaged.

Metal Bond

• Used on diamond wheels and for electrolytic grinding where a current must pass through the wheel.

Selecting Grinding Wheels

• When selecting a grinding wheel they are certain factors to be considered:

Grit size or GrainGradeType of BondStructure

Grit size

• The size of the abrasive grains determines the coarseness of fineness of the grinding wheel.

Grit Sizes Uses

Coarse 8-20 Roughing

Medium 30-60 General purpose

Fine 80-180 Finishing

Very fine 200-400 Jewelers

Grade

• This denotes the strength with which the bond holds the grit in place. The grade is indicated by letters of the alphabet, soft wheels having letters at the beginning of the alphabet and very hard wheels letters at the end of the alphabet.

Bond

• This is the material that holds the grit together. It may be one of the following: vitrified, silicate, rubber, shellac, synthetic resin or metal bond. Soft wheel- soft bond- grit readily breaks away under pressure, thus exposing fresh grit. Used for grinding hard materials. Hard wheels-hard bond used for grinding soft material.

Structure

• The structure of the wheel is an indication of the bond/abrasive ratio. The percentage of bond material varies between 10-30% of the wheel. This percentage will determine how far apart the abrasive grains are from each other.

Mounting Wheels

• Examine for any obvious surface defects• Test wheel for cracks by suspending by its

centre and tapping with a light bar. It should give a clear ring sound.

• Examine the lead bush for burrs• After mounting, run the wheel for some

minutes without load and without standing near.

Out of Balance

• This is cause when there are unequal forces acting on the circumference of the wheel, i.e. light on one side and heavy on the other. The remedy for this defect in a grinding wheel is truing and dressing.

Wheel Dresser

• For efficient cutting, the wheel must be free of glaze, run true and have a flat surface. To maintain this condition frequent dressing is necessary with a wheel dresser, which may be one of three types:

HuntingtonDiamond dresserCoarse abrasive stick

Huntington/star wheel

• This dresser consists of a set of star-shaped wheels and washers made of hardened steel which rotate in a heavy holder. The wheels and washers are replaceable when worn.

Diamond Dresser

• It consist of an industrial diamond brazed into a steel body. This tool is comparatively expensive but long-lasting. It is usually used for truing the wheels of precision grinders.

Coarse Abrasive Stick

• In use dressers are held firmly on the tool rest and in contact with the wheel while it is rotating. Cuts are taken over the whole face of the wheel until the original shape has been restored or a fresh layer of sharp grit exposed.

Dressing Wheels