This document provides information about off-hand grinders, including safety rules, types of off-hand grinders, their main parts, selecting appropriate grinding wheels, and wheel maintenance. The key types of off-hand grinders are pedestal, bench, and portable. Safety precautions for use include wearing protective equipment and ensuring guards are in place. Proper wheel selection depends on factors like grit size and grade. Wheel dressing helps maintain efficient cutting by exposing fresh abrasive.

1. Mechanical Engineering
Off-Hand Grinders

2. 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

3. 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.

4. 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

5. Types of Off-Hand Grinders
• Bench Type Pedestal Type
Portable type

6. 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.

7. Parts of the machine
• The main parts of the pedestal grinder are
the:
Body
Grinding wheels
Work rests and
Guards

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

9. 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.

10. 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

11. 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.

12. 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.

13. Abrasive
• These are materials used to cut softer
materials. They are used in the manufacture
of grinding wheels. They are:
Natural and
Manufactured

14. 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.

15. 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

16. 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

17. 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.

18. 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

19. 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.

20. 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.

21. 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.

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

23. Selecting Grinding Wheels
• When selecting a grinding wheel they are
certain factors to be considered:
Grit size or Grain
Grade
Type of Bond
Structure

24. 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

25. 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.

26. 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.

27. 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.

28. 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.

29. 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.

30. 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:
Huntington
Diamond dresser
Coarse abrasive stick

31. 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.

32. 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.

33. 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.

34. Dressing Wheels