Jigs & Fixtures with differences, Types of Jigs & Fixtures and Constructional details of Jigs & Fixtures
Jigs:
Jig is a special device that holds, supports the work piece
and guides the cutting tool during the operation performed.
Fixtures:
Fixtures holds, locates and supports the work piece securely
in the correct position with respect to the machine/cutter during operation and
does not guide the cutting tool.
Advantages of jigs and fixtures:
•Jigs and fixtures eliminate individual marking, positioning,
and frequent checking. This reduces operation time and increases productivity.
•They enable quick setting and proper locking of work.
•Jigs and fixtures facilitate easy and quick assembly.
•There is no need for skillful setting of the work piece and
tool.
•High production, reduction in scrap, and savings in labor
costs results in substantial reduction in work piece cost.
•The parts produced with their use are very accurate.
•Inspection and quality control of finished product is less.
•Higher speeds, feeds can be used for machining.
•Different machining operation can be done in single stroke.
Jigs:
In metalworking, a jig is a type of tool used to control the
location and motion of another tool. A jig's primary purpose is to provide repeatability,
accuracy, and interchangeability in manufacturing products. A jig is often confused with a fixture. A fixture holds the
work in a fixed location. A device that does both functions [holding the work and
guiding a tool] is called a jig. Jigs or templates have been known long before the industrial
age. There are many types of jigs, and each one is
custom-tailored to do a specific job. Many jigs are created because there is a necessity to do so
by the tradesmen, to increase productivity, to do repetitious activities and
to do a job more precisely & for faster rate of production. Easy to perform the operations even by unskilled workers. Layout and marking on individual parts eliminated.
Types of jigs:
Jigs are divided into two types:
•Open jigs - Work is done on only one side of work piece.
•Closed jigs - Work is done on more than one side of work
piece.
Main types of jigs:
•Template jig
•Plate jig
•Leaf jig
•Box jig
•Channel jig
1. Template jig:
It is a simple plate of metal which carries correct
locations. These types of jigs fits over, on, or into the work and is not
usually clamped. They may or may not have bushings. This will reduce the
marking time. Orientation of the whole pattern to work piece datum's may not be
as accurate as other types. They are usually not practical when locating
datum's are dimensioned.
2. Plate jig:
Plate jigs are similar to template jig. This jig consists of
a drill plate which rest on the component to be drilled For correct positioning
locating pins and clips are provided. A simple clamp will be there to hold the
jig over the work piece. Generally a base plate will not be available for this
type of jigs.
3. Leaf jig:
Leaf jig is a open type jig in which a leaf or a plate is
arranged to swing about a hinged point so that it completely clears the jig for
easy loading and unloading of the workpiece. In this, the drill bushes are
fitted into the leaf.
4. Box jig:
Box jigs, or tumble jigs, usually totally surround the part.
This style of jig allows the part to be completely machined on every surface
without the need to reposition the work in the jig. This jig is designed for
the components which need a very rigid support and difficult to be hold.
5. Channel jig:
Channel jigs are the simplest form of box jig. The work is
held between two sides and machined from the third side. In some cases, where
jig feet are used, the work can be machined on three sides.
Constructional details of jigs:
1.Base plate or jig body,
2.Drill plate or jig
plate,
3.Drill bushes,
4.Locating pins,
5.Clamps.
1. Jig body:
2. Jig plate:
3. Jig bushes:
They are used to locate
and guide drills. They are hardened and ground to exact sizes. Different types
of bushes used in jigs based on the requirements.
4. Locating pins:
They restrict the movement
of the component. Locators position the work piece with respect to the tool.
They facilitate easy loading and unloading of components. It must be assist the
operator for correct loading. Different types of locating pins are used
according to the shape of the components.
5. Clamps:
Standard bolts and
hinged bolts used for clamping. Various types of set screws also used for
clamping. C and U shaped slotted washers are commonly used. Standard clamping
components also used for clamping.
Types of fixture:
1. Vise fixture:
Vise is a mechanical
screw apparatus used for holding or clamping a work piece to allow work to
perform work. Vise usually have one fixed jaw and another parallel jaw which is
moved towards or away from the fixed jaw by the screw. The fixed jaw is moulded
with the vise body and movable jaw is mounted on a screw. Rotation of screw
causes the movable jaw to open and close. This jaw may include a dog hole to
hold a bench dog. This vise are made up of cast steel or malleable cast iron
for high strength.
2. Turning fixture:
Regular shaped work
pieces is easy to hold in lathe using chucks. Holding irregular shaped
components in lathe is very difficult. For simple irregular component can be
hold by for jaw chucks. But adjustment ofjaws in a four-jaw chuck is not easy
for production jobs. So, complicated shapes have to be necessarily held in
position with the help of turning fixtures. The fixtures are normally mounted
on the nose of the machine spindle or on a face plate.
3. Milling fixture:
Pressure of cut should
always be against the solid part of the fixture. Clamps should always operates
from the front of the fixture. The work piece should be supported as near the
tool thrust as possible. A milling fixture is a work holding device which is
firmly clamped to the table of the milling machine. Keys are provided on the
bottom of the base plate which are used for easy and accurate aligning of the
fixture on the milling machine table T-Slots. The fixture is fastened to the
machine table with the help of two T-bolts engaging in T-slots of the work
table. The setting block is fixed to the fixture which have reference planes.
4. Grinding fixture:
Every grinding operation
needs some sort of fixture. The work piece should be supported as near the tool
thrust as possible. Normally grinding divided into two groups,
(i) Cylindrical grinding:
a) Internal grinding,
b) External grinding.
(ii) Surface grinding.
Cylindrical grinding
utilizes mandrels for external surface grinding. The mandrel may be plain or
tapered. Mandrel is hardened and is held between centers of a machine. The work
piece is located and held on the mandrel with the help of the bore so that the
external surface may be machined truly concentric to the bore. For internal
grinding the chuck is the most standard fixture. For holding irregular shaped
component, the fixture designed same as turning or boring fixture. The work
piece can be held for machining on a surface grinder in the following ways:
• It may be held in a vice.
• The work piece may be held by means of a magnetic chuck or a vacuum chuck.
Here the work piece is held without any mechanical clamping.
• The work piece may be held in a special fixture same as milling fixture.
5. Indexing fixture:
Indexing fixtures are
very similar to indexing jigs. These fixtures are used for machining parts that
must have machined details evenly spaced. Such components are required to be
indexed equally as many times as the number of surfaces to be machined. The work
holding devices carry the indexing mechanism.
6. Welding fixture:
A welding fixture clamps
the parts together and simultaneously supporting them, the objective being to
effect a weld in the flat position. Joints must be readily accessible for
welding. The fixture should permit quick and easy positioning [by one hand, if
possible]. Whenever possible, a fixture should be enable all welds to brought
to a convenient welding position. Clamps must operate quickly. Screws and
moving parts should be protected against weld spatter. Methods of grounding the
work are important considerations in fixture design, since they affect arc
action, quality of weld.
Constructional features of a fixture:
These fixtures consist
of a base plate, standard clamps and locators, setting blocks and balancing
weights.
1. Base plate:
The base plate for a
milling fixture is provided with tensions at its bottom for proper location of
the fixture with the machine table through Tee slots.
2. Standard clamps and
locators:
These are provided for
clamping and locating the work pieces with the fixture as in the case of drill
jigs. The clamps used in the fixtures are very rigid and sturdy.
3. The setting blocks:
These are used to
position the fixture and work relative to the cutter before machining. A feeler
is introduced between the cutter and the setting faces of the block for correct
positioning of the cutter with the fixture.
4. Balancing weight:
This is used dynamically
balancing the irregular work piece fixed to the turning or cylindrical grinding
fixture. In the case of a turning fixture, normally the base plate of the
fixture is clamped to the face plate.
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