Jigs and Fixtures
Introduction
Fixtures : being used in machine shop, are strong
and rigid mechanical devices which enable easy,
quick and consistently accurate locating, supporting
and clamping of blanks against cutting tools and
result in faster and accurate machining with
consistent quality, functional ability and
interchangeability.
Jig : It is a fixture with an additional feature of tool
guidance.
Purpose of using Fixtures and Jigs
To eliminate marking, punching, positioning,
alignments etc.
Easy, quick and consistently accurate locating,
supporting and clamping the blank in alignment of
the cutting tool.
Guidance to the cutting tool like drill, reamer etc.
Increase in productivity and maintain product quality
consistently.
To reduce operator’s labour and skill – requirement.
To reduce measurement and its cost.
Enhancing technological capacity of the machine
tools.
Reduction of overall machining cost and also
increase in interchangeability.
Economic viability of jig and fixture for batch production.
Major elements of jig and fixtures.
Design Considerations for Jigs and Fixtures
Easy, quick and consistently accurate locating of the
blank in the jig or fixture in reference to the cutting
tool.
Providing strong, rigid and stable support to the
blank.
Quick, strong and rigid clamping of the blank in the jig
or fixture without interrupting any other operations.
Tool guidance for slender cutting tools like drills and
reamers.
Easy and quick loading and unloading the job to and
from the jig or fixture.
Use of minimum number of parts for making the jig or
fixture.
Use of standard parts as much as possible.
reasonable amount of flexibility or adjustability, if
feasible, to accommodate slight variation in the job -
dimensions.
Easy, quick and accurate indexing system if required.
prevention of jamming of chips, i.e. wide chips-space
and easy chip disposal.
easy and safe handling and moving the jig or fixture on
the machine table, i.e., their shape, size, weight and
sharp edges and corners
• easy and quick removal and replacement of small parts
• manufacturability i.e. ease of manufacture
• durability and maintainability
• service life and overall expenses
Principles or rules of locating in jigs and fixtures
Possible degrees of freedom of a solid body.
Methods of Location (3-2-1 Principle)
- A workpiece will be completely confined when banked against:- 3 points in one plane- 2 points in another plane- 1 point in a third plane- All three planes are perpendicular to each
other.
- Buttons should be as far apart as possible because Greater the spread, lesser alignment error.
12 degrees of freedom
3 pins arrest 5 dof
5 pins arrest 8 dof
6 pins arrest 9 dof
Arresting all degrees of freedom of a blank in a fixture.
7 dof arrested by V locator with stop pin
Workholder with multiple V locators
basic principles or rules need to be followed while planning for locating blanks in fixtures.
One or more surfaces (preferably machined) and / or
drilled / bored hole(s) are to be taken for reference.
The reference surfaces should be significant and
important feature(s) based on which most of the
dimensions are laid down.
Locating should be easy, quick and accurate.
In case of locating by pin, the pins and their mounting
and contact points should be strong, rigid and hard.
A minimum of three point must be
used to locate a horizontal flat surface.
The locating pins should be as far apart
as feasible.
Vee block and cones should be used for self-locating
solid and hollow cylindrical jobs.
For locating large jobs by rough bottom surface one of
the three pins may be replaced by a pivoted arm. The
pivoted arm provides wo contact points.
Locating by Vee block and cone
Clamping of workpiece in fixtures
While designing for clamping the following factors essentially need to be considered :
1. Clamping need to be strong and rigid enough to hold
the blank firmly during machining.
2. Clamping should be easy, quick and consistently
adequate.
3. Clamping should be such that it is not affected by
vibration, chatter or heavy pressure.
4. The way of clamping and unclamping should not hinder
loading and unloading the blank in the jig or fixture.
5. The clamp and clamping force must not damage or
deform the workpiece.
6. Clamping operation should be very simple and quick
acting when the jig or fixture is to be used more
frequently and for large volume of work.
7. Clamps, which move by slide or slip or tend to do so
during applying clamping forces, should be avoided.
8. Clamping system should comprise of less number of
parts for ease of design, operation and maintenance
9. The wearing parts should be hard or hardened and also be
easily replaceable.
10. Clamping force should act on heavy part(s) and against
supporting and locating surfaces.
11. Clamping force should be away from the machining
thrust forces.
12. Clamping method should be fool proof and safe.
13. lamping must be reliable but also inexpensive
Methods of Clamping
Clamping method and system are
basically of two categories :
1. General type without much consideration on speed of clamping operations.
2. Quick acting types.
General clamping methods of common use
Screw operated strap clamps
The clamping end of the strap is pressed against a spring which enables quick unclamping.
Clamping from side for unobstructed through machining (like milling, planing and broaching) of the top surface.
Clamping from side for free machining of the top surface.
Clamping by swing plates
Such clamping are simple and relatively quick in operation but is suitable for jobs of relatively smaller size, simpler shape and requiring lesser clamping forces.
Other conventional clamping methods include :
Vices like drilling and milling vices.
Magnetic chucks.
Chucks and collets for lathe work.
Quick clamping methods and systems
Cam clamping
Quick clamping by cam is very effective and
very simple in operation.
The cam and screw type clamping system is
used for clamping through some interior parts
where other simple system will not have access.
Cam Clamping
screw and cam clamping from
distance
Quick multiple locating and clamping of cylindrical
jobs.
Quick multiple clamping by pivoted clamps in series and
parallel is capable to simultaneously clamp number of rods
even with slight diameter variation.
Quick clamping by hydraulic and pneumatic force for
strong and light clamping respectively.
Light but quick clamping by bayonet type clamp.
Quick acting screw (bayonet type) clamping.
Use of quick acting nut for rapid clamping
Drill – jig bushing
Necessity
Slender and cantelever type cutting tools, mainly drills,
usually suffer from run – out due to possible errors in the
drill, sockets and drilling machine spindle and finally in
the overall alignment.
Such run out causes over sizing, out of roundness and
surface roughening of the drilled holes.
Such run out aggravates further with the increase in drill
speed (rpm) and the thrust force, specially if the drill is not
geometrically symmetrical.
This often leads to, in addition to poor product quality,
breakage of the drill by bending and / or buckling.
To reduce such problems, bushes are used in the jigs to
guide the drill bits.
The factors to be considered while designing for jig – bushing
The bushes, used to guide and properly locate drills,
reamers etc. are generally made of carbon or alloy steel
and made wear resistive by hardening to RC 60 and above.
Often bushes are also made from grey cast iron for
antifriction and protection of the tools.
The hardened jig bushes are finished outside by grinding
and inside by grinding and lapping if high precision is
insisted.
The bush’s length should be sufficient (≥ twice drill
diameter) and its diameter should be slightly larger than
the drill diameter
Design and construction should enable easy and quick
proper fitting and removal or replacement of the bushes
Bushes should not come out from its seat along with the
drill during its return.
Types of jig bushes
Depending upon nature of fitting, quick mounting and
replacement, job requirement etc., jig bushes are
classified into several types.
• Press fitted type
• Slip type
• Screwed type
Press fitted thin sleeve type bushes are generally used
for shorter runs and are not renewable.
Renewable type slip bushes are used with liner.
screw bushes, though renewable may be used without
or with liner.
Bushes may be
Without head
With head
With a flange being screwed on the bracket
Frequently replaceable bushes are provided with some
locking system.
Locking of frequently
replaceable bushes
Some special jig bushings are often designed and used as and when required.
two close holes – in one bush
two very close holes – using eccentric bush
two close holes – by two adjacent modified bushes
Types of Jigs
Open jig (Plate jig or drill template)
It is the simplest type of drill jig.
It consists of a plate with holes to guide the drills,
and may have locating pins that locate the
workpiece on the jig.
Jig body
Holds the various parts of a jig assembly.
Cap screws and dowel pins
Hold fabricated parts together.
Components of Drill Jig
Locating devices
Pins, pads, and recesses are used to locate the
workpiece on the jig.
Clamping devices
Locking pins
Inserted after the first hole is
drilled to lock or hold the
workpiece securely to the
jig plate while subsequent holes
are being drilled.
Template Jig
Template jigs are normally used for accuracy rather
than speed.
This type of jig fits over, on, or into the work and is
not usually clamped.
Templates are the least expensive and simplest type of
jig to use.
They may or may not have bushings. When bushings
are not used, the whole jig plate is normally hardened.
Plate jigs are similar to templates. The only difference
is that plate jigs have built-in clamps to hold the work.
These jigs can also be made with or without bushings,
depending on the number of parts to be made.
Plate jigs are sometimes made with legs to raise the jig
off the table for large work. This style is called a table
jig.
Plate Jig
Plate jig
Table jig
Sandwich jigs are a form of plate jig with a back plate.
This type of jig is ideal for thin or soft parts that could
bend or warp in another style of jig.
The use of bushings is determined by the number of
parts to be made.
Sandwich Jig
Sandwich jig
Angle-plate jigs are used to hold parts that are
machined at right angles to their mounting locators.
Pulleys, collars, and gears are some of the parts that
use this type of jig.
A variation is the modified angle-plate jig, which is
used for machining angles other than 90 degrees.
Both of these examples have clearance problems with
the cutting tool.
Angle-Plate Jig
Angle-plate jig Modified Angle-plate jig
As the drill exits the product being drilled, it has little
or no room for the drill point to clear the product
completely, produce a round hole all the way through
the part wall, and avoid drilling the part locator.
This is most noticeable, where an angled hole requires
additional clearance to the relieved portion of the part
locator.
Additional clearance here would allow the drill to
complete the hole and avoid drilling the relieved
portion of the locator.
Box Jig or Tumble 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.
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.
Leaf Jig
Leaf jigs are small box jigs with a hinged leaf to allow
for easier loading and unloading.
The main differences between leaf jigs and box jigs are
size and part location.
Leaf jigs are normally smaller than box jigs and are
sometimes made so that they do not completely
surround the part.
They are usually equipped with a handle for easier
movement.
Indexing Jig
Indexing jigs are used to accurately space holes or
other machined areas around a part.
To do this, the jig uses either the part itself or a
reference plate and a plunger.
Larger indexing jigs are called rotary jigs.
Types of Fixtures
Jigs and fixtures are made basically the same way as
far as locators and positioners are concerned.
The main construction difference is mass.
Because of the increased tool forces, fixtures are built
stronger and heavier than a jig would be for the same
part.
Classification of Fixtures
Fixtures are normally classified by the type of machine
on which they are used.
Fixtures can also be identified by a sub classification.
For example, if a fixture is designed to be used on a
milling machine, it is called a milling fixture.
If the task it is intended to perform is straddle milling, it
is called a straddle milling fixture.
The same principle applies to a lathe fixture that is
designed to machine radii. It is called a lathe-radius
fixture.
Plate Fixture
Plate fixtures are the simplest form of fixture.
The basic fixture is made from a flat plate that has a
variety of clamps and locators to hold and locate the
part.
The simplicity of this fixture makes it useful for most
machining operations.
Its adaptability makes it popular.
Angle-plate Fixture
The angle-plate fixture is a variation of the plate
fixture.
With this tool, the part is normally machined at a right
angle to its locator.
While most angle-plate fixtures are made at 90
degrees, there are times when other angles are needed.
In these cases, a modified angle-plate fixture can be
used.
Modified Angle-plate
fixture
Angle-plate fixture
Vice-jaw Fixture
Vise-jaw fixtures are used for machining small parts.
With this type of tool, the standard vise jaws are
replaced with jaws that are formed to fit the part.
Vise-jaw fixtures are the least expensive type of
fixture to make.
Their use is limited only by the sizes of the vises
available.
Indexing Fixture
Indexing fixtures are very similar to indexing jigs.
These fixtures are used for machining parts that must
have machined details evenly spaced.
Parts machined with an indexing fixture
Multistation Fixtures
Multistation fixtures are used primarily for high
speed, high-volume production runs, where the
machining cycle must be continuous.
Duplex fixtures are the simplest form of multistation
fixture, using only two stations.
This form allows the loading and unloading
operations to be performed while the machining
operation is in progress.
For example, once the machining operation is
complete at station 1, the tool is revolved and the
cycle is repeated at station 2.
At the same time, the part is unloaded at station 1 and
a fresh part is loaded.
Duplex Fixture
Profiling Fixtures
Profiling fixtures are used to guide tools for
machining contours that the machine cannot normally
follow.
These contours can be either internal or external.
Since the fixture continuously contacts the tool, an
incorrectly cut shape is almost impossible.
The operation in Fig. shows how the cam is accurately
cut by maintaining contact between the fixture and the
bearing on the milling cutter.
This bearing is an important part of the tool and must
always be used.