Work Holding Devices - R Mukhil 14M439 Final Edited

Work Holding Devices 1

Done by

R MUKHIL

(14M439)

Work Holding DeviceWork holding refers to any device that is used to a secure a workpiece and hold it in

place against the forces of machining.

The most basic work holding device is a simple clamp that is screwed into place on a

machining center’s table, but work holding can also involve complex fixtures that are

custom-built for particular parts.

Other common work holding devices include vises and chucks, as well as indexers or

rotary tables that are able to change the part’s position while it is held, so the machine can

reach various features of the part without the setup having to be changed.

In most machining applications, work holding also locates the part. In turning, for

example, a chuck not only holds the part securely but also holds it precisely on the

centreline of the spindle.

On a machining center, the vise or fixture that holds the part may also hold it in the

precise position and orientation where the machining program expects to find it. For this

reason, components such as locating pins and workstops that make it easy to place piece

after piece in the same location are also common work holding components.


Work Holding Devices

Dedicated workholding devices

Jigs

Fixtures

Modular

Universal

Vises

Chucks

Dedicated Work holding Devices Jig: A specially designed w/h device that determines location dimensions that is

produced by machining or fastening operations.

Fixture: A specially designed w/h device that holds the work during machining or

assembly operations and establishes size dimensions.

Chucks and vises are often used for constructing fixtures.

Conventional Fixture Designs

Work holding devices provide to fundamental functions, locating and clamping.

Locating refers to orienting and positioning the part relative to the cutting tool.

Clamping refers to holding the part in its proper orientation with enough force to resist

the force of cutting but not deform the part.

Example of a work holder

FIGURE 27-2 A CNC turning

center with two chucks, turrets

for

cutting tools, and C-axis control

for

the main spindle. The C-axis

control, on the spindle, can

stop it

in any orientation so the

powered

tools can operate on the

workpiece.

Design Criteria for Work Holders

Positive location

The workpiece is located by 3-2-1

principal.


Repeatability

Fool proofing

Adequate clamping forces

Gravity, cutting, inertial, centrifugal

Reliability

Ruggedness

Loading and unloading operations more abuse than machining

Replacement of parts that wear


Constructability

Use standard elements

Low Profile

Workpiece Accommodation

Ergonomics, Safety

Pinch points

30-40 lbs force to operate clamp

Tool path and operator’s paths should not cross


Part distortion

Flexibility

Accommodate a family of parts

Design Considerations for Work Holders

Clamping

Spread clamping forces over large area

Direct clamping toward work support and locating points

Cutting forces toward fixed area of w/h device

Perform as many machining operations as possible with each setup.

Design Considerations for Work Holders

Chip Disposal

Decrease tool life

Interfere with part loading

Loading/Unloading Time

Minimize

Work Holding Devices of Milling Machines

Tool Maker Universal Vise

Plain Vise

Swivel Base Vise

V – Block

Dividing Head

Rotary Table

Tilting Table

Angle Plate

Bolts & Clamps


Tool Maker

Universal Vise• The universal vise can be

swivelled in a horizontal

plane similar to a swivel

vise.

• It can also be tilted in any

vertical position for

angular cuts.

• The rise not being rigid in

construction is used mainly

in tool room work.

Tool Maker Universal Vise


Plain Vise

• Either a plain or swivel-

type vise is furnished with

each milling machine.

• The plain vise, similar to

the machine table vise is

used for milling straight

workplaces and is bolted to

the milling machine table

either at right angles or

parallel to the machine

arbor.

Plain Vise


Swivel Base Vise• The Work holding device

used for holding small and

regular workpieces that

can be swivelled(rotate)

about its base.

Swivel Base Vise


V-Block• V-Blocks are precision metal working jigs

typically used to hold round metal rods or pipes

for performing drilling or milling operations.

• They consist of a rectangular steel or cast iron

block with a 90-degree channel rotated 45-

degrees from the sides, forming a V-shaped

channel in the top. A small groove is cut in the

bottom of the "V".

• They often come with screw clamps to hold the

work.

• There are also versions with internal magnets

for magnetic work holding.

• V-blocks are usually sold in pairs.

V-Block


Dividing Head• An indexing head, also known as a dividing

head or spiral head, is a specialised tool

that allows a workpiece to be circularly

indexed ; that is, easily and precisely

rotated to preset angles or circular

divisions.

• Indexing heads are usually used on the

tables of milling machines, but may be used

on many other machine tools including drill

presses, grinders, and boring machines.

• Common jobs for a dividing head include

machining the flutes of a milling cutter,

cutting the teeth of a gear, milling curved

slots, or drilling a bolt hole circle around

the circumference of a part.

Dividing Head


Rotary Table

• A rotary table is a precision work

positioning device used in metalworking.

• It enables the operator to drill or cut work

at exact intervals around a fixed (usually

horizontal or vertical) axis.

• Some rotary tables allow the use of index

plates for indexing operations, and some

can also be fitted with dividing plates that

enable regular work positioning at

divisions for which indexing plates are not

available.

• A rotary fixture used in this fashion is

more appropriately called a dividing head

(indexing head).

Rotary Table


Tilting Table• The workpiece held in the

tilting table is used to tilt

the workpiece in desired

angle.

Tilting Table


Angle Plate• When working surfaces are

to be milled at right angles

to another face, angle

plates are used for

supporting the work.

• The angle plate bolted on

the table and the work

piece is supported on its

face by bolts and clamps.

Angle Plate


T-Bolts & Clamps

• Bulky work pieces of irregular

shapes are clapped directly on the

milling machine table by using T-

Bolts and Clamps.

T-Bolts & Clamps


Work Holding Devices of Shaping Machine Shaper Vise

T – Bolts & Clamps

Stop Pins & Toe Dogs

Strip and Stop Pins

Angle Plate

V – Block

Shaper Centres


Shaper Vise• Vise is a quick method of holding and

locating relatively small and regular

shaped work pieces.

Parallels

• When the height of the job is less than

the height of the jaws of the vise,

parallels are used to rise and seat the

workpiece above the vise jaws and

parallel with the vise.

Hold Downs

• Hold downs are grippers are used for

holding thin pieces of work in a shaper

vise.


Shaper Vise

Clamping work on the Table

The different methods employed to clamp

different types of work on a shaper table

are:

Stop Pins•A stop pin is a one-leg screw clamp.

•Stop pins are used to prevent the work

from coming out of the position.

Stop Pins



• While holding thin work on the

table, stop pins in conjunction

with toe dogs are used.



Strip and Stop Pins

• Work having sufficient thickness is

held on the table by strip and stop

pins.

Strip and Stop Pins


Shaper Centres• This is a special attachment used

for cutting equally spaced grooves

or splines on the periphery of a

round work.

Shaper Centres


Work Holding Devices of a Planing Machine

The work may be held on a planing machine table by the following methods:

1. By Standard Clamping

2. By Special Fixtures

Standard Clamping Devices: The standard clamping devices are used for holding most

of the work on a planner table. The devices are as follows:

1. Heavy Duty Vises

2. T-Bolts and Clamps

3. Stepblocks, Clamps and T-Bolts

4. Poppets or Stop pins and Toe dogs

5. Angle plates

6. Planer jacks

7. Planer centres

8. Stops

9. V-BlocksWork Holding Devices 29

Planar Table• The figure shows the

arrangement for holding a

large workpiece.

• Strap Clamp hold the

workpiece on all sides.

Planar Table


Planer Jack• A Jack is used to level up

the work to be machined

on a Planer Jack

Planer Jack


Special Fixtures• Special fixtures are used

for holding a large identical

pieces of work on a planer

table.

• Fixtures are specially

designed for holding a

particular type of a work.

By using a fixture the

setting time may be

reduced considerably.

Special Fixtures


Work Holding Devices of a Slotting Machine The work is held on a slotter table by a vise, T-bolts and clamps or by special fixtures.

T-bolts and clamps are used for holding most of the work on the table.

Fixtures are used for holding repetitive work.


Types of Jigs

There are several basic forms for jigs, some of the basic types are:

Plate Jig

Channel Jig

Ring Jig

Leaf Jig

Box Jig

Universal Jigs

Common Jigs

FIGURE: Examples of some

common types of

workholders—jigs.

Universal Jigs

FIGURE: Two types of universal jigs are manual

(bottom) and power-actuated (center). A

completed jig (on the top) made from unit right

below.

Conventional Fixtures

Conventional Fixtures

A Vise are general purpose fixtures mounted on subplates and can have their jaws interchanged base on part geometry.

Lathe Chucks are general purpose fixtures for rotational parts

FIGURE: The conventional or

standard vise (top left and right)

can be modified with removable

jaw plates to adapt to different

part geometries. These vices

can be integrated into milling

fixtures (right middle and

bottom).

Conventional Vises

Conventional Chucks

FIGURE: Quick-changing of the top jaws on a three-jaw chuck.

Modular Fixturing

Modular Fixtures are similar to conventional fixture, except they are more versatile.

Modular systems use dowel pins and T-slots to provide a rigid, adjustable fixture.

Standard elements are positioned to fit the part needs, such as

Riser blocks Vee blocks

Angle plates Cubes

Box parallels Supports

Locator pins Clamps

Modular Fixtures

FIGURE: Modular

fixturing begins with a subplate

(grid base) and adds locators

and clamps.

Modular Fixture

FIGURE: Dedicated fixture on the left versus modular fixture on the

right.

Setup and Changeover

To speed up changeover, master jigs or intermediate jigs can be used.

A Master Jig, is a jig that can be used to make a number of similar parts.

An Intermediate jig is a jig that is designed hold another jig that can be quickly

changed out for each part.

Master Jig

FIGURE: Master jig

designed for a family of similar

components. (a) Part family of

rounds plates (six parts, A–F);

(b) group jig for drilling,

showing adapter and part A.

Intermediate Jig

FIGURE: Example of the intermediate jig concept applied to lathe

chucks. The actuator is mounted on the lathe and can quickly adapt to

three different chuck types. (Courtesy of ITW Workholding)

Clamps

FIGURE: Examples of basic types

of clamps used for workholding. The clamp

elements come in a wide variety of sizes.

Power Actuated Clamps

FIGURE: Examples of

power-clamping devices:

(a) extending clamp;

(b) edge clamp.

Other Workholding Devices

Other workholding devices include

Assembly jigs

Used to keep ensure the final assembly meets the location and fit

Magnetic workholders

Limited in holding force, but ensures that there is no distortion of a steel

workpiece

Electrostatic workholders

Similar to magnetic chucks, but used on electrically coductive non-

ferromagnetic materials, limited clamping force

Vacuum Chucks

Works with any material, initial set up more time consuming.

Assembly Jig

FIGURE: Example of large assembly jig for an airplane wing.

The body of the wing and flap are held in the correct location

with each other and then the flap is mechanically attached.

Electrostatic Chuck

FIGURE: Principle of

electrostatic chuck.

Vacuum Chuck

FIGURE: Cutaway view

of a vacuum chuck. (Courtesy of

Dunham Tool Company, Inc.)

Documents

Work Holding Devices - R Mukhil 14M439 Final Edited