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MFG 316 Chapter 4 //Workholding Principles

�All devices that grip, hold, chuck,

or retain a workpiece in order to perform a manufacturing

operation.

�Force=hydraulic, pneumatic, electrical, mechanical

�Force multiplication by levers-

H=a/bxF

Workholding Principles

MFG 316 Chapter 4 //Workholding Principles

�Chuck-used during turning, boring, drilling, grinding...

�Vise-simplest workholder, with special jaws, special capabilities, varied methods of actuation

MFG 316 Chapter 4 //Workholding Principles

�Position or locate-definite relationship to the cutting tool

�Must withstand cutting forces and maintain location

�Workholder elements

⌧Locating element-position workpiece

⌧Structure/tool body-withstands forces

⌧brackets-attach workholder to machine

⌧Clamps, screws, jaws-apply holding force to workpiece

�All functions must have

⌧Required firmness of holding

⌧Accuracy of positioning

⌧Safety for operator and equipment

Purpose and function of workholders

MFG 316 Chapter 4 //Workholding Principles

�Physical characteristics

�Support against cutting

force

�Location of workpiece

�Strength of workholder

�Production requirements

�Safety Requirements

�Foolproofing

�Standard parts

�Repeatable location of

workpiece

�Required strength

�Flat-flange, step, face

�Cylindrical-holes, curved

�Irregular-all else

General Considerations Locating Principles

Workpiece Surfaces

MFG 316 Chapter 4 //Workholding Principles

� Plane-wrt a particular plane

� Concentric-wrt internal or external diameter

� Radial-supplemental to concentric

� Combined

Types of Location

MFG 316 Chapter 4 //Workholding Principles

�Degrees of Freedom

�Movement in plane, along major axes with the addition of roll,

pitch, yaw (12deg)

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MFG 316 Chapter 4 //Workholding Principles

3-2-1 Method�Base plane (3 points)�Secondary plane (2 points)�Tertiary plane (1 point)�3 degrees remain-for loading

and unloading

MFG 316 Chapter 4 //Workholding Principles

�Simplest fixture

�restrains 9 degrees

�added radial restrains 11

degrees

Concentric and Radial

MFG 316 Chapter 4 //Workholding Principles

� Position and number of Locators

� Redundant Locators

� Locational Tolerances

� Foolproofing

Basic Locating Rules

MFG 316 Chapter 4 //Workholding Principles

�External Locators

�Integral Locators

�Assembled Locators

�V Locators

�Adjustable Supports

Basic Types of Locators

MFG 316 Chapter 4 //Workholding Principles

�Locators

�Commercial Pin Locators

�Relieved Locators

�Conical , self adjusting

�Spring Locators, stop buttons

�Locating Pins

MFG 316 Chapter 4 //Workholding Principles

�Locating Nests

�Adjustable Locators

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MFG 316 Chapter 4 //Workholding Principles

�Cause wear and disturb proper location

⌧Make locators easy to clean

• raised above surface

⌧Make locators self-cleaning

• edges or projections to wipe part

⌧Protect locators

• entirely covered by workpiece

⌧Air blast, suction, gravity slides used to remove chips

�Burrs can cause jamming in the fixture

⌧Minor burr at entry to part

⌧Major burr at exit from part

Chip and Burr Problems

MFG 316 Chapter 4 //Workholding Principles

MFG 316 Chapter 4 //Workholding Principles MFG 316 Chapter 4 //Workholding Principles

� Clamps matched to the operation-simple best

� Purpose to exert force to hold part against locators and resist machining forces

�Simple clamps preferred

�Proper clamp for job

�Rough parts may require longer travel or digging in of clamp

�Clamp type determined by operation performed

�Clamps should not obstruct load and unload or cause problems

�Clamps that move or slip should not be used in production

�Clamps should be selected by frequency of setup

Clamping Principles

MFG 316 Chapter 4 //Workholding Principles

�Clamping and location for

drilling

⌧Must resist torque and pull-out force

Tool Forces

MFG 316 Chapter 4 //Workholding Principles

�Clamping and location for

tapping

⌧Must resist torque and pull-out forces

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MFG 316 Chapter 4 //Workholding Principles

�Clamping and location for broaching

⌧Cutting tool works with cutting force

MFG 316 Chapter 4 //Workholding Principles

�Clamping and location for milling

⌧Use of fixed jaw to resist cutting force

⌧Use of friction to resist cutting force

MFG 316 Chapter 4 //Workholding Principles

� Clamps hold against a locator

� Mechanical methods to multiply force

� Must counteract tool force

� Must be consistent with material

� Must be appropriate to operation

� Directed parallel to the operation

Clamping Forces

MFG 316 Chapter 4 //Workholding Principles

�Positioning clamps

�Must not distort part

�Contact part at most rigid point

�Clamp over a locator

�Use secondary supports when required

MFG 316 Chapter 4 //Workholding Principles

�Rigid vs. Elastic

�Strap Clamps

�Screw Clamps

�Cam Clamps

�Toggle Clamps

�Wedge Action Clamps

�Specialty Clamps

Types of Clamps

MFG 316 Chapter 4 //Workholding Principles

Chucks and Vises

�Chucking

Operations

⌧External/External

⌧Endwise/Centers

⌧Combination

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MFG 316 Chapter 4 //Workholding Principles

Chucks and Vises

�Chucking Nomenclature

⌧Arbor and Mandrel

�Lathe Chucks

�Solid Arbor and Mandrel

�Straight Mandrel

�Collets

MFG 316 Chapter 4 //Workholding Principles

�Vises

⌧Special Vise Jaws

⌧Independent Jaws

MFG 316 Chapter 4 //Workholding Principles

�Magnetic Chucks

�Vacuum Chucks

�Electrostatic Chucks

�Power Clamping

�Multiple Part Clamping

Nonmechanical Clamping

MFG 316 Chapter 4 //Workholding Principles

�Tool Bodies

�Cast Tool Bodies

�Welded Tool Bodies

�Built-up Tool Bodies

MFG 316 Chapter 4 //Workholding Principles

�Tooling Materials

�Precision Ground Material

�Aluminum Tooling Plate

�Precast Brackets/Shapes

�Structural Sections

⌧C-channel best

MFG 316 Chapter 4 //Workholding Principles

�Screws and Bolts

�Nuts, Washers, Inserts

�Dowels and Pins