Jigs and Fixtures in Manufacturing

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Questions and Answers

What is the primary function of a jig in the production process?

To guide the cutting tool (correct)

To support the workpiece only

To eliminate the need for fixtures

To perform the cutting operation

Which of the following elements is NOT typically part of a jig or fixture design?

Locating elements

Cutting elements (correct)

Clamping elements

Supporting surfaces

Which benefit is achieved by using jigs and fixtures in manufacturing?

Enhancing the operator's skill level

Increasing the weight of the workpiece

Eliminating the need for machine tools

Reducing overall machining costs (correct)

What does the 3-2-1 locating principle aim to achieve?

Restrict all degrees of freedom of the workpiece Signup and view all the answers

How many locators are necessary to completely arrest the six degrees of freedom of a workpiece according to the 3-2-1 principle?

Five Signup and view all the answers

What aspect of jigs and fixtures contributes to reducing the operator's labor and skill requirement?

Quick and accurate locating capabilities Signup and view all the answers

Which of the following is a benefit of using both jigs and fixtures in mass production?

Improved product quality consistency Signup and view all the answers

What is the primary role of clamping elements in jigs and fixtures?

To stabilize the workpiece during machining Signup and view all the answers

What type of locator is considered superior for accommodating slight variations in hole diameter?

Conical Locator Signup and view all the answers

Which locator is primarily used for workpieces with rough and un-machined surfaces?

Jack Pin Locator Signup and view all the answers

What must be avoided when designing locators in milling fixtures?

Using clamps to resist tool forces Signup and view all the answers

What is the purpose of setting blocks in relation to the workpiece?

To correct the relationship between the fixture and cutters Signup and view all the answers

What feature of diamond pin locators assists in radial location?

Arresting five degrees of freedom with one pin Signup and view all the answers

Which locator is specifically designed to hold and locate cylindrical workpieces?

Drill Bush Locator Signup and view all the answers

What is the primary function of vee locators?

To quickly locate circular and semi-circular workpieces Signup and view all the answers

What should be designed to resist all tool forces and thrusts in milling fixtures?

Locators Signup and view all the answers

What is the main purpose of feeler gauges in machining?

To achieve correct depth of the cut and lateral setting Signup and view all the answers

What advantage do ejectors provide during machining operations?

They speed up unloading from locators and increase production rate Signup and view all the answers

Which clamping device is based on lever principles to amplify clamping force?

Strap clamps Signup and view all the answers

What is a key requirement for using a swing washer clamp?

The hole must be larger than the nut for the clamp Signup and view all the answers

Which type of clamp is described as providing a quick operation through a shaped cam surface?

Cam clamps Signup and view all the answers

How does the heel pin function in strap clamps?

It serves as the fulcrum for the lever action Signup and view all the answers

What characteristic is true about cam clamps?

They are categorized into three types, including eccentric cam Signup and view all the answers

What is the primary effect of a proper clamping device on machining?

It directly influences accuracy and production cycle time Signup and view all the answers

What is the purpose of the sixth locator in Plane 3?

To prevent motion along the Z-axis. Signup and view all the answers

How many degrees of freedom must be fixed to locate the work piece in the fixture?

9 degrees of freedom. Signup and view all the answers

Which degrees of freedom are fixed by resting the work piece at three non-collinear points on the bottom surface (XY)?

+Z, Clockwise Rotation-X, Anticlockwise Rotation-X, C Rot-Y and AC Rot-Y. Signup and view all the answers

What feature characterizes a duplex fixture?

It uses only two stations for machining processes. Signup and view all the answers

Why are milling fixtures commonly used?

They match the geometric complexity of milled workpieces. Signup and view all the answers

What should be avoided when placing more than one locator on a surface?

Placing locators too close together. Signup and view all the answers

What is a key aspect of the design of milling fixtures?

They should permit machining of as many surfaces as possible without removing the part. Signup and view all the answers

What is the recommended strategy regarding tool changes when using milling fixtures?

Always change the cutter to suit the part. Signup and view all the answers

Study Notes

Introduction to Jigs and Fixtures

Jigs and fixtures are devices used for mass production of repeated parts.
Key functions include locating, clamping, supporting, and resisting cutting forces. Safety is also crucial.

Jig Overview

Both jigs and fixtures hold, support, and locate workpieces.
Jigs also guide the cutting tool.

Fixture Overview

Fixtures hold, support, and locate workpieces.
Fixtures have a reference point for setting the cutting tool, relative to the workpiece.

Purpose of Jigs and Fixtures

Eliminate marking, punching, positioning, and alignments.
Provide easy, quick, and accurate locating, supporting, and clamping of workpieces.
Guiding cutting tools like drills and reamers.
Increase productivity and maintain product quality.
Reduce operator effort and skill requirements.
Improve machine tool technological capacity.
Reduce machining cost and increase interchangeability.

Design Considerations for Jigs and Fixtures

Jigs and fixtures are manually or partially power-operated devices.
They are comprised of various elements: base, body/frame (with clamping features), locating elements, supporting surfaces/base, clamping elements, tool guiding frame/bushes (for jigs), auxiliary elements, and fastening parts.

Locating Principle

Workpieces have six degrees of freedom (or twelve, considering positive and negative movement in each category).
Locating requires arresting all six degrees of freedom for mechanical stability.
A single locator in plane 1 arrests linear motion along the X-axis.
A second locator in the same plane arrests rotary motion around the Z-axis.
Another in the same plane arrests rotary motion about the Y-axis.

Additional Locations

Placing a fourth locator in plane 2, perpendicular to plane 1, restricts linear motion along the Y-axis.
A fifth locator in plane 2 arrests rotational motion about the X-axis.
The sixth locator in plane 3 (perpendicular to both planes 1 and 2) arrests linear motion along the Z-axis.

Considering 12 Degrees of Freedom

To locate a workpiece, fix all 12 degrees of freedom (except three transitional degrees of freedom (–X, –Y, and –Z).
Resting a workpiece on three non-collinear points on the bottom surface (XY plane ) will fix +Z, clockwise rotation-X, and counter-clockwise rotation-X, C Rot-Y, and AC Rot-Y degrees of freedom.
Fixing the workpiece by two points on the side surface (XZ plane) fixes +Y, C Rot-Z, and AC Rot-Z degrees of freedom.
Lastly, fixing the workpiece at one point on an adjoining surface (YZ plane) fixes the +X degree of freedom.

Points to Ponder

If using multiple locators on a surface/plane, distribute them as far apart as possible.
Choose a surface for large locators, considering the workpiece's largest area.

Duplex Fixtures

Used in high-speed, high-volume production, where machining cycles must be continuous.
These use only two stations, revolving to continuously machine parts.

Milling Fixtures

Most common jigs in use today due to workpieces' geometric complexity.
Simplest type is a milling vise mounted on a machine table.
Fixtures become more complex as workpiece size, shape, or complexity increase.

Tool Changes in Milling Fixtures

Changing tools is preferable to moving parts for efficient operations.
Locators should resist all tool forces; clamps should not be used for this purpose.
Sufficient clearance is required for tool changes and part loading/unloading.
Fixtures should be low-profile to prevent twisting or springing during operation.

Setting Blocks

Used to accurately position workpieces relative to cutters, after fixtures are clamped.
Achieved through setting blocks and feeler gauges.
Setting blocks are fixed to the fixture.
Feeler gauges position cutters relative to workpiece reference surfaces/planes.

Ejectors

Used to remove workpieces from close-fitting locators (e.g., full nests, ring nests).
Speed up unloading, reducing in-tool time and increasing production.

Clamping

Essential for firmly restraining workpieces in jigs and fixtures to resist applied forces during operation.
Proper clamping directly affects the accuracy and quality of work and production time.

Strap Clamps

Utilize lever principles to amplify clamping force.
Tightening a stud transfers clamping force to the part.
The fulcrum of the lever is the heel pin.
Clamping force amplification depends on distances between the stud, part, and heel pin.

Screw Clamps

Faster clamping methods use swing/cee washers, especially when workpieces have bores.
Clamped components are quickly loaded/unloaded.
The hole for clamping should be larger than the clamping nut.

Cam Clamps

Provide clamping force based on the cam's contour.
The plate is pressed down against a spring, holding the part.
Cam clamps are quick and come in different designs (eccentric, flat spiral, cylindrical).
Indirect pressure clamping ensures stability and resistance to machining vibrations.

Toggle Clamps

Quick-acting mechanical linkages.
Utilized for their fast clamping/unclamping actions, and for work pieces clearances.
Provide force amplification suitable for clamping operations.

Equalizers

Useful when clamping force is needed at multiple locations.
Link arm systems are used to distribute clamping force evenly to multiple-clamp points.
Convenient when operator access to one clamp is limited.

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Description

Test your knowledge on the use of jigs and fixtures in manufacturing processes. This quiz covers the fundamental principles, benefits, and design considerations involved in the effective use of jigs and fixtures. Explore concepts like the 3-2-1 locating principle and the role of clamping elements.