Fixture Design Criteria Phase Ii
Fixture Design Criteria Phase Ii
Fixture Design Criteria Phase Ii
ABSTRACT
The overall objective of this project is to research technologies to improve and speed
fixture designs within Tinker AFB. The proposed product at the end of three phases is a
comprehensive Roadmap for Fixture design and sample of the process involving some
complex fixtures representing the major production areas within the center. Through the
Phase I Project an electronic fixture design manual was initiated. An interactive fixture
design advisor is developed in Phase II that provides a computerized animation of 11
fixtures actually designed and implemented at Tinker. This report summarizes the
progress-to-date of the project Fixture Design Criteria. Projected benefits of the project
include improved repair processes, reduced costs, and higher reaction speeds.
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INTRODUCTION
The overall objective of this project is to research technologies to improve and speed
fixture designs within Tinker AFB. The proposed product is a Roadmap for Fixture
design and a prototype of the process involving 5-10 complex fixtures representing the
major production areas within the center. Through the Phase I Project an electronic
fixture design manual has been initiated. Currently, this manual has fixture design detail,
the components of jigs and fixtures and a step-by-step design procedure. A CAD
animation has been developed and hyperlinked to show 3-D manipulation. This manual
has been so developed that a beginner may design a fixture from scratch. Another module
shows possible hydraulic chucks and fixture components with pricing. These manuals are
currently specific to machining fixtures and consider hypothetical scenarios. During
Phase II, 10 actual fixtures designed and implemented at the base by Tinker Shop
Engineers are studied, analyzed and computerized (animated) within manipulable,
interactive software VRML.
Basic fixture design for manufacturing applications envelopes two main aspects:
location and clamping. Between these two functions, the 6 (3 translation and 3 rotation)
degrees of freedom are constrained, while effectively positioning and orienting the part
during processing. The location of box-type parts is usually achieved using the 3-2-1
principle. This principle locates the primary plane by three non-collinear points, typically
widely spaced; the second plane is located by two points and the third plane by one point.
Cylindrical part axes are usually located using V-blocks while concentric locators are
used to locate priorly drilled holes. The cutting wrenches (forces) are supported by
effectively holding the workpiece, to minimize distortion or deformation of the object.
Chip clearance, ease of part loading and removal, use in multiple applications (versatility)
is often additional considerations in designing fixtures. Jigs also provide tool guidance in
addition to the location and clamping provided by fixtures. Usually sheet metal
fabrication and assembly often requires other types of fixtures than machining fixtures. In
any case, fixture design is most cost-justified for batch or mass production runs.
Considering this, the fixture designs for single-piece parts are better accomplished by
modular fixtures. Fixture design is typically a setup cost function, making it very
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valuable in flow time and indirect cost calculations. Due to the rapid response required in
many applications, the fixture design principles must be integrated and properly detailed
so as to facilitate the fast design development of a fixture. Flexible, palletized and
modular fixtures are quite common in today’s industry to maintain rapid tooling in the
agile environment.
This project researches the parts manufactured at the base and some fixtures currently
developed and employed for specific applications. Correlation between theoretical fixture
design principles and actual practical development at Tinker can be drawn through such
an exercise. At the end of the three phases of this project, a formal generalization of the
concepts and methodologies of fixture design for Tinker will be developed. Specific
constraints for each shop will again be identified. Hence, a step-by-step procedure for jig
and fixture design will be developed. In addition to a hard-copy fixture design manual, a
procedural CD-ROM/Web-page for fixture design principles for manufacturing of
different part geometries will be developed. The links with conventional CAD and CAE
systems will be explored to automatically integrate part design concepts and engineering
(stress, deflection, etc.) analysis. In summary, a roadmap to fixture design and/or existing
fixture modification will be developed.
BACKGROUND
This report presents a basic detail of the information compiled for the electronic
Fixture Design Advisor. Information useful to the design of work-holding devices for
different operations was compiled and presented in Phase I. The principles of design,
essential elements of fixturing, dos and don’ts, and different devices available for quick
fixturing were discussed in detail. Phase I also presented information on hydraulic
chucks, jigs, and fixtures, as requested by the sponsor.
The information was compiled in the form of a web page. The fixturing principles
were illustrated using pictures and animations. At the conclusion of Phase I feedback was
obtained from the sponsor and other Tinker officials. A need was felt to add a certain
amount of user interaction in the learning process and to allow the user to visualize
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flexible programming language. VRML can easily blend with HTML and JAVA making
the overall system very flexible.
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‘SIGGRAPH 96’ in New Orleans. VRML 2.0 Specification was accepted as an ISO
standard in April 1997.
VRML
A VRML file is basically a textual description of the VRML world. The VRML
file is edited in any text editor such as notepad or WordPad but is characterized by the
extension ‘.wrl’. The VRML file contains all the information to build shapes, position
shapes, and animate shapes and can also contain embedded java codes to add user
interaction and computation of data. Whenever a web browser (e.g. Internet Explorer,
Microsoft Corporation) with a special utility, called VRML plug-in, reads a VRML file
with ‘.wrl’ extension, it builds a world exactly as described by the file. The different
VRML plug-ins available for viewing VRML worlds are;
‘Cosmo® Player’ by Computer Associates, Inc.
‘Cortona® VRML Client’ by Parallel Graphics
Cosmo® Player is used for the project because of its user-friendly menus and good
functionality.
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The fixture shown above consists of three components: the part, the fixture and tightening
screw. Each of these parts in the assembly can be considered as node and the attributes of
the part as color contrast and shape coordinates can be considered field values. Using
animation node, sensor node, motion node, and other nodes in VRML specification user
interaction with the world is achieved. The different programmable nodes used for fixture
assembly are;
Viewpoint Node:
This node assigns different viewpoints in the VRML world. Different views for
the fixture have been created using this viewpoint node.
PlaneSensor Node:
This node allows the world to react to how a user moves the mouse cursor in the
VRML world. Assembly of fixturing elements has been created using this node.
Transform Node:
This node translates a particular shape in the world. All the parts in the world
have been repositioned using this node. The parts can be translated or rotated using this
node.
Animations:
The animation in the VRML world is a combined effect of TimeSensor,
PositionInterpolator, OreintationInterpolator, Transform nodes, and many more such
nodes. The animations also include use of script nodes, which either have VRML or Java
scripts.
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assembly, and lighting effects in VRML world were given by using existing nodes in
VRML specification. The animation of parts in the world was done using VRML.
Navigating through the 3D VRML world is very simple. Getting acquainted with the
VRML browser interface is in order. Following figure shows a sample VRML window.
The control panel for the VRML browser looks such as this:
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Click and then drag to rotate an object Click and then drag to
pan right, left, up, or down.
Use the Undo Move and Redo Move buttons to retrace your
steps.
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MODULE 1
Fixture Design Tutorial
This module contains the information compiled in Phase I. The content has been
revised so as to increase user interaction and enhance learning experience. Fixturing
principles are explained using 3 Dimensional VRML models. The information has been
re-organized to suit the structure of current fixture design system and facilitate easy
navigation through out the fixture design system. This module contains five sub-modules:
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conditions and the resources being utilized. Depending on the working environment,
fixtures can be classified as welding structure, built up structure and cast structure.
Locators, supports, clamps, cutter action on the workpiece, set blocks, and chip
clearance are some of the key factors that govern design of any jig or fixture.
Standardization is one more factor that controls the economy of the system.
Standardization enables multiple use of standard elements for fixturing variety of
products, thus bringing down cost. Modular fixturing paves the way to standardization in
fixture design. The information regarding all the above factors is detailed in the design
manual. Modular fixturing will be better explained in the third phase of the project.
The factors affecting the design decision for jigs and fixtures are further
explained. As the initial cost involved in any new process or change of process is an
added expense it should be justified by its returns. Setup is considered as non-value-
added time. Setup time constitutes the time from the last good part of the old setup to the
first good part of the new setup. The cumulative output in terms of production units or the
rate of production, manufacturing time, labor required, and ease of operation are each
important for setup analysis.
During the designing phase the technical points such as principle and method of
location, cutter action on the work, locators and clamps used, body design, and provision
for chip clearance should each be considered suitably. For instance, the suitable off-set
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blocks and gauges bring the cutter very close to the cutting point in a short time, which
reduces the total manufacturing time during mass production.
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highest importance. The proper use of clamps to resist multi-directional cutting forces,
the facilitation of effective cutter action, provision of chip clearance and cleanup
allowance, easy and repeatable part loading/unloading can also each be standardized
based on vast experience drawn by designers. The individual locators, clamps and surface
contact points may differ in shape, size or other variants but the working principle of
configuring a machining fixture is unchanged. Drawing up on past experiences can help
chart-out do’s and don’ts appropriately. Such charting has the following advantages:
This section of the on-line manual provides a fast reference of do’s and don’ts in
designing a fixture. It is composed of pictorial presentations comparing the correct and
incorrect ways of fixturing. Often, common mistakes can be avoided through these
charts. The presentation shows 18 different examples of locating and clamping
techniques.
Mounting Elements
Mounting elements form the body of the fixture. These include tooling plates,
which could be flat, square, round or angular and depend on the product specs. These
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plates form the fixture base for mounting the locators and clamps and the special
accessories if required.
Locating Elements
These are the most essential part of fixtures and serve to locate a part accurately
on the fixture so as to ensure flawless fabrication or inspection of the part. Different types
of locating pins for plane location, edge location and point location constitute locating
elements. 3-2-1 principles, v-location, radial and concentric location are each important
standardized and scientific principles.
Clamping Elements
Clamps secure the part firmly onto a fixture. They must also provide the requisite
support against cutting wrenches (forces). It is important that the clamps do not damage
the part surface or edges, and do not obstruct the cutting path or cutter. They should also
not cause unnecessary deformations, due to over clamping which could lead to part
quality compromises. For instance, special care needs to be taken for clamping parts for
lapping and grinding operations where elastic recovery can be quite significant.
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Hydraulic Chucks
In general, chuck is a device that holds a part on its outer or inner surface. In other
words, wherever rotary motion is involved in an operation, the holding device of the
rotating part may be called as chuck. The chuck may be a tool holding chuck or a work
holding chuck. In this section information regarding basic features of chucks, different
types of chucks, improvements in the design of chucks, various applications of chucks
and different manufacturers of chucks are collected and compiled for ready reference.
Lesser vibration,
Easier pre-setting,
In the shrunk fit chuck, the tool is inserted in the tool holder when it is hot and the
tool is made integral with the tool holder. In the hydro-mechanical chuck the gripping
force is generated by an external hydraulic pump on a self-locking mechanical chuck.
The gripping force is twice that of a shrunk fit chuck and three times that of a hydraulic
chuck. Choosing the type of chuck among the four types depends upon the number of
parts to be produced, holding force required, and the accuracy required. All this
information is compiled in this section..
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MODULE 2
Fixture Assemblies
This section makes extensive use of VRML technology to help understand
fixtures. It provides an interactive feel to designing fixtures. It contains various fixtures
built using VRML. A user can view a 3D model of the fixture and also the different
elements that make a fixture. The user is capable of zooming in the part, panning the part,
and rotating the part, as desired. The seek option in the VRML world allows the user to
actually zoom into any particular area of the 3D model. The virtual world gives a feeling
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as if the user is actually handling the parts. The 3D models are associated with different
views such as the top view, bottom view, right hand side view, front view and default 3D
view. Some fixtures also show some important close-up views.
The fixture assembly contains eleven fixtures obtained from Tinker AFB. These
fixtures have been reproduced in Pro Engineer® after carefully studying their
photographs acquired from Tinker shops. The photographs had to be obtained since
ready-hand drawings were not available. One reason for this was the actual interruption
of production that could have resulted if the fixture was transported. These are
specialized fixtures used for various operations such as grinding, lapping, turning and
inspection. Each of these fixtures have been designed and implemented by Tinker
Engineers. Mr. Steve Moore, Mr. Pat Grissom and Mr. Pat Harris were 3 of the fixture
designers that have designed the majority of these.
There are two grinding fixtures, four lapping fixtures, three turning fixtures, and
one inspection fixture. The complete assembly of each fixture system has been split into
different active window frames, so as to facilitate navigation in the system. While one
large frame shows the complete fixture to be assembled, another small frame presents the
individual part for 3D examination in the VRML world. The fixture and part models are
accompanied by brief explanations. The user can pick any part in the fixture assembly
window and try to assemble it. While doing the assembly, the user can study different
attributes of the fixture as well as each part, using zoom, seek, pan and rotate options in
the VRML browser. These models also include an animation option whereby the user can
see the fixture assembly rotating making it easier to visualize the assembled fixture
mounted on machine tool. Appendix contains the screen shots of different fixtures
modeled in VRML.
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MODULE 3
Build your own Fixture
In this module, the user will be allowed to build his/her fixture using standard
available components including locating pins, clamps, tooling plates, riser blocks, and
fasteners. The very fact that VRML can be clubbed with JAVA and HTML enhances the
capabilities of VRML. We plan to build the above system using JavaScript codes
embedded in VRML. We propose to build the fixtures using a modular fixturing system,
complete with user interaction. The modular fixturing system consists of several elements
which can be broadly classified into
Mounting Elements
These mounting elements both mount the various tooling plates to the
machine-tool table as well as the variety of tooling platform and blocks, locators,
supports and clamps to the mounted tooling plates.
Locating Elements
Locating components are used in conjunction with the tooling plates and
mounting elements to perform a variety of different workpiece location
supporting, and positioning functions.
Clamping Elements
These elements are used to hold the workpiece in its place during
machining. Clamps vary as per the requirement.
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The system will have standard components under each section which can be selected and
the fixture can be built. JavaScript will be used to obtain data from the user and build the
fixture in a virtual world. It is required to build an interface between VRML and Java.
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COST ANALYSIS
Fixtures, based on their application, can be broadly classified into three
categories:
1) Manufacturing Fixtures
2) Repair Fixtures
3) Inspection fixtures.
These fixtures are unique and have their own specific attributes. All the same, all
fixtures need proper location and clamping.
Manufacturing fixture design can be considered to consist of the following stages:
1) Part analysis
2) Fixture Design
3) Design Analysis
4) Manufacturing
5) Measurement and Prototype testing with and without part
6) Putting fixture to production
Things to remember while designing a fixture include
1) Application of the fixture (manufacturing/repair/inspection),
2) The Number of parts for which the fixture will be used,
3) The Level of accuracy required,
4) The criticality of the part with respect to the aircraft,
5) Replaceability, reusability and discardability of the fixture, and
6) Standardization of fixtures and fixturing principles.
The time and money spent on a fixture should justify its use. The entire procedure
can be captured by the flow chart illustrated below. It can be seen from the subsequent
pie-chart that the time and effort is maximum for design, which is eventually reflected in
its cost. Thus reducing the design time can help in achieving much savings.
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Start
Re-design.
Check for conformance with
requirements of part features.
(Design Analysis)
Conforms No
Yes / No
Yes
Virtual Testing
Pass test No
Yes / No
Yes
Manufacture
Yes
Use for desired purpose
End
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Re-work
(Mfg error)
Prototype testing
Manufacturing
Design Analysis
Fixture Design
Part Analysis
Design Process
Realization of fixture
(Concept to Prototype)
Time (cost)
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We can say that at every non-conformance point, (marked with red circles in the
flow chart) the design cost shoots up exponentially. The non-conformance cost in most
cases is 100 % of what has been already spent. The Gantt chart also shows the rework
time spent due to design and manufacturing errors.
Work done in Phase 2 includes development of fixtures in virtual environment.
The virtual environment helps the designer to virtually see each and every aspect of the
fixture and its sub parts thus helping the designer to have a better understanding of the
part and the subject. The standard fixtures so developed will also help fast visualization
of existing fixtures without actually finding and analyzing the fixtures in reality. The
virtual environment serves as a platform for analyzing the fixtures and reduces time and
resources spent on actually building the fixture and then testing them. Also, much time is
saved, since no fixtures need to be analyzed physically. Using the same cost analysis as
used for Phase 1, we consider that a fixture is realized over a span of 60 days, of which
nearly 40 days are spent on designing a fixture. It should be also kept in mind that for that
period of time few aircrafts costing 15 million each are sitting idle.
Assume 50 design engineers are working over them for 2 months at a rate of $50 per
hour.
Consider that there is a 30 % non conformance of fixtures. The production lines get
overloaded by at least 40%; however the increase in lead-time increases the cost (due to
idle aircrafts) by 30 %.
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time and money is also saved using proper guidelines for fixturing. Use of virtual
environment not only saves time, but also enhances the learning experience.
Cost Analysis
4,000,000.00
900,000.00
3,500,000.00
3,000,000.00
Cost $
2,500,000.00 1,235,000.00
Considering the total cost further reduces by 5%, we have a total cost = $1890642.00
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These numbers are hypothetical and have been used to demonstrate ball-park cost
savings rather than perform an actual cost analysis. All the same, it can be seen that
substantial monetary gains can be realized by the development of a virtual design tool
that cuts down the design cycle.
CONCLUSIONS
This report provides a brief synopsis of the work undertaken to build an on-line fixture
design roadmap. This roadmap, over the course of the two phases completed, has evolved
from a pure paper manual to a fully-documented, interactive, computerized soft-manual
showing the basic principles and components of fixturing. Animations and manipulability
of fixture elements using established fixture designs allow the user to better understand
the principles of design. During Phase III this manual will be further developed to
interactively design modular fixtures for different workparts. We believe that such a
system will be useful to operators, engineers and managers who are in-charge of setup
reduction and fixture design.
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APPENDIX
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Grinding Fixture 1
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Grinding Fixture 2
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Lapping Fixture 1
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Lapping Fixture 2
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Lapping Fixture 3
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Lapping Fixture 4
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Turning Fixture
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Inspection Fixture
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Machining Fixture
(Expanding Sleeves)
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Machining Fixture
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