29-01-2013, 03:59 PM
FIXTURE DESIGN CRITERIA: PHASE I
FIXTURE DESIGN.pdf (Size: 191.26 KB / Downloads: 95)
Introduction
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
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 todays industry to maintain rapid tooling in the
agile environment.
BACKGROUND
This report presents a preview of the information compiled for designing work
holding devices under different operating situations. The nature of operation, the
principle of design, the essential supporting elements required and the different type of
the available devices are discussed in detail. The work holding devices that are
elaborately discussed in the report are chucks of different types, and jigs and fixtures
used for various operations.
In the majority of machining operations, the design of the machine tool is such
that it provides a rotary or linear movement to the cutting tool. Drilling, Grinding,
Milling, Broaching, Turning and Planing are some of the examples of cutting operations.
In a Lathe, rotary movement is given to the work instead of the tool and the tool is
translated to remove material. In a milling machine, the cutting tool is rotated and
translated on a stationary workpiece to remove material. The workpiece must be suitably
located to permit cutting action, by restraining the necessary degrees of freedom.
Clamping or holding is applied to support the work against cutting forces and torques. In
cases where the work piece is rotating, the holding force required for the holding device
increases with increasing RPM. The importance of the work holder is briefly detailed in
the manual with regards to its design and construction.
DIFFERENT MODULES OF MANUAL
The work has been broken up into phases. This report documents the activities in
a phase I. Phase I is a compilation of theoretical principles of fixturing and standard
textbook reference, although some practical fixture designs are also included. The
second phase will research several fixtures used at Tinker and group them using part
family concepts to serve as ready reference for future designers. Some
validation/verification of principles in real-time will also be implemented at that time.
Together these phases will attempt to design a Roadmap for Fixture Design. The work
to date is divided in five modules and it has been compiled as such within an electronic
CD-ROM manual
HYDRAULIC CHUCKS:
In general, chuck is a device that holds a part on its outer or inner surface.
Typically, wherever the part has to be rotated in a lathe, the holding device is a chuck.
The chuck may also be a tool holding chuck in other situations. 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. This module was principally
developed at the request of the sponsor.
Tool holding chucks:
Concentricity, holding force, presetting, vibrations and rigidity are the basic
selection criteria when going for a new tool holding device. The different types of chucks
collet chuck, hydraulic chuck, shrunk fit chuck, hydro mechanical chuck - are described
in this section with their relative accuracies, advantages and disadvantages. The
advantages of using a hydraulic chuck over collet-type chuck is that it does not have as
many moving parts to hold the tool, and the pressure is applied uniformly throughout the
gripping area.
Work holding chucks:
Work-holding chucks are usually used in Lathes. Chucks are either manual or
power actuated. Air and oil is used to transfer power in pneumatic and hydraulic chucks
respectively. The numbers of jaws differ according to the usage. Two-jaw, three-jaw,
four-jaw and six-jaw chucks are available and they are chosen depending upon the shape
of the product and the type of operation required. Working principle of all the types of
chucks is explained in this section. The information of different manufacturers of chucks
along with their product varieties is also provided for ready reference.
DESIGN OF JIGS AND FIXTURES:
This section explains the designing of Jigs and fixtures and a visual presentation
is prepared to explain the details. The basic difference between jig and fixture is that the
former locates and holds the work piece while also guiding the cutting tool, whereas the
latter just locates and clamps the work piece. The jig may or may not be fastened to the
machine table whereas the fixture should be fastened to the machine table precisely.
The points to be considered before deciding to go for jigs and fixtures, as well as
the theoretical principles for designing of jigs and fixtures are explained next. As the
initial cost involved in any new process or change of process is an added expense it
should be justified by its returns. As far as jigs and fixtures are concerned, the cumulative
output in terms of production units, manufacturing time, labor required and ease of
operation is more important.