Transcript Chapter 21

Chapter 21
Fabricating Techniques
and Practices
© 2012 Delmar, Cengage Learning
Objectives
• Explain the various safety issues related to
fabrication
• List the advantages of using custom fabrication
parts
• Demonstrate an understanding of the proper
placement of tack welds
• Demonstrate the use of location and alignment
points when assembling a project
• Explain how to adjust parts to meet the tolerance
© 2012 Delmar, Cengage Learning
Objectives (cont’d.)
• Describe how to control weld distortion
• Lay out and trace parts
• Identify common sizes and shapes of metals used
in weldments
• Describe how to assemble and fit up parts for
welding.
© 2012 Delmar, Cengage Learning
Introduction
• Assembly of the parts to be joined
– First step in almost every welding operation
• Basic level: placing two pieces of metal flat on a table
and tack welding them together
• Higher level: assembly of complex equipment,
buildings, ships, or other large welded structures
– No matter how large or complicated the welded
structure
• It is assembled one piece at a time
© 2012 Delmar, Cengage Learning
Fabrication
• Process of assembling parts to form a weldment
– Often required by welders
– Number and type of steps required to complete a
project vary
• Welding projects start with a plan
• Plans can range from simple to very complex
– You must learn how to follow a set of drawings to
produce a finished weldment
© 2012 Delmar, Cengage Learning
Safety
• Primary concern for fabrication of weldments
– Much of larger fabrication work may need to be
performed outside an enclosed welding booth
– Several welders may work simultaneously
• Notify others of hazards
– Ventilation is important
– Lines must be flat on the floor and covered to
prevent trip hazards
– Make sure fabrication is stable
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Parts and Pieces
• Welded fabrications
– Made from precut and preformed parts or hand-cut
and -formed parts
• Preformed part advantages
– Cost, speed, accuracy, and less waste
• Custom fabricated part advantages
– Originals, prototypes, repairs, and custom jobs
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Layout
• Welder must lay out lines and locate points for
cutting, bending, drilling, and assembling
– Always start a layout as close to a corner of the
material as possible
– It is easy to cut the wrong line
– Some shops have their own shorthand methods for
identifying layout lines
– Process may be affected by several factors
• Material shape, part shape, tolerance, and nesting
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FIGURE 21-13 Marking parts makes it less likely
that the wrong one is welded in place.
La-Co Markal
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Nesting
• Laying out parts so the least amount of scrap is
produced
FIGURE 21-21 Parts nested for cutting; note the
small blank space left between the parts.
MultiCamPix
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Kerf Space
• Space created as material is removed during a cut
– Width varies depending on cutting process
FIGURE 21-23 Angle iron shear.
Scotchman
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Material Shapes
• Metal stock
– Wide variety of shapes, sizes, and materials
• Bill of materials form
– Plate: usually 3/16 inch or thicker and measured in
inches and fractions of inches
– Sheets: usually 3/16 inch or less and measured in
gauge or decimals
– Pipe: dimensioned by diameter and schedule or
strength
– Tubing sizes: always given as outside diameter
© 2012 Delmar, Cengage Learning
Overall Tolerance
• Amount that a
part can be bigger
or smaller than it
should be and still
be acceptable
FIGURE 21-35 Small errors on lots of
parts can become a big error on the
finished assembly.
© Cengage Learning 2012
© 2012 Delmar, Cengage Learning
Assembly
• Assembly process
– Requires proficiency in several areas
• Very large projects
– Corner or centerline is established as a baseline
• Start the assembly
– Select largest or most central part
– Identify each part and mark each piece for future
reference
– Layout lines and other markings can be made on
the base
© 2012 Delmar, Cengage Learning
Assembly Tools
• Variety of tools make assembly easier
– Clamps: temporarily hold parts in place
• C-clamps, bar clamps, pipe clamps, locking pliers,
cam-lock clamps, and specialty clamps
– Fixtures: aid in assemblies and fabrication of
weldments
• A well-designed fixture allows adequate room for the
welder to make tack welds
© 2012 Delmar, Cengage Learning
Fitting
• Adjusting parts of a weldment so they meet the
overall tolerance
– Not all parts fit exactly as they were designed
• Slight imperfections in cutting or distortion due to
welding, heating, or mechanical damage
• Some problems can be solved by grinding away the
area
• Other situations may require parts be forced into
alignment
• More aligning force can be applied using cleats or
dogs with wedges or jacks
© 2012 Delmar, Cengage Learning
Tack Welding
• Welds made during assembly to hold all parts of a
weldment together so they can be finished welded
– Several factors to consider
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Thickness of the metal
Length and shape of the joint
Welding stresses
Tolerance
Fitup
© 2012 Delmar, Cengage Learning
Welding
• Good welding requires more than just filling up
joints with metal
– Welding on an assembly should be staggered
– Keep arc strikes in the welding joint
– Striking the arc in correct location on an assembly
is more difficult than working on a welding table
– Be sure you have enough freedom of movement to
complete the weld joint
– Follow all the safety and setup procedures
© 2012 Delmar, Cengage Learning
Finishing
• Welder may be responsible for finish work
– Depends on shop size
• Considerations
– Most grinding is done with a hand grinder
– Grinders must be held securely
– Painting and other finishes release fumes such as
volatile organic compounds (VOC)
• Special ventilation may be required
– Most paints are flammable
• Must be stored well away from any welding
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Summary
• Completing work on a piece of equipment,
building, trailer, or other structure
– One of the greatest experiences as a
welder/fabricator
• Layout and fabrication techniques must be learned
• Welded structures
– Enduring monument to your skill as a craftsman
• Every time you build a project, do it as if it were going
to be on display
© 2012 Delmar, Cengage Learning