Seminar on Jigs & Fixture

ISO 9001 : 2000 Training Institution

SCHOOL OF POSTGRADUATE STUDIES SEMINAR ON JIGS & FIXTURE

TOPIC : WELDING FIXTURE DESIGN PGTE – 060031

WELDING FIXTURES




WELDING FIXTURES

Welding is a material joining process in which localized joining is produced along the surfaces of the work pieces. Joining is produced by heating the materials to suitable temperatures with or without the application of pressure.

Stress resulting from thermal expansion of work pieces and fixtures must be considered in the design of clamps and locators in the proper positioning of work pieces. (Therefore close tolerances cannot be obtained and should not be given in the drawing)

Handles subjected to heating should be made of insulating materials such as wood.

WELDING FIXTURE DESIGN CONSIDERATIONS

Welding spatter should not be allowed to fall on the threaded parts of the clamping elements. The parts near the welding area should not be threaded . Consequently toggle clamps without threaded elements are used widely in welding fixture. These can be operated quickly and provide ample clearance in the unclamped condition.




Coefficient of thermal expansion of metals may result in the cracking adjacent to the weld when tightly clamped. This difficulty may be overcome with the fixture only tack welding.

Welding fixture must properly locate and hold workpieces for a complete welding operation. It simplifies and/or eliminates handling and moving of the work pieces and of associated tooling.




Revolving the fixture is preferable to removing and turning the workpiece.

Vertical or overhead welding should be avoided.

When beginning a new project for welding fiixtures, consider these fixture design objectives: Choose Your Fixture Material Wisely Optimize the Welding Circuit Orient the Fixture to Maximize Deposition Rates Strive for Accessibility, Repeatability, Simplicity, and Dependability Consider Alternatives when Fixture Design Costs are Prohibitive

Fixture MaterialOne of the first steps in designing welding fixture is choosing its base metal. Factors include 1] initial cost; 2] long-term maintenance costs; and 3] special characteristics that make it suited to welding application, such as maintaining accuracy and part repeatability in an environment exposed to elevated heat and weld spatter.

Most commonly used fixture materials :

1] Mild steel,

2] High-carbon tool steel,

3] Aluminum,

4] Stainless steel, and

5] Copper

Each material has different characteristics that can affect productivity and quality

Mild Steel:

From an initial cost standpoint, square or rectangular structural steel tubing is economical for a majority of the fixture's framework.

Tool Steel:

From a wear resistance standpoint, fixture hard stops and locating points often are made of alloyed high-carbon tool steel to help resist deformation. Another commonly sourced wear-resistant alloy, aluminum-bronze, work-hardens and isn't as prone as tool steel to residual magnetism

Copper:Pure copper generally is avoided when hardness is a consideration; however, it's typically the best all-around material for electrical conductivity. Electrical conductivity is critical for arc welding stability, a desirable and necessary aspect for achieving maximum travel speeds. Because pure copper is such a good electrical conductor, alloys of copper and zinc (brass) or copper and tungsten carbide can be used to help improve copper's wear resistance. Aluminum and copper both have high thermal conductivity—the ability to conduct heat—so they often are used for heat sinks that conduct heat away from the workpiece and spread the heat over a larger surface area. This helps minimize workpiece distortion.

Optimize the Welding Circuit

Optimized welding circuit to maintain short arc lengths while reducing spatter, stubbing, arc flare, and arc outages to maximize travel speeds.Take special care to identify the optimum location of the work lead on the robotic welding fixture. Generally, the work cable and sense lead (if applicable) should be as close to the welding arc as possible, rather than placed indirectly through a series of bolted connections. Connecting directly to the workpiece is preferred. Make sure that the work cable and sense lead are separated from each other, and from any robotic or welding communication cables, to help ensure the best results.

Orient the Fixture to Maximize Deposition Rates

The way you position your fixtures directly affects your travel speeds and productivity. Position fixtures to the work so they take advantage of gravitational forces, which will help produce welds that are flat, uniform, and more easily made with higher deposition rates.

Choosing the Right Welding Process

Each metal joining process has unique characteristics and ranges of capabilities. In selecting a process for a specific joining operation, users should scrutinize the requirements and conditions involved.

Considerations include:1)Depth of penetration

2)Joint preparation 3)Cleaning 4)Inert gas

5)Vacuum environment 6)Weld proximity to heat-sensitive materials

For penetration beyond 0.100 in. without preparing the weld joint for filler metal, consider EBW.

For critical, heat-sensitive weld joints and dissimilar materials, LBW and EBW are suitable.

When distortion to any degree is unacceptable, LBW, EBW, and PAW are suitable choices.

For high volumes and long production runs, LBW may be appropriate.

Seminar on Jigs & Fixture

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