fixturing for robotic weldi

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resistance of copper. Novice fixture-builders sometimes overlook the electrical conductivity aspectand paint all surfaces, including the bolted work cable connection surface, which is the source ofimmediate start-up troubleshooting problems.

The thermal conductivity properties of aluminum and copper are high, referring to the ability of eachof these materials to conduct heat. For these reasons, these materials are often used as heat sinksto conduct heat away from the work piece and 'spread' the heat over a larger surface area tominimize work piece distortion. In a robotic welding system, thermal expansion properties shouldalso be considered. Thermal expansion refers to the fractional change in length of a material for aunit of change in temperature. Aluminum has the capacity to significantly change in length andvolume when heated. Fo r this reason, copper is more commonly used for heat sinks, and aluminumis typically avoided in an effort to strive for part repeatability. Work piece sub-assemblies can also bephysically arranged and programmed with advanced welding processes, designed for low heat-input,to minimize welding distortion tendencies.

Optimize the Welding Circuit

ring for Robotic Welding Productivity | Lincoln Electric http://www.lincolnelectric.com/knowledge/articles/content/fixturin

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Refined welding waveforms require an optimized welding circuit to maintain short arc lengths while reducing spatter, stubbing, arc-flare, andarc outages, all in an effort to maximize travel speeds.

Special care must be taken to identify the optimum location of the work lead on the robotic welding fixture. As a general rule, it is desirable tolocate the work cable and sense lead (if applicable), as close to the welding arc as possible, rather than indirectly through a series of boltedconnections. In the optimum scenario, connection directly to the work piece is preferred.

Connection provisions on the fixture to insure that the wo rk cable and sense lead are separated from each o ther, and from any robotic orwelding communication cables insure the best results. When more than one power source is welding simultaneously on a single part, eachpower source requires a work lead from the work stud to the work piece. Avoid combining all of the work leads into one lead to reduce thearc interference and induced magnetism, otherwise known as arc blow.

At this point, also consider the anticipated welding travel directions, with the preference to move in the direction away from the work lead, asshown. Connect all of the work sense leads from each power source to the work piece at the opposite end.

Orient the Fixture to Maximize Deposition Rates


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Next, examine how your fixtures position the work relative to a vertical orientation to take advantage of the force of gravity. When welding apart in the flat position, gravity is an ally. The finished welds are flat, uniform, and more easily made with higher deposition rates, that directlyincrease travel speeds and productivity.

When sheet metal applications are designed with lap or 'T' joints, simple positioning of the part to allow a 15-degree downhill torch motioncan result in travel speed increases of 10 - 25 percent, by using gravity as an ally and the fast-follow characteristics of many consumable /gas combinations.

Welding overhead may be appealing to reduce or eliminate weld spatter from sticking to the part, however, the spatter will inevitably stick tothe exposed surface of the fixture and tooling, requiring long-term maintenance attention. Welding overhead also requires overcoming theforce of gravity. Deposition rates are lower and it is more difficult to maintain proper weld contours.

Strive for Accessibility, Repeatability, Simplicity and Dependability

There are many clamping / locating options to choose from when you approach a fixture at the design stage. The least complex involvessimple manual clamping such as swing, push, or plunger clamps applied to a fixed or stationary table and are typically applied for short-run orprototype parts. In an R&D or short-run setting, these are very simple, low-cost methods to locate a part. The labor intensive nature ofmanual clamping is overcome by flexibility and versatility in these settings. Modular fixturing is a secondary option that provides benefits offlexibility while maintaining dimensional control.

On the other end of the spectrum, the more complex applications mightinvolve a dedicated fixture. These fixture installations are more complicatedwith higher initial costs, and frequently involve the installation and routing ofwiring, and pneumatic or hydraulic lines. Advantages of automatic clamping

include the reduction or elimination of labor involvement for actuation, partproximity sensing, and sequenced clamping.

Another fixture design alternative involves retrofitting or repurposing anexisting semiautomatic fixture, but this alternative must be approached withcaution. In a semi-automatic application, the operator often makesadjustments, on-the-fly, to accommodate variation in joint location andgeometry. When a robot is applied to the same fixture, any part movementdooms the assembly to the reject bin. If the fixture is predominantlydedicated to fillet and lap welds, the fixture can normally be reused,however outside corner welds and square butt joints often require seriousenhancements to the existing fixture to maintain repeatability.

In each of these options, a variety of additional objectives must beconsidered. For example, the fixture should be designed to operate at an

ergonomic height and reach and without the need for operator force during loading and unloading. Sufficient torch accessibility and visual


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clearance, supplemented by adequate lighting, should be an emphasis. Fixture surfaces are ideally designed to minimize flat surfaces to avoidthe collection of welding spatter from interfering with critical locating surfaces or actuating components.

The objective of the integration of the fixture and clamping / locating deviceis to insure that the weld joint location repeats, in a 3-dimensional spacerelative to the system, within +/- half the diameter of the applied weldingwire. For example, using 0.045" diameter wire allows a tolerance of +/-0.022".

Gap location and width must be consistent from part to part, with the sametolerances as the weld joint location. Beyond plus or minus half the diameterof the wire thickness, the weld size may be required to be increased tooffset the smaller weld throat that results. A larger weld may require 125 -

200% more weld metal than required if proper fit-up is maintained.

Obtaining good fit-up is critical to controlling costs. In order to obtain a goodfit-up consistently, emphasis must also be placed on preconditioningoperations such as cutting / shearing, machining, heat treating, and bending

/ forming processes.

Consider Alternatives when Fixture Design Costs are ProhibitiveThere are times when it is too expensive to manufacture parts to tighttolerances, or impractical to present a large fabrication to a robot fixture,

within thousandths of an inch. Small part runs may also prohibit the application of some fixturing options. In these cases, low-cost softwaresensory technology such as Touch Sensing or Through-The-Arc Seam Tracking can be employed to overcome deficiencies in a part or fixture.

In the application of Touch Sensing, the robot is programmed to touch thewelding wire to a number of locations on the fabrication to identify the part

location and orientation. With this information a master program can be3-dimensionally shifted to match-up with the new part orientation.

Through-the-Arc Seam Tracking is another option that can be enabled once therobot begins welding - the robot weaves across the weld joint and modifies itspath based on any joint location shift, perhaps due to distortion or partspring-back, for example.

Other complex sensing and tracking methods include laser-based visionsystems.

Summary

Robotic welding is economical when properly applied, but it can be terriblyinefficient and cost-prohibitive when simple fixture design considerations areoverlooked. Many productivity gains are realized, or lost, at the design stageand while parts fixturing performs a role that is simple, it is critical to the successof a system. Consider basic fixture design objectives to improve the productivityof your robotic welding system.

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fixturing for robotic weldi

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