Automated welding for job shops
July 12, 2001
A robotic weldingsystem represents a significant capital investment for a job shop.
A robotic welding system represents a significant capital investment for a job shop. The number and diversity of typical products that job shops produce may discourage any interest in automation, primarily because the company cannot figure out how to justify the system purchase and operation. Ease of programming and quick changeovers have reduced setup time and enabled job shops to automate.
Why Welding Automation?
Among the reasons most job shops purchase welding automation are:
- Cost reduction by direct labor savings. Even though direct labor may be a small percentage of the product cost, automating direct labor-intensive processes is one of the easiest ways to reduce product cost.
- Improved part quality. Today's consumers require high product quality and competitive pricing, which is impacted by the rework costs, scrap, and liability claims associated with poor quality. Properly designed and applied welding automation practically eliminates weld variations (given consistent simple parts) that may lead to part failure.
- Availability of skilled welders. Few young people today are becoming welders. For those who are, a job shop often serves (unwillingly) as a training ground for more lucrative jobs at multimillion-dollar corporations. A single departure from a small job shop operation can cause a significant upheaval.
Preparing Parts for Automated Welding
A human welder performs countless welding-related tasks and makes adjustments constantly. An automated system, with its axes of motion and computerization, will have a difficult time performing these tasks if the part is presented with any variations, such as joint location or geometry.
Following are a few simple design guidelines to help to improve automated welding results:
- Weld joint location must repeat in three-dimensional space relative to the system within plus or minus half the diameter of the welding wire being used. For example, using 0.045-inch-diameter wire allows a tolerance of ±0.023 inch.
- Weld joint gaps must be weldable, and gap location and width must be consistent from part to part, with the same tolerances as the weld joint location.
- Weld joints must be readily accessible to a robotic welding torch. Typical arc welding robot arms have six degrees of freedom.
Manufacturing processes must be selected that will provide the accuracy required. Springback variances on bend angles must be controlled, even when using die forming. Formed gussets in the part are a good solution. Parts should be located from an edge rather than from a hole, because through-hole locator pins can bind and make it difficult to remove the weldment from the weld fixture.
Job shops should encourage customers to design parts with fillet welds and lap joints. Groove and butt joints are fine but more difficult to control. Outside corner and square butt joints usually are not acceptable, unless serious tooling enhancements are anticipated.
There are instances when it is too expensive to manufacture parts to the tight tolerances and limitations required for automated welding, but that doesn't mean automation is out of the question. It means simply that there are more capital costs involved and a slower welding process. Sensory technology can be used to allow the system to find and follow the joint:
- Wire touch sensing. With this sensing method, the robot is programmed to move to a series of weld joints, apply a sensing voltage to the welding wire, and move the wire toward the part. When the wire touches the part, the voltage drops to zero. The robot control senses this lack of voltage, stops the robot's motion, records the XYZ coordinates of the end of the weld wire (tool point), and advances the robot program to the next touch.
- After a series of touches, the control recalculates the position of the found joint relative to the original taught program and then offsets the original program to the new coordinates. Search routines can be performed along one, two, or three axes. Touch sensing also can be used to verify part presence, clamp status, etc.
- To ensure a correct touch on lap joints, the surface must be at least 1/8 inch thick. Touch sensing cannot be used on aluminum or on butt joints, circular surfaces, or groove joints.
- Through-the-arc seam tracking. With this sensing type, the robot is programmed to weave the arc across the weld joint. As it does, the robot control samples the weld current. When the joint moves relative to the taught program, the wire stickout changes. This distance change results in a welding current change at the weld power supply. The robot control senses this change and offsets the program in the proper direction to bring the current back to the programmed level.
- This sensing technology is limited to welds of 5 millimeters or more, and the legs on a fillet weld must be 2 to 3 millimeters larger than the weld legs because of the robot arm's mechanical constraints. In general, weld travel speeds on 5- to 7-millimeter welds are faster without weaving. This technology does not work on aluminum or for butt joints or small groove joints.
- Other sensing methods include tactile probes, vision-guided systems, and variations on the methods discussed here. These should be evaluated on a case-by-case basis because of their complexity and costs.
Tooling and Fixtures
Weld fixtures are required to locate simple parts for the final welded assembly. The shop must integrate fixture quality and cost into the overall cost, lead-time, and complexity of the welding automation project concept.
The following are general guidelines to tooling complexity:
- The least complex (and least expensive) has no automation and involves manual clamping with limited dimensional control. This type of tooling typically is a fixture designed to hold a tacked-up assembly with a minimum number of loose pieces that need to be located.
- The most complex (and most expensive) fixturing method is a fully automated tool with complete control of all critical locating points in three dimensions. This tooling may feature part-presence sensing, clamp position control, sequenced clamping, pressure sensing, and more. Generally, all parts comprising the assembly are loose parts with few welded subassemblies.
- Other welding fixtures falls between these guidelines in both complexity and cost.
As with any process, simplification is a goal to strive for. The more complex the tooling, the more difficult it may be to program the robot to access part weld joints. Job shops should encourage customers to use part designs that feature self-locating, simple parts to reduce tooling content. Toy tabbing, slot and tab, locating studs, and part interlocking often can be helpful in reducing tooling complexity and cost.
Some sort of fixturing always is required when a new job is brought into the shop. For a one-time part run or a prototype part, angle iron clamped to a stationary or indexing table often is sufficient to locate the parts for robotic welding.
Considering welding automation often means throwing out some traditional ways of working that have been used for years. By staying open to exploring new ideas, job shops can find the level of welding automation most suited to their needs.