Comparison of methods for multi-bent tube inspection

Typically three methods are employed to inspect multi-bend tubes; using a fixture, an articulating measurement arm with probe or infrared touch probe, or the TubeInspect optical gage system.

Fixtures

• Historically tubes have been checked by means of precision-made no-go, go fixtures.
• Fixtures provide only non-numerical information which is less helpful for bending machine setup.
• Traditional fixtures do not allow for the implementation of statistical process control.
• Producing the first gage or any subsequent modifications requires considerable lead-time, and repeat costs.
• Gages are expensive to store and require maintenance due to wear. In addition, they require yearly recalibration.
• Fixed gages cost thousands of dollars for each tube. Flexible gages, which allow a fixture to be reassembled for different tubes, are available but cost much more and typically require significant setup time.
• Some producers of short run bent tubes rely on tower type fixtures. To use this type of fixture, instructions are printed out and a mylar is created by the programming department. The operator tapes or secures it to a steel table. An operator manually places a stand inside of drawn lines on the mylar and raises the shaft to a prescribed height using a height stand. This process is repeated for each straight location. This gage is then used as a go/no-go gage. The stack up of tolerances include the thickness of the lines draw on the paper, paper flexibility, flatness of the steel table, flatness of the stand, the ability of the operator to set up the position and height of the stand and the manufacturing tolerance of the hardware. The time required to produce the entire setup to include the making of the Mylar can be longer than the manufacturing time for a short cycle tube production run. With all of these problems you still have only a go/no-go gage giving you no numerical data to setup or optimize a bender.

Articulating Measurement Arms

• Usually consist of six or seven axis and are equipped with specializedt tube measuring software.
• Provide numerical measurement data that can be used in the setup of an automatic tube-bending machine.
• Most 6-axis arms have a range of from 2 to 12 feet. Ranges are in two feet increments. A 10-foot arm will measure a 10-foot diameter if fully extended. Therefore, to measure an 8 ft long tube, a 12-foot arm is required to allow for bends.
• For tubes that are longer than 8-foot, two methods that can be used. The arm can be relocated (“leap-frogged”) and last several sections of the tube remeasured. Alternatively, a linear rail (7th axis), generally floor mounted, is added to the system. This 7th axis adds additional range to the arm; however, it makes the system much less portable and adds cost.
• Articulated arms typically measure the straight sections and extropolate to calculate bend points. Measurement of changing radius tubes or bend-in-bend tubes are usually not possible.
• Articulated arms are slower than TubeInspect and require more training time.

Tube Inspect - an automated 3D tube measurement system:

• Measures tubes in seconds
• Provides complete 3D digital data for production control or statistical analysis
• Reduces operator variability.
• Has inspection software that will provide a COMPLETE analysis of tube shape and profile, not just at the bend points.
• Provides optional direct feedback to CNC tube bending machines, allowing for quick corrections of out of specification tubes and faster set-up of new tube designs
• Compares tubes directly to CAD models or to a master tube. A go/no-go output indicates any areas in or out of tolerance.

Comparison of different methods for multi bent tube inspection