SURPASSING TRIAL AND ERROR: ACCELERATING THE INSPECTION OF TUBE AND PIPE DIMENSIONS

Rare Breed: Tube and pipe bending shops must rethink their processes to improve accuracies and meet tighter tolerances at lower costs. Measurement technology designed specifically for these applications will not only increase their productivity and precision, but also dramatically reduce their scrap rates and lower their costs. Here’s how.

Tube and pipe bending shops are a rare breed in manufacturing and, as such, they face important challenges that are exclusive to their discipline. Measuring the exact dimensions of the tubes and pipes they bend is one such challenge in an industry where accuracy counts more than ever. A tube that does not meet specifications will likely head to the scrap bin, as it will not have the proper clearance to mate-up and connect with subcomponents.

In the tube and pipe sector, market forces that are demanding ever tighter tolerances with lower costs are forcing fabricators to rethink their processes to improve accuracies even more. Under this competitive pressure, manufacturers are discovering that advancements in measurement technology designed specifically for tube and pipe applications will not only increase their productivity and precision, but also dramatically reduce scrap rates.

THE TRADITIONAL APPROACH
The old school method of measuring tubes and pipes comes down to trial-and-error techniques that can waste precious time and resources. In cases where CAD drawings are available, the bend radius is given. A plotter then produces to-scale drawings that show the remaining specifications. Known in the industry as LRA (length, rotation and angle) or PTB (pull, twist, bend), these drawings must try to replicate in two dimensions what is happening in 3D space.

When a tube is produced, it is placed on the drawings and visually inspected. Tubes that do not meet spec are rejected and the CNC bender controls are tweaked and adjusted. When the adjusted tube is produced, it is brought back to the drawings and visually inspected as well. This process of adjusting the bender and inspecting the tube is repeated until the tube finally conforms to specification.

If CAD models are not available – as is often the case with reverse engineering applications – the centerline radius should be calculated. One method, as illustrated in Figure 1, is to measure the circumference of the bend by pressing a tape measure against its inner surface from where the bend begins to where it ends (the red area). This result is then multiplied by the number of times it would take to make a complete circle. In the simplified example, it is multiplied by 2 since the bend is 180 deg (multiply by 4 if the bend is 90 deg). The result will be the circumference, or ‘C’.

Working through R₂ = C/2π, where π is 3.14, yields ‘R₂’ as the inner radius. Adding one-half of the tube diameter (D) gives the centerline radius. The easiest way to measure the rest of the tube is to sketch its outline from various angles on cardboard. When a tube is produced, it is placed on the cardboard and visually inspected. Then, as mentioned  earlier, the iterative process of tweaking and readjusting the CNC bender takes place, remeasuring until the tube is in compliance.

THE MODERN APPROACH
Current technology is providing tube and pipe benders with attractive alternatives to the trial-and-error approach. For example, tubes can be inspected on the shop floor with the latest portable coordinate measuring machines (CMMs) that reference the standard X,Y,Z point data also used for measurement on traditional fixed CMMs in the quality control lab.

Using portable technology that is developed specifically for tube measurement enhances the inspection process even further. A good example of this is the Tube Inspection System from ROMER (North Kingstown, RI) that includes a portable CMM (also known as an articulating arm), non-contact tube probes, Data Overlay Camera System (DOCS) software, and an optional CNC bender interface – all of the tools needed to quickly and accurately measure tubes and pipes. This system also has the ability to measure other geometric features attached to tube components, such as brackets, flanges, or bosses.

To measure a tube using this system, the tube must be securely mounted. A tube probe is then passed over the tube at each point of bend change, in sequence from one end to the other. The probe’s infrared sensor identifies or ‘sees’ where these individual points are taken and locates them on the X,Y, and Z axes. The DOCS software platform automatically computes the geometries of the tube based upon the point locations in 3D space. Once this information is collected, the software compares actual data with nominal data and calculates the corrections. These corrections are either input into the bender manually or directly uploaded using an optional software module. The next tube produced automatically meets specification.

Reverse engineering applications are also simplified using this Tube Inspection System. By measuring two neighboring straights and then sweeping the bend with a tube probe, DOCS calculates the bend radius as well as the LRA data. The information is then programmed into the CNC bender to produce a virtually identical tube.

MANUAL MEETS MODERN
One of the issues with manually inspecting tube and pipe is the inaccuracy that is inherent with visual measurements. A tube may appear to meet tolerances when in fact it does not. If such a tube passes inspection, it may ultimately be rejected by the customer. However, tolerances that are smaller than what the human eye can perceive are easily measured with the Tube Inspection System so that corrections can be made before the product ships.

Costly time can be wasted by manually readjusting, remeasuring, and readjusting again with conventional methods. It is not unheard of for this manual tweaking process to take most of the day. Furthermore, every tube is scrapped while making these manual adjustments. If set-ups last most of the day, an entire scrap bin could potentially be filled by the time an accurate tube is produced. Advanced measurement technology addresses all of these issues by reducing the time and resources required to get tubes to meet specification.

The Tube Inspection System reduces inspection time to minutes instead of hours. It significantly reduces scrap because its first readjusted tube conforms to specification. Accuracy is greatly improved and, depending on the length of the portable arm being used, can be as tight as ±0.0009 in (0.023 mm). Also, tube probes are a fraction of the cost of laser scanners which are used – though not recommended – in tube inspection by manufacturers that have already invested in a portable CMM.

Tube and pipe bending shops employing the latest CNC software and hardware could better leverage this asset with a portable CMM providing on-demand inspection. Considering the amount of time, energy, and scrap to be saved, it is an investment that merits serious consideration. Tube and pipe bending shops may be a rare breed, but they deserve customized inspection solutions to make life easier.