With digital measuring arm...

Citation

(2002), "With digital measuring arm...", Aircraft Engineering and Aerospace Technology, Vol. 74 No. 2. https://doi.org/10.1108/aeat.2002.12774baf.002

With digital measuring arm...

Forging technology evolves from art to science as Wisconsin manufacturer captures shape of rough forgings with measuring arm and uses this data to find "best fit" for machining.

At the moment of ignition, two solid-fuel rockets produce 35 million horsepower to propel NASA's Space shuttle off a Florida beach, on its way to orbit. At 24 miles up, the fuel is spent, and the rocket boosters are jettisoned. Eventually, they fall back into the ocean to be recovered and reused. During the burn, pressure inside the booster is huge, but the casing and nose dome hold firm— through as many as 20 consecutive launches.

The rocket bodies resist the loads of launch and splashdown because they are forged from D-6 tool steel to be nearly indestructible. All of the cylinders and domes that form the solid rocket motor cases are forged by the Ladish Co. of Cudahy, WI, in the second largest forging facility in North America. Most of the items that the company makes become part of something airborne. As a result, Ladish is accustomed to making parts to instrument- like tolerances—even if they are 12 ft in diameter and weigh over 5,200 lbs.

Holding dimensions

Maintaining design specifications on rocket forgings requires team effort on the part of Ladish manufacturing and quality engineers. They work in the "no-mistake zone" of manufacturing because these parts are too costly to scarp.

The domes present, perhaps, the greatest challenge to Ladish for dimensional control. The part must support a jet engine on top of it that ignites the solid fuel within the rocket at liftoff, then support aerodynamic and splashdown loads. The upper section of the dome is a half-sphere that is about.400 in. at the thinnest part. This rests on a thick mounting ring at the base that is later attached to the rocket casing. The dome (with attached jet engine) fits beneath the nose cone.

Forming of each dome begins by pressing a billet of tool steel into a pancake shape, to give radial orientation to the metal grain structure and increased strength. Then, the pancake is pressed into a heated draw ring to form the spherical contour. Finally, the dome is machined smooth on all surfaces.

Machining from experience

In the past, Ladish technicians measured each newly forged dome with a portage CMM to guide them in setting up the piece to be machined. This technique worked reasonably well, but was slow, taking two weeks to get an image of a single dome. The problem was that the image generated was not always complete enough. "The data gathered by the CMM was two-dimensional only and did not produce the complete image needed without more time-consuming mathematical interpolation or manual point plotting in CAD by engineering," said Jeff Bacon, Die Engineer at Ladish.

The precision required to turn a forging into a machined component was jeopardized. "This placed the burden of responsibility on the skill of the machinist to cut a good part," said Ron Snyder, Post Forge Engineering Supervisor at Ladish. They needed a more comprehensive means of assessing the exact shape of each forging prior to machining.

Digital "cloud"

Two years ago, Ladish quality engineers found a vastly simpler way to generate a digital image of the rough-forged dome—and get a vastly more detailed image in the process. They brought in a FaroArm, a portable, 3-D measuring system that is the mainstay of design and quality checking programs in many automotive, metal fabrication, and aircraft industries. The arm is an articulating instrument that employs optical encoders at the joints to provide X-Y-Z position and I-J-K orientation data to a computer. Dimensional tolerances are as close as + - 0.001 in, and it can measure any point within its spherical reach (Plate 1).

Measured data are fed into the companion software for the arm, known as CAM2 Measure, as streams of points or as splines. This software operates seamlessly with contemporary CAD and imaging packages, enabling users to recreate an image of the surface of almost any object. In addition, this software can use the original CAD design file of the part as a quality "template" so that users can see instantly where a measured part is out of spec with the design.

Plate 1 Ladish technicians use FaroArm 3-D measuring instrument to gather dimensional data on raw forged body and dome parts for Space Shuttle booster rockets. Ladish manufactures both the semi-spherical dome and cylindrical body sections of the rockets

Ladish uses the arm for conventional dimensional checking, with two notable twists. One involves the quality of the data collected. Most manufacturers check 100 or fewer points on an object, because that is necessary to ensure dimensional quality for the items that they make. For instance, the maker of a fabricated metal tank for an off- highway vehicle requires 50 points be checked. For the dome, however, extreme detail is needed to best position the forging to ensure machining a compliant part. For this reason, Ladish gathers what they call a "digital cloud" of data—up to 200,000 streaming points on each piece.

After it is collected in CAM2 software, this point cloud is inserted into surfacing software to create an accurate 3-D reproduction of the dome. Then, quality and die engineers inspect this digital picture in minute detail, often looking at areas of the overall image that are only 1/8 in. across. The software highlights the surface of the dome with colors that represent normal distances from forged surface data to finished part (Plate 2). A red spot, obviously, signifies a section that is too high or too low and may have to be adjusted before the part can be machined. "With the FaroArm, we gather 2000 times the data in one-fifth the time," explained Bacon, "and then use it to create layout marks for the machinist. We have not shifted a single part during machining since using the arm for layout."

Plate 2 Technicians gather very dense digital data on rocket forgings so that they can create a highly detailed CAD image of the part prior to final machining

This highlights another way that the work at Ladish differs from that of other arm users. Most employ the arm to measure parts after an item is manufactured. Ladish, however, uses it to guide the overall forming process.

The attention to detail pays off by eliminating potential machining issues(Plate 3). "The image that we get from inspection gives us realistic dimensions to work with and lets us align the part for the widest latitude on the CNC machine," explained Snyder.

Plate 3 This is a digital "cloud" of data gathered by Ladish technicians for a rocket dome (left) and body (right) sections

Machining from a picture

Ladish begins the imaging process by striking eight punch marks along the OD of the forging. These are locator marks for the FaroArm, much like pins sunk in a tree by a surveyor. Because the largest arm is 12 ft and the circumference of the forgings is more than twice that length, the arm has to be jumped from one locator point to another to completely measure the part. The total digitizing process takes two days. "When we are finished, we can use any of the punch points to locate the part's true center," said Eric Modrynski, Forge Shop Engineer at Ladish.

After examining the 3-D image of the part, inspectors define the machined envelope and the part is transferred to a CNC cutter(Plate 4). Then, a cutting program is written from the digitized image to machine the part. Finally, the part is aligned with the CNC machine, using the punch marks to reference, and machining begins.

Plate 4 Engineers convert the data cloud created with the arm into a 3-D surface image of the part to be machined. This enables them to inspect the part for out-tolerance points, and to write a program to cut the raw forging into its finished shape on CNC equipment

If, on the other hand, the image shows that the part has areas that are too high or too low, it can undergo a controlled rework to yield an acceptable machinable profile. After that, a new set of locator marks is punched into the part and a new digital image is created and inspected (Plate 5).

Plate 5 Finished rocket dome reflects the quality of the machining process developed by Ladish

"This has given us a 'no mistake' way to align and machine these parts," asserted Ed Pastorek, Supervisor of Product Assurance, Quality and Technology at Ladish. "Now, we know exactly what metal we have to take off."

The new inspection technique has all but eliminated the possibility of producing an unusable part. "Because the great detail that we get from this technique, we are better able to see sections of a rough forging that require secondary reshaping. This way, there is no chance to miss an area that would be out of tolerance," explained Pastorek.

Ladish Co., Inc. and FARO Technologies