Start With Waterjet For More Effective Milling
by David Millson, contributor
For aircraft components machined out of plate stock, this shop roughs with abrasive waterjet before finishing with conventional milling. Removing roughing from the machining center cuts cost and lead time.
However, fewer isn’t always better. AFG, or Aerospace Fabrications of Georgia (Dallas, Georgia), recently discovered it could streamline its process for machining aircraft components out of solid aluminum by inserting a setup on an abrasive waterjet machine ahead of the setup on the machining center. The waterjet machine cuts the part to its rough shape so the machining center doesn’t have to. This approach not only saves time, it also results in material savings that reduce the cost of each part.
Why Waterjet Roughing
“Material handling and fixturing for any waterjet operation is typically more forgiving than for any other CNC machining setup,” Mr. Grizzle says. No fixturing at all was required to cut the window frame out of the large plate on the waterjet because of the mass of the stock. Gravity and friction held the workpiece in place. The 50 hp waterjet cut the rough outline of the part at a rate of 1 inch per minute using 60,000 psi and garnet abrasive. After cleaning, the part was ready for the five-axis machining center.
“If we had done the roughing on the five-axis machine, we’d have added an hour to each piece,” says Mr. Grizzle. Using the machining center alone also would have wasted material, with the material that might otherwise deliver a fifth piece ending up in a pile of chips. “That means the overall material cost per piece would be higher,” he says. “In bidding situations, nested waterjet roughing is part of our competitive edge.”
Other savings from waterjet roughing are not so easy to quantify. “Because of its more hoggish use of materials, conventional roughing won’t allow us the economy to cut an extra piece to have on hand—to build in an ‘oops!’ factor, so to speak—so we can still complete a customer’s order in case of a machining error,” he says. “Also, with waterjet there is a saving in wear and tear on the equipment. We use it not just in aluminum, but also in materials that are hard to rough—even in three-axis applications—to save our CNC mills and cutters. And when compared to five-axis machining, the waterjet is normally less expensive to run, its initial cost being only a fraction of the price of a five-axis mill.”
An Aluminum Window Frame Part
An aluminum window frame part is roughed with waterjet and finished through five-axis milling. Where large plate stock is used, waterjet allows cut-out blanks to be nested for maximum material efficiency. Nesting this part let the shop get one extra piece out of the plate.
Five-axis machining was a must. The frame conforms not only to the semi-cylindrical curve of the aircraft’s fuselage, but it also must accommodate the fore-to-aft sweep. Even so, the geometry existed only in blueprint form. Though it reached production in the 1990s, the plane was being designed as early as the ’60s. Translating all of its part designs into CAD would not have been cost-effective for Boeing, so Mr. Grizzle worked from the print to create the 3D model of the part to be used for CNC programming.
The frame is machined from 2.5-inch thick 7050-T7651 aluminum plate. While the shop formerly used standard-size plate for this part, it now machines the part out of plate cut to size so it can deliver the part on a JIT basis. When AFG did machine the part from larger stock, nesting the part for waterjet cutting let the shop get five pieces from one plate. Milling alone would allow the same plate to deliver only four pieces. However, even when AFG doesn’t use nesting, Mr. Grizzle says other advantages make waterjet roughing attractive.
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Waterjet Programming
Most users don’t think of Mastercam as a system for programming waterjets. A waterjet machine usually comes with its own programming system. However, AFG prefers to perform all of its programming off-line, using one system for all of its machines.
This 1-inch thick titanium part is cut to its final dimensions and surface quality using one pass on the waterjet machine.
“We treat the stream diameter just as we would a small diameter cutter,” Mr. Grizzle says. A 0.040-inch nozzle becomes a 0.040-inch end mill. “Also, we can pierce holes using the software’s drill cycles and cut contours using 2D contour toolpaths. We even incorporated the automatic slowdowns for corners and arcs into the Mastercam post.”
On material up to 1 inch thick, the waterjet machine can give AFG the surface quality it needs for a finish cut, even in titanium. These finish-cut parts in particular argue for programming in the shop’s standard CAM system. One of the reasons the shop chose this software is because the tool paths it generates are associative. When the part model changes, the tool paths change automatically. This is an important advantage to AFG because customers often revise their part files. By using Mastercam for waterjet cutting, the shop extends this same benefit to parts that rely on waterjet for their critical machining passes. Thanks to associativity, when the customer makes a change, the path of a milling cutter or a waterjet nozzle can change along with it.
About the author: David Millson is editorial consultant for CNC Software/Mastercam of Tolland, Connecticut.
The Five-Axis Milling Cycle
AFG technicians refer to the surface of the window frame closest to the aircraft’s skin as the “exterior,” which is where five-axis machining begins. Because the stock has to be thick enough to accommodate the part’s thickness plus the full range of the exterior and interior curvature, the original aluminum weighs 150 pounds. After machining, the part weighs 15 pounds. On the exterior side, five-axis milling with a 3-inch face mill imparts the curvature, and a 1-inch end mill cleans up rough spots left by the waterjet cutting in the exterior’s pocket contours.
With the frame still fixtured, next comes the drill tool path for more than 200 holes through which rivets will attach the frame to the inside of the aircraft’s skin or to other airframe members. With few exceptions, each hole is angled differently.
When the pockets, profiles and holes are complete, the part is flipped for work on the more complex interior. Tools used here include a 1-inch end mill to finish the form, as well as a specially ground keyway tool used to cut channels in the window frame.
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