Aerospace components cut from the solid.
At first blush it appears illogical to carve a solid 120 4 1/2-thick block of aluminum weighing 450 lb down to a part that scarcely weighs 22 lb. There must be another way. But Ed Crozier, Algonquin Parts Inc, South Norwalk, CT, says no, and explains the reasons logically and with ease.
Algonquin Parts has been a manufacturer of machined parts for the aerospace and missile industry since 1941. It specializes in producing difficult, contoured parts in the small lots generally mandated by the nature of the aerospace business. An extensive knowledge of metals and their properties is mandatory, since the procedures followed in machining a piece are often more important to the final part configuration and tolerance than the accuracy of the machining system as it moves the cutter through its assigned path. This becomes more evident when you consider a part, for example, that is machined from a solid 4-thick block to small, varying cross sections, some of which are only 0.090 thick. How do you prevent the part from assuming a set that differs from that described by the drawing?
Figure 1 shows a spar slat for the leading edge of a McDonnell Douglas Corp E4 Phantom aircraft wing, used during takeoff and landing for additional lift. It is cut from the 4 1/2-thick, 120-long block of solid material shown behind it. The cross section of the machined part varies from one end to the other, and the part is cut from the block on the bias. It demonstrates the potential problems with deformation that can exist.
To ensure stability in the final configuration of parts such as these, Algonquin departs from general industrial practice, which suggests that all operations possible should be performed during any one setup--both roughing and finishing. In a first operation, generally performed on a six-spindle manually operated milling machine, most of the excess material is hogged out, leaving varying amounts of stock for the finishing operations. These first cuts introduce deformation. To combat this, the part is then allowed to self-age for one to two months. During this period, the material gets harder and stabilizes. After aging, the reference points are re-established and the parts are then finished on one of Algonquin's eight Hillyer machining centers equipped with three-axis contouring capability. Those operations that require drilling or grooving at special attitudes with respect to the machine axes are handled as a follow-on operation on machines especially set up to accommodate them.
In explaining some of the structural reasons for carving from the solid, Ed Crozier said that the wings and wing elements of an aircraft have to be flexible. "The Lockheed C5A wings, for example, droop considerably when the aircraft is on the ground. But the wing tips raise 13 ft to reach flight attitude when the aircraft is airborne. This flexibility is obtained from the properties introduced by hot rolling the metal. The cast-aluminum billets are hot rolled into slabs. The heat and rolling process eliminate gas pockets and shrinkage cavities, elongate the coarse grains of the original cast structure, and then break them up into smaller ones. In addition, the smaller grains are squeezed closer together, elongating any internal cavities and eventually eliminating them completely by welding their surfaces together.
"The result is a material in slab form that is stronger, tougher, and more defect-free. The strength lines are continuous and parallel, allowing machining operations to be performed without introducing any weakening effect to the basic material. The result is flexible strength--the element that makes flight possible. The stock for each part that requires this characteristic is then sawed from the slab in a shape that approximates the envelope size and contour.'
In the background of Figure 1, three Fairchild A10 aircraft engine pickup bulkheads are shown. The engines, equipped with two wraparound support rings, mount to the left and right of this part. The front ring of each engine is located in the right and left clevis, and two bolts on each side secure them in place. Another similar rib, located 80 aft of this one, supports the rear ring of each engine, also secured by bolts in the same fashion.
The former for an F18 Hornet aircraft is shown in Figure 2, positioned on the top of a contour-sawed block similar to one from which the part was machined. The curved portion of the part is designed to fit the engine. When machined, the block weight is reduced from 200 lb to a finished part weighing only 12 lb. Before machining, the solid block is loaded into its fixture, located by dowel pins.
One side of the part was roughed and finished in one setup on a Hillyer. A close-up view of the operation is shown in Figure 3 to illustrate the amount of material removed and the nature of the contouring work.
The wingfold rib of the F4 aircraft is shown in Figure 4 during the first operation. The wings of the F4 fold when in storage and are unfolded and locked for flight. This part is the hinge and locking mechanism that mounts on the inboard wing section; it mates with one mounted on the outboard section. Flexible strength cannot be tolerated here. Instead, rigid strength is required to allow close, dependable fits and a strong, rigid coupling. A forging fills the bill in this case.
In producing the part, shown in the background of Figure 5, approximately 0.300 of material is removed from the forging shown in the foreground. This is done in two machining-center operations. The hinge and lockpin holes are drilled and finished as follow-on operations on special machines set up for that purpose.
One of the largest parts in terms of table area is 72 long 56 wide and requires a 48-dia counterbore to be profiled with a small end mill. The machine accuracy required is 0.001 at any point. The longest part is presently the 120-long wing slat; the smallest part currently produced encompasses a 1 1/2 3 8 envelope.
Ed Crozier says that handling of the wide size range is complicated by the fact that he does not manufacture a final product, and so does not have the luxury of selecting and justifying equipment based on an expected annual volume of known products. Instead, he selects it based on its flexibility to do his kind of work. A twin-column machine was especially interesting for that reason. Crozier states: "It can handle our biggest payloads efficiently, yet it can also handle our smallest economically.'
There is another factor considered in evaluating the finishing machines: The work that comes from the multispindle roughing machine has widely varying depths of stock to be removed, and the tooling and the machine must be rigid enough to handle the job to the close tolerances required.
"We are a production shop, so we cannot afford to baby a machine,' says Crozier. "It has to be capable and ready to go full tilt when our schedule calls for it.' For info on the machining centers from Hillyer Machining Center Div, Acme Technologies Group, Acme-Cleveland Corp, Mountainside, NJ, circle E11.
Photo: 1. The part shown was machined from the 4 1/2-thick, 120-long block of aluminum shown in the background. The finished part weighs 22 lb; the block weighs 450 lb. The parts shown in the background support the engines for a Fairchild A10 aircraft.
Photo: 2. This F18 Hornet aircraft part, weighing 12 lb, is shown on a contour-sawed block similar to the one from which it was machined. The stock weighs 200 lb.
Photo: 3. This close-up view of the F18 Hornet part illustrates the complexity of the part and the amount of material that is removed.
Photo: 4. The wingfold rib of a McDonnell Douglas F4 aircraft and its fixturing are shown on the table of a machining center.
Photo: 5. Two finished wingfold ribs and a forging from which they are machined are shown in this view.
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|Title Annotation:||Algonquin Parts Inc.|
|Publication:||Tooling & Production|
|Date:||Jan 1, 1984|
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