Although there are several ways of bringing together parts of an aircraft, Boeing devised a new and somewhat different type of production often referred to as multiline production. This system is predicated on the fact that an aircraft in its final form is extremely extravagant of space. Multiline represents the antithesis of the elongated single production line.
The divergence of methods used by various manufacturers of large type aircraft reflects differences in production philosophy. In Seattle, the Boeing plant was designed specifically for quantity production of large four engine planes. In designing the building and laying out the flow of parts through the various stages of production, thought was directed to the fact that an aircraft of any kind is complicated, and that the building of a military type requires the utmost of flexibility in manufacturing.
Because every square foot of floor space represents a real investment, the most economical type of manufacturing would seem to be one which makes the best possible use of this floor space. In adopting this philosophy, Boeing has achieved an outstanding record of production in terms of pounds of aircraft per square foot of floor space and per worker. By keeping most of the operations under one roof, the flow of materials from raw stock through the sequence of manufacturing operations to the final finished airplane is considerably shortened.
The chief ideas guiding the Boeing assembly plan are:
Source of all parts manufactured in the plant is in the extensive machine and forming shops spread across the rear. Raw stock and small ready made parts are stored along one side of the building. Opposite is the receiving and subassembly station for subcontracted outer wing panels, control surfaces, engines, propellers, tires, etc. A wood shop manufactures plywood flooring, walkways, and ammunition boxes.
The tool and die shop is located in a building adjoining the fabrication, subassembly, and assembly areas, and is equipped for rapid production of new tools. The tooling system is designed to keep pace with the continual flow of changes which pour in during production of any military aircraft. A new die can be in operation within two weeks of a change request. The philosophy has been to put tooling engineers in cooperation with design engineers who are to draw up proposed changes.
Two things are accomplished by this, the first being that tool designs can be drawn up almost simultaneously with the engineering drawings; second, that changes will be planned with practical tooling in mind. This latter is a whole subject in itself,and much profitable work has been done to guide progressive design thinking.
Modifications on current production are controlled as fully as possible in the block system of application, which allots certain changes to be applied with a definite serial number, with another group of of alterations to begin on a later designated serial number. This produces uniform ships within groups, which is preferable to modifications being strung out, with each successive plane a little different from its predecessor.
A variety of crank and hydro presses ranging from five-to seven-ton capacity turn out all the smaller formed and perforated parts, while heavy presses up to 5,000-ton capacity handle large sheet and heavy parts. Drop hammers have been practically eliminated, which has speeded production in some cases from two parts an hour to four a minute. Many parts have been redesigned for this type of production, such as the tail gunner's door, which used to require a jig, riveting, and welding. Redesign for draw and form has eliminated 26 parts and considerable assembly time.
A great advance has been made in retarding action of heat-treated dural. After forming, heat treating of a part causes some warpage. By putting the part immediately under refrigeration, the period of setting up is interrupted. When ready for use, it is remove from dry ice and put on a restrike die which trues the form, and a short time after the heat treat sets up to maximum strength.
Two of the nearly quarter million specially developed tools are the automatic spot welder for making up large sheets from smaller sections, and a multiple hydro punch press for cutting out slots in the circumferential stiffeners in one operation. The welder performs 62 automatic spot welds a minute, moving the sheets half an inch every 2/10 sec, spending 8/10 sec on the weld. The circumferential hydro punch, nicknamed "octopus," increases production of stiffeners 45 times. Both have been passed on by Boeing for use by other manufacturers.
In the second floor subassembly section space and time saving really begins, for assemblies worked out here are often as complete as they will be when finally assembled. An example is the wiring procedure, which uses clearly marked wiring panels with colors and lines to mark the leads. Accomplishments are threefold: First is simplification of the process to makes use of unskilled help; second is a finished product which requires a minimum of reaching and dragging back and forth in installation, the wiring being slapped into place all in a piece at the proper time; and third is complete pre-testing by means of terminal plates which test every wire of the circuit before removal from the template. With over six miles of wire in 8,000 pieces, failures average less than three per airplane. This system has likewise been passed on to other manufacturers producing the B-17.
In subassembly, tubing is shaped to necessary contours and flared, then terminals are prepared for connection. Other subassemblies include all the smaller components of the ship. Outer wing panels, all control surfaces, nacelles, wing tank doors, and ribs are subcontracted. Small frame parts for the fuselage are made up as subassemblies, ready to be assembled on the main floor. Cabin top shells, dorsal fins, stinger turrets, leading edges of wing inner sections, and bulkheads are all made in subassembly. An upholstery department prepares insulation and inner covering for the ship.
Between the fabrication an subassembly areas (in the rear of the plant) and the major assembly area is a processed parts store which, in addition to parts, is occupied by most of the completed subassemblies ready to be incorporated in larger structures. Wing components are stacked beside the wing jigs on the main floor.
There are five main divisions to the major assembly plan
Both sections of the fuselage are fabricated in two-story jigs lined up across the rear of the floor as closely as efficiency permits, and it can be seen from the illustrations that although spacing is close, accessibility has nowhere been sacrificed. Actually, waste motion is reduced to a minimum because tools and materials are always within easy reach.
The rear section is built in one piece. Previously fabricated bulkheads and dorsal fins come down to be joined with the structures. At the forward end of the rear section, provisions are made for the structural splices which will unite fore and rear sections. These consist of beam splices at top and bottom just under the skin. Sheet joints at these points are left unriveted until the two pieces are united. An additional horizontal plate attachment at bottom further increases strength of the joint.
When structurally complete, front and rear sections are lifted one row forward by overhead bridge-type cranes, which span wide floor areas and greatly contribute to the ease and flexibility of assembly. In this area the units are supported in mobile dollies, where complete interior installations are made with exception of seats, turrets, and guns. Wiring, control cables, and everything requiring easy access, is put in place before the two units are joined.
This process is simplicity itself. The forward sections are moved ahead to the front of the space allotted, and placed in stationary supports. Rear sections are then brought over by crane, lowered onto mobile dollies, and pushed up to rear of the forward sections. Jacks incorporated into these dollies enable exact height adjustment to be made when the two sections are pushed together. Splices are mated, secured, and riveted. The circular bulkhead and flange are tightly bolted together and finished with a torque wrench. All connections of wires and other lines are easily managed by special terminals and junction boxes.
At this stage the bomb racks are installed in the bomb bay, and the wing control cables, wires, and plumbing connections are made ready for rapid attachment to the mating units in the wing. By this means the numerous reaching and fitting operations have been eliminated from final assembly. The bombardier's and avigator's instruments are installed forward, bombardier's seat put in place, and the transparent nose fastened on. Windows are installed in the cabin. The stinger and other turrets are not put in until final assembly.
The wing inner section jigs, occupying a large area nearly midfloor, are built together in blocks of eight. One structural support in each block extends down 40 ft to bedrock and furnishes the key reference point to which all the other levels are checked. This is done by markers sighted-on by surveyor's transit, and adjustments are made at the base of each jig support by means of a nut. Adjustments can be made to .0005 in, and they are checked after removal of each completed wing before beginning of work on the next.
Three working levels on these jigs give access to all parts. Thickness of the wing is such that internal working space is ample. The front spar, which is built complete as a subassembly, is inserted at the bottom, with the rear spar at top. Ribs are fixed in place by spacing jigs. Control and flap motor brackets are installed, followed by the corrugated sheets which lie just under the skin. As the latter are applied, the engine nacelles are admitted through doors in the lower platform and joined to the wing. Landing gear brackets go on and the skin is completed everywhere up to the flap section on the lower side.
At this stage of completion, the jig is opened and the wing lifted by crane to the next station forward, where all the final installations are made. Landing gear struts, exhaust tail pipes, turbo-superchargers, exhaust heaters, control cables, flaps, tanks, lights, radiators, leading edges, plumbing, and fuselage connections are all finished in this area, and finally, even the engine installation is completed before the unit ever touches the fuselage.
This brings the wing section abreast of the joining fixture, complete except for the outer wing panels, which go on later. Many parts installed here come in from outside the plant, feeding in at this point from the side of the building.
The nearly complete fuselage is now lifted forward from its station just to the rear and placed on jack supports, while the two wings are brought over and bolted on, with the ready made connections of cables, wires, and plumbing hooked up, and the fairing strips applied. The rudder, stabilizers, elevators, and wing outer sections are installed, wheels put on, turrets put in, propellers mounted and there's the ship.
Following a few items like pilots' seats, bomb bay and rear turret doors, testing of landing gear and flap operation, and checking of the instruments with an electric test bench, the ship is lowered to its wheels and rolled forward for a final inspection.
Perhaps the most characteristic quality of this type of production is its flexibility. As contrasted with long-line work, the B-17 units are made so complete in major subsections that they can be brought together almost anywhere there is a free space. The cranes covering wide floor sections make possible this freedom of movement, while rendering constant direction of flow relatively unimportant. Some stages of assembly vary considerably in floor position according to space available. If certain processes are to be speeded, this assembly can be spread to other areas which are not dependent on line stations. Tools and materials are easily moved to accommodate.
Behind all this fluid activity is a perfect timing. If one division falls behind, it is as instantly apparent as an empty space in a line. Speeds are geared to each other and competition is promoted to keep all departments up to their best.
The production record on this world famous aircraft has been for strategic purposes, kept well under cover. But it can be revealed that it is such as to strengthen our belief in the soundness of the system and organization which has made this ship appear in force on all the battlefronts of the world.
Testifying to the efficiency of the multiline production system is the fact that Boeing, a first winner of the Army-Navy "E" award, has since received a star signifying a second six months of production excellence. This plant is currently producing eight times as many Flying Fortresses as it did the month before Pearl Harbor. This has been done without expanding the manufacturing facilities.
This article was originally published in the July, 1943, issue of Aviation magazine, vol 42, no 7, pp 125-129, 309-310, 313.
The PDF of this article [ PDF, 13.2 MiB ] includes a photo of a B-17F in flight, seen from 2 o'clock low, seventeen captioned detail photos of manufacturing processes, and a diagram showing the overall fabrication plan at Boeing.
Photos are not credited, but are certainly from Boeing.