Among the more important developments which have aided mass production of aircraft have been the mechanized assembly line, simplification in tooling, steel tooling and production of planes in small sections.
Progress in this field at the San Diego plant of Consolidated Vultee Aircraft Corp, has resulted in meeting heavy production schedules and a striking reduction in man-hr per plane in turning out a B-24 Liberator. On January 1, 1943, direct labor hours for a B-24 in the San Diego plant and feeder shops amounted to 18,000. A year later this figure had been cut to 9500 hr. Today, the number is less than 8000. It is significant that these improvements followed a year (1942) of accelerated B-24 production during which man-hr per plane had been slashed from 40,000 to 18,000.
During 1940, seven B-24s were produced in the San Diego plant; in 1941 there were 177; approximately 1200 were built in 1942, due principally to expansion in tooling and experience acquired in volume production. Output accelerated throughout 1943 when about 2400 B-24s were made. Production leveled off in 1944 and schedules have been maintained despite a substantial reduction in the number of employes.
Total production of heavy military planes in the San Diego plant during the war period has amounted to approximately 8000 units, including nearly 2000 Catalina PBYs and over 200 Coronado PB2Y-3 flying boats. The following broad specifications for the tooling program have been a factor of great importance in production records established at Convair.
A feature of aircraft production is the constant flow of design changes in each model, or conversion to an entirely new type of plane. To minimize the difficulties in scheduling these changes through production, one key is used in the San Diego plant, that of coordination of parts with 3/8" tooling holes. These reference holes, found in all primary structural parts, are identically laid out, regardless of the time or method of manufacture of the parts. All changes in production technique are referred back to the basic tooling holes.
In establishing these points of coordination on structural drawings, close cooperation is observed between tool planning and design engineering. The latter groups establish water and buttock line dimensions for certain "stay clear" areas, which are indicated on the basic loft layouts. In general, these "stay clear" areas are shown as a circle of 1-1½" in diameter. It is understood by Engineering that no accessory equipment or structural changes will ever be made in this area inasmuch as this is reserved solely for Tooling Department's use. Even if plane contours change and new frames are substituted, the "stay c1ear" area pattern will remain fixed.
Actual centers for tooling holes in the "stay clear" area are located to the nearest even ½" dimension by use of a grid plate. This plate includes eleven lines of No 30 bushed holes with eleven holes in each row located on precise 1" spacings. Lines through the centers of these holes are dimensionally parallel, both horizontally and vertically, in relation to the outside perimeter of the plate. With ½" offsets at the corners of the plate, it is possible to set hole centers to either inch or half-inch increments according to the positioning of the plate on loft grid lines.
Use of the grid plate on detail templates, together with charted dimensions of the tooling holes, obviates the necessity of jig boring templates. This is possible because templates are first drilled to a 3/16" opening for hydropress locating pins. Parts are subsequently drilled out by jigs to 3/8" for assembly operations.
For establishing permanent locations of the tooling holes to be used on permanent or temporary assembly tools, master index plates are made. These plates show body offsets, and include tooling holes for all stations of the fuselage. From these master index plates, individual station tooling plates and transfer plates are made. No changes are allowed in the tooling holes once the master index plates and tooling plates have been made.
For each model airplane constructed at the plant there are three tooling phases. Within four to six months after start of tooling on a project, the first phase tools and the first plane can be delivered. This tooling consists of temporary, detail, and assembly tools whose only relationship to the final tooling will be the use of the same loft and tooling hole dimensions.
Assembly tooling at this stage consists of table-top fixtures which will individually locate all primary structural elements for sub-assembly work such as belt-frame segments, floor structures, door openings, astrodome and turret structures. Parts are located by contour with blocks set to lines on reproductions of the original loft layout. Jigs are provided on the fixtures for drilling tooling holes. All other holes are initially laid out by hand. The tooling holes allow the various subassembled segments of belt-frames and flooring to be transferred to temporary final assembly tooling, generally one or two major bucks. These bucks locate components by means of tooling pins which are coordinated to tooling plates, as are the component jigs.
Maximum capacity of a tooling setup of this nature is considered about one unit every two days. As schedules accelerate, further decentralization is necessary to avoid congestion in working conditions usually found in the close confines of a buck. Moreover, installation of equipment in complete fuselages rather than segments gives rise to problems of station loading. Consequently, concurrent with initial production, second phase tooling is developed for release to production.
In the second phase of tooling, shop-stationed tooling personnel modify the existing table top fixtures through improvements. These changes consist of adding drill plates to drill out splice attaching holes and providing sheet metal templates to simplify layout of holes in the structure itself. Templates are located from the tooling hole jigs through the medium of pull pins. All holes of splice plates involving coordination of segments are developed from tooling plates.
The next step in the second phase tooling program is construction of sequence No 2 panel fixtures in the tooling dock. This equipment is a steel framed structure 60' x 15' x 10' equipped with horizontal, vertical and transverse steel straight edges. It is used for constructing three-dimensional assembly fixtures from loft information in a minimum of time and to tolerances not possible with former methods of building fixtures. The tooling dock has revolutionized the methods of building and duplicating fixtures and has been a great factor in speeding up tooling for new models.
The second phase fixtures, depending upon the limits of major breakdown of the airplane, are trunnion or picture frame type, accessible from both sides. Fixtures are integrally self-contained and are independently suspended without attachment to the floor. Skins, stringers, and belt-frame segments are constructed in these fixtures. Tooling holes in the sequence No 2 type fixtures coincide with the locations established in the table-top fixtures inasmuch as both have been laid out from the same basic tooling plates.
From the sequence No 2 fixtures, completed panels are mated into major segments of the fuselage in sequence No 3 type panel fixtures. These differ only from the No 2 fixtures in that they are larger and contain fewer locators which are positioned only at key attaching points. The latter fixtures are also built in the tooling dock and use 3/8" tooling holes and splice holes as the basis of coordination with other sections.
In order to obtain maximum production on a model, third phase tooling is employed which consists of permanent detail tooling and a further breakdown in the panel fixtures known as a sequence No 1 fixture.
Permanent detail tooling generally consists of heavy dies for blanking and piercing holes for full size coordination where no contours are present on the matching parts, or 1/32nd undersize where contours are present. Thus, by using predrilled setups on the detail parts, the table-top fixtures can be replaced with simple holding fixtures for riveting and clean-up operations.
Sequence No 1 fixtures are also "tooling dock" set and are used in setting-up skins and stringers for automatic riveting. As such, they allow removal of stringer and skin locating pins on sequence No 2 fixtures, an operation which can be accomplished without removal of tools from production.
This third phase tooling generally can be completed within a year after commencement of a project. According to the degree of refinement of detail tooling, any required maximum production capacity can be obtained.
By overlapping tooling improvements with initial production it is possible to work design changes into planes and thus build them to changing military needs. The extensive tooling program, built after the above pattern, has made it possible to utilize thousands of semi-skilled workmen in building precision products at the San Diego plant. It has enabled rapid acceleration of schedules whenever a new model is placed in production.
Illustrative of progress made in the fabrication of many detail parts which require burring is a machine developed for this purpose in the San Diego plant. The machine handles flat sheets which have required many thousands of man-hr each month in burring holes by hand tool methods. The new equipment accomplishes this work in a fraction of the time formerly required. A spar web 24" × 156" containing 962 holes can be burred in 5 sec. Experience in operation of the machine indicates a tremendous reduction in burring time. Estimated total man-hr saved annually will be about 130,000. In addition to cutting man-hr for burring, the machine will obviate the necessity of using large numbers of hand tools, air motors and drills, with substantial savings in maintenance on these items.
The machine is designed to remove burrs and ragged edges from sheet metal which are formed by drilling, punching and shearing operations. It handles satisfactorily not only aluminum-alloy sheets, but also stainless steel and other steel sheets which can be drilled. On the large machine now in operation two semi-skilled workmen are employed who feed the sheets into rollers on one side and remove the burred sheets on the opposite side. Belt delivery of small parts is being provided for on a smaller unit. Table-top trucks and adjustable height rollers for handling long and heavy sheets will speed up burring work in larger machines.
Burrs are removed by conveying the sheet metal over a highly tempered steel knife by means of power driven rollers. The blade can be precision adjusted to within .002", resulting in practically complete removal of burrs. Any gauge sheet metal may be processed simply by changing the vertical distance between the rollers.
Sheet metal is started into the machine between rubber covered rollers, spaced according to gauge thickness. The rubber covering allows the burrs to pass between the rollers without being flattened against the sheet metal or being forced into the holes. As the sheet progresses over the knife and through the machine a set of power driven steel rollers flatten any remaining burrs on the sheet. A burr of several thousandths may be left around holes. These are flattened against the surface as the sheet moves between the steel rollers. In this process the remaining burrs are work hardened, causing them to become brittle. In subsequent assembly operations these brittle burrs are ejected from the holes when rivets or bolts are inserted.
An innovation in riveting technique has been made by the San Diego plant with development of a new type bucking assembly for production riveting.
Designed to replace half of the conventional manual riveting team, the device when fully developed will save many thousands of man-hr. The assembly consists of a "floating" steel bar, which is positioned against the shank end of the rivet. Two or more spring-actuated reacting hammers which operate against the back side of the bar absorb impulses from the conventional air-driven riveting gun.
The device is not limited in size or shape and can be adapted to many types of riveting assemblies. It can be set up in a jig to function automatically on practically all of the rivets in the assembly. When used in this manner, the floating portion is in the shape of an elongated steel bar, extending behind each row of rivets. It is supported to each end of the fixtures by a device which is pivoted to the framework of the jig.
The bar does not have to be straight nor horizontal. It may be curved or offset to accommodate the configuration of any line of rivets. A gauge on each supporting device controls the size of the upset head, resulting in uniformity and preventing over-riveting.
An important advantage of the new method of riveting is the elimination of "oil-canning" or deforming the skin. In manual riveting and bucking, failure of the two workmen to coordinate their work often results in deformation of thin sheet metal parts with adverse effects on aerodynamic characteristics of the plane. A feature of the new assembly is that the operator can determine when to stop hammering to obtain a perfectly driven rivet because there is a decided change in the sound of his blows.
Accurate countersunk riveting is also possible with the jig riveting assembly. This special type of riveting has required the services of the most highly skilled rivet teams. With the new equipment, unskilled workmen can drive a perfect row of countersunk rivets without dimpling the skin surfaces.
The tooling cost of the Convair riveting assembly is reasonable in view of the savings it can effect.
For some jobs where no jig or fixture equipment is available, manual bucking bar adaptations have been created. These lightweight tools incorporate reacting hammers, so that all a workman has to do is support the assembly in the proper position. The reacting hammers absorb the impact of the rivet gun blows. Adjusting bolts make it possible for the reacting hammers to absorb all or part of the energy expended by the rivet gun.
When used on long rows of rivets, the manual bucking bar has a face shaped like a long, thin rectangle. This enables the workmen to cover several rivets simultaneously, thus eliminating the likelihood that he will support one rivet while the gun operator drives another, a frequent cause of dimpling. The tool has proved effective in reducing fatigue.
Much of the efficiency being maintained in the San Diego plant is attributable to the highly developed scheduling system. Synchronization of a plant producing large aircraft with their thousands of individual parts is a complex job. Accurately planned schedules prepared weeks in advance for every department assures steady and uninterrupted flow of detail parts to subassembly departments and these components to major and final assembly lines. Manufacturing schedules are developed for the San Diego plant as follows:
Volume production requires as many simultaneous operations as possible. Production Control, therefore, analyzes the Assembly Planning sheets in relation to the floor plan layout to ascertain if it is possible to work on some assemblies concurrently with others. The three major components, for example, of the B-24 are made at the same time; wing center sections, tail, and nose fuselages. After all the work which can be done simultaneously is determined, numbers known as priorities are assigned to each assembly station and to each Assembly Planning Sheet.
These priority numbers are consecutively numbered from the end of the manufacturing activities, back to the first assembly operation. A chart is then prepared which shows the relative concurrency for each Assembly Station by placing its number in the horizontal position according to its priority number. The horizontal positions are serially numbered from top to bottom according to priority numbers. This chart, in effect, is the usual "Christmas Tree" type which shows assembly sequence from detail parts through subassembly and major assembly stages to completion of the airplane.
The foundation is thus established for the manufacture of one airplane. The next problem is to relate a rate of production to the method of production. This is done by a Master Priority Chart in which the required delivery date is the beginning point and from which all work is figured backward through the various assembly manufacturing operations, which are expressed in terms of priorities. The volume of these assembly activities must be accelerated beyond the required delivery rate during the first period of manufacturing so that the assembly lines will be entirely filled. Thereafter, rates at which assembly work is completed in the various lines becomes identical with the rate of delivery.
The standard manufacturing times allowed for each assembly are then totalled to show when the first assembly operation must be performed in order that it can be delivered to its next assembly point in accordance with the schedule and manufacturing scheme. An allowance is made for each schedule issued to the shop which adjusts the assembly standard time to the actual manufacturing efficiency. Schedules are then prepared for each activity involved.
Behind these assembly operations is the manufacture of thousands of detail parts used in making the assemblies. Detail part scheduling is handled in another manner and is called the "time interval-cycling method" in which detail department shop loading is balanced by both delivery lot sizes and production quantity requirements. It is imperative that the assembly and detail part scheduling methods be keyed exactly into each other to prevent shortages from occurring on assembly lines.
Schedules for each model being made in the San Diego plant are prepared many weeks in advance. Superintendents and foremen are guided in directing work by consulting the schedules prepared for their individual departments. Each major assembly such as the wing center section, tail fuselage section and nose fuselage section has a time table prepared for it from the time it starts until it reaches the mating station on the final line.
This article was originally published in the October, 1944, issue of Industrial Aviation magazine, vol 1, no 5, pp 26-34, 36-37, 93-94.
The original article includes a portrait of the author and 9 photos of the San Diego assembly line.
Photos are not credited but are certainly from Consolidated Vultee.