One hundred thirty thousand pieces and 700 separate parts flow through 197 sub-sub-assemblies into two main sub-assemblies of the aft section and leading edge which come together in the main assembly, to which is added the doors and landing gear fairings to achieve final assembly of the Republic P-47 wing section, as accomplished by Servel, Inc, Evansville, Indiana.
Fabrication of the Thunderbolt is somewhat unique, in that wing sections are independent of fuselage construction, being separately produced, bolted and locked by wing hinges to fuselage plates, requiring unusual precision in assembly. Withstanding the stresses and strains of the unusual speeds developed by this fighter-pursuit plane and its combat maneuverability, there are few types of airframe construction that require higher qualities of engineering and craftsmanship than the P-47 wing. Without experience in aviation in 1942, Servel Inc, has achieved eminence in both quality and quantity of wing construction. In 1943, it increased its wing production 667%, originating "cycle timing" by which worker crews are brought to the work at 23 final assembly stations, reverse of Willow Run, where work was brought to the crews. By this method, Servel has accomplished this important production acceleration in less time, less investment in plant equipment, savings in man hours, less inspection time, rework and repair.
The "cycle-timing" system is new from the standpoint of its application in the Servel plant. Before this method of production was started, few of the operators had a specific operation or group of operations to handle repetitiously. The fact that there was no trained personnel for this type of work available in the Evansville labor area made it essential that jobs be made as repetitious as possible to reduce training required for each individual employee and to permit higher degrees of specialization on individual jobs. The "cycle system" accomplished this. After a relatively few weeks of operation, efficiency and skill of individual operators increased to the point where Servel is now producing its peak quantity with overall reduction in the total number of employees of approximately 40%.
Literally everything has been attempted to produce savings in production cost by the management, according to W R Campbell, superintendent of the Servel Wing Section Plant; reducing absenteeism, savings in direct labor cost, origination of special jigs and fixtures, streamlining final assembly, creating trouble-shooting coordination, lessening of tool abuse, incentive awards and work suggestions. From a day's plant observation, 80 specific items of production savings are detailed. No doubt there are others.
The shop engineering setup is vitally important to cost reduction. This has led to central control of hole information and skin sizes. It has facilitated the necessary changes to eliminate manufacturing variations which result in excessive filing and hand fitting. Rivet pattern changes have been made to improve quality or to simplify riveting operations. Unnecessary holes, rivets and operations are removed wherever possible with resultant time saving. It is necessary to have a supply of blank parts without any holes to make a good many of the repairs caused by errors in fabrication. this has been another responsibility of the shop engineering department.
There have been a large number of engineering changes made and placed into effect on the P-47 wing. Latest model of the Thunderbolt, the P-47D, has a new wing section. To develop added combat range, Republic Aviation engineers increased the internal gas supply by redesigning the wing section, increasing the span by 18 in and adding 22 sq ft more area. It is the duty of the Servel shop engineering group to follow these new parts and changes and make necessary tryouts to prove that they will work, as well as to assure that tooling and operations are fully ready to effect the change on schedule. A further function of the shop engineers is to make necessary arrangements to utilize parts which are not entirely to the drawing and may require special handling. Parts of this nature can cause an extreme amount of trouble if not followed closely and if installers are not fully informed.
Through follow-up, investigation and report of various troubles encountered by individual foremen, shop engineers have taken a large responsibility from the foremen, permitting them more time for actual personnel supervision with resulting increases in efficiency. In the investigation of various troubles, shop engineers issue individual reports on each which shows:
Another function of the engineers is to investigate closely those complaints of inspection, wherein the quality is not sufficiently high. The prevention of such recurrences tends to quickly improve overall quality. Further stressing of quality by this group has been handled by talks at various employee conferences held throughout the plant.
Through similar investigations and reports, shop engineers obtain correct usage of various supplies such as bolts, nuts, washers. This phase may appear relatively unimportant. However, through misuse of a bolt 1/16 in too long, it is conceivable that the plant could run into a material shortage on such bolts which might cause a shutdown.
A group of shop engineers has been set up to coordinate the efforts of all foremen and other supervisors of the various departments in reducing tool abuse, to assure the correct type of portable tools in the right places at the right time and to maintain accurate control over all portable tools. It follows this one problem exclusively.
Tool boards which provide mounting accommodations and silhouettes for all tool requirements such as portable drill motors, rivet guns, rivet squeezer, extension lights, are placed at various floor locations as required. These act as depositories for the tools used by both day and night shifts. Through the use of these boards it has been possible to obtain full two-shift use of each tool. This eliminates duplicate sets of tools, as well as all loss of time usually required to check tools in and out of tool cribs. The number of tool crib attendants has been decidedly reduced. By strictly controlling the tools on the boards and their identification by tool number and silhouette, these boards provide the correct quota for each operation or group of operations. They assure that the proper type of tool required in specific locations is always there. This cuts down run-around confusion.
On the P-47 wing all skin rivets are flush. This means that each of some 12,000 rivets has two or more dimple or countersink operations performed before driving, an enormous dimpling output per day. Dimpling machines became heavily overloaded long before peak production was obtained. It was necessary to increase machine efficiency to relieve this overload. An approximate 33% efficiency increase was obtained by the substitution of automatic machine control for manual control. These relatively simple attachments have repaid their cost many times over.
The machines, as originally received from the manufacturer, were equipped with foot operated controls which demanded pressure for each individual cycle of the machine. Design of the automatic control is extremely simple and has proven exceptionally dependable over a period of more than a year.
The male punch including the pilot which fits into the hole to be dimpled will support the weight of the sheet until such time as it enters the hole to be dimpled. Upon the pilot entering the hole, the weight of the sheet to be dimpled falls on the pressure pad insert, which depresses the lever, which in turn operates the close limit switch, thereby closing the control circuit of the machine and causing a complete dimpling cycle to take place. This control does not increase speed of machine cycle, but does completely eliminate the time required for operator reflex as well as to guard against operation of the machine when the die pilot is not in the hole to be dimpled.
Machining, forming and refining flanges for the main spars of the P-47 wing entails much machine time and many man hours. Recognizing the possibilities for cost reduction in this field, Servel has added a number of refinements and changes to special machines and tooling originally designed to handle this job. Approximately 5% time saving was obtained by addition of run-out plates to the ends of contour cams. These plates determine proper dimensions and setting of each cut. Through addition of locating holes in the rough extrusion, accuracy has been immensely improved on all dimensions. thus, a 5% saving on machines and a 10% saving in refining operations has been accomplished.
Increasing the sprocket ratio of the lead screw on milling machines has enabled Servel to obtain a 33% increase in feed. In conjunction with increased feed, it was found beneficial to reduce the number of blades in the cutters from 6 to 8 down to 2 or 4. This has resulted in a decided decrease in grinding time. It has also added materially to tool life through an average increase approximating 20% in number of cuts made per cutter. A 25% saving in machine "down" time has been effected in changing cutters. Use of Carboloy inserts in cutter blades won an additional 12½% time saving in cutter grinding.
Through increased feed and relatively high speed at which cutters operate, quite heavy bearing loads and severe conditions are indicated, under which the bearings function. Installation of special screens were made to close all openings into motor bearings, preventing chips, particles of grit and excessive dirt from entering the system, a prevalent cause of short circuits and burned out bearings. Approximately 35% motor repair saving was realized, in addition to reduction in machine down time for repair. Use of screens to close openings in motor bearings is not particularly new. However, in this particular instance it was not obvious that me aluminum particles were floating through the air in such quantities as to cause motor trouble. When the cause was finally traced down, the installation of these screens removed much of the hazard.
Using hand clamps, double drive arbors, and threaded draw bars to hold cutter arbors rigidly in the drive spindles resulted in an approximate 25% saving in machine down time and in refining time as well. Use of cam clamps definitely reduced the time required to clamp extrusions in place in spar mills. Double drive arbors enabled Servel in some operations, to use two motors to drive the same cutter; consequently the power available to drive the cutter was increased which permitted increased feed. Threaded draw bars used to hold cutter arbors into the drive spindles reduced the amount of down time resulting from the arbors sliding in the spindles and becoming scored, requiring replacement as well as facilitating removal of arbors or sharpening cutters and changing setups.
The straddle mill operation, originally done on large special mills, was removed from these machines and placed on hand mills which have reduced load on the special milling machines, resulting in a 33% actual machine time saving. Cam plates used on these mills are heavy and must be handled carefully to avoid distortion and damage. Servel has realized a 3% handling time advantage by incorporating hand holes to facilitate handling. The special mills, as originally received, were equipped with 15-hp motors which were changed to 20-hp, with resultant improvement in accuracy, smoothness of cuts, reduced repair cost and overall increased machine efficiency approximating 24%.
Use of adjusting screws on the side and front of the heads to control lost motion and reduce vibration, installation of a chain curtain with bar arresters at the rear of the cutting head which prevents offal and chips from being scattered, extension of coolant lines to keep chips worked back to the conveyor, increase in diameter of nozzles on coolant lines, addition of extra feed-screw steady rests, improvement of gauges for checking radius cuts, addition of adjusting screws on cam plates to determine proper machining dimensions all have contributed to greater accuracy and quality in the machining operation of spar flanges.
Following machining operations, spar flanges pass through a series of refining operations. The refining department was rearranged to obtain straight line production, eliminating lost motion of the original layout. Use of efficient file holders and scraper handles have shown minor time saving gains in removing chatter marks and bringing the spar flange to the proper dimension at those points where the milling might be off slightly.
Following refining operations, the spar flange is formed to required contour. In this department, forming fixtures were rearranged to eliminate lost motion. A new design of blocks to remove twist was inaugurated. Use of heavy punch presses and forming dies for preforming operations in the heavier sections have all shown an appreciable time reduction. In most operations of this nature hydraulic straightening presses are utilized; through the use of the punch press, Servel was able to cut down the number of operations through its incorporation of several hydraulic operations into one punch press operation and thereby gained considerable time saving and increased accuracy in forming.
Trailing edge assemblies represent relatively difficult problems due to confined space for riveting and light gauge material. Due to the nature of some of the light gauge parts involved, there were storage problems which have been accommodated by providing special racks, bins, and cabinets. Provision of this material-handling equipment has reduced lost motion, eliminated a goodly proportion of skin scratches, provided centrally located storage which minimizes material loss. The change has shown overall time saving.
Rearrangement of burring tables, use of special jigs to hold long extrusion strips while countersinking, special air wrenches for tightening nuts, replacing of hand squeeze tools with power squeeze tools on some riveting operations and substitution of dimpled holes for countersunk holes all have shown a reduction in lost time as well as quality improvement. In several locations use of a vibrator and bucking bar to drive rivets was not providing the desired quality of work, so special yokes for squeeze guns were designed. These were sawed out from flat slabs of machine steel in a variety of external shapes and contours to fit the requirements of the operation for which they were designed. This resulted in rivets being driven faster by one person, saving over 50% in man hours on this operation.
Various miscellaneous items, such as reworking of the skin drill template to eliminate trimming one edge of the skin, placing repair responsibility on each repair man, construction of catwalk for workers on skin drilling operations have lessened worker fatigue and decreased drill breakage. Providing hangers for small fixtures used on the fixtures to eliminate safety hazards and reduce lost motion have further contributed to appreciable overall time saving,
Throughout the wing assembly when the job was first started, use of flexible shaft drills was quite prevalent. It was difficult to purchase sufficient angle drills. Using this type of drill resulted in numerous elongated holes and other errors. To overcome this costly and time consuming drilling method, Servel's tool room designed and built angular heads for pneumatic drill motors, permitting elimination of flexible drills at all of the more critical locations.
A great deal of time has been saved in bench work on skins through use of toggle clamps in place of "C" clamps. Bench work on skins usually requires cutting and filing to a scribed line. This demands a means of holding the sheet that is being filed. Servel found that substituting quick acting toggle clamps in place of usual "C" clamps and special bars with wing nuts, resulted in an appreciable net time saving.
Squeeze guns in preference to vibrators wherever possible has saved scrap, eliminated "oil cans" in skins and speeded up the job. Application of squeeze guns in many cases required design and development of special yokes which were made and proven out as the job progressed. The requirement that individual riveters sell their own work to inspection has improved overall quality and reduced rivet replacement decidedly.
In some instances it was found possible to assemble a series of ribs and other parts into a large sub-assembly where they originally had been installed in the main assembly as individual parts. One such area so revised was that of the gun bay. Here all ribs and mounts, gussets and stringers are assembled into one unit, then located in the main assembly. This has resulted in more accurate location, a great saving of time on main fixtures where such saving is important. This has eliminated using gun trunnion drill fixtures which originally cost so much time and trouble and were in constant need of repair.
Originally, after removal from general assembly fixtures, wings were worked on in a horizontal or flight position for addition of doors, tips and fairings. A cradle fixture was devised so that panels are now handled in upright position. This requires approximately one-third the original space.
In several locations, it was formerly necessary to drive heavy heat-treated rivets with vibrators which had to be compact, due to inaccessibility. To overcome this difficulty a hydraulic squeeze gun was designed and built in Servel's toolroom. These have made short work of this formerly difficult operation.
Originally, it was necessary to scribe, snip and file all access door openings on various skins after they had been fitted. This operation consumed too many man hours. It was revised so that the operation now entails drilling two locating holes in the slug of each door. From this hole registry on a blanking die, all doors are now blanked on a punch press.
Originally, skins were applied to a framework built of ribs, longitudinal stringers and access door frames. The technique now is to assemble stringers and access door frames to skins, this subassembly of the skin then being installed on the ribs. In conjunction with this skin subassembly, fixtures have been devised which match perfectly; one to drill the framework to size, another to dill the same rivet holes in the skin sub-assembly full size. All dimpling and countersinking is completed before either of the two parts are put together. Once the framework is completely dimpled and countersunk, it is now possible to place a skin subassembly in place and start riveting immediately. This has provided an efficient means of skin assembly where the method is applicable. Riveting on skin assemblies, formerly done by vibrators requiring approximately 1 hr per panel, is now accomplished on gang riveting machines which rivet five and six panels per hour.
Originally, it was necessary to fit landing gear fairings with wing in flight position. It is now possible to fit these with the wing still in vertical position; making necessary allowances so that when the wing is returned to flight position, correct clearances are provided.
Cost reduction is a constant practice, participated in by all employees at Servel. Every worker has the realization that increased earnings come only from increased production of the nature that hold repair, abuse, time and direct labor expense at minimum. To achieve this state of worker mind, there must be a sense of teamwork between management, supervision, and workers.
This article was originally published in the June, 1945, issue of Industrial Aviation magazine, vol 2, no 6, pp 69-71, 102-103.
The original article includes 7 photos of shop-floor activity.
Photos are not credited, but are certainly from Servel.