Having decided to hold the body, in in Fig. 94, one must go still further, for the methods that might be employed in machining the two bores could produce inaccurate alignment. When casting the body, cores are placed in the mold to produce the bores. This leaves a scale inside the bores, and, as the cores invariably shift, the resultant bores either are not in line, or, if in line, are not concentric with the outside of the body. In either case the bore is eccentric if the outside of the body runs true when attached to the faceplate.
Valve-caste Brass- reqd piece no. 1
Fig. 98. Typical Tool Data Sheet.
If a roughing cutter, Fig. 95, is used to machine the large bore nearly to size, and if the large bore is not running true.
Fig. 96. Inside Hole of Carbureter Body Received before Using Roughing Cutter.
The cutter follows the hole, and, after a finish reamer machines the large bore, the diameter may be correct but the bore is still running eccentric. Therefore, to insure the roughing cutter starting centrally, the outer end of each bore is recessed to cutter size for a short distance, as in Fig. 96, using a single-pointed boring tool. The roughing cutters mounted in the turret head of a monitor lathe can then be used to rough out the bores, due allowance being made to leave enough stock for the finish tool to clean up the hole. For absolute alignment of bores a single-pointed tool should be used to finish-size both bores, for a reamer cannot be depended upon to keep the hole concentric, although it produces the desired diameter.
Another method of machining the bores would be to rough out both bores to remove the scale, then finish the large bore and insert in it a concentric hardened-steel bushing, Fig. 97, the central hole in which is used to guide the cutter for finishing the small bore. The student, however, can readily see wherein there are chances for inaccuracies to creep in when the bushing is employed. "How many chances for errors?" would make an excellent examination question. The answer is three, as follows:
(1) difference between diameter of bushing and bore in body;
(2) difference in diameters of hole in bushing and of reamer; and:
(3) eccentricity of hole in bushing.
The points that must be considered by the designer to produce the carbureter body, or similar work where holes must be in absolute alignment or concentric with outside, and where holes must be perfectly round, are as follows:
(1) Use a holding device that does not distort.
(2) Lay out the sequence of operations so that the work can be completed in the least number of operations.
(3) Do all machining that is feasible at one setting, to insure concentricity.
(4) Do not trust a reamer to final-size holes that must be in line with each other.
(5) Do not machine one end then reverse the work and machine the other end, if possible to avoid, for any error caused in either end is doubled when the work is reversed.
(6) When finish-sizing holes or circular outsides that must be in absolute alignment, use a single-pointed tool, not guided.
(7) It makes no difference whether the work revolves and the single-pointed tool is stationary, or whether the single-pointed tool revolves in work that is stationary.
The valve a, Fig. 90, of necessity must be made just as accurately as are the bores in the body, and the same ideas must be followed out as were outlined in connection with the body, i.e., do all turning possible at one setting, using a single-pointed tool to guard against eccentricity, etc.
To complete the outline of tool design - supplementing the question of why a certain design is adopted - and to enumerate the detailed methods that must be followed by the designer, assume that you have been given a blue print, Fig. 98, which gives the detailed dimensions of the three pieces shown in Fig. 90, and that you are instructed to proceed with the tooling up for a large production job.
The first step is to procure a long strip of paper and rule it, or to have a quantity printed, as in Fig. 99, for a data sheet. The next move is to select piece No. 1 and lay out the sequence of operations by mentally going over each operation necessary to complete the piece satisfactorily. The order of operations as finally decided upon is enumerated on a pad in numerical order, Fig. 100.
After all pieces have been thoroughly gone over many times and the final operations written out, the next step is to outline the tools required. On the data sheet are written the operations and all the tools required, even to standard drills, reamers, etc., as in Fig. 99. The tools are then assigned a number for record purposes.
Before designing each tool, a careful classification should be made so that the tools designed first shall produce a secondary unit. For example, a product is made up of pieces parts, which in turn go to make up a secondary unit, and the secondary units finally make up the primary unit, which is the completed product. A graphical illustration is partially shown in Fig. 101. The object in designing the tools so that a secondary unit may be completed is that, as fast as the tools are made, trial models should be made from the tools and the trial models, if they are piece parts of a secondary unit, can be assembled one to the other, making an assembly of a secondary unit which is a check on the tools. If a tool were made to produce the float in a carbureter, and the next tool made produced the air valve, these tools could not safely be allowed to run off a quantity of pieces until all tools were completed and a trial model assembled from pieces made from the tools. In the arrangement in Fig. 101 the 1-inch carbureter is of course the primary unit; the throttle-body assembly and the float assembly are both secondary units that go to make up the primary unit, while the pieces that make up the secondary units are piece parts. If the designer will systematically lay out the tools and parts as outlined, the danger of missing some tool or piece may be reduced to the minimum.
Fig. 99. Order of Performing Operations.
Another point that governs the design of tools, and in a sense comes within the tool designer's domain to a great extent, is the design of the product. In up-to-date engineering departments the product designer consults the chief tool designer before turning over to the tool designer the finished product for tooling up. For instance, a model device may have been perfected of sheet metal, but certain parts of this device are held together by screws, which means tapping operations and the cost of screws besides the slow operation of putting in the screws.
Fig. 100. The Units and Their Classification in Any "Tooling-Up" Process.
The tool designer, due to his training, notes that these certain parts could be held together satisfactorily by punching and bending down an ear, which in turn fits in a slot and is then riveted over, or bent over, eliminating the tapping and the cost and handling of screws.
Fig. 101. Diagrammatic Layout of Carbureter Operations into Units.
The successful designer does not depend entirely upon his own ideas, but obtains the views of all interested. There is a shop phrase that "there are forty-nine ways of doing every job", and a designer who places himself upon a pinnacle and refuses to confer with designers and tool-makers under him or with the foreman who is to use the tool always wonders why he does not advance, and this type of designer will be found too frequently looking for a new position. Oftentimes the foreman who is to use the tool may recall some particular job that is identical with the one at hand, and can greatly aid the designer. Therefore, the designer must not allow personal pride or conceit to govern his work, but should make it a practice to get every idea and to listen to every suggestion possible. While a suggestion may not be applicable to the job at hand, it should be mentally retained and sooner or later may be employed.
Observation plays an important part in making a successful designer. For instance, should the designer see in operation some complicated machine or fixture, he should at least make it a point to note mentally one or more movements, even if the entire principle of the machine cannot be grasped at a casual glance. The reading of journals devoted to the mechanical field is one of the greatest aids to success; every article contains some unique kink or valuable point, and a reader may grasp in a few minutes' reading what has required years of travel and experience for the author to gather.