Problem Of Sequence Of Operations

The economical manufacture of a product does not depend solely upon the proper design of tools; the sequence of operations through which the product passes also plays a very important part. Therefore the designer may tool up for a certain product, and the tools may be of the best design imaginable - embodying quick loading and unloading features and clever locking devices, all of which tend to make the rapid handling and completion of the parts all that may be desired but the actual economical feature can be lost through not doing the proper operation first, and it too often happens that a third tool is made necessary to correct discrepancies that are caused by the improper sequence of operations.

The carbureter body, Fig. 90 - with its thin delicate walls, the two bores which must be absolutely in line, and an outside of rough casting scale - is an excellent product to demonstrate the necessity of carefully laying out the proper operations before designing the tools. The difference in diameters between the inside bore of the body and the outside diameter of the valve a is limited to not more than 0.002 inch.

The following line of reasoning must be followed to be a successful designer:

Fig. 91. Carbureter Body Clamped to Faceplate.

Manner Of Holding Work. Distortion By Chuck Jaws

The first thought naturally would be to grip the body in a chuck having special jaws to hold it, but the fact that the outside of the body is a rough casting and the walls are thin precludes the use of chuck jaws, for the reason that one small high spot on the casting would cause the pressure of the jaws to center on this high spot, resulting in distortion of the body; then, if the body were finished while under distortion, the finished hole would be out of round when the pressure is released. This fact makes it necessary for the designer to depart from the usual design of tools and to devise special tools, depending almost entirely upon his inventive ability, and upon a fund of experience and knowledge of every conceivable mechanical device From his knowledge he must mentally select a certain principle involved possibly in some one jig, and another principle or movement embodied in some other fixture, and so on, until by putting these together he can complete a satisfactory tool to meet the requirements of the operation at hand. Therefore a method other than chuck jaws must be devised for holding the body for the first operation. The thought occurs that the wide base b, Fig. 90, is an excellent surface to clamp against a lathe faceplate containing special holding fixtures as in Fig. 91. However, the designer immediately discards this method, for it is noted that the larger bore cannot be machined in this position, and, as the two bores must be in absolute alignment, it would mean that the work would have to be machined first on one end and then on the other, which is decidedly wrong, for eccentricity will creep in. Following the rule of machining as many surfaces as possible at one setting, a device must be employed that holds the work by the small end, so that both diameters can be finished at the same setting and while the body is not under distortion. By so doing, absolute alignment is possible.

Fig. 92. Holding Small End of Carbureter Body by Means of Chucking Piece.

Fig. 93. Carbureter Body with Special Lugs for Holding.

Fig. 94. Carbureter Body Held by Faceplate with Special Lugs.

Fig. 95. Lathe Set Up for Roughing Cutter.

Use Of Chucking Piece

The problem now at hand is to satisfactorily hold the body by the small end. A chucking piece a, Fig. 92, could be cast on the body simply to provide a holding member, then after the body is machined the chucking piece could be cut off. This chucking piece could distort the body in two ways:

(1) by the pressure of the chuck jaws on the chucking piece;

(2) since in all castings there are internal strains and, to a great extent, the scale prevents these internal strains exerting themselves, the process of roughing out the bores to remove the strains, then final-finishing the bores, and lastly, severing the ring or chucking piece would further release the strains, and the finished bore on the small end of the body would not be round.

Special Lugs

Knowledge of these facts leads the designer to add a special lug a, Fig. 93, on each side of the body, and the body is finished complete at one setting, as in Fig. 94, the lugs then being cut off, or, if possible, left on the body. The design of the body or product at hand can often be cleverly arranged so that the members added as a holding means look like part of the intended design.