The commutating fields are connected in series with the armature and carry the full machine current. They are therefore made of heavy copper somewhat like the series section of the main field. Plate K shows these field coils and the spool.
The coil is made up of several turns of copper each made up of twenty-two laminations of thin copper strip five-eighths of an inch wide and seventy-five-thousandths of an inch thick, Fig. 13. The coil is insulated by separating the turns with separating strips of veneered maple, shown in the side view, Fig. 1. This veneering, being thin pieces of wood glued together with alternate pieces having the grain at right angles, makes a very stiff firm support, which will not warp or twist out of shape when heated. Placing the coils in this way, they are open to air circulation all over and are easily kept cool. The ends of the coil must be supported firmly from the spool. This is accomplished by means of the two copper clips. Notice Section AA, which shows how the coil, clip, and insulation are assembled with relation to each other. Notice also how the ends of the coils are spread to form slots for the connection bars (see left-hand view Fig. 13).
The spool, Figs. 2 and 3, is similar to that for the main field. The body consists of sheet steel bent to fit the pole piece and having the ends turned down to hold the collars, and the whole held in place by means of rivets in one side. The collars are of veneered maple and are drilled for dowel pins which hold the separating strips. The separating strips, Fig. 4, are also of veneered maple and are very carefully dimensioned to fit the coil. The slots must have the proper slope and each of the strips on one side must be different.
It is possible that one might go over this whole plate without giving a thought to the assembly of the coils. For example, on first thought it might seem proper to wind the coil on the assembled spool. A little thought will show, however, that this would not give a good smooth piece of work and would hardly be satisfactory. It is almost universally true that coils for electrical machinery are wound on forms and assembled afterward. By studying the construction as given in this plate, it will be seen how easy it is to take such a form-wound coil, set the separating strips into place, and slip this down over the spool body which already has one collar in place. The other collar can be placed in position and the ends of the spool body bent down over the top collar.
It is true that this work has nothing to do with shop practice, and yet the draftsman must sooner or later attain a position where he sees things from the shopman's viewpoint. The above paragraph illustrates how well the draftsman realized the method to be pursued in assembling these coils. The spool might easily have been drawn so that the labor necessary to assemble it with the coil would have been double that necessary as it is drawn. As an example of what this means, the following is typical: In a certain shop, two similar electrical devices were being made, one for much heavier service than the other. An investigation of costs revealed the fact that the smaller one was costing twice as much as the larger one. Further investigation in the drafting room showed that the whole trouble was that the designing draftsman had laid out the smaller device so that the principal casting was very hard to mold in the foundry and harder still to finish in the machine shop. Simple changes in the drawing by a man familiar with shop methods made the costs of the two pieces comparable. So it will be seen that the draftsman must consider not only the pattern maker, but the foundryman, the machinist, and the assembler as well.