Now we come to the commutator, clamping rings, and spider or shell. Plate F is a splendid example of detail work where the draftsman must work out dimensions to the finest point, considering only not the foundry man, the machinist, and the assembler, but the ultimate result to be obtained and the use to which this part of the machine is to be put. Here we have a device which must consist of 216 copper bars insulated from each other, from the supporting shell, and from the clamping rings, having a given length and wearing depth and assembled in a cylinder having a diameter of about 23 inches. It is obvious that the only way these bars can be held in place is by clamping rings drawn up against beveled surfaces on the ends of the bars, with proper insulation between the individual bars and between the bars and the rings. Insulation must also be placed between the bars and the shell.
It will be seen that the dimensions can only be expressed in decimals, if accurate results are to be obtained. Note now the width of the top of each bar - .2963 inch shown in Fig. 5 - and, taking this dimension and the insulation thickness between each bar of .0382 inch, check back and find the circumferential length. Now compare this figure with the circumference of a circle whose diameter is 23 inches. It will be found that the thickness of the bars has been figured so closely that the total error in the length of the circumference will only be a matter of thousandths of an inch in a total length of some six feet. Such work is only possible by means of very fine gages. A reference will be found to a gage number, which means that the tool maker must make a gage accurate within one ten-thousandth of an inch, which will be used by the workman in making these bars.
It should be noted that all dimensions between concentric surfaces are referred to radial or diametrical distances. This certainly shows that the draftsman understood his business. The whole construction is such that diameters or radii are the fixed dimensions, and any attempt to give these dimensions in any other way would cause the shopman to calculate the diameters with great chance for error.
It might be well at this point to emphasize the importance of giving information on the drawing in such a way and in such completeness that the shop man need ask no questions; in fact, modern shop practice requires that the workman work entirely from the drawing and the dimensions given there, and under no circumstances is it permitted the shopman to make any calculations. It should also be remembered that it costs much more to make additions to or changes on a drawing than the same work would have cost, if done when the drawing was made in the first place. And so we see that even so small a thing as one of these commutator bars is given closest attention, and each detail is worked out so that when the whole thing reaches the assembler it will be as easy to put together as if it were two pieces instead of several hundred.
This plate is another good example of how the draftsman may completely describe the whole by showing only a part. The assembled view, Fig. 1, loses nothing by showing a section of less than half the commutator; in fact, to spend time and money showing more would certainly be wasteful. The same may be said of the other parts shown on this plate.
The assembly shown gives the general scheme of placing the parts of the commutator together so as to perform the proper functions. From this, the other details can be developed.
First we must have a shell for supporting the bars and rings. This can be made in the form of two concentric cylinders with supporting arms between, Fig. 2. The size of the shaft has already been decided upon, so we can start with this dimension. It is not desirable to make this piece fit the shaft throughout its entire length since it would require a difficult job of boring in the shop and would add nothing to the finished piece. The casting is therefore cored out by the foundry man, so that there will be two bearing surfaces each 4½ inches long to be bored, but the central portion of the hub need not be finished.
This saves considerable time in the machine work. The thickness of the hub is only enough for mechanical strength to carry the commutator. The same is true of the arms and outer shell. In other words, as little material is used as possible to give the desired strength, with a proper factor of safety. Thus we have the arms 1 inch thick and, of course, the full length of the shell. The outer shell, having a number of holes through it for the studs for drawing up the rings, must be somewhat thicker than the hub. Also the rings must be considered and these holes spaced so that they will not come too close to the inner edge. The various holes, being rather hard to show clearly, are described in notes at one side of the drawing. These notes give size of hole, threading data when necessary, and any other information as to number, location, or depth that may be required.
It should be noted that those dimensions which must be machined very closely are given, as in other plates, to three and four decimal places. The usual notes and marks as to finish and fits are included.
The clamp rings, Figs. 3 and 4, are fairly simple as to form, but accurate and careful work is shown in making the drawings, and the information must be quite as clear and complete as for the more complicated-looking pieces. Note how many dimensions are given and the reasons for them - for instance, the angles of the cone faces which must check with the corresponding angle on the commutator bars, and the radius of the curve between the cone face and the vertical face which must be such as to turn the insulating cone without any tendency to break it. Since these rings are finished all over, a single note to that effect saves the time of putting the usual marks on all finished surfaces.
Surely the shopman will have little trouble in building this commutator from the drawing, with its wealth of detail information. How many dimensions could be omitted and how many lines left out and still be sure of the information being complete? Just enough is shown, just the right number of dimensions are given, to give complete instructions to the shopman at the smallest possible expense. This stands for efficiency in drawing and in building from the drawings.