It is naturally difficult for the student to determine what constitutes "complete instructions to the workman", and this knowledge can only be fully acquired by experience, both in drawing room and shop. Association, however, with the shop men who use drawings, a careful observation of their operation of tools, and a general familiarity with handling of material in a shop, help wonderfully in getting the right point of view and proper spirit for making a good drawing. When one stops to think about it, to give instructions without having the least idea of how the workman will go about it to follow them, seems the height of foolishness, yet that is what the student who tries to make working drawings wholly from book rules is doing. He should use his book knowledge as a guide and constant help, but he should be a "shop man" first, last, and all the time. When he has acquired the habit of constantly putting himself in the workman's place, his drawings will be right and will convey "complete instructions to the workman".
In the ordinary run of shop work there are several classes of workmen who have to use drawings. Broadly classed, they are as follows:
Pattern makers, Blacksmiths, Machinists (including Tool-makers, Special Machine men, and Erectors), Order and Receiving Clerks.
These several workmen will use the same drawing, and the instruction which it conveys must be so arranged that each can readily pick out the portion which he needs to enable his work to be properly done. The general requirements of each are discussed below and form the basis for the style and methods of dimensioning drawings used in common practice, and illustrated in this book.
The pattern maker, on receiving a detail drawing of a piece, first proceeds to copy it full size, divided up into such sections as are convenient, upon his work board. This board is merely a large smooth table top, set up on a couple of horses. Sometimes brown paper is tacked on this board and the pattern drawing made on it, but more often the pencil lines are made directly on the surface of the board and the board resurfaced for future work. He does not make a finished drawing, but with his straight-edge, large dividers, and compasses he lays out enough to enable him to see and measure the detail at all points.
This pattern layout is made for a number of reasons. Molten cast iron, when it cools, shrinks about 1/8" per foot, so the pattern has to be made larger than the figures on the drawing call for. In order to save calculation for each dimension, a "shrink rule" is used, each foot of which is made 12 ⅛" long. The pattern maker uses this rule in all his work, and thus makes his layout on a "shrink" basis.
Wherever the drawing calls for finished surfaces, the stock of the pattern has to be increased by ⅛" or more, and this addition has to show on the pattern maker's drawing. In order to get the casting out of the sand of the mold, "draft", or taper on the pattern, has to be allowed. As the draftsman cannot always predict just how the piece will be molded, the "draft" is not shown on the office drawing, and the pattern maker, therefore, has to make the allowance and show it on his drawing. All fillets, sizes of cores and core prints, details of core boxes, and loose pieces of the pattern have to be carefully worked up on the pattern drawing. The result of this special pattern layout often is that certain minor changes have to be made in the shape and size of the piece to permit the pattern to be properly built and readily molded in the foundry, for a good pattern maker has also to be a good foundryman. We thus see that, as far as the pattern maker is concerned, the drawing must be very complete as to detail, both inside and out, and carry dimensions for all surfaces, cores, fillets, corners, etc.
These dimensions must be in even figures as far as possible, as a pattern maker's rule seldom reads finer than 1/16". Gear work is a specialty, and decimals are allowable, and there are certain other cases where odd dimensions cannot be avoided. In arranging the dimensions on the drawing, the more knowledge the draftsman has of pattern making, the more conveniently will he figure the drawing for the pattern maker. He will, in figuring the interior of a casting, think of the core box which will be made, and will be sure that he gives the length, breadth, and depth of the cavity, and all corners, bosses, and lugs projecting into it, with simplicity and clearness. He will give dimensions for all sloping lines and odd-shaped curves definitely and carefully, thinking all the while of the pattern maker and his tools, the square, straight-edge, dividers, and compasses. He will avoid thin edges, and frail projections, and awkward intersections. The consideration of such points as these is what makes a good drawing for the pattern maker to use, and greatly reduces the cost of the pattern, for pattern making is relatively high-priced labor. Some of these points are really points of machine design, but it is not possible for a good detail drawing to be made without using to a small extent, at least, the elementary principles of design. These the student can unconsciously acquire by familiarizing himself with actual shop work.
The blacksmith sometimes uses a pattern for a forging. This is to enable him to lay aside the drawing for pieces which are to be made in large numbers, and set his calipers quickly and accurately from a pattern. Simplicity of shape is of even more importance to the blacksmith than to the pattern maker. The stock material of the blacksmith consists of straight bars of iron and steel of round, square, and oblong cross section. All bosses, lugs, hubs, or sudden variations of shape have to be produced by "upsetting" or welding, either of which is a process involving time, care, and expense. Forgings, therefore, should be, as far as possible, simple, straight pieces, with few bosses or lugs, and when bends are necessary they should be of the simplest nature.
In forging a piece the blacksmith has to work quickly, and has no time to read or measure fine dimensions, it is therefore useless to expect him to read any finer dimensions than 1/16"; special attention should be paid to giving him over-all dimensions, not only for cutting off the stock, but for enabling him to make his measurements quickly while the piece is hot and gripped by his tongs on the anvil. The blacksmith has to make about the same allowance of extra stock for finished surfaces as the pattern maker.
The machinist uses only a few of the figures on the average drawing, while the pattern maker and blacksmith use practically all of them. The machinist is concerned only with finishing the piece, and views the drawing with regard to the machine work upon it. In order to finish the surfaces accurately in proper relation to one another it is necessary to choose some fundamental face of the piece, first finish that, and then use it as a gauging surface from which to work the others. The draftsman, if he is reasonably familiar with shop work, can usually foresee what this gauging surface will be. This has an important influence on his dimensions, for he should so give the dimensions that the machinist will find them convenient and consistent with all his operations on the piece.
When special tools, jigs, fixtures, and automatic devices are applied for the finishing of pieces in large numbers, the method of dimensioning is apt to be somewhat different from that on the general run of machine work. A free use of notes on the drawing, specifying the nature of finished surface desired, or the kind of fit, or any special points in connection with the machining of the piece, is valuable to the machinist. It is not good economy to spend any more labor on securing a finished surface than the purpose for which it is made requires. For example, in turning up a shaft with a number of bearings along it, most of its surface being free, care should be taken to finish the parts for the bearings to an exact diameter, but for the balance of the length a smooth surface with the diameter reasonably accurate is all that is necessary. The drawing should specify this difference of finish so that the machinist will not waste time on the piece.
The special operations on a piece, such as cutting of gear teeth, grinding and "lapping" of shafts, cutting of threads, etc., are all done subsequent to the main finishing of the piece. For example, the casting for a cut gear is first bored, the hub faced, and the solid rim turned and faced to the dimensions on the drawing. This produces the "gear blank". The subsequent operations of cutting the teeth on an automatic gear-cutting machine, and keyseating the hub on a keyseater, require additional instructions as to the style of cutter, number of teeth, dimensions and style of keyway, etc.
Machine shops are differently equipped for doing the same kinds of work, and this has an important influence on the manner of placing the finishing dimensions on drawings. Thus, some shops have rotary planers instead of the regular reciprocating platen type. Some have turret lathes, screw machines, and horizontal boring mills, while others have only lathes. Some have grinding machines, both for fiat surface and cylindrical work, the final finishing cuts being taken on these machines, after the pieces have been roughed out on the lathe and planer. Grinding machines are now regularly built to take a heavy cut and coarse feed for roughing out the work, thus often dispensing entirely with the lathe. Milling machines, though found in all shops, are used in widely different scopes. Some use the milling machine almost entirely, to the exclusion of the planer, specially heavy machines being adapted for this purpose. Locomotive shops differ in their equipment and practice from stationary engine shops; machine tool and automatic machinery builders have little in common with the equipment of shops for manufacturing heavy power transmission machinery; steam pump shops are wholly different in their equipment from that of an establishment building electrical machinery.
All these differences have an important bearing on the style of drawings needed, and especially so on the methods of giving the dimensions for the use of the machinist. Without attempting to give an idea of how to control each case, which would be well nigh impossible, suffice it to say that the student should become impressed with the fact that he must study the workman constantly in order to be able to give him upon the drawing the necessary "complete instructions".
The order and receiving clerks are very easily satisfied as far as their part in the use of a working drawing is concerned. They simply need a designating mark, a pattern number for a casting, and a piece number for all other parts, together with the material and number wanted of each piece, in order that the proper orders may be written, and the material identified for recording its receipt in the shop or field. The number wanted is often given only on a separate "bill of material" which accompanies the drawing, but it is also quite general to note on the drawing in the title of each piece, the number wanted for one complete machine, whether billed elsewhere or not.