A shop drawing is a drawing which gives all the information necessary to lay out, cut, punch, and rivet the piece shown. It is the medium by which instructions are conveyed from the engineer's office to the shop. It must convey full, accurate and explicit instructions for every operation. It must be so clear and explicit that no further explanations are needed to enable the shop to correctly interpret it, and the information must be given in such form that only one interpretation is possible. The draftsman making a shop drawing must constantly bear in mind that the man at the shop will work entirely from this drawing; that he does not have access to the sources of information which are consulted by the draftsman in making the drawing, and that what might be clear in connection with these other drawings will be blind or uncertain to the shop man not familiar with them. The draftsman should further understand that it is distinctly the duty of the shop man not to read into the drawing anything not there, and that consequently the responsibility is entirely upon the draftsman to make his drawing so complete that such action will be unnecessary and impossible. Neatness in execution of a shop drawing is desirable, but accuracy and clearness are absolutely essential.

Shop drawings differ from general detail drawings in that they do not show the different parts of construction assembled, but cover only one piece. For instance, an engineer making a drawing to send to the drafting room where the shop details are to be prepared, would show a column with the girders and beams framing into it, just as they would appear when assembled. In this way he would establish the relations of the different members and would determine the character of the connections and any special features of the details. The draftsman detailing for the shop, however, would make the column on one sheet, each beam and girder on separate sheets, and the different members forming the whole structure would appear only as individual pieces, their relations one to the other being given by an assembly or erection drawing.

In a large shop the columns, beams, and girders would be fabricated in entirely distinct departments and the men in the different departments would not know that those different pieces when assembled, fitted into each other. The responsibility for correctly laying out these pieces so that they will fit together is upon the draftsman.

Measurements on shop drawings are always carried out as close as one-sixteenth of an inch, and sometimes to one thirty-second. An error of one-sixteenth may be sufficient to make it impossible to assemble the pieces in erection, as steel cannot be cut and drilled at the building except at considerable expense of time and money. Such errors are costly.

The student should clearly understand the importance of the work of the shop draftsman and should always be imbued with the idea that he is the last authority to pass upon all the various points determining the instructions of the shop and the last sentinel to discover and prevent errors. Drawings are almost always checked by some other than the man who makes them, but no man will make a successful draftsman unless he does his work without a thought of being saved from errors by the checker.

The making of templets, and the way in which a shop uses a detail drawing have already been explained. The draftsman should always detail as far as possible in accordance with standard shop practice, as in this way much templet, work can be eliminated and thus time and expense saved, and the work will be more quickly fabricated because of the familiarity of the shop with the details. The standard forms differ somewhat in the different shops, but the Carnegie standards are essentially the same as all others; these have been given in Steel Construction, Part I. A great many conditions arise in which standard forms cannot be used, in which cases as simple details as practicable should be employed.

Scales Used in Details. Details of plate and box girders and of trusses are almost invariably made to scale, generally 3/4, 1 or 1 1/2 in. to the foot. Details of columns are generally made to scale as far as the connections for beams and the head and foot of columns are concerned. The length along the shaft from top to bottom and between connections at different levels is generally not to scale.

Details of beams are rarely drawn to scale, but the position of holes and of shelf angles, etc., are shown in the proper relation to each other and to the whole beam. That is, if the beam shown is a 12-in. beam 16 ft. long, the elevation of the beam might be drawn to a scale of 1 1/2 in. to the foot as regards the height of beam, while as regards the length it might be drawn at no definite scale, simply made to come within the limits of the sheet. In locating holes in this elevation, if there was a horizontal line of holes in the center of the beam it should show in the center of space limiting the height of beam; if another line 2 in. off from the center, it should be shown at 1/6 of the depth from the center line. Similarly to spacing holes along the length of the beam a set of holes centrally located as regards the length should show in the center of the sketch, and another set 2 ft. from the center should show 1/8 of the whole length from the center.

In other words, the beam is detailed according to the scale of the sketch which represents the beam, but this will not be the same scale vertically as horizontally and will not be the same scale for any two sketches.

The reason for the above absence of scale in beam sketches is that these details are almost invariably made on a standard size of sheet, say 12 X 18 in. One sheet may have beams varying in depth from a 7-in beam to a 15-in. beam, and in length from 6 ft. to 20 ft. To accommodate all such varied conditions to the same size sheet it is necessary to adopt a standard size of sketch representing all sizes and lengths of beams, and locate details on this sketch simply by the eye, so as to show the details in proper relations as outlined above. In many drafting offices these beam sheets are printed with a blank elevation and plan and end view of a beam ready for the draftsman to fill in the details.