The tracing, of course, cannot be sent into the shop for the workmen to use, as it would soon become soiled and in time destroyed, so that it is necessary to have some cheap and rapid means of making copies from it. These copies are made by the process of blue printing, in which the tracing is used in a manner similar to the use made of a negative in photography.

Almost all drafting rooms have a frame for the purpose of making blue prints. These frames are made in many styles, some simple, some elaborate. A simple and efficient form is a flat surface usually of wood, covered with padding of soft material, such as felting. To this is hinged the cover, which consists of a frame similar to a picture frame, in which is set a piece of clear glass. The whole is either mounted on a track or on some sort of a swinging arm, so that it may readily be run in and out of a window.

The print is made on paper prepared for the purpose by having one of its surfaces coated with chemicals which are sensitive to sunlight. This coated paper, or blue-print paper, as it is called, is laid on the padded surface of the frame with its coated side uppermost; the tracing is laid over it right side up, and the glass pressed down firmly and fastened in place. Springs are frequently used to keep the paper, tracing, etc., against the glass. With some frames it is more convenient to turn them over and remove the backs. In such cases the tracing is laid against the glass, face down; the coated paper is then placed on it with the coated side against the tracing cloth.

The sun is allowed to shine upon the drawing for a few minutes, then the blue-print paper is taken out and thoroughly washed in clean water for several minutes and hung up to dry. If the paper has been recently prepared and the exposure properly timed, the coated surface of the paper will now be of a clear, deep blue color, except where it was covered by the ink lines, where it will be perfectly white.

The action has been this: Before the paper was exposed to the light the coating was of a pale yellow color, and if it had then been put in water the coating would have all washed off, leaving the paper white. In other words, before being exposed to the sunlight the coating was soluble. The light penetrated the transparent . tracing doth and acted upon the chemicals of the coating, changing their nature so that they became insoluble; that is, when put in water, the coating, instead of being washed off, merely turned blue. The light could not penetrate the ink with which the lines, figures, etc., were drawn, consequently the coating under these was not acted upon and it washed off when put in water, leaving a white copy of the ink drawing on a blue background. If running water cannot be used, the paper must be washed in a sufficient number of changes until the water is clear. It is a good plan to arrange a tank having an overflow, so that the water may remain at a depth of about 3 or 4 inches.

The length of time to which a print should be exposed to the light depends upon the quality and freshness of the paper, the chemicals used, and the brightness of the light. Some paper is prepared so that an exposure of one minute, or even less, in bright sunlight, will give a good print, and the time ranges from this to twenty minutes or more, according to the proportions of the various chem-cals in the coating. If the full strength of the sunlight does not strike the paper, as, for instance, if clouds partly cover the sun, the time of exposure must be lengthened.

Blue-print paper should not be exposed to bright actinic light except during the process of printing. It is not, however, so sensitive as the ordinary dry plate and may be handled in a subdued light if the exposure is very brief. When not in use the paper must be kept in a dry, dark place and should be hermetically sealed.

A more modern type of blue-print machine is the electric machine, usually arranged in the form of a vertical cylinder of glass, around which is placed the sensitized paper and the tracing, and along the longitudinal axis of which travels an electric arc lamp at a uniform speed. This speed is so adjusted that the right exposure is given to the sensitized paper. There are several other types of electric blue-printing machines, all based upon the same idea of uniform exposure of the sensitized paper to the rays of one or more arc lamps. These machines are a positive necessity to modern drawing offices, because of the uncertainty of sunlight, and, therefore, limited capacity for turning out prints. With the electric machines there is no limit, and they may be run to their full capacity 24 hours a day if desired.

Formula For Blue-Print Solution

Dissolve thoroughly and filter.

A. Red prussiate of potash............................2 ounces.

Water............................................1 pint.

B. Ammonium-citrate of iron..........................4 ounces.

Water............................................1 pint.

Use equal parts of A and B.

Formula For Black Prints

Negatives

White lines on blue ground; prepare the paper with:

Ammonium-citrate of iron....................................................................................................................

40 grains

Water...........................................

1 ounce

After printing wash in water.

Positives

Black lines on white ground; prepare the paper with:

Iron perchloride.............................................................................................................................

616 grains

Oxalic acid.....................................................................................................................................

308 grains

Water..............................................................................................................................................

14 ounces

Develop in

Gallic acid.......................................................................................................................

1 ounce

Citric acid........................................................................................................................

1 ounce

Alum................................................................................................................................

8 ounces

Use 1 ounces of developer to 1 gallon of water. Paper is fully exposed when it has changed from yellow to white.

Assembly Drawings

We have followed through the process of making a detail drawing, from the sketches to the blue print ready for the workmen. Such a detail drawing or set of drawings shows the form and size of each piece, but does not show how the pieces go together and gives no idea of the machine as a whole. Consequently, a general drawing or assembly drawing must be made, which will show these things. Usually two or more views are necessary, the number depending upon the complexity of the machine. Very often a cross section through some part of the machine, chosen so as to give the best general idea with the least amount of work, will make the drawing clearer.

The number of dimensions required on an assembly drawing depends largely upon the kind of machine. It is usually best to give the important over-all dimensions and the distance between the principal center lines. Care must be taken that the over-all dimensions agree with the sum of the dimensions of the various details. For example, suppose three pieces are bolted together, the thickness of the pieces, according to the detail drawing, being 1", 2" and 5" respectively; the sum of these three dimensions is 8" and the dimensions from outside to outside on the assembly drawing, if given at all, must agree with this. These over-all dimensions serve as a check and relieve the mechanic of the necessity of adding fractions.