Essentials of sheet metal work and pattern drafting

This book is written in answer to numerous requests for information regarding courses in sheet metal, suitable for grade and high schools, coming from school people who are introducing sheet metal work into the regular school curriculum. Realizing the need for a correlated course in practical sheet metal work and pattern drafting in text form, it is offered, therefore, to industrial teachers, students, apprentices, sheet metal workers and others interested in this subject.

The problems presented follow the lines of the best shop methods, derived from years of experience in teaching the subject and as a practical sheet metal worker. The course is outlined and the problems are presented in such sequence that the processes and machine operations are reviewed with each new problem.

It is not expected that the problems as given will be strictly copied, but rather that they will make clear the methods and processes that may be applied in the construction of similar problems. The proper sequence, so necessary for successful instruction in sheet metal pattern drafting, is an important feature of this course. Many of the problems are only partly solved, which prevents the student from copying from the text and compels him to develop his powers to think as well as to draw. It is desired that the work shall be regarded as a collection of data presenting the essentials of sheet metal working, rather than an attempt to produce a series of models.

When the student or workman is required to make articles simple in form, from sheet metal, the pattern can be made directly on the metal from given measurements. If he is required to make an article round in form with flaring sides, or an article having an irregular shape, it is highly important to make a full-sized drawing and to develop the patterns. This necessitates operations with the drafting board and drawing instruments, which will be taken up later in this course. After the pattern is developed on detail paper, it may be transferred to the sheet metal and the work of construction begun.

Use of Patterns - If two or more pieces from a pattern are desired, do not prick through the paper pattern to obtain each piece. When one pattern is cut from metal, it should be used as a pattern whether two or a dozen pieces are required. Place the metal pattern upon the material, using a scratch awl, and scribe a line around the pattern. If the pattern is large, a weight should be placed upon it to keep it from moving, but if the pattern is small the weight is not necessary, as the pattern can be held in position with the fingers of the left hand while using the scratch awl with the right.


CHAPTER II - CUTTING PATTERNS AND TEMPLATES

Hand Shears - Sheet metal patterns are cut from metal by means of shears or snips of various shapes and sizes. The shears in general use for light work is known as the straight hand shears, or snip, having a left-hand cut, the length of the cut commonly being 3½ inches, an illustration of which is shown in Figure 3. This shears, when taken in the right hand, has the lower blade on the left side of the shears, and cuts at the left side of the upper jaw. The position of the jaws enables the sheet metal worker to /follow the cutting line accurately, as it is always in full view.

Another straight shears, known as Lyon pattern snip, shown in Figure 4, is well adapted for regular work. The jaws are pointed and rounded, permitting the metal to pass freely when cutting curves, scrolls, and circles. Circular snips, shown in Figure 5, are well adapted for cutting small circles and openings of various shapes in sheet metal. The popular size has a length of cut of 3 inches. A bench shears, shown in Figure 6, is used for cutting heavy material. This tool is much larger than the ordinary hand shears. When in use it is fastened in the bench by inserting the prong in the bench plate, Figure 7, or a hole of the proper size cut in the bench for this purpose. This shears has a right-hand cut, with the lower blade on the right side of the shears. Note the difference in the position of the upper blades in the right hand shears in Figure 3, and the right hand bench shears in Figure 6.

The double cutting shears, shown in Figure 8, is adapted to cutting off round and square pipes, bottoms of pails, cans, etc. A hole is. punched in the article to be cut, and the point of the lower blade inserted, after which the cutting is done in the regular manner, leaving the edges clean, and smooth.

Squaring Shears, - Turning to cutting machinery, Figure 9 shows a modern squaring shears which is recommended for this course in cutting strips, squaring tin. and making long straight cuts across sheets of metal when shearing material for the construction of pipes and articles cylindrical in form.

If the shears do not respond in cutting material of heavier gauges within the rated capacities claimed by the manufacturer, the blades should be set farther apart. The lower blade must be set back from the upper, though not far enough to burr the edge of the material. This adjustment can be made by releasing slightly the bed bolts that hold the bed of the shears to the legs, and by loosening the two front bed screws. The bed can then be shifted on its seat towards or away from the upper cutting blade until the proper position is secured. The blades can be easily removed for grinding, and when dull they should be returned to the factory for grinding. After being ground they are fastened securely to their frames and adjusted so that they will cut paper the entire length.


CHAPTER III - FOLDING EDGES AND SEAMING

One of the most important processes in sheet metal working is that of seaming. Seams of various kinds are used, depending on the strain to be resisted and the equipment on hand for constructing them. The machine in general use for bending the edges of sheet metal for seaming is known as the adjustable bar folder, as shown in Figure 16. The following edges and seams are extensively used in light sheet metal work:

Single Edge - This edge, as shown in Figure 17, is used in constructing seams and hemming the edges of sheet metal. In forming this edge in the folder, set the gauge to the required width, then insert the metal in the machine, holding it firmly against the gauge with the left hand. Grasp the handle with the right hand and bring the folding bar over until it rests on top of the machine. The handle of the machine is now brought to its former position and the metal removed from the machine, completing the operation.

Double Edge - The double lock, shown in Figure 18, while used in certain work, is most commonly utilized to strengthen sheet metal forms. When used for this purpose it is known as a double hemmed edge. This edge is formed in the folder in the same manner as the single edge. After the latter is formed, the sheet is turned over, then the single edge is placed in the machine against the gauge and the operation is repeated.

Wire Edge - It is often necessary to increase the strength of articles made from sheet metal by inclosing a wire in certain of their edges. The edges for this purpose must be rounded as shown in Figure 19. To form an open or round lock for wiring, set the gauge on the folder equal in width to two and one half times the diameter of the wire to be used. Using the wrench, loosen the lock screw to the right on the back of the machine, and by moving this screw to the right or left in the slot the wing is raised or lowered. In adjusting the machine, lower the wing equal in width to the diameter of wire to be used, fasten the lock screws firmly, then turn the edge in the usual manner.

Lap Seam - In Figure 20 is shown the ordinary lap seam, as used in the construction of small cylinders, square pipes, cornice miters, etc. This seam is usually soldered or riveted.


Folded Seam - In making this seam, a single edge is turned on the metal, and the edges are hooked together as shown in Figure 21, after which they are hammered down with a wooden mallet. Seams that are malleted down smooth are stronger and easier to solder than when uneven. Seams of this kind are used in laying flat seam tin and copper roofing.

Grooved Seam - With light material, the grooved seam is the universally used method of joining the edges of sheet metal. This seam is frequently used in joining two flat sheets of metal, making longitudinal seams in round and square pipes, and vertical side seams in sheet metal articles having a flaring or cylindrical surface. An illustration of this seam, showing the construction, is seen in Figure 22. When joining two flat pieces of metal by this method, set the gauge on the folding machine to the width of the edge required, and turn a single edge on the sheets as shown at A. Hooking the edges together as shown at B, the seam is laid on the horn of the grooving machine (Figure 23). The rolls run over the seam lengthwise, completing the seam as shown at C When the grooving wheel is run over the seam, an offset is made in the upper sheet e at m, which prevents the seam from coming apart. The seam is finished by placing it on a mandrel stake, pounding it with a wooden mallet, closing it down, and leaving the seam tight and smooth.


DOWNLOAD FREE SHEET METAL BOOK:
Essentials of sheet metal work and pattern drafting