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(a)

Fig. 70 Drawing Boards, (a) battened type, viewed from below, (b) with "clamped" ends. Both have harder inserts at the working end. (c) typical tapered tee-square.

than wood to work on. This doesn't matter very much, as you always need a backing sheet of stiff paper anyway, but it does mean that you must use draughting tape (a low-tack adhesive material) instead of drawing pins to hold down the paper. Now, drawing pins are 'old hat' these days -indeed, it is not easy to get the good quality type with large domed heads. But there are disadvantages to the use of tape. First, unless the paper is really good quality even the least tacky tapes will tear the surface. Worse still, it has a nasty habit of turning up an edge and sticking to the tee-square. Even when the board is used continuously it takes several years before the corners need attention when using pins, and these are probably the best for casual users.

Set squares (triangles in USA) are needed to draw lines at right angles to those done with the tee-square, and a pair, one at 45° and one 60° - 30°, is needed. They are sold from 3 inches up to 15 inches tall, but a pair about one-third of the width of the board is a good compromise. However, you may need to draw lines at other angles, and a single adjustable square can replace both. Mine (Fig. 71) are of celluloid, but most today are perspex. The same photo shows a very useful - indeed, indispensable - device, the erasing shield. That seen is of nickel silver, but most are of plastic. Placed over the drawing it enables you to rub out just part of a line without straying over the adjacent parts. (I haven't mentioned the eraser; an ordinary india-rubber will do, but it is best to spend a few pence more on a proper draughtsman's quality.)

If all your work is drawn full-size an ordinary ruler will serve provided that it is divided into thirty-seconds or 0.5mm, but a proper engineer's scale is otherwise essential. Two of mine are seen in Fig. 71, one covering full and half sizes on one side, with 1/4 and 1/8 on the back. The other scale is engraved in tenths and fiftieths of inches on one edge and in millimetres on the other. Those seen are posh, having xylonite edges, but plain boxwood is just as good. There are other scales to be had, but either of these will serve alone, depending on whether you work in imperial or metric.

You need to be able to draw circles, of course. You can get templates for this purpose - a lot of holes surrounded by perspex in calibrated sizes. Avoid them - they may be fair enough for artworkers, but are quite useless for precision work. You must have a pair of compasses. You can, of course, spend the earth on an elaborate cased set, but a single pair which will spread 4 inches, perhaps together with a lengthening bar, will serve. I would advise that you obtain a divider point that can be substituted for the pencil at the same time. All-important dimensions should be picked up from the scale with dividers and transferred to the paper by 'pricking'. These can be expensive, but quite well engineered instruments can be bought for a few pounds. Cheap compasses, that spread their legs in use, are a waste of time and money. You may find difficulty in drawing small circles below about 6mm or 1/4 inch with these, and if you find you are spending a lot of time over them then look for a second-hand pair of spring bows. These have screw adjustment and can be set to very small radii with precision. Not essential, but highly desirable as soon as you can afford them, see Fig. 71(b). You may feel rather overwhelmed at this list, but most can be ob-

Fig. 71 Adjustable and fixed set-squares; erasing shield;two 12in engineer's scales, (b) Compasses with extension; "Spring bow" compasses; Engineer's scales, (c) small clamped drawing board and tee-square with set squares of comparable size: traditional and "lead holder" pencils; and on the right, the author's pencil sharpening file.

tained second-hand from local paper advertisements. I advise buying the set-squares new as these must be square to be effective (this doesn't matter with the tee-square so long as it is straight). Fig. 72 shows two second-hand sets of instruments; the larger cost £3 and included three small set-squares. The smaller, a set of three very fine spring bows, cost £8 from an antiques fair. The former is a very good buy for anyone just starting up, while the spring bows were a collector's bargain, being very fine quality indeed. Good drawing instruments don't wear out; I am still using my set bought in 1929 - a complete outfit which cost £3-17s-6d

Laying out the drawing

You will need a backing sheet on the

Fig. 72 Second-hand drawing instruments. The set on the right is more than adequate for the beginner.

drawing-board first otherwise hard pencils will make grooves in the board. Thick ^ cartridge paper, or a couple of sheets of ordinary drawing paper, will do. I use the backs of old and now redundant drawings.

If the drawing you are to make is small, you can use a correspondingly small sheet of paper, but I prefer to use the full size of the board - there will always be other drawings needed which can be set on the

Fig. 73 Successive steps in the making of a detail drawing.

unused part. Pin or tape the top left-hand corner, draw the sheet tight towards the bottom right-hand corner and pin that; then the top right and finally the bottom left corners (reverse if you are left-handed). Now, if you are likely to take a fair time over the drawing (a day or so) then I recommend that you set the paper on the board the night before and retighten it before starting; paper does move with changes in weather. However, if the job will only take an hour or so this is not necessary. Make sure that your pencils are sharp and that your hands are clean - and the sleeves of whatever garment you are wearing

Start by estimating the amount of space you will need and how far apart the views will have to be. Trained draughtsmen will do this instinctively, but it pays to make a very rough sketch on some scrap paper. Now look at Fig. 73. We are going to make the drawing of the link seen at the bottom of the illustration. Start as at (a) by laying out, very lightly, the position of the centrelines. Use a hard pencil - 4H in my case - and exert no pressure. Let the bare weight of the pencil draw the lines. Compare this with your preliminary sketch, and check that you will be able to get in all the necessary views and their dimensions. Satisfied, proceed to step (b). Use a 3H pencil this time, but again, very lightly, to block in the outlines. Draw the large circles first, so that you can project the other views from them, and set out all other widths using dividers if you have them, or failing that, your compasses. Do not attempt to set out (e.g.) the width of the bosses or the shank by setting the scale rule across and using the point of your pencil. In fact, treat the setting out as you would a marking-out job in the workshop. Set out the radii for the fillets with your compasses, and put a very faint ring round the spot where the compass points lie - the larger fillets, at least.

Check this and make sure that the various lines join up smoothly. Don't hesitate to rub out and redraw. The lines should be just visible and no more at this stage. The most common fault, with me at any rate, is to find that parallel lines are not quite equidistant from the appropriate centreline. Once satisfied you can then 'line in', as at (c). I used a 2H in making the illustration at this stage, although 3H would have served. You now know where the centres of the fillets should be, so, having drawn the larger circles next draw the fillets and marry the straight lines to them to get a clean outline. You can also add the hidden detail at this stage, though on a more complex drawing you should have indicated this at the earlier stage, (b).

The final step is to set out the dimension lines and dimensions, and finish off the drawing. Note that the various lines should be set out very lightly at first and not lined in until you are sure that you can get them all in and that the drawing will be clear. I used a 3H for the dimension lines and leaders, and H for the figures and letters, but you may find it easier with HB if you keep it really sharp. Add the machining marks, title and the symbol to show the type of projection used, put a border round the drawing, date it, and the job is done. There is no need to rub out the faint lines of stage (b).

This was a very simple example, of course, but it is exactly the procedure used for even the most complex drawing. On a sheet carrying a number of details you would, of course, treat each item as a separate drawing, following the Fig. 73

procedure for each in turn. The object is to make a clear drawing, not crowded, but without having the views so far apart that they do not relate one to another. But it is not a bad rule to take more space than is really necessary rather than less. Clarity is the all-important objective.

Reproducing drawings

The prints used in an engineering workshop are made by using the drawings - traced in ink on film as a rule - as photographic negatives. In the old days these were blueprints, the paper being coated with a ferro-prussiate compound. This, when exposed to strong light (sunlight or arc light) and washed with water turns dark blue, the image of the lines on the tracing appearing white. Such prints last for ever and the only disadvantages are (a) that it is slow, and the wet prints must be dried and (b) many workshop fluids (including tea) bleach the blue to white. Unfortunately blue print paper is no longer available (though it is easily made) and modem prints appear as black or dark brown lines on a white background. This is a dry process and prints can be used immediately they are made. The process does, however, need special equipment outside the means of most amateurs. With both processes it is possible to obtain fair prints from pencil drawings on tracing or detail paper (but not from thick cartridge paper) provided the lines are not too thin.

Plain paper copying services are now available in most towns. The latest plain paper copying machines use continuous paper of up to 1000mm width and have very fine resolution. A further advantage of even these very large machines is their ability to enlarge and reduce thus enabling quite large drawings produced full-size on the drawing board to be quickly and easily reduced to a convenient size for use in the workplace. A scan through the 'Reprographic Services' section of the 'Yellow Pages' will locate a suitable service. Of course, second-hand copiers do turn up from time to time, my own is one such and it has the advantage that it can reduce and enlarge; I can bring all prints down to A4 size, which is handy in the workshop.

Section 9

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