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recognised abbreviations save space, No.3M, J6, or No.2 B &S.

Fig. 48 shows various methods of dimensioning other tapers. At (a) is the conventional symbol for a taper, and used in this way the taper quoted on it is always measured on the diameter or width of the object. The symbol may be used even when sufficient data is given by dimensions, simply to call attention to the fact that there is a taper. Note in examples (b), (c) and (d) there must be no more than three defining figures: two diameters and the length, or one diameter, the angle and the length. To give two diameters and the angle would require some mathematics by the machinist and is not approved.

The special case shown at (e) does appear to have four dimensions, but one

Fig. 48 Methods of dimensioning tapers. The international symbol is shown at (a) -note that it has a centreline and that the taper is always stated on the diameter or the width.

equipment indeed, but for the record, would be stated in degrees, minutes and seconds of arc, thus: 27° 35' 30". Angles less than one degree should always show the zero degrees, thus: 0° 25'. (e) Tapers

The dimensioning of tapers can be tricky. There is no problem with the 'standards' -Morse, Jacobs, Brown & Sharpe etc - and

NORMAL CHAMFER fl if U>

ANGLE OTHER THAN 45deg.

Fig. 49 Chamfers. The angle must always be defined to a surface if other than 45deg.

of these merely defines the position of one of the diameters. It is permissible to describe a taper 'to fit part xxx', but it must still be fully dimensioned; the 'to fit' is, in effect, an implied tolerance. Chamfers can be dimensioned simply by stating the width and the angle, noting that this angle must be defined to a surface as in Fig. 49 it is other than 45°.

Setting out dimensions

There are a few basic and very important rules here, coupled with a vast range of good and bad practices. These

Fig. 50 Setting out dimensions. The points are explained in the text, but note that method (e) is "permitted" only in exceptional circumstances.

Fig. 50 Setting out dimensions. The points are explained in the text, but note that method (e) is "permitted" only in exceptional circumstances.

are inevitable, as each drawing is different, and what may be quite acceptable on one „ may make another almost unintelligible. First, the rules.

(1) Figures and letters should stand at right-angles to the dimension line, and slightly away from them, as at Fig. 50 (a). The earlier practice of setting the figure within the length of the line as at (b) is not recommended, but is acceptable when circumstances demand. Figures set on vertical or near vertical lines should be arranged to be read from the right-hand side of the drawing sheet, as at (c). Figures should NOT be set as shown at (d).

Contravention of this rule is one of the most common faults in amateur circles. It springs, I believe, from a reluctance to use stencils in the vertical plane or at an angle —another argument for using freehand figures. But it is quite WRONG. On the face of it, it doesn't matter, but if I were to write down all the examples of workshop mistakes which I know to have sprung from this practice it would take several pages.

Clearly there will be occasional exceptions. Where such are absolutely unavoidable then the figures should be set within the dimension line as at Fig. 50e. As seen in Fig. 50f there is an awkward area in which it is difficult to set dimensions, and this should be borne in mind when making the drawing.

(2) Wherever possible dimensions should be set outside the object lines, with the longest dimensions outside the shorter ones, as Fig. 50g and (h). Where a dimension can be applied to two views of the object it should be set between the views, as at (h). Note that the extension lines should spring from one view only.

(3) Where there is no room for the figures on the dimension line itself one of the arrangements of Fig. 50j should be used.

(4) If a feature on a drawing is not drawn to scale the dimension figures must be underlined, Fig. 50k. (This circumstance may arise when the design of a part undergoes a minor alteration.)

(5) Angles should be dimensioned with the figures disposed horizontally, as shown at Fig.50l. In the case of a large angle, however, the figures may be set along the arc if this adds clarity to the drawing.

(6) Circles must be dimensioned by giving the diameter but arcs by their radius. An exception may be made to this latter rule when the arc is to be formed by machining e.g. with a slot drill. In that case the practice shown in Fig. 50m may be followed.

(7) The rules for dimensioning keyways are shown on Fig. 51. These are, I think, clear enough, the obvious way. The dimensions on the drawing correspond to the measurement you would take with your rule or calipers - not a bad plan to follow in all dimensioning, (a) and (b) are for ordinary keys, the rest for the Woodruffe type.

(8) A very important rule. If any dimension must be altered once a print has been issued it must be crossed out, NOT erased, and the new figure set alongside. A reference letter should be set with the new figure and a note at the foot of the drawing should give the date of the alteration and its nature. Thus: (C) 21/11/02. 1/2 becomes 12mm; Ballrace changed to type FAG6001.

This rule need not, of course, be followed while you are actually making the drawing, but even if it is one which will be used only in your own workshop it is prudent to follow the rule once you have started using the drawing.

General dimensioning practice

Unless good practice is followed confusion can still result even if all the rules are strictly observed. To detail this in words would take many pages, so I have tried to distil the most obvious points into a few examples, see Fig. 52. At (a) is the correct way of dimensioning a cylinder; (b) and (c) are to be discouraged as a waste of space, extra work and the drawing is not very clear. Illustrated at (d) is the principle of setting a <0

wheel wheel

shaft

Fig. 51 Dimensioning keyways. Note the difference between plain and "woodruffe" keys when dealing with the shaft.

wheel

out the dimensions clearly. It also illustrates the use of Xb ' meaning diameter thereby saving space. Note that each dimension is set where it will be measured and that even though the two flats dictate the use of end elevation the diameters are still shown on the front view. At (e) is shown the use of a 'note' that gives all the necessary data about the holes without any dimensioning on the object itself. This is good practice for symmetrical pieces, but will not serve if the holes are not equally spaced or differ f-

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