Info

Half size (1/2)

Half size (1/2)

Quarter size (1/4)

1/5

1/8

1/10

1/12

1/20

Structural drawings may be reduced even more than the above e.g. 1/96 or 1/ 100 if metric. On imperial drawings these ratios may be stated as 'x inches to the foot' and these are also used by ship modellers. But for workshop drawings such scales should be avoided, the scales being stated as shown in the table above (e.g. 1 inch to the foot as 1/12). In passing, it should be noted that there is no such thing as a one inch scale. One inch to the what? Foot, mile, rod, pole or perch? Avoid such statements; although they are in common usage, they can only cause confusion to someone used to thinking in, for example, metres.

Providing the scale of the drawing lines up with those given in the table you will be able to buy boxwood scales all marked out with feet and inches, or metres and millimetres, at the correct scale. Which you choose is up to you, but my own experience has taught me to use full size for choice if the sheet of paper is large enough (in which connection the standard sizes of paper are given at the end of the book).

General principles

Most of these are obvious, but still need stating, for many are often forgotten. The basic principle is that sufficient, but no more than sufficient, dimensions should be given to allow the part to be made.

(a) Each dimension should appear once only. An exception may be made where a long object has complex features at each end, when repetition may ease interpretation of other dimensions.

(b) Dimensions should be so arranged that it is unnecessary to arrive at one of them by addition or subtraction of others.

(c) On no account should any dimension be obtained by scaling the drawing. The reason for this is that the reproduction processes used cause a dimensional change in the paper. (The exception to this rule arises when a model is being designed from an undimensioned general arrangement drawing. However, when this is necessary it is only prudent to establish some known dimension and check that no serious error will result.)

(d) As far as possible each feature of the part should be dimensioned on a single view rather than on several. However, it is preferable to show a further view rather than to load a single one with too many dimensions. Fig. 40 (a) is preferred to 40 (b).

(e) Functional dimensions i.e. those that affect the way in which the part works, should be clearly distinguished from the less important dimensions, as suggested in the sketch, Fig.4l. (f) It is preferable to use 'general notes' at the foot of the drawing rather than individual notes having identical meaning to each part. For example:

All fillets to be 5mm rad.

Drill sizes in mm., all other dimensions

Fig. 40. Examples of "preferred" (a) and "undesirable" (b) methods of dimensioning. Fig. 41 "A" are "Functional"; "B" are "Non-functional" and "C" is an "Auxiliary" .Fig. 42 Use of a note to provide dimensional information. Fig. 43 . Extension and dimension lines in current "preferred" practice. Fig. 44. Dimension lines as found on older drawings - NOT good practice. Fig. 45 Use of dots to emphasise point from which dimension is measured. Fig. 46 Dimensions should never lie on centrelines.

Fig. 40. Examples of "preferred" (a) and "undesirable" (b) methods of dimensioning. Fig. 41 "A" are "Functional"; "B" are "Non-functional" and "C" is an "Auxiliary" .Fig. 42 Use of a note to provide dimensional information. Fig. 43 . Extension and dimension lines in current "preferred" practice. Fig. 44. Dimension lines as found on older drawings - NOT good practice. Fig. 45 Use of dots to emphasise point from which dimension is measured. Fig. 46 Dimensions should never lie on centrelines.

2"

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