4.3.1. Arrangement of the Dimensions
− Show each dimension only once!
− Do not show dimensions at hidden edges of the object! − Enter external dimensions in any case (see Fig. 4.50.)!
− Show dimensions which belong together at one and the same site, i.e. where the partial shape to be dimensioned is represented most clearly! (Fig. 4.46.).
Fig. 4.46. a wrong, b correct − All dimensions can be classified in two categories:
Fig. 4.47. 1 − dimension of size
2 − positional dimension
This grouping into dimensions of size and dimensions of position facilitates both the reading of drawings and the independent dimensioning.
− Entering dimensions in drawings is largely governed by points of view of manufacture and function (see Section 4.3.2.).
4.3.2. Reference Systems for Dimensioning
The final condition of the workpiece to which the dimensions refer can be achieved by differend methods of manufacture. Since dimensioning is also governed by the production, there may be different ways of
dimensioning the same workpiece correctly, depending on the method of manufacture. Analogously, this also applies to the influence of the function on the way of dimensioning.
As starting lines for dimensioning, such lines are selected which are essential for manufacture and for the function. These lines are called reference lines.
Reference lines are starting lines for dimensioning. They are determined by manufacture or the function. The dimensions are related to these lines.
Frequently, the following items are selected as reference lines:
Supporting surfaces, contact surfaces, symmetry lines of the objects and symmetry lines of partial shapes. In practice, usually manufacturing and functional points of view must be observed for dimensioning the drawing of an object. Thus, in Fig. 4.48., the reference lines characterise both contact surfaces (function) and the starting surfaces for manufacture (see also Figs. 4.48., 4.49. and 4.50.).
Fig. 4.48. Two surfaces as dimensional reference lines x Dimensional reference lines, y 2 thick
Fig. 4.49. A surface and the line of symmetry as dimensional reference line x Dimensional reference lines, y 3 thick
Fig. 4.50. Two lines of symmetry as dimensional reference lines x Dimensional reference lines y 1 thick
For objects whose shape calls for at least two views, sometimes three reference lines must be established (Fig. 4.55.). The designer will lay down the dimensions in such a way that the component will properly function with other parts after mounting. The drawing which is submitted to the manufacturer contains the dimensions according to the conditions of manufacturing engineering.
Example 1
Figure 4.51. and Figure 4.52.
Fig. 4.51. Dimensioning according to functional and technological points of view 1 Recessing
Fig. 4.52. Dimensioning according to technological points of view 1 Mounting in lathe for tooling the right−hand side
2 Mounting in lathe for turning the journal having a diameter of 20 and a length of 14 Example 2
Figure 4.53. and Figure 4.54.
Fig. 4.53. Drawing for scribing and drilling with drilling machine x 4 thick
Fig. 4.54. Drawing for boring by means of a boring mill (dimensioning by coordinates) 4 thick, x/y dimensional reference lines (x abscissa, y ordinate)
4.3.3. Succession of Dimensioning
The succession of the individual steps of dimensioning is dependent on the sequence of operations. In the sequence of operations, one partial shape is produced at a time. These partial shapes are determined by dimensions of size and dimensions of position. Although the dimensions are required in manufacture in the sequence of basic shape/partial shape 1/ partial shape 2/ etc., it is more expedient from the drawing point of view to enter them in the inverse order! In this way, the room for the dimensions can be observed better − that is to say, the smaller dimensions are located closer by the object, the larger dimensions are located a larger distance away from it (cf. Figs. 4.55. and 4.56.).
Repetition
1. On the basis of Figs. 4.46. (right) and 4.55. show dimensions which have to be entered in the drawing together in one and the same view.
Substantiate this!
2. What are reference lines?
3. What are the points of view that govern the specification of reference lines for dimensioning a workpiece representation?
4. Determine the reference lines for dimensioning in Figs. 4.46. (right−hand side), 4.53. and 4.55.
5. Determine the dimensions of size and of position of the partial shapes and of the basic shape in Figs. 4.49. and 4.52.
Fig. 4.56. a Sequence of operations:
0. Providing reference surfaces for the basic shape 1. Producing the basic shape
2. Milling the shoulder 3. Milling the slot
4. Harking the holes and drilling the holes b Sequence of entering the dimensions:
0. Providing dimensional reference lines according to procedure of manufacturing and/or function
1. Dimensions of size of the holes to be drilled and positional dimensions of the holes to be drilled
2. Dimensions of size of the slot (positional dimensions are omitted) 3. Dimensions of size of the shoulder (positional dimensions are omitted) 4. Dimensions of size of the basic shape
4.3.4. Locating Holes
In workpieces with may holes close by each other at the same distance, a comprehensive dimensioning would be very time−consuming and less informative (Fig. 4.57.).
Fig. 4.57. 1 to be kept free
If this manner of dimensioning is required for certain reasons, then the dimensioning chain must not be closed!
Closed dimensioning chains should be avoided!
The dimensioning chains for the equal location of holes usually are represented in a simplified manner, as is shown in Figs. 4.53. to 4.61.
Fig. 4.58.
Fig. 4.60. Rectangular openings, dimensioned from edge to edge
Fig. 4.61. Spacings between holes given in the form of angular measures
If holes are located at the same spacing on a pitch circle, then the angular measure for the distance need not be given (Fig. 4.62.). The indication of one angular measure will suffice (Fig. 4.63.). When the locating is clear and subject to but one interpretation, it will suffice to represent one hole fully while the other holes are only specified in their position (Fig. 4.63.).
Fig. 4.62. Four holes an a hole (or index) circle 1 Index circle,
Fig. 4.63. Six holes on a hole circle
4.3.5. Sort Drawing
In order to achieve a rational method of representation for parts of the same shape but different dimensions, the part is drawn only once and is provided with dimension letters instead of dimension figures. The
dimension figures for the sorts in question are given in a Table (Fig. 4.64.).
Fig. 4.64. Drawing of grades x grade, y 2 thick