Don't forget to give donation to paypal at "harywiranata@yahoo.co.id" even just 1 dollar A drawing should provide a complete specification of the component to ensure that the design intent can be met at all stages of manufacture. Dimens…
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A drawing should provide a complete specification of the component to ensure that the design intent can be met at all stages of manufacture. Dimensions specifying features of size, position, location, geometric control and surface texture must be defined and appear on the drawing once only. It should not be necessary for the craftsman either to scale the drawing or to deduce dimensions by the subtraction or addition of other dimensions. Double dimensioning is also not acceptable.
Theoretically any component can be analysed and divided into a number of standard common geometrical shapes such as cubes, prisms, cylinders, parts of cones, etc. The circular hole in Fig. 14.1 can be considered as a cylinder through the plate. Dimensioning a component is the means of specifying the design intent in the manufacture and verification of the finished part.
A solid block with a circular hole in it is shown in Fig. 14.1 and to establish the exact shape of the item we require to know the dimensions which govern its length, height and thickness, also the diameter and depth of the hole and its position in relation to the surface of the block. The axis of the hole is shown at the intersection of two centre lines positioned from the left hand side and the bottom of the block and these two surfaces have been taken as datums. The length and height have also been measured from these surfaces separately and this is a very important point as errors may become cumulative and this is discussed later in the chapter.
Dimensioning therefore, should be undertaken with a view to defining the shape or form and overall size of the component carefully, also the sizes and positions of the various features, such as holes, counterbores, tappings, etc., from the necessary datum planes or axes.
The completed engineering drawing should also include sufficient information for the manufacture of the part and this involves the addition of notes regarding the materials used, tolerances of size, limits and fits, surface finishes, the number of parts required and any further comments which result from a consideration of the use to which the completed component will be put. For example, the part could be used in sub-assembly and notes would then make reference to associated drawings or general assemblies.
British Standard 8888 covers all the ISO rules applicable to dimensioning and, if these are adhered to, it is reasonably easy to produce a drawing to a good professional standard.
Dimension and projection lines are narrow continuous lines 0.35 mm thick, if possible, clearly placed outside the outline of the drawing. As previously mentioned, the drawing outline is depicted with wide lines of 0.7 mm thick. The drawing outline will then be clearly defined and in contrast with the dimensioning system.
The projection lines should not touch the drawing but a small gap should be left, about 2 to 3 mm, depending on the size of the drawing. The projection lines should then continue for the same distance past the dimension line.
Arrowheads should be approximately triangular, must be of uniform size and shape and in every case touch the dimension line to which they refer. Arrowheads drawn manually should be filled in. Arrowheads drawn by machine need not be filled in.
Bearing in mind the size of the actual dimensions and the fact that there may be two numbers together where limits of size are quoted, then adequate space must be left between rows of dimensions and a spacing of about 12 mm is recommended.
Centre lines must never be used as dimension lines but must be left clear and distinct. They can be extended, however, when used in the role of projection lines.
Dimensions are quoted in millimetres to the minimum number of significant figures. For example, 19 and not 19.0. In the case of a decimal dimension, always use a nought before the decimal marker, which might not be noticed on a drawing print that has poor line definition. We write 0,4 and not .4. It should be stated here that on metric drawings the decimal marker is a comma positioned on the base line between the figures, for example, 5,2 but never 5·2 with a decimal point midway.
To enable dimensions to be read clearly, figures are placed so that they can be read from the bottom of the drawing, or by turning the drawing in a clockwise direction, so that they can be read from the right hand side.
Leader lines are used to indicate where specific indications apply. The leader line to the hole is directed towards the centre point but terminates at the circumference in an arrow. A leader line for a part number terminates in a dot within the outline of the component. The gauge plate here is assumed to be part number six of a set of inspection gauges.
Figure 14.2 shows a partly completed drawing of a gauge to illustrate the above aspects of dimensioning.
When components are drawn in orthographic projection, a choice often exists where to place the dimensions and the following general rules will give assistance.
Start by dimensioning the view which gives the clearest understanding of the profile or shape of the component.
If space permits, and obviously this varies with the size and degree of complexity of the subject, place the dimensions outside the profile of the component as first choice.
Where several dimensions are placed on the same side of the drawing, position the shortest dimension nearest to the component and this will avoid dimension lines crossing.
Try to ensure that similar spacings are made between dimension lines as this gives a neat appearance on the completed drawing.
Overall dimensions which are given for surfaces that can be seen in two projected views are generally best positioned between these two views.
Remember, that drawings are the media to communicate the design intent used to the manufacturing and verification units. Therefore always check over your drawing, view it and question yourself. Is the information complete? Ask yourself whether or not the machinist or fitter can use or work to the dimension you have quoted to make the item. Also, can the inspector verify the figure, in other words, is it a measurable distance?
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Figure 14.3 shows a component which has been partly dimensioned to illustrate some of the principles involved.
Careless and untidy dimensioning can spoil an otherwise sound drawing and it should be stated that many marks are lost in examinations due to poor quality work.
Dimensioning of Features Not Drawn to Scale
This method of indication is by underlining a particular dimension with a wide line as indicated in Fig. 14.4. This practice is very useful where the dimensional change does not impair the understanding of the drawing.
Chain Dimensioning and Auxiliary Dimensioning
Chains of dimensions should only be used where the possible accumulation of tolerances does not endanger the function of the part.
A plan view of a twist drill stand is given in Fig. 14.5 to illustrate chain dimensioning. Now each of the dimensions in the chain would be subject to a manufacturing tolerance since it is not possible to mark out and drill each of the centre distances exactly. As a test of drawing accuracy, start at the left hand side and mark out the dimensions shown in turn. Measure the overall figure on your drawing and check with the auxiliary dimension given. Note the considerable variation in length, which results from small errors in each of the six separate dimensions in the chain, which clearly accumulate. Imagine the effect of marking out say twenty holes for rivets in each of two plates, how many holes would eventually line up? The overall length is shown in parentheses (157) and is known as an auxiliary dimension. This dimension is not one which is worked to in practice but is given purely for reference purposes. You will now appreciate that it will depend on the accuracy with which each of the pitches in the chain is marked out.
Parallel Dimensioning
Improved positional accuracy is obtainable by dimensioning more than one feature from a common datum, and this method is shown in Fig. 14.6. The selected datum is the left hand side of the stand. Note that the overall length is not an auxiliary dimension, but as a dimensional length in its own right.
Running Dimensioning
Is a simplified method of parallel dimensioning having the advantage that the indication requires less space. The common origin is indicated as shown (Fig. 14.7) with a narrow continuous circle and the dimensions placed near the respective arrowheads.
Staggered Dimensions
For greater clarity a number of parallel dimensions may be indicated as shown in Fig. 14.8 and Fig. 14.9.
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Dimensioning Circles
The symbol Ø preceding the figure is used for specifying diameters and it should be written as large as the figures which establish the size, e.g. Ø65. Alternative methods of dimensioning diameters are given below. The size of hole and space available on the drawing generally dictates which method the draughtsman chooses.
Dimensioning Radii
Alternative methods are shown where the position of the centre of the arc need not be located. Note that the dimension line is drawn through the arc centre or lies in a line with it in the case of short distances and the arrowhead touches the arc.
Dimensioning Spherical Radii and Diameters
Dimensioning Curves
A curve formed by the blending of several radii must have the radii with their centres of curvature clearly marked.
Dimensioning Irregular Curves
Irregular curves may be dimensioned by the use of ordinates. To illustrate the use of ordinates, a section through the hull of a boat is shown (Fig. 14.14). Since the hull is symmetrical about the vertical centre line it is not necessary to draw both halves in full and if the curve is presented in this manner then two short thick parallel lines are drawn at each end of the profile at right angles to the centre line. The outline is also extended slightly beyond the centre line to indicate that the shape is to be continued. Ordinates are then positioned on the drawing and the outline passes through each of the chosen fixed points.
Unidirectional and Aligned Dimensions
Both methods are in common use.
Unidirectional dimensions are drawn parallel with the bottom of the drawing sheet, also any notes which refer to the drawing use this method.
Aligned dimensions are shown in parallel with the related dimension line and positioned so that they can be read from the bottom of the drawing or from the right hand side (Fig. 14.16).
Angular Dimensions
Angular dimensions on engineering drawings are expressed as follows:
(a) Degrees, e.g. 30°. (b) Degrees and minutes, e.g. 30° 40′. (c) Degrees, minutes and seconds e.g. 30° 40′ 20″.
For clarity a full space is left between the degree symbol and the minute figure also between the minute symbol and the second figure.
In the case of an angle less than one degree it should be preceded by 0°, e.g. 0° 25′.
Figure 14.17 shows various methods of dimensioning angles.
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Tapers
In Fig. 14.18 the difference in magnitude between dimensions X and Y (whether diameters or widths) divided by the length between them defines a ratio known as a taper.
and may be expressed as rate of taper 0.25:1 on diameter.
The ISO recommended symbol for taper is, and this symbol can be shown on drawings accompanying the rate of taper,
The arrow indicates the direction of taper.
When a taper is required as a datum, it is enclosed in a box as follows:
Dimensioning Tapers
The size, form, and position of a tapered feature can be defined by calling for a suitable combination of the following:
The rate of taper, or the included angle;
The diameter or width at the larger end;
The diameter or width at the smaller end;
The length of the tapered feature;
The diameter or width at a particular cross-section, which may lie within or outside the feature concerned;
The locating dimension from the datum to the cross-section referred to above.
Care must be taken to ensure that no more dimensions are quoted on the drawing than are necessary. If reference dimensions are given to improve communications, then they must be shown in brackets, e.g. (1:5 taper).
Figure 14.20 gives four examples of the methods used to specify the size, form, and position of tapered features.
1. Dimensioning two mating tapers
When the fit to a mating part or gauge is necessary, a tried and successful method used in manufacturing units is to add the following information to the feature(s).
1 'To FIT PART NO. YYY'. 2 'TO FIT GAUGE (PART NO. GG)'.
When note 2 is added to the drawing, this implies that a 'standard rubbing gauge' will give an acceptable even marking when 'blued'. The functional requirement whether the end-wise location is important or not, will determine the method and choice of dimensioning.
An example of dimensioning two mating tapers when end-wise location is important is shown in Fig. 14.21.
For more accurate repeatability of location, the use of Geometric Tolerancing and a specific datum is recommended. Additional information on this subject may be found in BS ISO 3040.
Dimensioning Chamfers
Alternative methods of dimensioning internal and external chamfers are shown in Fig. 14.22.
Dimensioning Squares or Flats
Figure 14.23 shows a square machined on the end of a shaft so that it can be turned by means of a spanner.
The narrow diagonal lines are added to indicate the flat surface.
Part of a spindle which carries the chain wheel of a cycle, secured by a cotter pin, illustrates a flat surface which is not at the end of the shaft (Fig. 14.24).
Dimensioning Holes
The depth of drilled holes, when stated in note form, refers to the depth of the cylindrical portion and not to the point left by the drill. If no other indication is given they are assumed to go through the material. Holes in flanges or bosses are generally positioned around a pitch circle (PCD) and may be spaced on the main centre lines of the component (on centres) or as shown below equally spaced off centres. Holes are usually drilled off centres to provide for maximum access to fixing bolts in the case of valves and pipeline fittings. Special flanges need to have each hole positioned individually and an example is given with three tapped holes (see Fig. 14.25).
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