and Mechanical Engineering Drawing Subject Reports. CAPE® Geometrical and ENGINEERING MECHANICS AND DRAWING - Text Books Online. Central Machine Tool Institute, Bangalore Joint Director (Mech Engg), BIS Section 11 General Principles of Dimensioning on Technical Drawings. No part of this book or parts thereof may be reproduced, stored in a retrieval .. applications are: building drawing for civil engineers, machine drawing for.
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So - leave all hatching to the very last, when all associated details have first been checked carefully. A section view is always associated with a corresponding outside orthographic view, as can be seen in fig The location of the cutting plane is shown on this outside view.
The direction of the arrowheads indicates the direction of viewing from the cutting plane. The section view is located according to the projection, first or third angle, in the same way as the rest of the views on the drawing.
There are a number of common errors with section drawing which are summarised in figure Study these carefully so as to appreciate and avoid these errors. Dimensioning Mechanical drawings should always be drawn accurately, whether full size or scales. The finished shapes must then be annotated with dimensions that fully define all of the geometric values needed to manufacture the component or build the assembly.
Fig 13 shows the part already seen, with all dimensions added. It is important when laying out the drawing to anticipate the amount of space that will be required for the dimensions and spread the views accordingly. Much of the detail of the dimensioning method is self-explanatory from the figure, but note the points of detail listed below. Dimensioning is closely related to tolerancing, which is covered separately in the next section of these notes.
Never rely on someone being able to scale off from the drawing. It should be placed on the most appropriate view for clarity. The dimension leader has one arrowhead that touches the arc.
The abbreviation R or RAD should precede the dimension. Fig 14 Narrow spaces. It is not possible in practice to manufacture items to the exact size stated by a single number. Fortunately some variation is always permissible whilst still preserving satisfactory functioning. The maximum permissible variation is known as the tolerance value. Each dimension on a drawing must include a tolerance value.
This can be either: Specific tolerance values are expressed thus - the upper value should be written above the lower value Use General Tolerances wherever possible, reserving Specific tolerances only to those dimensions where they are essential for satisfactory functioning. All tolerance values should be expressed to the number of decimal places intended, even when one limit is zeros eg If two or more dimensions are contiguous lying alongside each other then tolerances build up on related dimensions.
At least one of the portions must be undimensioned - this is termed an open dimension. This portion carries the maximum cumulative tolerance value, so must be chosen with care. Reducing the tolerance value implies greater precision of the item, with consequent increase in manufacturing cost. Therefore it is important to choose the largest tolerance value possible consistent with satisfactory functioning.
Geometric Tolerancing is defined by BS as the maximum permissible overall variation of form or position of a feature. Examples are those which refer to single features, such as the flatness of a face, or the concentricity of a shaft, and those which refer to related features, such as the squareness of a face relative to a reference face. It is beyond the scope of these notes to discuss geometric tolerancing, other than to make students aware of it. For details of the topic consult either BS Part 3 or any of the text books referred to in the introduction to the booklet.
An engineering assembly drawing sometimes termed an arrangement drawing shows how a number of individual components fit together into an assembly. Separate drawings, termed detail drawings, describe the components themselves. An example of an assembly drawing is shown in the Sample Drawings section of this manual. Study this carefully in conjunction with these notes. If the assembly drawing includes proprietary bought-in items that can be adequately described by a title or reference then detail drawings of these are not required.
Common examples of such bought-in items are standard bolts and nuts, bearings, seals, switches, and suchlike. An assembly drawing may show outside views or sectional views of the assembly.
Cross-sections are often the most informative since they reveal how components fit inside each other. If more than one view of the assembly is required, then the rules of orthographic projection for the layout of the views apply just as for detail drawings. Similarly, BS conventions must be followed. No manufacturing dimensions should appear on assembly drawings - always reserve these for the corresponding detail drawings.
However, certain dimensions may appear: This number is placed in a circle balloon of convenient size typically 12 mm minimum diameter and joined to the appropriate part by a leader line. The leader terminates either with an arrowhead if it just touches a component edge, or a dot if it goes onto the component. It is conventional to assign an approximate order of priority to the parts, using low part numbers for the major components eg main housing, main shaft and the lower number for minor components bolts, washers, etc.
If more than one identical item occurs on the assembly eg several identical bolts only one of these is numbered and the total number required specified on the parts list see below. When two or more components are very close to each other their numbered circles may butt to each other and share a single leader eg a bolt, nut, washer combination. A parts list sometimes called a bill of materials must be included on each assembly drawing, showing the information for each part under the headings listed below.
The preferred position of this is on the right hand side of the drawing, in such a manner that later additions can be made to it. All items require an entry for the first three columns. Components that are to be manufactured must have their material specified. Bought-in items should have their supplier code or similar entered in the Remarks column. Conventions are simple symbolic shapes used for frequently occurring items such as bearings, springs, etc.
A selection of the more common ones is listed below and overleaf. The figures below show the simplified diagrammatic conventions for representing some common mechanical engineering items. Various other data also appears on the label, the exact content depending on the type of the drawing. For manual drawings, students are recommended to use the standard Faculty drawing label.
These can be bought as stick-ons from the Bookshop. For a neat appearance, engineering drawings should be bordered, with the label carefully aligned with one corner usually the bottom right of the drawing. Standard routines are available in the Faculty CAD systems to automatically generate labels and borders - details and instructions are included in the CAD course notes. Notes are frequently added to drawings.
Their usual content comprises general information not readily or conveniently conveyed as dimensions. Notes of general character should be grouped together and not spread over the drawing, commonly at a convenient corner of the drawing. Typical such notes are:. Notes relating to special details should appear near the relevant feature, but not so near as to crowd the view.
Underlining of notes is not recommended. Where emphasis is required, larger characters should be used. The space between lines of lettering should be not less than half the character height but, in the case of titles, closer spacing may sometimes be unavoidable.
The neeed for every dimension on every individual parrt to have ann associatedd tolerance has h already been nooted. The tolerance vaalues to usee depend on o many prractical connsiderations, including functionn, the need for fo interchanngeability off parts, and cost. A cylinndrical shafft or pin fittiing into a ho ole is the most m commonn example. But these teerms tight and a loose are inexact terms.
All the informattion given below b is bassed on BS The fulll BS can bee viewed inn CD form in n the LRC Reservee Collection,, but the detaails given inn theses notees, plus addiitional matteer in the reco ommended course textbook, shoould suffice most coursee needs. In a clearance fit an internal member fits in an external member typically as a shaft in a hole and always leaves a space or clearance between the parts.
In an interference fit the internal member is larger than the external member such that there is always an actual interference of metal. A transition fit is intermediate between the other two, and may result in either a clearance or interference condition. A specified fit is achieved in practice by controlling the relative size and tolerance of each of the two mating parts. The main figure shows the condition where the basic size of the hole remains constant, and the basic size of the shaft is varied to achieve the desired fit condition.
With the basic hole system, the minimum hole is taken as the basic size, an offset from this is assigned, and tolerances are applied on both sides of, and away from, this offset.
The following terms apply for ISO metric fits to BS, some of which are illustrated in the figure. Basic size, or dimension, is the theoretical size of the part hole or shaft from which the actual limits of size are derived by the application of offsets and tolerances. For a fit between 2 parts the value of basic size is the same for the two mating parts.
Deviation is the difference between the basic size and the hole or shaft permitted toleranced size; it comprise 2 components:. Upper deviation is the difference between the basic size and the permitted maximum size of the part. Lower deviation is the difference between the basic size and the minimum permitted size of the part.
Nominal size is the designation used for convenient general identification and is usually expressed in common round values.
The actual size is usually slightly different from the Nominal. Allowance refers to the mating condition of the two parts. It represents the tightest permissible fit and is simply the smallest hole minus the largest shaft. For clearance fits, this difference will be positive minimum clearance , while for interference fits it will be negative maximum interference.
Refer again to the figure illustrating Tolerances and Limits. Each tolerance condition is characterised by its Offset from the basic size, and the Extent range of the tolerance values. The principles set out above are made specific by a prescribed set of tolerance values, specified in BS Refer to the table overleaf.
Note specific examples such as H8, c11, etc. Tolerance zone refers to the relationship of the tolerance to basic size. To distinguish holes from shafts, holes are always designated with upper case letters, and shafts with lower case.
The full range spans letters A — Z, centred at H and lower case for shafts. For holes, A yields a hole well above the Basic Size, and V well below. For shafts this is reversed. The range of the tolerance values is specified by the International tolerance grade IT and is indicated by a number from 0 — Each number provides a uniform level of accuracy within the grade, with 0 giving a very tight tolerance range and 16 a slack tolerance range.
Thus the combination of Fundamental Deviation and tolerance grade uniquely defines a Tolerance zone. When a tolerance zone for a hole is combined with a tolerance zone for a shaft then a definite class of fit results.
This ensures that irrespective of the size of the units, large or small, the same fit is achieved. This is not necessary in practice, and BS advises that a selection of only ten fits, with a unilateral hole basis, covering diameters up to mm, will prove suitable for the great majority of applications. Normally use only these preferred fits.
The Hole Based system is recommended for most applications as it is usually convenient to make a standard size of hole with a drill or a reamer and then produce the shaft to an appropriate diameter to suit it. All holes suitable for a unilateral hole basis system have the tolerance letter code H.
The Shaft Based system is sometimes used for preference though, particularly when stock bar material is used for the shaft, or if several parts having different fits, but one nominal size, are mounted on a common shaft diameter. Refer to the BSA table. Note that the basic size is divided in size ranges, for example, 6 to 10 mm diameter.
This means for size of holes or shafts over 6 up to and including 10 mm the figures in that Row should be applied to the basic size to achieve a given tolerance zone and class of fit. Note that for ease of reading the values are given thousandths of a millimetre.
Example 1 Achieve a Close running fit for a basic size of 7. Values need to be taken from the row: Example 2 Again achieve a Close running fit, this time for a basic size of These values result in a clearance ranging from: But, in both cases, the class of fit, and hence the manner in which this pair of parts will perform together, is the same.
Thus any pairing of holes and shafts whose diameters lie within the range given by these values are equally acceptable for achieving the class of fit, and hence functional performance, that is required. All of the notes so far have referred to fits in cylindrical terms, ie holes and shafts.
This is by far the most common occurrence, but this system is also adaptable to fits between parallel surfaces, typically rectangular drive keys in rectangular slots.
The notes so far have explained the principles and representations of limits and fits. The designer however is faced with the more fundamental task too of selecting the correct fit to use. This requires considerable experience, but some general guidelines should be noted.
Refer back to table 2, which shows the preferred subset of ISO fits to choose from. These have been divided into 3 categories of fit: Clearance Transitional Interference. Clearance fits are for use when there is movement in the form of running or sliding conditions between two mating parts.
The choice of which specific clearance fit to use depends mainly on the degree of precision necessary for effective functioning. Transitional fits are intended to control the relative location ie accuracy of relative positioning, between two stationary parts. Interference fits are used where there is a need to maintain a definite contact pressure between two stationary parts, so as to ensure no relative movement between them under all load conditions.
The information on a drawing has to satisfy the sometimes differing needs of all those who refer to it. Someone who is manufacturing a part needs to see the actual tolerance values, and should not be expected to ascertain these from a fit.
Conversely, at the design stages of a project, the designers are primarily interested in the class of fit, rather than actual values, and so prefer to read that data. However, in the context of a design information drawing, that will often show both mating parts, show the class of fit on the dimension thus:. The simple tolerance conditions implied by the general note do not, in fact, cover every tolerancing eventuality.
Some of the possible inaccuracies are listed below. Inaccuracy … an error of … Any dimension may not be exactly as indicated Size The hole may not be positioned exactly as indicated by the Location dimension The hole may not be exactly square with the top or bottom Form faces The hole may not be perfectly straight eg slightly bowed Form The hole may not be perfectly round eg slightly oval Form Any of the flat sides may be slightly bowed or out of square Form.
But equally, it is possible for any or all of these errors to be present, but only in such small amounts, that the part does function correctly — in which case the errors are acceptable. It is not possible with conventional linear tolerancing to cover all of the inaccuracies listed above. This leads to the need for a more comprehensive tolerancing system that can unambiguously deal with geometric inaccuracies of these sorts, especially those of Form.
Geometrical tolerances are used to convey in a brief and precise manner complete geometrical requirements on engineering drawings. They are applied selectively over and above normal dimensional tolerances when it is necessary to control more precisely the form or shape of some feature of a manufactured part, because of the particular duty that the part has to perform.
This is achieved by defining the size and shape of a tolerance zone as opposed simply to a toleranced dimension within which the surface or median plane or axis of the feature is to lie.
The full BS can be viewed in CD form in the LRC Reserve Collection, but the details given in theses notes, plus additional matter in the recommended course textbook, should suffice most course needs. BS defines the method of indicating Geometric Tolerancing by using a range of standard symbology and notation.
Table 1 summarizes the range of feature characteristics that can be controlled with this technique, and the associated symbols. These notes do not consider each type of geometric tolerance control in detail, but summarise the main features of the system, and show a few illustrative examples.
As necessary, the recommended coursebook and the BS should be consulted for more detail.
Characteristic to be Symbol Type of tolerance Symbol toleranced number Straightness 1. Roundness 3 For single Form features Cylindricity 4. Angularity 9 For related features Position Maximum material condition 14 Boxed dimension dimension which defines true position Table 1: BS Part 3 Symbols.
Geometrical tolerances are indicated by stating the following details in 2 or 3 compartments in a rectangular tolerance frame, in a prescribed sequence:. Fig 1 Two examples of geometric tolerance frames are shown in figure 1.
As can be seen from figure 1, the tolerance frame may, as necessary, include the third compartment for specifying a datum. Table 1 clarifies when a datum is, or is not required. Form control such as straightness of an edge, for example is self-contained, and references nothing else. But parallelism, for example, must involve two related features, the one being controlled — the controlled feature - , and a reference feature — the datum feature.
When a datum is used, it too must be indicated on the drawing.
The chosen letter for the datum is put into the square box. Fig 2. Fig 3. Refer back to figure 1. The left-hand frame defines a Flatness tolerance, of value 0. This means that a surface thus designated, although nominally flat, is actually permitted to tilt or undulate away from perfect flatness. Perpendicularity is the condition when a line, plane, or surface is at right angles to a datum feature. The tolerance zone is usually the space between two parallel lines or surfaces; it can also be the space contained within a cylinder.
All tolerance zones are perpendicular to the datum feature. The magnitude of the tolerance value is the specified distance between these parallel lines or surfaces, or the diameter of the cylinder. In the case of figure 4, the LH end face is controlled to be perpendicular to the horizontal axis of the RH end portion, labelled datum B.
Some imperfection is permitted, the face can tilt or undulate away from perfection, but must remain within 2 parallel planes, equi-spaced from the perfect face, that are 0. Positional Tolerance This is a common form control, and is accompanied by a true position dimension. This true position represents the position of the perfect centreline running through the feature. Figure 5 shows an example of positional tolerance control. A true position dimension is distinguished by enclosing the dimension values in a square box, as above.
By definition, no tolerance value attaches to this true value. Rather, the associated feature, commonly a hole, is allowed to deviate from its theoretical exact position as defined by its positional tolerance zone. The tolerance zone can be the space between two parallel lines or planes, a circle, or a cylinder. The zone defines the permissible deviation of a specified feature from a theoretically exact position. The tolerance value is the distance between the parallel lines or planes, or the diameters of the circle or cylinder.
The notes so far have assumed that the surface used as a datum is feasible for measuring from. In the case of flat surfaces this is usually so, but this is not feasible for surfaces that are curved, eg automobile body panels.
In such cases it is not practical to designate an entire surface as a functional datum because accurate and repeatable measurements cannot be made from it.
In order to define a practical datum plane, appropriate points or areas are selected indicated and then indicated on the drawing. These are termed datum targets. Manufacturing processes and inspection utilise these datum targets.
The symbol for a datum target is a circle divided by a horizontal line. The lower part identifies the datum target. The upper area may be used only for information relating to datum target.
If the datum target is: Note too the positional tolerance on the hole, with its true position dimensions 25,40 and positional tolerance control frame information. All symbols for datum targets appear on the drawing view which most clearly shows the relevant surface.
Geometric tolerances should only be used selectively on the dimensions of parts. The decision to use, and values for, geometric tolerances in any particular instance should be based on careful consideration of design criteria as functional requirements, or interchangeability of a part. They should always be considered for surfaces that come into contact with other parts, especially when close tolerances are applied to the features concerned.
Note though that geometrical tolerances should be applied only when real advantages result, when normal methods of dimensioning are considered inadequate to ensure that the design function is kept, and especially where repeatability must be guaranteed. The indiscriminate use of geometrical tolerances could increase costs in manufacture and inspection.
As always, tolerance values should be as wide as possible, consistent with satisfactory functioning. Schematic Drawings Schematic drawings are those that define the logical interconnection between components in a circuit; electrical wiring diagrams and pneumatics systems diagrams are examples of schematic drawings.
There is no concept of scale or dimensions in these drawings, they merely show schematically the components of the circuit and the interconnections between them. The principal applications for schematic drawings are pneumatic and hydraulic circuits, electrical circuits, and process plant circuits.
The first two only are considered in these notes. A fluid power system contains all the necessary components for providing power and control for a particular need. The working fluid is usually either a hydraulic oil or compressed air. The system designer needs to define, size and specify all the necessary components, connecting pipework, control units and power source for achieving the desired functions. The standard method for describing the complete system is a schematic drawing. All components are shown on the drawing in their rest state.
Set values eg pressures , where applicable, should be included on the diagram. Many CAD systems include libraries that contain a range of BS symbols , allowing the quick and convenient construction of the diagrams. BS defines a large number of components in the form of symbols.
Note the following features of fluid power schematic drawings:. The nature of most symbols is straightforward, except perhaps for the Directional Control Valves. These valves change over their internal port locations according to the flow directions required.
The valvve has 5 ports numbered 1 to 5 as shoown. Port 1 is i permanenttly connectedd to the fluid d source. In fig the power fluid f is routeed from 1 to 4, and simulltaneously poort 2 is routeed to port 3, providing p ann exhaust route r for thee fluid to the right of the piston.
Now stuudy fig Note N that thee connectionns coming froom the valvee to the cylinnder have beeen reversed.. Thus thee piston unit now moves from right too left.
In both h figures the piston is shoown at the co ompletion off its strokee. It is tediious to draw w two separatte diagrams for the two conditions. In this Fifty-third Edition some errors are rectified. The earlier Fiftieth Edition of this text-book is thoroughly revised, extensively enlarged, completely updated.
It has been one of the most comprehensive revisions since the book was first published. As a result, all the drawings have been redrawn with utmost intelligibility. Many new examples, drawings are incorporated along with some new text matter. Chapter on Computer Aided Drafting CADr is entirely rewritten with inclusion of 50 self-interactive and self-learning practice modules. This book accompanied by a computer CD as a novel pedagogical concept, containing 51 selected audiovisual animation modules presented for better visualization and understanding of the subject.
The solutions to exercises of Chapter 17, Isometric Projection and Chapter 20 Conversion of Views are given in this edition. Share B. Leave A Reply Cancel Reply. Save my name, email, and website in this browser for the next time I comment.
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