Chapter 6: Thread and Fasteners – Machine Drawing with AutoCAD

Chapter 6

Thread and Fasteners

Chapter Outline

A wide range of fastening devices are available to connect two or more components together. Threaded fasteners remain the basic assembly method employed in industry despite the advances in other fastening techniques, such as welding, rivetting, brazing, soldering, clutches, keys, and so on. In this chapter our attention will be restricted to different types of threaded fasteners and their uses in industry.

Threaded screws as fastening devices must be an old practice because they find mention even in the writing of Archimedes (278—212 BC) where he used the principle of a screw in a practical device to raise water. In the medieval centuries, screw threads were generated by means of a crude lathe and dies and the heads were prepared by forging. The major problem of the earlier threaded fasteners (bolt and nut) was the absence of interchangeability of the parts. The nuts had to be tied to their own bolts. The first attempt to standardise screw threads took place only in the 19th century. As a result of the efforts of various engineering societies, Bureau of Standards, and others, different standards that fulfil the basic requirement for interchangeability of threaded products have been set.

We shall first explain the basic principle of threaded fasteners and subsequently discuss various types of fasteners commonly used and their representations in orthographic drawing by AutoCAD. We will introduce the user to Block and Wblock commands that are very efficient in creating threads and fastener shapes and store them in the AutoCAD library for use later.

THREAD TERMINOLOGY

A screw thread is a ridge of uniform section that follows a helical path on the external or internal surface of a cylinder. Such a thread is known as a straight thread. If the thread is formed on a conical surface, it is referred to as taper thread. Various parts of screw thread are illustrated in Fig. 6.1 and are defined below.

External thread   An external thread is formed on the external surface of a cylinder—Fig. 6.1b.

Internal thread   An internal thread is formed on the internal surface of a hollow cylinder as shown in Fig. 6.1a.

Crest   The pick of a thread is called the crest—Fig. 6.1b.

Root   The valley of a thread is called the root—Fig. 6.1b.

Outside or major diameter    It is the diameter at the crest of a thread measured at right angle to the axis of the screw—Fig. 6.1a.

 

FIG. 6.1   Thread terminology

 

Core or minor diameter    It is the diameter at the root of a thread measured at right angle to the axis of the screw—Fig. 6.1a. It is the smallest diameter of a screw thread.

Nominal diameter    It is the diameter of the cylindrical piece on which the thread is cut—Fig. 6.1b. Usually, a screw is specified by this diameter.

Depth of thread    The depth of thread is the distance between the crest and the root, measured perpendicular to the axis—Fig. 6.1a.

Thread angle    The thread angle is the angle between the flanks (sides) of the thread measured in the axial plane—Fig. 6.1b.

Pitch    It is the distance measured parallel to the axis from a point on one thread to the corresponding point on an adjacent thread—Fig. 6.1b.

Lead    It is the distance a threaded part moves axially, with respect to a fixed mating part, in one complete revolution.

Single thread    A single (single start) thread is one with lead equal to pitch—Fig. 6.2a.

Double thread    A double thread (double start) is one with lead twice the pitch—Fig. 6.2b.

Multiple thread    A multiple thread (multi thread) is one where the lead is an integral multiple of the pitch, that is, two or more helices form the thread—Fig. 6.2c.

 

FIG. 6.2

 

FIG. 6.3

 

Right hand thread    A thread is a right hand thread if, when viewed axially, it moves away from the observer when turned clockwise. Unless otherwise mentioned, threads are, in general, right hand—Fig. 6.3a.

Left hand thread    A thread is a left hand thread if, when viewed axially, it moves away from the observer when turned counter clockwise—Fig. 6.3b. A left hand thread is always marked on a drawing. The thread slopes are in the reverse direction for two types of threads as can be seen in Fig. 6.3.

THREAD FORMS

When a threaded fastener is cut into two halves along its axis, the shape of the thread obtained while viewing perpendicular to the cut section is referred to as the ‘Thread Form’. Different thread forms have different uses and histories. Some of the widely accepted thread forms are illustrated in Fig. 6.4.

 

FIG. 6.4   Different thread forms

 

With the advancement of technology, it became essential to provide for standardisation in the thread form and its pitch on screws of any diameter. Screw thread standards in England evolved from a paper presented by Sir Joseph Whitworth to the Institute of Civil Engineers in 1841. On the other hand, in USA, screw-thread standards first emerged following a report presented by Willam Sellers to the Franklin Institute in 1864. Unfortunately these two standards had different, uncommon features. Therefore, they were not interchangeable. After long deliberation, in the year 1948, unified thread standards were finally agreed upon by standardisation committees of Canada, United Kingdom, and United States of America. Later, the International Standard Organisation (ISO) adopted the unified thread standard as the ISO inch screw thread system along with an ISO metric screw thread standard. Both systems have many things in common. Fig. 6.5 shows the external and internal thread profiles for the unified system. There are slight variations in the two types of thread (internal and external) profiles as can be seen from Fig. 6.5. Once the pitch is known, other relevant dimensions of the profile can be obtained easily.

 

FIG. 6.5   Unified thread profile

METRIC THREAD

The Bureau of Indian Standards has adopted a unified screw thread profile based on the metric system as the standard thread profile for use in India and designated it as the Metric Screw Thread. It also recommends the pitch of the thread instead of threads per inch as one of the standard parameters in specifying a screw thread.

Metric threads are specified in a drawing by the letter M followed by the nominal size (Basic Major Diameter) and pitch, both expressed in millimetres. For example, a 20 mm diameter, 1.25 mm pitch fine screw thread is expressed as M20 × 1.25. A 20 mm diameter, 1.5 pitch, course screw thread is expressed as M20. It is customary not to mention the pitch of a course thread. If thread length is required to be specified, it is added after the pitch and, an × is used to separate the length of the thread from the rest of the designation. For example, M10 × 1.25 × 25 indicates a thread length of 25 mm. Table 6.1 gives the values of pitch commonly used for screw threads with nominal sizes ranging from 6 mm to 39 mm.

 

Table 6.1   I. S. O. Metric Screw Threads

Designation Pitch (mm) Core Diameter (mm)
6 1.0 4.773
7 1.0 5.773
8 1.25 6.466
10 1.5 8.160
12 1.75 9.853
14 2.0 11.546
16 2.0 13.546
18 2.5 14.933
20 2.5 16.933
22 2.5 18.933
24 3.0 20.320
27 3.0 23.320
30 3.5 25.706
33 3.5 28.706
36 4.0 31.093
39 4.0 34.093
OTHER THREAD FORMS

It has already been mentioned that other screw forms are still being widely used for various industrial purposes. Their thread profiles with intricate detail are shown in Fig. 6.4.

A sharp ‘V’ shaped thread form, shown in Fig. 6.4, is excellent for general assembly and for adjustment purposes. However, this thread form is not good for transmitting high loads and is seldom used. The square thread and a similar form (Acme, Buttress, and B&S worm thread) are developed to transmit motion or power and to keep the forces in line with the axis of the thread. Fig. 6.6a exhibits various types of threads with different pitch values. The square thread is being replaced by Acme threads as it is easier to manufacture the latter. The Acme thread is stronger at its root compared to the square thread and is particularly used where split nuts are employed, for example, in the lead screw of a lathe.

The unique profile of a knuckle thread is provided by rounding off the corners of the square thread. This thread form is used on bottle caps, glass jars, electric light sockets, and so on. The dardelet thread is a self-locking thread, originally designed by a French military officer. The buttress thread is designed to take the load in one direction only.

THREAD REPRESENTATION

A true representation of a thread is not essential for a working drawing. Since the true thread form is helical (Fig. 6.6a), it would be tedious and time consuming to draw a thread literally. In actual practice, threads are, therefore, represented by symbols.

There are three ways to graphically represent threads in a drawing: detailed, schematic, and simplified. Fig. 6.6b shows the three representations. A detailed representation looks like actual threads. A schematic representation is less realistic than a detailed representation whereas a simplified version is a simple symbol used to designate a thread on the drawing. The choice of thread representation depends on individual preferences. The resulting drawing should clearly be understood by everybody. In the following sections, we shall discuss only the simple symbol used to designate a thread representation reccommended by the Bureau of Indian Standards (SP: 46-1988) for metric screw threads and square threads.

 

FIG. 6.6   Thread representation

EXTERNAL THREADS

External threads are represented by two continuous lines drawn parallel to the thread axis as shown in Fig. 6.7a. The inner lines indicate the minor or root diameter of the threads. It should be remembered that only in the case of larger size threads, can true depth of thread be shown without creating any confusion. The extent of the thread length is specified by the major diameter of the threads. The runout of the threads is represented by lines drawn at an angle of 30° or 40° to the axis. In the side view of external threads, the minor diameter is indicated by an arc covering a little more than three quarters of the circumference as shown in Fig. 6.7.

 

FIG. 6.7   External thread representation

 

External thread in a sectional view is shown in a similar fashion as illustrated in Fig. 6.7b, with only exception of the thread limit line being drawn as a dashed line. The hatch lines are extended upto the major diameter, as shown in the figure.

INTERNAL THREADS

Internal threads are shown by two dashed lines indicating the major and minor diameters of the threads, as shown in Fig. 6.8. In case of a sectional view, these lines become continuous. In a side view, the major diameter becomes an arc, differentiating the internal thread from the external thread.

 

FIG. 6.8   Internal thread representation

DESIGNING SCREW THREAD IN DRAWING

The method of designating metric screw threads in a drawing is shown in Fig. 6.9. In case of a square thread, the type, size, and pitch are clearly labelled (Fig. 6.10).

 

FIG. 6.9   Thread designation

 

FIG. 6.10   Square thread

THREADED FASTENERS

For the assembly of different parts of a machine or structure, we use various kinds of devices known as fasteners. They may vary in shape and size, depending on their usage, from ordinary nails and glue to screw, bolt, nut, rivet, key, coller, and so on.

Fasteners are, in general, divided into two categories-

  1. Temporary fasteners
  2. Permanent fasteners

Temporary fasteners are widely employed, connecting different machine parts and other engineering components which are frequently dismantled. Examples of this type of fasteners are nuts and bolts, screws, studs, and so on.

On the other hand, when the parts are assembled through rivetting, welding, soldering, and brazing, they cannot be separated without breaking the joining elements. These type of fasteners are known as permanent fasteners.

 

FIG. 6.11   Bolted joint

Bolted Joint

A bolted joint, in general, consists of two parts—a bolt, or a screw, and a nut—besides the components to be joined. Such a pair is known as screw pair and is shown in Fig. 6.11a joining two components. Fig. 6.11b presents a photographic view of a nut partially screwed into a bolt. The helical grooves shown on the outer cylindrical surface of the bolt represents the thread of the screw.

Nuts

Nuts are usually manufactured in hexagonal or square shape, though other forms are also available to suit particular purposes. The hexagonal or square shape enables the nut to be gripped properly by means of a spanner to turn it. The hexagonal nut is the most common because during tightening, the spanner needs to be turned through an angle of 60° before it can get another hold. Hence, it is more convenient to use a hexagonal nut instead of a square nut. If the number of edges (for example, septagonal, octagonal) are increased, the spanner has greater tendency to slip. Therefore, those nuts are seldom recommended.

Hexagonal nut    Fig. 6.12a shows the two views of a representative hexagonal nut with conventional proportions. The upper portion of the nut is chamfered by an angle of 30° to 45° with the base of the nut. In the top view, the inscribed circle is formed only by chamfering. The dimensions of the hexagonal nut cannot be expressed exactly in terms of the nominal diameter of the bolt. However, for ordinary engineering drawing, it is convenient to assume the following approximate dimensions of the nut in terms of the nominal diameter D (Table 6.2). The actual proportions for nuts and bolts may be obtained from the table published by BIS. Fig. 6.12b gives the solid model of a hexagonal nut where the chamfered edges may be noted.

 

FIG. 6.12   Solid model and 2-D views of a nut

 

The following section explains step by step how to draw a nut using AutoCAD.

Drawing a Nut

All the necessary dimensions of the nut are specified in Table 6.2.

  1. Start with the top view. Instruct AutoCAD to draw a hexagon using the Polygon command with edge = D. Add the inscribed circle. Two circles are added to represent the screw thread of the nut, the diameter of the inner circle being the core diameter of the thread. The outer circle is drawn with a thin line (the line width can be adjusted in AutoCAD) and partly cut (approx. 75%) as per drawing convention.
  2. Draw the projection line to form the front view taking T = D (use point filter, if necessary). Mark point A-A on the upper face. Draw line AB at 30 to the upper face arising due to chamfering. Draw line BB and get the edges of the central face at points CC.
  3. Use the Arc command (three-points method) to draw the central arc, touching line AA at K. In manual drawing, the radius of the arc is assumed to be 1.4 D approximately. In AutoCAD, no such assumption is necessary. Similarly, draw two smaller arcs (Fig. 6.13).
  4. Project the side view. The distance between the outer edges will be equal to the across flat width (W) of the hexagonal nut. Obtain points FF of the vertical edges. Complete the arcs by the three-points option, the second point being the center point L. Note that only two faces of the nut can be observed in the side view and the chamfered corners can be seen only in the front view and definitely not in the side view. (Fig. 6.14).
  5. Finally, put the dashed lines in the front and side views to indicate the screw thread (Fig. 6.15).

 

FIG. 6.13   Hexagonal nut—projections

 

FIG. 6.14   Construction of the side view

 

FIG. 6.15   Drawing screw threads

 

Square nut    The procedure for the development of a block for a square nut (Fig. 6.16) is almost similar to that of the hexagonal nut described earlier. In fact, the across flat widths are identical for both types of nuts. The drawing steps being the same are not repeated here. Fig. 6.16a exhibits two views of a square nut when two faces are equally visible to the observer whereas Fig. 6.16b depicts the same nut when only one face is seen in the front view.

 

FIG. 6.16   Square nut

 

Table 6.2   Rough Rule Dimensions

No. Description Dimensions
1. Height or thickness of the nut, T T = D
2. Distance across diagonally opposite corners 2 D
3. Angle of Chamfer 30°
TIP

Keep the Ortho mode and Osnap on and use Polar tracking for precision and convenience.

In AutoCAD, once the nut is drawn, you can create the nut as library object and then store it as blocks. Later, using the Insert command, you can insert the blocks at any desired location of the drawing. Since there is no limit to the number of times you can insert a block, the drawing time is reduced considerably compared to manual drawing.

Other Types of Nuts

In engineering practice, we use other types of nuts for special purposes. The purpose of these nuts along with their drawings indicating the proportions of functional parts with respect to nominal diameter are mentioned below.

Flanged nut    This is essentially a hexagonal nut with a washer (a flat circular disc) so that a larger bearing surface can be provided. It is widely used in automobile industry (Fig. 6.17).

Cap nut    This is a hexagonal nut with a cylindrical cap at the top to protect the end of the bolt from corrosion (Fig. 6.18).

 

FIG. 6.17   Flanged nut

 

FIG. 6.18   Cap nut

 

FIG. 6.19   Dome nut

 

Dome nut   This is another version of a cap nut with a spherical dome at the top (Fig. 6.19).

Types of Bolts

A bolt is a cylindrical piece of metal with threads at the tail end for sufficient length and a head. The head may be of different shapes such as hexagonal, square, and so on depending on the purpose it serves. When a bolt length is specified, it does not include the thickness of the head. When a nut is screwed on the threaded end, we obtain a nut and bolt joint as shown in Fig. 6.11.

Hexagonal-headed bolt    This form of bolt (Fig. 6.20) is extensively used in industry. The upper end of the hexagonal head is chamfered at an angle of 30°. The thickness of the head varies between 0.7D to 0.8D. The other dimension of the head is same as that of the nut.

 

FIG. 6.20   Hexagonal-headed bolt

Creation of Bolt

First, we draw the bolt on the drawing screen. The method of drawing the bolt head is similar to that of hexagonal nut. The end of the bolt is in general rounded with a radius (R) equal to the nominal diameter of the bolt. The two concentric circles in the side view represent the metric thread. Once the two views are completed we can make a block for the two views and store them in the hard disk as library objects. (For details, see section on Blocks).

Other Forms of Bolts

According to the requirement, various types of bolts are available, a few of which are described below.

Square-headed bolt    This type of bolt (Fig. 6.21) is generally used when the head has to be accommodated in a recess so that there is no chance of rotation of the bolt head.

 

FIG. 6.21   Square-headed bolt

 

Cylindrical or cheese-headed bolt    The head of this type of bolt is cylindrical. The rotation of the bolt is prevented by means of a pin—Fig. 6.22a—inserted into the shank, just below the head. Another variation of this type of bolt is shown in Fig. 6.22b in which the pin is inserted in the bolt head. In both cases, the projected portion of the pin fits into the groove of the matching component.

 

FIG. 6.22   Cylindrical or cheese-headed bolt

 

Counter shunk-headed bolt    Sometimes it is essential that the head of the bolt must not project above the surface of the connected piece. In such circumstances the counter shunk-headed bolt is useful. It may be provided with a snug or may have a square cross-section to prevent it from rotation (Fig. 6.23).

 

FIG. 6.23   Counter shunk-headed bolt

 

Stud-bolt or stud    Fig. 6.24a and b show the studbolt and stud bolted joint respectively. A stud is thus basically a cylindrical shank having threads at both ends. The length of the nut-end thread (N) should be slightly more than the thickness of the nut to be used. The thread length at the other end, (M), generally known as the metal end, is at least equal to the diameter of the stud (D). In between the two threads is the plain part, the length of which depends on the thickness of piece A to be connected with piece B. The diameter D1 indicates the dimension of the clearance hole provided in piece A to pass the stud. From the figure, it is clear that place for the bolt head is no longer necessary and the length of the bolt is reduced considerably. Studs are invariably used to connect cylinder covers of various types of engines to engine cylinders.

 

FIG. 6.24   Stud bolt

 

The stud may have one or two collers to give bearing surface and support the adjoining parts. Accordingly, a stud is known as a single coller (Fig. 6.25) or double coller stud. Sometimes, the central portion of the stud is square to facilitate gripping of the stud during screwing or unscrewing (Fig. 6.26).

 

FIG. 6.25   Single coller stud

 

FIG. 6.26   Square central portion in stud

 

When a stud is connected to a block, a tapped hole has to be generated in the block in the following two steps.

  1. First, a hole is drilled—Fig. 6.27a—in the block with a drill having a diameter equal to the core diameter of the thread of the stud.
  2. Subsequently, the inside thread is cut with the help of a tap. It may be noted that the end of the hole is conical on account of the pointed end of the drill—Fig. 6.27b.

 

FIG. 6.27   Drilled and tapped holes

LOCKING ARRANGEMENT

In any assembly, it has to be ensured that the nuts do not get loose under vibration due to dynamic load. In such cases, it is essential to take precautions to properly secure the nuts at their respective positions with some kind of a locking device. Some of the frequently used locking devices are discussed below.

Lock Nut

Sometimes a nut, chamfered on both the hexagonal faces, is used in addition to the ordinary nut, as shown in Fig. 6.28. This is known as a lock nut. Its thickness, in general, is less than that of the standard nut. First, the lock nut (A) is screwed on the bolt as tightly as possible. Next, the standard nut (B) is screwed on so as to touch nut A. Keeping nut B in its position (with a spanner), nut A is unscrewed through a small angle. The two nuts are thus locked together as the thread in the nuts get wedged against those of the bolt and are thus prevented from slackening (Fig. 6.28). If both the nuts are properly screwed in, the upper nut carries the whole of the tensile load on the bolt. Accordingly, the standard nut should be placed on top. As lock nut A is thinner (0.6D), a thin spanner is necessary to turn it backwards. Since a thin spanner is not always available, the lock nut is often placed above the standard nut in industry, though it is not recommended. Fig. 6.28a highlights how the clearance between the nuts and bolt get distorted following the introduction of the lock nut.

 

FIG. 6.28   Distortion in the clearance between nuts and bolt on introduction of lock nut

Split Pin

Sometimes, a nut is locked by means of a pin passing through the center of the bolt once the nut is fully tightened. This method provides a very secured locking arrangement. The pin—Fig. 6.29a—is made up of a semicircular cross-section. After the pin is driven in, it is split open at its tail end—Fig. 6.29b.

 

FIG. 6.29   Nut with split pin

 

FIG. 6.30   Slotted nut

Slotted Nut and Castle Nut

Figs. 6.30 and 6.31 show the two types of nuts used as locking devices. A slotted nut is hexagonal with slots cut at the upper end so that a split pin can be inserted through the slot. The pin passes through the drilled hole made in the bolt and then opens up at its end.

The castle nut is widely used in automobile and locomotive engines where it is subjected to dynamic and impulsive loads.

 

FIG. 6.31   Castle nut

Washer

A washer is a cylindrical metal piece usually placed below the nut to provide a smooth bearing surface and reduce the pressure of the nut by spreading it over a larger area. Two varieties of washers (plain and chamferred) are available in the market. Fig. 6.21 shows both types.

The drawing of the washer is very simple as can be observed from the figure.

 

FIG. 6.32   Washer

Set Screw

Set screws are extensively used to prevent relative motion among a coller, pulley, gear and shaft on which they are mounted. The screw is screwed into the tapped hole (Fig. 6.33) made on the coller, pulley, gear and so on and then passed on the shaft, thus producing a strong clamping action that prevents relative motion between the two parts.

Set screws have various forms as shown in Fig. 6.34. The head as well as the end of the set screw may be different, depending on their usage.

 

FIG. 6.33   Set screw

 

FIG. 6.34   Various types of set screws

CREATING BLOCKS FOR REPETITIVE USAGE IN AutoCAD

AutoCAD Library

The shape of all temporary fasteners are standardised though their size may vary, depending on the location and design consideration. In a drawing, these standard parts may be used many a time. For example, a joint with a nut and bolt or through rivet may appear at several places in a drawing. In case of a manual drawing, the same feature has to be reproduced again and again. This is applicable for any frequently used symbols, components, or standard parts. This problem can be avoided by using a special feature provided by AutoCAD (or any other computer-aided drafting software).

If the shape of an object consists of various complex entities and much time has been spent in creating it with proper geometrical dimensions, it is much more convenient to store that combined shape as a Block and save it with a name. Once the block is created, you can insert it at any desired location in the drawing. Regardless of the number of objects required to create it, after its creation, a block becomes one single object. When a block is inserted in a drawing, AutoCAD actually inserts a reference to the original block definition, rather than completely copying the block itself, thus reducing the requirement of memory size of the drawing file.

Once a block has been created, it can be used just like another object and can easily move copy, be scaled, or rotated. One great advantage of using a block is that object snap can be used on any individual object of the block though it cannot be edited individually.

In fact Block can be used to build a standard library of frequently used symbols, components, and standard parts. From the library it is very easy to insert the same block numerous times at a desired location instead of recreating the drawing elements each time. Thus, a custom library of objects required for different applications may be created. For example, if you are concerned with gears, you can generate blocks of gears and then integrate these blocks with the Custom menu. In this manner, a library of objects useful for specific application environments may be created and stored so that these objects can be inserted in the drawing whenever necessary, thus saving time and labour. In the following section, we shall discuss the Block command and its application to organise the drawing tasks in a systematic way. The examples will be limited to fasteners only for this chapter although the idea can be expanded to various other objects as well.

CREATING BLOCKS FROM INDIVIDUAL OBJECTS

Before the creation of any block, advance planning is required to fix up the base points and insertion points. Let us explore these concepts.

 

FIG. 6.35   Choosing the Base points

Base Points and Insertion Points

When a block is created, a base point has to be defined which is, in fact, the reference point for the block. For example, in order to generate a block of the top view of a hexagonal nut, point A may be assumed to be the base point as shown in Fig. 6.35. During the insertion of a block, you have to specify an insertion point and all the individual member objects of the block will be inserted in their proper places relative to the insertion point. For the front view of the nut, point B (Fig. 6.35) may be a suitable point as base point. The base point may not necessarily be on the objects of the block but it should be conveniently fixed making the insertion process easy.

Creating a Block by Saving a Block within the Current Drawing

 

Before creating the block you will have to draw the objects at their proper relative positions and with correct measurements.

After creating the objects, follow these steps.

 

1. Choose the Block or Make Block (from the Draw toolbar) command to initiate the Block command.

 

The Block definition dialog box, as shown in Fig. 6.36, appears on the screen.

 

FIG. 6.36   The Block Definition dialog box

 

2. Type the name of the block in the Name text box.

3. Click the Select Objects button. The dialog box will disappear temporarily from the screen to allow you to select objects for the block. Use any object selection method and press Enter to return to the dialog box. You will get a miniature image of your selected objects at the bottom right corner of the objects box as a confirmation of your selection.

4. Next, define the insertion base point. By default the base point is 0, 0 (or 0,0,0 for 3-D drawings). To define any other point, choose the pick point button and AutoCAD will temporarily return you to the original drawing and choose Object Snap mode to define your base point. AutoCAD will promptly return you to the dialog box that displays the X, Y coordinates of the base point part. You can also type in the coordinate value directly against the X, Y, Z slots.

5. If you do not want to delete the objects that formed the block then check Retain; otherwise, check Delete. To convert the objects to a block, check Convert to block.

6. AutoCAD automatically creates an icon based on your block which will be displayed later in the preview box if you use the AutoCAD Design Center. You may click Do Not Include an Icon if you do not use it.

7. You should choose the insert units keeping in mind the future uses. If you want to insert the block into a drawing where millimetres is used, mention millimetre here.

8. You may enter a brief description for the block to be used by the AutoCAD Design Center for ready reference.

9. Click OK to return to your drawing.

 

The block is now ready to be used for as many times as you need it.

Inserting Blocks

You can use the Insert command for inserting a block into your current drawing. The Insert dialog box is displayed when you initiate the Insert command. For invoking the Insert command you can do one of the following.

 

 

FIG. 6.37   The Insert dialog box

 

Once the Insert command is initiated, the insert dialog box, as shown in Fig. 6.37, appears on the screen. Follow the steps mentioned below.

  1. Specify the Block name to be inserted in the Name text box.
  2. Specify scale and rotation angle according to your requirements. With proper scaling, you can enlarge or reduce the overall block size to be placed on the screen.
  3. Indicate the insertion point, that is, the point in your drawing where the block is to be inserted.
  4. Click the Specify Onscreen box to choose the point onscreen. A moving image of the block will follow as you move your mouse.
  5. Click OK to resume your drawing with the Block inserted at the desired location.

Saving Block as Separate Drawing Files

When you create a block with the Block command, the resulting block can only be used within the current drawing. However, it is possible that the blocks thus created could be used in other drawings as well. For this purpose, create the block as a separate drawing file by creating it through the Wblock command. This Drawing File (or Block) can be inserted easily into another drawing as required. Let us explore the method in detail.

On initiating the Wblock command, the Write Block dialog box is displayed (Fig. 6.38).

 

FIG. 6.38   The Write Block dialog box

 

The different headings found in the dialog box with details of their input requirements are discussed below for creating a Wblock file.

Source    Here you must define the source objects by selecting one of the following buttons under the Source heading of the dialog box.

Block    It will specify the name of a Block that has already been defined in the current drawing. Select the block name in the adjacent drop down list and the selected block will be saved to a file.

Entire drawing    This option will select the entire drawing and will save it to a file. But remember that only objects shown in the current space (Model/Paper) are included. Unused (but defined) blocks, line types, text styles, named views, user coordinate system are not icluded in the new file.

Objects    This is the same as creating blocks with the Block Definition dialog box. You will have to specify the base point also.

Defining the destination file    You must specify the file name and the folder in which the block is to be saved in the destination heading of the dialog box.

Specify the Insert units    In the Insert units drop down list, select the units to which the block should be scaled when dragged and inserted.

NOTE

When you select a block as the source object, AutoCAD automatically assigns the name of the block for the new file name. You can rename it if you like.

Minsert Command

With the Minsert command you can insert multiple instances of block or file in a rectangular pattern combining the functions of Array and Insert commands. On initiation, the command will prompt you for the following inputs.

  1. Block name
  2. Insertion point
  3. Scale factor
  4. Rotation angle
  5. Number of rows and columns
  6. Distance between the rows and the columns

The rotation angle will determine the orientation of the resulting array and when created, the entire array will be treated as a single object. Though it is highly advantageous to use the Minsert command for a small file size, it has the following limitations.

  1. It can create only a rectangular array.
  2. To modify the position of the blocks, the entire array has to be erased and redefined. However, you can redefine the original block and update all the instances automatically. You may also change the position of the rows and columns through the Properties window.

 

Exploding Blocks

Sometimes it may be necessary to modify a block. You can do so by exploding any instance of an inserted block. The exploded block will go back to its original component objects, retaining the original scale factor. However, the original block definition will remain in the drawing and can be further used for insertion of additional copies of block. Invoke the command by any one of the methods mentioned below.

Select the block and press Enter.

Redefining Blocks

Sometimes, especially in industry, the specification or design parameters of some of the components of a machine may be changed after the drawing is created. To accommodate such changes it may be necessary that all the instances of an inserted block are redefined and the drawing updated, reflecting the change (s).

AutoCAD has a simple way to achieve this goal by the following steps.

  1. Start the Block command.
  2. In the Block Definition Dialog Box, under Name, select the block you want to redefine.
  3. Under Base Point, specify the base insertion point.
  4. Under Objects, click the Select Objects button, select the objects to be included in the redefined new block, and press Enter.
  5. Redefine the Insert units and Descriptions, as needed.
  6. Click OK. AutoCAD displays a dialog box asking confirmation for the redefinition.
  7. Click Yes and AutoCAD automatically updates all instances of the redefined block in the current drawing file (that has been modified later).
NOTE

You cannot explode non-uniformly scaled blocks (inserted using different X,Y, and Z scale factors) unless the value of the EXPLMODE system variable is set to 1.

For updating all instances of a block that were inserted from a separate drawing, you will have to use the Insert command.

  1. Start the Insert command.
  2. In the Insert dialog box, click the Browse button and specify the separate drawing file.
  3. Click OK. AutoCAD displays a dialog box asking your confirmation of the redefinition of the block (conforming to the modifications to the original drawing file).
  4. Click Yes. AutoCAD immediately updates all instances of the redefined block in the current drawing.

Creation of Bolted Joint Using Block

Once the block for different views of nut, bolt, and washer are available in the hard disk, it is an easy task to draw a bolted joint comprising a hexagonal headed bolt, nut, and washer at any desired location of the drawing. The insertion base points of the blocks should be selected carefully—preferably some well-defined points on the drawing objects, for example, center of a circle, corner of a rectangle, and so on—so that the insertion of the block at any location becomes accurate. All the dimensions of nut, bolt, and so on are more or less proportional to the nominal diameter. Therefore, while inserting the blocks of nut, bolt, and so on, choose the necessary scale factor and generate nuts and bolts of any size. In fact, most of the nuts and bolts used in this book have been created by the Block command.

CREATING A SYMBOL LIBRARY

Finally, we have come to the most important use of making a block—to create a Symbol Library of frequently used machine components, for example, nut, bolt, key, cotter, gear, flanges, and so on. Follow the procedure given below for this purpose.

Create a directory on your disk, with a defined name, for example, Part Sym, and make individual files. Then, either,

  1. draw all the symbols on one sheet and use the Wblock command to create individual Block files and save them under a folder, or
  2. draw all related symbols on one sheet and name them as MECH-SYM or ELEC-SYM or PLUMB-SYM. Block the symbols in one sheet individually and save the drawing sheetwise as a whole. To insert the symbols, simply put in the name, (say, PLUMB-SYM), into your current drawing at 0, 0 and then use Insert to access the block you want.
EXERCISE
  1. Define with sketches the following terms used in connection with a screw thread—i) core diameter ii) outside diameter iii) pitch iv) nominal diameter v) depth of thread vi) thread angle vii) root viii) crest ix) external thread and x) internal thread.
  2. What do you mean by lead in a screw thread? Explain the working principle of left hand thread and right hand thread.
  3. Name the different types of thread forms with their usage.
  4. Draw dimensioned sketches of the following screw threads—Square, Acme, Buttress, and Whitworth.
  5. Draw the principles of internal and external unified threads with dimension.
  6. What is a metric thread? Show a metric thread with nominal diameter equal to 10 mm.
  7. In how many ways can you represent a thread? Explain with sketches.
  8. Draw three views of a hexagonal nut for a 22 mm diameter bolt. Store the views in the AutoCAD library as Blocks.
  9. Draw three views of a hexagonal-headed bolt with 22 mm diameter and, 120 mm long, with a hexagonal nut and washer. For the nut, use the block created in Question 8.
  10. Why is locking arrangement necessary for a nut? Explain, with sketches, the different types of locking arrangements used in practice.
  11. Draw three views of a 24 mm diameter stud, 120 mm long, with a hexagonal nut and a split pin.
  12. Draw three views of a square nut for a 24 mm diameter bolt. Add them in AutoCAD library as Blocks.
  13. Develop the solid model of the nut mentioned in Question 12.
  14. Draw the views of each of the following nuts. i) flanged nut, ii) cap nut, iii) dome nut. Take D = 24 mm.
  15. Draw two views of a cheese-headed bolt and counter shunk-headed bolt. Assume D = 22 mm and bolt length = 120 mm.
  16. Develop the solid model view of the counter shunk bolt mentioned in Question 15.
  17. What is the function of set screws? Sketch various types of set screws available in practice.
  18. Show clearly by means of suitable sketches the difference between the following: i) bolt and nut fastening, ii) stud-bolt and nut fastening, iii) tap-bolt fastening.
  19. Draw sectional top and front views of two 22 mm thick plates fastened together by means of 20 mm diameter hexagonal bolt, a hexagonal nut, and a washer. Mark all the necessary dimensions.
  20. Draw sectional front and top views of two 20 mm thick plates fastened together by means of 20 mm diameter stud, a square nut, and a washer. One of the plates has a tapped hole to fit the stud. Mention all the important dimensions in the drawing.