JJ204 Topic 1

SCREW THREAD

1.0  Introduction

All elements of the thread influence the strength and interchange ability of screw thread, but the pitch, angle and effective diameter are much more important than the other elements

1.1   Elements of a thread

1.1.1. Major Diameter

            It is the largest diameter of the thread. This is the distance between the crests of the thread measured perpendicular to the thread axis.

1.1.2.          Pitch/Mean Diameter

            The diameter of the thread used to establish the relationship, or fit, between an internal and external thread. The pitch diameter is the distance between the pitch points measured perpendicular to the thread axis. The pitch points are the points on the thread where the thread ridge and the space between the threads are of the same width.

1.1.3.          Minor Diameter

            It is the smallest diameter of the thread. This is the distance between the roots of the thread measured perpendicular to the thread axis.

1.1.4.          Thread Angle

            This is the included angle of the thread form.

1.1.5.          Pitch

            It is the distance between the same points on adjacent threads. This is also the linear distance the thread will travel in one revolution.

1.1.6.          Root

            The surface of the thread that joins the flanks of adjacent threads. The distance between the roots on opposite sides of the thread is called the root, or minor diameter.

1.2.     Measuring the major diameter

To measure major diameter of the screw, a micrometer, with anvils of a diameter sufficient to span two threads, may be used To eliminate the effect of errors in the micrometer screw and measuring faces, it is advisable first to check the instrument to a cylindrical standard of about the same diameter as the screw. For such purposes a plug gauge or a set of ‘Hoffman’ rollers is useful.

1.2. Measuring the minor/core diameter

The diameter over the roots of a thread may be checked by means of a special micrometer adapted with a shaped anvil or a micrometer may be used in conjunction with a pair of vee pieces (steel prisms).  The second method is recommended USED The steel prisms on the micrometer are pressed into the thread groove.  The ends of the prisms are slightly curved and parallel to the root thread.  It is important , when making the test, to ensure that the micrometer is positioned at right angles to the axis of the screw being measured, and when a large amount of such work is to be done, a special ‘floating bench micrometer’ is used.  It is because; it supports the screw and incorporates the micrometer elements correctly located, as well as providing means for suspending the vee prisms.

1.4.    Measuring the mean/pitch/effective diameter

The three-wire method is recognized as one of the best methods of checking the pitch diameter because the results are least affected by any error which may be present in the included thread angle. For threads which require an accuracy of 0.001 in. or 0.02 mm, a micrometer can be used to measure the distance over the wires. For threads requiring greater accuracy an electronic comparator should be used to measure the distance over the wires.

In the three-wire method, three wires of equal diameter are placed in the thread; two on one side and one on the other side. The wires used should be hardened and lapped to three times the accuracy of the thread to be

inspected. A standard micrometer may then be used to measure the distance over the wires. For greatest accuracy, the best size wire should be used.

The hard round bars (wire) with the same size are positioned opposite to the screw thread groove .  The distance is measured between the outside of the round bars.  The most suitable wire size is 0.57735p.  In Fig. 1.7 P is the pitch of the screw thread. The suitable wire size is quite hard to get, usually a size bigger than 0.57735p wire size will be used.

1.4.1. Best Size Wires.

            Wires which touch the thread at the pitch diameter are known as "Best Size" Wires.  Such wires are used because the measurements of pitch diameter are least affected by errors that may be present in the angle of the thread.

            The above analysis for the distance over wires holds good provided the wire touches the thread somewhere on its right side, and provided the thread angle is correct.  The extremes of wire sizes which touch on the straight sides and which can be measured For ISO metric, unified and Whitworth threads these limiting sizes are given in Table 1.1

Table 1.1.  Wire sizes for thread measurement ( p = pitch of thread)

Thread Form	Max. Wire	Min. Wire	‘Best Wire’	Size range for Best wire
ISO metric and Unified	1.01p	0.505p	0.557p	0.534p 0.620p
Whitworth	0.853p	0.506p	0.564p	0.535p 0.593p

1.5.             Optical Comparator

An optical comparator or shadowgraph  projects an enlarge shadow onto a screen where it may be compared to lines or to a master from which indicates the limits of the dimensions or the contour of the part being checked.  The optical comparator is a fast, accurate means of measuring or comparing the work piece with a master. It is often used when the work piece is difficult to check by other method.  Optical comparators are particularly suited for checking extremely small or odd-shaped parts, which would be difficult to inspect without the use of expensive gauges.

Optical comparators are available in bench and floor models, which are identical in principle and operation. Light from a lamp passes through a condenser lens and is projected against the work piece.  The shadow caused by the work piece is transmitted through a projecting lens system, which magnifies the image and casts it onto a mirror.  The image is then reflected to the viewing screen and is further magnified in this process.

The extent of the image magnification depends on the lens used.  Interchangeable lenses for optical comparators are available in the following magnifications: 5 x, 10 x, 31.25 x, 50 x, 62.5 x, 90 x, 100 x, and 125 x.

A comparator chart or master form mounted on the viewing screen is used to compare the accuracy of the enlarged image of the work piece being inspected. Charts are usually made of translucent material, such as cellulose acetate or frosted glass.  Many different charts are available for special jobs, but the most commonly used are linear-measuring, radius, and angular charts.  A vernier protractor screen is also available for checking angles.  Since charts are available in several magnifications, care must be taken to use the chart of the same magnification as the lens mounted on the comparator.

Many accessories are available for the comparator, increasing the versatility of the machine. Some of the most common ones are tilting work centres, which permit the work piece to be tilted to the required helix angle for checking threads; a micrometer work stage, with permit quick and accurate measuring of dimensions in both direction; and gauge blocks, measuring rods, and dial indicators used on comparators for checking measurement. The surface of the work piece may be checked by a surface illuminator, which lights up the face of work piece adjacent to the projecting lens system and permits this image to be projected onto the screen.

1.5.1. To check the angle of a 60^o thread using an optical comparator

1.              Mount the correct lens in the comparator.

2.              Mount the tilting work centres on the micrometer cross-slide stage.

3.              Set the tilting work centres to the helix angle of

           the thread.

4.              Set the work piece between centres.

5.              Mount the vernier protractor chart and align it horizontally on the screen.

6.              Turn on the light switch.

7.              Focus the lens so that a clear image appears on the screen.

8.              Move the micrometer cross-slide stage until the thread image is centralized on the screen.

9.              Remove the vernier protractor chart to show a reading of 30^o.

10.           Adjust the cross-slides until the image coincides with the protector line.

11.           Check the other side of the thread in the
            same  manner.

Note: If the threaded angle is not correct or square with the centre line, adjust the vernier protractor chart to measure the angle of the thread image.  Other dimensions of the threads, and width of flats, may be measured with micrometer measuring stages or devices such as rods, gauge blocks and indicators.