 I welcome you for the series of lecture on metrology. Now we will start module number 6 on screw thread metrology. In this module we will be discussing about the general introduction to screw threads covering the thread making processes and how they are classified and what are the various applications of screw thread those things we will be studying. And then we will also discuss about the various screw thread terminology and then we will move on to the measurement of screw thread elements. In this lecture 1 we will be covering the general introduction to screw threads. We will cover general applications of screw threads and classification of screw threads, thread cutting methods and how they are designated. Then we will study the screw thread terminologies various terms used related to screw threads. And then we will discuss about screw thread measurement that is measure diameter measurement, minor diameter measurement, thread angle measurement, pitch measurement and effective diameter measurement. Now briefly we will try to understand the general aspects of screw threads. These are the most important machine elements and are used in screws, bolts, nuts, threads, tapped holes and other power transmitting devices. They are very convenient for joining and sealing purposes. They are used as coarse type for bracket fitments and as very fine type of type for micrometer heads and for precision moving mechanisms. It is basically a helical ridge produced by forming a continuous helical groove of uniform cross section on an external or internal surface of a cylinder or a cone. Now you can in this photo we can see the helical ridge produced and the cross section is uniform for all the threads. Now a screw thread formed on a cylinder is known as straight or parallel screw thread while the one formed on a cone is known as tapered thread. You can see here some tapered threads here. This is external tapered thread and this is internal tapered thread. Now what are the general applications of screw threads? They are used for fastening purpose screws, nut and bolts and threads are used for temporarily fixing one part onto the other part and they are used for joining coaxial joining of rods, tubes etc. by external and internal screws and they are also used for clamping. They strongly hold an object by a threaded rod as in C clamps, vices, tile stock on lathe bed. They are used for controlled linear movement that is for travel of slides and movement of work tables in milling machine, shaping machine, CNC machine tools etc. And these screw threads they are used for transmission of motion and power. For example lead screw used in a lathe is used for transmitting motion and power and they are also used for converting rotary motion to translation that is rotation of the screw causing linear travel of the nut which have wide use in machine tool kinematic systems. Screw threads are used for position control in instruments. Screws enabling precision movement of the work table in microscopes or examples and screw threads are used in precision movement of precision measurement of length that is the threaded spindle of micrometer is an example for this. Now to get the very slow rotation we can use the screw threads in the form of as in the example of gear and worm wheel and to exert heavy force the screw threads are used example is mechanical process. The screw threads are also used in conveying and squeezing materials in screw conveyors, injection molding machines and screw pumps and in controlled automatic feeding in mass production assembly the screw threads are used. Now let us study how these screw threads are classified. They are classified according to the location that is whether these screw threads are made on the external surface as in bolts or whether they are made on the internal surface for example nuts. These threads can also be classified according to the configuration that is whether they are straight helical for example bolts and struts, tapered helical as in the drill check, also radial on scroll as in the self centering check. Screw threads can also be classified according to the direction of helix that is whether they are right handed or left handed. According to the form they are classified as V threads normally 60 degree and 55 degree thread angle is used. Acme thread where in the angle is 29 degree they are classified as square thread which are generally used in the power screws, butters thread, worm thread like this depending upon the form they are classified and depending upon whether they are full circle or semi circular threads. So semi circular threads are being used in the circulating type of bolts and screws. Now according to the standard whether they are BSW ability standard with per threads or metric threads according to the number of start whether it is single start or double start or like this multi start screw threads. Now according to the spacing of threads I can say TPI threads number of threads per inch whether it is 6 TPI, 12 TPI like that we can classify and depending upon the pitch also we can classify. So pitch is pitch or lead it is the distance between two successive threads or length of travel of net for one rotation of screw. So pitch can be 1 millimeter pitch or 2 millimeter pitch or 3 millimeter like that. According to the compactness or fineness of threads we can classify them as general threads and pipe threads and verifying threads which are used for leak proof applications and according to the segmentation whether they are full form threads or off turn threads example is off net used in lathe and then sector threads which are used in the jaws of lathe checks. Now how these threads are made there are many methods very common method is thread machining they can be turned they can be milled and thread grinding thread tapping. So these are some thread machining methods and the threads can also be made by thread rolling operation. Now let us move to the screw thread terminologies various terms are used in with respect to screw threads internal screw threads as well as external screw threads. Now this picture shows some of the terminologies you can see the profile of the screw thread and the distance between one point on the screw thread to the corresponding point on the next thread this is known as pitch and then this peak portion of the thread is called crest and in the bottom most that is the lowest point on the screw thread profile is known as root and then we have this sloping surface which connects crest with the root is called the flank and then the angle between the perpendicular line perpendicular to the axis and the flank is called flank angle and double the flank angle is known as thread angle. Now this is the pitch line so this pitch line it bisects the thread such a way that the thread material space is equal to the air space. So this line is called pitch line and this diameter is known as pitch diameter and addendum so the distance between the pitch line and the crest point is known as addendum and similarly the distance between the root and the pitch line is called didendum. Now pitch it is the distance from a point on a screw thread to a corresponding point on the next thread measured parallel to the axis so this is very important when we measure the pitch measurement should be made parallel to the axis. Now you can see here this is a single start screw thread so where in lead is equal to pitch. Now what is lead it is the distance a screw thread advances in one turn when we rotate the screw thread by one turn what is the distance movement of the thread that is called lead for a single start lead is equal to pitch whereas in the case of two start screw thread lead is equal to twice the pitch. Now thread form is the cross section of the thread cut by a plane containing the axis for example in the case of metric thread we will be having a v profile like this. So this is the thread form or thread profile in case of metric thread this included angle which is known as thread angle is equal to 60 degrees and in the case of Witworth thread the included angle is 55 degrees and how these screw threads are designated the metric threads they are designated in this fashion one example is shown here m10 times 1. So m indicates that the thread is metric type and 10 indicates 10 mm measure diameter and this 1 indicates a pitch of 1 millimeter. Now various thread forms are used in the practice is a sharp v wherein we have a sharp crust here. So it is slightly modified it is made rounded or flat then it is called unified thread and then in these two cases sharp v and unified included angle is 60 degree whereas in the case of Witworth thread the included angle is 55 degree and then modified square 10 degree included angle these are modified square threads used in the power screws and acme threads wherein the included angle is 29 degree. So similarly we have modified buttress and then knuckle threads so like this different thread forms are available. Now what is the measure diameter we can understand from this diagram the it is the measure diameter is the diameter of an imaginary cylinder coaxial with the screw which just touches the crusts of an external thread you can see here we have the crust points here and we have the crusts here. So the imaginary cylinder will pass over these crust points just touches the crusts of an external thread or roots of an internal thread it is also known as nominal diameter in case of internal thread you can see these are the roots and this is the root. Now the distance between root here and root at this place so this distance is minor diameter and distance between the crust to crust so this is a major diameter. So minor diameter is the diameter of an imaginary cylinder coaxial with the screw which just touches the roots of an external thread or crust of an internal thread so this is also known as root diameter or core diameter. Now let us understand what is the meaning of effective diameter or pitch diameter it is the diameter of an imaginary cylinder coaxial with the axis of the thread and intersects the flanks of the thread such that width of threads and width of spaces between the threads are equal. So this is the line which is passing through the thread profile and it cuts the flank such that the width of thread is equal to width of space. So if you pass the line in such a manner then that is called pitch line and from pitch line to the other pitch line on the other side so this diameter is pitch diameter. The flank is the thread surface that connects crust with root so the crust is connected with the root this sloping surface is known as flank and then the depth of thread it is the distance between crust and root measured perpendicular to the axis of screw. So the axis of screw will be somewhere here and we have to pass a line perpendicular to the axis and the distance between the root and the crust. So this is the depth of thread so this is depth of thread now angle of thread so angle it is the angle between sides of thread that means angle between two flanks measured in axial plane you can see here we have a flank here we have another flank here so the included angle between two flanks is known as thread angle and Helix angle is angle that the thread makes when with plane perpendicular to the thread axis so this is the thread axis so we have the thread axis here and then this is the ridge or the thread okay it makes an angle with this perpendicular so this angle is known as Helix angle and flank angle is of the included angle of the thread and then we discussed about the addendum and didem didendem then sometimes we say right-handed thread so when the screw thread advances when turned clockwise it is called right-handed thread and when it advances when turned in the counterclockwise direction it is called left-handed thread. Now let us move to the measurement of thread elements in order to find the accuracy of screw thread reduced it is necessary to measure the various thread elements so thereby we can say whether the thread produced is as per the specification or not so normally these are the elements measured to check whether the screw thread is okay or not the measurement of measure diameter measurement of minor diameter and measurement of pitch diameter or effective diameter pitch measurement, thread angle measurement and thread form is also measured. Now let us study how the major diameter of a screw thread is measured so this instrument is known as bench micrometer we can see the very rugged cast iron base of the instrument wherein there is a slide which can be moved parallel to the axis of the bench micrometer this is a slideable body which houses the micrometer head a large diameter micrometer is fitted and this is the anvil so when we rotate this thimble the anvil moves in or out and then we have a table so this table also can be moved in this direction to accommodate the workpieces and this table surface is a flat surface which can be moved up and down again to accommodate the workpieces of different sizes. So left hand side we have the body which houses the anvil this anvil also can be moved in and out to accommodate workpieces of different sizes we can see there is a fiducial indicator so when we place the workpiece between these two anil anmils and we rotate the micrometer head the anvil will move and then the it will act some pressure on the workpiece that is transmitted to this anvil and finally it is transmitted to the fiducial indicator and this indicator will indicate what is the pressure or force applied on to the workpiece this is very essential to conduct the experiments measurement experiments at the same pressure always it is very essential that we should not over pressurize the screw threads so when the anvils when they should just touch the crust if you over pressurize the screw thread what happens is anmils will try to crush the crust and the screw thread gets deformed and we do not get the proper reading. So to apply even pressure for all the workpieces this fiducial indicator is provided and on this table surface we can we can put some fixtures to hold the workpieces. Now this shows a schematic diagram of bench micrometer the micrometer head with the scale on the thimble and with the scale on the barrel and then we have the measuring anvils and clamps for measuring anvils fiducial indicator the anvil and fiducial indicator it is a slidable so that we can accommodate the workpieces of different sizes we can also see there is a fixture holding centers are there to hold the screw threads. Now it is very essential that the axis of these holding centers should be perpendicular to the axis of the bench micrometer. Now in order to use this bench micrometer to measure the measured diameter normally the bench micrometer it is used as a comparator that means a standard cylinder normally a plain plug gauge having approximately same diameter as the measure diameter of the thread to be measured is used as a standard cylinder for setting the micrometer and over the standard cylinder micrometer reading is taken and it is recorded as R1. So we use the standard cylinder to establish for comparative measurement in order to reduce the possible errors and then after taking reading R1 we have to remove the standard cylinder from the bench micrometer and we have to insert the thread to be inspected between the centers and again the micrometer reading R2 is noted down. Using these bench micrometer it is possible to measure the measure diameter to an accuracy of plus or minus 0.001 millimeter such a fine accurate measurement is possible with the bench micrometers. Now I can see here SC is the diameter of the standard cylinder normally plain plug gauges are used as standard cylinders and R1 is the reading on the standard cylinder and R2 is reading on the thread to be inspected and then D the measure diameter of thread can be calculated using this relationship S plus or minus R1 minus R2 whether we use plus or minus it depends on whether the standard cylinder diameter is greater than measure diameter of thread or smaller than measure diameter of thread. You can see here the measurement process initially we have to take the measurement using standard cylinder and the reading that is obtained is R1 and then we have to remove the standard cylinder place this thread to be inspected and then again you take the reading so this reading will be R2 and then we can conveniently use an outside micrometer also for measurement of measure diameter. So the care should be taken to see that the crests of the thread are not over pressurized so when we move the anvil we should take care that as soon as the anvil just touches the crest of thread we should stop the rotation of thimble. So again procedure is same reading is taken with the standard cylinder that is R1 we get and then standard cylinder is removed and screw thread is placed between anvils and reading R2 is taken and again using this relationship we can calculate the measure diameter of the thread. Now we will have a numerical problem while measuring the measure diameter of an external thread a 35.5 millimeter diameter line plug gauge is used as standard the micrometer readings over the plug gauge and screw thread or 9.376 millimeter and 11.876 millimeter respectively. Now we have to find the thread measure diameter now the data that is given is R1 is 9.376 millimeter this is the micrometer reading on the standard cylinder and then we have R2 is equal to 11.876 millimeter this is the micrometer reading on the thread gauge screw thread and then S that is standard cylinder diameter is 35.5 millimeter. Now in the measurement process of this type normally smaller diameter standard is used that is diameter of standard is smaller than the measure diameter of screw. So in that case we use this relationship D is equal to S plus R2 minus R1. So the S value is 35.5 and R2 value is 11.876 millimeter and R1 value is 9.376 millimeter. So the calculation will give us the measure diameter of this screw thread as 38 millimeter. Now we will move to measurement of minor diameter the minor diameter is measured again by a comparative process that means we use a standard cylinder and we take the measurement and then we put the screw thread to be inspected again we take the reading and then by using the relationship we calculate the minor diameter and floating carriage diameter measuring machines can be conveniently used for measurement measurement of minor diameter. The details about the floating carriage machine we will discuss after some time. So use of prisms is made while measuring the minor diameter these prisms they look like small v pieces and they make contact with the root of the thread. The prisms are made of hardened steel and are made in several sizes having suitable radii at the tips. Now you can see here these are the prisms. So this is the prism being used on screw thread as well as on the setting cylinder. The prisms will look like this so the isometric view of the prism I have written here. So this the sloping surface of the prism will not make any contact with the flank of thread as we can see here. So this portion I am enlarging so this is the v prism and then we have a small radii here at the tip and then we have the screw thread profile. So the sloping surface of the prism will not make any contact with the flank of the screw thread. The contact will be there only at the root. Now using these prisms we can conveniently measure the minor diameter of external screw threads. There is an arrangement in the floating carriage machine. There is an arrangement to place these prisms. Hooks are provided. Using hooks we can suspend these prisms and we can conveniently measure the minor diameter. Now the procedure is similar to the measurement of a measure diameter. Reading R1 is taken with the standard cylinder placed between two prisms and then standard cylinder is removed and threaded workpiece which is to be inspected is mounted between the centers of the instrument and then reading micrometer reading R2 is noted down and then using this relationship we can find the minor diameter of the thread. Again whether we use plus or minus symbol depends upon the size of the standard cylinder whether it is the diameter is greater than the minor diameter or diameter is smaller than the minor diameter. Now using this relationship we can find out the minor diameter of screw thread. Now we will have a numerical problem here. While measuring the minor diameter of an external thread, a 30.5 millimeter diameter flying plug gauge is used as a standard. The micrometer readings over the plug gauge and thread are 15.376 millimeter and 13.521 millimeter respectively. We are required to calculate the minor diameter that is small d. Now the data that is given is R1 is equal to 15.376 millimeter and R2 is 13.521 millimeter and diameter of the standard is 30.5 millimeter. So in this case larger diameter standard is used and minor diameter is equal to d minus. Since the diameter of standard is larger we are using this negative sign d minus R1 minus R2 is equal to where to feed these values and finally we get the minor diameter that is 28.645 millimeter. Now we will move on to the measurement of internal threads. Let us understand how to measure the major diameter of internal thread and minor diameter of internal thread. The minor diameter can be measured by using tapered parallels and also by using rollers. Now measurement of major diameter of internal thread. Now for this we use a method called casting method. You can see in this diagram this is the work piece having internal threads whose major diameter is to be measured. Now we have to take a replica of this internal thread. So how do we get the replica of internal thread? You can see the arrangement. This is the work piece with the internal thread on both sides of this work piece like this. We have to keep wooden blocks and then we have to pour either plaster of paris or dental wax or sulphur also can be used. We have to put the plaster of paris mixed with water or dental wax into this cavity and we should allow it to settle and then once it dries we have to remove the wooden blocks and then we have to remove the solidified plaster of paris piece. Without rotating the plaster of paris we have to just lift it. We should take care that while pouring the plaster of paris it is the, so this is the centre of the work piece. You can see the level of, this is the level of plaster of paris. It is not crossing the centre of or it is not crossing the radius of the work piece. Such care should be taken so that we can easily remove the solidified plaster of paris. Now this casting will have the thread profile and then we using the bench micrometer we can measure the measure diameter of internal thread. Again in the cast that is obtained we should measure R1 and R2 and then we should use this equation to find out the measure diameter. Now how do we find the minor diameter of internal thread? For that two methods are suggested. So first method is using taper parallels. This is used when the diameter is less than 20 millimetre. So taper parallels and micrometer are used for measurement of internal thread. So these taper parallels are pairs of wedges having rounded and parallel outer edges. So when we look from this side the parallels will look like this. So they have a rounded edge. So this curved surface will come in contact with the screw thread. So like this. Again another wedge is used. Two a pair of wedges are used. Again this is the rounded edge and this is the screw thread. Now the distance between the outer edges of the parallels can be changed by moving them in this fashion. So using this arrangement internal diameter of the screw threads of different sizes can be measured. Now using the micrometer the distance between the outer edge, rounded edge is measured. So that gives the minor diameter of internal thread. Now using rollers or balls we can measure the internal thread, minor diameter of internal thread. So this is used when the screw thread diameter is more than 20 millimetre. So precision rollers or balls are inserted inside the thread as shown in this schematic diagram and slip edges are inserted between the rollers or balls. So these are the rollers or balls of same diameter. They are in contact with the internal thread and the gap between the rollers or balls is filled with slip cage and then the minor diameter is calculated by adding the diameter of these two rollers plus the slip cage width. Now let us conduct an experiment to show the measurement of minor diameter of internal thread. You can see a component having internal threads. You can see these are the internal threads. Now we are required to measure the minor diameter of the internal thread. So for this I am using two steel balls and slip gauge box. So initially we should measure the approximate minor diameter using a vernier, vernier caliper. So the approximate minor diameter as measured using a vernier caliper is 55, 56, 58, 58 millimetre and then we have to add the vernier scale reading. So it is 20th reading, 20th line is coinciding with the main scale mark. So the vernier reading is 20 into 0.02. We have to keep the two steel balls inside where to measure what is the diameter of steel ball. We can see steel ball diameter is 12 millimetre and then we have to note down the vernier reading. The second steel ball diameter also should be measured to ensure that both are having same diameter. Again you can see the diameter is 12 millimetre and then we have to note down the vernier reading. Now we should measure the approximate distance between the two steel balls so that we can easily build the slip gauge assembly. You can see the approximate distance between steel balls is 33 millimetre and then we should see the coinciding division. I am building the slip gauge assembly and slip gauge should be filled between the steel balls. I am taking 20 millimetre slip gauge, 9 millimetre slip gauge and then 1.005 millimetre slip gauge, 2 millimetre slip gauge I am ringing and then 1.23 millimetre slip gauge and finally 1.006 millimetre slip gauge. Now the assembly I am trying to insert between the two steel balls so it is not entering the gap. So the width of assembly is greater than the gap between steel balls so we have to again reduce the width. Now I am taking the 1.001 millimetre slip gauge, I removed 1.006 millimetre slip gauge. Now you can see it is entering. The assembly is entering in the gap between two steel balls. So now this is the width of the slip gauge assembly that is 33.231 millimetre. The total width is 33.231 millimetre and then we have to add diameter of this steel ball and diameter of this steel ball to get the minor diameter of internal thread. Now in the experiment we observed that ball diameter is 12.56 millimetre and gap between balls is 33.231 millimetre. So the minor diameter is 2 times the ball diameter plus the gap between balls. So finally we get 58.351 millimetre. So this is the minor diameter of internal thread. Let us summarize this lecture. In this lecture we discussed about the different methods of making the screw thread and how they are classified and what are the various types of screw threads normally used. We also discussed about screw thread terminology and then we studied about the measurement of major diameter and minor diameter of external screw thread and internal screw thread. With this we will conclude this session. Thank you.