surface integrity

Surface integrity means it is a condition of surface of a component after manufacturing process. After manufacturing process the surface may be consisting of smoother surface or surface with irregularities. These conditions depends on the process characteristics of manufacturing process we have selected for the component.

     Generally, a component is made by casting operation or any other machining operations. By taking care of the process characteristics for these two processes we can get the desired surface finishing for the component. For the casting operation what are the process characteristics means preparation of proper mould cavity with high compact strength, passage ways with smoother surface inside, proper cross sectional areas for the passage ways which provides uniform flow of the molten form of metal, getting totally converted form of metal etc. For machining operation process characteristics are constant speed ratio between worpiece and tool, uniform feed rate and proper depth of cut etc. So, maintaining all these process characteristics in optimum condition we can avoid the irregularities may take place on the surface of component.

    Surface integrity is considered in two forms based on the irregularities and reasons for getting irregularities on the surface of a component after manufacturing process. Those two forms are Topography characteristics and surface layer characteristics. The topography characteristics are related to the irregularities related to roughness, waviness, flaws and lays. Surface layer characteristics are those irregularities which are due to the high heat applied to component which causes the change in crystal structure, grain structure and ultimately causes the variation of properties of metal of component.

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Measurment of Pitch:

     A pitch value a screw thread can be measured in so many ways like by using screw pitch gauges, screw plug gauges and also by Tool Maker's Microscope. The screw pitch gauges are provided in a form of number of blades. Each blade provided with different thread profiles having different pitch values. The pitch value of each thread profile is mentioned on the blade. After manufacturing desired threads, the pitch value is checked by matching the profile with the number of pitch gauges.  

Screw pitch gauges

     A pitch value of external threads can also be determined by using Tool Maker's Microscope. In this method the pitch value can be measured by using two micrometers provided in longitudinal and traverse direction. By using the same cross mark, we should place the cross mark on any crest point or root point and then we should move that cross mark to the successive crest or root point by using the longitudinal micrometer. In between these two positions we should take micrometer readings as initial and final reading. The difference between these two readings is the desired pitch value.

Using a Pitch measuring machine:
     A pitch measuring machine consists of two vertical slides provided with two screws. These two screws are provided for fixing a threaded component. It means we have to adjust the component in between the two screws of slides. These slides be sliding on the guide of base of this machine. This machine is also consisting of an indicator provided with stylus. According to the movement of the stylus the reading on the indicator will be changing. This indicator is provided on a guide way which will be sliding in traversing direction. Here one of the sliding is provided with a micrometer head.

     After adjusting the threaded component in between the screws, we should insert the stylus of indicator in the thread and we should make the reading on indicator as zero. Then we should rotate micrometer so that the thread component moves in longitudinal direction. So, the reading on indicator changes. We should move the component until the reading again becomes zero. It means the stylus moved from a point on a flank surface to the next successive point on the next flank surface. Here, in between these two positions we should take two micrometer readings. The difference between two micrometer readings is the desired pitch value.

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Measurement of Thread angle:

     A Thread angle of a given screw thread can be determined by using Tool Maker's microscope. The microscope gives an enlarged view of the thread profile. In Tool Maker's microscope an illuminating unit is provided below a measuring table. This illuminating unit will focus the edge profile or contour shape of the thread. By getting the contour shape we can find the thread angle.
     The Tool Maker's microscope is provided with a cross mark(x) on its primary eye piece. In this cross line, we have to adjust one line to one flank surface as it is aligning to it. In this position we should take the protractor disc reading as initial reading. Then after we should rotate the protractor as the same line of cross mark aligns with the opposite flank surface. In this second position again we should take the protractor reading as final reading. The difference between the two readings will be the desired angle value.

Tool Maker's Microscope

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Measurement of Minor diameter of External and Internal Threaded component

Minor Diameter of External Threaded component:
   
     The minor diameter of external threads can be determined by using a special type of micrometer which is provided with number of V-peices. These V pieces are changeable according to the root diameter of the thread profile. This method is used for British standard and British association threads. While measuring the minor diameter, the V pieces are connected to micrometer anvil and the screw is adjusted in between the anvils as tip diameter of V pieces matches with the root diameter of thread profile. If it is not exactly matching we should change the V pieces until a perfect match is obtained. Once the perfect match is obtained, then we should take the reading on micrometer which gives the minor diameter of screw or external threaded component.

 

Measurement of Minor Diameter of Internal Threaded component(Nut):
     Minor diameter of an internal threaded component can be measured by two methods as they are by using Tapered Parallels and using Rollers & Slip gauges.

     In the method of using tapered parallels, the tapered plates are placed at initial position on each other as they give minimum height and then they are inserted into a nut component. After inserting in a nut, they are slide on each other as the height be increasing. The sliding of tapered parallels is done until their top and bottom surfaces touch the crest points of the internal threads. Once they touch there will be no movement and in this position we should find the height of the plates by using micrometer. The micrometer anvils are to be arranged on the plates and reading should be taken.This reading will give the minor diameter of the Internal threaded component(nut).

    In the method of using rollers and slip gauges, the two rollers are adjusted inside a nut with the help of slip gauages. It means in between the rollers number of slip guages are arranged as per gap taking place between the rollers. Here the diameter of rollers and width of slip gauages are known. By adding all these values we can find the minor diameter of an internal threaded component or a nut.

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Measurement of major diameter of internal thread

     There is no perfect method for finding the elements of internal threads. But even we can find them by using screw pitch gauages and screw plug gauges. By using pitch gauges we can measure or check pitch value and by using the screw plug gauge we can measure or check major diameter.

                                                                Screw pitch gauges

                                                                         Screw plug


    Another approximation method is preparing the cast of the internal thread profile and following the same procedure as for the external threads. For preparing the cast of the internal thread, the nut is fixed in between two wooden blocks with certain gap. Through this gap molten form of metal like wax or sulphur is poured into the nut. Before pouring the molten metal, the internal thread profile should be properly cleaned and brushed from all the dust with oil. The molten metal should be poured upto a level of less than the radius of internal thread for easy removal. After solidification we have to remove the metal by lifting it up and then we should go for the measurements.

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Measurement of Major diameter of a screw thread

     The major diameter of a screw thread can be determined by a normal micrometer by carefully adjusting the screw in between in two anvils of micrometer. But there is a difficulty of pressure application during adjusting the screw in between the anvils. Due to this pressure the chance of errors in measurement becomes more. So, to avoid this a special instrument that is bench micrometer used.

     A bench micrometer consists of two anvils, one is connected to fudicial indicator and another is connected to micrometer head. The fudicial indicator confirms the application of uniform and light pressure while adjusting the screw in between the two anvils. The micrometer gives the desired reading. Generally for finding the major diameter of a screw, a setting cylinder is used which a standard piece equal to the major diameter. It is used to avoid pitch errors in a micrometer screw if any. First, micrometer reading is taken for setting cylinder and the reading is assumed as R1. Next, the micrometer reading is taken for required screw and the reading is assumed as R2. The final measurement is obtained by the following formula without any errors.
                        Major diameter = D±(R2-R1)
                                                                         D = Setting cylinder diameter
                                                                         R1 = Micrometer reading on setting cylinder
                                                                         R2  = Micrometer reading on screw thread

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Effect of pitch error

    The pitch error causes increase in effective diameter of bolt or screw and decrease in effective diameter of nut. If δp is the pitch error then increase in effective diameter of bolt or decrees in effective diameter of nut is given by the relation for whit-worth thread = 1.921xδp.

     If  δθ1 and δθ2 are errors in flank angles in opposite directions, then increase or decrease in effective diameter of bolt or nut = 0.0105xp(δθ1+δθ2).
     We can also say that, the errors in pitch and angle are useful for the alternation of effective diameter. But there is a limit to the errors for accurate work. The maximum limit for the equivalent effective diameter is = 1.921xδp+ 0.0105xp(δθ1+δθ2).
     The virtual diameter can be defined as the sum of simple effective diameter and maximum limit for the equivalent effective diameter. 
     If any difficulty is taking place in an assembly of two bolt and nut with pitch errors in bolt, by increasing the effective diameter of the nut we can assemble the two bolt and nut. In this the pitch error is distributed over the length of engagement.

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Errors in Threads

     Errors in screw threads are related to the five elements of the screw threads. They are major diameter, minor diameter, pitch diameter, pitch and thread angle. If any errors are taking place in these five elements the produced screw is rejected. So, these elements are also be checked with proper gauging system carefully. The threads are produced by a point cutting tools.
     The errors in major and minor diameter cause interference of the mating threads, less root section, less wall thickness and poor contact of the flanks, which ultimately cause the weak in strength of the component. The errors in effective diameter also cause the interference of the flanks.
     The errors in pitch and thread angle also cause the progressive tightening of the mating parts due to the interference of the flank surfaces.

     Let us discuss some important errors in thread forms. They are

1. Drunken error
2. Pitch errors 

    Drunken Error:  It is error due to the irregular form of helical groove on a cylindrical surface. In this case pitch measured parallel to the axis is always same, but problem is with the thread is not cut to its true helix.
Due to this flank surface will not be as a straight edge, it will be as curved form.

     Pitch errors: 
    The pitch errors are due to improper ratios of cutting tool velocity to rotating velocity of the workpiece. these pitch errors are again classified as

Progressive pitch erros
Periodic pitch errors
Irregular errors

     Progressive errors: In this the pitch error results increasing of major or minor diameter or decreasing of major or minor diameter. It means the error may either in increasing order or decreasing order.
     Periodic errors: In this the pitch error causes the errors to repeat at certain time of interval.
     Irregular errors: These are the errors randomly take place on threads without any specific reason. These are the combination of all the errors take place on threads.
    

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Screw Thread Metrology

   
 A Screw Thread Metrology deals with the measurements of the threads. To understand what are the measurements we can take from screw threads, we should know about the terms or elements of the threads. The Screw Thread Terminology gives the list of the terms and their definition those terms.


   Screw Thread Terminology:
The following are the terms of the screw threads:

1.  Screw thread
2.  External thread
3.  Internal thread
4.  Mulitple start thread
5.  Axis of thread
6.  Left hand or Right hand thread
7.  Form of thread
8.  Crest of thread]
9.  Root of thread
10.  Flanks of thread
11.  Angle of thread
12.  Flank angle
13. Pitch
14. Lead
15. Thread per inch
16. Lead angle
17. Helix angle
18. Depth of the thread
19. Axial thickness
20. Fundamental triangle
21. Truncation
22. Addendum
23. Dedendum
24. Major diameter
25. Minor diameter
26. Effective diameter

     Screw Thread: It is defined as a helical ridge which is formed by a continuous helical groove of uniform cross section on the external or internal surface of the cylinder or cone.
     The threads formed on cylinder are known as straight threads and the threads formed on a cone or a fulcrum of a cone are known as tapered threads.

     External Thread: Threads formed on the outside of the workpiece body are known as external threads.
Ex: Bolts and Studs etc.

     Internal Thread: The threads formed on inside of the workpiece body are known as Internal Threads.
Ex: Nuts

     Multiple start screw thread: It is a screw which is made by forming two or more than two helical grooves or teeth with equally spaced and similarly formed in an axial section on a cylinder.

     Axis of thread: It is an imaginary line which takes place longitudinally through the center of the screw.

     Right hand or left hand thread:  The Thread is placed in such a way that its logitudinal axis is normal to the observer and the thread is rotated in clockwise direction, if it is moving away from the observer, then it is a right hand thread; if it is moving towards the observer, then it is left hand thread.

     Form of thread: It is an edge shape of one complete thread as seen in axial section.

     Crest of  thread: It is the top most point of the groove forming threads.

     Root of thread: It is the bottom point of the groove forming threads.

     Flanks of threads: These are the straight edge surfaces which joins the crest to root.

     Angle of thread: It is an angle between two opposite flanks or slopes of a thread measured in an axial plane.

     Flank angle: It is an angle between individual flanks and a plane perpendicular to the axis of the thread. This angle is similar to the half of the thread angle.

     Pitch:  It is the distance between the two successive crest points or root points measured parallel to the axis of the thread.

     Lead: It is the distance moved by the screw for one complete revolution with respect to its mating part. Some times the lead is equal to the pitch but not always.

     Thread per inch: It is the number of threads taking place per one inch. It is reciprocal of the pitch.

     Lead angle: It is an angle made by helical curve of the thread at pitch line with a plane perpendicular to the axis of the thread.

     Helix angle: It is an angle made by the helical curve of the thread with the axis of the thread.

     Depth of thread: It is the distance between crest point and root point which is measured along a plane perpendicular to the axis of thread.

     Axial thickness: It is the distance between two opposite faces of the same thread which is measured along the pitch line.
   Pitch line is a line which divides a thread profile into two equal parts.

     Fundamental triangle: This is made by extending the flanks and joing the points B and C. Thus the triangle ABC is referred to as fundamental triangle. The point A is called as Apex.

     Truncation: The truncation at the crest s the radial distance from the crest to the nearest apex of the fundamental triangle. Truncation is given on both sides of a thread profile that is at crest and root.

     Addendum: It is the radial distance between crest point and pitch  line.

     Dedendum: It is the radial distance between root point and pitch line.

     Major diameter: It is defined as the diameter of an imaginary cylinder which passes through the crest points of the thread.

    Minor diameter: It is defined as the diameter of an imaginary cylinder which passes through the root points of the thread.

Mean or Effective or Pitch diameter: It is defined as the diameter of an imaginary cylinder which passes through the pitch line of the screw threads.

    

     

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