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Gandhinagar Institute of Technology

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1 Gandhinagar Institute of Technology
Mechanical Measurement and Metrology Mechanical Engineering 4th : B - 2 Prepared By :Bhautik Malviya ( ) Pavan Narkhede ( ) Darshit Panchal ( ) Guided By: Prof. Manish Meghani

2 Screw thread- definition
a screw thread is the helical ridge produced by forming a continuous helical groove of uniform section on the external or internal surface of a cylinder or a cone. 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 threads.

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4 Classification of thread
1) According to the surface on which the threads are cut 1] External thread : These thread are cut on the surface of a cylindrical bar.

5 2] Internal thread : These thread are cut into the surface of cylindrical hole of bar or cone.

6 2)According to the direction of rotation of the threaded cylinder with respect to engagement with the other part : 1] Right handed thread : A right handed thread is one in which the nut must be turned in a right handed direction to screw it on.

7 2] Left handed thread : A left handed thread is one in which the nut would be screwed on by turning it to the left .

8 According to number of starts
Single start threaded : A start threaded screw one in which there is a movement of one thread for one complete turn round the screw or bolt. Pitch and lead are same in this case.

9 Multi start thread : There is a movement of more than one thread . For double start thread lead is twice the pitch. For triple start thread lead is three times the pitch. Lead =pitch x Number of start

10 Specification and forms of screw threads

11 1] Vee threads : British Association (B.A.) threads are exclusively used for small screw like in clocks etc. thread is rounded V-form. British standard whitworth (B.S.W.) threads are used in general machine construction. 2] transmission threads : Square threads are most commonly used for transmission of power as in vices, clamps and in jacks

12 Acme thread is stronger than square thread and easy to make it
Acme thread is stronger than square thread and easy to make it . It is used in lead screw of lathe machine for transmission of power. Buttress thread resist heavy axial loads, loads acting in the direction of the axis Example of uses of buttress thread occur in quick acting vices and in breech mechanism of guns.

13 MEASUREMENT OF VARIOUS ELEMENTS OF THREAD
To find out the accuracy of a screw thread it will be necessary to measure the following: 1) Major diameter. 2) Minor diameter. 3) Effective or Pitch diameter. 4) Pitch 5) Thread angle and form

14 Measurement of major diameter
The instruments which are used to find the major diameter are by Bench micrometer Bench micrometer

15 Measurement by Bench micrometer:

16 Ordinary micrometer: The ordinary micrometer is quite suitable for measuring the external major diameter. It is first adjusted for appropriate cylindrical size (S) having the same diameter (approximately).This process is known as ‘ gauge setting’ . After taking this reading ‘ R the micrometer is set on the major diameter of the thread, and the new reading is ‘R2

17 Measurement by Bench micrometer:
For getting the greater accuracy the bench micrometer is used for measuring the major diameter. In this process the variation in measuring Pressure, pitch errors are being neglected. The fiducial indicator is used to ensure all the measurements are made at same pressure. The instrument has a micrometer head with a vernier scale to read the accuracy of 0.002mm. Calibrated setting cylinder having the same diameter as the major diameter of the thread to be measured is used as setting standard. After setting the standard, the setting cylinder is held between the anvils and the reading is taken

18 Measurement of Minor diameter of Internal threads:
The Minor diameter of Internal threads are measured by Using taper parallels 2. Using Rollers.

19 Measurement of Minor diameter of Internal threads:
Using taper parallels: For diameters less than 200mm the use of Taper parallels and micrometer is very common. The taper parallels are pairs of wedges having reduced and parallel outer edges. The diameter across their outer edges can be changed by sliding them over each other.

20 Measurement of Minor diameter of Internal threads:
Using rollers: For more than 200mm diameter this method is used. Precision rollers are inserted inside the thread and proper slip gauge is inserted between the rollers. The minor diameter is then the length of slip gauges plus twice the diameter of roller.

21 Measurement of screw thread angle (Flank angle)

22 Measurement of effective diameter
Effective diameter measurement is carried out by following methods. 1.One wire method 2. two wires method 3. three wires method. 4. Micrometer method.

23 Two wire method: The effective diameter can not be measured directly but can be calculated from the measurements made. Wires of exactly known diameters are chosen such that they contact the flanks at their straight portions. If the size of the wire is such it contacts the flanks at the pitch line, it is called the ‘best size’ of wire which can be determined by geometry of screw thread. The screw thread is mounted between the centers & wires are placed in the grooves and reading M is taken. Then the effective diameter E =T+P where T =M-2d, & P is a value which depends on diameter of wire, pitch & angle of the screw thread.

24 Two wire method: M M-Dimension over the wire

25 Two wire method:

26 Two wire method:

27 Two wire method:

28 Two wire method: P AP=OP-OA

29 Three Wire method The three-wire method is the accurate method.
In this method three wires of equal and precise diameter are placed in the groves at opposite sides of the screw. In this one wire on one side and two on the other side are used. The wires either may held in hand or hung from a stand. This method ensures the alignment of micrometer anvil faces parallel to the thread axis.

30 Three Wire method

31 Three Wire method This method is more accurate than two wire method as it ensures alignment of micrometer faces parallel to the thread axis. Here, three wires of exactly known diameters are used, one on one side & the two on the other side. The wires may be held in hand or hung from a stand. From the fig, M=diameter over the wires E= effective diameter (to be found) d= diameter of wires, h=height of wire center above the pitch line, r=radius of wire, H=depth of thread, D=major diameter of the thread.

32 Three Wire method E

33 Three Wire method

34 Three Wire method

35 Errors in thread: A rise during the manufacturing or storage of threads Error may cause due to the following elements in thread 1.Major diameter error : Cause reduction in flank contact and interference with the matching threads 2.Minor diameter error : Cause interference, reduction of flank contact 3.Effective diameter error : If it is small, the threads will be thin on the external screw and thick on the internal screw

36 4.Pitch error : Defined as total length of thread engaged is either too high or too small
Types: I .Progressive error Ii .Periodic error Iii .Drunken error Iv .Irregular error

37 I . Progressive error: Pitch of the thread is uniform but it is longer or shorter to its nominal value Causes Incorrect linear and angular velocity ratio Incorrect gear train and lead screw Saddle fault Variation in length due to hardening

38 ii. Periodic error: Repeated itself at regular intervals along the thread causes Non uniform tool work velocity ratio Teeth error in gears Lead screw error Eccentric mounting of the gears

39 Iii . Drunken error: Repeated once per turn of the thread in drunken thread In drunken thread, pitch is measured parallel to thread axis If the thread is not cut to true helix, drunken thread error will be formed

40 Iv . Irregular error : Varies in irregular manner along the length of thread
Causes : Machine fault Non uniformity in the material Cutting action is not correct Machining disturbances

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