IPE 381 Chapter:13 Measurement of Screw Thread

Transcription

1 IPE 381 Chapter:13 Measurement of Screw Thread Abdullah-Al-Mamun Lecturer, Dept. of IPE Outline Common terms Pitch Errors Errors in Measurement Effective Dia Measurement 1

2 Screw Thread A SCREW THREAD is a ridge of uniform section in the form of a helix on the external or internal surface of a cylinder. the PITCH of a thread P is the distance from a point on the thread form to the corresponding point on the next form, measured parallel to the axis Elements of a Vee-form Thread A vee-from thread is composed basically of the following elements. Major or outside diameter Minor or root diameter Form, particularly flank angles Pitch Simple effective diameter 2

3 Screw Thread Screw Thread 3

4 Lead & Pitch LEAD = 1P LEAD = 2P LEAD = 3P 4

5 Errors in Measurement There are at least five important elements which require consideration: Major diameter Minor diameter Effective diameter Pitch and angle of the thread form Errors on the major and minor diameters will cause interference with the mating thread Errors on the effective diameter will also result in weakening of the assembly pitch and angle errors are also not desirable as they cause a progressive tightening and interference on assembly Errors in Measurement Drunken Thread: Thread drunkenness is a particular case of a periodic pitch error recurring at intervals of one pitch 5

6 Pitch Errors in Screw Threads Progressive Pitch Error: This error occurs when the tool work velocity ratio is incorrect though it may be constant. Periodic Pitch Error: This type of error occurs when the tool work velocity ratio is not constant. Irregular Errors These arise from disturbances in the machining set-up, variations in the cutting properties of material etc Erratic Pitch. This is the irregular error in pitch and varies irregularly in magnitude over different lengths of thread. Progressive Error. When the pitch of a screw is uniform, but is shorter or longer than its nominal value, it is said to have progressive error. Periodic Error. If the errors vary in magnitude and recur at regular intervals, when measured from thread to thread along the screw are referred to as periodic errors. 6

7 Effect of Pitch Errors An error in pitch virtually increases the effective diameter of a bolt or screw and decreases the effective diameter of a nut. Error in Flank angle also increases the effective diameter of Screw. Let us consider, δp as the error in pitch and δθ 1 and δθ 2 are error in flank angle. Effect of Flank Angle Errors Nut of perfect form mating with a screw having a flank angle error δθ on one flank only. 7

8 B θ A C For Flank Error δθ 1 and δθ 2 the Correction factor = CP(δθ 1 + δθ 2 ) For Whithworth Thread, C= and θ=55⁰ so, cot(55⁰/2)=1.921 So, Virtual Effective Diameter = Simple Effective Diameter δP P (δθ 1 + δθ 2 ) 8

9 Measurement of Major Dia bench micrometer is used to measure Major Diameter D c =Calibrated diameter of the setting cylinder R c = Micrometer reading on setting cylinder R t =Micrometer reading on thread Then, Major Diameter = D c +(R t -R c ) Effective Dia Measurement (i) Thread micrometer method. (ii) One wire and two wire method. 9

10 Two Wire Method From the Fig-14, E d =T+2x In the ABC, AB= BC cotθ But, BC=¼ pitch=¼ P Therefore, AB= ¼ P cotθ In the ADE, d AE = DE cos ecθ = cos ecθ 2 Now, x =AB-AF and AF =AE EF =AE - d/2 Therefore, D d AF = (cos ecθ 1) 2 where, P d x = cot θ (cos ecθ 1) 4 2 P= Nominal Pitch; D=Wire Diameter; θ= Flank Angle A E C A B Two Wire Method 10

11 Matrix Pitch Measuring Machine Three Wire Method The wires are placed in the threads and a micrometer is used to measure over the top of the wires. M = E P + 3W M = the measurement over the wires E = the pitch diameter P = the pitch or (1/TPI) W= the wire size 11

12 Best Size Wire This wire is of such diameter that it makes contact with the flanks of the thread on the effective diameter or pitch line. Worked Example: Problem 13.1: Derive an expression for Best Size Wire Thank You 12