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IPE 381Chapter:13

Measurement of Screw Thread

Abdullah-Al-Mamun

Lecturer, Dept. of IPE

Outline

• Common terms

• Pitch Errors

• Errors in Measurement

• Effective Dia Measurement

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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 ThreadA 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

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Screw Thread

Screw Thread

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Lead & Pitch

LEAD = 2PLEAD = 1P

LEAD = 3P

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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 interferencewith 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

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• 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.

Pitch Errors in Screw Threads

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.

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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 δθ1

and δθ2 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.

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B

CA

θ

• For Flank Error δθ1 and δθ2

the Correction factor = CP(δθ1 + δθ2 )

• For Whithworth Thread, C=0.0105 and θ=55⁰ so, cot(55⁰/2)=1.921

• So,

Virtual Effective Diameter = Simple Effective Diameter + 1.921δP + 0.0105P (δθ1 + δθ2 )

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Measurement of Major Dia

bench micrometer is used to measure Major Diameter

Dc=Calibrated diameter of the setting cylinderRc= Micrometer reading on setting cylinderRt =Micrometer reading on thread

Then, Major Diameter = Dc+(Rt-Rc)

Effective Dia Measurement

(i) Thread micrometer method.

(ii) One wire and two wire method.

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Two Wire Method

From the Fig-14, Ed=T+2x

In the ∆ABC , AB= BC cotθ

But, BC=¼ pitch=¼ P

Therefore, AB= ¼ P cotθ

In the ∆ADE ,

Now, x =AB-AF and AF =AE – EF =AE - d/2

Therefore,

where,

P= Nominal Pitch; D=Wire Diameter; θ= Flank Angle

θθ ecd

ecDEAE cos2

cos ==

)1(cos2

−=∴ θecd

AF

)1(cos2

cot4

−−= θθ ecdP

x

BC

A

E

D

A

Two Wire Method

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Matrix Pitch Measuring Machine

Three Wire MethodThe wires are placed in the threads and a micrometer is used to measure over the top of the wires.

M = E - 0.86603P + 3W

M = the measurement over the wires E = the pitch diameter P = the pitch or (1/TPI) W= the wire size

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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