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CONTENTS
No Content Page
1.0 Title 2
2.0 Objective 2
3.0 Introduction 2
4.0 Apparatus 6
4.1 Industrial Apparatus 7
5.0 Experimental Procedure 8
6.0 Results and Data Analysis 9
7.0 Discussion of Results 12
8.0 Conclusion 14
9.0 Recommendation 15
10.0 References 16
1.0 TITLE : SCREW THREAD MEASUREMENT
2.0 OBJECTIVE
To measure the Major Diameter, Minor Diameter and the Effect Diameter of a Screw Plug
Gauge using Floating Carriage Measuring Machine.
3.0 INTRODUCTION
Floating Carriage Diameter is also commonly known as Effective Diameter Measuring
Micrometer. It use Bench micrometer to hold the workpiece as they look like they are
floating. This instrument is used for accurate measurement of Screw Plug Gauges
dimension such as Major Diameter, Minor Diameter and Effective Diameter. All these
dimensions such as Major Diameter, Minor Diameter and Effective Diameter. All these
dimensions have an important role in the screw plug gauges, since the accurancy and
interchangeability of the component depends on the gauges used. To minimize the effect
of slight errors in the micrometer screws and measuring faces, this micrometer is
fundamentally used as comparator.
Figure 2 : Thread Terminology
2
a) Major Diameter : This is the diameter of an imaginary cylinder, co-axial with the screw,
which just touches the crests of an external thread or roots of internal threads. Also
known as Nominal diameter.
Referring to the Figure 3.0
If the micrometer reading over Standard Cylinder = R1
The micrometer reading over Screw Pug Gauge = R2
The diameter of Standard Cylinder = D1
Therefore, Major Diameter of Screw Plug Gauge, Dmajor= D1+(R1+R2)
Figure 3.0 : Screw Terminology
b) Minor Dimension : This is the diameter of an imaginary cylinder, co-axial with screw
which just touches the roots of an external thread. Also referred as root or core
diameter.
Referring to the figure 3.1
If the micometer reading over Standard Cylinder and Core Prisms = R3
The micrometer reading over Screw Pug Gauge and Core Prisms = R4
The diameter of Standard Cylinder = D1
Therefore, Minor Diameter of Screw Plug Gauge, Dminor= D1+(R3+R4)
Figure 3.1
3
c) Effective diameter : Also known as Pitch Diameter. It is the Diameter of imaginary
Cylinder,co-axial with the axis of the thread and intersects the flank of the thread and
intersects the flanks of the thread such that width of the threads and width of spaces
between threads are equal.
Referring to the Figure 3
If the micometer reading over Standard Cylinder and Wires = R5
The micrometer reading over Screw Pug Gauge and Wires = R6
The diameter of Standard Cylinder = D1
Therefore, the Diameter with the wire D3= D1+(R5+R6)
Hence, the Effective Diameter, Deff = D3+P where P is the difference between Effective
Diameter and the diameter under the wires as shown in Figure 4
The P calue is a constant depending on the diameter of the wires (d), the pitch (p) and the
type of screw thread, it is found as follows:
Metric Threads (60°) P = 0.86602p –d
Whitworth threads (55°) P = 0.9604p – 1.16568d
British Association Threads (47.5°) P = 1.13634p – 1.43295d
Note : Pitch, p given is 3.5mm
4
4.0 APPARATUS
1. A Floating Carriage Diameter Measuring Machine
2. A Set of Core Prism
3. A Set of Calibrated Screw Thread Measuring Wires (Wire Cylinders)