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screw thread measurement
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Measurement of Screw Threads
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
Bench Micrometer – Major Diameter
Measurement of Minor Diameter
Use of prisms to measure minor diameter
Measurement of thread form
Screw Thread Projection
Screw thread projection
Projection normal to thread axis causes interference
Turning thread through helix angle avoids interference but foreshortens pitch and distorts profile
Microscopic flank angle measurement The flank angle is defined as the angle made between the straight
portion of the thread flank and a line normal to the thread axis.
Effect of Flank Angle Errors
Nut of perfect form mating with a screw having a flank angle error on one flank only.
Fouling with a screw having a flank angle error
)(2sin 21
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Pitch errors in screw threads
Progressive pitch error Periodic pitch error
Measurement of Pitch Error
Fiducial indicator used on pitch-measuring machine
Pitch Measuring Machine
Effects of Pitch Errors
anpEd cot
Simple Effective Diameter
Best Wire Size
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