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

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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

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Simple Effective Diameter

Best Wire Size