GUIDES
FUNDAMENTALS OF
DRILLING & TAPPING
ON SALE NOW
FUNDAMENTALS
OF THREADING ON A LATHE
ON SALE NOW
FUNDAMENTALS OF
WORKING BETWEEN CENTERS
ON SALE NOW
FUNDAMENTALS
OF MILLING
COMING SOON
FUNDAMENTALS
OF SAWING
COMING SOON
METALWORKING
SHOP TERMS
COMING SOON
SCREW THREAD TERMS
Cutting screw threads is among the most common of lathe operations. Before attempting such work, however, the operator should have some familiarity with the fundamental principles of threads and the types in general use in machine shops. Such familiarity will make it comparatively east to understand the processes and calculations involved. Throughout this and succeeding paragraphs, the following terms and definitions will be used in discussing threads and thread cutting:
Screw thread: A helical ridge of uniform section on the external or internal surface of a cylinder or cone.External or male thread: A thread on the outside of a member.
Internal or female thread: A thread on the inside of a member.
Major diameter: The largest diameter of a screw thread.
Minor diameter: The smallest diameter of screw thread.
Pitch diameter: The diameter of an imaginary cylinder, the surface of which would pass through the threads at such points to equalize the width of the threads and the width of the spaces cut by the surface of the cylinder.
Pitch: The distance from a point on a screw thread to a corresponding point on the next thread, measured parallel to the axis. Pitch in inches equals 1 divided by the number of threads per inch (N).
Lead: The distance a screw thread advances axially in one turn. On a single-thread screw, the lead and pitch are identical; on a double-thread screw, the lead is twice the pitch; on a triple-thread screw, the lead is three times the pitch, etc.
Fig. 1 American national screw thread forms and parts.
Thread angle: The angle included between the sides of adjacent threads, measured in an axial plane It equals 60o in fig 1.
Helix angle. The angle formed by the helix of the thread at its pitch diameter with a plane perpendicular to the axis.
Crest: commonly called "flat": The top surface joining the two sides of a thread.
Root: The bottom surface joining the sides of two adjacent threads.
Side: The surface of a thread which connects the crest and the root.
Axis of a screw: The center line through a screw.
Depth of thread: The distance between the crest and root of a thread, measured perpendicular to the axis.
Note: In connection with the foregoing definitions study fig. 1 carefully to become familiar with the various parts of screw threads.