Tag Archives: Helix Angle

DEFINITIONS AND TERMS in the ROLLED THREAD WORLD

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As you’ve come to expect, a good roll threading process produces a uniform, precise surface without tears, chatters or cutter marks.

How is it done? In roll threading, steel is extruded to form the threaded portion, instead of being removed as in done in cut threading. The product is fed or “rolled” through threading dies to form the threads. To help make this process even more clear, here are some roll threading terms and their definitions.

Class of Thread: alphanumerical designation indicating the standard grade of tolerance and allowance specified.

Crest: top surface joining two sides of thread.

Depth of Thread Engagement: radial distance, crest to crest, by which thread forms overlap between two assembled mating threads.

Helix Angle: The angle made by conical spiral, or helix.

Major Diameter: distance across the crests of thread of the major cylinder.

Minor Diameter: root diameter of thread of minor cylinder.

Nominal Size: designation for general identification based on the major diameter.

Pitch: axial distance from a point on one screw thread to the corresponding point on the next screw thread. Pitch is equal to the lead divided by number of thread starts.

Pitch Diameter: On a straight screw thread, diameter of an imaginary cylinder surface that passes through the threads at such points to make thread width and the width of the spaces cut by the surface of the cylinder equal. On a taper screw thread, diameter at a given distance from a reference plane perpendicular to the axis of an imaginary cone, the surface of which would pass through the threads at such points as to make thread width and the width of the spaces cut by the surface of the cone equal.

Profile of Thread: contour of a screw thread ridge and groove delineated by a cutting plane passing through thread axis. (Also called form of thread)

Root: bottom surface joining two sides of thread.

Root Diameter: diameter of an imaginary cylinder bounding the bottom of the roots of a screw thread. (minor diameter of thread)

Thread Series: Groups of diameter/pitch combinations distinguished from each other by the number of threads per unit of measurement.