Machinery's Handbook, 31st Edition
Taps 979 Interrupted Thread Tap: A tap having an odd number of lands with alternate teeth in the thread helix removed. In some designs, alternate teeth are removed only for a portion of the thread length. Land: One of the threaded sections between the flutes of a tap. Lead: Distance a screw thread advances axially in one complete turn. Lead Error: Deviation from prescribed limits. Lead Deviation: Deviation from the basic nominal lead. Progressive Lead Deviation: (1) On a straight thread, the deviation from a true helix where the thread helix advances uniformly. (2) On a taper thread, the deviation from a true spiral where the thread spiral advances uniformly. Tap Terms
Pitch
Tap Crest Basic Crest
Basic Height of Thread
Angle of Thread
Max. Tap Major Dia.
Min. Tap Major Dia.
Basic Major Dia.
Flank
Basic Minor Dia.
Base of Thread
Basic Pitch Dia.
Basic Root
Relieved to Cutting Edge
No Relief
Eccentric Relief Concentric Margin
Cutting Edge Cutting Face
Heel
Eccentric
Con-Eccentric Relief
Concentric
Land
Negative Rake Angle Negative Rake
Zero Rake
Positive Rake Angle
Radial
Positive Rake
Positive Hook Negative Hook Fig. 3. Tap Terms Left Hand Cut: Rotation in a clockwise direction from cutting when viewed from the chamfered end of a tap. Length of Engagement: The length of engagement of two mating threads is the axial dis tance over which two mating threads are designed to contact. 0 Deg. Hook Length of Thread: The length of the thread of the tap includes the chamfered threads and the full threads, but does not include an external center. It is indicated by the letter “B” in the illustrations at the heads of the tables. Limits: The limits of size are the applicable maximum and minimum sizes. Major Diameter: On a straight thread, the major diameter is that of the major cylinder. On a taper thread, the major diameter at a given position on the thread axis is that of the major cone at that position.
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