Machinery's Handbook, 31st Edition
Taps 981 Pitch Diameter (Simple Effective Diameter): On a straight thread, the pitch diameter is the diameter of the imaginary coaxial cylinder, the surface of which would pass through the thread profiles at such points as to make the width of the groove equal to one-half the basic pitch. On a perfect thread, this coincidence occurs at the point where the widths of the thread and groove are equal. On a taper thread, the pitch diameter at a given position on the thread axis is the diameter of the pitch cone at that position. Point Diameter: Diameter at the cutting edge of the leading end of the chamfered section. Plug Tap: A tap having a chamfer length of 3 to 5 pitches. Rake: Angular relationship of the straight cutting face of a tooth with respect to a radial line through the crest of the tooth at the cutting edge. Positive rake means that the crest of the cutting face is angularly ahead of the balance of the cutting face of the tooth. Negative rake means that the crest of the cutting face is angularly behind the balance of the cutting face of the tooth. Zero rake means that the cutting face is directly on a radial line. Relief: Removal of metal behind the cutting edge to provide clearance between the part being threaded and the threaded land. Relief, Center: Clearance produced on a portion of the tap land by reducing the diameter of the entire thread form between cutting edge and heel. Relief, Chamfer: Gradual decrease in land height from cutting edge to heel on the cham fered portion of the land on a tap to provide radial clearance for the cutting edge. Relief, Con-Eccentric Thread: Radial relief in the thread form starting back of a concen tric margin. Relief, Double Eccentric Thread: Combination of a slight radial relief in the thread form starting at the cutting edge and continuing for a portion of the land width, and a greater radial relief for the balance of the land. Relief, Eccentric Thread: Radial relief in the thread form starting at the cutting edge and continuing to the heel. Relief, Flatted Land: Clearance produced on a portion of the tap land by truncating the thread between cutting edge and heel. Relief, Grooved Land: Clearance produced on a tap land by forming a longitudinal groove in the center of the land. Relief, Radial: Clearance produced by removal of metal from behind the cutting edge. Taps should have the chamfer relieved and should have back taper, but may or may not have relief in the angle and on the major diameter of the threads. When the thread angle is relieved, starting at the cutting edge and continuing to the heel, the tap is said to have “eccentric” relief. If the thread angle is relieved back of a concentric margin (usually one- third of land width), the tap is said to have “con-eccentric” relief. Right Hand Cut: Rotation in clockwise direction for cutting when viewed from the chamfered end of a tap or die. Roots: The surface of the thread that joins the flanks of adjacent thread forms and is iden tical to cone from which the thread projects. Screw Thread: A uniform section produced by forming a groove in the form of helix on the external or the internal surface of a cylinder. Screw Thread Inserts (STI): Screw thread bushing coiled from diamond-shape cross- section wire. They are screwed into oversized tapped holes to size nominal-size internal threads. Screw Thread Insert (STI) Taps: These taps are over the nominal size to the extent that the internal thread they produce will accommodate a helical coil screw insert, which at final assembly will accept a screw thread of the nominal size and pitch.
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