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Table 4-9.  Standard Drill Sizes
TM-9-254 Gearcase Transfer M548 M548A1 (
Figure 4-55.  Hand Tap Details
TM 9-254 4-27.  Hand Tap Nomenclature. a. Angle of Thread.  The Angle included between the flanks of the thread measured in an axial plane. b. Back Taper.  A slight axial relief on the thread of the tap which makes the pitch diameter of the thread near the shank somewhat smaller than that of the chamfered end. c. Basic.  The theoretical or nominal standard size from which all variations are made. d. Chamfer.  The tapering of the threads at the front end of each land of a tap by cutting away and relieving the crest of the first few teeth to distribute the cutting action over several teeth.  When the tapering amounts to 7 to 10 threads, the tap is called a "Taper" Tap; 3 to 5 threads, a "Plug" Tap; and I to 2 threads, a "Bottoming" Tap. e. Chamfer Relief.  The gradual decrease in land height from cutting edge to heel on the chamfered portion, to provide clearance for the cutting action as the tap advances f. Crest.    The  top  surface  joining  the  two  flanks  of  a  thread.    The  crest  of  an  external  thread  is  at  its  major  diameter, while the crest of an internal thread is at its minor diameter. g. Cutting Face.  The leading side of the land in the direction of rotation for cutting on which the chip impinges. h. Dryseal.    A  pipe  threaded  fuel  connection  for  both  external  and  internal  application  designed  for  use  where  the assembled product must withstand high fluid or gas pressures without the use of a sealing compound, or where a sealer is functionally objectionable. i. Flutes.    The  longitudinal  channels  formed  in  a  tap  to  create  cutting  edges  on  the  thread  profile  and  to  provide  chip spaces and cutting fluid passages. j. Height of Thread.  The distance between the crest and the base of a thread measured normal to the axis. k. Helical Flute.  A flute with uniform axial lead and constant helix in a helical path around the axis of a cylindrical tap. l. Hook Face.  A concave cutting face, usually specified either as Chordal Hook or Tangential Hook. m.    Chordal Hook Angle.  The angle between the chord passing through the root and crest of a thread form at the cutting face, and a radial line through the crest of the cutting edge. n. Tangential Hook Angle.  The angle between a line tangent to a hook cutting face at the cutting edge and a radial line to the same point o. Interrupted Thread.  A tap having an odd number of lands, with every other tooth along the thread helix removed. p. Lead.    The  distance  a  screw  thread  advances  axially  in  one  complete  turn.    On  a  single  lead  screw  or  tap,  the  lead and pitch are identical.  On a double lead screw or tap, the lead is twice the pitch, etc. q. Threads Per Inch.  The number of threads in one inch of length. r. Pitch.  The distance from any point on a screw or tap thread to a corresponding point on the next thread, measured parallel to the axis The pitch equals one divided by the number of threads per inch. s. Pitch Diameter.  On a straight thread, the diameter of an imaginary coaxial cylinder, the surface of which would pass through the thread profile at such points as to make equal the width of the threads and the width of the spaces cut by the surface of the cylinder.  On a Taper thread, the diameter at a given distance from a reference plane perpendicular to the axis of an imaginary co-axial cone, the surface of which would pass through the thread profile at such points as to make equal the width of the threads and the width of the spaces cut by the surface of the cone. 4-60  Change 6

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