Threads and Fasteners Thread Symbols Thread Symbols Screw Thread Terms External Thread A thread on the outside of a member as on a shaft Internal Thread A thread on the inside of a member as in a hole Major Diameter The largest diameter of a screw thread Minor Diameter The smallest diameter of a screw thread Pitch The distance from a point on a screw thread to a corresponding point on the next thread measured parallel to the axis The pitch P is equal to 1 divided by the number of threads per inch Thread Symbols Screw Thread Terms Lead The distance a screw thread advances axially in one turn or revolution Crest The top surface joining the sides of two adjacent threads Root The bottom surface joining the sides of two adjacent threads Depth of Thread The distance between the crest and the root of the thread measured normal to the axis Form of Thread The cross section of thread cut by a plane containing the axis Thread Symbols Screw Thread Forms Various forms of threads are used to hold parts together to adjust parts with reference to each other or to transmit power Thread Symbols Right hand and left hand threads A right hand thread is one that advances into a nut when turned clockwise figure a A left hand thread is on the advances into a nut when turned counterclockwise figure b A thread is always considered to be right handed unless specified otherwise A left handed thread is always labeled LH on a drawing Thread Symbols Single and Multiple Threads A single thread is composed of one ridge and the lead is equal to the pitch figure a A double thread is composed of two ridges and the lead is equal to twice the pitch figure b A triple thread is composed to three ridges and the lead is equal three times the pitch figure c Thread Symbols External Thread Symbols Simplified external thread symbols are shown in figure a and figure b The threaded portions are indicated by hidden lines parallel to the axis at the approximate depth of the thread whether in section or elevation Schematic external thread symbols are shown in figure c and figure d Schematic threads in elevation figure d are indicated by alternate long and short lines at right angles to the center line When the schematic form is shown in section figure c it is necessary to show the Vs It is not necessary to show the Vs to scale To draw the Vs use the schematic thread depth and let the pitch be determined by the 60 degree Vs Thread Symbols Internal Thread Symbols Internal simplified and schematic thread symbols are shown below Note that the only difference between the schematic and simplified internal thread symbols is in the section views Thread Symbols Drawing Simplified Threads Step 1 Draw the major diameter and locate the thread length of full threads Step 2 Draw the minor diameter by marking off the thread depth D from the table Step 3 From the minor diameter draw the 45 degree chamfered ends Thread Symbols Drawing Schematic Threads Step 1 Draw the major diameter and locate the thread length of full threads Step 2 Locate the minor diameter D from the table and draw the 45 degree chamfered ends Step 3 Draw lines to represent the crest of the threads spaced at a distance equal to P from the table Step 4 Draw lines to represent the root of the threads centered between the crest lines to the minor diameter D Thread Symbols Detailed Representation Metric Unified and American National Threads The representation for metric Unified and American National Threads is the same since the flats are disregarded The steps in drawing these threads are shown below Thread Symbols Thread Notes The thread note for Unified and American National screw threads is shown in figure a The thread note for metric screw threads is shown in figure b Metric screw threads are designated by the letter M metric profile followed by the nominal size major diameter and the pitch both in millimeters Thread Symbols Thread Notes Thread Symbols Tapped Holes The bottom of a drilled hole is conical in shape formed by the point of the twist drill figure a and figure b When an ordinary drill is used in connection with tapping it is referred to as a tap drill On drawings an angle of 30 degrees is used to approximate the actual 31 degrees The portion of the drill depth shown beyond the threads includes several imperfect threads produced by the chamfered end of the tap For drawing purposes the distance A may be drawn equal to three schematic thread pitches figure c and figure d Thread Symbols Since the tapped thread length contains only full threads it is necessary to make this length onlly one or two pitches beyond the end of the engaging fastener In simplified and schematic thread representation the threads are omitted in the bottoms of tapped holes to show the ends of the fasteners clearly