Helix Is the spiral grooves cut into the surface of cylinders. This is the same form made – by wrapping a copper wire around a cylinder to form a spring. External Refers to threads found on a bolt, cap screw, wood screw, etc. – Internal Refers to threads found on a nut or tapped inside a part. – Major Diameter Is the largest diameter of a thread. Measured from crest to crest. – Diameter Is the smallest diameter of a thread. – Is the shallowest thread cut. It can be rounded or flat. – Root- Is the deepest thread cut. It can be rounded or flat. Pitch The pitch of a thread is the distance from one point on the thread to the – corresponding point on the next form. The pitch of a thread is usually expressed in tables in terms of the number of threads per inch. Lead Is the distance a thread moves in one revolution. – Single Threads Has a lead equal to its pitch – Multiple Threads Has a lead to more than one pitch. Multiple threads permit rapid – advancement of parts. For example, a double thread will advance twice as far as a single thread in one revolution. Class of Fit There is four standardized classes of fit. The term fit refers to how closely – the screw fits in the threaded hole. That is the amount of play between the two parts. Class 1: Loose fit, used for rough work. Class 2: Free fit, general-purpose use of most bolts and nuts. Class 3: Medium fit, used for the better grades of work, such in automobiles. Class 4: Close fit, used where a very snug fit is required, as in aircraft engines. Thread Depth Refers to the distance from the top of the thread (crest), to the bottom of – the thread (root). Thread Angle The angle formed by the walls of the thread. The angle created by the – slope of the thread is a standard 60 degrees. Right-Handed (RH) Advances into a nut when turned clockwise. All threads are – understood to be RH unless designated specifically LH in the thread note. (LH) Advances into a nut when turned counterclockwise. Left-Handed – Figure 1. Screw Thread Terms. Thread-Cutting Tools: R1 (378), R3 (309, 316-319) Smaller threads may be cut by hand and larger threads by a lathe.
Helix – Is the spiral grooves cut into the surface of cylinders. This is the same form made by wrapping a copper wire around a cylinder to form a spring. External – Refers to threads found on a bolt, cap screw, wood screw, etc. Internal – Refers to threads found on a nut or tapped inside a part. Major Diameter – Is the largest diameter of a thread. Measured from crest to crest. Diameter – Is the smallest diameter of a thread. – Is the shallowest thread cut. It can be rounded or flat. Root- Is the deepest thread cut. It can be rounded or flat. Pitch – The pitch of a thread is the distance from one point on the thread to the corresponding point on the next form. The pitch of a thread is usually expressed in tables in terms of the number of threads per inch. Lead – Is the distance a thread moves in one revolution. Single Threads – Has a lead equal to its pitch Multiple Threads – Has a lead to more than one pitch. Multiple threads permit rapid advancement of parts. For example, a double thread will advance twice as far as a single thread in one revolution. Class of Fit – There is four standardized classes of fit. The term fit refers to how closely the screw fits in the threaded hole. That is the amount of play between the two parts. Class 1: Loose fit, used for rough work. Class 2: Free fit, general-purpose use of most bolts and nuts. Class 3: Medium fit, used for the better grades of work, such in automobiles. Class 4: Close fit, used where a very snug fit is required, as in aircraft engines. Thread Depth – Refers to the distance from the top of the thread (crest), to the bottom of the thread (root). Thread Angle – The angle formed by the walls of the thread. The angle created by the slope of the thread is a standard 60 degrees. Right-Handed (RH) – Advances into a nut when turned clockwise. All threads are understood to be RH unless designated specifically LH in the thread note. Left-Handed (LH) – Advances into a nut when turned counterclockwise. Figure 1. Screw Thread Terms. Thread-Cutting Tools: R1 (378), R3 (309, 316-319) Smaller threads may be cut by hand and larger threads by a lathe.