SCREW THREADS

Draftsmen use different methods to show thread on drawings. Figures 4-8 through 4-11 show several of

Figure 4-7.-A keyseat and keyway.

Figure 4-8.-Simplified method of thread representation.

Figure 4-9.-Schematic method of thread representation.

Figure 4-10-Detailed method of thread representation.

Figure 4-11.-Tapered pipe thread representation.

them. Now look at figure 4-12. The left side shows a thread profile in section and the right side shows a common method of drawing threads. To save time, the draftsman uses symbols that are not drawn to scale. The drawing shows the dimensions of the threaded part but other information may be placed in "notes" almost any place on the drawing but most often in the upper left corner. However, in our example the note is directly above the drawing and shows the thread designator "1/4-20 UNC-2."

The first number of the note, 1/4, is the nominal size which is the outside diameter. The number after the first dash, 20, means there are 20 threads per inch The letters UNC identify the thread series as Unified National Coarse. The last number, 2, identifies the class of thread and tolerance, commonly called the fit. If it is a left-hand thread, a dash and the letters LH will follow the class of thread. Threads without the LH are right-hand threads.

Specifications necessary for the manufacture of screws include thread diameter, number of threads per inch, thread series, and class of thread The two most widely used screw-thread series are (1) Unified or

Figure 4-12.-Outside threads.

National Form Threads, which are called National Coarse, or NC, and (2) National Fine, or NF threads. The NF threads have more threads per inch of screw length than the NC.

Classes of threads are distinguished from each other by the amount of tolerance and/or allowance specified. Classes of thread were formerly known as class of fit, a term that will probably remain in use for many years. The new term, class of thread, was established by the National Bureau of Standards in the Screw-Thread Standards for Federal Services, Handbook H-28.

Figure 4-13 shows the terminology used to describe screw threads. Each of the terms is explained in the following list:

HELIX-The curve formed on any cylinder by a straight line in a plane that is wrapped around the cylinder with a forward progression.

EXTERNAL THREAD-A thread on the outside of a member. An example is the thread of a bolt.

INTERNAL THREAD-A thread on the inside of a member. An example is the thread inside a nut.

MAJOR DIAMETER-The largest diameter of an external or internal thread

AXIS-The center line running lengthwise through a screw.

CREST-The surface of the thread corresponding to the major diameter of an external thread and the minor diameter of an internal thread.

Figure 4-13.-Screw thread terminology.

ROOT-The surface of the thread corresponding to the minor diameter of an external thread and the major diameter of an internal thread

DEPTH-The distance from the root of a thread to the crest, measured perpendicularly to the axis.

PITCH-The distance from a point on a screw thread to a corresponding point on the next thread, measured parallel to the axis.

LEAD-The distance a screw thread advances on one turn, measured parallel to the axis. On a single-thread screw the lead and the pitch are identical; on a double-thread screw the lead is twice the pitch; on a triple-thread screw the lead is three times the pitch GEARS

When gears are drawn on machine drawings, the draftsman usually draws only enough gear teeth to identify the necessary dimensions. Figure 4-14 shows gear nomenclature, and the terms in the figure are explained in the following list:

PITCH DIAMETER (PD)-The diameter of the pitch circle (or line), which equals the number of teeth on the gear divided by the diametral pitch

DIAMETRAL PITCH (DP)-The number of teeth to each inch of the pitch diameter or the number of teeth on the gear divided by the pitch diameter. Diametral pitch is usually referred to as simply PITCH.

NUMBER OF TEETH (N)-The diametral pitch multiplied by the diameter of the pitch circle (DP x PD).

 ADDENDUM CIRCLE (AC)-The circle over the tops of the teeth.

OUTSIDE DIAMETER (OD)-The diameter of the addendum circle.

Figure 4-14.-Gear nomenclature.

CIRCULAR PITCH (CP)-The length of the arc of the pitch circle between the centers or corresponding points of adjacent teeth.

ADDENDUM (A)-The height of the tooth above the pitch circle or the radial distance between the pitch circle and the top of the tooth.

DEDENDUM (D)-The length of the portion of the tooth from the pitch circle to the base of the tooth.

CHORDAL PITCH-The distance from center to center of teeth measured along a straight line or chord of the pitch circle.

ROOT DIAMETER (RD)-The diameter of the circle at the root of the teeth.

CLEARANCE (C)-The distance between the bottom of a tooth and the top of a mating tooth.

WHOLE DEPTH (WD)-The distance from the top of the tooth to the bottom, including the clearance.

FACE-The working surface of the tooth over the pitch line.

THICKNESS-The width of the tooth, taken as a chord of the pitch circle.

PITCH CIRCLE-The circle having the pitch diameter.

WORKING DEPTH-The greatest depth to which a tooth of one gear extends into the tooth space of another gear.

RACK TEETH-A rack may be compared to a spur gear that has been straightened out. The linear pitch of the rack teeth must equal the circular pitch of the mating gear.

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