Introduction The complete aero‑engine and the airframe consists of many parts which, unless bonded together.by one of the metal joining processes or held solely by their fit, are held together by various types of fastening devices. Bolts, nuts, studs, rivets, taper pins, cowling fasteners, ball and fork end joints, etc., are typical examples of such fasteners. Many variations of these devices are used in general engineering, but for use on aircraft they are made to rigid specification and their sizes and design have been standardized as far as possible. Screw thread is described here separately as it forms the basis of number of securing devices

Screw ThreadsA screw thread is a groove of uniform cross section, which followsThe path of a helix on the surface of a cylinder or cone. Male (external) screw thread is formed by cutting a helical groove into the surface of a round bar; a Female (Internal) thread is formed by cutting a helical groove in a cylindrical hole. When the external thread is cut into the normal diameter or the rod, it is termed a 'minus' thread. Where it is important to avoid reducing the strength of the round bar, the end to be screwed is formed to a larger diameter and the thread stands above a surface and is termed a 'plus' thread

Screw Nomenclature

THREAD FORMS

The four common types of thread form are the Vee, Square, Acme, and the Buttress. 1.       The Vee Thread gives the greatest strength for common usage, such as bolts, and is easily produced by taps and dies2.       The square thread stands up well to constant wear and heavy loads ,such as met with screw jacks. This type of thread is readily cut on a lathe.3. The Acme Thread is for use on the lead screws of lathes to facilitate the opening and closing of a split nut4. The Buttress Thread is used where the thrust is required in one direction only, as with the bench vice screw. 

The Vee Thread

The square thread

The Acme Thread

The Buttress Thread

            

 

Types of threads 1. British Standard Whitworth

It is a symmetrical ‘V' thread of 55o included angle ie. thread angle 055o). With an equal radius at the root and crest of 0.1373 x pitch.

2. British Standard Fine(B.S.F.)The thread form is identical to BSW but differs in the TPIBSW has 24 TPI whereas BSF has 32 TPI

3 British Association (B.A.) Similar form to BSW and BSF but has thread angle of 47 1/2o

4. American National Threads (ANC , ANF)The American Thread is another 60 o angle thread which is truncated. There are 2 standard series; the course (A.N.C.) and Fine (.A.N.F.).

5. Unified Thread ( UNC & UNF) The unified system or screw threads, introduced by Canada, USA and UK was provided to give a common standard between the three countries.As per the American thread with a thread angle of 60o

6. Metric Thread In MM . The thread angle is 60 o and is truncated

Screw threads are identified by their profile. Some example are: -

3. Whitworth Thread

2. Metric Fine Thread

1. Metric Thread

4. Trapezoidal Thread

Screw Nomenclature

6. Round Thread

5. Buttress Thread

Effective Diameter Tolerance

This is derived from a three-part formula, which takes account of diameter, pitch and length of engagement

 

Major Diameter Tolerance

With external threads the tolerance on major diameter is derived solely from a formula based on pitch.

 

Minor Diameter Tolerance

The minor diameter tolerance on external threads is related directly to the effective diameter tolerance.

Animation Presentation

Thread Forms, Dimensions and Tolerances for Standard Threads Used in Aircraft

Measuring Screw Threads

E .g BSW

PITCH = 1 NO. OF THREADS PER INCH

H = 0.9601 X Ph = 0.6403 X PR = 0.1373 X PH/6 = 0.1601 X P

Screw Pitch Gauge This type of gauge is used to check the profile or to determine the number of threads per inch of a given thread. The gauge consists of a number of blades,each of of which is cut with a given number of teeth per inch, the teeth being accurately shaped to the profile a standard screw thread. Screw thread gauges are available for testing B.A. , Whitworth, B.S.F. , Unified threads, etc., and each is etched with the type of thread and the number of threads per inch. Before gauging, the threads of the component to be checked must first be cleaned. The component is then held up to the light and the gauge placed in position. If the thread is accurately cut, no light should be visible between the gauge and the component threads. If the thread of the component 'out‑of‑pitch', the gauge will not fit properly on the threads

Screw Pitch Gauge

 Objective: -

At the end of this lesson the student will be able to define self-tapping screws and dowels.

 

Self Tapping Screws, Dowels

Self Tapping Screws, Dowels

Self-tapping screws are used to secure thin gauge sheet metal

where nuts and bolts are impracticable.

They are screwed into a hole of the correct diameter and form their

own thread.

The screw head may be slotted or cruciform;

four types are in common used.

Examples of self-tapping screws are Parker Kalon (P-K),

Barber and Calon (B-K).