Ex. 19 Answer the questions:

1. When and where was Sir Joseph Whitworth born?

2. What can you say about Sir Joseph Whitworth’s education?

3. What do you know about Sir Joseph Whitworth’s inventions?

4. Why was Queen Victoria proud of Sir Joseph Whitworth?

5. What did Sir Joseph Whitworth design for army?

Ex. 20 Speak on the following problems:

1. Sir Joseph Whitworth’s career.

2. Sir Joseph Whitworth’s inventions.

3. Whitworth rifle.

VII. Oral Practice.

Ex. 21. Prove that Sir Joseph Whitworth is a famous inventor.

Supplementary reading

Texts for written translation.

Read the texts and translate them in writing. Use a dictionary.

Screw thread

A screw thread is a helical or tapered structure used to convert between rotational and linear movement or force.

A screw thread may be thought of as an inclined plane wrapped around a cylinder or cone. The tightening of a fastener's screw thread is comparable to driving a wedge into a gap until it sticks fast through friction and slight plastic deformation.

In most applications, the thread pitch of a screw is chosen so that friction is sufficient to prevent linear motion being converted to rotary, that is so the screw does not slip even when linear force is applied so long as no external rotational force is present. This characteristic is essential to the vast majority of its uses.

Internal and external threads illustrated using a common nut and bolt. The screw and nut pair can be used to convert torque into linear force. As the screw (or bolt) is rotated, the screw moves along its axis through the fixed nut, or the non-rotating nut moves along the lead-screw.

 

Screw thread, used to convert torque into the linear force in the flood gate. The operator rotates the two long vertical bolts (via bevel gear).

Screw threads have several applications:

• Fastening
• Fasteners such as wood screws, machine screws, nuts and bolts.
• Connecting threaded pipes and hoses to each other and to caps and fixtures.
• Gear reduction via worm drives
• Moving objects linearly by converting rotary motion to linear motion, as in a screw jack.
• Measuring by correlating linear motion to rotary motion (and simultaneously amplifying it), as in a micrometer.
• Both moving objects linearly and simultaneously measuring the movement, combining the two aforementioned functions, as in a leadscrew.

In all of these applications, the screw thread has two main functions:

• It converts rotary motion into linear motion.
• It prevents linear motion without the corresponding rotation.

Standard threads

Standards for machine screw threads have evolved since the early nineteenth century to facilitate compatibility between different manufacturers and users. Many of these standards also specified corresponding bolt head and nut sizes, to facilitate compatibility between spanners and other driving tools.

Nearly all threads are oriented so that a bolt or nut, seen from above, is tightened (the item turned moves away from the viewer) by turning it in a clockwise direction, and loosened (the item moves towards the viewer) by turning anticlockwise. This is known as a right-handed thread, since the natural screwing motion for a right-handed person is clockwise, and is the default because most people are right-handed. Threads oriented in the opposite direction are known as left-handed. There are also self-tapping screw threads where no nut is required.

Left-handed threads are used:

• Where the rotation of a shaft would cause a conventional right-handed nut to loosen rather than to tighten due to fretting induced precession, e.g. on a left-hand bicycle pedal.
• In combination with right-handed threads in turnbuckles.
• In some gas supply connections to prevent dangerous misconnections, for example in gas welding the flammable gas supply uses left-handed threads.
• In some instances, for example early ballpoint pens, to provide a "secret" method of disassembly.
• In some applications of a leadscrew, for example the cross slide of a lathe, where it is desirable for the cross slide to move away from the operator when the leadscrew is turned clockwise.

Unless stated otherwise, all standards below specify right-handed threads.

ISO standard threads

The most common threads in use are the ISO metric screw threads (M) and BSP threads also called G threads for pipes.

These were standardized by the International Organization for Standardization in 1947. Before that, there were separate metric thread standards used in France, Germany, and Japan, and the Swiss had a set of threads for watches.

 

 

Generating screw threads

 

Page 23 of Colvin FH, Stanley FA (eds) (1914): American Machinists' Handbook, 2nd ed. New York and London: McGraw-Hill. Summarizes screw thread rolling practice as of 1914.

There are various methods for generating screw threads. The method chosen for any one application is chosen based on constraints—time; money; degree of precision needed (or not needed); what equipment is already available; what equipment purchases could be justified based on resulting unit price of the threaded part (which depends on how many parts are planned); etc.

In general, certain thread-generating processes tend to fall along certain portions of the spectrum from toolroom-made parts to mass-produced parts, although there can be considerable overlap. For example, thread lapping following thread grinding would fall only on the extreme toolroom end of the spectrum, while thread rolling is a large and diverse area of practice that is used for everything from microlathe leadscrews (somewhat pricey and very precise) to the cheapest deck screws (very affordable and with precision to spare).

The various methods are summarized below.

Thread cutting

The excess material is cut away, with taps and dies for most smaller diameters, or with single-point thread-cutting on a lathe for larger ones (or smaller ones needing very high concentricity).

Thread rolling

The material is extruded into a male thread through mechanical pressure as the screw blank is rolled between a matched pair of flat dies. (See Cold forming.) Thread rolling is more common for high-volume production, and produces threads of diameters typically smaller than one inch. Also, materials with good deformation characteristics are better used with rolling; these materials include softer (more ductile) metals and exclude brittle materials, such as cast iron. A rolled thread can often be easily recognized because the thread has a larger diameter than the blank rod from which it has been made. (However, necks and shoulders can be cut or rolled to different diameters, so this in itself is not a forensic give-away.) Also, the end of the screw usually looks a bit different from the end of a cut-thread screw. Rolled male threads tend to be slightly stronger than cut male threads. Thread rolling is a very economical way of producing large quantities with good dimensional accuracy. The cost of thread rolling depends on the quantity; the more parts made, the cheaper the unit cost.

Thread forming

This is the female-thread analogue of the male-thread-rolling process described above. The material is extruded into a thread through mechanical pressure by a tap that is similar to a cutting tap except that it has no flutes. Instead of cutting, the tap squeezes the material out of its way. Formed female threads tend to be slightly stronger than cut female threads.

This process is more often employed in soft, ductile metals (such as aluminum) than in hard, brittle metals (such as cast iron).

Thread casting

The threads take the shape of whatever mold or die that the (liquid or gas) material is poured into. When the material freezes into a solid, it retains the shape. Material is either heated to a liquid (or rarely a gas), or mixed with a liquid that will either dry or cure (such as plaster or cement). Alternately, the material may be forced into a mould as a powder and compressed into a solid, as with graphite.

Cast threads in metal parts may be finished by machining, or may be left in the as-cast state. (The same can be said of cast gear teeth.) Whether or not to bother with the additional expense of a machining operation depends on the application. For parts where the extra precision and surface finish is not strictly necessary (although it might be nice), the machining is forgone in order to achieve a lower cost. With sand cast parts this means a rather rough finish; but with molded plastic or die-cast metal, the threads can be very nice indeed straight from the mold or die.

Thread grinding

Thread grinding is done on cylindrical grinders using specially dressed wheels matching the shape of the threads. Although expensive, threads produced by grinding are highly accurate and have a very fine surface finish with applications such as ball screw mechanisms used for precise movement of machine components.

Technically, thread grinding is a subset of thread cutting, as grinding is a true metalcutting process. Each grain of abrasive functions as a microscopic single-point cutting edge (although of high negative rake angle), and shears a tiny chip that is analogous to what would conventionally be called a "cut" chip (turning, milling, drilling, tapping, etc.). However, among people who work in the machining fields, the term cutting is understood to refer to the macroscopic cutting operations, and grinding is mentally categorized as a "separate" process. This is why the terms are usually used in contradistinction in shop-floor practice, even though technically grinding is a subset of cutting.

Thread lapping

Rarely, thread grinding will be followed by thread lapping in order to achieve the highest precision and surface finish achievable. This is an ultra-deluxe toolroom practice, rarely employed except for the leadscrews or ballscrews of high-end machine tools

 

Unit 7

Gears

I. Language

Ex.1. Remember the following words and word combinations:

 

gear   toothed wheel angular velocity ratio   driving shaft driven shaft rate   intersecting shaft non-coplanar curved helical gear     bevel gear eccentric gear   herringbone gear   screw gear   spur gear   worm gear plane incline converge revolution   strength speed width treat grind (ground, ground) accurate smooth   running cone   resemble   frequently body forward motion   connect external gearing   internal gearing   rack-and-pinion gearing   serve lathe right angle

зубчате колесо, приладнувати зубчате колесо коефіцієнт кутової швидкості ведучий вал відомий вал швидкість, рівень   перетинаючий вал некопланарний вигнутий, зігнутий вінтовe колесо (гелікоїдальне косозубе колесо) конічна передача ексцентрична зубчата передача шевронне зубчате колесо гвинтове зубчате колесо   циліндричне прямозубе колесо черв’ячне колесо площина нахиляти сходитися кругове обертання, оборот сила, міць швидкість ширина обробляти шліфувати точний рівний, гладенький, плавний робота конус   скидатися на   часто тіло, кістяк рух уперед   з’єднувати зовнішнє пристосування внутрішнє пристосування передача шестерної і зубчатою рейкою служити токарний верстат прямій кут

шестерня, зубчатое колесо, сцепляться зубчатое колесо коэффициент угловой скорости ведущий вал ведомый вал величина, скорость, степень пересекающий вал некопланарный кривой винтовое колесо (геликоидальное косозубое колесо) коническая передача эксцентрическая зубчатая передача шевронное зубчатое колесо винтовое зубчатое колесо цилиндрическое прямозубое колесо червячное колесо плоскость наклонять сходиться круговое вращение, оборот сила, прочность скорость ширина обрабатывать шлифовать, протирать точный ровный, плавный, гладкий работа, эксплуатация конус, деталь конической формы походить, иметь сходство часто тело, корпус поступательное движение соединять внешнее зацепление   внутреннее зацепление   передача шестерней и зубчатой рейкой служить токарный станок прямой угол