You are going to prepare a short talk with 3 members of your team of the most popular machine-tool type, milling machine. As the first step, discuss the answers to these questions and make notes.

1. What does the type of milling machine depend on?

2. While milling, what kinds of movements can be performed?

3.  There are two basic configurations of a milling machine: vertical and horizontal. What underlies this classification?

4. Also of note are two subcategories of vertical mills: a bed mill and a turret mill. What are the key features of them?

5. Are there advantages of horizontal mills over vertical ones?

 

Divide your notes into sections. Prepare the talk with your partners in a team. Divide up the tasks between all the team members. One way of doing this will be to have a different person to present each section of the talk.

Give your talk to the class and answer questions. These phrases will help you to communicate.

First of all

To begin with

Let’s look at

I like to move on to

My aim in this talk

Finally

I’d like

 

MACHINE SHOP ESSENTIALS

1. Read the text. Find evidence in that “Lathes and mills are complimentary machines”

 

Machine shop essentials

There are between 15 and 36 milling machine designs or styles, depending on who is counting, but the focus of this study is the Bridgeport-style vertical knee mill because they are most often used in shops doing prototyping and R&D work. They outnumber all other designs combined. This design has so much to offer that it has been copied in every industrialized country. At one time there were no less than thirteen separate Spanish companies building Bridgeport-style mills. A working knowledge of a Bridgeport-style vertical milling machine also provide? a good start for operating any other style milling machine. Lathes and mills are complementary machines. While lathes rotate the workpiece and produce a cylindrical cut, milling machines move work into a rotating cutter and make a straight line cut. Lathes and mills are both capable of boring large-diameter holes, but mills are better at placing holes anywhere on the surface of the work. Although one can sometimes make do with just a lathe or mill, a well-equipped shop must have both machines. The lathe cutting tool is in continuous contact with the work and so makes a continuous cut. Milling machines are just the opposite. They use multi-tooth cutting tools and their cutting action is intermittent as each tooth takes a bite. Metal is removed in small individual chips. Unlike lathe cutting tools, end mills, the most common cutting tool for Bridgeport-style mills, cannot be sharpened freehand because they must be perfectly symmetrical. Sharpening them requires special fixtures and shaped grinding wheels. Smaller shops send their cutters out for sharpening. Adding a digital readout (DRO) is a great convenience to any milling machine. It reduces the need to repeatedly stop the mill to make measurements and lowers the chance of errors. For production applications, there are large, expensive milling machines with three or more axes under computer control. Some machines perform all operations including automatic tool changing. However, today there is an intermediate step between a manual mill and a fully automated one.

 

Make a list of words to describe each of the types of the machine tools.

Pair off and discuss strengths and weakness of both configurations of machine-tools: lathes and mills. For example one section of the talk could be. “Mill orientation is the primary classification for milling machines”.

 

Unit 15

CHEMICAL ROCKETS

Read through the text. Make a list of words pertaining to the features of rockets.

Chemical Rockets

Chemical rockets are unique in that the energy required to accelerate the propellant comes from the propellant itself, and in this sense, are considered energy limited. Thus, the attainable kinetic energy per unit mass of propellant is limited primarily by the energy released in chemical reaction; the attainment of high exhaust velocity requires the use of high-energy propellant combinations that produce low molecular weight exhaust products. Currently, propellants with the best combinations of high energy content and low molecular weight seem capable of producing specific impulses in the range of 400 to 500 seconds or exhaust velocities of 13,000 to 14,500 ft/sec. Chemical rockets may use liquid or solid propellants or, in some schemes, combinations of both. Liquid rockets may use one (monopropellant), two (bipropellant) or more propellants. Bipropellants consist of a combination of a fuel (kerosene, alcohol, hydrogen) and an oxidizer (oxygen, nitric acid, fluorine). The liquids are held in tanks and fed into the combustion chamber where they react and then expand through the nozzle. In contrast, solid propellants are an intimate mixture containing all the material necessary for reaction. The entire block of solid propellant, called the grain, is stored within the combustion chamber. Combustion proceeds from the surface of the propellant. A chemical rocket engine is little more than a gas generator. The rapid combination (combustion) of certain chemicals results in the release of energy and large volumes of gaseous products. The gas molecules generated have considerable energy in the form of heat. In ordinary chemical rocket engines, the temperature of the resulting gases can rise higher than 5,500 degrees Fahrenheit. For chemical systems in general, liquid propellants provide higher specific impulses than solid propellants. We call liquid Hydrogen and liquid Oxygen high energy propellants because of the large energy release during combustion and the high transfer of thermal energy into directed kinetic energy of the exhaust stream.. The total impulse of a rocket is the product of thrust and the effective firing duration. A typical shoulder launched short-range rocket may have an average thrust of 660 pounds for an effective duration of 0.2 seconds.