Lesson 2 Tolerances and Fits
Big Mouth
Directions: Read the following words. Mark the one whose underlined part has NOT the same pronunciation as the given phonetic symbol.
Big Ears
What Did He Find?
Directions: Decide whether the following statements are true or false according to what you have heard.
1.Newton returned to the family farm to understand the world and the nature better.
2.He used the methods he had learned in Cambridge University to do experiments.
3.He created a new method known as integral calculus to measure areas curved in shape.
4.He believed that gravity could be measured.
5.He discovered the exact relationship between speed and gravity.
6.He measured the mass of the Earth and the Moon, and the distance between them.
Light and Colors
Directions: Choose the best answer according to what you have heard.
1.What did Newton use to study light and colors?
A.A three-sided piece of glass called a prism
B.A telescope
C.A curved mirror
D.A special device made by himself
2.What did Newton learn by studying light?
A.Light travels at a very fast speed.
B.Colored light is actually the beam of sunlight.
C.Faraway objects seen through a telescope seem unclear because of the curved glass lenses.
D.The prism causes a change in direction of the light beam.
3.What did Newton eventually invent?
A.The largest telescope
B.The first telescope
C.The most advanced telescope
D.The reflecting telescope
4.What is mentioned in the passage about the Newton’s reflecting telescope?
A.It used a curved glass lens instead of a curved mirror.
B.It produced much clearer pictures than the old kind of telescope.
C.It changed the colors of a rainbow into white light.
D.It created a circle of colored light around an object.
The Greatest Book
Directions: Fill in the blanks according to what you have heard.
Newton began to write a book (1). It was published in 1687. Newton called his book The Mathematical Principles of Natural Philosophy. The book is considered the greatest scientific work ever written.
In his book, Newton explains (2). The first law is that an object not moving remains still. And one that is moving continues to move at an unchanging speed, so long as no outside force(3)it.
Objects in space continue to move, because nothing exists in space to stop them.
Newton’s second law of motion describes (4). It says force equals the (5) of an object, multiplied by the change in (6)it produces in an object.
His third law says that for every (7), there is an (8)reaction.
From these three laws, Newton was able to show (9). He proved it with easily understood (10). Scientists everywhere accepted Newton’s ideas.
Big Mind
Tolerances and Fits
Tolerances
Interchangeable manufacturing allows parts made in widely separated locations to be brought together for the end assembly. That the parts all fit together properly is an essential element of mass production. Without interchangeable manufacturing, modern industry could not exist, and without effective size control by engineer, interchangeable manufacturing could not be achieved.
However, it is impossible to make anything to the exact size. Parts can be made to very close dimensions, even to a few millionths of an inch or thousandths of a millimeter, but such accuracy is extremely expensive.
Fortunately, exact sizes are not needed. The need is for varying degrees of accuracy according to functional requirements. A manufacturer of children’s tricycles would soon go out of business if the parts were made with jet-engine accuracy, no one would be willing to pay such price. So what is wanted is a means of specifying dimensions with whatever degree of accuracy is required. The answer to the problem is the specification of tolerance on each dimension.
The tolerance is the total amount that a specific dimension is permitted to vary; it is the difference between the maximum and minimum limits for the dimension. Manufacturing involves applying a series of operations to components ( parts, subassemblies, etc. ). There operations are intended to ensure specific geometry on work pieces. Dimensions in engineering drawings specify ideal geometry: size, location and shape. Because variations exist in both process and material, the manufacturing process creates a part that has an approximate geometry of the ideal. Tolerances are introduced to specify and control the variations. With the advent of assembly lines, it became critical to manufacture interchangeable parts. The use of replacement parts for maintenance operations also requires the interchangeability of parts. Tolerances are used to ensure the parts have this property.
Two types of tolerances are often used: design tolerances and manufacturing tolerances. Design tolerances are related to the operational requirements of a mechanical assembly or of a component, whereas manufacturing tolerances are mainly designed for a process plan for fabricating a part. Manufacturing tolerances must ensure the realization of design tolerances.
Both design and process engineers are concerned about the effects of tolerances. Designers usually specify tight tolerances to ensure the performance and functionality of the design. Process engineers prefer loose tolerances, which make parts easier and less expensive to produce. Therefore, tolerance specifications become a critical link between design and manufacturing. Good tolerance design ensures quality products at low cost.
Fits
In engineering when a product is designed, it consists of a number of parts and these parts mate with each other in some form. In the assembly, it is important to consider the type of mating or fit between two parts which will actually define the way the parts are to behave during the working of the assembly.
The fit between two mating parts is the relationship which results from the clearance or interference obtained. There are three classes of fits, namely, clearance, transition and interference.
Take for example a shaft and hole, which will have to fit together. In the simplest case, if the dimension of the shaft is lower than the dimension of the hole, then there will be clearance. Such a fit is termed as clearance fit. Alternatively, if the dimension of the shaft is more than that of the hole, then it is termed as interference fit. However, depending upon the possibilities of dimensions, at times there will be clearance and other times there will be interference. Such a fit is termed as transition fit.
Power Speech
Dialogue 1
Mike: We are pleased to receive your order. In order to guarantee the products’ quality, I would like to have a talk with you about the drawing.
Lisa: OK. What would you like to question about?
Mike: Are the dimensions of the drawing in inches? Is the thread the major fit size?
Lisa: Yes. The thread size must be guaranteed and the major sizes must be between the positive 0.005 inch and the negative 0.005 inch, the minor sizes must be between the positive 0.01 inch and the negative 0.01 inch.
Mike: Do you provide the stock and the tool?
Lisa: We provide the stock, and you can buy the tool in local markets or in other countries.
Mike: Is it necessary for us to send you the machine samples?
Lisa: No. We can check them on the contract date, but you may finish the order before the contract date.
Mike: OK. We will finish the order. Hope we have a nice cooperation.
Lisa: Me too. Happy cooperation.
Mike: Thank you. See you later.
Lisa: See you.
Dialogue 2
Manager: Did you put the samples on the table? Assembly Department is waiting for them for a testing.
Jack: Everything that I made up to now is all on your table, but I noticed that some did not reach the diameter.
Manager: Ah, I see. The shaft is thick, and it can not pass through the snap gauge. How can that be?
Jack: I guess the turning cutter has worn away. What about these ones?
Manager: These ones? They can pass the snap gauge, but their surface quality is not fine enough. Anyway, they have to be reworked.
Jack: I think the turning cutter needs to be replaced. Don’t worry. I’ll call the technician to solve the problem at the moment. In the meantime, I’ll have those samples reworked on another lathe.
Manager: That’s good. Thank you, Jack.
Jack: You are welcome.