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Milling
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Milling is the process of removing metal by feeding the work past a rotating multipoint cutter. As the cutter rotates its teeth engage the advancing work clamped on the milling machine table and remove chips from its surface, thereby imparting to the work the form and size specified in the drawing.
The rotation of a cutter is called the primary cutting motion while the forward movement of the work is known as the feed motion. Both of these motions are available on a milling machine.
The primary cutting motion, i.e. rotation of the cutter, is characterized by the number of spindle revolutions per minute (rpm) and determines the cutting speed. The feed motion is characterized by the amount of table travel per minute with the clamped work. This magnitude is expressed in millimeters per minute (mm/min).
There exist two main methods of milling: conventional milling and climb milling.
In conventional milling the tooth doesn’t begin the cut at point. It tends to slide for a minute distance across the surface machined by the preceding tooth. Depending on the spring of the arbor, the degree of deformation of metal being machined, the cutter runout and the sharpness of the cutting edge, this sliding action will continue until the thickness of the layer of metal before the tooth becomes sufficient to prevent further slip. Only at this point does the actual chip formation begin. Ample coolant is required to dissipate the heat generated by this sliding action. The fact that the tooth begins to wear before it engages the metal is a distinctive disadvantage of this milling method.
In climb milling the tooth engages the metal at the point of maximum chip thickness and leaves the work at the point of minimum thickness so that the tooth begins to cut at once without sliding. Less heat generated in this method and less wear permits higher milling speeds.
Although climb milling is a more progressive method, it can be employed only when there’s no backlash in the table feeding mechanism and when the work is rigidly clamped. This method can’t be used for rough castings and forgings because of the hard skin that quickly dulls the cutter.
Climb milling is the most expedient procedure for machining surfaces of thin workpieces in slitting and cutting off operations.
EXERCISES
1. Find in the text synonyms for the following English words:
To take off, workpiece, to revolve, shape, to start, to go on, real, to need, drawback, immediately, to allow, velocity, advanced, to use, firmly.
2. Find in the text antonyms for the following English words:
Stationary, backward, to finish, advantage, more, soft, slowly, to sharpen, the least.
3. Choose the definition that fits each word or phrase as it is used in the text:
| teeth | 1. Hard white objects inside your mouth that you use for biting and for chewing food. 2. Narrow pointed parts that form the edge of a tool or machine. 3. The necessary power and authority to be effective or to make people obey you. |
| revolution | 1. A situation in which people completely change their government or political system, usually by force. 2. A sudden or major change, especially in ideas or methods. 3. The movement of something in a circle around something else, either once or continuously. |
| degree | 1. A unit for measuring temperature. 2. A unit for measuring angles. 3. An amount of something such as a feeling or a quality. |
| casting | 1. The process of choosing people to act in a film, play etc. 2. An object formed into a particular shape by pouring a liquid into a mould. |
| skin | 1. The outer layer of a person’s or animal’s body. 2. The outer layer of a fruit or vegetable. 3. The outer layer of something, for example a plane or boat. 4. An informal name of a skinhead. |
4. Answer the following questions:
1. What is milling?
2. What motions are available on a milling machine?
3. What milling methods do you know?
4. Why is ample coolant required in conventional milling?
5. What milling method is considered to be a more progressive one?
6. In what cases can’t climb method be used?
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