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THE PUNCHED TAPE IN NC MACHINES AND ITS CODING SYSTEM

The punched tape is a standardized tape of a fixed width and infinite on which NC machining instructions can be coded in the form of holes and spaces following the binary code. The punched tape used in NC is one inch (1”) wide. It is standardized in accordance with the EIA.

There are eight regular columns of holes running the lengthwise direction of the tape. There is also a ninth column of holes between the third and fourth regular columns. However, these holes are smaller and are used as sprocket holes for feeding the tape. Row positions run across the tape, normal to the edge.

The coding of the tape is achieved by either the presence or absence of a hole in the various positions. Because there are two possible conditions for each position (i.e. with the presence or absence of a hole) this coding system is called the binary code. It uses the base two number system which can represent any number in the more familiar base 10 or decimal system.

The NC tape coding system is used to code not only numbers but also alphabetical letters and other symbols (+) plus and minus (-) signs etc. the most common input medium is the punched tape. The tape has become standardized so that tape punchers are manufactured to prepare the NC tapes, while tape readers are manufactured to read the tapes, thus enabling compatibility of both units. The punched tape is standardized by the Electronics Industries Association (EIA).

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In NC, a punched tape must always be generated irrespective of whether the part program was done manually or with the assistance of a computer. However, with either method of part programming, the punched tape is the net result of the programming effort.

MATERIALS USED IN MAKING PUNCHED TAPE

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NC punched tapes can be made of different materials ranging from,

Ordinary paper tapes

Mylar-reinforced paper tapes

Mylar coated aluminum

Certain plastic

The ordinary paper tape is cheaper but not durable. Hence, it is not appropriate for storage and repeated use. The most used for initial preparation and testing of the part program is the ordinary paper. Its use may be temporary after which a stronger production tape, is duplicated with more durable materials. The initial paper tape is then retained as the master copy.

MAGNETIC TAPES

Instead of punching holes on the tapes, magnetic dots are used on magnetic tapes. The polarization of the tape indicates the binary digit zero or 1. Its advantages are that:

It can store voluminous information in a small space.

It is faster and more durable.

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The magnetic tape reader consists of an electromagnetic. Each magnetized spot generates an electric impulse.

THE PUNCHED TAPE CODING SYSTEM

NC tape coding can be defined as the conversion of the basic symbols that are contained in the part programming set of instruction into a set of hole patterns on the tape following the binary code, to give what is called a punched tape.

The part program is usually converted into a sequence of machine tool action by means of;

  1. The input medium i.e. the punched tape which contains the program of instruction, and
  2. The controller unit, which interprets the information from the input unit and uses it to control the machining actions.

There are two basic methods of preparing the punched tape or coding the tape. The first method is associated with manual part programming, there the operator tapes directly from the part programmer’s handwritten list of coded instructions using a typewriter-like device called the flexo-writer. This leaves the copy of the program together with the prepared tape.

The second method is used with the computer assisted part programming system.

In the manual method, the tape punch machine operates in conjunction with a typewriter-like machine called the flexo-writer. Each character typed on the flexo-writer is converted into a set of hole-patterns in the tape by the tape punch machine. Thus the actual coding of the tape is done by the tape punch machine, which has been designed and fed with a program that understands characters, numbers and some specific symbols in terms of holes and spaces.

The NC tape coding system is used to code not only numbers but also alphabetical letters and other symbols of plus and minus signs. In the binary system, there are only two numbers, 0 and 1. The meaning of successive digits in the system is based on the number 2 raised to successive powers.

 

 

Originally posted 2018-12-06 14:04:08.

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