CNC PART PROGRAMMING

CNC part programming is concerned with the planning and documentation of the sequence of processing steps to be performed on a Numerical Control machine. It is usually accomplished by a person whose title is part programmer. The planning portion of part programming requires knowledge of machining (or other processing technology for which the NC machine is designed) as well as geometry and trigonometry. The sequence of processing steps in NC involves a series of movements of the processing head with respect to the machine table and work part.

The documentation portion of part programming involves the input medium that is used to transmit the program of instructions to the CNC controller unit (the MCU). The most common input medium is used over the last 30 years is 1 wide punched tape. Recently other ways of entering the program to the MCU have been developed. The use of magnetic tape and floppy disks has been growing in popularity since they represent more modern storage technologies for numerical control. The advantage of these input media is their much higher data density. For example, one floppy diskette is capable of storing the equivalent of several thousand feet of punched tape.

In addition, techniques for transmitting the program directly from a central computer to the individual machines in the factory have been introduced. This form of program input is called direct numerical control (DNC)

NC WORDS

Following is a list of the different types of words used in the formation of block. Not every NC machine uses all the words. Also, the manner in which the words in a block are given in the order below:

SEQUENCE NUMBER (N-WORDS). This is used to identify the block

PREPARATORY WORK (G-WORDS)

This word is used to prepare the controller for instructions that are to follow.

For example, the word G02 is used to prepare the NC controller unit for circular interpolation along an arc in the clockwise direction. The preparatory word is needed so that the controller can correctly interpret the data that follow it in the block. Some typical examples of G-words are given in the Table below.

Some Common G-Words

CODEPREPARATORY FUNCTIONS

G00Used with countering systems to prepare for a point to point operation

G02Linear interpolation in contouring systems

G03Circular interpolation, clockwise

COORDINATES (X-, Y-, Z-, WORDS):

These give the coordinate positions of the tool. In a two-axis system, only two of the words would be used. In a four or-five-axis machine, additional a-words, b-words would specify the angular positions.

FEED RATE:

This specifies the feed rate in a machining operation. Units are mm/min or inch/min.

CUTTING SPEED:

This specifies the cutting speed of the process, the rate at which the spindle rotates. Units are revaluations per minute. In a machining operation it is usually desirable for the tool engineer to specify the speed in terms of the relative speed of the tool and work. The units would be metre per minute. It is there necessary for the part programmer to make the conversion from using the machining process equations given.

TOOL SELECTION:

This word would only be needed for machines with a tool turret of automatic tool changer. The t-word specifies which tool is to be used in the operation. For example, T05 might be the designation of 6mm drill bit ion turret position 5 on an NC turret drill.

MISCELLANEOUS FUNCTION:

The m-word is used to specify certain miscellaneous or auxiliary functions which may be available on the machine tool. Of course, the machine must posses the function that is being called. A partial but representative list of is given below. The miscellaneous function is the last work in the block. To identify the end of instruction, an end of block (EOB) symbol is punched on the tape. Some typical M-Words:

CODEMISCELLANEOUS FUNCTIONS

M03Start spindle in clockwise direction

M04Start spindle in counter clockwise direction

M05Stop spindle

TAPE FORMATS:

The organization of words within blocks is called the tape format. Three tape format seem to enjoy the most widespread use:

1. Word address format

2. Tab sequential format

3. Fixed block format

The tape for mat refers to the method of writing the words in a block of instruction. Within each format there are variations because of differences in machining processes, type of machine, features of the machine tool, and so on.

WORD ADDRESS FORMAT:

In this format, a letter precedes each word and is used to identify the word type and to address the data to a particular location in the controller unit. The X-prefix identifies an X-coordinate word; an S-prefix identifies spindle speed and so on. The standard sequence of words for two-axis NC system is

M-word

G-word

X-word

Y-word

F-word

S-word

T-word

EOB

However, since the type of word is designed by the prefix letter, the words can be presented in any sequence. Also, if a word remains unchanged from the previous block or is not needed, it can be deleted from the block.

FUNCTION (G CODES)

G Codes are instruction describing machine tool movements.

G00Fast Traverse

G01Linear interpolation (cutting feed)

G02Circular Interpolation (clockwise)

G03Circular Interpolation (counter clockwise)

G04 Dwell

G20Imperial (Input in Inches)

G21Metric (input in mm)

G28Go to Reference

G40Cutter Compensation Cancel

G41Cutter Compensation Right

G42Cutter Compensation Left

G50Coordinate Setting

G70Finishing Cycle

G71Stock Removal in Turing

G72Multiple Facing

G73Pattern Repeating

G74Peck Drilling

G76Multiple Thread

G81Drilling Cycle

G90Turning Cycle

G94Facing Cycle

G96Const. Surface

G97Var. Surface

G98Feed per Minute

G99Feed per Rev.

MISCELLANEOUS FUNCTION (M CODES)

M Codes are instructions describing miscellaneous functions like calling the tool, spindle rotation, coolant on etc.,

M00Program Stop

M01Optional stop

M02Program end

M03Spindle On

M05Spindle Stop

M06Tool Change

M08Coolant On

M09Coolant Off

M10Vice Open

M11Vice Close

M13Spindle Forward, Coolant on

M15 Spindle Reverse, Coolant on

M25Quill Extend

M26Quill Retract

M30Programme End

M38Door Open

M39Door Close

M98Subprogram call

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