CNC Part Programming Ll

8/13/2019 CNC Part Programming Ll

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30. CNC Part Programming II

In the previous section, fundamentals of programming as well basic motioncommands for milling and turning have been discussed. This section gives anoverview of G codes used for changing the programming mode, applying

transformations etc.,

30.1 Programming modes

Programming mode should be specified when it needs to be changed from absoluteto incremental and vice versa. There are two programming modes, absolute andincremental and is discussed below.

30.1.1 Absolute programming (G90)

In absolute programming, all measurements are made from the part originestablished by the programmer and set up by the operator. Any programmedcoordinate has the absolute value in respect to the absolute coordinate system zeropoint. The machine control uses the part origin as the reference point in order toposition the tool during program execution !igure "#.$%.

30.1.2 Relatie programming (G91)

In incremental programming, the tool movement is measured from the last toolposition. The programmed movement is based on the change in position betweentwo successive points. The coordinate value is always incremented according to thepreceding tool location. The programmer enters the relative distance between currentlocation and the next point !igure "#.&%.

30.2 !pindle "ontrol


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The spindle speed is programmed by the letter '(' followed by four digit number, such as ($###.There are two ways to define speed.$. )evolutions per minute )P*%&. +onstant surface speedThe spindle speed in revolutions per minute is also nown as constant rpm or direct rpm. The change

in tool position does not affect the rpm commanded. It means that the spindle )P* will remainconstant until another )P* is programmed. +onstant surface speed is almost exclusively used onlathes. The )P* changes according to diameter being cut. The smaller the diameter, the more )P*is achieved- the bigger the diameter, the less )P* is commanded. This is changed automatically bythe machine speed control unit while the tool is changing positions. This is the reason that, thisspindle speed mode is nown as diameter speed.

"#." oops and /nconditional 0ump G&1%The unconditional 0ump is used to repeat a set of statements a number of times.2xample3 4$#

In the above example, the program statements from 45# to 4$## are repeated once when thestatement 4$6# is executed. /sually the G&1 is used after a mirror statement. Illustrative examplegeometry and its program are given below !igure "#."%.

2xample3

!ig. "#." Illustrative example for programming loops


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30.# $irroring

The mirroring command is used when features of components shares symmetry about one or moreaxes and are also dimensionally identical. 7y using this code components can be machined using asingle set of data and length of programs can be reduced.

G$# cancellation of mirroring imageG$$ *irror image on 8 axisG$& *irror image on 9 axis

G$" *irror image on : axis2xample3

!ig. "#.; Illustrative 2xample for mirroring


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30.% !&i'ting origin

G


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!ig. "#.6 Illustrative 2xample for scaling

30. Pattern rotation


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Pattern rotation is used to obtain a pattern of similar features. G5" code is used torotate the feature to form a pattern.(yntax G5" Aa, where 'a' is the angle of rotation. This command is cumulative, andthe angle gets added up on time the program is executed. (o all the rotational angleparameters should be cancelled using the code G5".

The unconditional 0ump code G&1 is used in con0unction with this code to achieve thedesired rotation.

The following example !igure "#.5% depicts the case of a pattern which needs to beprogrammed through G5".2xample3

!ig. "#.5 Illustrative 2xample for Pattern rotation


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30.* +ool sele"tion

Tool selection is accomplished using 'T' function followed by a four digit numberwhere, first two digits are used to call the particular tool and last two digits are used torepresent tool offset in the program. The tool offset is used to correct the values

entered in the coordinate system preset bloc. This can be done @uicly on themachine without actually changing the values in the program.

/sing the tool offsets, it is easy to set up the tools and to mae ad0ustments

,eed rate "ontrol

+utting operations may be programmed using two basic feed rate modes3$. !eed rate per spindle revolution&. !eed rate per time

The feed rate per spindle revolution depends on the )P* programmed.

CNC Part Programming Ll

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