Tool Information:
Tool
information is given by a word beginning with T followed by numerical code for
tool position in tool turret or tool magazine when automatic tool changer is
used. In some machines where manual
changing facilities exist, a common instruction T …is given which means that
the machine will stop for a tool change and can be restarted by operator by
operating a switch.
Speed & Feed Data:
Speed is
designated by letter S followed by the RPM of the spindle.
Sometimes, the Magic Three rule is used to program the speed data. The
rule is as follows:
i) Add 3 to the number of digits on the left of the decimal of numerical value of the speed. This will give the first digit of the coded speed value.
ii) Next two digits in the coded value are first two digits of the numerical value of the speed.
For
example if the speed is 640 RPM, then the coded value will be S664.
i) Count the number of zeros after the decimal and subtract this number from 3. This will be the value of first digit in coded value.
ii) Next two digits in the coded value will be the first two non-zero digits in the feed rate.
For
example feed rate of 0.025 mm/min will be coded as F225
Other Symbols:
Each
information block for the operation begins with operation number, coded as N
followed by two or three- digit-number and ends with a symbol EOB which means
end of block. In writing it is often indicated as *.
Interpolation:
Two types of interpolation are possible in NC machines, I) Linear & ii) Circular. For these the preparatory codes are G01, G02 or G03.
In
linear interpolation the coordinates of the destination are provided in the
block along with the code G01 and other machining parameters. The data
processing unit calculates the slope and intercept for the straight line along
which the tool is supposed to move.
For
circular interpolation, besides the destination coordinates, it is required to
provide the coordinates of center of the arc, with respect to the starting point
of the arc. The data processing unit calculates the radius of the circle of
which the arc forms the part and determines various points required for the
interpolation.
Example:
a)
Linear Interpolation:
Let the cutter be at some point and it is required to proceed to a point (50,60) along a straight line. Then the instruction will be:
N20 G01 X50000 Y60000 F100……
b)
Circular Interpolation:
Now, suppose it is required to proceed along an arc of a circle whose center is (80,60) to a point (80,90) then the instruction will be:
N21
G02 X80000 Y90000 I30000 J0
Where I & J correspond to coordinates of the center with respect to starting point (50,60).
NC Motion Control Systems:
Point –to –point Motion Control
Let us say that the machine has to drill two holes on a plate, coordinates of the holes are (50,80) and (199, 45) with reference to one corner of the plate. First the machine is set at the reference corner and the datum is recorded. Now X and Y information is supplied for the first hole, the machine table moves to position the work piece under the spindle to drill the first hole. When this is complete the drill moves up and then the information is read for next hole and similarly the next hole is drilled. In this system machining operation starts after the tool reaches the destination.
Straight Line Motion Control:
In point
to point system, after finishing the machining operation at a point the tool
moves to nest position at fastest feed rate possible since it is moving in air.
But for cutting motion along lines (milling), feed rates have to be controlled.
In straight milling only data required are coordinates of starting and
finishing points.
Continuous Control (Contouring Control)
It is a more complex system in which the tool moves from point to point as well as performs straight line machining operations. In such system there is simultaneous control of more than one axis movement of tool. The path of the cutter is continuously controlled to generate the desired geometry on the work piece. This system can perform almost all kind of jobs. This is often called Contouring NC. Contouring represents highest level of control. It is used for milling and turning operations.