Cycles used in CNC Machining Operations

Using Cycles

Canned cycle or fixed cycle may be defined as a set of instructions, inbuilt or stored in the system memory, to perform a fixed sequence of operations. The canned cycles can be brought into action with a single command and as such reduce the programming time and effort. Canned cycles are used for repetitive and commonly used machining operations. The canned cycles are stored under G code address. G81 to G89 are reserved for fixed canned cycles and G80 is used to cancel the canned cycle. Commonly available fixed cycles for lathe operations are canned cycle for turning, threading, and rough turning, finish turning and threading these are explained below.

Fixed Cycle for Turning

As discussed in the previous chapter, the depth of cut is limited in CNC machines also.  In order to machine the component to required dimensions, a number of cuts may have to taken. One way of writing a part program for achieving the required diameter on a CNC lathe machine has been discussed earlier i.e. by repeating the same steps. However, since the same steps are being repeated every time, the part program becomes unnecessarily lengthy, occupies large computer memory and the part program has to spend more time in writing the part program. In order to save part programming time and computer memory, fixed cycle for turning are available in the control system. The programmer has to first write an instruction block to position the cutting tool at the starting point and then call the fixed cycle for turning as follows: -

N5 G81 X-2.0 Z-30.00 F 200 EOB
Where,
G81 is the code for the fixed turning cycle
X -2.0 denotes that the depth of cut is 2 mm
Z-30.00 denotes that the length to be machined is 30 mm.

The cycle is executed as follows: -

• Step 1.  The cutting tool moves by 2 mm in the X direction at a given feed rate of 200 mm / min. i.e. it takes required depth of cut.
• Step 2.  The cutting tool moves 30.00 mm in negative Z direction at feed rate of 200 mm / minute.
• Step 3.  The cutting tool moves back by 2 mm in X direction at rapid traverse.
• Step 4.  The cutting tool moves back in Z direction by 30 mm at rapid traverse.

So after the cycle has been executed, the cutting tool is re-positioned at the same point from where it started. Also it may be noted that the four tool motions have been accomplished by a single instruction block, which may otherwise have been written in, four instruction blocks.

To understand the use of a fixed turning cycle, consider the component shown in figure "Turning Cycle". It is a case of simple step turning where it is required to reduce the diameter from 20 mm to 16 mm and 12 mm.

The steps required to make this component are: -

• Turn to 16 mm diameter over a length of 50 mm
• Turn to 12 mm diameter over a length of 35 mm.

The depth of cut should not exceed 1.5 mm and the speed of the work piece is 300 rpm and feed rate is to be kept at 200 mm/minute. Program can be written as follows for Figure "Turning Cycle".

N1 G91 G94 G7l M03 8800 EOB N2 G00 X20.00 Z0 EOB
N3 G81 X-2.00 Z-50.00 F200 EOB N4 G81 X-4.00 Z-50.00 F 200 EOB N5 G81 X-6.00 Z-35.00 F 200 EOB N6 G81 X -8.00 Z-35.00 F 200 EOB N7 G80 EOB
N8 G00 X 25.00 Z 10.00 EOB N9 M02 EOB

Threading Cycle

In order to call and use a fixed cycle for thread cutting on a CNC lathe machine, the information is given as follows:

N2 G84 X-O.3 Z-30.00 K 1.5 EOB
Where,
G84 is the fixed cycle code for thread cutting. X 0.3 is the depth of cut in one pass
Z -30.00 is the length of thread
K 1.5 specifies the pitch of the thread

The following four tool motions are executed with the use of above instruction.

• The cutting tool moves 0.3 mm in negative X direction at a feed rate depending on the pitch of the thread to be cut.
• Threads are cut over a length of 30 mm. The feed of the cutting tool is automatically set according to the pitch of the thread to be cut, programmed under address K.
• The cutting tool retracts by 0.3 mm at a rapid feed rate.
• The cutting tool moves back to the starting position at a rapid feed rate. To illustrate the use of fixed cycle for thread cutting, consider that threads with pitch of 0.75 mm are to be cut over a length of 50 mm. The work piece is shown in figure "Threading Cycle" Here again let us assume that the starting point is at X = 25 and Z = 5. The part program for this component using fixed threading cycles is given below. (Depth of cut in first two passes is 0.2 mm and in the third pass it is only 0.05 mm).

Threading Cycle Program. Threading Cycle Program is given as follows and the tool path is shown in figure "Threading Cycle".

N001 G9l G94 G7l M03 8500 EOB N002 G00 X 25.00 Z 5.00 EOB
N003 G84 X-5.2 Z-50.00 K 0.75 EOB N004 G84 X-5.4 Z-50.00 KO.75 EOB N005 G84 X-5.45 Z-50.00 KO.75 EOB N006 G80 EOB
N007 G00 X6.00 Z25.00 EOB N008 M02 EOB