Cycles in CNC Milling Machine


CNC Milling Cycles

CNC milling machine provided with machining software program in ready form by the manufacturer. These are called machining cycles. These cycles are already programmed in the form of parameters of a particular operation. Whenever the stored programmed similarity arises, simply the cycles called on the computer and necessary parameter centred then the program is ready. These cycles save valuable programming time and computer memory space and make it very convenient to the operator.

Drilling Cycle. Fixed cycle for drilling a hole is applicable; where the complete drilling is completed by giving information in a single block.

To understand the use of drilling cycle, consider the work piece shown in figure "Drilling Cycle" below. The part program for this component is given below:

N01 G71 G94 G90 EOB
N02 G00 X10.00 Y 15.00 EOB
N03 G00 Z-10.00 EOB
N04 G81 Z-50.00 M03 S 800 F 150 EOB
N05 G80 EOB

N06 M02 EOB
drilling cycle
The drilling cycle is explained below:
(a) N01 - Metric mode feed rate in mm/min and absolute coordinate system
(b) N02 - Positioning block i.e. position the drilling tool at X = 10.00 and Y= 15.00
(c) N03 - Drilling tool moves to reference plane in rapid traverse. The reference plane is selected above the work piece surface to avoid the drill striking the work piece while moving in rapid traverse.
(d) N04 - Call drilling Cycle. The spindle starts rotating at 800 rpm in clockwise direction and the hole is drilled at the required position at the given feed rate of 150 mm I minute. The drilling tool is positioned at reference plane after the drilling operation is completed.
(e) N05 - The drilling cycle is cancelled
(f) N06 - End of program

Deep Hole Drilling Cycle (Peck Drilling Cycle). When the depth of hole is more (l/d > 10) it is desirable to withdraw the drill from the hole at regular intervals to avoid clogging due to chips. This is called wood peck drilling. In the CNC machining centres, peck drilling cycle is available. By using the peck drilling cycle, the drill is retracted upto reference plane at rapid feed rate every time after drilling the hole to a specified incremental depth. Consider the work piece shown in figure "Peck Drilling Cycle" below. The final depth of the 10 mm diameter hole is 70 mm, the reference plane is 10 mm above the surface and the over travel required is also 10 mm. The total movement of the drill is 90 mm. However, the hole is not drilled in a single pass. Each time the drill is fed to a specified depth and withdrawn to reference plane before again feeding the drill further into the work piece. Here the total tool travel from reference plane to final position of the drill is programmed as Z value and the incremental depth after which the tool has to be withdrawn is programmed as K valve. The typical format for using deep hole drilling cycle is given below:
Peck Drilling Cycle
N01 G71 G94 G91 M03 S 1000 EOB
N02 G00 X 10.00 Y 10.00 EOB
N03 G00 Z-10.00 EOB
N04 G82 Z-90.00 K 25.00 F 100 EOB
N05 G80 EOB
N06 M02 EOB
Here the deep hole drilling cycle is called using G82

Boring Cycle. In the boring cycle the boring tool is fed to the required depth at the given feed rate. When the tool has reached the required depth, the rotation of the tool is stopped and the tool is withdrawn at a rapid feed rate upto the reference plane. The programming format for using boring cycle (G83) is as under:
N001 G9l G7l M03 S 600 EOB
N002 G00 X 10.00 Y 10.00 EOB
N004 G83 Z-60.00 F 100 EOB
N005 G80 EOB
N006 M02 EOB

Threading (Tapping) Cycle. The tapping operation, involves positioning of tap at required X and Y position, moving it rapidly to reference plane and feeding into the predrilled hole in the work piece at given feed rate. The spindle rotation is then reversed and the tap is brought back to reference plane at the programmed feed rate. The spindle rotation is again reversed to prepare for next tapping operation.

Fixed cycle for tapping is available on CNC machining centres. The use of tapping cycle is illustrated with the help of figure "Tapping Cycle" below. The part program using tapping cycle (G84) is given below:
Tapping Cycle
N001 G71 G91 M03 S 500 EOB
N002 G00 X10.00 Y10.00 EOB
N003 G00 Z-10.00 EOB
N004 G84 Z-25.00 F60 EOB
N005 G80 EOB
N006 M02 EOB