The CNC machines are divided in to four types they are
- Based on type of motion
- Based on control loop
- Based on power supply
- Based on the position of the system
Based on the type of the motion systems they are divided into two types, they are:
- Point to point system
- Continuous path
Point to point systemAdvertisement:
In the point to point system the work piece or tool must be moved from one place of the point to the other place of the point, this is to perform the required task. Based on the next position the tool must be moved to obtain shape. In this process there is no importance for the feed rate and the path of the cutting tool. The accuracy must depend on the basic length unit which generally ranges between the 0.001 to 0.0001 inches.
Continuous path system:
The continuous path system is also known as the contouring system. During the performance of the cutting operation the motion of the work piece with respect to the cutter, the cutting operation is takes place.
The machine tool includes the routing, milling etc. At the time of machining process the tool and the work piece must be controlled continuously. The control system must accept the information related to the position and velocities of the machine slides. Feed rates are to be programmed.Advertisement:
Based on the control loop are also divided in to two types they are
- Open loop system
- Closed loop system
Open loop system;
In the open loop control system there is no feedback controller and it uses the stepping motor for driving the lead screw. Stepping motor is responsible for the input pulse whose output shaft rotates with a fixed angle. The accuracy of the system mainly depends upon the ability of the motor. Stepping motor frequency depends upon the load torque.
If the load torque is higher, the frequency would be lower. In the machine tools, due to the cutting forces the load torque is excessive. The open loop system is suitable for where the tool force does not exist.
The terms used in the system are
Step angle α =
N = no of pulse required for one revolution.
Total angle A= n
n= no of pulse
Closed loop system:
The closed loop control system consists of feedback subsystem. This is used to screen the actual output. The feedback system is to be in a digital or analog format. The digital system is used to screen the output variations with the help of the electrical pulses. The analog is used to measure the physical variables vibrations like velocity and the positions. The closed loop system must be very accurate, for the reason that the feedback system is to be automatically compensating the variations in real time. They are able to provide resolution of 0.0001 inches. Closed system requires a control device. Compare with the open loop system the closed loop system is considered to be very expensive and more complex.
Based on the type of the power supply also they are divided in to three types they are
- Hydraulic power supply
- Pneumatic power supply
- Electric power supply
Comparison of three power supplies
|1||Working speed||5 m/s||4 m/s||10 m/s|
|2||Achievable Force||High||Limited and nearly 20 kN||High|
|6||Over load safety||Complete||Complete||Complete|
|8||Change of forces||Simple and accurate||Simple and accurate||Simple and complex|
During the time of heavy loads for immediate requirement of the motion the hydraulic system is used. It allows the fuels to reach quickly.
For a wide range of easy and continuous reaction of the speed and forces the pneumatic system is used. The position of the fluid must be repeatable and accurate. For more than three positions they prefer pneumatic system.
The electrical system consists of both the contouring and positioning machines. For the requirement of the continuous motion the electrical system must be preferred. They need to control the torque and direction either by using the AC or DC motor. By varying the armature or field supply they controlled the speed of the DC motor. They are mainly used in the small machine tools, due to the heat losses in clutches.
Based on the type of the position of the system they are also divided in to two types they are
- Incremental system
- Absolute system
In the incremental system the distance is measured from one point to the other (next) point.
In the absolute system all the moving commands are stated from a reference point.
Advantages of incremental system:
- Inspection of the program is considered to be easier
- By changing the signs of position commands mirror image programming is simple.
Advantages of Absolute system:
- Interruption would not affect the position
- Dimensional data should be change easily.
CNC parts Programming:
The design of the product must be initially performed manually with the help of the computer language. Part program must have knowledge of the machining process and abilities of machine tools.
G and M Codes
Preparatory Function– It involves actual tool moves
|G02||Circular interpolation clock wise||M||T|
|G03||Circular interpolation clock wise||M||T|
|G05P10000||High- precision Contour Control||M|
|G05.1||Al Advanced preview control||M|
|G06.1||Non uniform rational B-splines (NURBS) machining||M|
|G07||Imaginary axis designation||M|
|G09||Exact stop Check, non- modal||M||T|
|G10||Programmable Data input||M||T|
|G11||Data Write Cancel||M||T|
|G12||Full circle interpolation clock||M|
|G13||Full circle interpolation counter clock||M|
|G17||XY plane selection||M|
|G18||ZX plane selection||M||T|
|G19||YZ plane selection||M|
|G20||Programming in inches||M||T|
|G21||Programming in millimeters||M||T|
|G28||Return to home position||M||T|
|G30||Return to secondary home position||M||T|
|G32||Single-point threading long hand style||T|
|G33||Constant pitch threading||M|
|G33||Single point threading long hand style||T|
|G34||Variable pitch threading||M|
|G40||Tool radius compensation off||M||T|
|G41||Tool radius compensation left||M||T|
|G42||Tool radius compensation right||M||T|
|G43||Tool height offset compensation negative||M|
|G44||Tool Height offset compensation Positive||M|
|G45||Axis Offset Single increase||M|
|G46||Axis offset angle decreases||M|
|G47||Axis Offset double increases||M|
|G48||Axis Offset double decreases||M|
|G49||Tool length offset compensation cancel||M|
|G50||Define the maximum spindle speed||T|
|G50||Scaling function Cancel||M|
|G52||Local Coordinate system||M|
|G53||Machine coordinate system||M||T|
|G54 to G59||Work coordinates||M||T|
|G54.1 to P1 to P48||Extended work coordinate system||M||T|
|G61||Exact stop check, modal||M||T|
|G62||Automatic corner override||M||T|
|G64||Default Cutting mode||M||T|
|G70||Fixed cycle, multiple repetitive cycle, for finishing||T|
|G71||Fixed cycle, multiple repetitive cycle, for roughing (Z-axis emphasis)||T|
|G72||Fixed cycle, multiple repetitive cycle, for roughing (X-axis emphasis)||T|
|G73||Fixed cycle, multiple repetitive cycle, for roughing, with pattern repetition||T|
|G73||Peck drilling cycle for milling – high-speed (NO full retraction from pecks)||M|
|G74||Peck drilling cycle for turning||T|
|G74||Tapping cycle for milling, left-hand thread, M04 spindle direction||M|
|G75||Peck grooving cycle for turning||T|
|G76||Fine boring cycle for milling||M|
|G76||Threading cycle for turning, multiple repetitive cycle||T|
|G80||Cancel canned cycle||M||T|
|G81||Simple drilling cycle||M|
|G82||Drilling cycle with dwell||M|
|G83||Peck drilling cycle (full retraction from pecks)||M|
|G84||Tapping cycle, right-hand thread,M03 spindle direction||M|
|G84.2||Tapping cycle, right hand thread,M03 spindle direction, rigid tool holder||M|
|G84.3||Tapping cycle, left hand thread,M04 spindle direction, rigid tool holder||M|
|G85||boring cycle, feed in/feed out||M|
|G86||boring cycle, feed in/spindle stop/rapid out||M|
|G87||boring cycle, back boring||M|
|G88||boring cycle, feed in/spindle stop/manual operation||M|
|G89||Boring cycle feed||M|
|G90||Fixed cycle, simple cycle, for roughing (Z-axis emphasis)||T|
|G92||Position register (programming of vector from part zero to tool tip||M||T|
|G92||Threading cycle, simple cycle||T|
|G94||Feed rate per minute||M||T|
|G94||Fixed cycle, simple cycle, for roughing (X-axis emphasis)||T|
|G95||Feed rate per revolution||M||T|
|G96||Constant surface speed (CSS)||T|
|G97||Constant spindle speed||M||T|
|G98||Return to initial Z level in canned cycle||M|
|G98||Feed rate per minute (group type A)||T|
|G99||Return to R level in canned cycle||M|
|G99||Feed rate per revolution (group type A)||T|
Miscellaneous functions- it involve actions necessary for machining.
|M02||End of program||M||T|
|M03||Spindle on (clockwise rotation)||M||T|
|M04||Spindle on (counter clockwise rotation)||M||T|
|M06||Automatic tool change (ATC)||M||T|
|M10||Pallet clamp on||M|
|M11||Pallet clamp off||M|
|M13||Spindle on (clockwise rotation) and coolant on (flood)||M|
|M23||Thread gradual pull-out ON||T|
|M24||Thread gradual pull-out OFF||T|
|M30||End of program, with return to program top||M||T|
|M41||Gear select – gear 1||T|
|M42||Gear select – gear 2||T|
|M43||Gear select – gear 3||T|
|M44||Gear select – gear 4||T|
|M48||Feed rate override allowed||M||T|
|M49||Feed rate override NOT allowed||M||T|
|M52||unload last tool from spindle||M||T|
|M60||Automatic pallet change (APC)||M|
It gives an identification related to the numbers for each block. It is usually a good run through process to increment each block number by 5 or 10; this is to permit additional blocks to be introduced for the future changes if essential.
Letters used for milling and turning:
|A||Absolute or incremental position of A axis (rotational axis around X axis)|
|B||Absolute or incremental position of B axis (rotational axis around Y axis)|
|C||Absolute or incremental position of C axis (rotational axis around Z axis)|
|D||Defines diameter or radial offset used for cutter compensation. D is used for depth of cut on lathes. It is used for aperture selection and commands on photo plotters.|
|E||Precision feed rate for threading on lathes|
|F||Defines feed rate|
|G||Address for preparatory commands|
|H||Defines tool length offset;
Incremental axis corresponding to C axis (e.g., on a turn-mill)
|I||Defines arc centre in X axis for G02 or G03arc commands.
Also used as a parameter within some fixed cycles.
|J||Defines arc centre in Y axis for G02 or G03arc commands.
Also used as a parameter within some fixed cycles
|K||Defines arc centre in Z axis for G02 or G03arc commands.
Also used as a parameter within some fixed cycles, equal to L address.
|L||Fixed cycle loop count;
Specification of what register to edit using G10
|N||Line (block) number in program;
System parameter number to be changed using G10
|P||Serves as parameter address for various G and M codes|
|Q||Peck increment in canned cycles|
|R||Defines size of arc radius, or defines retract height in milling canned cycles|
|S||Defines speed, either spindle speed or surface speed depending on mode|
|U||Incremental axis corresponding to X axis (typically only lathe group A controls)
Also defines dwell time on some machines (instead of “P” or “X”).
|V||Incremental axis corresponding to Y axis|
|W||Incremental axis corresponding to Z axis (typically only lathe group A controls)|
|X||Absolute or incremental position of X axis.
Also defines dwell time on some machines (instead of “P” or “U”).
|Y||Absolute or incremental position of Y axis|
|Z||Absolute or incremental position of Z axis|
Click Here To Know More About: Terms Used In CNC Machining