Cnc control systems
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This document discusses different modes and control systems for CNC machines. It describes point-to-point control which involves individual axis motions like drilling. Continuous-path control allows simultaneous multi-axis motion for contouring. Interpolation methods compute intermediate positions to smoothly connect points, including linear, circular, helical, parabolic and cubic forms. Open-loop systems use stepper motors while closed-loop systems add feedback control for greater accuracy.
Cnc control systems
- 2. Mode of the CNC Control System • Point-to-point path The point-to-point control system is often referred to as the positioning system. 1. Axis path. 2. 45° line path. 3. Linear path.
- 3. In the linear path motion mode, the controller has the capability to synchronize the motion in X and Y directions to generate a linear path. The most common applications of the PTP control system are drilling, boring, reaming, tapping and sheet metal punching.
- 4. The cutting of arcs and angles other not possible PTP system, than 45° is with the but it can be approximated by a series of straight-line cuts. The tolerance, or deviation of the actual path from the true path, can be specified in three ways.
- 5. Continuous-path The continuous-path control system is also known as the contouring system. It involves simultaneous motion control of two or more axes. The contouring system is more complex because each axis of motion requires separate position and velocity loops. The contouring along a predefined tool path is implemented by means of interpolation, in which the system generates a set of intermediate data points between given coordinate positions
- 6. CNC Interpolation An interpolator provides two functions: It computes individual axis velocities to drive the tool along the programmed path at the given feed rate. It generates intermediate coordinate positions along the programmed path. There are five types of interpolation: linear, circular,helical, parabolic, and cubic.
- 7. Linear Interpolation Linear interpolation requires three parameters: start point coordinate, end point coordinate and the speed command for each axis. In two-axis linear interpolation, the interpolator calculates the speed commands, in pulses per second, for the X and Y axes in such a way that it maintains the speed ratio between the X and Y axes equal to the ratio of the required incremental distance (dx / dy).
- 8. Theoretically, linear interpolation can be used to cut all types of tool paths, including straight lines, circles, arcs, curves, and helical contours, etc.. However, it takes much more data to cut contours other than straight lines.
- 9. The use of other interpolation method results in a substantial reduction of data to be processed. Curves that can not be defined mathematically can only be approximated by using linear interpolation.
- 12. Circular Interpolation The interpolator computes the axial velocity components and produces a sequence of reference pulses for each control axis of motion. The advantage of circular interpolation is its ability to generate an arc in a single program block. In some NC controls, circular interpolation is limited to a 90° arc in a single block.
- 13. The information required for programming a circular interpolation includes: (1) (2) (3) coordinates of the start point and end point, radius of direction the arc or coordinates of the arc centre, and in which the tool is to proceed (CW or CCW). interpolation is limited to the two-axis plane.The circular
- 16. Helical Interpolation It combines the two-axis circular interpolation and a linear interpolation along the third axis.
- 17. Helical interpolation:- Producing a large-diameter hole is a common application for many shops, and there are numerous methods that can be used to achieve the end result. However, there are often numerous obstacles to completing the process cost effectively. Horsepower consumption is frequently a concern in these types of applications, especially on the more common 20 horsepower and below machine tools. These machines are capable of high speeds and feeds, but rigidity is sacrificed to the extent necessary to accomplish the quick movements. Using conventional means, making large diameter holes is hard on the machine... Function and purpose:- Command G02 or G03 with a designation for the third axis allows synchronous circular interpolation on the plane specified by plane-selection command G17, G18 or G19 with the linear interpolation on the axis.
- 18. Description:- For helical interpolation, movement designation is additionally required for one to two linear axes not forming the plane for circular interpolation. The velocity in the tangential direction must be designated as the feed rate F. Programming Format:- G17 G02 (or G03) X___ Y___ I__ J__ P__ F__ ; Or G17 G02 (or G03) X__ Y__ R__ P__ F__ ; X = Arc ending point coordinates X axis Y = Arc ending Point coordinates Y axis I & J = Arc center Coordinates P = Number of pitches F = Feed rate R = Arc Radius Example:- G28 U0. W0. Y0. ; G50 X0. Z0. Y0. ; G17 G03 X100. Y50. Z-50.0 R50. F1000. Notes:- Plane selection:- As with circular interpolation, the circular interpolation plane for helical interpolation is determined by the plane selection code and axis addresses. The basic programming procedure for helical interpolation is selecting a circular – interpolation plane using a plane selection command (G17, G18 or G19) and then designating the two axis addresses for circular interpolation and the address of one axis for linear interpolation.
- 19. Parabolic Interpolation Parabolic interpolation uses three noncollinear points to approximate free-form curves. It can be used to cut either planar or spatial curves. It is primarily used in mold and die making, where free-form designs are preferred over precisely defined shapes.
- 20. Cubic Interpolation It is of the third order and can be used to generate complex tool path for machining complicated shapes such as automobile sheet metal dies with a relatively small number of programmed points. However, it is very complex and requires considerable computing power and a large memory.
- 21. Open-loop Control Systems Open-loop systems normally use stepping motors as the drive devices to move the machine slide. Due to the advent of precision ball screw and stepping motor control technology, open-loop control can be refined to 0.001 in. resolution, which is accurate enough to be used in many precision positioning and light-load contouring applications.
- 23. Closed-loop Control Systems A feedback loop is implemented to monitor the actual output and correct any discrepancy from desired output. Both analog-type and digital-type can be applied. Most modern closed-loop NC systems are able to provide very fine resolution of 0.0001 in.