Category Archives: CNCMachines

CNC Coordinate System | Cartesian Coordinate System

CNC machines Axis:

The real parts of a CNC program involves the input of co-ordinates of the tool endpoint to produce any machining profile, subsequently it is necessary to follow a proper co-ordinate system.

01 - CNC milling axis - Cartesian coordinate system01 - CNC turning axis - cartesian coordinate system

Cartesian Co-ordinate System:

All the machine tools make use of the Cartesian coordinate system for the purpose of simplicity. The guiding coordinate system followed for assigning the axes is the familiar right hand coordinate system. The fundamental axes to be assigned are the rectangular axes and the rotary axes. One could use his right hand to arrive at these alternate variable positions of the same right hand coordinate system.

Designating the axes:

Z-Axis and Motion:

Location:

The Z-Axis movement is either along the shaft pivot axle or parallel to the shaft pivot axle. In the case of machine without a shaft / spindle such as shapers and planers, it is identified as the one perpendicular to the table, which passing through the controlled point (e.g., the cutting tooltip).

Direction:

The tool moving far from the work-holding surface towards the cutting tool is assigned as the positive Z direction. This means in a drilling machine the drill moving into the workpiece is the negative (-) Z direction. This aids in decreasing the possible accidents because of the wrong part program entry in the coordinate signs.

When there are several shafts and slide ways:

In such cases, one of the shafts, preferably the one perpendicular to the work holding surface may be chosen as the standard shaft. The primary Z movement is then close to the primary shaft. The tool movements of other spindle quills or sometimes slides, these movements are termed as secondary and tertiary movements, and they are designated as U, V, W and P, Q, R respectively. For other machines such as shaper, planer etc., the positive (+) Z movement increases the clearance between the work surface and the tool holder.

01 - Vertical Axis Milling Machine Cartesian coordinate system

X – Axis:

The X – Axis is the principle motion direction in the positioning plane of the cutting tool or the workpiece.

Location:

It is perpendicular to the Z – Axis and should be horizontal and parallel to the work holding surface wherever possible.

Direction:

When looking from the principle spindle to the column, the positive (+) X is to the right. For turning machines, it is radial and parallel to the cross slide. X motion is positive when the tool withdraws from the axis of rotation of the workpiece. For other machine tools, the X – Axis is parallel to and positive along the principle direction of movement of the cutting or the guided point.

Y – Axis:

It is perpendicular to both X – and Z – axis and the direction is identified by the right hand Cartesian coordinate system.

Rotary Motions:

A, B and C movements define the primary rotary motions.

Location:

These movements are located about the axis parallel to X, Y and Z respectively. If it in count to the above mentioned primary rotary motions, there are secondary rotary motions. These should be designated as D or E regardless of whether they are parallel or not to A, B and C motions.

Direction:

Positive A, B and C are in the directions which advance right hand screws in the positive X, Y and Z directions respectively. The fingers of the right hand point towards the positive direction of the rotary motions. All the above mentioned (e.g., X, Y, Z; U, V, W; P, Q, R; A, B, C and D, E) is the reference to a point, the movement of which is sought to be controlled. This point is generally the tip of the cutting tool. Often the tool point may not move in some directions, e.g., the quill of the spindle of a vertical milling machine moves in the Z direction but not in the X and Y directions. In such problems, the work surface is usually motioned in a direction opposite to the one projected for the tool, e.g., the table of the milling machine holding the workpiece may be moved in – X and – Y directions. Such movements of machine elements, – X or –Y are signified as + X or + Y respectively. Prepared letters can thus be used for all the afore-mentioned motions to show the corresponding reversed directions for moving work surfaces.

Numerical Control Making A New Technology | Why Computers Are Involved In Numerical Control

Why Computers are involved in NC

One method, which is quite elaborate, for translating and storing in-formation on a tape is as follows. Using a special language, which consists of numbers and a limited number of words having no more than six letters, the person doing the programming writes the instructions for a complete cycle. These instructions in the special language are then punched on cards, and these cards are fed into a computer which translates the special language into punched holes on a paper tape. This is a method for transferring information between man and machine or between two machines. A computer may even make calculations as it translates the special language into the necessary commands on the tape.

01 - Why Computers Are Involved In Numerical Control

Computers are not necessary for preparing tapes for numerical control, but they accomplish the preparation much faster and eliminate human mistakes. The preparation of the tapes consists of problems which are to a great extent repetitive and well suited to the use of high-speed computers. Command information thus is placed on a tape in the desired sequence, usually in sections known as "command blocks." For a machining operation, one command block will pertain to one particular cut or surface to be produced. As the tape advances through the equipment known as a "programmer," or "controller," the command blocks are arranged in proper sequence. Thus, when one command block is completed, the next command block will have arrived at the place where the controller will send some of its signals to the machine. A controller, using the command in-formation on a tape, or other device, controls a machine with desired timing and sequencing and makes a limited number of logical decisions. Two different types of signals are employed for sending information between a controller and a machine. These two types of signals are known as "analogue" signals and "digital" signals. Analogue signals are continuously sent, and changes in the quantity being measured cause corresponding changes in the signal. As a common example, an instantaneous present value of a measurement may be continuously indicated. As changes take place in this measurement, the continuous signal follows the changes. Another example of an analogue signal is the continuous comparison of a present dimension of a workpiece with its desired dimension. A digital signal consists of one or more incremental, discrete pulses. The pushing of a start button is an example of a digital signal. A desired dimension may be transmitted as a definite arrangement of pulses, if a digital signal is to be used.

Digital signals can offer a higher degree of accuracy, but they require more complicated equipment in comparison with that needed for analogue signals. Analogue control is continuous and the control action is directly proportional to the desired machine action. Digital control is given in steps. Numerical control uses digital signals, and it comes largely under the classification of digital control. The continuous control of a tool location in relation to the workpiece may be accomplished by pulses which position a stepping switch. Analogue signals may be made avail-able to a controller of a numerical control system by the use of an analogue-to-digital converter.

01 - Numerical Control Making A New Technology

A control system is known as a "closed-loop system" when feedback is incorporated. For example, if a controller is controlling the position of a machine table, feedback will be a signal back to the controller giving the location of the machine table. Thus a controller can check the results of its controlling action and send appropriate signals when needed. A controller in a closed-loop system can be described as having some judgment. Feedback provides a means for overcoming tendencies of a machine to drift away from a desired condition. Control systems without feedback are not self correcting, and they are known as "open loop systems."

Controllers vary considerably in complexity and controlling ability. Controllers for analogue control systems are the simplest. A more complicated controller may have a computer incorporated with it to solve problems which occur during the controlling action. Such a controller can make a wider range of decisions.

When used with machine tools, numerical control is employed to control the following: all cutting speeds, the complete path and feed rare of a cutter in relation to the fixture or workpiece, and all auxiliary functions such as turning cutting fluid off and on.

Numerical control has been developed for use with various machine tools. One of these machine tools is known as a turret drilling machine. A six- or eight-position turret is mounted on the spindle so that preset drills and other cutting tools may be automatically changed. A work-piece is loaded into a fixture, and once started, numerical control selects cutting tools, positions the workpiece in a horizontal plane relative to the cutting tool, and clamps the table while machining is performed, controls the depth of travel for each cut, controls the flow of cutting fluid, and controls other auxiliary functions. The table position in a horizontal plane is controlled by two servo mechanisms, one for each coordinate axis. Many holes may be machined in a workpiece during a complete cycle. A fixture is used for locating and holding the workpiece, since drill bushings, which are used in drill jigs, are not needed. When the machining cycle is completed, the table is returned to its loading and unloading position. Machine setup is simple and fast. A tape, which has been made for the workpiece, is installed in the controller, the fixture for the workpiece is fastened to the table of the machine, and the table is positioned so that the cutting tool is located in space at its predetermined starting position relative to the fixture. The fixtures used are considerably simpler and less costly than drill jigs, which otherwise would be required for the workpiece. Other machine tools which have been adapted for use with numerical control include jig-boring machines, milling machines, and lathes.