A Brief Guide to CNC Machining

In this brief guide, we break down a very complicated process, CNC machining, into its basic elements to give you a better idea of what the process entails.

 

What is a CNC machine?

A CNC machine, or computer numerical control machine, is a machine which uses a computer to control tools. The original numerical control machines were developed in the 1940s and 1950s to create parts for the aircraft industry. These early machines produced templates from data on punched tapes. Now, computer software, such as a CAM program, is used to control the movements of the machine.

How Do CNC Machines Work?

CNC machines usually feature a version of a CAD software program. A CAD program, or computer-aided design, is software which allows you to draw what you want to cut. The drawing is either 2D or 3D and when completed it a creates a code for the CNC machine to read.

CNC machines work by following co-ordinates along axes. Basic machines move along one or two axes, but advanced machines, and those creating 3D items, will move along three axes. The machine’s tools follow thousands of co-ordinates, cutting and shaping as they move.

Before a CNC machine begins its first task, a trial run is conducted. This trial run is referred to as cutting air and is incredibly important as any mistakes could result in a damaged machine or part.

Parts of a CNC Machine

Input Devices: These are the devices which are used to input the part program in the CNC machine. There are three commonly used input devices and these are a punch tape reader, magnetic tape reader or a computer.

Machine Control Unit (MCU): This is the heart of the CNC machine as it controls the action of the CNC machine.  To do so:

  • It reads the coded instructions fed into it.
  • It decodes the coded instruction.
  • It implements interpolation ( linear, circular and helical ) to generate axis motion commands.
  • It feeds the axis motion commands to the amplifier circuits for driving the axis mechanisms.
  • It receives the feedback signals of position and speed for each drive axis.
  • It implements the auxiliary control functions such as coolant or spindle on/off and tool change.

Machine Tool: A CNC machine tool always has a slide table and a spindle to control position and speed. The machine table is controlled in X and Y-axis directions and the spindle is controlled in the Z-axis direction.

Driving System: The driving system of a CNC machine consists of amplifier circuits, drive motors and ball lead screw. The MCU feeds the signals (i.e. of position and speed) of each axis to the amplifier circuits. The control signals are then augmented (increased) to actuate the drive motors which then rotate the ball lead screw which positions the machine table.

Feedback System: This system consists of transducers that act like sensors. It is also called the measuring system. It contains position and speed transducers that continuously monitor the position and speed of the cutting tool’s location at all times. The MCU receives signals from these transducers and uses the difference between the reference signals and feedback signals to generate the control signals for correcting position and speed errors.

Display Unit: A monitor is used to display the programs, commands and other useful data of CNC machine.

Some limitations of CNC controlled machines

Depending on their age and sophistication, CNC machines can be limited to the capabilities of their control and drive systems. Most CNC controllers only understand straight line movements and circular arcs. In many machines, the arcs are restricted to the principal XYZ planes as well. Rotary axis movements can be considered like linear movements, just degrees instead of distance. To create arc movements or linear movements that are at an angle to the principal axes, two or more axes must interpolate (move precisely in a synchronized manner) together. Linear and rotary axes can also interpolate simultaneously. In the case of five-axis machines, all five must be perfectly synchronized – no easy task.

The speed at which the machine controller can receive and process the incoming data, transmit commands to the drive system, and monitor the machine’s speed and position is critical. Older and less expensive machines are obviously less capable in this, much in the same way that an older computer will work less well and more slowly (if at all) on demanding tasks than a newer one.

Conclusion

CNC machining is a complicated task that requires a trained engineer to get the desired results.  We hope this brief guide gives you at least an elementary understanding of this complicated process.

 

 

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