CNC lathe

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modern CNC lathe, in this case also suitable for drilling
Haas Automation CNC lathe

A CNC lathe is a type of lathe and a CNC machine , i.e. a lathe in which the tool movements are controlled with numerically stored programs (CNC = Computerized Numerical Control ). In contrast to an NC lathe, most CNC machines have a memory for programs and an interface with which CAD drawings can also be read in and programmed directly.

Basic functions

A CNC lathe has a Computerized Numerical Control (German: computer numerical control). Computerized means that the processing information is read into the memory of the controller and can be used and saved again and again for processing without additional information being supplied. In contrast to this, with NC machines (Numerical Control), the control information must be read in from an external information carrier (for example, punched tape) in sets with each processing.

Numerical means that all target specifications are communicated to the control in coded form and are constantly compared in the work process by means of control devices :

programming

When working with CNC machines, it is important to create the necessary CNC program before machining. This can be done either directly on the machine (workshop programming), using data carriers (CD, floppy disk, laptop) or via a special network (DNC). Programming is basically in accordance with DIN 66025, and the manufacturer's instructions must also be observed. The beginning of a program is described with Oxxxx or with % xxxx , whereby the x are replaced by digits. You can also start or end z. B. be an order number. A so-called technology line is then created; information on tool number T…. , Spindle speed S… and the direction of rotation of the spindle (M03 clockwise / M04 counterclockwise). In order to guarantee an economical process, CNC programs should be as short and clear as possible. Letters (except N) are called addresses . An address with digits 0–9 is a command. The commands are entered in the order of the actual tool movements. Several commands for a machining step are summarized in one sentence (line).

Addresses

The most important addresses (letters) in a CNC program:

  • T ... calls the corresponding tool (e.g. T0101 or T01, T = Tool)
  • S ... selection of the spindle speed (two to six digits, e.g. S800 S = Speed)
  • M ... so-called modal functions (one to three digits, depending on the manufacturer, e.g. M08 coolant supply ON M = Miscellaneous)
  • G ... travel commands (one to three digits, manufacturer-dependent, e.g. G00 straight tool movement in rapid traverse G = Go)
  • X, Y, Z, U, V, W, I, J, K, C are coordinates to which the tool moves

So-called auxiliary parameters for length, angle and other additional functions are manufacturer-dependent, but are available on all CNC machines. The route and control commands can also be given as single digits, e.g. B. G1. G0 , M3 . A CNC control usually recognizes both spellings.

sentences

CNC blocks can begin with N and a block number. They contain all commands for a processing step and end with an end of sentence character (or line break ). The next record is only started when the previous one has been processed.

Example of a CNC program line:

G 01 X 135.5 Z 7.2 F 0.05 A 150 ;

The commands have the following meaning here:

  • G 0113 Work step, straight travel of the tool carrier
  • X 135.5 Move to absolute coordinates X135.5
  • Z 7.213 Move to absolute coordinates Z7.2
  • F 0.05 … with a feed rate of 0.05 mm / revolution
  • A 150 … at a 30 ° angle

These details are only an example of a section from a possible program with absolute coordinate values.

Zero points

The machine zero point is the start of the coordinate system specified by the manufacturer. Each CNC program normally refers to the adjustable workpiece zero . The workpiece zero point X (and Y ) is always the turning center on lathes. The workpiece zero point Z normally lies on the plane surface of the finished workpiece. This zero point is set manually in the control in Z when the machine is set up, whereby an allowance of approx. 1/10 mm is taken into account for a subsequent finishing process. The workpiece zero point can be shifted with program commands if necessary.

Axes when turning

Coordinate system,
blue = working plane G18

The reference axis when turning is the axis of rotation of the main spindle around which the workpiece and the chuck rotate. This is known as the Z-axis and points from the chuck towards the tailstock.

The plan axis X and the Y axis are perpendicular to the Z axis. The X-axis normally does not show the path of the transverse axis, but the diameter of the workpiece. The main cutting edge of the turning tool is set to the center of the workpiece in the Y-axis direction during set-up and is usually exactly in the working plane G18 or XZ plane. In special exceptional cases, such as parting turning, the turning tool can also be set slightly below the turning center.

Rotation angles around the main axes are designated with A (rotation around X), B (rotation around Y) and C (rotation around Z). Other axes can u. a. be those of tool turrets, steady rests, powered tools, counter spindles or tailstocks.

Tools

For CNC machines, as for other lathes, there are a large number of tools for almost all common operations. The most important tools are various indexable inserts for the machining of round or edged material as well as drills and taps. For some years there have also been driven tools with the help of which z. B. can also perform milling work on conventional CNC lathes or engrave . Special measuring tools for measuring produced parts should also be mentioned. One of the most important features of a CNC-controlled machine is the possibility of using several different tools for a variety of machining operations in one operation, without having to set up the tools during the operation. Even with simpler machines, up to 99 tools can be programmed. The disadvantage with such machines, however, is often the lack of space in the machine's work area, so that often only a maximum of four to five tool holders are used. Special machines (CNC with tool turret , machining centers) can accommodate up to 50 tools in already integrated holders.

automation

On more modern machines, extensive automation is possible with the help of feeder and removal systems, in some cases machines can also carry out measurements independently, which further relieves the operating personnel and further improves productivity. Such operations can be designed very economically, as qualified personnel are often only required for setting and setting up the machines.

use

On the one hand, working on CNC lathes makes work easier for the machine operator, since a program that has been saved once is processed again and again, while a lathe operator on a manually or mechanically controlled conventional lathe always has to observe the machining process and intervene if necessary. On the other hand, the preparatory work is also more complex, because when setting up or setting up, the processes of the later work have to be thought in advance (and in some cases programmed).

Only simple functions such as the coolant supply or an automatic tool change are actually controlled by a PLC (i.e. without feedback).

advantages

  • Productivity increase
  • High quantities with consistent quality
  • Reduction of tool wear (through constant conditions)
  • No manual intervention required (apart from tool offsets)
  • Constant production times (production can be planned)
  • CNC machines can be connected to each other (manufacturing systems)
  • Great variety of processing options
  • Multi-machine operability
  • Machining complex workpieces
  • Further improvement of automation through robots , loaders, embedding in network systems (DNC) , etc.

disadvantage

  • High acquisition costs (machine plus control), but these are getting lower and lower due to the huge drop in prices in the electronic control sector. From an economic point of view, purchasing a conventionally automated machine tool is only worthwhile in exceptional cases
  • High development requirements for work preparation
  • Due to the complexity of the systems, maintenance and service usually have to be carried out by external service providers
  • Monitoring work while production is running becomes routine
  • Different assignment of control and route commands from individual manufacturers

literature

  • CNC technology. Vocational school edition, Christiani Verlag, Konstanz 2009, ISBN 978-3-86522-427-9 .
  • Manufacturing process. Part 1, turning, milling, drilling. Springer Verlag, 2008, ISBN 978-3-540-23458-6 .
  • Dietmar Falk: CNC Compendium PAL turning and milling. Westermann, Braunschweig 2010, ISBN 978-3-14-235027-1 .
  • Peter Schierbock: Formulas and tables for metal engineering professions. Bildungsverlag EINS, Troisdorf 2009, ISBN 978-3-8239-7140-5 .

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