Cnc machining nm09/2

4.7 Program validation

4.7.1 Computer graphics

Advantages

1. 
   Visual of Tool path geometry.
   
2. 
   3D graphics will also show Z AXIS motion of milling cutters.
   

Limitations

1. 
   Graphical resolution is often limited to the resolution of the CRT (monitor).
   
2. 
   Graphical tolerances are often greater than what the MCU will permit. Often what works on a computer graphics display will cause error messages from the MCU.
   

4.7.2 MCV graphics

Advantages

1. 
   MCU graphics are relative to the machine slide controls so that what the graphics accept is what the machine accepts.
   
2. 
   MCU graphics often allow ‘dry run’ validation at a rapid traverse speed. This saves time in proving long programs.
   
3. 
   MCU graphic often include a clock which will time the process as if it were actually cutting.
   

Limitations

1. 
   MCU graphics can not detect other problems such as incorrect tool offsets, incorrect program zero or any errors in the physical dimensions of the workpiece.
   
2. 
   MCU graphics are often limited to two axis displays which, in the case of machining centres, does not allow the checking of Z axis motion.
   

4.7.3 Single block machining

Single block validation requires the machine to be manually switched to single block mode via the MCU soft keys. With this feature set each block is first read and then executed by the operator pressing the cycle start button. During all rapid traverse motions it is good practice to manually override the feed rate to about 10-20% of the feed rate. This will slow the tool traverse to a speed which allows the operator to make any emergency stops well before a crash.

Advantages

1. 
   All axis motion are seen in relation to the size and location of the workpiece.
   
2. 
   Feeds and speeds can, be validated and also fine tuned during the cutting cycle using the manual override.
   

Limitations

1. 
   Can be time consuming in the first instance.
   
2. 
   Error messages will often ‘lock up’ the program until the edits are made.
   
3. 
   Because the tool dwells in the same position between each block ‘ringing’ will occur and surface finish may be impaired.
   

4.7.4 Printer/Plotter path

Normally you would find that any computer terminal would be connected to a printer or a plotter which could be used to generate the tool path of a part program. Depending on the required accuracy and resolution of the printout the printer used could be something as simple as a nine pin dot matrix printer or something as sophisticated as an ink jet drum plotter.

Advantages

1. 
   A visual full size plot of the tool path is produced as a permanent record.
   

Limitations

2. 
   Plots are only two axis therefore other axis such as the Z axis are not shown
   
3. 
   Like computer graphics the tolerance for plotting are quite large which again means that what ‘appears’ to be correct may call up MCU error messages.
   

4.7.5 Pen plotting program path

This method is generally used to plot a part program for a Machining Centre. The process requires a spring loaded pen/marker to be manually driven down to the Z zero reference face of the workpiece. Once at Z zero, the Z axis is locked using the MCU soft key/switch (this has the effect of ignoring all Z axis commands) the table is then driven back to the ‘machine home’ in the X and Y axis and the program started either in ‘feed rate’ or ‘dry run’ mode.

Advantages

1. 
   Tool path is relative to the workpiece.
   
2. 
   Accuracy is relative to the program data processed by the MCU.
   
3. 
   Miscellaneous codes can be proved.
   

Limitations

1. 
   This method can only validate X and Y axis coordinates.
   
2. 
   MCU will often ‘lock up’ the program at errors requiring editing. This in tum often requires the program to be restarted from the beginning if a sequence restart facility is not available.
   

4.8 The computer

The following items are designed to visit some key points, terms and definitions.

Types of computers

4.8.1 Mainframes and supercomputers

In computer jargon, large computers are called mainframes. Mainframes have assess to billions of characters of data and can process data very quickly -millions of instructions per second. The price of a mainframe varies from several hundred thousand dollars to over $10 million. Clearly with that kind of price tag, you will not buy a mainframe to use as an electronic diary. Generally they are used by governments, big business and industry.

4.8.2 Minicomputers

Computers with less storage than mainframes are called minicomputers. They are slower but less costly than mainframes, around $10,000 to $50,000. Minicomputers were intended to be small and serve some special purpose. Improvements quickly turned them into a very versatile machine. Today the line between minicomputers and mainframes is somewhat blurred, to a point where the name, mini, no longer seems to fit. The term super mini has been coined to describe minis at the top end of the spectrum —up to $500,000. These are widely used by government and business.

4.8.3 Microcomputers

Small computers such as desk top, personal or home computers are called microcomputers. For many years the computer industry put much, if not all, of its research effort into big computers. Small computers, and those suggesting a niche for them, were scoffed at. Time has proved them right however. Relatively inexpensive, small computers are now readily available.

4.8.4 Mini-microcomputers

The trend to smaller computers continues. Small mini-computers better known as Laptops are now part of the computer market. Many are as powerful and have the same capabilities as a PC. The advantage to Laptops with their size, weight and the fact they can run on battery power, is they can be taken anywhere. Many sales engineers do calculations and enter orders on their Laptops on-site and transmit the information to their office over a telephone using a modem.

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