instead of X10.0
Another common error is the change of G-code mode, for example:
| G00 Z2.0 | ... rapid to 2 mm above top of part |
| Z-5.0 F150.0 | ... feed to 5 mm deep at 150 mm/min |
| ... |
Feedrate is NOT effective in rapid mode
Missing Decimal Point
Also a very common error is a missing decimal point. This is an area that requires a little deeper explanation. Try to answer this question - what is the decimal equivalent of a program entry of X34720 ?
Don’t rush with the answer yet - ask yourself another question first - what units are used in the program? Machine shop programmers and operators who work with both metric and imperial units have to think twice. Here is some background for better understanding.
In the very early stages of numerical control, the decision was made to limit every dimensional entry into the maximum of eight numerals (8-bit format), ranging from 00000000 to 99999999. At the same time, virtually all machine NC (hard wired) controls have been set to accept leading zero suppression as default. Its definition is quite clear:
Leading zero suppression allows all zeros before the first significant digit to be dropped
The example of X34720 is in reality equivalent to X00034720, with insignificant zeros dropped. Decimal point programming did not become widespread in part programming until the middle or even late 1980s (!). Until then, the decimal point had to be implied.
The rule was as simple as it was logical:
For METRIC units, the implied decimal point is THREE decimal places left of the last of 8 digits
For IMPERIAL units, the implied decimal point is FOUR decimal places left of the last of 8 digits
In both cases, the number of decimal places indicates the smallest resolution - the smallest amount of motion possible for each system of units. That means the example of X34720 is equivalent to:
▪Metric units:
X34720 is equivalent to X34.720 (= X34.72) mm
▪Imperial units:
X34720 is equivalent to X3.4720 (= X3.472) inches
For those interested in the trailing zero suppression (not a required knowledge), the same program word X34720 would have to be written as X0003472 with the following results:
▪Metric units:
X0003472 is equivalent to X34.720 (= X34.72) mm
▪Imperial units:
X0003472 is equivalent to X3.4720 (= X3.472) inches
Trailing zero suppression is not used on CNC machines, because of its impracticality.
As this section covers the subject of logical errors that cannot be normally detected by the control system, there is at least one notable exception:
In G41/G42 cutter radius offset mode, the control system will issue an alarm when the cutter radius is greater than the inside arc radius
More on the subject of Cutter Radius Offset is covered in a separate comprehensive chapter (page 135).
Entering the wrong offset amount into the control system is also an error that can have serious consequences. For every dimensional problem, check not only the program but check the offset settings as well.
If parts to be machined frequently change from one unit system to another (metric to imperial or vice versa), a common error is to use a frequently used dimension is one format and using it in the other format.
For example,
G99 G81 R2.0 Z-10.0 F200.0
... is a normal metric example. In imperial units, the feedrate start point in R-level will be 2 inches, and - even worse - the depth of cut will be 10 inches. Feedrate will also be extremely fast, at 200 inches per minute.
Many other programming errors can be listed, but the most common have been covered.
| 4 | CONTROL SYSTEM |
The control system is the heart and brain of all CNC machines - it is their main component. In fact, this whole handbook is about the control system of a CNC machine. The abbreviation ‘CNC’ stands for Computer Numerical Control, and is commonly referred to as the control or the control system or the control unit and the even more common expression - CNC Control, in short. Although the expression CNC control does not follow any exact rules of the English language because of literal redundancy (Computer Numerical Control control), it is still a commonly accepted form of expression, at least in informal communications, in some cases including this publication.
Regardless of any specific description applied, the control system of any CNC machine can be a complex entity and often intimidating for new users. There are many aspects to the control system that are used by skilled professionals in different fields. For example, an electronic expert will look for different features of the system than a mechanical expert. Features relating to the CNC programmers and CNC operators will share many features, but each area of interest will always be unique. From a more practical perspective, both CNC operators and CNC programmers look only for those control features that are required in their respective fields.
Looking at a CNC machine (mill, lathe or other), the control system is identified by its visual components, in the form of a large panel, where its most noticeable features are the display screen and a keyboard or a keypad. Various switches, buttons, lights, etc., are also part of the control system. A short summary lists the most important visual features of any control system:
▪Display screen (monitor) with soft keys
▪Power ON/Off switches
▪Keyboard
▪Handle
▪Push buttons - Toggle switches - Rotary switches
▪Confirmation lights
▪Alarms and Errors
▪... any several others
Each of these features is covered in this handbook in sufficient detail. While the overall control