3- Fixed Cycles on the Mill - Rules
For a canned cycle to operate correctly, the programmer needs to know the code words that the cycle uses. The following is an example of how a G81 canned cycle is properly written:

G81 X1. Y1. Z-.50 R.1F6. G99

Five words in the block above are used in milling canned cycles and are explained below.

  1. The G word defines the type of cycle.
  2. The X and Y indicate the hole location.
  3. The Z word is the depth of the hole.
  4. The R word is to set the rapid plane
  5. The F word is the feed rate.

Note: the use of the word "word" to describe a meaningful string of programming text is derived from the EIA word-address programming protocol. The original NC programming language.

G98 tells the tool to retract to the initial plane.

The initial plane is the Z level when the canned cycle was called (not the rapid plane). G98 is good for avoiding clamps as long as the initial Z was higher than all tooling and clamps.

G99 tells the tool to retract to the rapid plane when the cycle is finished. Canned cycles already retract to the R plane but a G99 is a safe way to remember where the tool is going after it has completed the operation. Using G99 cancels G98 and visa versa.

G98 and G99 can be inserted in block of code anywhere in the canned cycle. This allows the programmer to shift at will between planes.

Note: the rapid plane is sometimes called the "retract plane" but the R word is used the same

Canned cycles are modal. Once they are turned on they stay on until cancelled. This means that every block of code after the start of a canned cycle will be interpreted as being part of that canned cycle process.

Always use a G80 command to cancel a canned cycle (note that the G80 is used after all movements are completed in the example on the next page). If not cancelled with a G80 command, every movement after the intended hole(s) may machine more unwanted holes or crash the machine.

All canned cycles will:

  1. Rapid to a specific X and Y location;
  2. Rapid to the Z rapid plane specified;
  3. Machine the hole to the G code specified.
  4. Exit the hole.

Here is an example of a properly written G81 canned cycle to drill four holes.

  • N008 G81 X1. Y1. Z-.2 R.1 F6 G99
  • N009 X1.25 Y1.25
  • N010 X1.5
  • N011 Y1.
  • N012 G80