If you’ve already learned all of the Preparatory and Miscellaneous function codes, it’s time to move on to the Letter codes for CNC programming. Most of the letters of the alphabet are used on milling machines.
Just like the G and M codes, not every machine uses the same Letter codes. Also, there are several letters that are used in more than one function, but that depends on the input units.
Below is a list of the most commonly used letter codes when programming on a milling center. However, I recommend reading through your machine’s manual to confirm that they have the same function, or if your machine uses different letters/codes.
- A – Rotary or indexing axis around the X-axis (unit in degrees)
- B – Rotary or indexing axis around the Y-axis (unit in degrees)
- D – Cutter radius compensation offset number
- F – Feedrate function (may vary)
- G – Preparatory command (G-code)
- H – Tool length offset number
- I – Arc center modifier for X-axis (radius)
- J – Arc center modifier for Y-axis (radius)
- K – Arc center modifier for Z-axis
- L – Repetition count for subprogram/fixed cycle
- M – Miscellaneous function (M-code)
- N – Block or sequence number
- O – Program number
- P – Subprogram number call; Work offset number (used with G10); Dwell time in milliseconds; Block number in main program when used with M99
- Q – Depth of peck in fixed cycles G73 & G83; Shift amount in fixed cycle G76 & G87
- R – Retract point in fixed cycles
- S – Spindle speed in Rotations per minute (RPM)
- T – Tool function
- X – X-axis coordinate value designation
- Y – Y-axis coordinate value designation
- Z – Z-axis coordinate value designation
Some letters have multiple uses that you may have to incorporate in your program. “P”, for example, can call out the time that you want to dwell (pause) with a tool, or it can call up a subprogram number.
It’s up to you to learn these if you want to know how to create and edit programs. A lot of the letters are easy to remember, so if you already memorized all of most of the G/M codes then this is a piece of cake.