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CNC Basics · Chapter 4

Drilling by hand

When you drill a hole in a wall, you automatically do the same steps: set the drill, start it, press it in slowly, pull it out again. The machine does nothing automatically. You write every step into the program. Later you learn shortcuts. But if you know the single steps, you understand what the shortcut does and you can find errors.

A hole in four steps

Every hole follows the same sequence:

  1. Move over the hole with rapid traverse, in the air.
  2. Just above the surface with rapid traverse, for example to Z2. From here on you have to drill slowly. Until then, rapid traverse saves time.
  3. Drill with G01 down to the depth.
  4. Retract with rapid traverse to the safe height or, if the next hole comes right away, to Z2.

The tool must be changed and the spindle switched on beforehand.

(One hole: four steps)G90 G21 G17T2 M6S1500 M3G00 X40 Y25   (1. over the hole)G00 Z2        (2. just above the surface)G01 Z-12 F100 (3. drill 12 mm deep, 100 mm/min)G00 Z50       (4. retract)M5M30

G01 and the feed rate F

G01 is the working move: the machine moves in a straight line, but at the feed rate F instead of rapid traverse. F says in millimetres per minute how fast the tool advances. Inside the material you may only move with G01.

Without F the machine does not know how fast to drill and shows an alarm.

Depth

The depth is the Z of G01. It is negative, because the material lies below Z0: Z-12 means 12 mm deep. The workpiece is 40 mm thick. If you drill deeper than Z-40, you break through the bottom and trigger an alarm.

The machine remembers the feed

F stays valid until you write a new one. So you do not need it at every hole. As a guide for drills in aluminium: T2 with F 50 to 200, T3 with F 40 to 150. A feed that is too big can break the drill, one that is too small makes it hot.

(F stays valid until a new F)G90 G21 G17T3 M6S900 M3G00 X30 Y20G00 Z2G01 Z-10 F80  (F80 from here on)G00 Z2G00 X60 Y20G00 Z2G01 Z-10      (still F80)G00 Z50M5M30

Several holes

For several holes you repeat the four steps. Between two holes, retracting to Z2 is enough, because there you are still above the material. Only after the last hole do you go to the safe height. Short paths save time, so you drill holes in a row one after the other.

(Three holes: retract only to Z2 in between)G90 G21 G17T2 M6S1500 M3G00 X20 Y40G00 Z2G01 Z-6 F100G00 Z2G00 X50 Y40G00 Z2G01 Z-6 F100G00 Z2G00 X80 Y40G00 Z2G01 Z-6 F100G00 Z50M5M30

Common mistakes

  • G01 without F

    At the first G01 in the program the machine needs an F. If it is missing, it shows an alarm.

  • Positive depth

    Z10 moves up, not into the material. For the depth you write a minus sign: Z-10.

  • Moving sideways out of the hole

    If you go to the next hole right after drilling, you drag the drill sideways through the material. That is forbidden with rapid traverse. Pull out first.

  • Rapid traverse instead of a working move

    The drilling itself belongs in G01. Moving into the material with rapid traverse is stopped with an alarm.