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CNC Basics · Chapter 3

Tool and spindle

A kitchen mixer has different attachments: whisk, dough hook, blade. You fit the right one and set the speed. A CNC machine does the same. It has a magazine with tools and a spindle that turns the chosen tool. In the program you say which tool and how fast it turns.

Selecting and changing a tool

The spindle is the rotating part of the machine that holds the tool. Two words belong together:

  • T2 selects tool number 2 in the magazine.
  • M6 carries out the tool change: the selected tool goes into the spindle.

In this course there are six tools. You already know the first three from the tasks: T1 centre drill Ø6, T2 drill Ø6 and T3 drill Ø8.5.

(Load tool 2 and start the spindle)G90 G21 G17T2          (select tool 2)M6          (change the tool)S1500       (speed 1500 rpm)M3          (spindle on, clockwise)M5          (spindle off)M30

Speed and direction

S1500 sets the spindle speed: 1500 revolutions per minute. The spindle is not running yet, though. For that you write:

  • M3: spindle on, turning clockwise. All drills turn this way.
  • M4: spindle on, turning counter-clockwise. You only need that for tapping threads.
  • M5: spindle off.

Without a tool in the spindle or without S the machine shows an alarm.

The right speed

The thicker the tool, the slower it has to turn, because its edge runs through the material faster. Too fast makes the tool hot, too slow and the cutting edge is not guided properly. You will learn how to calculate the speed exactly in a later chapter. For the start a rule of thumb is enough:

  • T2 (Ø6) in aluminium: 1000 to 2000 rpm.
  • T3 (Ø8.5) in aluminium: 700 to 1400 rpm.
(The thicker the drill, the slower)G90 G21 G17T2 M6S1500 M3      (6 mm drill: 1000 to 2000 rpm)M5T3 M6S900 M3       (8.5 mm drill: 700 to 1400 rpm)M5M30

Changing safely

When you change the tool, it must stand above the material, at Z0 or higher. The order is always: move up, spindle off, change, set speed, spindle on. The lines with G01 in the example drill a hole. How they work is explained in the next chapter.

(Change safely: up, spindle off, change, spindle on)G90 G21 G17T2 M6S1500 M3G00 X20 Y20G00 Z2G01 Z-8 F100  (drilling, see the next chapter)G00 Z50       (up to the safe height)M5            (spindle off)T3 M6         (change the tool)S900 M3       (new speed, spindle on)M5M30

Common mistakes

  • Only T, no M6

    T2 alone only selects the tool. It goes into the spindle with M6. If you write M3 after it, you get the alarm "no tool loaded".

  • Spindle without speed

    M3 without S does not run. Write the speed before switching on.

  • Changing inside the material

    The machine refuses a change at depth. First move up with rapid traverse, best to Z50.

  • Wrong tool for the hole

    If you forget the change, you drill with the old tool. The hole then has the wrong diameter. Before every hole, check which tool is in the spindle.