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17 Canned Cycles for Drilling
148
9. with G98, rapid-traverse retraction to the initial point
10. -
17.1.9 Rigid (Clockwise and Counter-clockwise) Tap Cycles (G84.2, G84.3)
In a tapping cycle the quotient of the drill-axis feed and the spindle rpm must be equal to the thread
pitch of the tap. In other words, under ideal conditions of tapping, the quotient
must be constant from moment to moment
where
P is the thread pitch (mm/rev or inches/rev),
F is the feed (mm/minute or inches/minute),
S is the rpm of spindle (revolutions/minute).
The spindle speed and the feed of the tapping axis are controlled completely independently in left-
hand and right-hand tapping cycles G74 and G84, respectively. Accordingly, the above condition
cannot be fulfilled to full accuracy. This is particularly applicable to the bottom of the hole where the
feed of the drill axis and the spindle speed ought to be slowed down and stopped in synchronism,
and accelerated so in the opposite direction. This condition cannot be fulfilled from a controlled
point of view in the above case. The above problem can be eliminated by a spring tap, that would
compensate for the fluctuations in the value of quotient .
A different principle of control is adopted in drilling cycles G84.2 and G84.3 enabling rigid tap
(tapping without spring). There the control maintains quotient constant from moment to
moment.
The control will regulate only the speed of the spindle in the former case, in the latter case its
position is also controlled. The movements of the drilling axis and the spindle are linked through
linear interpolations in cycles G84.2 and G84.3. In this way quotient can be maintained
constant in the acceleration and deceleration stages as well.
G84.2: Rigid tapping cycle
G84.3: Rigid counter tapping cycle
The above cycles are only applicable with machines in which the spindle is fitted with an encoder,
and the main drive can be fed back for position control (parameter INDEX1=1). Otherwise the
control will return error message 3052 ERROR IN G76, G87 when the mode is called.
The variables used in the cycle are
G17 G84._ X
p
__ Y
p
__ Z
p
__ R__ F__ S__ L__
G18 G84._ Z
p
__ X
p
__ Y
p
__ R__ F__ S__ L__
G19 G84._ Y
p
__ Z
p
__ X
p
__ R__ F__ S__ L__
The spindle comes to a halt at the end of the cycle, if necessary, it has to be re-started by the
programmer.
The feed and the spindle rpm have to be specified in conformity with the thread pitch of the tap.