
B–62073E–1/042. INTERFACE BETWEEN CNC AND PMC–NA OR NB
– 196 –
(c) Threading with a desired gear ratio
In threading, the single rotation signal of the spindle is required to synchronize the spindle and the Z–
axis. When a motor with built–in position coder is used to execute threading with a desired gear ratio,
the single rotation signal of the spindle must be sent to the CNC unit as illustrated below. (The figure
below shows a sample system containing a FANUC spindle motor and spindle amplifier.)
CNC
Position coder signal
Signal converter
Feedback (P)
Position detecter
Spindle motor
M:N
Z
L
Tool
Work
1 revolution signal
1 revolution signal detecter
Spindle AMP
M: Number of position coder gear teeth (pcs.)
N : Number of spindle gear teeth (pcs.)
L : Thread pitch (mm/rev)
P : Number of pulses received by position coder (pulse)
Z : Byte move distance (mm)
Spindle
∆Z can be calculated as follows:
Z +
L
4096
P
M
N
CAUTION The gear ratio parameters must not be changed during threading or other machining.
NOTE 1 Under the following conditions, the threading error per pitch does not exceed the value
obtained by the expression shown below:
⋅ There is no deviation due to servo delay or acceleration/deceleration delay.
⋅ The spindle rotates at constant speed.
⋅ The mechanical system is free from deviation.
The threading error can be reduced by smoothing (setting the number of times sampling is
executed in parameter 5611). (If parameter 5611 is set to 4, the error can be reduced to 1/16
of its value.)
The threading error will not accumulate because it is the maximum value per pitch.
NOTE 2 When the gear ratio is specified as shown in the figure of (c) above, the position of the single
rotation signal of the spindle deviates by the amount calculated by the following expression:
360/4096/780×S (degrees) S: Spindle speed (rpm)
L : Screw pitch (mm/rev)
L
4096
M
N
(mmńrev)
(This expression is obtained by setting P to 1 in the expression described above.)