HEIDENHAIN SW 548328-05 DIN Programming - Manual (Page 6)

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 The spindle synchronization option G720 synchronizes the shaft
speeds of two or more spindles so that they rotate synchronously
with a gear ratio or a defined offset.(see "Spindle synchronization
G720" on page 380)
 In combination with the synchronization (G720) of main spindle and
tool spindle, the new "Hobbing" cycle (G808) is available for milling
external teeth and profiles.(see "Hobbing G808" on page 520)
 With G924, a "fluctuating speed" can now be programmed to
prevent resonance (see "Fluctuating spindle speed G924" on page
374)