Yes, youcanuse this step-down transformer for your CNC controller.
Youcan calculate by using the simple electrical power formula.
Power (Watt or W) = Voltage (Volt or V) * Current (Ampere or A)
The sign * means multiply
For your case.
Your CNC controller need power = 24V * 6.3A = 151.2W. Calculate back to 120V it need the cuurrent = 151.2W/120V = 1.26A
The step-down transformer can supply power up to 200W (At the output of 120V, the cuurrent is 200W/120V = 1.66A and it can handle output peak current up to 1.8A)
Hope this can help.