PDA

View Full Version : Any reason not to turn the nut instead of the screw?



abn
01-16-2007, 04:18 AM
I notice that the machines I see turn the leadscrew while the nut remains stationary...is there any mechanical reason why one shouldn't keep the leadscrew fixed and turn the nut to move a machine member?

ptjw7uk
01-16-2007, 04:53 AM
Just it's easier to turn the screw otherwise how would you get the pwer to the nut if it is on the carriage.
Peter

John Stevenson
01-16-2007, 06:36 AM
Some CNC's and a lot of horizontal mills do this.
Motor on the knee for a mill driving the screw, only real advantage on a mill is you can reduce the table width if the motor is underneath.
Most horizontal mills are like this as it's easier to get power from a central 3 way gearbox to the table.

Disadvantages are that it's mechanically harder to do as the nut has to be in bearings that can control rotation and thrust often making it quite large.

One application where it does score is on a manual and CNC machine.
In CNC mode the motor drives the nut usually via a toothed belt drive, in manual or teach mode the handwheel engages the nut via a clutch, or bevel drive and allows manual movement.
Bevels and clutches are interlocked to prevent simultaneous use.

.