mill 4th axis

Hello.
I have a 3T knee mill 1000mm x axis 400mm y/z axis.
What motor reducer you suggest for a rotating axis ?
Stepper with encoder and planetary reducer is good option ?

This is really a big big question. I've noticed most of the commercial 4th axes are worm gears driven by servos in the 30:1 to 100:1 range. That said, I've read good things about harmonic drives driven by servos or steppers. The biggest concern I had when deciding on my 4th axis was backlash. Harmonic drives and commercially built worm drives are really low backlash. I've always favored servos for their built in feedback, but steppers can also run encoders. I've never been a fan of open-loop axes of any sort.

TLDR: get the largest harmonic reducer you can afford and build a 4th axis around it.

The two (basic) issues with rotary axes are backlash and compliance.  Eliminating both gets very expensive.

A planetary reducer - especially a low-cost one - will have backlash, and so will a worm drive.  Both mechanisms are generally suitable for uni-directional indexing, but bi-directional movement will result in artifacts or dimensional errors.  Features machined from multiple orientations will not line up.

Worse still, if off-centerline cutting forces are applied, the part can rotate even with no commanded movement.  Imagine drilling straight down along the centerline - there's no rotational force on the rotary axis so all goes fine.  However, if you try to drill holes offset from the rotational axis the cutting forces will cause the rotary axis to move as the force takes up the mechanical backlash.  Same thing happens when side-milling above the centerline.

This problem can be reduced or eliminated by adding a brake to the rotary axis directly on the output shaft (platter).  A motor brake (stepper or servo) will stop back-driving, but not eliminate cutting forces taking up the backlash.

The above may not be an issue for light machining close to the centerline... but even very low-load engraving will result in artifacts due to backlash.  My mill came with a worm drive 4th axis and I quickly abandoned it due to the backlash issue.

Harmonic reducers eliminate backlash (more or less), and have come down in price dramatically in the past 4-5 years.  The downside with harmonic reducers is compliance.  Due to the nature of the flexible strain wave gear/cup, machining which imposes a high rotational force can cause the flex gear to twist under load.  Once the force is removed the reducer 'springs back' in to the desired position, but this flex can cause tool chatter or dimensional errors.  Generally speaking, a bigger reducer will flex less than a smaller one under the same rotational force since the larger flex cup/gear is stiffer.

As with planetary or worm drives, a suitable brake will stop chatter or flex in a harmonic reducer.  But a brake cant work during continuous rotational machining.

For light(ish) machining that isn't really far off the centerline, harmonic reducers are an easy solution to the backlash problem.  I now have a 32mm (through-bore) harmonic drive in a 6" diameter rotary axis and it works quite well.  But if I had wanted a 8" or larger mechanism - or I had a more powerful mill using larger tools - I would probably have gone up to a 40mm (or larger) reducer to increase the off-centerline stiffness.

At the upper end of rotary axis mechanisms are cam-roller drives and direct drives (torque motors).  Cam-roller mechanisms, especially preloaded versions, have zero backlash and are extremely stiff, fast, and accurate.  Direct drive motors are also excellent, but may not be as fast as cam-roller drives unless wound for higher speed (at the potential loss of holding torque).

Both are eye-wateringly expensive and are not as common on the surplus market as harmonic reducers.

In addition to info from spumco.

Commercial suppliers of 4th axis usualyl differenciate between positioner (brake opens, thing rotates, brake closes)
Movable axis which allow light milling up to heavy milling. (workpiece weigth torqure)
Movable axis that are even fast enough to do turning operations. (sometimes even with direct drive)

If a positioner is "good enough" it makes things a lot easier.