Advice on 4th Axis

[m_c]

So scrap of paper calcs.
3KW at 24000RPM, gives 1.19Nm of torque. I just used the second calc down on this page - https://planetcalc.com/1908/ , as I can never remember the full formula! Other online calcs are available, that was just the first one I found.

It's worth a quick mention, that the speed you run the spindle at is pretty irrelevant, as this kind of electric motor is a constant torque source, so the torque remains reasonably constant up to the motors rated speed.
The only thing you do need to allow for, is the VFD may allow the motor current to go above the setting for a short period of time before tripping due to an over current situation, which IIRC is typically 150%, so the spindle may be able to output 150% of that rated torque briefly (torque is directly proportional to current).

For simplicity, we'll just round up to 1.2Nm for now.

Now running a 12.7mm cutter, we have to work out the force at the edge of the cutter.
Force = torque/radius.
radius = 6.35mm, or 0.00635m
1.2/0.00635 = 188.97637795N, but we'll round up to 189.

Now we know the cutter force, you can then use that figure to work out the torque needed for a 4th axis.
Torque = Force x radius.
80mm or 0.08m, torque = 15.12Nm
100mme = 18.9Nm
150mm = 28.35Nm

Now you have those figures, you need to consider a few things.
They are based on the spindle running at full power - This isn't likely.
All cutting forces being at the full cutter radius and perfectly tangential to the rotary axis. Again unless you happen to be taking a very shallow cut and somehow still manage to use full spindle torque, it's not likely.
So in that context the torque figures are likely to be high.

However, you need enough torque to prevent the cutter from grabbing and spinning the work, which is where you need a reasonable safety margin, as due to inertia in the spindle, the torque in such a situation could be very high, but as you'll be working in wood, I would very much doubt that would be as big a problem as a cutter suddenly digging in to a lump of steel/alu which will overcome an axis far more than a cutter digging into a bit wood.

Personally, I'd probably suggest aiming for 20Nm.
If you're making it yourself, 4:1 reduction belt drive should be doable. Combine it with either a big Nema 34 10/12Nm stepper motor for simplicity (and not much speed, but you don't really need speed for this kind of application), or a 750W servo (3000rpm will only be ~2.4Nm, but peak torque will typically be over 7Nm) if you want more speed. You could use a 1000W servo, but it all depends on how much money you want to spend.


Other option I forgot to mention, was Arc do a rotary table with stepper motor - https://www.arceurotrade.co.uk/Catal...-Stepper-Motor
However it would be slow, and using a rotary table for woodwork will likely lead to it eventually filling up and jamming with sawdust. Metalworking stuff really isn't that suited to woodwork, unless it is very well sealed.