Noob looking for little advice...

[m_c]

There's a reason why lathes with live tooling have huge horse power motors. It's not that you're ever likely to use 30+hp while turning, it's so they still have enough torque to maintain a stationary position.

Given you're working with wood, the forces involved shouldn't be as high as metal, but you can still calculate the potential forces. If you know how big your live tooling motor is, and how much torque it can produce, you can then calculate the potential maximum force on the cutter edge. You can then take that force and calculate the torque the spindle needs to produce to hold steady (plus add a safety margin).

For ideas, search out the In-turn (there's a huge build/development thread on the mach support forum), which is a dual speed 4th axis add-on.
However, having a suitably geared motor only connect for positional work is a valid option. At low speed, having the high speed motor still connected shouldn't be a problem. It will add a bit more inertia, but I'd be looking at keeping the low speed at under 60rpm, so that additional inertia is going to be minimal, and reduces the level of complexity you need for changing speed.
The big problem is how you handle backlash in your positional motor drive system.

[MRob]

Nice thanks - I have found the thread, or another related thread, lots to dig through there.

You mentioned you didnt think that extra inertia will be a problem, but that backlash in the positional motor will be - so, where is this backlash coming from?

The servos I was looking at last night were in the 1.5-3kW range, not exactly 30+hp! Damn that would be a chunky servo :). I'll see if I can figure out my torque needs then.

[m_c]

Looking at figures, say you have a live tooling spindle capable of 1Nm (pretty high for anything wood cutting).
Now you have a 10mm diameter tool, which means the force is acting at a radius of 5mm, or 0.005m. So 1Nm/0.005m gives us potentially a 200N force at the cutter edge.

Now if you apply that 200N at a radius of say 50mm, or 0.05, it translates to 200Nx0.05m or 10Nm.
And that's before you consider what can happen if the tool grabs the work and takes a far larger cut, at which point you have the additional inertia of the live tooling spindle to drag the workpiece around, so you need to allow a reasonable safety margin. And any backlash will increase the likelihood of grabbing, unless you stick to conventional milling.

Off course, those figures are dependant on the tooling being able to transmit that much torque without snapping. It's worth spending some time running some figures through a speeds and feed calculator (FSWizard online I think gives torque figures, if not HSM Advisor has a free 30day trial which definitely does).

[MRob]

OK, I understand, ratio of forces from the diameter of the cutting head to the diameter of the workpiece - good to know. I'll check out the resources.

Think I gave my specifications wrong to begin with too - I see that people are classifying the slow rpm rotary as another axis to the high speed. So, I need continuous on the 4th axis and I would very much like to be able to change the angle of my tool, but that doesnt need to be continuous. And perhaps I could add this in later. 5 axis then.

... just to get a "finger in the air" idea though, how big a project do you think this is? Can I do it on a budget of under £2k??

[magicniner]

If it's your primary spindle on a lathe it's your Z Axis, not your Fourth Axis.
If it's not then add a diagram as I'm lost! ;-)

[MRob]

All these axis! OK so, two axis for positioning the tool (X and Y), then a rotary table axis on the tool (no idea what thats called... doesnt need continuous, maybe just change angle a couple of times per part). Then the workpiece spindle axis which should be able to both go at speed, and precision angle controlled (which I always thought was the C??).

[magicniner]

(which I always thought was the C??).

There are VMC, HMC and Lathe Conventions, Mill/Turn adds complexity and a degree of the manufacturer making it up as they go along.
On a lathe the main spindle is on the Z Axis.