Noob looking for little advice...

[magicniner]

Something which sprang to mind for your PS is engaging the stepper with a clutch with interlocks to cut power to the spindle motor before the clutch can be engaged, especially easy if your stepper is driving the head through a belt as the driven pulley can be on bearings and run freely with the clutch disengaged.

[MRob]

Thanks for the reply - I did look into whether there were any off the shelf CNC clutch mechanisms that I could buy but couldnt find anything. I mean I could probably design something, or use e.g. lawn mower clutches but it is still essentially using steppers to mechanically engage each drive, one at a time. Which I can do, I just thought maybe there was another way! The only alternative that I saw elsewhere, was a suggestion to use a variable reluctance stepper, which is happy to freewheel, but my inclination would still be to disengage the main motor when the stepper is active to reduce mass being driven and backlash. At which point I might as well put both drives on a clutch. Perhaps I am being over the top with it though, and over-thinking the problem? Anyway, the interlock is an interesting idea, I hadnt thought of that!

[magicniner]

In that case a DC Servo with encoder might make more sense, then you can run it as a positional axis or at a set speed.

[MRob]

I looked into servos before, but the combination of speed (3500rpm), power (500W+ ideally) and high accuracy together in one package seemed like a stretch, my concern was it would be always be inferior in one area - or least without paying an arm and a leg, and then adding the control complexity on top... But perhaps I just gave up too soon, I'll have another look into it, thanks.

EDIT yeah definately gave up too soon, can get for £400 or so, much better option.

[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).