How strong should my motor be

Here is something to get started:  Torque= Inertia x acceleration.  The acceleration is based on what the final turntable speed is and how fast to get there from rest.  The inertia of a circular disk is .5x mass x radius x radius or 1/2 M R^2.  You should also add the mass of the rotating parts that hold the wood. So for example say you want to have 1 rev/4 sec final speed. And you want to get there in 3 sec. If you control the motor for constant acceleration, accel = 1 rev/4sec/3 sec or 1 rev/12 sec^2. The inertia is .5x4x.24x.24= .111 kg m^2. The torque is .111 x 1rev/12 x 2*PI rad/rev  = .06 Nm or 8.5 oz inch. Now that is ideal torque at shaft of turntable not counting friction or drag. Once the motor gets to speed the torque only overcomes friction.  But just before it gets to final speed it is still accelerating. Round the 8.5 up to 10 oz inch and look for a motor graph of torque vs speed at a certain voltage (V you plan on using)  where the curve is above the point (10 oz inch, .25 rev/sec). Torque drops off with speed.  Yes you can use pulleys and belts to increase or decrease Torque motor  / Torque turntable.  Also all this is only looking at torque you need to make sure the motor will do the other things you want like max speed, voltage, etc.

Thank you so much, JB. Very informative, very helpful.

Now I know what I need to consider. It didn't enter my head to think how fast it would end up; I was only concerned with overcoming the inertia without making smoke. In my mind, any desired final speed would be well within its capacity if only it could get something that heavy moving. Like you say, only friction to overcome once in motion.

Now you've given me more to think about; greater control, how fast, gearing issues etc.

Thanks again,

Len