Why do you want to make sawdust? The dust is like the lowest value of the wood industry. Simply a by product of making usefull stuff outa trees.
Besides it seems to me that your going to have to use a pile of fuel to make sawdust with a tub grinder.
If you need green sawdust, go to a sawmill, tell them what you want and they will usually either give it ti you, or you might pay a little for a loading fee. If dried sawdust is what your looking for, the mill might have that if they do any finish work, but if they dont cabinet shops, and places that make spindles and stuff usually have a ton of it to get rig of.