I have a large round of wood (horizontal slice of a tree) which is about 20 inches across (circle diameter) and 8-9 inches deep.
I want to take the centre out of the wood, either by cutting a circle which is maybe 4-6 inches smaller in diameter than the piece (thus leaving 2-3 inches of wood around the outside) or by making some kind of cut that follows the shape of the wood (since its not a perfect circle). The second option here would be my preference but I can work with the first.
I'd also like to avoid cutting it into pieces or cutting into it from the edge and then having to re-glue but if thats the only option that might be a possibility. Even if I slice it in half somehow though I still have the problem of hollowing out two very large semi-circles.
Wherever possible I would like to keep the wood inside relatively intact so I can do the same thing to the centre-area I've taken out on a smaller scale (rather than it all being just wasted).
The options I have considered / tried are:
1) Bandsaw - I don't have one of these with a large enough throat I could actually cut the depth of wood on it. Also this would mean entering from the side and manipulating a very heavy and large piece of wood accurately moving it around to place the cut along the side.
2) Drilling adjoining holes - I have tried to drill holes through with a spade bit but have had issues. For one my spade bit is not long enough. Assuming I could get an extender of some sort and get it through to the other side its very hard to keep it 'on track' while drilling an adjoining hole since the rotation of the bit seems to want to pull it into the adjoining hole drilled previously. Once this happens it destroys the 'walls' separating the two touching holes I am trying to drill and makes it near impossible to keep on track. I have a pillar drill that might be able to help with this issue but then I am back to manipulating a very large piece of wood on a drill platform which is much too small for it.
3) Drilling separate holes then cutting through - I have also tried to drill holes which are separated by a small distance (e.g. 10mm). This is much easier to drill the holes and my plan was to use a keyhole saw to cut through from one hole to the other but making these cuts is very onerous, for one its hard to get the pressure required to get the cut through without trying to balance the wood so that the saw is cutting downwards (otherwise I'm trying to cut sideways or in some weird position) and I also have problems with the depth of the wood, particularly if the hole is not drilled all the way through.
4) Jigsaw - I have some quite long bits for my jigsaw but nowhere near long enough. I bought a reciprocal saw as I was able to get a 12" blade for it but it seemed to cut very slowly, suffer from absolutely horrible amounts of vibration, and I still had the problem of getting the blade through (admittedly could be resolved with an extender or a longer spade bit on my drill) and because the blade was so long I had my doubts about it being able to make the turns required to cut the circle.
5) Chainsaw - for a brief time (before I actually handled a petrol chainsaw) I had considered making plunge cuts with a chainsaw through the wood but now see a number of issues. For one oil from the blade might affect the final finish on the wood and plunging a chainsaw into an 8-9" cut just seems to be asking for the chain to bind and then either get perma-stuck in the wood or develop into some unanticipated and likely not very funny distribution of kinetic energy towards the delicate meatbag holding it.
6) Lathe - I don't have one but also I can't imagine it would take the weight of such a large chunk of wood.
7) Circle-cutter drill bit - I once had one of these that went maybe 10-11" across maximum. I haven't since looked or found a larger one but in any case the one I had wouldn't have been able to cut deeply enough to take the entire centre out, so all I would get is a circle groove which isn't much use. My previous experience with hole saws and circle cutters is things get more difficult the deeper you get so I would think this would be problematic.
I'm running low on ideas at this point as to how I get it done. The drill-and-saw-through method might work but it seems an inordinate amout of work. Probably the best I can think of is to spend a lot of money on a large bandsaw with a large throat depth, then build some kind of large adjustable height platform which would allow me to make the cut from one side, turn it round and then exit from another or the same side, then re-glue.
If someone has a solution whereby I can cut it into two halves (also not easy, best I can come up with here is some major elbow grease with a long-blade handsaw or again back to a bandsaw with large throat depth?), and then hollow it out then that would be interesting for other reasons too since I seem to often find myself wanting to hollow out shapes/curves/semicircles of wood which wouldn't be doable on a lathe.
Update for anyone interested:
I wanted to try some of these options before accepting an answer but it's taken me a while to get round to it.
I tried my preferred option by making a bow saw from a modern blade and some custom wood. The saw turned out OK and was able to saw a test piece no problems but because the depth of the cut it had to be able to make to cut the outer ring of the wood was so large it ended up being a very tall saw (around 80cm):
This made it very cumbersome to actually use. If the saw is held higher up then its there's way too much leverage to force the blade along and make the cut and the saw blade just sticks and the whole saw tilts instead. If held lower down then without the wrist strength of Hercules and Popeye combined the saw is just too hard to keep straight. Cutting anything other than horizontally had similar problems of managing the weight and tilt. I did eventually figure out a way around this by hooking a broom handle off a rafter and using it as a slide that I could lift and lower to adjust the angle of the saw. With this I was able to get the saw going but found that for some reason it was very difficult to get it to follow the curve of the wood. The back side of the cut seemed to curve slightly but the front side just wanted to go straight. Even given quite significant torsion on the blade (see pic) there seemed to be no way to get it to curve. Possibly the wood was just too thick for it to make the curve.
What I did find worked somewhat were auger bits which tracked very well (distinct from forstner bits which I hadn't realised and haven't tried) but which did need a wall-powered drill to have the power to get them through repeatedly and successfully. I also found drill bit extenders which would allow me to get them all the way through although the extenders themselves are quite wide and mean I have to use a wider auger bit than I would like. Even so, drilling so many holes and trying to get them to link up all the way through the depth of the wood is not easy and is very likely to end up needing further efforts to join the holes up from the underside due to them not being perfectly straight.
Auger bits are probably the best way to get the job done given my original requirements but it seems the only relatively quick and easy option is a bandsaw with a large throat depth, although possibly cutting the whole thing in half with a chainsaw and chopping bits out with that might be a good way to go if the centre wasn't important.