I have a big slab which is a cross section of a large tree trunk. The slab is about 10 inches thick and 3 feet in diameter, so it is large, heavy piece of wood. I cannot lift it. I moved to its current location by walking it. It took two men to load it into the car.

I want to smooth the top perfectly flat. The problem is that it is very rough cut and there are gashes by the chain saw. Trying to use a rasp to even it out would take forever.

Therefore I am thinking maybe to use a router. The problem is that I would need some kind of rail system to keep the router moving in a level plane over the surface. Also, what kind of bit would be used for doing something like this? Is there a better approach than using a router? If a router should be used, how can I move it X-Y and keep Z constant over a large surface?

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    Google "router planing sled", that should get you going. There are a lot of DIY versions, as well as commercial ones. – Eli Iser Apr 17 '17 at 12:02
  • I agree with the router planing sled. There are a million articles and videos about making them. The type of bit you want to use is called a "bottom cleaning bit". It's basically a large diameter short length straight bit. – SaSSafraS1232 Apr 17 '17 at 16:57

There are some very good Youtube videos that exist on how to do router planing. Some links to help you on your way:

This one planes a tree slab similar to what you want to do.

This one has a similar method to the other video, but shows how this can be done on a larger scale.

A standard router bit should work fine, just make sure it is sharp and running at the right speed so there wont be any ugly tearout or burn marks. A wider router bit will not necessarily make for a quicker job, keep that in mind. Just make sure to get the slab oriented correctly so you will have as little planing as possible to do.

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I would plane it flat. It will require sharp planes because of the end grain and lot of work. Make sure it is cured out before you start or it will warp on you later.

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