Any rules of thumb how deep a mortice hole should be. I gather the tenon should be a 1/3 of the width of the timber so on a piece of wood 42mm deep a 14mm tenon. How deep should the mortice hole be. The posts where the mortices will be cut are 70mm square and the tenon timber is 68mm x 42mm. I have a router with a 14mm bit but it's only 25mm long , I know you can pull them out of the chuck a bit but i'm still looking at a max of 35mm, can you buy longer if need be