I need to drill a 10mm hole 200mm through timber 50mm wide. If these dimensions sound odd it's for a pin through a right angle finger join to another piece of the same size.
I start centred 25mm from the three nearest vertical faces but even with a drill guide and some care it is very difficult to emerge centred in the opposite face.
If my drill press had 200mm of travel it would be just the ticket, but it doesn't.
Is there a usual solution for this type of positioning problem?
Here's the jig I eventually created from one 50mm length and two 300mm lengths of 50mm angle iron. Holes are 10mm diameter with centres 25mm from nearby faces. Cut, then drill, then assemble as illustrated clamped around a sample job. Tack weld, remove timber and strengthen welds. Do not weld the inside seam, it would foul on the job when using the jig.
Clamped flush onto the work-piece, this jig positions an auger bit and aligns it parallel to the 200mm tall vertical face. When the board is cupped, place it against the concave side for stable alignment. The guides are 100mm apart so a 190mm auger bit will drill about 80mm into the work-piece.
The jig can then be removed and the hole used to align the bit for drilling through with adequate precision.
For optimal results combine this jig with the technique described in the accepted answer. For cupped boards this will require a double ended version of the jig due to the need for opposite chirality .