I have a severely twisted beam (approx. one inch). Looking to make a tenon on end to fit into a mortise joint. Any suggestions on how to achieve this.

scribe it.

Scribe it, snap reference lines on it and square rule it, plane the twist out, or cut it into firewood. It may keep twisting.

how does the grain run on it? is at at a heavy angle to the faces, or is it visibly spiraled? If so, throw it away, it will never be stable.

If it's just angled a little bit or slightly spiraled, it may be done twisting now (assuming it is dry or close to it) and so you could use it. My opinion, as stated earlier, is that scribing it would be the way to go. But then again, that's my opinion for frame layout in general!

Remember though, with scribing you still need a reference (aka datum) line or else your scribing will be totally arbitrary and will never fit right.

Hand plane a flat section in the middle of the timber.

Place a level across this section(for now it does not have to be level).

Go to one end of the timber and place a framing square on it with the tongue hanging down, shim the square until it is in plane with the level at the center of the timber.

Gently slide the square so the tongue just touches one edge of the twisted timber. It may be the top edge or the bottom edge.

Mark 1-1/2" or 2" on the top face and on either side based off the now carefully placed square which is in plane with the center level.

Repeat on the other end and snap the lines connecting those points. Use these lines as reference/datum lines in either scribe or square rule.

Do not use the faces of the timber as a reference by holding the square on them, only the now dictated snapped lines should be used.

How long is the beam? If it's say, over 16' or so and not too girthy you may be able to take the twist out of it. I once had an 8x8x20' DF tie beam that was so twisted that when I assembled posts to it one post end was 3' off the floor deck. With gentle persuation I managed to unwind the beam. The tenons were housed so there was no danger of twisting them off. That was 3 years ago and no massive twist checks or signs that it was forced straight.

I had a few wild spruce timbers I had to deal with recently and here's what I did:)

plumb the timber in the center (will take out twist evenly on both ends)





plumb the ends mapping only the lines you need  (no sense of snapping too many lines or measuring off the center line)







I use the plumb lines on each end to establish the level lines on the other two faces if needed, and again only snap the lines I need ie. instead of a center line, I snap 4" off the edge for 2" M/T layout which becomes inside line of the M/T.