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      Advice on tools and methods for mortising big timbers in non-structural situations. February 11, 2010

I am building int. large beam truss work with 8x12, 10x14, and 10x12 doug fur lumber. What is the best way to mortise for intersections and locking together?

Forum Responses
(Cabinet and Millwork Installation Forum)
From contributor J:
You are designing trusses and want engineering for the connections? If they are load bearing, you better talk to an engineer. If they are for decoration, then butt joint them and put a metal strap across the top of them.

From contributor K:
What does the "int." in large beam trusses refer to? Is that interior large beam trusses? If so, and you want to assemble a timber frame indoors you will have an impossible time with this if you’re not experienced with timber framing to start with. Generally, you need a lot of men or a forklift to move the timbers around, a crane to erect the frame and clear space around it to do the assembly. Please explain your situation more in depth! I have done a little timberframing over the years and am a timberframe enthusiast.

From the original questioner:
I do not want to know how to connect them I want to know what the best tool is to mortise them I have a beam saw, an O&M band saw. Is a chain mortise tool worth the price to mortise beams? All the beams are decorative only no structural.

From Gary Katz, forum technical advisor:
A good router with a long upcut spiral cutter and a few templates for the mortises; circular saw and sharp chisels for the tenons. We've done it that way lots of times - typical finish carpentry tools.

From contributor K:
I have a Sauer 1 1/2" chain mortiser and it worked well on white oak timbers. Be sure to keep the chain tight and lubed. Before I had that I also drilled with a 1 1/2" spured auger bit in a big Milwaukee 1/2" right angle drill then used a corner chisel and slick to smooth the cheeks of the mortise. I went to the mortiser because I had enough jobs to pay for it. I like the chain mortiser it worked well once I got used to it. I paid $ 2,500.00 for it so, obviously buy one if you are going to us it enough to pay for it. Otherwise, drill then chisel. The router idea Gary mentioned isn't a bad one but it will only take you so deep before you still need to chisel. Since you are not building structural, maybe the router with the longest straight bit available would be all you need. Then peg it.

From Gary Katz, forum technical advisor:
Ditto contributor K. That's what I was figuring. Since it's not structural, the joints don't need to be that deep.

From contributor D:
Make a template and route them out - near exact. Take a sharp gouge and round out the edges of the tenons.

From contributor S:
I use a 1/2" upcut spiral router bit for the mortise. I will scribe a layout on the beam plunge rout in the center, and then climb cut small passes until I get to the layout lines. I leave plenty of line and clean up the sides with a 3/4" paring chisel. I cut the tenons with a 1/2" pattern cutting router bit and a straight edge.

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