Tricks to cutting large moritse and tenons.

[Joe Hillmann]

In the near future I am going to need to cut about 18-20 mortise and tenon joints in 4x12 beams. The parts that will get the tenons will only be 3 feet long so I will be able to cut them with a band saw and table saw without too much effort. The part that is going to be work is cutting the mortises which will be on 18 foot long beams.

Since this is a one time job I don't want to invest in specialty tools that are expensive and won't get any further use.

At the moment my plan is to mark out the mortise, drill the corners, cut out the bulk with a sawsall and clean and square everything up with a chisel. I am planing to use wedges to pull everything tight.

Does anyone have any tricks that will make cutting the mortises easier?

[Dick Strauss]

How about drilling out all of the mortises and squaring the corners/webs between drill holes?

You might find someone willing to drill them for you with a chainsaw mortiser if you can find someone that does timber framing.

[Steve Rozmiarek]

I'd hog out the mass of the waste with a forstner bit, so you can overlap holes. Might need a longer bit than normal, but they exist. Clean up the corners with the chisel, and there ya go, timber frame.

[Joe Hillmann]

I will take a look at the forstner bit idea, being able to overlap the holes would allow for much less clean up with a chisel. I will also ask around to see if I can find someone in the area with a chainsaw mortiser.

I have a feeling once I figure out how to best cut them it won't be much hard work just time consuming work.

[Richard Coers]

Drill out the waste, then with a homemade fixture and guide bushing, clean up as much as you can with a router. It will give you perfect walls to guide a chisel for deeper cleanup.

[Peter Quinn]

You might be able to rent timber frame equipment depending on your area, if not I'd call a timber framer and get a price on getting the work done. Some of the chain moritsers are portable, they clamp to the beam and go from there, so they might come to you. That IMO would be the least painful most expeditions way if available. Baring that I'd make a rig from a bench top drill press, somewhat close to working level, something with rollers or such that allows you to move either the press or the beam to get the slots you need. Tight overlap, not much clean up. You might be able to do that with a hand drill, but the over lap will not be as tight.

[Tom M King]

Look on my "structural" page on my website. I made some simple templates for a plunge router. Go as deep as you can with a router, and then hog out the rest of the depth with plumbers bits and drill. Then square it up with chisels.

It's not that bad of a job.

[pat warner]

Mortice, how deep, + any demand on the corner radius?

[Joe Hillmann]

They will be about 3x7 and probably all the way through the 4" width of the 4x12. I don't know what you mean by corner radius.

I am debating if I want to run the tenon all the way through and use a wedge driven in to pull everything tight or if I want to draw bore it and then only go 3 inches deep. I would prefer to drawbore it but with wedges I can tighten it up if gaps appear as the beams dry. The problem with wedges is they will be sticking out and there is a good chance of getting hurt on one.

[Kevin Jenness]

For a 4" deep through mortise without using a chain mortiser, I would mark out both faces and hog out the center from both faces with a drill bit or bits as close as possible to the edges. Then cut the mortise outlines using a router with a guide collar and template from one face, deepen that with a 2" top bearing pattern bit and come in from the opposite face with a 2" bottom bearing pattern bit. Square up the corners with a chisel, or round the tenon corners. Take your time with the routing; the long bits are subject to deflection if forced. If using a chain mortiser I would plunge in from the far (wedged) face of the beam to hide any blowout behind the tenon shoulders, and perhaps score the opposite face just outside the mortise to suppress tearout when plunging through depending on the material.

What are you making?

[Joe Hillmann]

It is the bed and rails of a bandsaw mill that I am building. I was planning to build a temporary bed out of 2x construction lumber so it could cut 9 foot logs but was given a load of 20 foot logs. My plan is to set up the mill with no bed, just attaching the rails to the black top in my parking lot so I can mill the first two logs to make the beams for the bed.

[Jim Matthews]

I would recommend laminating the beams from smaller stock.

That way, you could just leave a space where you want the mortise.
I would use LVL as it's very straight, and there will be little alignment quibbles.

legdetail.JPG

[Steve Rozmiarek]

I would recommend laminating the beams from smaller stock.

That way, you could just leave a space where you want the mortise.
I would use LVL as it's very straight, and there will be little alignment quibbles.

legdetail.JPG

I agree, you are going to want something that behaves nicely. Lvls will, and because it's not for arts sake, this approach would be far easier, and more stable.

[Gary Kman]

Register all of the joints on the beam off the same plane. That is to say if you have two mortises or a mortise and a tenon 12' apart on a beam that has a 2 degree twist between them you can't just clamp a fixture on to the spot where you are working. The twist or bow may not look like much on the beam but the joints aren't forgiving and the mating parts will really amplify the error.

[Mark Bolton]

We do them similar to Kevin's method but I don't rely on the opposing face to be parallel with the first especially with timbers. I use a u shaped jig to mill from both sides with a guide bushing and a 1/2 x 3 straight bit in the router. You can get about a touch under 5" this way.

If your not guaranteed to have timbers with parallel faces using the bottom bearing bit to clean up from the opposing face can result in a slight kink in a deep mortise.