Dovetailing Mortise and Tenon Timber Joists

[mnboatman]

Though I am not finished with my current shed project, I'm thinking about building a bigger cabin with a  post and beam style in the future.
I came across this router jig system for timbers:


It looks slick but is too expensive:
http://www.timberwolftools.com/tools/arunda/arunda.html

Has anyone used this product?
Has anyone made their own router jig for mortising loft floor joists?

[Redoverfarm]

Rather up front joint.  I am sure you could make your own template and with the use of a collared bit it could be done for a fraction of the cost.  As with anything else the "layout" determines the quality of the fit.  So if you incorporated layout notches/lines in your jig you will get a good fit 90 deg on your stock.

[rick91351]

The biggest problem I see with this system and a one time cabin craftsman is the price of the bits plus the router that it would take to turn them.  I have never seen a 220 powered router in any the big box stores that I know of. 

There are only a couple 110 wired routers I know of that might handle that bit.  Like a PC 7518 or a Milwaukee 5625-20 router that would or might turn that bit well enough to be satisfactory.  Both of those $300 or so shopping around.  Milwaukee advertised at 3.5 the PC 3.25 horses.  That or you find a 220 powered router on the used tool racks some where    or pay like a $1000 for a new one.  Unless someone here knows of an outlet that handles them cheaper.

You could remove as much by hand with a couple cuts with a worm drive saw angled to give you a little material to rout out and a good sharp timber framing chisel.  Then using a 3 plus horse 110 router  basically to finish it out.  As far as a jig with a little messing around with some hard board or sheet metal you could craft one I would think.

Thinking out side the box you basically can do the tenon with a worm drive or a skill saw.  Mortising that would not even be that hard with some good measured markings and a little time and practice         

[Don_P]

Jigging that like John said would work fine. I do have the large PC router but could do it with a 1-1/2 hp by wasting out the majority with a straight sided bit in multiple passes and then the dovetail in a final pass.

But, this joint sucks. If the wood is well seasoned I'll take half of that back.

A drop in dovetail does two things that I don't like. If the wood is green the joist width will become narrower as the wood dries. The male tennon part of the dovetail will also become narrower. The mortise in the length of a girder is not going to change appreciably in size. A timberframe I was the carpenter on was built using this joint on floor joists and roof purlins in green oak. The tennons were ~7/8" long, the mortises were 1" deep. It's been about 5 years and the floor joists have shrunk and withdrawn from the mortises, the worst is 5/8"... think about how much wood is left holding that joist up  . I'm not happy and trying to figure out how to pull it back together without disturbing things.

Next, a drop in joist weakens the girder more than was previously thought, our guts were off. It used to be thought that the effective section to be used when sizing these girders was an inverted T shape if joists were coming in from both sides of the girder. In full size beam breaking they found that the real working beam in a summer beam type arrangement is the rectangular section left between the mortises, the strap left under the mortises does not seem to strengthen the beam appreciably if the top strap has been cut by the mortise. Long story short on that mortise, if joists come in on both sides of the beam, reduce what you are calling the beam width by 2" when you are sizing your beams.

This one takes lots more hands to assemble but is considered the best joint. From Hewett;

"The ultimate joist end joint, believed to afford the maximum possible mechanical efficiency in cases where the neutral axis of the main joist is level with the tennon's soffit. This was, apparently, developed by Master Richard Russel, and was used for the side purlins of the roof of King's College Chapel at Cambridge in 1510-12. It is the soffit tennon with diminished haunch."   also known as a tusk tennon.

Because that joint does not break the top fiber of the beam and the removed wood is in the neutral axis the beam is not derated. This joint in testing was the strongest and nearly outlasted the beam before the tennon snapped.

But if you were wanting to buy new tools this does still leave a(better) opportunity.  ;
http://www.timberwolftools.com/tools/makita/M-7104L.html
Or a chisel and mallet.

PS, look carefully at that joint, you could argue that Master Russell also invented the I beam. He understood the forces within the beam and carved away the fluff.

[rick91351]

Thanks Don I forgot all about a chain mortiser.

I was wondering about how this would weaken the mortised timber?  But it did not look to me as if it was removing a dangerous proportion or ratio depending on width and depth of the timber.   However I can see a big problem especially is there were a number of these joints.  Then back to the joint is this tenon deep enough, or meaty enough?  An other problem I see is if they have very high moisture content when the house acclimates itself it is hardly going to fit like a glove.     As you stated   "But, this joint sucks. If the wood is well seasoned I'll take half of that back." 

Thanks
Rick

[NM_Shooter]

Wow... that is crazy expensive.  I'd like those jigs, but you know what I'd like more?  Access to beams that are as straight as the ones in the video. 

I bet you could make your own jigs, and use a smaller HP router.  just hog out the opening first with a straight bit, then use the dovetailing jigs to clean the corners using multiple passes.  I wonder if Woodworkers has bits that large?

[davestreck]

Or a chisel and mallet.

Second.

IMO, there is no reason to invest in overly-complicated power tool jigs for even the most complicated timber frame joints (unless you are a production shop banging out many complete frames as a business). Any joint, even complex dovetails, tusk tenons, or wedged and keyed scarf joints can be cut with simple hand tools: mallet, chisel, auger bit, crosscut saw, and framing square. And most simple frames don't require more than a few joints: mortise and tenon, half-lap, tongue and fork, and maybe a shouldered dovetail or two. All of these can be mastered easily with a little practice. Just buy one of Jack Sobon's books and shop eBay or flea markets for some decent framing chisels. And don't be overly concerned about a few screw-ups. As my old foreman used to say, "We aint building a friggin' piano here"

Regardless of what people say, timber framing is not rocket science, and many of the power tool "advances" touted in woodworking magazines actually add more trouble and expense than they are worth. When I built my first real timber frame (an 18x34 barn/workshop), I started off thinking that routers, chain mortisers and the like would speed up the job and make my joints more accurate. By the end of my project, I was using hand tools almost exclusively. Not because I am a Luddite, or because I am an expert hand tool user, but because in the end hand tools did a better job, were more controllable, and were more pleasant to use.

BTW, I also second DonP on the tusk tenon. Excellent joint, far superior to a housed dovetail. I wish I'd used them on my frame.

[Whitlock]

But, this joint sucks. If the wood is well seasoned I'll take half of that back.

Joints have there place if used right do they not?

Were a summer beam (in tension) ties into the wall beam I just don't see how a tusk tenon can be stronger.

With a tusk tenon the only thing that is holding the tension is the relish behind the peg.

I think the tusk tenoned summer beam would be a lot harder to put in place also.

Other than useing it for a summer beam I can't think of anywere else in a frame I would use a dovetail.


Later,W

[Don_P]

The frame I worked on used them for all drop in joists and purlins. The machining tolerence on the joinery was ~.003" when the package arrived green (Hundegger K2).

This was the worst one but as you can see there is little vertical or horizontal capacity left.

Their nice straight timber looks to be white pine glulam.

[Whitlock]

That sucks Don....

What are they 8x8's and why such short tenons??? That's right it was a kit...

That is going to need to be fixed as you know... can you get them back together with turnbuckles or pipe clamps and slip wedges down the sides of the dovetails?

[Don_P]

I'm not sure what constitutes a kit, or why that would drive a joinery decision. One group considers it a kit if the trees did not come from the land and were not worked on site by the homeowner. This timberframe company hand cuts some frames and CNC cuts others. Is one a kit and the other something else? They're all kits from the narrow perspective above. The machine is more accurate if that matters. ... the problem would be that perspective. The term kit is used prejudicially by some and I'm not sure that it amounts to anything. I view them all as material suppliers. Some are good, some are no better than Lowes.

This timberframe company, like most, provides an erected frame wrapped in SIPS for their portion of the build. I worked with them as a laborer for the on site work and then carried this house to completion. I have several repair methods in mind, the frame is not mine and it is not my call.

The joint is the same as the one under discussion in the op. It can be produced by a CNC machine's milling heads or by a handheld router... or a chisel and mallet. How the joint is produced is of no matter, the geometry and use of the joint is my issue. Why wasn't the tennon longer? A longer drop in tennon of any style that breaks the top fiber weakens the beam further and if the tails were of the same angle would withdraw the same amount.

The tusk tennon is more difficult to assemble but its' mortise does not break the upper edge of the beam. The beam is nearly as strong as an unmortised beam where a beam with these drop in mortises is effectively 1" narrower than full width per side and if the dovetails were longer the beam would be effectively even smaller. This can be done, it's all in the detailing of the design. High tension in timberframes, even historically, has been resisted by metal. They generally tried to avoid tension joinery. This was not a frame tension problem, these joists run from gable to gable.

So there's my anecdote. Long story short, I'd explore all my options. If you read up, this is not unique. I've sat through the same discussions in conferences with TF engineers.

[Whitlock]

Kit = Not your design.

Just thought I might learn something here