Wedged 1/2 Dovetail Thru Tenon Question?

[jander3]

I am working up a little "how to" article for our website.  I am curious as to how others approach the following on the a Wedged 1/2 Dovetail Thru Mortise & Tenon.  Any input is appreciated?  I will share the article here when it is complete.

What size wedge do you use?

Do you taper the mortise on the wedge side?

What technique do you use to cut the dovetail angle in the mortise?

[gdaddy01]

I will be watching

[shinnlinger]

I have never seen this in timberframe construction, but I 'aint seen everything.  When I am doing a thru tenon in woodworking or hanging an axe, I will cut the tenon 2/3rds of the tenon depth and precut the wedge so it it will be flush when max depth vs blindly guessing if it has bottomed out.  I might run the tenon a bit long so I can cut it off flush after to clean it up.  I like a bit of give in my frames to deal with expansion and contraction, so pegs are a better fit for me and a simpler joint.

[Brian_Weekley]

When I did mine, I studied the drawings shown on pages 4-6 and used it to help proportion my wedges and tenons:

https://www.ncptt.nps.gov/wp-content/uploads/2004-08.pdf

[jander3]

Brian...did you drill at an angle or cut a normal mortise and use a saw and chisel to get the dovetail and wedge angle?  Or some other technique?

[Brian_Weekley]

Jon,

I didn't try to drill the mortise at an angle.  I just used my boring machine to drill out the straight, vertical portion of the mortise.  Then, I used a hand saw to cut sloped kerfs to match the angle and chisels to remove the material.  I did made a plywood template of the tenon to represent the dovetail angle.  This template was extremely useful to check and refine the mortise and housing to get the fit just right. You can also see a line drawn on the template to check the housing. There's definitely a lot going on with the reductions on two sides of the beam, the sloped shoulder of the housing, and the angles on top and bottom of the mortise to fit both the dovetail and the wedge!  These were the most complicated joints in my frame.  They were fun!

Brian

[jander3]

Thanks Brian...I approach it the same way.  Great idea on the template.  I've seen folks drill the angle but that seems like too much jacking around.  I am working on a how to instruction that will help students understand this joint. I hope to have it finished in a week.  When you look at the drawings and read about it, it is hard to wrap your head around the dovetail and wedge, to there really only a few steps.  I plan to simplify this for our students.  Trying to decide if we will include this joint on our student built frame in June. I am thinking yes as it is important to understand this one.

[Brian_Weekley]

This may be obvious, but something useful for your instruction...  Since the dovetail on the beam must slide into the mortise on the post and then drop down to seat, there needs to be enough clearance in the top of the mortise so it will fit (A > B).  Based on the nominal dimensions I used to cut my joints, the angles were 10.9 degrees for the dovetail and 5.4 degrees for the wedge.

[timberwrestler]

I don't want to complicate things too much, but if the timber is out of square then things can get tricky in the laying out this joint.  I drew this up a while ago.  So there's one square edge shown by the triangles below.  The out of square-ness is exaggerated to show how it can affect the 3 different sloped lines.  

The general approach is to lay out the key points on the outside of the post.  Then lay out the slopes on the sides of the posts.  Then connect those points on the face with the housing.  

If it seems odd, think about how the planes of the 3 slopes relate to the reference plane on the outside of the post.



 



 



 

[Heartwood]

The "Learn to Timber Frame" book has pages 97-104 showing the layout of this joint in step-by-step detail, including another drawing of what timberwrestler is explaining.
3/4 in 8 slope for the wedge and upper part of the mortise.

 

 

[timberframe]

Steve Chappel's book has a good section on this joint as well.