the Carpentry Way: French Connection 10                                                          

French Connection 10

    
Continuing on with the drawing work on the 19th century layout exercise of a three-legged bench with various arrangements of scissor braces - or Saltaire crosses as they are also termed.

I started with brace pair 1, in which the faces of the two brace sticks are in the same plane as one another:


There are some tricky bits to the layout of these, especially in regards to developing the end cuts of the sticks to fit around the post and beams to which they fit at a compound angle. The problems can be solved for you with a 3D drawing program, however I prefer to exercise my powers of visualization and work the drawing in a 2D format, as if it were a large sheet of paper, developing the angles and then transferring the information directly to the sticks:


Satisfied with side #1, I moved to what I am calling side #2:


With side two, the two brace sticks are rotated so that their front and back sides are plumb. Certain aspects of side #2's layout are simpler than side #1, however there are complications as well.

One of the complications with this project involves tenons. The drawings in the text show tenons on many - but not all - of the sticks, however I learned well from my previous 4-legged Mazerolle sawhorse build to take such drawings with a grain of salt. While a connection with a mortise and tenon might be shown, that doesn't mean it is possible to actually assemble the piece with joinery in such a configuration! indeed, in terms of the French carpentry models I have looked at, the vast majority of the connections in a piece like this do not employ tenons but are spiked together. Still, I want to see if I can design the piece in such a way that it can go together without recourse to metal fasteners. This may be a case of if wishes were horses.

The first tenon complication i am looking at involves how the lower brace ends connect to the lower ends of the legs. The perspective drawing in the text shows a through tenon:


Let's look a little closer at that connection:


Indeed, if considering only the tenon on brace set #1, then the mortise would be configured as a compound angled parallelogram, and this is exactly what I had after detailing that tenon and mortise:


It's not an easy connection to fabricate, but this is how the French would seemingly prefer to do things, so I'll give it a shot.

There is the lower end of the the right side brace from set #2 to connect to this same area though, and the plan drawing in the text indicates that the tenon ends from the two braces are to pass through the post and meet in line on the exit face:


It's a little unclear whether the tenon from brace set #1 is cut short to let the tenon from brace set #2 pass in front, or vice-versa. Both seem to be indicated as 'possibilities'.

If you look at another corner of the plan drawing, where the brace sets #2 and #3's lower ends will meet at the post, the same sort of connection is indicated:


Sounds like a cool plan to bring two tenons through to meet in one location, however in reality, the shapes of the tenons are completely opposite to one another in this location where brace set #1 and #2 lower ends meet the post:


The parallelogram-shaped tracing on the face of the leg is the outline of the brace from pair #2, which is gray and enters from the left side in this view. You can see that it's form is completely opposite to the parallelogram shaped mortise produced by the yellowish brace from pair #1, and they are at different heights. If I formed a tenon on the gray brace's end and ran it through the post, then it surely would not look the same as what is shown in the perspective drawing shown a few images above.

The book shows that one of the tenons could be held back so as to let one exit out on the face, and it would appear the book means that to be the tenon from brace pair #1. So, following that train of thought, let's trim that gray tenon back so that only the yellow tenon comes through:


 With the front face of the post removed, you can see how I've trimmed the grey tenon back so as to allow the yellow one to pass through to the exit.

Now with the face put back on, the appearance would be fairly similar to the perspective drawing shown in the book, however the gray brace would leave a slit down below the yellow tenon:


That grey brace end tenon could be trimmed a little differently to hold it off the face, but what is a far bigger problem in my view is the sort of mortise one would need to cut for the grey brace:


Compound angled parallelogram shaped mortises are annoying enough to contemplate, but to do one which is partially blind as well does not appeal to me so much.

A look down the brace mortise for the yellow stick reveals an easier path to cut:


Anytime you can cut right through it will be easier to execute. I'm not exactly enthused about the prospect of cutting a blind compound mortise, though I've done them before. It's just very hard to get a clean and accurate result.

So, with that apparent roadblock staring me in the face, I decided to look in some of my other French drawing materials to see if I could find some other approaches to the same problem. Delataille's work Art Du trait Pratique de Charpente does show several iterations of the three-legged bench. On on the cover of volume I (and not found otherwise in the text) is shown a through-tenoned connection at that lower post location in a bench done with each brace pair rotated in the same manner:


Here he shows a tenon for each stick poking though the face of the post.

That approach seemed like it was worth a try at least:


That looks like it would work, however when you remove the braces, the mortises that must be done in the post are structurally weak. here's the mortise for the yellow brace:

 And here's the mortise for the grey brace:


So, that doesn't seem like much of a solution from my point of view at least. One could squeeze the tenons in a little closer to one another, and thicken up the leg a bit to improve the situation, but I'd rather see if there might be a better way to go.

What I have come up with at this stage is to have the tenons emerge on the leg's front face aligned in the same central location, and lightly lapped over one another, the grey brace's tenon trimmed back partially:


It's not what Mazerolle showed in his perspective sketch, but it would work.

Remove the post and you can see what needs to go on with the tenons:


And with the yellow brace out of the way, the bizarro cut on the bottom of the grey brace's tenon is apparent:


What this means in the bigger picture is that the grey brace's tenon must insert fully before the yellow brace tenon can be inserted more than about halfway in. Not sure at this point whether such an assembly sequence will be possible. In fact, considering the entire assembly sequence, as far as what is going to be possible, where there can be tenons and where there can't be tenons, is going to be a tricky matter on this piece. You certainly can't go faithfully off of what the book shows, and that is one of the major challenges when working with such material. Delataille is no better, as some of his illustrations of three-legged benches are clearly showing impossible assemblies.

All for today. Thanks for coming by and if you have any good suggestions as far as connecting this beast together, I'm all ears. On to post 11

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