Stretcher to Post Joinery

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markberg
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Re: Stretcher to Post Joinery

Thu Feb 11, 2016 3:40 pm

Long time lurker here and on your blog. Have commented on your blog a few times, but not for a long while.

This is all great stuff, and much appreciated, but I do have a question regarding the Watari Ago joint.

It seems to me, although I may be missing something very obvious, that you would need to make a vertical chop within the mortise to form that "hump" in the middle, and I can't see how you would do that. How do you go about cutting that mortise?
toscano
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Re: Stretcher to Post Joinery

Thu Feb 11, 2016 6:50 pm

Perhaps crank-neck chisels or just using the bevel-side of a regular chisel?
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Chris Hall
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Re: Stretcher to Post Joinery

Thu Feb 11, 2016 7:07 pm

markberg wrote:It seems to me, although I may be missing something very obvious, that you would need to make a vertical chop within the mortise to form that "hump" in the middle, and I can't see how you would do that. How do you go about cutting that mortise?
The mortise is simple to cut. First you cut a through mortise. Let's say the jaw of the joint houses upwards 15mm, and in from the post face each side by 15mm as well. All you have to do is cut a 15mm long, 15mm deep housing at the bottom of the already cut through mortise, on each side, and the joint is produced.

Assuming a 120mm tall stretcher, here is the layout:
Nuki to post connections mortise sequence 1.jpg
Nuki to post connections mortise sequence 1.jpg (473 KiB) Viewed 7488 times
The arrow on the post marks the finish position of the stretcher after it is inserted.

Mortise through, leaving the 15mm notch allowance:
Nuki to post connections  mortise sequence 2.jpg
Nuki to post connections mortise sequence 2.jpg (471.9 KiB) Viewed 7488 times
Cut notch, 15mm x 15mm:
Nuki to post connections  mortise sequence 3.jpg
Nuki to post connections mortise sequence 3.jpg (476.95 KiB) Viewed 7488 times
The cutting of the housing on each side leaves the middle bit of the post inside sticking up proud by 15mm.

All that remains is to add the taper to the upper mortise wall for the wedge to be accommodated.

The stretcher inserts in the high position:
Nuki to post connections mortise sequence 4.jpg
Nuki to post connections mortise sequence 4.jpg (465.27 KiB) Viewed 7488 times
Slides through:
Nuki to post connections mortise sequence 5.jpg
Nuki to post connections mortise sequence 5.jpg (448.94 KiB) Viewed 7488 times
Drops down:
Nuki to post connections mortise sequence 6.jpg
Nuki to post connections mortise sequence 6.jpg (460.11 KiB) Viewed 7488 times
And then the wedge is put in. Make sense?
markberg
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Re: Stretcher to Post Joinery

Fri Feb 12, 2016 9:03 am

Thanks Chris. All very clear except for the mechanics of cutting the 15mm x 15mm notches.

It looks to me that you would have to make vertical as well as horizontal cuts to form that notch, but there's clearly no room in there for a chisel to make vertical cuts. I look at that and see the need for a paring chisel with the blade at 90 degrees to the shank, and I do have one of those to clean out the bottom of mortises. Is that the sort of thing you would use?

Or am I way off?
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Brian
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Re: Stretcher to Post Joinery

Fri Feb 12, 2016 9:35 am

Mark,

I would make the cuts with the chisel facing bevel down, personally, that would likely allow room to produce the cuts.
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Chris Hall
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Re: Stretcher to Post Joinery

Fri Feb 12, 2016 9:49 am

markberg wrote:Thanks Chris. All very clear except for the mechanics of cutting the 15mm x 15mm notches.

It looks to me that you would have to make vertical as well as horizontal cuts to form that notch, but there's clearly no room in there for a chisel to make vertical cuts. I look at that and see the need for a paring chisel with the blade at 90 degrees to the shank, and I do have one of those to clean out the bottom of mortises. Is that the sort of thing you would use?

Or am I way off?
I would use a router to cut the notches to depth. Quick and precise - routers excel at decking such surfaces flat. I wouldn't hog out 15mm depth in one pass though. Either cut the notch in 2~3 passes of increasing depth, or chop/drill most of the way there and trim the bottom with a router. A short length bit with top bearing to roll around the mortise walls would be ideal.

If you want to use a hand tool, then a hand router (witch's tooth) would work fine too, after chopping most of the way there. A soko-zarai nomi - which you mention you own - would also come in handy if you didn't have the hand router. People in Japan were hand chopping these joints for many years before the advent of powered tools, so there are certainly ways to tackle the cutting without an electric cord.
markberg
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Re: Stretcher to Post Joinery

Fri Feb 12, 2016 10:32 am

Yes, a hand router since I generally stick with hand tools. I was missing something obvious.

Thanks.
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Chris Hall
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Re: Stretcher to Post Joinery

Fri Feb 12, 2016 9:41 pm

A crank neck chisel would work very well also to clean the bottom the housing.

Please let me know if I am over-explaining things.
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Chris Hall
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Re: Stretcher to Post Joinery

Sat Feb 13, 2016 5:41 pm

Now we can take a look at the two previous joint mechanisms (half dovetailed and cogged), except this time secure the stretchers using two wedges instead of one.

There are a couple of different ways to go about it, and both involve inserting a wedge from each side. Here is the first type, where the wedges are gapped apart inside the post:
Nuki to post connections double wedge a.jpg
Nuki to post connections double wedge a.jpg (585.3 KiB) Viewed 7444 times
In the background we see an example where the opposed wedges enter a mortise with a straight-across upper mortise wall, and in the foreground we have a version in which the upper end grain wall of the mortise is sloped to match the upper surfaces of the wedges.

While the version in the foreground has a bit better mechanical connection between the parts, as a result of the fact that the middle of the mortise dips downward to the midpoint, we must make the mortise taller overall and the wedges considerably thicker to allow enough space for the stretcher to enter upon assembly. Trade offs....

Connections in which the wedges are kept from touching one another especially suit situations where the post is of comparatively deep section and the mortise for the wedge to slip into, and have good bearing with, is long. The longer the better.

Consider for the moment the flip side, where the post is less deep than in the illustrated example - like a 2x6 timber for example. In order to keep the wedges apart from one another they would have to be extremely short, and that mean less security in the connection all around.

The other form of connection is to have the wedges slide by one another. Now, if the work is going to be concealed in a wall, under a floor, or something similar, we can pay less mind to appearances. Here is something appropriate to that scenario:
DSC05414.jpg
DSC05414.jpg (183.61 KiB) Viewed 7444 times
For finish work, furniture or the like, the above work would be considered ugly. Too much stuff sticking out, even if the joint intersections were cut more cleanly, the pressure on the skinny end of the wedge as it emerges is likely to cause that skinny end to not lay flat upon the wedge below.

So, the clean solution is to make the wedges slightly different from one another in form, and bring each one just up to a hair shy of the wedge tip making a mortise exit:
Nuki to post connections double wedge b.jpg
Nuki to post connections double wedge b.jpg (360.6 KiB) Viewed 7444 times
Note the wedges carefully, as the configurations do vary from one to the other.

To judge this connection properly, so that when the wedges are driven in tight, and do not evidence any tips poking out to view, is a matter of some finesse.

Now, in the article mentioned about a Van Arsdale sawhorse design, I do recall that the leg on that piece was made rectangular, and tapered on its narrow sides from base to top. Presumably this was done in interest of making the joint between the leg and the sill piece, or foot, as wide and strong as possible. That makes sense on account of the fact the the joint between post and foot is the weak spot in that type of sawhorse. Sometimes though, solving one thing with one aspect leads to challenges elsewhere in a structure.

In the next sketch, I have taken the former post section of 120mm, and made it half as deep, at 60mm:
Nuki to post connections double wedge c.jpg
Nuki to post connections double wedge c.jpg (407.48 KiB) Viewed 7444 times
Another view, without the post sectioned shows the completed connection:
Nuki to post connections double wedge d.jpg
Nuki to post connections double wedge d.jpg (279.05 KiB) Viewed 7443 times
I'm illustrating the cogged joint version, though the half dovetail connection is much the same in terms of the ramifications of marking the post section shallower than the stretcher is tall. Simply put, the double wedges have to ramp up at a much more extreme angle than before - halving the post depth makes the wedge slope double. Such steep wedge ramps make the wedges themselves more liable to come loose as a result of frame movement - - which can easily happen on a sawhorse when loaded with a heavy shiftable mass like a large timber. So, in order to keep the wedge angle more reasonable, one has the recourse of making the wedges longer, sticking out bother entry and exit faces, and having the ugly look, or going to a single wedge lock solution. I'd be inclined in that latter direction myself if faced with such a choice. It's a sawhorse and is utilitarian, but clean joinery is worth doing I think regardless.

The question is then: are either form of stretcher joint suited, really, to connecting to posts which are shallow in section? No. Does making the post shallower in section benefit these types of joint in any way? No.

There are other connection options better suited to the situation of a stretcher meeting a post of shallow section, however we have some more variants to look at with the standard square post yet, so that will form the subject of the next installment in this thread.
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Brian
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Re: Stretcher to Post Joinery

Sat Feb 13, 2016 7:03 pm

Thanks Chris, this thread has been incredibly helpful.

I like the connections that you used on the sawhorse project (still planning to build a pair of those before proceeding through a series of doors for my house, though it may be a little while). The combination of cross bracing and a low stretcher makes for a very sturdy structure. I do not see cross bracing employed much with regard to furniture and often times I assume it is due to the complexity of layout and cutout.

When building race car frames it was a no-no to make a square structure that did not include cross bracing, a sheer plate, or at the very least gussets in the corners. I suppose that shoulders and through connections are the functional equilvalent of gussets, but it still surprises me that very little crossbracing is delayed in furniture.

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