Chris Hall's Carpentry Study Group - Fifth Project

[Yxoc]

G'Day Sam - thank you for your interest and kind words.
I don't believe that this joint is special to any style of structure, but is one of a number, all varying by different amounts, that deal with this part of the building. Any hipped roof will need to deal with the issues presented here in one way or another. I am certainly no expert though, and will try and get a more definitive answer.

Regarding my own goals for this work - yes I definitely would like to build a gazebo/gate/outbuilding using this knowledge one day. I am a sucker for the Japanese aesthetic when it comes to the built environment, and their use of wood in structures is the most sophisticated, as far as I know. I do deeply desire to have something, Japanese inspired, built on my property. This goal is quite aspirational though and is some way off.

Also, I am a maths geek, I love maths and this is a very practical application of Geometry and Trig (Hey! Where did all the readers go....?). I find this sort of work is very good for helping oneself visualise geometrical problems in 3-D.

Lastly, I find that working on these projects is advancing my technical skills a lot, solving problems that I wouldn't come across in many wood working fields. So my general woodworking skills are benefitting greatly.

Regards

Derek

[Yxoc]

Moving along...

I didn't draw attention to the fact earlier, but the abutments of the Keta lap are given a small mitre - you can see it in the previous couple of Sketchup graphics. This treatment is purely to protect the joint during the several test assemblies that would occur as the joint is fitted. Without the mitre, end grain rubs against face grain and the former is much stronger. To effect this mitre the sides of the joint first need to be 'waisted', the width of the piece is brought in and then the step can then be cut to 45 degrees.
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In the above photo you can see the thickness had been brought in, I removed the bulk of the wood with some saw cut and then a chisel, then used a router plane to get a consistent level surface. Next, the little step thus created can be mitred, I used a 45 degree paring block to assist here:

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And here the work on this piece is complete for now:
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And finally, sitting atop its post:
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[Yxoc]

The next port of call is the Keta piece which will form the lower half of the lap joint. Like the lower piece, this one also needs a mortice chopped beforehand. In this piece the mortice is a 30mm by 30mm square that will pass the whole way through. Importantly though, the entry and exit hole for this mortice both sit in what will become waste, because this will ultimately be the middle piece in the three way lap. The flow on is that I need to make the mortice as straight and accurate as possible into the centre section of the piece. I started by drilling from both ends with an auger bit. The diameter was something close to an inch, which allowed for any accidental drift of the bit to remain within the mortice dimensions:
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Try squares aligned with the auger helped me achieve a straight hole and the channels met in the middle quite nicely:
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Afterwards my Mates from Narex helped to bring the mortice down to final dimensions:
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Then I followed a similar process for cutting out the lower Keta half:
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[Yxoc]

And finally the family gets together for a photo shoot...
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[Pam]

And finally the family gets together for a photo shoot...
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So why didn't you cut away the now missing part before starting the mortise? I understand your approach when cutting mortises in things like curved chair legs or if you were to eventually cut away from both sides (in which case I'd cut away from one side, do the mortising, then cut away the other side), but not in this case.

Pam

[Yxoc]

G'Day Pam,
Thanks for your very relevant question. Firstly let me tell you that I wouldn't always promote my solutions to problems as world's best practice. In this case you definitely could cut away the waste for the lap joint and then chop the square mortise. The main reason I didn't do so in this case is that the lap joint surface, once cut, is a compound slope. It may not be really clear from the photos but the joint slopes from front to back and from left to right as well. The resulting 'print' of the mortise onto that surface would not be a square but a diamond shape. I felt that it would be easier to work with two orthogonal surfaces, and accept the greater requirement to not let the mortise 'drift' with depth than try and accurately mark out on a sloping surface. As it turns out my way has worked well enough - who knows, with retrospection I may have chosen to do it differently. If you were using machinery such as a hollow chisel mortiser, it may be easier to cut the mortice second (speculating here because I've never used one) but mine is a modest hand tool shop.

Regards

Derek

[Yxoc]

One other thing Pam,
The other side of this piece will be cut away because a sloping trench will be cut on this top piece to accept the passage of the hip rafter.

Derek

[Pam]

G'Day Pam,
Thanks for your very relevant question. Firstly let me tell you that I wouldn't always promote my solutions to problems as world's best practice. In this case you definitely could cut away the waste for the lap joint and then chop the square mortise. The main reason I didn't do so in this case is that the lap joint surface, once cut, is a compound slope. It may not be really clear from the photos but the joint slopes from front to back and from left to right as well. The resulting 'print' of the mortise onto that surface would not be a square but a diamond shape...

Yes, I did see the slope, and I see how that could effect the chopping angle. However, it doesn't make any angle difference logically if you chop an extra 3" or not. Straight from top to bottom is also straight from middle to bottom.

Pam, who's only on lesson I

[Pam]

Derek, I've been thinking about a bit of refinement that could make chopping straight a little easier. How about cutting out/off/whatever the now missing part, but not sloped? Instead make it parallel with the bottom, chop the mortise, then cut the slope.

Pam

[Yxoc]

Pam,
That would definitely work, but at the penalty of adding a lot of extra processing. Perhaps I have overstated the difficulty of getting that mortice straight through the thickness of the wood. I raised it as an important consideration, but in practicality after drilling and then chopping to achieve the rough dimensions, I then used a square paring block and pared from both ends to get plumb surfaces and a straight hole.

I should add that chopping the mortice through a sloped entry surface doesn't add difficulty to the chopping, mainly it makes the marking of the entry hole more difficult.

"Pam, who's only on lesson I" - This statement has me a little confused...?

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And with the post fitted:
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The top Keta piece is still sitting proud by about half a mm, so the joint needs a little bit of fine tuning yet but I am pleased with the first trial fit. Next the trench for the hip needs to be cut

[Pam]

..."Pam, who's only on lesson I" - This statement has me a little confused...?...

Glad to hear all is working well. By the above, I meant I just got



Japanese Carpentry Drawing, Volume I. Having a little difficulty finding the time, but...

Pam

[Yxoc]

Pam,
That's great!!

That's basically the mathematics volume from memory. Depending on your love/hate of Maths, that volume could go quick or slow. I don't want to presume either way but if you have questions about anything, drop me a pm and I'm more than happy to help.

Good luck!

Derek

[Pam]

Pam,
That's great!!

That's basically the mathematics volume from memory. Depending on your love/hate of Maths, that volume could go quick or slow. I don't want to presume either way but if you have questions about anything, drop me a pm and I'm more than happy to help.

Good luck!

Derek

Thanks, Derek. I've always been very good at math, excluding geometry, so an updating review of tangents et al is very welcome. I suspect it's going to be very slow going though.

Pam

[Yxoc]

Pam,
Another Maths geek - It's good to not feel so alone.

Regards

Derek

[Pam]

Pam,
Another Maths geek - It's good to not feel so alone.

Well, when I was 12. Mother wrecked it all by holding me back when informed that I tested 4 grades higher in math, "oh, she's too immature." At that point I stopped communicating about anything that mattered and never again asked for anything from my parents. The thing I'd wanted most was to get a lot closer to independence, completely foiled. I did learn that if I never asked for anything, people often showered me with goodies.

Pam