Tuesday, June 30, 2009

Comment Reply

I received an interesting comment to yesterday's post, and discovered that my reply was too long to be posted, so I have decided to create a new post. Here is the comment by Derek Cox:

G'Day Chris,
I am thoroughly enjoying this lamp build, probably the most of all your posts so far...

I noted earlier your use of the half laps with mitered abutments and I had thought that they must be undesirable in one aspect in that they reduce the cross section of the joining members and therefore decrease overall strength. They look nice though and I was pondering the main reason for their use. Are they mainly used here so that finish planing will not create gaps in the joint like it would with normal half laps; is there some mechanical advantage such as resisting twisting in the plane of the joining pieces or is it largely aesthetic?

Regards

Derek Cox


Here's my reply:

Hi Derek,

glad to read your greeting from the land of Oz and that you are enjoying the thread so far. Also, great question! I've been wondering if someone was going to be curious about that.

You are quite right that the mitered half laps - any housed half lap for that matter - reduce the cross section of the pieces at the joint and thus the strength. I strive to compensate for this at the outset, if I know that I will be using these joints, by designing the pieces for finished size in view of their reduced cross section and resultant lowered strength after cut-out. I would observe that the lap itself is really a bigger culprit than the housing in terms of reducing the joint strength.

I am doing all I can to retain as much strength at these joints as possible, and thus my earlier comment a few posts ago that the initial half lap with the hijiki was not ideal with a 0.25" housing reduction on each face, and my recent decision to add a cap to the top of the post which also served to stiffen up the connection for the hijiki. The lantern is a prototype after all, so I am learning, revising, and fine-tuning as I go. The more important piece, for structural strength to carry/resist the loads is the second tier level of beams, and I have made the lap in the beams a fair bit stronger, using only a 0.125" housing, and that's despite the fact that it is a slightly smaller section than the pieces used for hijiki.

In any case, the purpose of housing the joint, let it be said, is twofold: depending upon the case it allows more of one timber's grain to be carried (not just the portion in the half-lap), and (a little more to the point here) is that it allows for shrinkage and swelling, through seasonal moisture cycling, to be concealed in the joint. That said, mahogany is renowned for having very slight in-service movement, so that would not appear to be as much of a concern in this case.

So, why use these kind of joints then? While they do, I agree, look nice, in this case virtually none of these joints will be visible once the lantern is assembled. Another aesthetic aspect which favors the use of mitered abutments, and is definitely a factor in my decision to use them, is that they allow for clean flowing chamfers of the arrises of the involved pieces. I will be making a slight chamfer on the pieces, about 1/16' or so, and the miters allow that to happen harmoniously where they intersect. This is not the main reason I choose the mitered abutment however.

The finish planing aspect IS one of the reasons, yes, though still NOT the main one for me. I have left myself just a pass to plane on each face, so the potential for gaps after finish planing is but slight (hah -I'm hoping!).

The MAIN reason I like to use the mitered abutments in a half lap relates to assembly, as I can fit a tighter joint with the mitered abutments. How so? Well, with a non-mitered abutment, you will have un-yielding end grain on one piece pressing/rubbing against the comparatively soft face grain of the other piece. If you are trying for a tight fit, it is easy for the face grain to get torn or marred by the end grain of the other as the two surfaces slide past one another, especially when several trial fittings may be involved. While the vulnerable surface can be 'killed' by pre-compressing with a hammer, and re-dampened later to swell the grain back, sometimes the results aren't quite as satisfactory. With the mitered abutments, a similar surface density of is found on each abutment (about 45˚ to end grain), and the joint can be fitted relatively tightly without concern for damage to the abutting surfaces.

I've had experiences, especially with softwoods, with the face grain portion getting a bit torn up by the end grain in housed half laps and that certainly makes a mess of a joint in a hurry. This is a lesson I bring forward from timber framing. The twisted half lap (neji-gumi) with mitered abutments is thus my preferred connection for the wall plate underneath the hip rafter.

A final reason one might choose the mitered abutments is in relation to timber framing and the behavior of such a joint under horizontal-plane shear loading. The miters, I believe, would provide a decrease in a potential stress riser (as opposed to a 90˚ abutment) and I think the joint would better resist those sorts of loads, and that extends to temporary loads as might occur during raising, where a timber might need to be wiggled from side to side - the mitered abutment is less likely to show up any damage (wood denting from the end grain pressing into the face grain when the timber presses one side to the other - or worse damage).

As an aside, I would add that in timber framing connections at the corners of the wall plate, there aren't a lot of choices, fundamentally, in how to join the pieces. I would argue that the half lap (or it's more refined version of twisted half lap) leaves more meat from each piece, and gives better support to each piece in the connection than the other two common choices, which are M&T (no projection of nose beyond the joint, at least not on both pieces), or sliding dovetails with a detached nose on one so as to give each a projecting nose (the detached nose with dovetail is so weak that it ultimately is little but decoration). And of course, these joinery choices become more critical when a hip rafter is introduced on top of the joint and must take some material out of it to be interfaced.

I hope that explanation made good sense to you. To reiterate: the main reason I use the mitered abutments in a housed half lap is to be able to fit the joints up tight, through several trial fits, without fear of damaging the surfaces. Secondary, and still important reasons why one might choose this type of joint relate to the fact that chamfering or other treatments of the arrises of the intersecting pieces can be seamlessly accommodated, and that finish planing will not spoil the fit.

Cheers,

Chris

Note to readers: I will have a second post later today, and then I will be away from my desk and unable to post for about a week.

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