Monday, December 28, 2009

Battari Shōgi 4

Continuing now with the build of the fold-up bench for the Boston Children's Museum - I hope the jumping back and forth between projects isn't making anyone dizzy! Previous installments in this and other threads can be found to the right of the page in the archive section.

Where I last left off, the cut out of the housed tusk tenons for the frame cross-pieces had commenced. While there are numerous ways to connect the crosspieces to the frame rails, and the original which I am replacing used some form of simple glued blind tenon, the superior joint for this form of connection is the tusk tenon. I say superior in terms of structural logic, not the usual $$$/efficiency/get it out the door matrix of consideration. This type of joint finds common application in the framing of timber floors, where it is typical to connect the main floor beams with a series of closely spaced joists, which are of smaller section. Since the main floor beams are important structural members, cutting mortises and housings into them to receive a large number of smaller floor joists can easily result in excessively weakening the beams. one solution to that would be to up-size the main beams to the point where the remaining material after all the mortising was complete would be adequate to carry the load, however this is not a wise use of material. The quick and dirty connection, if I might characterize it in that manner, is to chop housings so that the joists can simply drop vertically into the beam - this can be seen in this particular chestnut beam - this the remnant of a frame present in the house in which I live, right in the living room:

This sort of simple housed joint is very common; indeed, many brand-new frames incorporate it. Notice too the propagation of cracks for the corners of the mortises in these boxed heart beams- hah- that's another topic so I'll hold off on walking further down that track.

While the drop-in joist is convenient to frame, since the joists are easily installed after the rest of the frame is erected, it removes a lot of material from a crucial section of the beam. You see, any beam, oriented horizontally and loaded vertically, behaves under load very much like a steel 'I-beam' - an 'I' beam in fact is extremely rational in configuration, material placed onlywhere it is needed. The upper surface of a beam is under compression, and the lower surface of the beam is under tension. The middle axis of the beam, halfway between upper and lower faces, is called the neutral axis (as opposed to the "Axis of Evil"), and is subject to neither compression nor tension. The best place to remove material in the beam for a housing or mortise is therefore in the section where the loads are minimal - the neutral axis is such a place. Since the top surface of a beam is under compression, chopping mortises into that portion of the timber unduly weakens it.

Now, the development of joinery for timber work has undergone a long evolution, and this is well-documented in the case of English timber framing by Cecil Hewett, now deceased. In most cases a similar process unfolds - various joints are used, and time - i.e., the observation of older buildings and how the structures performed, as well as calamitous failures in some cases, and subsequent analysis of such effects by thoughtful framers, led to incremental improvements in the joints. Eventually each joint would reach a penultimate form of maximum mechanical efficiency - by some carpenter or another. It was not necessarily the case that one carpenter's perfection of a given joint form, through their deep understanding of the joint and insight into how to effect the highest form, would become necessarily widely-shared or understood. other carpenters may have continued using inferior forms of the joint, because they thought it was traditional, or it was what their master had taught them, etc. Once the penultimate form of joint was achieved, wherever and whenever it may have been, it was often the case afterwards that carpenters who failed to study the historic examples, or who misunderstood the logic of the form, would create a new version which was in fact inferior. Like some sort of wave, the process of joint evolution appears to rise to a pinnacle of excellence and then recedes (crashes?) to a debased form. Sometimes the lessons are lost all together, and sometimes, though the lessons are there to be seen in older buildings, if the carpenter chooses not to think about such matters then points of history are somewhat moot now aren't they? One of the aspects to living on the east coast which I like is that there are many older wooden buildings to observe, to see what worked and what didn't. The thing that puzzles me is when I see new buildings going up which seem to not reflect the obvious lessons available- again, another topic.

In terms of floor joist connections, the tusk tenon was the ultimate evolved form - I'll quote Cecil Hewett directly, from his masterwork English Historic Carpentry:

"The ultimate joist end joint, believed to afford the maximum possible mechanical efficiency in cases where the neutral axis of the main joist is level with the tenons soffit. This was, apparently, developed by Master Richard Russell, and was used for the side purlins of King's College Chapel at Cambridge in 1510~12. It is the tenon with diminished haunch, in which the sole object of the tenon is to prevent end-float. Industrial stress tests were conducted on a half-sized model of this joint..."

Those stress test observations, conducted by Dr. D. M. Brohn, are summarized a page or so later,

"This joint behaved more favorably than any of the other three in that there was virtually no rotational movement of the joint up to failure. At a jack load of 40kN there did not appear to be any movement of the joint. Towards the end of the test it appeared that the beam would fail before the joist. There was already substantial splitting along the face of the beam, and one of those splits coincided with the underside of the joist. Close to failure this split opened up and the portion of the beam below the joist was effectively spanning between the supports of the beam. This was deflecting substantially. In the end, however, the tenon broke across the weakest section, diagonally from the face of the main beam to the junction between the haunch and the top face of the tenon."

While in the case of this bench project the Wenge is leagues strong enough that I would have no worries whatsoever about employing simple drop in crosspieces; nevertheless I choose to follow best practice - which to mean means choosing the form of joint which is most mechanically efficient for the application: in this case, the tusk tenon. The most mechanically efficient joint is also, often, the one which uses material most efficiently as well. This Wenge is precious stuff and I am building a piece to last for a long time - there's no other way to go about it I feel than making careful choices about joinery.

The Japanese also employ this tusk tenon joint - they have a few different terms for it: kiba hozo (牙枘) "tusk tenon", dan-gata hozo (段形枘) "stepped form tenon", and kake-tsuki hozo (掛け付き枘) "seated penetrating tenon", depending upon the details of how the joint is configured. This joint can be done blind, or through, housed or not, pegged or wedged. I choose to wedge it in this instance, as I feel the tenon has rather a small amount of relish for pegging, generally speaking.

So, I wonder if some readers are being kept in suspense, wondering what sort of tenon I am blathering about - so, without further ado, here's some photos of the tusk tenons after most of the cut out had been effected:

A close-up:

I processed the tenons entirely with a router and a solid carbide spiral cutter.

I find the Wenge processes quite cleanly with carbide, and was able to get consistent accuracy of +/- 0.005" with tenon thickness:

The next step was to drill some small holes at the apex of the slots where the wedges will be eventually driven, these small holes act as stress-riser eliminators, preventing the wedge's kerf from traveling further up into the stick when the wedge is inserted:

I'll leave the kerfing for the wedge slots until later- I'm planning to saw these with my 240mm ryoba - possibly in what will be its sayonara performance.

With the tenons largely completed, I moved on to the mortises and their housings in the main frame rails. First I roughed out the mortises by drilling with a 10mm Festool brad point:

Even though the drill was virtually new and quite sharp, within a couple of holes the bit was smoking hot - this stuff is tough to drill! It took me longer than I expected to drill out the mortises. Eventually however, that stage was completed and I moved on to clean the mortises to spec. dimension using my router with edge guide:

Once the mortises were done, I got to work on the housings along the inside faces of the main frame rails:

Then I made a simple MDF jig to process the sloped abutments:

The result:

Then it was time for the massacre of the chisels:

A step followed inevitably, and with some frequency, by:

Well, that's the 15 picture limit reached for today's post dear reader. Hope to see you next time!

--> go to part 5

Thursday, December 24, 2009

Tréteau XII

Returning now to the Mazerolle sawhorse build, part 12. Back to the noxious Canarywood. Today I processed the cuts for the interior x-brace mitered half laps. In a previous installment, I rough cut the laps on all the pieces, and now the wood has had a couple of days to settle, I can proceed with taking the cuts out to the line. For previous installments in this series, please refer to the 'Blog Archive' located on the right of this page.

My first step was to make an MDF template to clean out the lap trenches to the required dimension. After that, I routed the housings on the sides of the pieces so that they were within a few thou of the width of the lap trenches. Here's the scene after that stage:

Next I made up a paring block using a piece of oak I had in my scrap pile. The lap is not at 45˚, so two miter bevel angles are required. Once the paring block was ready, I clamped it in place and commenced paring, staying slightly fat of the line:

I also made some use of a small shoulder plane to work the abutments:

Here's one of the mitered laps, now very close to final form:

Once a pair was done, I offered the joint up to see how it fit, adjusting with chisel and shoulder plane as necessary:

As the joint came together a little added persuasion was employed, both with a clamp and by tapping with a mallet:

Here the lap is a hair away from fully-seated:

Not too bad, though there appears to be a slight opening at the right miter - one pare too many unfortunately:

The ink lines, by the way, were very slightly out from where they had to be in terms of the angle described. After I discovered this issue, rather than scraping off the marks, I used a pencil to lay another line on top, which you can see if you look closely. Then I found a precise reference point and relied upon the router jig to cut the correct angle. Thus the discrepancy between marked line and cut line.

I gang-cut a pair of sticks at a time during this step - the result is that in a couple of spots the ink line, or a slight step between surfaces, is giving an impression of a gap when there is in fact no gap (though the one at the right miter is in fact a gap, no two ways about it). I will show these laps again after I have planed the pieces clean and chamfered them, and hopefully they will look better.

Here's how the other one came out:

I think there is a very slight opening at the bottom miter, but it will likely disappear when I chamfer the pieces. All in all, the mitered laps came out acceptably, though they are short of perfection.

Here's another view - the Canarywood is certainly quite striking, no?:

With the pieces assembled, I could now check with a straightedge and confirm my pieces were aligned as they should be:

Things are looking good as far as alignment goes, and in fact I may work the subsequent cuts (the tenons) on this piece using the assembled unit as a basis instead of working the individual sticks, as that approach should even out any errors. The Canarywood is very rigid, so I can't rely upon anything to ''bend to fit' later.

So, two pairs of interior braces are now lapped, and I will move on to completing the laps for the other sets of braces in upcoming posts in this series, hopefully without any over-pares.

Thanks for dropping by today, and Merry Christmas for those of you that celebrate the event.

--> Go to post XIII if you dare!

Wednesday, December 23, 2009

French Connection 7

Work continues on my two current joinery projects, and more posts will be forthcoming shortly.

Today I wanted to share with readers a couple of interesting French carpentry videos I came across recently.

This first is a pleasant audio visual experience featuring a CAD depiction of a clocher, or spire, which had suffered distortion as a result of settling at a foundation point. Some of these spires - a minority - were actually made purposely with twist, while others twisted by accident. Carpentry lesson: by fussy about the foundation details! These distorted spires look so wonderful!

Here's the link:

The next one is a bit longer and narrated in French, and concerns the layout of a delightful form of under-eave cantilevering that the French employed in 19th century carpentry. This type of layout is covered in the Mazerolle book, but in only in relation to balconies - here the layout is for the roof support for a lucarne, or dormer. An interesting video to watch, and I link it as it shows quite well what can be accomplished by the study of layout in carpentry, and gives a view to some aspects of the French method, featuring full-scale layout on the lofting floor. The video begins with a visual survey at a past compagnon masterpiece with a double geometrical staircase inside another form of spire:

Happy viewing!

Tuesday, December 22, 2009

Battari Shōgi 3

Back to the Wenge frame for the machi-ya produce stand. The word 'wenge' by the way, does not rhyme with 'henge', rather it's pronunciation sounds like a popular analgesic balm:

In common with the advert above, Wenge is a bit of a pain to work, pretty much as hard as stone, and very readily inflicts splinters (like the spiked club in the picture above) which rapidly fester if left in place for more than a few minutes. Not so much fun to work, and my plan is to attack it with carbide cutters as much as possible. It will be interesting to see how the final planing goes, however after that step I intend to apply Tung Oil to the frame pieces, and my approach will be to wet block sand the pieces in the oil. The oil/swarf mix will fill the pores of the wood, and as far as I know it's one of the better ways to polish this particular material. We'll see - maybe the planing won't be as bad as I'm apprehending. I can't have any chance of slivers in the finished piece, so I'm paranoid about that.

One of the first tasks to deal with was to cut the respective frame pieces to the same lengths. So, over to my shiny Martin t73 programmable sliding saw with digital motorized fence I went - - oh, wait a minute, I don't have one of those! Dang!

Okay, plan 'b' - I dragged the sawhorse outside and used some clamps and my Hitachi circular saw to gang cut the pieces:

It was bright out there with a sunny clear sky and reflection off of the snow crust. This gang-cutting seemed to work fairly well, though I know it is not an absolutely accurate method. The saw did make clean cuts in the stone, er, Wenge, all the same.

Next it was time to gang cut the long frame rails:

I set those frame pieces aside for the time being, and tackled the support block cut-out, which was facilitated by my Festool Jigsaw (it did the job, but the blade got smokin' hot:

Re-clamping allowed me to complete the other cut:

After doing the same cuts on the other support bracket, I used the mdf template prepared earlier and my router table to take the pieces to their finished overall shape:

I'm pleased with the form. I'll let those sit a couple of days and then have a bunch more work to do on them before they are ready. They will get a little fancy actually - I'm saving the details on that as a surprise.

Next I commenced the cut out for the frame crosspieces, of which there are six pieces. I am going to use housed tusk tenons for the connections, which I will double wedge instead of pegging from above. Here are the six crosspieces processed through stage one, which is a reduction to a housing 1.5" wide - the mortise housing will be 0.25" deep when completed. The tenon will be 0.5" thick and emerge dead-center on the long frame rails:

I feel like I got a decent amount completed today - see y'all next time. --> go to part 4

Monday, December 21, 2009

Tréteau XI

The next stage in the 19th Century French sawhorse build is to rough cut the half-laps for the braces. Previous installments in this thread are indexed in the 'Blog Archive' to the right of the page, so readers new to this blog might want to have a gander over there first.

I choose here not to take the saw to the line. My reasoning is that the half-lap does cut halfway into the timber, and there may be growth or drying stresses released by the cut. It occured to me that the piece could un-bow slightly causing the width of the lap to open up a bit, which would potentially spoil the fit a bit. So, my plan is to rough cut the half-laps, let the material rest a couple of days, and then trim the half-laps to dimension with high accuracy using a router and jig. With the router i can get repeatable and accurate results, so it is my preference to use it for the final cuts anyhow. After the lap trenches are done, then I will complete the lap joint mitered abutments, the cuts for which I intend to do with a chisel and paring block. I will need to make 6 different blocks to accomplish that, I do believe.

So, today some photos of the sawing and chop out. First I started with the interior x-braces, three saw kerfs per lap:

Then I chopped the waste out using a 10mm bench chisel and a small plane hammer:

Setting those aside, I then proceeded with the short side Croix de Saint André pieces:

What you can see in the picture are the new and exciting 'sawing socks' ® - part of the newly introduced Sawing for Teens line of fashionable wear. I'm sure most of you have seen the infomercials by now with Britney Spears, and what with Christmas just around the corner a pair of sawing socks ® would make a fine gift for the young aspiring woodworker in the family.

:^) Happy holidays everybody!

More ch-ch-ch-choppin';

And then bump up a chisel size for a quick clean out at the bottom:

Long side braces, rough-cut laps now done:

After the chips had flown, here is my tidy pile of rough-lapped pieces, the interior x-braces at right, the long side braces in middle, and the Croix de Saint André pieces to the left:

I'll let those bits hang out in the closet for a while, to move and groove as they see fit (probably not too much as the wood didn't seem to move appreciably while I cut it), and in the meantime I will return to the work on the battari shōgi and that delightful (?) Wenge awaiting some cutting work.

Thanks for dropping by today. --> Go to post XII

Sunday, December 20, 2009

Battari Shōgi 2

In the previous post, I described in detail the meaning of battari-shōgi - a folding bench installed on the front of Kyōto-area merchants houses, and showed some examples of these benches. Today I'll share with the readers the bench at the Bostons' Children's Museum as I found it, and begin the process of constructing a replacement.

The bench is made from Southern Yellow Pine, I do believe, and was constructed at the same time as the machi-ya installation into the museum. Thirty years of wear has left the frame of the bench quite eroded, and the panels with a delightful texture:

Nearest to view, the short-side frame member also has a long crack on it, though that did not become apparent until I had the piece back at home and commenced work upon it to remove the panels. I will keep the panels and construct a new frame. The existing frame joinery is a butt-jointed affair with screwed and plugged connections at the corners. The panels are held in a groove in the frame and are affixed to the cross-pieces with domed brass finishing nails. The panels attach to each other along their edges with double-pointed nails.

Here's a look at the bench folded up:

The cross pieces on the bench are tenoned and glued to the long frame rails. I plan to replace them with wedged through-tenoned parts, probably without glue (or I'll use rice-paste glue).

The hinges for the fold-out legs are formed out of 2"x2" stock - here's the right side one:

The other end:

The 2"x2" hinge pins are screwed to a cross piece at one end, and mortised into the frame at the other. Later on, I had the pieces apart:

The cylindrical part of the pins is not lathe-turned but formed using - presumably - chisel and plane. They are not perfect cylinders by any stretch, and therefore the fit to the corresponding holes in the legs is somewhat loose. I don't plan to duplicate that aspect of the previous fabrication.

Here's a look at the other end of the bench, folded up, showing, for one thing, the lovely metal catch that serves to keep it in place (one at each end):

At the bottom of the picture can be seen one of the two support brackets. The long side of the frame at the rear terminates in a pin, about 1.5" diameter, which fits into a socket in each bracket. Like the singing leg hinges, these main hinges were not made with precision, and likely had a somewhat sloppy fit from the get-go. Thirty years of being raised and lowered had caused the pin and socket interface to become quite worn, as one would expect and so loose that I was worried the bench could slip right out of the hinge. The looseness of the hinge points had led in turn to a lot of wear on the bottom edge of the bench frame - about 1/8" of wood had eroded away from the edge, also visible in the above picture (and the third picture above).

Here's a picture of the pair of support brackets, showing the condition of the sockets:

The brackets were fastened to the building by carriage screws and metal screws. The connections were very firm and took a little careful investigating to discover. I'm planning to use timber screws to fasten the replacements into position.

And as for the replacement, well, which wood to use. The museum agreed that replacement of the SYP with the same, or another soft wood, was not worth the trouble and expense. I suggested that, given the extraordinary wear and tear on the piece, compared to a 'normal' situation, it would make sense to construct the replacement frame out of a much tougher wood. There were a few possible choices in that regard, and it was recognized that the ideal choice would be a wood which matched the dark stained color somewhat closely, as a stained piece would suffer the issue of needing somewhat regular renewal of the finish, re-surfacing and so forth.

After some thought, I concluded that Wenge (millettia laurentii) would be an ideal choice. Wenge, a member of the legume family of plants, is found primarily in the open forests in the Southern Regions of Tanzania and Mozambique, and also occurs in periodically-inundated swampy forests in that region as well. It is a tree of medium size, growing 50~60 feet in height with a trunk diameter in the 30~36" range. It is a highly bend- and shock-resistant wood, that seasons without much distortion. As with many woods, the harvest of this tree is apparently not sustainable at present and it is only intermittently available from suppliers. A lot of the available timber is processed into veneer and flooring, and the bark is harvested in it's local area for the reddish sap, termed kino, an astringent and medicinal. It's fairly expensive to buy. I have not had much experience working the wood, though I know it is a bit challenging to polish. I happened to have one plank already on hand in my small reserve pile - it had found use previously as a planing beam. I ended up buying another 4 foot section of an 8/4 plank at the local hardwood supplier and I had enough to make the frame. This is precious material and I have a responsibility to make very careful and considerate use of it.

Here's a few pictures of the material, the first showing how it's natural color compares to the stained SYP:

Most of what I have to work with is near-perfect rift grain, though, as the above picture shows, the brackets have flat grain.

Some of my other pieces, now jointed and milled to dimension - I took the wood to the shop down the street and worked with one of their employees for an hour to process the pieces:

I've already started in on the layout- the first pieces I tackled were the main frame components:

The mechanical pencil lead is clearly visible on the dark surface so long as the lighting is sufficient.

Because the Wenge is leagues stiffer and denser than the SYP, it did not need to be as chunky. The original frame rails were 2.25" (50mm) by 5" (125mm) - I've slimmed these dimensions down to 1.75" x 4.5". The frame is still going to be stouter than it needs to be on structural grounds, however the aesthetic norm needs to be addressed as well. It's a compromise.

I found the original support brackets to be a bit on the rough side - the outer lines were not sinuous and there were intermittent flat sections in a couple of places, and they had been formed with little more than chisel and saw. I looked at a bunch of other battari-shōgi examples to compare their support brackets, and found that there was a diversity of forms. Thus, I felt safe in massaging the form a little to obtain something a little cleaner looking, while still keeping to a semblance of the original lines. I have made templates for the new pieces, and in the following photos you can see how the old and new shapes compare:

One key design issue were the main bench hinges. I fired up the SketchUp to explore the joinery. I told the museum that I would be stepping the quality of construction of the frame up a notch or two.

The points where the rear frame corners connect are the tricky bit. After some manipulation, I settled on a twin-tenoned construction with a lipped mitered return on top and a floating hinge pin 1" in diameter:

Notice the small curved section removed at the bottom edge which will allow the piece to smoothly rotate around the support ledge on the support bracket.

The twin tenons will also have root stub tenon, or mechi, which will serve to resist any tendency of the part to twist and to reinforce the tenons:

Here's the opposite piece which will be mortised:

The pin is to be a separate piece which I will glue into place. I considered machining the pin out of the solid frame piece, but I don't have access to such a large lathe (the ideal way to fabricate such a piece), and by using a reversible glue, like hide glue, any wear on the pin could be repaired by replacing only the pin. And the pin, for that matter, along with its mating socket (which will also be an insert) will likely last for a very long time as I am going to make them out of lignum vitae. The hinge should last, effectively, forever.

More to here for part 3