Thursday, February 19, 2009

A Desk job suits me sometimes

I mentioned in an earlier posting that I had gone to see Doctor H for a shoulder problem, upon return from doing some work in California, and I happened to have a timber frame conference shirt on at the time of my appointment. He started asking me about that, and a few days of discussion later, I had landed my very first furniture commission.

The good doctor had been excited by my description of timber frame carpentry, and liked the idea of a structure held together with wooden joinery only. While the reality of timber framing is that usually there are some metal fasteners in a structure, I told the doctor that I was confident that I could make a reception desk for his clinic that was fully demountable and used no metal fasteners whatsoever. The clinic was to open in 3 months. I received a deposit on the work and materials and off I went.

After some discussion with the doctor, somehow we made a choice to go with Purpleheart (peltogyne pubescens) for the frame of the desk. I had worked with it a little before, and knew it to be a very strong, rot-resistant wood that happens to be very purple. Many people, upon seeing this wood, assume it must be painted or stained. It comes from Central America primarily, where it is used in various applications where tough and durable wood is required, like dock pilings. It is not easy to work with, when you're cutting joinery and hand-planing the material. I was inexperienced at making furniture, and my choice in material reflected that.

I could obtain some dried and dimensioned purpleheart from a local lumber yard chain, but their price was a bit high. I discovered another supplier who had large 2.5" x 16" chunks of it, but not dried. The price was really good. I figured I could get around the problem of the wood being green by constructing my own de-humidifying kiln. I figured 3~4 weeks in such a kiln would bring the moisture content (m.c.) right down, and in that time I could undertake doing full scale drawings of the piece. I did those drawings on sheets of door skin ply, just like when doing geometrical drawings for hip roof work and other complicated constructions.

I built the kiln under my workbench against the wall, and put the purpleheart planks in there, then sitting at 18~22% m.c.. I ran the dehumidifier for 4 weeks, checking with my moisture meter frequently, and at the end, I had the material at 6~7% m.c. Even better, it was nice and straight still. The drawings were complete by this point, so I set to work slicing and dicing the purpleheart into the various pieces needed for the desk. I was building the desk very much like a timber frame, with a sill, posts, connecting girts, through girts (nuki in Japanese), and so forth. I found the wood to be well-behaved when sawn, springing or warping minimally after cutting up. Things were looking good, as they often do before going haywire.

After a week or so into the cut-out, I noticed that the purpleheart was oozing sap - a deep purple-black sap. It oozed it all over the place, face grain, edge grain, end grain. I'd never seen this before in purpleheart, so I initially just wiped the wood down with alcohol (or something like that), and that seemed to clean it up nicely. Trouble was, a few days later the sap returned. I wiped again. The sap returned again. A feeling of mild panic began to creep over me. I researched on the net and could find no other descriptions of this phenomenon with purpleheart. I began to wonder if I had some sort of weird batch of the stuff.

And then I figured it out, though reading more on the subject of drying wood. It seems that in the drying cycle, with many species, the heat has to be sufficiently high to crystallize the sap in the wood. I had experienced this issue, in fact, many times before with Douglas Fir timbers, which are infamous for pitch pockets. You can create the most beautiful timber frame, the house gets all buttoned up, the owner installs some lovely carpets, and then all of a sudden, it may be months later, a beam may suddenly begin to ooze out globs of pitch. All over the new carpet naturally. Pitch pockets in fir, if discovered in the cut-out phase, can be plugged with patches, 'stopped' as they used to say in the trade. Kiln drying isn't really an option for timbers larger than about 6"x6", though if available, radio-microwave frequency drying is a good solution.

Anyway, I realized with the purpleheart, that despite 4 weeks in my kiln and a low m.c. of 6~7%, had never been subject to drying temperatures at the required 180˚, for the right amount of time, for sap crystallization to have occured. Six weeks in, I realized I had a pile of bleeding purpleheart and there was no way to stop the bleeding without another session in a kiln, a kiln that could develop the necessary temperatures. No local kiln space happened to be available at that moment, naturally. I realized I was half-way into the job, and would have to start afresh. Now the panic wasn't so mild anymore. Terror loomed!

I explained the matter to the doctor, and reached an agreement with him where we would split the cost of the new wood.We agreed that a sap-bleeding desk wouldn't be a great motif for a medical clinic....

That replacement material was my potential profit on the job neatly swallowed, but it was a generous offer on his part to split the cost. I made a decision, based on conversations with a few more knowledgeable furniture makers, to move away from the purpleheart. I went down to the hardwood supplier and loaded up a bunch of Honduran Mahogany, about $1500. Six weeks to go until the clinic opened.

Take 2.

What followed was one of the tougher periods in my life as I attempted to meet the deadline. I worked 300 hours a month of the next 2 months. In the end, I was 2 weeks late in delivering the reception desk. The receptionist, who had been suffering with a temporary 2x and plywood desk for the 2 weeks was becoming quite hostile towards me. It was stressful. I had underbid the job as well, so I in fact completely ran out of money three weeks before it ended and had to borrow money to stay afloat and keep food in the fridge.

Finally though, the work came to an end. The Honduran Mahongany had proven to be a much better choice than the purpleheart, and I can see why it is one of the pre-eminent furniture woods.

I was definitely worse for wear by the end, and had developed tendonitis in my elbows from the excessive work schedule. Sleep had been difficult some nights. My girlfriend broke up with me because I was wigged right out, and had no energy for our relationship. It came out okay in the end though, since I managed to make it up with her and we're married now.

What follows in this thread, which will spread over several postings, then is an account, not of how I made the desk, but how it was assembled, a process that took 2-1/2 days.

Running on fumes physically and mentally, I set about to assemble the reception desk, and fortuitously, it was the weekend and the clinic was closed. I moved the parts into the clinic on Friday afternoon:

It was quite a tidy pile - I think around 140 separate parts. Then I set up a small table - the current reception desk - with some of my tools:

Just like a timber frame, the sill was the first item to be set up:

I apologize for the rotated image, I tried a bunch of tricks to deal with it, but it just stays rotated for some reason when I upload it. I think having been originally taken by the camera in a sideways position is the reason for this problem, but I'm not entirely sure. Also, I wasn't wielding the camera for these shots, so they are not always the pictures I would have chosen to take, but hopefully they will suffice.

As you can see, the desk is turning a corner - in fact it is in a 'J' shape, with one wing some 9' long, in three 'bays', a central bay, and then a shorter wing of 2 bays. If you make a framed structure in the form of a ladder-frame like this desk sill, one that turns a 45˚ corner at the front, there are several ways to deal with it. If I simply turned the frame through a 45˚at the miters, then while the front of that turned section would be the same as what you see in the picture, the inside of that turn, where the receptionist worked, would come to a point, thus forming a 90˚ inside corner. I didn't think that having a 90˚ inside corner was particularly ergonomic, or attractive way to do it, so I chose a different, more involved solution. The receptionist was going to work at the inside corner, that's where her computer and keyboard would be positioned, so the inside of the frame had to be turning 45˚ and be wide enough to work at comfortably.

The front of the frame does turn in 45˚, however, the actual angle of abutment between the pieces is un-equal. The un-equal miters solved the problem, and the net result in this case that all the pieces in the central bay are about 10% wider than the pieces they connect to at the side wings. In the picture above, the part that turns in the foreground is some 10% wider than the longer sill it connects to. This is hardly noticeable to the viewer of course, but it does allow for clean flowing connections at the miters. This is an example of where complexity in design and execution can produce a seamless, elegant result, I think. You be the judge.

Now it was time to put a few 'posts on, as well as the receivers for the grill bars, or koshi, which, as you can see, have numerous dovetail mortises to receive the lower ends of the koshi:

You can see in this picture the wedge that locks the sill joint has been started. This joint is similar to the one described for the vanity in earlier posts, but with one wedge. The sill joint itself has a female dovetail mortise that, as the joint is squeezed tight by the insertion of the wedge, grabs a hold of the male dovetail tenon on the end of the post. Now, normally in timber frame construction I would never connect a post to a sill or beam with a joint like this - in fact I strive to avoid dovetails altogether in timber frame work. Further, I don't like to employ mud-sills for that matter in timber frames. In this case though, having a sill allowed the piece to be slid around easily on the carpeted floor, and with the sill only being 1.375" thick, a regular tenon on the post would have left inadequate relish for conventional pegging (and it would have been very hard to mortise for the peg, at 0.25" square right through the sill, or even insert a peg though such an assembly). So, you can see that although my basic design approach is like a timber frame structure, I make changes in joinery to suit the nature of the project.

For the posts that exist in the middle of the sill run, I used a over-size dovetail mortise that allows the post to drop in vertically, then a parallelogram-shaped wedge is driven in from the front to lock the dovetail tight:

You can see a little tub of wax on the floor - I used the wax to lubricate the Wenge wedges and pins. That was my last sideways picture - I will only post up the pictures that are oriented correctly. There's lots of pictures, so not much will be missed.

Once I had a run of posts in place, I placed the piercing stretchers, nuki. These slid right through a mortise in each post, then once though all posts in a run, drop down onto a cogged and housed lap joint. A pair of crossed wedges later go in above the nuki, and lock it down into position. Nuki are used in Japanese building wall systems, buried inside the walls and covered by plaster:

Where the nuki meet at the corner posts, they cog-lap onto each other. In the above picture, we are sliding the short nuki into place above the longer one, the end of which is visible on the side of the post.

Here is the frame, now with front sills down, all front post in, and nuki connected:

I should mention that I had some help - a friend of mine, Alan, another cabinetmaker on the island, kindly volunteered his time, un-asked, to help me put the desk together over the 2-1/2 days. He had his nephew in town, so he was along too, and Doc H even was helping. I think he got a real kick out of being able to participate. Another friend of Al's held the camera.

I tightened up a few connections, and trimmed the excess off the sliding parallelogram pins:

I might add, that generally I leave most connections loose for as long as possible during assembly for joined structures.

Then it was time to fit the rear sill pieces, and the rear posts:

The vice grips, an essential tool in any kit, were for pulling the temporary fixing splines I employed back out of the connections. I had to invent some unique connection to bring this desk together - here I'm tapping in a peg, which goes through a internal cross-spline in a rear sill miter:

Like the front, the rear posts had male dovetail tenons which were fixed, at the miters, by the squeezing together of the miter, each half of which comprised half of the dovetail mortise. The intermediate sill connections, visible at the right of the above picture, were fixed by drop-in-and-slide dovetails, wedged from the side.

More to come, of course: Part II.


  1. Chris

    Don't know if you will ever read this, since its an old post... I've had experience with Doug. fir pitch pockets; one in a beam that oozed for years, luckily on a tile floor. Years later a carpenter I was working with installed a monstrous D. fir mantle. The next day there was a glob of pitch on the hearth. The NEXT day I brought my heat gun and put a piece of scrap ply on the hearth. The pitch ran like water with the heat! Not sure if we drained it or crystalized it, but we had no more problems.

    Now you caught me rereading old posts!


  2. Thanks for the comment Tom- I am glad to receive them no matter how old the posting.

    Sounds like your strategy of using the heat gun solved the problem that time - good thinking!



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