Tuesday, November 10, 2015

A Ming-Inspired Cabinet (7)

Last post in this thread took place back in September.

After acquiring some fabulous shedua stock for the drawers, I have been sidetracked with other projects. In the past couple of weeks, I have been able to turn my attention to this sideboard build, which will likely consume the next 4-6 months of shop time. Not just shop time, a fair amount of home time as well, as that is where I do my CAD work.

The past 10 days or so have been largely about working through the drawing with a fine-tooth comb, in preparation for rough cutting of stock. This is one of those initial stepping stones across the river I must traverse to realize a successful outcome in this design build process. After so many hours have already gone into a design, and having received the go-ahead from the client, one could be forgiven for being eager to get to work slicing up the material.

However, in this case, maybe more particularly than with other projects, I am dealing with material which is more or less irreplaceable. Take a look around for wide and thick slabs of bubinga and you aren't going to come up with much these days. The cessation of raw log exports from Cameroon, plus the economics of wood drying being what they are, have made the combination of wide and thick bubinga stock rather difficult to come by in what is otherwise a fairly readily available material.

I wanted to be totally certain about how I would slice and dice a slab like this:


About 600 lbs.  of material lying there, 50" wide, 9' long, and 3" thick. You only get one chance with cutting up a chunk like this.

While the design of the cabinet was largely resolved, as I worked on obtaining some take-offs of each part I began to have some second thoughts about one of the framing details.

Just for a refresher, here is that design as it stood in September:


The niggling detail that wasn't sitting right with me was the negative space created by the pillow block layer on the support stand. The pillow blocks act as spacers between a pair of beams, atop of which the cabinet carcase would be placed.

While I liked the aesthetics of that arrangement, and the practical consideration of the even air circulation around the beams that associated, there was a drawback. The drawback is that the pillow blocks are little more than spacers, and thus only modestly tie together the two beams in a structural sense. What you have there is essentially a horizontally oriented 'ladder'. And that ladder, when you get down to it, is not as optimally stiff as it might be be if an alternate structural arrangement were to be considered. A ladder is poor when it comes to shear loading, as would be experienced with weight bearing downwards.

Given that this is a sideboard, one with reasonable capaciousness in the volume of stuff that can be accommodated, it is not beyond the realm of consideration to see that the cabinet could become heavily loaded at some point in time. Added to this, there is time itself, a constant, as is gravity. What I fear is that over time the weight of the cabinet and its contents might tend towards deforming the middle of the cabinet slightly downward. I'm thinking the effects of this are the sort of thing that might start to accrue after 20, 30, or 40 years. Certainly, if you look at old furniture pieces in museums, one can find no shortage of examples where gravity and the proclivities of wood to move have made decidedly clear effects. And those effects are rarely if ever on the positive side.

The problem with some slight amount of deformation in the middle of the cabinet down the line is that it would have strong adverse knock-on effects in terms of the fit of the double bifold doors and the many drawers inside the cabinet. If the doors and drawers are to be fitted with reasonably close tolerances, then one also exposes the fitted parts to fitting problems if slight deformation in the surrounding casework were to occur down the line.

I concluded therefore that this area of the support stand framing was deserving of a second look.

While I had a certain attachment to the use of pillow blocks as a tie-in to the previously built side and coffee tables for the client, it was also obvious that from a normal standing position the pillow blocks were largely concealed from view. To get a look at the negative space framed by those pillow blocks would involve laying on the floor - after all, the support stand for this cabinet is all of 14" tall. In short then, the aesthetic aspect of the pillow blocks was a minor aspect.

From my understanding, for every increase in depth of a beam, stiffness is improves by a square of that increase. The current arrangement featured a pair of beams, one at 1.375" thick, and another at 1.125" thick, a total of 2.5" of material.

To wit:
For a beam with rectangular cross section, bending (flexural) strength is a function of the square (second power) of the depth (height). A similar, though more complicated, relation applies for I-beam shapes.
With all other factors the same, a rectangular beam with depth of 12 inches has a bending strength that is 4 times that of a beam with 6-inch depth.
Bending strength of the beam is dependent on the "section modulus".
From: http://www.structural101.com/Beam-Design---Basic.html

If you start looking at the most efficient arrangement of material to make a beam stiff, you end up at an 'I' -beam. So, with that ideal in mind, I reconfigured the lower beam into an inverted 'T' shape which tongues and grooves into the upper beam.

Here's the revised lower beam form:


This change more or less gives the lower beam a depth of 3", so as to make a total of 4.125" for total depth of the two beams connected together. The previous design gave me 2.5" and now there is 4.125", an increase of 65%. Without going into detailed calculations, I can expect, as a ballpark, an increase in stiffness with the new arrangement on the order of 2.72 times greater than previous.

Aesthetically, the change is slight, and, as noted above, most of the change will be out of view until one's eyes are below knee height:


The confluence of parts at the cabinet corner remains pleasing to my eye:


Feeling sufficiently confident about the design of the stand, and unlikely to make significant changes to the proportions of the cabinet carcase atop the stand, I felt I could start in on the cut out today of the parts comprising the lower stand.

The big slab was sufficiently heavy that I asked a couple of guys from the adjacent shop to help me lower it down to the floor. Once down, I marked some lines, snapped some ink down, and got busy with the larger Makita circular saw:


The board leaning against the posts is one of the wide Goncalo alves boards I am hoarding.

Able assistance to the cut out task was afforded by the larger re-saw:


Here are some of the rough-cut legs - keep in mind that I'm making two of these cabinets:


The rest lay piled up after cutting out - the stock for the front door panels, the stretchers, and what will be the inverted 'T' section lower beams:


What remained from the slab was still a chunk, still pretty heavy, but entirely flatsawn:


The standing piece to the left, also 10/4 stock, is a piece left over from a Ming-influenced table I built a few years back. That material will be sliced up further to become part of these two cabinets.

The sawing was sufficiently vigorous that the Makita 380mm saw began  to complain. Smoke is never a good sign. Stripping the machine down afterwards was straightforward however.

Best case scenario: it looks like I will need to replace the rear arbor bearing and the brushes:


Worst case scenario is that the motor will need rewinding, which is another way of saying that I will need to buy a new saw. Smoke generally means that the insulation around the windings has burned off.

The arbor windings don't look too bad, though there is some soot on the commutator surface:


Both brushes and bearing are fairly cheap and easily sourced, so hopefully this saw will be up and running in another week or so. Maybe that is just wishful thinking though. Fortunately it got me through some critical steps today, so I won't be needing it for a while. Those long rip cuts in a hard wood are tough on a saw, and I guess I must have pushed it a little too hard today.

All for now. Be prepared for a bunch more posts on this thread as the build gets underway over the next few months. On to the post 8 .

13 comments:

  1. YAY! Woodworking!

    I think you're design change is a great one, I like the solid brace it looks right.

    Amazing that so much of this cabinet is coming out of one big slab of bubinga.

    Brian

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    1. Brian,

      sounds like some pent up needs there - I take it that lots of posts in a row on machinery is not your cup of tea?

      Amazing also is that despite having that large slab of bubinga, other bubinga stock will be required to complete the cabinet. The flatsawn mid section of that big slab is probably not going to be completely usable.

      ~C

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    2. I have been refreshing the blog daily. :)

      cheers,
      Brian

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    3. Whaat?! You should be checking every 15 minutes. I mean, c'mon, let's be reasonable!

      ~C

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  2. I'm looking forward to another build thread! I hope that this winter is a good bit milder for you. I was shivering in Florida reading your posts last year.

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    1. Anton,

      appreciate that, and what's this talk of winter? Not sure I'm ready to acknowledge such an outrageous situation, however truth be told, thoughts have been turning towards an electrically heated jacket...

      ~C

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  3. Oh no, not the Makita! That thing has such a great Streamline Moderne look to it.. Well, tools are meant to be used, yadda yadda..

    Since I'm apparently in a "reminding" mode, please don't forget during the toolbox discussion in the Forum you were going to take a picture of the shipping box that saw came in for me - I was curious about what the lid and inner support structure looked like.. TIA Darrell

    PS I think the big Mafell cuprex motors have overload protection - just sayin' ;)

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    1. Darrell,

      well, thinking the Makita 380mm has probably cut its last board.

      Too late on the shipping crate photos i'm afraid.

      And the Mafell would be nice, but at 4 times the price of the Makita, given my occasional use of the tool it is probably not justifiable as a purchase anytime soon. Now, if Mafell USA prices were remotely competitive with Mafell prices in Europe, it might be a different story.

      ~C

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    2. The Festool TS 75 could almost have sliced up that slab. It's a shame that none of the track saws go deeper than 2 3/4".

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    3. Anton,

      The Festool would have been very close to being able to cut through in one pass, but not quite.

      I think the Makita crap-out was not just about these rips cuts in this slab, but all the other long rips in various other timbers, including a bunch of bubinga, that took place previously. Wear and tear adds up and I pushed the saw a little harder than might have been prudent. The motor burned out at the end of two long rips in a row in 3.125" stock without a break.

      ~C

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    4. Anton,

      I believe that all of the Mafell saws, even the 16" model (MKS 185Ec) will fit on the Mafell track. so you can have a track saw with 6"+ of cutting depth...

      ~C

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  4. Can't resist commenting on the structural aspects of this post!

    You're right about stiffness being proportional to the square of the depth, and that an 'I' is an efficent beam shape. The thing to keep in mind, though, is that the web (the vertical middle portion) is critical to ensure continuity between the flanges (top and bottom). To use an analogy, a phone book is not that stiff if you bend it, because the pages just slide against each other. If you glued all the pages together, it would be multiple orders of magnitude stiffer.

    So, your two beams separated by pillow blocks are only about half as stiff as a single beam of 2.5" would be, but replacing the pillow blocks with the solid 'web' is about 13 times as stiff as original two beams. These are all estimates of your geometry but that's the gist of it. This also assumes that the 'tongue and groove' you describe between all the pieces is locked in along the full length.

    Glad the build has resumes, and looking forward to reading about it!

    Daniel G

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    1. Daniel,

      thanks for your comment. Assumptions however have lead you to a land called 'Erroneous'. I've visited that land many times myself and have a good idea s to its contours.

      I realize that if I glued the dado of upper beam to the vertical web of the lower inverted 'T' section, I would end up with a bonded 'I' beam, which would be the stiffest arrangement by far, however I will not be gluing the two parts together. That decision not to glue comes about from two concerns, one being that the intended assembly directions of the two beam layers (the lower assembles as a lap joint, the upper one does not) are not the same, and that a glued assembly is not a readily demountable/repair-able assembly, and I strive to avoid that form of construction in my work.

      I have thought of various wooden interlocks I could make between the two beams, but wish to avoid anything showing on the upper surface, and that limits the possibilities somewhat. Besides, if the interlock does not achieve a truly non-sliding connection between upper and lower beam, it is likely to be of marginal help in increasing stiffness. I've concluded therefore that further reinforcement is unnecessary.

      Removing the pillow block layer and replacing it with a solid vertical web from the lower beam realizes a significant increase in stiffness, however not as ultimately stiff as would be possible if I glued the two beams together (or, indeed, machined the entire assembly from one piece of wood).

      Trying to balance the factors which are important (stiffness, ease of assembly, demountability, aesthetics) is a bit of a juggling act at times.

      ~C

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